tool design theory (dcd)

TRAINER GUIDE - II

TOOL DESIGN THEORY (DCD)

(5

TH SEMESTER)

DTM

MSME TOOL ROOM INDO GERMAN TOOL ROOM

AHMEDABAD

SUBJECTS &

COURSE CONTENT

In

DIPLOMA IN TOOL & DIE MAKING (DTM)

VERSION - 0

HRS.PLANNED

160

A Introduction A1 A.1.1.1 Die Casting Background

A.1.1.1(a) What is Die Casting

A.1.1.3(a) Ciassification Of Castings

A.1.1.3 Principle Of Die Design And Process

A.1.1.3(a) Advantages Of Die Casting Techniques

A.1.1.3(b) Technique of filling die cavity

A.1.1.4 Comparision of die casting with other products.

B Die casting operations B1 Gravity die casting B.1.1.1 Introduction

B.1.1.2 Gravity die casting process

B.1.1.3 Limitation of permanent mould castings

B.1.1.4 Principle Of permanent mould casting

B.1.1.4(a) Progressive solidification

B.1.1.4(b) Minimum turbulance

B.1.1.4(c) Air and gas clearance

B.1.1.5 Suitable casting metals for GDC

B.1.1.6 Selection of mould materials

B.1.1.7 Permanent mould casting machines

B.1.1.8 Gravity die casting mould

B.1.1.9 Principle of mould design

B.1.1.10 Solid graphite permanent mould

B2

Pressure Die casting (cold

chamber) B.2.1.1 Introduction

B.2.1.2 Pressure die casting machines

B.2.1.3 Classification of die casting machines

B.2.1.4 Cold chamber machines

B.2.1.5 Cold chamber machines and parts

B.2.1.6 Classification of cold chamber die casting machines

B.2.1.7 Horizontal cold chamber die casting machine

B.2.1.8 Vertical cold chamber die casting machine

B.2.1.9 Comparision of cold and hot chamber process

B.2.1.10 Process parameters and controls

B.2.1.11

Cold chamber die casting processing metals and

alloys

Over view of mass

production of Casting Parts

Produced by various

Casting Process

SUB CODE

SUBJECT OBJECTIVE : This subject under Core Technology Group intended to teach the trainess and make them to understand & apply the concept, principle and procedure of

knowledge in design of Die Casting Dies and other metal casting process for different types of ferrous and non ferrous alloys.

UNIT

CHAPTER

NO.

THEORY /PRATICENAME OF THE SUBJECT

TOOL DESIGN THEORY - DIE CASTING DIES

PR

AC

TIC

E

TO

PIC

S &

SU

B T

OP

ICS

THEORY

TOPICS & SUB TOPICSCHAPTER P

AR

T

B.2.1.12 Cold chamber die casting die

B3

Pressure Die casting (hot

chamber) B.3.1.1 Introduction

B.3.1.2 Classification of die casting machines

B.3.1.3 Hot chamber machines

B.3.1.4 Hot chamber machines and parts

B.3.1.5 Classification of hot chamber die casting machines

B4 Defects and remedies of die casting componentsB.4.1.1

Introduction to various defects of die casting

components

B.4.1.2 Identification of defects

B.4.1.3 Classification of defects

B.4.1.4

Description for different types of defects and their

causes and remedies

C

Elements of die casting and their

function C1 Feeding system C.1.1.1

Introduction of feeding system of die casting

components

C.1.1.2 Definition of elements and fuction of feed system

C.1.1.3 Feed system for gravity die casting dies

C.1.1.4 Feed system for hot chamber castings dies

C.1.1.5 Feed system for core chamber casting dies

C.1.1.6 Classification of gate systems

C.1.1.7 Principle of feed system

C.1.1.8 Balancing of feed system

C2 Cooling system C.2.1.1 Introduction to cooling of die casting dies

C.2.1.2 Definition of elements and fuction of cooling system

C.2.1.3 Principle of cooling of die casting dies

C.2.1.4 Classification of cooling systems

C3 Ejection system or techniquesC.3.1.1 Introduction

C.3.1.2 Various elements of ejection techniques and fuctions

C.3.1.3 Various ejection techniques

C.3.1.4 Principle of ejection system

D Material handling D1 Pre-casting D.1.1.1 Introduction

D.1.1.2 Principle of metal casting technique

D.1.1.3 Classification of metal casting technique

D.1.1.4 Pre-casting techinque

D.1.1.5 Pre-casting techinque related equipments

D2 Post-casting D.2.1.1 Introduction of post-casting technique

D.2.1.2 Principle of metal casting technique

D.2.1.3 Classification of post-casting techniques

D.2.1.4 Trimming

D.2.1.5 Post machining

D.2.1.6 Surface decoration

D.2.1.7 Coating

D.2.1.8 Deburing operations

D.2.1.9 Post-casting related equipments

E Maintenance, safety and storage E1 Maintenance, safety and storage with respect to die casting die and machineE.1.1.1

Indroduction to understand the necessity of

maintenance, safety and storage of die casting dies

and machines

E.1.1.2 Concept of safety

E.1.1.3 Concept of maintenance

E.1.1.4 Concept of storage

E.1.1.5 Safety of die casting die

E.1.1.6 Safety of die casting machines and their equipments

E.1.1.7 Safety of personnel

E.1.1.8 Check list for maintenance of die and machines

E.1.1.9 Storage of die casting die

F Specification F1 Specification of die, material and machinesF.1.1.1

Introduction to use and application of the specifications

pertaining to die casting dies, materials and machines

for tool design data

F.1.1.2 Die casting die specification

F.1.1.3 Die casting metal specification

F.1.1.4 Machine specification

F.1.1.5 Specification for processing

G Computer aided information analysis G1 Introduction of simulation and analysis packagesG.1.1.1

Introduction to use and application of simulation

package

G.1.1.2 Concept of process parameters

G.1.1.3 Classification of simulation packages

G.1.1.4

Principle of selection of process parameters using

software packages

D D2 TD 1.2,5.3d

D

Preparation & Work /Data

Sheet & Mold, Material

&Machine TD 2.1 Introduction to Use & Work sheet for Mold Design

D TD2.2 Definition , Concept & Principle &

D TD2.a Mould ,plastic material Specification

D TD2.2b Estiimation & material

D TD2.2c Machine hours

D TD 2.2d process parameters

D TD 2.2e

Component Geometry dimensional and tolerance &

Constraint

D TD 2.2f Mold design

D TD 2.2g

work/data sheat format for the design parameter with

respect to Mold , machine material & process

parameters

D TD 2.3 Work sheet for ijection mold

D TD 2.4 Work/Data sheet for mold Compression

D TD 2.5 Work/ Data sheet for parameters

D E1 TD 2.6 Work/Data sheet for Blow mold

E Conceptual Design Sketching Conceptual TD 1.1 Introduction

E TD 1.1a Application & atternative Conceptual Design

E TD 1.2 Definition & Concept &

E TD 1.2a Conceptual Design

E TD 1.2b Evaluation

E TD 1.3 Procedure

E TD1.3a

Develop alternative Conceptual design Using design

Parameters

E F1 TD1.3c Select the optimal Design

F Design & Moulds

Draw the assembly and

Details diagrams & mold TF1.1a Flow Chart for Development & Design

F TF1.1b Preparation Design Data Sheet

F TF1.1c Preparation Concept Drawing

F TF1.1e Assembly Drawing method (TA 1.1.c)

F G1 TF1.1f Details Drawing method

G Mould Data Bill & Materials TG1.1(a) Introduction to Bill & Material

G TG1.1(b) Elements & Bill & Material

G TG1.1 ( c )

Preparation & Bill & Material by Appropriation Selection

& Material, Material size

G G2 TG.1.1 (d) Representation & Standard parts in Bill & Material

G Mould Data TG. 2.1 (a) Introduction to mould Data

G TG.2 .1 (b) different Element & mold Data

G H 1 TG.2.1 ( c) Preparation & mold data

H CAD/CAE TH 1.1 (a) Introduction to CAD/CAE SoftWare

H TH 1.1(b) mould Design Soft wares

H TH 1.1 ( c ) Application & Softwares in plentic Processing

H H2 TH 1.1 ( d)

Different Types & Application SoftWarer for the plastic

processing

H

Introduction to Design &

mould wih CAD TH .2 .1 (a) Different types & soft wares for mould Design

H TH 2 . 2 (b) Difference between 2D Drawing to 3D Solid models

H TH 2.3 ( c ) Sequence & mould Design Using CAD Package

H TH 2.4 ( d ) Details Drawing from Assembly Drawing

H TH 2.5 ( e) Associativity between 3 D model to 2D Drawing

H TH 2.6 ( f) Advantage & 3D model CAD Soft wares

- 1 -

INDO-GERMAN TOOL ROOM, AHMEDABAD

DIE CASTING TG-2THEORY

UNIT-1CHAPTER – A1

OVERVIEW OF MASS PRODUCTION OF CASTING ARTS PRODUCED BY VARIOUS

CASTING PROCESS

- 2 -


CHAPTER OUTLINE

A1.1.1 Die casting Background

A1.1.1 (a) what is Die Casting

A1.1.2 Classification of Castings

A1.1.3 Principle of Die Design and Process

A1.1.3 (a) Advantages of Die Casting Techniques

A1.1.3 (b) Technique of filling Die Cavity

A1.1.4 Comparison of Die Casting with other Process

- 3 -


A1.1.1 DIE CASTING BACKGROUND

Die casting are among the highest volume, mass - produced items manufactured by the metalworking industry. Die casting are important components in thousands of consumer, commercial and industrial products such as automobiles, household appliances, recreation, hobby and leisure -time products, farm and garden equipment, elec trical equipment and ordnance, general hardware, power tools, computers and other business equipment, instruments, toys, novelties and a great many other too numerous to mention. In fact, die casting have greater utility and are used in more appl ications than components produced by almost any other metal forming process.

Die –casting is a process involving the injection of molten metal at high pressure (as opposed to casting by gravity pressure). It is beloved to have begun sometime durin g the middle of the 19 th century. According to records, in 1849, Sturges patented the first manually operated machine for casting printing type.

Another 20 years passed before the process was extended to casting other shapes. The casting of printer’ s type led to patents which eventually resulted in development of the Linotype machine by Ottoman Megenthaler.

The earliest commercial applications for die castings occurred in 1892 when parts were produced for phonographs and cash registers. Mass product ion was further encouraged when the H.H. Franklin Company began die casting Babbitt alloy bearings for automobile connecting rods shortly after the turn of the century.

Various compositions of tin and lead were the first die casting alloys. Their importance and use declined, however, with the development of zinc alloys just prior to World War I. Aluminum alloys for die casting made their commercial debut about 1914. Magnesium and copper followed shortly thereafter.

During the 1930s, many of the alloys we know today had become available. Modern science and technology, metallurgical controls and research are making possible still further refinements resulting in new alloys with increased strength and stability.

Through the years, many significant technol ogical improvements have been made to the basic die casting process, to die steels and to die construction, as well as in casting machine design. Improvements have not only extended the capability and production capacity of the process, they have been tremendously effective in expanding die casting applications into almost every know market.

A1.1.1 (a) WHAT IS DIE CASTING

Die casting is a manufacturing process for producing accurately dimensioned, sharply defined, smooth or textured -surface metal parts. It is

- 4 -


accomplished by forcing molten metal under high pressure into reusable metal dies. The process is often described as the shortest distance between raw material and finished product. The term, “die casting” is also used to describe the finishe d part.

The term “gravity die casting” refers to castings made in metal molds under gravity head. It is known as permanent mold casting in the U.S.A and Canada. What we call “die casting” here is know as “pressure die casting” in Europe.

Ever since m an discovered that metals could be melted, he had tried to form these metals into shapes useful to him by pouring the liquid metals whose shape they retain during and after solidification. The casting of molten in moulds is one of the oldest methods developed by man to shape metal objects.

These are called molding or casting and are classified depending upon the molding method, mould material or casting process employed.

A1.1.2 CLASSIFICATION OF CASTING

1. Sand Castings:a. Green sand molding b. Dry sand molding c. Shell molding d. High pressure molding (using high pressure molding machine)e. Floor and pit molding

2. Metal mould castings:a. Permanent mould (gravity die casting)b. Semi- permanent mould casting (using metallic molding machine)

3. Plaster mould casting.4. Investment casting.

In metal mould castings, the mould consists of two or more parts, is used repeatedly for the production of many casting of t he same form. Where as in the other molding processes the mould is destroyed for each casting produced. A great deal of these type of casting processes are still employed for the production of castings.

The summary of each of these molding and cas ting processes is furnished for comparative study at the end of this chapter. Each method has its own advantages and disadvantage like finish, dimensional accuracy etc.

In order to produce cast articles more efficiently the permanent steel mould was developed. The molding of non - ferrous metals and their alloys with relatively low melting temperatures in permanent steel mould under pressure is called Die Casting. Castings produced by pressure die casting process are distinguished by their characterist ic accuracy smoothness and surface quality The die castings are made with minimum expenditure of metal and they are accurate in sized to

- 5 -


the extent that very little or no subsequent machining is necessary after removal of the gate and flash . Th e die castings made with hot or cold chamber machines are called Pressure Die Casting.

A1.1.3 PRINCIPLE OF DIE DESIGN AND PROCESS

First, a steel mold capable of producing tens of thousands of castings in rapid succession must be made at least two secti ons to permit removal of castings. These sections are mounted securely in a machine and are arranged so that one is stationary ( fixed die half) while the other is moveable ( injector die half ) To begin the casting cycle, the two die halves are clamp ed tightly together by the die casting machine. Molten metal is injected into the die cavity where it solidifies quickly. The die halves are drawn apart and the casting is ejected. Die casting dies can be simple or complex , having moveable slides, co res, or other sections depending on the complexity of the casting.

The complete cycle of the die casting process is by far the fastest known for producing precise non -ferrous metal parts. This is in marked contrast to sand casting which requires a new sand mold for each casting . while the permanent mold process uses iron or steel molds instead of sand it is considerably slower, and not as precise as die casting.

A1.1. 3 (a) ADVANTAGES OF DIE CASTING TECHNIQUES

Gravity Die Casting:-

The die is built up of parts or elements made of metal ( generally cast iron or steel ) . The design is adopted to the shape of the article required to be produced, so as to enable easy assembly, pouring and extraction These operations constitute a cycle of operations which when repeated in a certain rhythm, determine the output rate of the equipment The various operations of assembly and disassembly of the die may to some extent be mechanized. In this process liquid flows into the die entirely under its own weight. It is form this features that the term “gravity Die Casting” was coined.

Pressure Die Casting:-

This technique is a development over the gravity die casting and has the following characteristics.

a. The die is mounted between the two plates called “PLATENS’ of press, generally of horizontal type, by means of which it is closed and opened.

- 6 -


b. The movement of the die follows that of the machine, and this determines the general directions of assembly.

c. The liquid metal is generally injected by the action of a piston, which forces it through the die from a compression chamber This is referred to as a hot chamber, if it is situated inside the molten metal which is heated by a furnace forming part of the assembly a cold chamber if it is fed with metal which has been melted in a furnace separate from the machine.

Advantages of Pressure Die Casting Process:-

Die casting components parts, decorative trim, and /or finished prod ucts offer many features, advantages and benefits to those who specify this manufacturing process.

1. Die casting provides complex shapes within closer tolerance than many other mass production processes.

2. Die castings are produced a t high rates of production. Little or no machining is required.

3. Die castings can be produced with thinner walls than those obtainable by other casting methods …. much stronger than plastic injection moldings with the same dimensions

4. Die casting provide parts which are durable, dimensionally stable, and have the feel and appearance of quality.

5. Die casting dies can produce thousand of identical castings within specified tolerances before additional tooling may be required.

6. Zinc castings can be easily plated or finished with a minimum of surface preparation.

7. Die castings can be produced with surfaces simulating a wide variety of textures.

8. Die cast, surfaces, as cast, are smoother than most other forms of casting.

9. Holes in die castings can be cored, and made to tap drill sizes.

10.External threads on parts can be readily die cast.11.Die castings provide integral fastening elements, such as bosses and

studs, which can result in assembly economics.12. Inserts of other metals and some non-metals can be die cast in place.13.Corrosion resistance of die casting alloys rates from goods to high.14.Die castings are monolithic. They combine many functions in one,

complex shaped part. Because die castings do not consist of separ ate parts, welded or fastened together the strength is that of the material, not that of threads or welds, etc.

15.More complex shapes can be produced by the pressure die casting process than gravity die casting, Ex Carburetor.

- 7 -


16.Since the dies are fil led by pressure, castings with thinner walls, greater length to thickness ratio and greater dimensional accuracy can be produced.

17.Production rates are higher in pressure die casting, especially when multiple cavity dies are used.

18.The castings are p roduced as almost completely finished parts, the investment in inventory and factory floor space reduced to a minimum.

19.Dies for pressure die casting can produce many thousands of castings without significant change in casting dimensions.

20.Metal cost is often lower than in other casting process, because pressure die casting permits components of thinner sections.

21.Many die casting can be plated (finished) with minimum surface preparation.

22.Some Aluminum alloy pressure die castings can be develop ing higher strength than compared sand castings.

The Principle Limitations of the Pressure Die Casting Process: -

1. Casting size is limited. The casting weight seldom exceeds 50 1b and normally is less than 15 lbs.

2. Depending on the casting contours and gating, difficulty may be encountered with air trapped in the in the die. Trapped air is principle cause of porosity.

3. The die casting facilities, consisting of the machine, the auxiliary equipment and the dies are r elatively expensive. Because the die castings are small, large quantities of castings are required for the process to be economical.

4. Commercial use of the process is limited to metals having melting temperatures not higher than these of copper - base all oys, with few exceptions.

Dies can be produced for simple and complex parts. Parts having external undercuts or projections on side walls often require slides which increase costs. In many cases however, resultant savings of metal or other advantages such as uniform wall sections, offset the extra cost or effect a net economy in overall costs. This is especially true when large quantities are involved.

A1.1.3 (b) TECHNIQUE OF FILLING DIE CAVITY

As the name “ Pressure Die Casting” implies, injecti on of the molten metal into the mould or die cavity is done under pressure. The thin walls as well as the various bends around the corners and the edges of complicated die castings offer considerable resistance to complete filling of the mould or die. Therefore it is necessary that the metal moves through the die with high velocity before it settles in the mould cavity. The air present in the cavity has to be displaced by the entering metal. The air could be displaced by providing air vents in the die or by connecting the cavity to vacuum before the metal is injected into the die.

- 8 -


Vacuum die castings are used only for small parts made of low or high melting light alloys.

Pressure die casting necessitates a particularly care fu lly study of the design and shape of the articles to be produced.

The choice of technique involved for the production of a given article is governed by many factors, the most important of which are as follows :

1. Mechanical properties2. Dimensional accuracy 3. Complexity of shape4. Surface condition5. Number of casting to be produced 6. Production time 7. Production cost

Certain considerations of strength, precision or surface condition indicate a particular technique. It is always important that the production cost of the article desired should be estimated. The production cost must take into account.

a. Cost of mould b. Cost of the injection machinec. Cost of subsidiary operations (melting. trimming and machining)d. Actual weight of the metal.

The careful study of the production cost of an article based on the above factors and related to the number of articles to b e produced determines the limit of viability of a particular technique to make the process economical.

The advent of mass production has made possible the study and practical application of mechanized means of pressure die casting. As a result of considerable capital investment it is possible to mass produce articles at extremely competitive prices. For example, an electric coffee mill may be sold at a price lower than that of most of its component parts, if these were to be produced individually other than the die casting process.

A1.1.4 COMPARISONS OF DIE CASTING WITH OTHER PRODUCTS

Plastics Injection Molding: -

Compared with plastic injection moldings, die castings are stronger , stiffer, more stable dimensionally, more heat resistant, and ar e far superior to plastics on a properties /cost basis They help prevent radio frequency and electromagnetic emissions. For chrome plating, die casting are much superior to plastic, are completely resistant to ultra -violet rays, weatheri ng , and stress

- 9 -


cracking in the presence of various regents. Manufacturing cycles for producing die castings are much faster than for plastic injection moldings, Plastics, however, may be cheaper on a unit volume basis, have color inherent pro perties which tend to eliminate finishing , are temperature sensitive, and are good electrical insulators.

Sand Castings:-

Compared with sand castings die casting require much less machining; can be made with thinner walls; can have all or nearly all holes cored to size; can be held within much closer dimensional limits; are produced more rapidly in dies which make thousands of die castings without replacement; do not require new cores for each casting; are easily provided with inserts die cast in place; have smother surfaces and involve much less labor cost per casting Sand castings, on the other hand, can be made from ferrous metals and from many non -ferrous alloys not suitable for die casting. Shapes not producible by die castin g are available in sand castings; maximum size can be greater; tooling cost is often less and small quantities can be produced more economically.

Permanent Mold Castings:-

Compared with permanent mold castings, die cast ings can be made to closer dimensional limits and with thinner sections; holes can be cored; are produced at higher rates with less manual labor; have smoother surface and usually cost less per die casting. Permanent mold casting involves somewhat lower tooling costs; can be made with sand cores yielding shapes not available in die casting.

Forgings:-

Compared with forgings die castings can be made more complex in shape and have shapes not forgeable; can have thinner sections; be held to close r dimensions and have coring not feasible in forgings. Forgings, however, are denser and stronger than die castings; have properties of wrought alloys; can be produced in ferrous and other metals and in sizes not suitable for die castings.

Stampings:-

Compared with stampings, one die casting can often replace several parts. Die casting frequently require fewer assembly operations; can be held within closer dimensional limits; can have almost any desired variation in section thickness; involve less waste in scrap; are producible in more complex shapes and can be made in shapes not producible in stamped forms. Stampings, on the

- 10 -


other hand, have properties of wrought metals; can be made in steel and in alloys not suitable for die casti ng; in their simpler forms, are produced more rapidly; and may weigh less than die castings.

Screw Machine Products:-

Compared with screw machine products, die castings are often produced more rapidly; involve much less waste in scrap; can be ma de in shapes difficult or impossible to produce from bar or tubular stock; and may require fewer operations. On the other hand, screw machine products can be made from and alloys which cannot be die cast; they have the properties of wrought metals; and they require less tooling expense.

There are some comparison tables for Die Casting Process with other process with respect to Process, Die/Mold, Cost, Design and application etc., continued in the next page.

- 11 -


- 9 -6

- 9 -



CHAPTER: A1 OVERVIEW OF MASS PRODUCTION OF CASTING ARTSPRODUCED BY VARIOUS CASTING PROCESS

Table : SUMMARY OF MOULDING AND CASTING PROCESSES

Process *Choice of materials Complexity of part Number of castings

relative to tool life

Casting size or weight

Sand 1, 2 ,3, 4, 5, 6, 7, 8, 9, 10,

11,

Considerable limited by pattern

drawing. No limit with cores.

Wide range, type of pattern

depends upon total/casting.

28 gms. To

Shell Mould 1, 2, 3, 4, 5, 6, 9 Considerable, limited by

removal of mold from pattern.

Less limited with cores.

High metal patterns have a

long life.

28 grms – 45 gms and 387

cm2

Permanent Mould 1, 3, 4, 5, 6, 7, 8, 10, 11 Limited, restricted by the rigid

molds. Ability to eject casting

limits shape

Moderate to high , casting

metal affects life of mold.

Several grams - 23 kgs.

Die Casting 4, 5, 7, 8, 10, 11 Moderate, limited by design of

movable cores

High, mold life affe cted by

casting metal.

Several grams – 33 kgs. In

aluminium 90 kgs. In zinc

usually under 7 kgs.

Plaster moulding 4, 5 Considerable, possible to make

mold of several pieces,

expendable mold

Moderate, depends on

pattern material

28 grms to several kgs. In

most of material.

Investment Casting 3, 4, 5, 6, 9 Considerable, very complex

patterns can be assembled

from pieces.

Moderate, type of pattern

mold depends upon

number of castings.

Under 28 gms to 45 kgs.

Best for parts under 0.8 kg.

Centrifugal Casting 1, 3, 4, 5, 6, 9 Casting of circular periphery

most favorable. Almost any

shape can be cast.

Low to moderate Upto several kgs.

- 10 -6

- 10 -




Process Min. section mm Min. dia cored hole mm Surface finish Microns Precision and tolerances

Sand 3 – 6 depending upon

metal

4, 5 – 6 6 , 25 - 25 1, 5 – 4, 2 mm depending

upon metal & casting size.

Tolerance of ± 0.25 mm

possible on some parts.

Shell Mould 1, 5 for most materials. 3 – 6 Some what better than

sand

± 0, 003 mm/mm 0,075

total possible on some

dimensions.

Permanent Mould 2.38 most materials 4, 5 – 6 2, 5 – 6, 25 ± 0,015 mm/mm for firs t

25 mm 0.025 mm to 0.05

mm for each additional 25

mm

Die Casting 0.625 0.76 – 4.5 depending upon

metal

1 – 2, 5 ±0,025 – 0,125 mm

depending upon material.

Plaster moulding 0, 750 12.5 0,75 – 1.25 ±0.005 – 0.010 mm/mm or

less

Investment Casting 0.750 0.50 – 0.750 0.25 – 2.12 ±0.005 mm/mm

Centrifugal Casting 0.750 4.3 - 6 2.25 – 6.25 or as in sand Same as permanent

mould.

- 11 -6

- 11 -




(1) Gray Iron, (2) Malleable Iron, (3) Steel, (4) Aluminium alloys, (5) Copper alloys (6) Nickel alloys, (7) Zinc alloys,

(8) Magnesium alloys, (9) Heat and corrosion resistant alloys, (10) Tin alloys, (11) Lead alloys.

Process Tool costs Direct Labour costs Finishing costs Field of application

Sand Low Wide range, much hand labour

required.

Wide range, high to low,

depends upon cleaning,

snagging and machining

required.

Singular and batch production of

medium & large components of cast

iron, cast steel, not precision.

Shell Mould Low to moderate Moderate Low, often only a minimum

required.

Batch & mass production of cast iron,

steel components. To reduce the cost

of machining.

Permanent Mould Medium Moderate Low to Moderate Batch Production.

Die Casting High Low to medium Low, little more than

trimming necessary.

Mass production of small components

of Auminium, zinc, magnesium,

copper alloys. To reduce the cost of

machining.

Plaster moulding Medium High, skilled operators

necessary

Low, little machining

necessary.

Batch Production.

Investment Casting High High, many hand operators

required

Low, machining usually not

necessary.

Steel, alloyed steel, small batch, small

size to reduce the expensive

machining.

Centrifugal Casting Medium Moderate Low to moderate Singular batch production bearing

sleeves of bearing alloys cast iron

tubes.

- 12 -6

- 12 -




DIE CASTING COMPARISON WITH OTHER PRODUCTION PROCESSES

Process Process defined Materials Rate of

production

Size & weight of

parts

Strength

of parts

Wall

thickness

Complex

ity

Other

characteristics

Die

casting

Castings made by

forcing molten metal

under external pressure

into a metal die or

mold.

Lead, tin,

zinc

magnesium

, aluminium

and copper

alloys.

Very high,

upto 500

shots/hr

possible with

some parts.

No real size

limitation. Size

depends upon

casting equipment

available. Present

max. sizes run : 15

lb for aluminum, 10

lb for magnesium

and 30 lb for zinc.

High unit

strength.

Very thin; upto 1

in or more max.

From

simple to

very

complex.

Inserts of almo st

any metal can be

embedded in

castings.

Permanen

t mold

casting

Castings produced by

pouring molten metal

under a gravity head

into metallic molds.

Iron,

manesium,

aluminum

& copper

alloys.

Relatively

low. Not a

high

production

process.

Usually medium or

large parts. Between

die, castings and

sand castings.

High Not so thin as

die casting but

much heavier

sections

possible.

Usually

not so

complex

as die

castings.

Inserts can be

used.

Sand

Casting

Castings made by

pouring molten metal

under a gravity head

into molds prepared by

packing molding sand

around a suitable

pattern.

Principally

iron,

magnesium

, aluminum

& copper

alloys.

Low. Not a

high

production

process.

Medium to very

large.

Less than

die castings

or

permanent

mold

castings.

Must be heavier

than die

castings &

permanent mold

castings can be

cast in sections

1 ft or more.

Housings

& hubs

represent

average

degree of

complexity

.

Inserts seldom

practical.

- 13 -6

- 13 -




Plaster mold

casting

Castings made by

pouring metal under

a gravity head into

molds made of

gypsum with

strengthening &

setting agents

added.

Any nonferrous

material having

a melting point

of less than

2000 F, except

magnesium in

large sizes.

Low. Not a

high

production

process.

Relatively small. Equal to

sand

castings.

Not as thin as

lead, tin or zinc die

castings, but

sometimes equal

to die castings of

aluminum,

magnesium &

brass.

Usually not

so complex

as die

castings or

permanent

mold

castings.

Precision

investment

casting

Castings made by

pouring molten

metal into refractory

or ceramic molds

formed around wax

patterns. Patterns

are removed by

melting in the

process of firing of

the refractory.

Iron, zinc,

magnesium,

copper alloys &

especially hi gh

alloy steels.

Usually

lowest of

all

processes

Small parts only.

Max weight of part

about 101b or up

to 201b by special

techniques.

Section size

usually limited to 7

in. or less.

Equal to or

better than

permanent

mold

castings.

0.040 in. min. 1/16

in. prescribed min.

tolerance on walls

no less than 0.005

in. min.

Intricate

shapes not

readily

made by

machining,

forging or

sand casting

can be

produced.

Min. thickness

of trailing edge

equal to 0.015

in. min. &

preferably

0.025 in.

inserts not

practical.

- 14 -6

- 14 -




COMPARISON OF DIE CASTING WITH OTHER PRODUCTION PROCESS (Contd…)

Process Appearance & finish Cost Applications

Die casting Excellent. Can be finished with variety

of mechanical, plated, chemical or

organic finishes.

High equipment cost, high tool cost, &

low labor cost. Low part cost on high

activity items. Machining, grinding &

other operations usually not

necessary.

Structural parts, machine elements &

decorative members & parts for automotive,

business, machine electrical appliance, & all

other high production industries making both

industrial & consumer products.

Permanent mold

casting

Usually machined or ground but left with

base metal surface.

Medium equipment cost, high tool

cost, high labor cost. Fairly high part

cost.

For parts similar to sand castings but which

must have superior surface finish, closer

tolerances, & better strength in as cast

condition.

Sand casting Inferior to die or permanent mold

castings. Usually machined or ground

but left with base metal surface.

Low tool cost, high equipment cost,

high labor cost part cost between

those of die castings & precision

castings.

Gears, framing members, housings motor

blocks & structural members when cast

structure having relatively low strength &

resistance to impact satisfactory. Usually

limited to cast iron & cast steel for industrial

equipment.

Plaster mold

casting

Excellent Low tool cost, low equipment cost,

high labor cost, fairly high part cost.

Various engineering parts, mostly of brass

alloys.

Precision

investment casting

Equal to die castings, but usually left

with base metal surface.

Low equipment cost, low tooling cost,

high labor cost, high part cost.

Small intricate parts made in limited

quantities, usually from high alloy metals

such as stainless steel. Inconel Hastelloy

etc.

-1 --1 -16



CHAPTER: B1 GRAVITY DIE CASTING

-1 -

UNIT-2CHAPTER – B1

GRAVITY DIE CASTING

-2 --2 -16




-2 -

CHAPTER OUTLINE

B1.1.1 Introduction

B1.1.2 Gravity Die Casting (GDC)

B1.1.3 Limitations of Permanent Mold Casting

B1.1.4 Principle of Permanent Mold Casting

B1.1.4 (a) Progressive Solidification

B1.1.4 (b) Minimum Turbulence

B1.1.4 (c) Air and Gas Clearance

B1.1.5 Suitable Casting Materials for GDC

B1.1.6 Permanent Mold Casting Machines

B1.1.7 Selection of Mold Material

B1.1.8 Gravity Die Casting Mold Life

B1.1.9 Principle of Mold Design

B1.1.10 Solid Graphite Permanent Mold

-3 --3 -16




-3 -

B1.1.1 INTRODUCTION

This is a casting process in which the mould is permanent and same can be repeatedly used for making thousands of identical components.

B1.1.2 PERMANENT MOULD/GRAVITY DIE CASTING

In permanent mold casting, a metal mold consisting of two or more parts is repeatedly used for production of many castings of the same from The liquid metal enters the mold by gravity Simple removable cores are usually made of metal, but more complex cores are made of sand or plaster when sand or plaster cores are used , the process is called semi permanent mold casting.

Permanent mold casting is particularly suitable for the high -volume production of castings with fairly uniform wall thickness and limited under -cuts or intricate internal coring. The process can also be used to produce complex castings, but production quantities should be high enough to justify the cost of the molds. Compared to sand casting, permanent mold casting permits the production of more uniform castings with closer dimensional tolerances, superior surface finish, and improved mechanical properties.

B1.1.3 LIMITATIONS OF PERMANENT MOULD CASTING

Permanent mold casting has the following limitations: -

§ Not all alloys are suitable for permanent mold casting§ Because of relatively high tooling costs, the process can be prohibitively expensive

for low production quantities§ Some shapes cannot be made using permanent mold casting, because of parting line

location undercuts, or difficulties in removing the casting from the mold§ Coatings are required to protect the mold from attack by the molten metal

Metals that can be cast in permanent molds include the aluminum, magnesium, zinc and copper alloys and hypereutectic gray iron.

B1.1.4 PRINCIPLE OF PERMANENT MOULD CASTING

Compared with permanent mould casting, Die Casting can be made to closely dimensional limits and with thinner sections, holes can be cored, are produced, higher rates with less manual labor, have smoother surfaces and usually cost less per die casting. Permanent mould casting involves some what lower tooling cost, can be made with sand cores yielding shapes not available in die casting.

-4 --4 -16




-4 -

In permanent mould casting, a metal mold consisting of two or more parts is used repeatedly for producing many castings of the same form. The liqui d metal enters the mould by gravity. (The process does not, however, include pouring of ingots in metal moulds). Simple cores area made of metal, but more complex cores are made of sand or plaster. When sand or plaster cores are used, the process is called Semi permanent mold casting.

Removal of Casting from Molds: -

After a casting has solidified, the mold is opened and the casting is removed. To facilitate release of the casting from the mold, a lubricant is often added to the mold coating. The use of as much drafts as permissible on all portions of the casting makes ejection easier. For many castings, ejector pins or pry bars must be used. Core pins and cores should be designed so as not to interfere with removal of castings from the mold.

B1.1.4 (a) PROGRESSIVE SOLIDIFICATION

Casting requires a feeding system whic h consists of gates, runners, risen etc. Risers must be connected to heavy section of the casting. The process of solidification must be in such manner that the casting freezes from the further most point progressively towards the risers. If this principle is not satisfied, shrinkage, porosity, cavitations or surface depressions can occur which may causes the rejection of the component.

B1.1.4 (b) MINIMUM TURBULENCE

This principle must be observed during the filling of die cavity with molten metal. Turbulent filling of die will leave air bubbles and oxide films entrapped within the casting. The turbulence can be controlled by suitably designing the runner system. This principle must be satisfied for setting a sound casting.

B1.1.4 (c) AIR AND GAS CLEARANCE

For gravity die casting the care must be given to air and gas clearance otherwise entrapped air or gas will cause defects in the components (blow holes, surface depression etc)

B1.1.5 SUITABLE CASTING METALS FOR GDC

Metals that can be cast in perma nent moulds include aluminum, magnesium, zinc and copper alloys, and hypereutectic gray iron.

-5 --5 -16




-5 -

Aluminum alloys : - Aluminum alloys have low density, which combined with their oxide-film-forming characteristics, make them flow some what sluggishly. The shrinkage of aluminum alloys during solidification is relatively large, and provision must be made for ample metal feed during solidifi cation. After solidification, aluminum alloys are soft at elevated temperature, and castings may distort during removal from the mold.

Magnesium alloys : - Magnesium alloys are less castable than aluminum alloys, and have relatively poor feeding characteri stics in thin -wall castings. Also, the castings are more sensitive to hot shortness (brittleness at elevated temperature) than are aluminum alloys castings. Generous fillers are required when the casting contains large bosses or when one section of the cas ting is much larger than another. Sharp casting detail cannot be obtained with magnesium alloys, and shapes that shrink on to mold sections are susceptible to cracking and should be avoided.

Copper alloys : - Copper alloys solidify at high temperatures, an d some have narrow solidifications ranges. They shrink on to cores and other mold elements, and shrink on to cores and other mold elements, and must be ejected from molds as soon as possible.

Zinc alloys: - Zinc alloys can be cast in permanent molds, but because the castings are usually made in large quantities, they are more often die cast.

Gray iron: - Gray iron is used successfully in high -volume production of small (28 g to 13.5 kg, or 1 oz to 30 lb), simple castings. However, more complex gray iron c astings, with internal coring and marked changes in section, have also been successfully made by the permanent mold process.

Maximum Size of Casting: -Practical sizes of permanent mold castings are limited by cost. The maximum sizes

that have been cast differ among the casting alloys.

Aluminum alloys: - In high production, permanent mold castings weighing up to 13.5 kg (30 lb) are made from aluminum alloys in casting machines. However, much larger castings can be produced.

Magnesium alloys: - It despite their comparatively low cast ability; have been cast in permanent or semi -permanent molds to produce relatively large and complex casting. For instance, 8 kg (17.7 lb) housing for an emergency power unit was poured from alloy AZ91C in a semi -permanent mold. The mold utilized vertical parting and an oil -sand core to develop the vanes and internal surfaces of the casting. Surface finish of the casting varied from 6.4 to 12.7mm (250 to 500min.).

Copper alloy: - These permanent molds casting weighing over 9 kg (20 lb) rarely can be justified.

Gray iron: - Production of gray iron castings in permanent mold is seldom practical when the castings weigh more than 13.5 kg (30 lb).

-6 --6 -16




-6 -

B1.1.6 PERMANENT MOLD CASTING MACHINES

Permanent mold casting machines can be customized to automatically or manually operating. They are basically simple in construction.

Manually operated machine: -

Manually operated permanent mold casting machines may consists of a simple “book” mold arrangement, such as that shown in fig.1 or for castings with high ribs or walls that require mold retraction without rotation, the machine shown in fig. 2 can be used. With Either type of machine, after the casting solidified the mo ld halves are separated by manually releasing the eccentric mold clamps.

Automatic machines: -

For high-volume production, the manual drives are replaced by two -way hydraulic mechanisms. These can be programmed to open and close in a preset cycle. Thus, except for pouring of the metal and removal of castings, the operation is automatic.

A method of permanent mold casting has been developed in which the metal is not ladled by hand. This is called the Wessel Process . The equipment for which is shown in fig. 3. In this method, the permanent mold is mounted on rails against the end face of the tilting reverberate furnace. As the furnace is tiled about an axis bear its center of gravity, metals flows through a pouring hole in ht wall of the furnace in the mold. The assembly remains in its tilted position for a predetermined interval, then returns to the s tarting position. Tilting is done by means of a hydraulic cylinder.

.

-7 --7 -16




-7 -

Molds are parted vertically, parallel to the direction of metal flow. One mold half slides on the mounting rails; the other, which is hinged, swings away from the mounting rails, pulling the casting and sprue with if to leave the pouring hole clear for the next cycle. Core manipulation and casting ejection are the same as in conventional practice.

B1.1.7 SELECTION OF MOULD MATERIALS

Four principle factors affect the selection of material s for permanent molds and cores: -

§ The pouring temperature of the metal to be cast § The size of the casting§ The number of castings per mold§ Cost of the mold material

Mould Materials: - As indicated in Table A, gray iron is the most commonly used mold material. Aluminum or graphite molds are sometimes used for the small -quantity production of aluminum and magnesium castings, and graphite or carbon linear on steel are sometimes used for molds for casting copper alloys ( see also the section “Solid Graphite Molds” in this article).

With aluminum or magnesium casting alloys, it is not unusual to obtain 100,000 castings, or more, per mold; however, molds for copper or gray iron casting alloys have a shorter life because of the higher pouring temperatures required. Pouring temperatures for specific metals are as follows:

-8 --8 -16




-8 -

Gray iron molds without tool steel inserts are satisfactory for long production runs of aluminum and magnesium castings that will be magnesium casting that be machined extensively and for which s urface finish is not a major consideration. In the casting of zinc, well over 00.000 pours are possible in a gray iron mold (die casting is usually selected to produce zinc castings such large quantities).

Mold Inserts: - Full or partial mold cavities ins erts of the same material as the mold , or of a different material, are sometimes used to obtain longer mold life, or to simplify machining handling or replacement. Inserts can also be used for venting, cooling thin walls, and heating portions of the mold o r the full cavity area . Inserts made of cast -to-shape gray iron are used for casting complex alu minum and magnesium parts that r ange in surface area from 320 to 2900 cm 2 (50 to 50 in. 2). Tolerance on these parts range from 0.76 to ±1.5 mm (± 1.5 mm ± 0. 030 to ± 0.060 in.). Inserts last for 5000 to 20.000 pours, depending on casting complexity.

Core Materials: - Core materia ls are recommended in Tables B and C on the basis of performance over a wide range of coring requirement for small and large cores. An expendable core is used when the location or shape of the core does not permit its removal from the casting or when an in tricate design can be obtained at less cost with materials for such cores. These materials are listed below in order of increasing preference:

§ Sand (oil – bonded or resin –bonded, shell, car bon dioxide-silicate)§ Plaster§ Graphite and carbon

Table A recommended permanent mold MaterialsCasting alloy No. of Pours

1000 10,000 100,000

for small Castings (25 mm or 1inch Maximum dimension)

Zinc Gray Iron: 1020steel Gray Iron: 1020steel Gray Iron: 1020steel Aluminum Mg. Gray Iron: 1020steel Gray Iron: 1020steel Gray Iron with AISI-H14

inserts: 1020steelCopper Gray Iron Gray Iron Alloy Cast IronGray Iron Gray Iron (a)Gray Iron (a) Quantity not Poured

for medium and large castings (upto 915mm or 36Inch maximum dimension)Zinc Gray Iron:AISI-H11 (b)Gray Iron: AISI-H11 (b) Gray Iron: AISI-H11 (b)Aluminum Mg. Gray Iron Gray Iron Gray Iron with AIS-H11/H1(c)

inserts: 1020steelCopper Alloy Cast Iron Alloy Cast Iron Alloy Cast Iron (d)Gray Iron Gray Iron (a) Gray Iron (a) Quantity not Poured

METAL/ALLOY TEMPERATURE, °C (°F)ZincAluminumMagnesiumCopperGray Iron

465-620 (870-1050)675-790 (1250-1450)705-790 (1300-1450)980-1230 (1800-2250)

1275-1355 (2325-2475)

-9 --9 -16




-9 -

Table B Recommended Materials for small cores (<75mm or 3In in Diameter & 255mm or 10In. long) for Permanent mouldsCasting alloy Recommended core materials (a)

Zinc Sand Plaster, Gray Iron 1020steelAluminum Mg. 1010 or 1020 steel Sand Plaster H11 Die steel or Equivalent (b) carbon (c)

Copper Sand 1020steel Gray Iron Plaster (d) Graphite9(c) Gray Iron Sand Graphite Carbon and Gray Iron

(a) Materials are listed descending order of preference (b) Hardened to 40-45HRC (c) for use with relatively few Pours (d) for Castings of Aluminum Bronze.

Table B Recommended Materials for Large cores (<75mm or 3In in Diameter & 255mm or 10In. long) for Permanent mouldsCasting alloy No. of Pours

1000 10,000 100,000

Zinc Gray Iron: 1020steel Gray Iron: 1020steel Gray Iron: 1020steel Aluminum Mg. Gray Iron: Gray Iron with Gray Iron: Gray Iron with Gray Iron: Gray Iron with

1020steel insert (b) 1020steel or H11 Insert (b) H11 Insert (b),Sand Plaster (b) Gray Iron: 1020steel H11 Die steel,

Copper Sand Sand Quantity not Poured Gray Iron Sand, Graphite, Sand, Graphite,Quantity not PouredCarbon Gray Iron Carbon Gray Iron

(a)Material listed in descending order of preference, (b) except for openings with complex shape, which required expandable sand cores.

B1. 1.8 GRAVITY DIE CASTING MOULD LIFE

Mold life can vary from as few as 100 pours as many as 250,000 pours (or even more), depending on the variables discussed later in the section. A mold for an aluminum piston for example, can be accepted to produce 250,000 casting before requiring repair. After the production of 250,000 more castings, the repaired mold will require a major overhaul.

Mold life is likely to be longer in the casting of magnesium alloys than in the casting of aluminum alloys of similar size and shape; this is because molte n magnesium does not attack ferrous metal molds however the difference the difference in mold life for magnesium alloys depends to a great Extent on the effectiveness of the mold coating used in the casting of gray iron, mold coating used in the casting of gray iron, mold life is excepted to be short compared to the casting of similar shapes from aluminum alloys.

-10 --10 -16




-10 -

Molds are often fabricated from cast iron is cause casting the mold close to the finished shape can decrease machining costs. In ad dition cast iron is much more resistant to attack by molten aluminum than steel; however is weld able and easier to repair than cast iron. Therefore steel molds are often used for high-production castings.

Major variables that affect the life of Permanent molds are: -

§ Pouring temperature: The hotter the casting metal is poured, the hotter the mold is operated which leads to rapid weakening of the mold metal.

§ Weight of casting: Mold life decreases as casting weight increases. The decrease in mold life with increasing weight of casting is shown in Fig. 2 for 25 mm (1 in.) thick gray iron mold used for casting of gray iron.

§ Casting shape : Mold walls are required to dissipat e more heat from casting having thick sections than from those having thin sections When there is a significant variation in the section thickness of a casting , a temperature differential is set up among different portion of the mold. As t he temperature differential increases mold life decreases.

B1.1.9 PRINCIPLE OF MOLD DESIGN

A mold design has a marked effect on mold life. Variation in mold -wall thickness causes excessive stress to develop during heating and cooling which in turn causes premature mold failure from cracking Abrupt changes in thickness without generous fillets also cause premature mold failure . Small fillets and radii lead to reduced mold life because checking and cracking as well as ultimate failure , often start at these points.

Usually less draft is required on external mold surface than on internal mold surface than on internal mold surfaces because of the shrinkage in the casting A 5° draft is desirable but 2 ° on external and 3 ° on internal mold surfaces can be used Lower draft angles, however decrease the number of castings that can be made between mold repairs The effect of draft angle on the life of cores and molds used for producing aluminum alloy castings is shown in Fig 3

-11 --11 -16




-11 -

Projections in the mold cavities contrib ute greatly to reduce mold life These projections become extremely hot. Which increases the possibility of extrusion , deformation and mutilation when the casting is removed I t is sometimes possible to extend mold life by using inserts to replace worn or broken projections.

Undercuts on the outside of a casting complicate mold design and increase casting cost, because additional mold parts or expendable cores are needed. Complicated and undercut internal sections are usually made more easily with expendable cores than with metal cores, although collapsible steel cores or loose metal pieces can sometimes be used instead of expandable cores.

Numbers of castings per mold is a major consideration in designing the mold; the objective is to have the optimum number of cavities per mold that will yield acceptable castings at the lower cost. Except for vary small and thin castings, as the weight of the metal being cast per mold increases, the cycle time of the machine also increases. These increases, however, are not directly proportional. A mold with the maximum number of cavities often will produce more castings per unit of time than a mold with a smal ler number of cavities that was designed to operate on shorter cycle.

For relatively simple castings, cavities may be placed one above the other as shown in the below fig. the metal then flows through the lower cavities to till to those above this permits maximum utilization of the machine platen area available.

-12 --12 -16




-12 -

Mold Design and Dimensional Variations:-

The dimensional accuracy of permanent mold castings is affected by short -term and long -term variables Short -term and long –term variables. Short –term variables are those that prevail regardless of the length of run: -

§ Cycle- to – cycle variat ion in mold closure or in the position of other moving elements of the mold

§ Variations in mold closure caused by foreign material on mold faces or by distribution of the mold elements

§ Variations in thickness of the mold coating § Variations in temperature distribution in the mold§ Variations in casting-removal temperature

Long –term variables that occur over the life of the mold are caused by: -

§ Gradual and progressive mold distortion resulting from stress relief, growth and creep

§ Progressive wear of mold surface primarily due to cleaning

Dimensional variations can be minimized by keeping heating and cooling rates constant, by operating on a fixed cycle, and by maintaining clean parting faces. It is particularly important to select mold cleaning procedures that remove a minimum of mold material.

The mold thickness and the design of the supporting ribs both affect the degree of mould warp age at operating temperatures. Supporting ribs on the back of a thin mold will warp the mold fa ce into a concave form. This mold –design error can alter casting dimensions across the parting line by as much as 1.6 mm (1/16 in). Adequate mold lockup will contribute to the control of otherwise severe warp age problems.

Mould erosion resulting from me tal impingement and cavitations due to improper gating design both contribute to rapid weakening of the mould metal and to heat checking. These mould design errors contribute to rapid dimensional variation during a long run mechanical abrasion due to insuf ficient draft or to improperly designed ejection systems also contributes to the raped variation of casting dimensions.

The dimensions of many mold and core componen ts change at a relatively uniform rate; there fore, it is possible to estimate when rework or replacement will be required To maintain castings within tolerances. It is sometimes necessary to select mold -component materials on the basis of their wear resistance.

Cooling methods: - Water cooling is more effective than air cooling but it substantially decreases mold life.

-13 --13 -16




-13 -

Heating cycles: - Generally, a continuous run. in which the mold is maintained at a uniform temperature provides maximum mold life. Repeated heating and cooling over a wide temperature range will shorten mold life.

Preheating the mold: - This is done to operating temperature with a gas flame or electric heaters, and it greatly increases mold life. Thermal shock i s one of the principle causes of mold failure.

Mold coating: - This protects the mold from erosion and soldering by preventing the metal from contacting mold surfaces, thus increasing mold life ( see discussion below).

Mold materials: - See Table 1.

Storage: - Improper storage can lead to excessive rusting and pitting of mold surfaces which will reduce mold life.

Cleaning: - The common practices for cleaning mold are abrasive blasting , dipping in caustic solution and wire brushing Dipping in caustic can be hazardous to the operator. Wire brushing and abrasive blasting can cause excessive mold wear if not carefully controlled. Glass beads are the safest abrasive blast material; their use minimizes dimensional changes due to erosion from the abrasive blast.

Gating: - A poor gating system can greatly reduce mold life by causing excessive turbulence and washout at the gate areas.

Method of mold operation: - Although the same materials are used to make moldsand cores for both automatically operated equipment and hand –operated equipment the life of the tool materials on hand –operated equipment is shorter because of the abuse the tooling must withstand Tools for automatic equipment may last up to twice as long as for hand-operated equipment.

End use of casting : - If the structural function of a casting is more important than its appearance, a mold can be used for more pouring before being discarded.

Surface Finish: - Surface finish on castings is determined primarily by the roughness of the mold coatings and is essentially independent of the surface achieved in machining the mold. A specified surface finish for the part has little bearing on the selection of the mold material unless heat checking and pitting ( in high –volume production ) cause mold surfaces to become rougher than the mould coating A tool steel is also selected where it is desirable to maintain a high polish on the mold as for casting zi nc parts to be chromium plated.

-14 --14 -16




-14 -

Supplementary Heating or Cooling: - Supplementary heating or cooling of all or selected portions of a mold, to minimize extremes of temperature during the operating cycle , will usually increase mold life and result in the most efficient operation of the mold regardless of the metal being cast Supplementary heating is used to :

§ Bring the mold to operating temperature especially where the quantity of heat to be removed is small

§ Equalize the temperature throug hout the mold so as to avoid thermally induced stresses

§ Avoid chilling of metal being cast

Supplementary cooling is used to: -

§ Hold mold temperature at its required level especially where the quantity of heat to be removed is large

§ Permit casting to be done at a faster rate, thus increasing production§ Equalize the temperature throughout the mold thus avoiding thermal stresses

Erosion: - Erosion of mold materials varies with the metal being cast and the mount of metal flowing into the mold

Scaling: - Scaling of molds is most likely to occur when the molds are not in actual operation- for example if they are overheated when being prepared for production; this can be prevented by providing preheating torches with a reducing

Pitting and Corrosion: - Pitting and corrosion usually occur in storage where moist and corrosive conditions cause chemical attack. This is especially true when molds are stored without prior removal of the mold coating However, pitting may occur during service if the mold cavity is not well protected with a mold coating.

Mold Coating: - A mold coating is applied to mold and code surface to serve as a barrier between the molten metal and the surface of the mold while a skin of solidi fied metal is formed Mold coatings are used for five purpose: -§ To prevent premature freezing of the molten metal§ To control the rate and direction of solidification of the casting and therefore its

soundness and structure§ To minimize thermal shock to the mold material§ To prevent soldering of molten metal to the mold material§ To vent air trapped in the mold cavity

Types: - Mold coating are of two general types insulating and lubricating Some coatings perform both functions A good insulating coating can be made from (by weight) one part sodium silicate to two parts colloidal kaolin in sufficient water to permit spraying. The lubricating coatings usually include graphite in a suitable carrier Typical compo sitions of 15 mold coatings are called in Table D Coatings are available as proprietary materials.

-15 --15 -16




-15 -

The various requirements of a mold coating are not always obtained with one coating formulation. These requirements are often met by applying different coat ings to various locations in the mold cavity.

TABLE -D) Typical compositions of coatings for permanent moldsComposition, % by weight (remainder, water)

Insulators Lubricants

Coating Requirements: - To prolong mold life a coating must be non corrosive. It must adhere well to the mold and yet be easy to remove. It must also kee p the molten metal from direct contact with the mold surfaces.

Coating Procedure: - The mold surface must be clean and free of oil and grease. The portions to be coated should be lightly sand blasted. If the coating is being applied with a spray, the mold should be sufficiently hot (205 °C or 400 °F) to evaporate the water immediately.

B1. 1.10 SOLID GRAPHITE PERMANENT MOULDS

Permanent molds can be machined from solid blocks of graphite instead of cast iron or steel The low coefficient of thermal expansion and superior resistance to distortion of graphite make it attractive for the reproduce idle production of successive casting made in the same mold. Because graphite oxidizes at temperatures above 400 °C ( 750 °F) molds would wear out quickly even it used for nonferrous casting TO protect

Casting No.

Sodium Silicate

Whiting FireClay

MetalOxide

Diatom-aceousearth

SoapStone(a)

Talk(a)

Mica(a)

Graphite BoricAcid

1 2 4 12 8 43 7 7

4(b) 12 95 5 11 2 56 9 4 147 11 178 4 23 59 7 1 23 20 210 23 1011 30 512 18 4113 8 16 6214 715 20 53

-16 --16 -16




-16 -

the molds and to extend their service lives they are usually coated with a wash, which is normally made of ethyl silicate or colloidal by forming minute cracks in their surface .

Graphite pe rmanent molds are used for a variety of products ( notably bronze bushings and sleeves) and graphite chills are often inserted in molds to promote progressive or directional solidification The use of graphite as a permanent mold material is perhaps best demonstrated in the casting of chilled iron railroad car wheels ( the Griffin wheel casting process) as shown in Fig 5 Graphite is a particularly suitable mold material for this process It produces castings with closer tolerance than can be achieved with sand molding and the high thermal conductivity of graphite chills the metal next to the mold face very efficiently giving it a wear – resistant white iron structure.

However because graphite erodes easily pouring the me tal into molds from the top under the influence of gravity causes unacceptable mold wear. The technique has been used to make ferrous casting weighing up to 410 kg (900lb).

129



CHAPTER: B2.1 PRESSURE DIE CASTING(COLD CHAMBER)

1


PRESSURE DIE CASTING

229




2

CHAPTER OUTLINE

B2.1.1 Introduction

B2.1.2 (a) Pressure die casting machine

B2.1.2 (b) Classification of die casting machine

B2.2.1 (a) Cold chamber machine

B2.2.1 (b) Cold chamber machine & parts

B2.2.2 Classification of cold chamber die casting m/c

B2.2.3 Horizontal cold chamber machine

B2.2.4 Vertical cold chamber machine

B2.2.5 Comparisons of hot and cold chamber process

B2.2.6 Process parameter & control

B2.2.7 Cold chamber die casting processing metals and alloys

B2.2.8 Cold chamber die casting die

329




3

B2. 1.1 INTRODUCTION

Pressure die casting technique is a development over the gravity die casting Die casting is a process in which molten metal is injected into a precisely dimensioned steel mould with in which pressure is maintained until solidification is completed The die casting according reproduces with high fidelity the finest detail of the impression within which it was formed.

Die casting offers the users a means of obtaining dimensional accuracy and good surface finish in several alloys with wide range of mechanical and physical properties In the pressure die casting

1. The die is mounted between the two “ platen” of a press, generally of horizontal type by means of which it is closed and opened .

2. The movement of die follows that of ma chine and this determines the general directional of assembly.

3. The liquid molten is generally injected by the action of piston which forces it through the die from a compression chamber. This is referred to as HOT CHAMBER if it is situated inside the molten metal which is heated by furnace forming port of the assembly a COLD CHAMBER if it is fed with metal which has been melted in furnace separates from the machine.

B2.1.2 (a) PRESSURE DIE CASTING MACHINES

There is a wide choice ranging from small hot chamber machines intended for the production of hardware to massive cold chamber machines intended for the production of hardware to massive cold chamber machines with sophisticated equipment suitable for the automatic production of large and complex casting of high quality.

The function of pressure die casting Machines: -

1. To Hold the two Halves of the die together2. Inject molten metal under pressure into the die3. Close and open die halves to permit removal of finished castings4. Eject the casting

The die costing machines essentially consists of a frame on which the die and the actuating equipment are mounted The actuating device opens and closes the die and the actuating equ ipment which in all modern machines are hydraulic cylinder and piston or hydraulic cylinder and toggle arrangement The injection system of the machine forces the molten material into the die under pressure The modern machines are equipped to give slow plunger movement a variable speed filling stroke and an intensified squaring pressure before solidification is completed The ejection system consist of knock out rod or plates hydraulic cylinder and rack and pinion The power cylinders are actu ated either hydraulically or pneumatically from pressurized accumulator.

429




4

B2.1.2 (b) CLASSIFICATION OF DIE CASTING MACHINES

There are generally two types of the die casting machines as identified by the placement of the injection chamber or metal pumping systems.

1. HOT CHAMBER (GOOSE NECK) MACHINE2. COLD CHAMBER MACHING

i) HORIZONTAL ii) VERTICAL

B2.2.1 (a) COLD CHAMBER MACHINES

For the die casting trade however the introduction of cold chamber machines was a considerable step forward No t only could Aluminum and magnesium alloys be die casted successfully on such equipment but large and complex zinc alloys components could be produced.

The metal is heated to pouring temperature in a separate holding furnace and transferred to the shot cylinder by ladling The metal injection chamber is separate from the melting temperature of the alloys These machines are called Cold Chamber Casting Machines and also know as Plunger Casting Machines.

These cold chamber machines are subdivide d into two types on the position of the injection chamber as: -

1. Horizontal cold chamber machines.2. Vertical cold chamber machines.

The molten is in contact with shot cylinder and plunger for only a short period of time in cold chamber machines and is relatively cooler.

Cold chamber injection system can be used for metals that can be die cast. They are usually used for alumin um, magnesium and copper based alloys. Molten aluminum has a tendency to react with iron when it comes into contact with any steel at all times, resulting in contamination of the alloy and causing production of inferior castings.

This injection system is relatively free from attack of molten metal since, it is not submerged in the metal bath. This helps the use of high injection pressures and they range form 8000- 30000 psi (560 – 2100 kg/cm2) Optimum plunger speed varies with:

a) The alloy being cast.b) The size and shape of casting. c) The design of runner and gate.

529




5

B2.2.1 (b) COLD CHAMBER M/C AND PARTS

Horizontal Cold Chamber Die Casting M/C: -

Following are the most important characteristic of a horizontal cold chamber machine: -

1) Injection System.2) Accumulator. 3) Intensifier.4) Ejection.5) Core pull.6) Pump.7) Clamping.

629




6

729




7

Plunger speeds range from 150 to 900 ft. per minute Shot chamber and plunger of standard sizes could be fitted to the machine depending on the volume of the metal required for the casting (shot capacity).

Horizontal Cold Chamber Machines: -

The shot chamber is mounted horizontal with pouring hole in the top of the chamber wall.

829




8

The Injection System (cold chamber machine): -

The Metal sleeve through which the Injection plunger passes through is located in the fixed platen of the casting machine. It projects through the platen into the fixed die side and leads into the running system ( As shown in the figure below) At the o ther end of the sleeve there is a hole into which the measured amount of molten material is poured.

The shot piston advances material into the runner system and die cavity along the sleeve. The pouring sleeve and plunger tip are subjected to repeated expansion and contraction on each shot, and thus are subjected to thermal stress

The sleeve is made of Nit riding steels e.g. EN -31etc. It is case hardened to 70 HRC having a case depth of 0.37 mm. The bore of sleeve is honed to 0.001 into 0.00 A 5° taper at front end per side for last o.5 in to give easier flow through plunger.

The plunger tip is generally made of copper alloys like copper Beryllium As due to high temperature and relatively soft metal it peens over at front edge to give good seal The Injection plunger is actuated by hydraulic power.

The Operating cycle of Horizontal cold chamber machine system: - The die is closed and locked and the cylinder plunger is in retracted position. The molten metal is ladled through the pouring hole of shot chamber The Injection cylinder is actuated and the plunger moves forward.

The plunger movement takes place in 3 stages: - At the first stage the plunger advance slowly covering pouring toles to prevent splash back and also pushes the material through sleeve and into the runner System displacing the air.

In the second stage the plunger pushes with high speed the material into the cavity. The plunger rushes to fill the material into the cavity,

At last in third stage when the cavity is filled the pressure boiled up value is actuated to bring high pressure source to Compact the casting This final forward travel of plunger tip forces the biscuitexcess metal free of shot chamber and aide in stripping of the casting from

929




9

the cover die The plunger retracts and casting is ejected and again the mold is ready for next shot2) Accumulator:- Accumulator is gas /oil vessel which, the accumulat or is an energy storing device which can supply large volumes of fluids at pre-determined pressure The gas used is generally Nitrogen

3) Intensifier:- It is a pressure m ultiplier which at the end of the injection piston travel increases the pressure on the molten metal in the shot chamber and thus on casting The Intensifier prevents voids and porosity in thick wall castings.

4) Ejection: - A Hydraulic cylinder is mounted on a moving platen to push out the solidified casting off the die impression A piston rod is coupled upto Ejector assembly to push and retract Directly cylinders are also used only to push the plate forward.

Where the sl ides are to be polled out by a hydraulic cylinder on a die electrical sequence is a must in machine for sensing that slides are pulled out first before the ejection stroke takes place.

1029




10

When the die opens the casting is ejected by the forward movement of the ejector plate The force required to actuate the ejection system may be provided by –

a) Mechanical knockout plate and knockout pinsb) Rack and pinionc) Hydraulic cylinder

Mechanical knockout System: -1. Ejector platen 2. Ejector pin plate3. Return pin4. Ejector pin5. Cover die plate 6. Cover platen 7. Casting 8. Ejector Die half9. Knockout pin10. Knockout plate

1129




11

This has a heavy steel plate secured to the toggle carrier of the die casting machine knockout pins that are in contact with the ejector pin plate pass through the holes in ejector platen As the die opens the ejector platen approaches the knockout plate. The knockout pins then contact the knockout plate and pushes the ejector plate of the die forward and the continuing travel of the ejection system pushes the casting and the sprue from the die . When the die closes the ejector pins are retracted by the die return pins.

Some die casting machines have adjustable knockout pins on either side or passing through the moving platen which could be made to contact the extended knockout plate of the ejector system of the die to actuate the ejection of the casting.

Rack and Pinion Ejection system: -

1. Ejector pin plate 2. Return pin 3. Ejector Die Half 4. Cover die half 5. Die cavity 6. Runner 7. Sprue spreader8. Sprue 9. Gate 10. Fixed core 11. Ejector pin 12. Pinion13. Rack

The ejector mechanism with rack and pinion is fastened to the ejector (movable) die half Guide pins with rack teeth are attached to the ejector pin plate. A pinion engages the rack teeth on the machine to eject the casting. Standard ejector boxes with rack and pinion assemblies could be used. This system is rather obsolete.

Hydraulic Ejection System: -

1. Cover platen2. Cover die half 3. Casting 4. Ejector pin 5. Return pin 6. Ejector pin plate 7. Cylinder rod 8. Hydraulic cylinder9. Ejector die half 10. Ejector platen

1229




12

This system includes a hydraulic cylinder mounted on the back of the ejector platen. The cylinder rod protrudes through the platen to the ejector pin plate. When the ejector ( movable) platen moves sufficiently to allow removal of the casting from the die, a limit switch cause the cylinder rod to advance, forcing the casting out of the ejector half o the die. The advantage of this system over the mechanical system is that the ejector pins can be retracted with the die in the o pen position. In this position the die can be easily cleaned and inspected with the pins retracted.

Hydraulic retraction of ejectors avoids the interception of side cores with the ejector pins before the side cores are moved in by finger cams in the die closed position. This system allows the loading of inserts in the moving half when ejector pins are located under the inserts.

5) Core pull: - When there are slides to be pulled by hydraulic cylinder to from impression detail in the direction other than in line with the machine movement separate facilities are provided for the hydraulic hose connection.

6) Pump:-The hydraulic pump provides the motive power when hydraulic pump operates it perform two functions:

Firstly its mechanical action creates a partial vacuum at the inlet of pump which enables atmospheric pressure in the reservoir to force liquid through the in let into the pump.

Secondly it delivers this liquid to the pump outlet on forces into the hydraulic system.7) Clamping: -The two halves of a die mounted on the platens of the machine must be clamped together to prevent the leakage of molten metal at the die parting line. The clamping system provides the basis for rating the die casting machines. A 200 ton machine is one that can exert clamping force or locking pressure of 200 tons on the die halves. The clam ping system consists of stationary platen and movable platen, two to four accurately ground tie rods a locking mechanism.

1329




13

(Fig. 17 Locked position / Unlocked position)

The stationery and movable platen have ‘T’ slots or holes for clamping the die halves. The cover half of the die is mounted on the stationery platen.

1429




14

The stroke and power transmitted by a hydraulic cylinder combined with the mechanical advantage of the link system is used for locking the die. The locking action begins when the two faces of the die come in contact and compression at the faces is established. When the three outer link pins are aligned and the cross head link position is normal to the alignment of the pins maximum force on the die equals tensile forces in the tie rods.Hydraulic Locking systems: -

This die locking system combines a hydraulic cylinder stroke and dual action. The system consists of three hydraulic cylinders two of, which furnish the forces necessary to drive the wedges that lock the two die halves together.

(Fig. 19 Hydraulic locking system)The third cylinder is a die closing cylinder which moves the ejector platen to the

point of die contact , and at this position the wedges become engaged with the mating wear plates at the rear of the central screw in the locked position. The wedge cylinders are then actuated to force wedges between the wear plates and rear platen. The locking force depends upon the wedge angle, the size of the cylinders and the pump pressures.Die opening Adjustment: -

1529




15

This is made by various mechanical devices. The simplest design consists of two nuts one on each side of the toggle carrier, thread on each of the rods machine. To close the die opening. All front adjusting nuts are loosened and the rear adjusting nuts arecSorrespondingly tightened to force the toggle system forward till it locks into position.

The other device uses a large bull ge ar on the rear face of the toggle carrier for adjusting the die opening The bull gear on the rear face of the toggle carrier for adjusting the die opening. The bull gear drives the tie rod nuts that are fastened to the toggle carrier.A fourth de sign incorporate nuts with sprocket teeth at their periphery to engage a chain drive.

The locking force should be adjusted over the tie bars to provide uniform locking pressure on the die and avoid over –stressing tie rods. Some die casting machine manufacture provide dial indicators to maintain equal locking force on each of the tie bars.B2.2.2 CLASSIFICATION OF COLD CHAMBER DIE CASTING MACHINES

The cold chambers machines are sub divided into two types based on the position of the injection chamber are

1) Horizontal cold chamber machine2) Vertical cold chamber machine.

B2.2.3 HORIZONTAL COLD CHAMBER MACHINE

The short chamber machine is mounted horizontal pouring. Hole in the top of the chamber wall.

The operating cycle of the horizontal cold chamber shot system starts with the die closed and locked and the cylinder plunger in retarded position. The molten metal is ladled

1629




16

though the pouring hole of the shot chamber. Then the injection cylinder is actua ted and moves the plunger forward. The plunger tip first closes off the pouring hole and then forces the metal into the die . After solidification of the metal in the cavity the die is opened and the plunger moves in the direction of initial trav el to complete its full stroke. This final forward travel of the plunger tip forces the biscuit (excess metal) free of shot chamber and aids in stripping the casting from the cover die. The plunger retracts the casting is ejected from the ejector die and the machine is ready for the next cycle after the halves of the die are closed and locked.

In the shot cylinder of the horizontal injection machine the plunger tip should be adjusted to be very near the pouring hole. Most plunger tips are made of H 13 tool steel or nit riding alloy steel to resist heat and wear.

1729




17

B.2.2.4 VERTICAL COLD CHAMBER MACHINES

In this m achine the injection system consists of a vertical shot chamber (fig. 15 connected directly to the cover die half by a sprue bushing. There are two plunger in this machine a hydraulically actuated lower plunger covers the bushing hole in the vertical cold chamber while metal is being ladled into the chamber from above. After the pressure builds up, the lower plunger retracts, uncovering the sprue –bushing hole, the metal is forced through the sprue bushing into the die. After dwell cycle for Metal solidification, the upper plunger is with drawn while the lower plunger raises and shears off the remaining slug of metal (biscuit) and ejects it.

The molten metal in a vertical chamber machine moves in a compact mass as the plunger advances. This minimizes metal turbulence and casting of low porosity could be produced.

Casting with high density specifications, or where the use of inserts is required and the parts that can best be made with centre gating and better finish, the die casting are recommended to be made in vertical machines . Some examples of casting for which centre gating is most convenient or most effective are those having thick centre hubs and thin outer sections , such as wheels and blower impellers.

(Fig. 14 Equivalent stages in the casting cycle of a cold chamber machine (End wick))(fig.15)

1829




18

B2.2.5 COMPARISON OF HOT AND COLD CHAMBER PROCESS

The surface quality the density of the cast structure and the other mechanical properties are most the same for parts made by the processes . It is found that cold chamber castings. Made of various alloys such as Aluminum silicon and Zn alloys, s how some what higher strength value. This is due to chemical properties (iron picks up) of the corresponding alloys in the molten condition and also due to higher pressure.

B2.2.6 PROCESS PARAMETERS AND CONTROL

As long as the die lasts, brasse s are quite easy to diecast. Because of the good cast ability of the alloys high quality die casting can be faithfully reproduced. With proper process control and die design good productivity can be achieved of high quality parts.

In any die cas ting process there are eight major process variables , and the die casting of brass is no exception These variables are :

1. Alloy Content 2. Metal Processing 3. Injection Velocity and pressure4. Die Temperatures5. Casting Ejection Temperatures 6. Cycle Timing7. Lubricants 8. Tie bar Loading

Except for the lubricant and tie bar loading each variables is discussed below in terms of its effect on the casting quality, operation efficiency, and methods of control as related to die casting the brasses.

Alloy Content and Processing: -

The alloy compositions must be maintained within the specification ranges. It is most important for the alloy to be prepared and handled correctly. Extraneous materials must be kept out of the holding furnace and the melt must not be overheated. Overheating may cause excessive loss of the zinc from the alloy.

Ideally, each heat of alloy should be checked but this is usually confined to the breakdown furnace or a certified analysis from th e ingot supplier. Methods of analysis may vary, but a spectrographic quant meter is probably the fastest and easiest but this type of equipment is not ordinarily present in a die casting shop Most die caster purchase their metal from an ingot maker who guarantees the composition.

1929




19

In reverberatory and rocking arc furnaces, separation of heat source and metal result in a local heat concentration at the surface of the liquid metal. The high heat concentration tends to increase the loss of zinc from the metal. When all factors are considered, induction heating is usually preferred to indirect heating. The breakdown furnace is usually run continuously. Overnight the charge in the furnac e is kept at 1/3 to 1/ 4 of the capacity of the breakdown furnace is dispensed at any on time. If the charge in the furnace. It is held at a temperature as close to the liquids as possible. This practice serves to reduce fuel costs and zinc loss. Prior tothe start of the shift, the furnace is charged with a mixture of about 1/2 new ingot metal and 1/2 trimming and /or chips. The temperature is raised to about 110 °C above the melting point of the alloy where it is held until needed by the die casting machine operator.

The metal is dispensed to the individual holding furnaces at each machine by transfer crucible or ladle which is filled from the breakdown furnace. Usually an overhead rail system or a traveling crane is used to move the transfer crucible from the breakdown furnace to the holding furnaces.

A low frequency electric induction, two chamber unit type of melting furnace has been introduced into a number of shops. One chamber is used for melting and the other for holding. The two chamber furnaces are desirable for shops whose product line involves different alloy compositions because a central break down furnace limits versatility regarding composition changes.

Injection Velocity and Pressure: -

The velocity of the plunger determines how long the molten metal is in contact with the relatively cold shot sleeve and runners, leaving less heat in the metal which must travel to the far reaches of the cavity . Therefore, changes in plunger velocity alter the thermodynamic casting cycle. And the most scientifically engineered gating design is worthless if the plunger velocity is incorrect.

The plunger velocity governs gate velocity and must be adjusted to compensate for any change in plunger diameter. The actual velocity of the plunger is the net result of the interaction of many variables such as: -

1. Hydraulic System Pressure2. Hydraulic Fluid Temperature3. Alloy Temperature4. Plunger Size 5. Cold Fit 6. Casting Rate & Charge Size 7. Temperature of Shot Sleeve8. Control Valve Opening 9. Control Valve Response ( repeat ability)10. Die Temperature 11. Plunger to Shot Sleeve Fit 12. Temperature of Plunger 13. Water Cooling Flow14. Plunger Lubricant (type & application)

2029




20

Die Temperature: -

The high fluidity of the brasses makes them easy to cast. The short die life result in very high tooling costs and poor utilization of plant and equipment. Ironical ly, the ease of casting contributes to the short die life Since it is possible to get good castings from a cold die after only 3 to 6 shots it is tempting to do so,. Even a preheated die may require several shots to establish the necessary thermal gradients through the die.

The magnitude of the stresses that cause heat checking is a function of the difference between metal injection temperature and the initial die temperature. The essence of the problem lies in the fact that with each injected metal , if the whole die is heated up it would simply expand. But, since the surface heats and expands while the base material remains as before, the surface experiences extreme compressive stress which results in physical mechanical deformation of the die material.

The total amount of cavity surface deformation becomes a direct function of the temperature change.

The studies conclude that the hotter the die the longer it will last. This operating temperature to life relationship is believed to hold true until a die is heated above the phase transformation temperature ( 500 ° C for H - 13) of the die material Higher temperatures supposedly reduce die life.

Die temperature control is also important to casting quality Internal porosity and blisters are dependent upon die temperature patterns. Zoned temperature control, could be very effective although it might involve heaters as well as cooling media.

If the die is too cold at the time of metal injection, it will contribute to cold shuts and lack of fill. Premature freezing in critical casting gate, or runner areas may impair shrinkage feeding even though adequate pressure is applied by the plunger. Conversely, i f the die is too hot the injected brass may “solder” to the die there may be excessive Flash or the re could even be bending of delicate cores. Zoned temperature control can practically eliminate premature freezing of sections in the “feed path”

Die temperature control is very important to the successful die casting of brass, even though it may not be always apparent to the operator of the machine.

Casting Ejection Temperature: -

Casting ejection temperature is a very critical variable. If the casting is too hot, it may deform when ejected, blister, “weep” lead (if it is high lead alloy), solder to the die, or burst in a heavy section. If one area only is too hot, if may have excessive porosity either internally or on the surface. Similarly, if the casting is below the proper ejection temperature, it may seize to the die and cause drag marks or crack the casting.

2129




21

Cycle Timing: -

Cycle timing is a critical factor in maintaining die and casting ejection temperature. The DT/CT controller establishes a process dependent feed back control of one point in the cycle Zoned die temperature control will compensate for some fluctuation in cycle timing . But these systems are limited. It is still necessary to achieve a uniform cycle. Die open time and ladling time should be as constant as possible.

A temperature monitoring device called a “pacer” has been developed by Dow Chemical Company to help control cycle timing. When the die gets too hot, a red light comes on the die is too cold and he must speed up A green light shows that the temperature is in the desired range. Because of the relative ease with which brass castings can be made in a low temperature die, some temperature sensitive system is necessary to avoid making shots into a “cold” die Such control devices will improve casting quality and optimize output while maximizing die life. Since die life is the key to economical die casting of brasses, the operating personnel must be trained in the reason for and use of the instrumentation.

B2.2.7 COLD CHAMBER DIE CASTING PROCESSING METALS AND ALLOYS

Alloy Metallurgy: - As a molten al loy freezes it usually passes through two stages Solidification begins at the liquids temperature; then as cooling proceeds, the quantity of solid metal increases and the alloy is in a pasty of solid metal increases and the alloy is in a pasty condition, finally it solidifies completely at the solid us temperature.

Fig. shows the equilibrium diagram of the well -know aluminum silicon alloys. It will be seen that with 8% silicon the liquids is at 6050 ° and the solids 577°C, so there is a freezing range of 28°C. At about 11.7 % silicon, the liquids and solids coincide and melting point is the lowest for the series. This composition is called the eutec tic, and the alloy solidifies at one fixed temperature, in a similar way to the freezing of a pure metal. A part from the exceptional cases of alloys whose composition is precisely that of their eutectic all alloys used in die casting freeze over a range of temperatures, from a few degrees for zinc alloy to over a hundred degrees for magnesium alloy. The table below shows the liquids and solids temperatures of typical die casting alloys.

2229




22

--------------------------------------------------------------------------------------------Alloy Liquid s °Centigrade Solids--------------------------------------------------------------------------------------------Zinc alloy BS 1004A (4.1 % A1.) 387 382Aluminum-silicon alloya 8% Si 605 577b 10% Si 590 577c 11% Si 585 577d 12% Si 582 577Aluminum –magnesium alloys a 5% Mg 635 590b 7.5% Mg 625 560c 10.0% Mg 610 525

Magnesium alloy MAG 7 596 468Brass60 % Cu 39.5 % Zn 0.5 % A1. 910 900

Aluminum Bronze89 % Cu 10 % A1. 1 % Fe 1053 1040

Nickel Aluminum Bronze84 % Cu 10 % A1. 3 % Fe 3

3 % Ni 1060 1049--------------------------------------------------------------------------------------------

The aluminum alloy LM 6 containing 10-13 per cent silicon illustrates the effect of freezing range in gravity die casting This alloy specification spans the theoretical eutectic composition of 11.7 % and the changes in freezing range on entire side of the eutectic cause wide variations in the form of shrinkage, during solidification. Furthermore the composition of the eutectic is displaced by fast cooling rates and by the effect of traces of sodium which are added t o modify the structure and improve the properties. To avoid excessive scrap due to shrinkage, both the silicon and sodium contents must be matched to the particular casting, and then controlled with in narrow ranges. In gravity die casting production it isnecessary to select and specify the composition which gives satisfactory results. If a gravity foundry accepted ingot with , say 12.5 per cent silicon from one supplier and 10.9 per cent silicon from another, either consignment separately might produce acceptable die casting, but if they were mixed, the average silicon composition would become 11.7 per cent and this , being the eutectic composition, would prove troublesome to gravity diecast because, because of difficulties in feeding.

In pressure die casting aluminum - silicon alloys, these problems are partly overcome by the pressure follow – up applied during freezing. Even so, critical castings subject to radiographic inspection necessitate close composition control to avoid shrinka ge defects. In general the freezing range of the alloy is not so important in pressure die casting as in gravity. For example the magnesium based alloys have long freezing ranges but are not difficult to pressure die cast. Other factors such as ho t- shortness, discussed below, are of greater importance.Hot Shortness: -

2329




23

In pressure die casting, molten metal is injected into a die at the speed of an express train, and is rapidly compressed in the pasty stage, to a coherent mass. Continued rapid cooling reduces the temperature of the metal by a few hundred degrees and so used to Obtaining accurate castings of good surface appearance, that they rarel y visualize the metal turbulence inside a die every time a pressure casting is produced.

Stresses are set up because of contraction on solidification and subsequent cooling. Certain alloys which tend to crack under these stresses are called hot-short, for example 70/30 brass is hot –short and so cannot be diecast with out cracking ; 60/40 brass is not hot-short and is comparatively easy. The aluminum alloys with 10 per cent magnesium are hot –short and are quite difficult to gravity diecast and still more difficult to pressure diecast.

The Effect of Melting Point on Die Life: -

Since the metal is only in contact , in this molten state , with the die surface for a fraction of a second , the tendency to dissolve the die steel is not vary significant. The principle effect of melting temperature concerns the thermal fatigue of effect of melting temperature concerns the thermal fatigue of the dies, which deteriorate gradually over a period of time, discussed in h eat checking. Small cores are replaced after a few thousand shots, ejectors require to be replaced ejector guides ground and die forms republished and additional draft or radii provided where die wear has occurred in important cavities.

Design features, section thickness and volume of the die casting and quality of the die making make it difficult to define an accurate relationship between melting point and die life. Lead and tin alloys, melting at fewer than 300 °C, give almost indefinite die lives. Zinc alloy, cast at about 400 °C, will often give several hundred thousand castings while, with good component design combined which good die -making , a life of o ver a million shots is possible. In casting aluminum at about 650°C it is likely that running repairs will have to be done quite regularly, but total die lives of from 100,000 to 200,000 are excepted, and may be exceeded quite considerably.

Magnesium has a similar melting point to that of aluminum, but it does not erode steel dies like aluminum so the die life is normally about 25 per cent greater for an equivalent job. The effect of this longer die life on total production cost is discussed on page 257. Brass pressure dies will endure abo ut 10,000 to 20,000 shots – sometimes more under ideal conditions.

When gravity die casting aluminum bronze, with a melting point of over 1000°C die lives of about 10,000 to 20,000 are expected and, for long runs, special materials such as the Mnemonics are economically worthwhile , these heat resisting materials give much longer die lives under gravity die casting conditions at such elevated temperatures.The Effect of Other Physical Properties: -

Fluidity thermal conductivity, specific heat and latent heat of the metal affect the Die-casting properties. The solubility otherwise of ferrous metals in the alloy because they have comparatively low melting points, but also because they do not dissolve the ferrous material from which the goosenecks and plunger are made. Molten pure zinc dissolves iron,

2429




24

so that this zinc alloy is suitable for casting in hot chamber machines. However, if the melt temperature is not kept down, rapid solution can occur.

Magnesium has a melting point similar to that of aluminum but it does not readily dissolve ferrous materials, so it can be die casting using goosenecks similar to those employed for zinc alloy die casting. Aluminum dissolves ferrous materials so a hot chamber machine for aluminum would require a gooseneck made of a ceramic material such as silicon nitride or titanium dibordieAluminum Alloys: -

Die Casting Alloys: -

It is customary to refer to particular alloy s by their specification number, without reference to the type of raw material used to produce the alloy. In practice this may be wholly secondary wholly primary on a mixture of both. The official national standards for foundry alloys in all the major countries simply specify the required limits of chemical

Composition but they impose no restrictions on the choice of raw material, leaving the ingot producer to decide what materials are technically economically suitable.

Pressure Die Casting Alloys: -

Per Cent Composition (Single Values are maxima)

BS 1490 Cu Mg Si Fe Ma Ni Zn Pb SnLM.2 0.7-2.5 0.3 9.0-11.5 1 1 0.5 2 0.3 0.2LM.5 0.1 3.0-6.0 0.3 0.6 0.3-0.7 0.1 0.1 0.05 0.05LM.6 0.1 0.1 10.0-13.0 0.6 0.5 0.1 0.1 0.1 0.05LM.20 0.4 0.2 10.0-13.0 1.0 0.5 0.1 0.2 0.1 0.1LM.24 3.0-4.0 0.30 7.5-9.5 1.3 0.5 0.5 3.0 0.3 0.2

The alloys most used in other countries for die casting are shown in the Table on 13. Generally the specifications shown in any one line are similar but not necessarily identical. In the USA the alloy to specification ASTM A.380.1 commercially as 380 alloy (comparable with LM .24 in UK), is by far the most commonly used alloy for die casting.

Properties: -

A part from suitability for the particular design and casting method, which should be a subject of consultation at an early stage of the conception of the casting, the choice of alloy will depend upon the excepted subsequent treatment and service requirements of the casting. The suitability of the alloys to meet particular requirement s are briefly reviewed below, mainly in relation to those more commonly used, but reference is made where appropriate, to special purpose alloys.

2529




25

Castability: -

With the modern high pressure die casting machines difference in casting characteristics are not of great significance, although the aluminum-magnesium alloys such as LM5, tend to suffer from hot-tearing and are considerably more difficult to diecast. All the alloys listed previously on page 215 have favorable characteristics for casting in permanent moulds. For very large, thin or complicated castings LM6 is the most suitable alloy but the need for modification treatment may be a slight disadvantage.

Mechanical properties: -

The strength and hardness of the four pressure die casting alloys LM.2 , 6, and 24 are roughly of the same order, with a slight advantages towards LM.24 Under ideal conditions with a test bar only 6mm in diameter, tensile strength of the order of 300 N/mm2 ( 20 tons per sq.in approx.) could be obtained but as a general rule for designing a component with one of these pressure diecast alloys a tensile strength of say 10-12 tons per sq.in. (150-170 N/mm2) should be expected.

The superiority in ductility of LM6 May sometimes is useful if a casting needs subsequently to be formed or rivotted. For permanent mould casting t here is a Much wider range of mechanical properties. The alloys LM4, 6, and 27 offer moderate strength and hardness which adequate for the greater proportion of castings; LM 6 has a some what lower proof strength and but much higher elongation. The alloys LM9 and 25 which are heat -treated, satisfy most requirements when higher strength hardness is required. LM25 which is specified in four conditions is particularly versatile in its range of mechanical properties.

When high shock resistance is needed LM6 Will usually satisfy requirements, but if greater strength is also required LM22 ( usually gravity diecast and then heat treated ) is suitable For castings of simple shape the alloy with 10% magnesium LM 10 —one of the most difficult alloys oven to gravity diecast might be considered. The fatigue properties of cast aluminum alloys tend to lie within a fairly narrow band and no one alloy offer an outstanding advantage. In particular it should be noted that, while the tensile strength of an alloy may be double by heat -treatment, the fatigue strength may be improved only marginally. When permanent mould castings are required for high temperature service, one of the special purpose alloys, LM 13 or LM 26, should be u sed, but where moderate temperatures of 200 or 250°C are excepted, alloys LM2, 4, 24 and 27 may be selected. The composition of all these alloys was shown in the tables on page.Corrosion Resistance: -

Weathering tests have shown that there is little difference in the resistance to atmospheric attack of the common die and permanent mould casting alloys. For use in marine applications or in chemical and food manufacture the alloys LM6, 9 and 25 offer greater resistance than the others listed before. Provided that they are suitable from other considerations, LM5 and LMO (pure aluminum) provide even greater resistance to chemical attack.

Machinability: -

2629




26

The die casting alloys LM2 and LM2 4 are significantly easier to machine than whilst LM20 is slightly superior to LM6. OF the permanent mould casting alloys, LM4 and LM27 Machine very well LM25 When heat –treated is a almost as good, LM 9 and LM6 cause rather more tool wear, and the relative softness of the latter alloy is disadvantage in certain machining operations such as the cutting of threads.

Physical properties: -

There is little to choose in terms of thermal expansion between the alloys commonly used for permanent mould or pressure die casting. As will be seen from the table this thermal expansion is of the order of 20 x 16-6 per °C. If a lower thermal expansion is of the order of LM 13, which has a coefficient of 19 x 10 -6 can be used, but the special piston alloy LM29 has a coefficient of only 16x 16-6 per OC.

If electrical conductivity is an important requirement there is an advantage in using LM6, 9, 20 and 25 of the common alloys ( a much higher conductivity is, of course, offered by pure aluminum).

Magnesium Alloys: -

Alloys: -

In the U .K. the preferred alloy for commercial die casting and permanent mould casting is MAG 7 to BS. 2970; this also know as Electron “C” alloy. The nearest U.S. Specification for this purpose is AZ91B, the German Mg A19 Zn1, the French GA9z1 and the Russian ML5.

A similar material , but with a tighter specification and generally only issued for permanent mould casting, is MAG 3, for which the aeronautical used specification is L123, though this also implies higher inspection standards; MAG 3 is the near equivalent of the United States AZ 91C, the German Mg A1 9 1, then French F10 alloy and the Russian ML6. The die casting

Equivalent C is AZ 91B which implies a beryllium content of 0.0015% in the ingot alloy. The composition and properties of the two alloys MAG 3 and MAG7 are shown below: -__________________________________________________________Alloy to Alloying Element % Maxim Impurities %BS2970 Aluminum Zinc Manganese Copper Silicon Iron Nickel___________________________________________________________MAG 3M(AZ91CNearest) 9.0-10.5 0.3-1.0 0.15-0.4 0.15 0.3 0.05 0.02MAG 7M(AZ91BNearest) 7.5-9.5 0.3-1.5 0.15-0.8 0.35 0.4 0.05 0.02

2729




27

____________________________________________________________

The Uses of Magnesium Die Casting: -

The uses of magnesium die casting derive essentially from their weight saving effect, through in many instances the saving in machining effect, t hrough in many instances the saving in machining costs alone can be sufficient justification. It is certain however that, in our present fuel and power economy drive, any weight reduction is of worthwhile benefit especially where inertia plays a part in stopping and starting , for example textile machinery or transport vehicles.

Since magnesium casting alloys are not aggressive in their corrosive action toward mild steel or low alloy steel components, the hot chamber machine has definite adv antages in that the benefits of automating the process, originally derived from zinc alloy practices, can be applied to magnesium production.

The general pattern of the use of magnesium die casting is quite extraordinary. It appears that, once a user or indeed a country has realized that magnesium is quite safe to use and that the fire hazard is not a bogey ( it has been used for cooking pots and frying pans) the metal is accepted and used

Component Design: -

A uniform section (generally of 2-4 mm) is desirable with the minimum of thickened sections. These, if unavoidable, should be lightened by means of coring if at all practical, with ribs being preferable to thickened sections if strengthened areas are required.

To take full advantage of the high casting rates available with the use of magnesium alloy, side cores should be omitted, and apertures formed either by opposing die block design or by a secondary machining or press tool operation. Thus the planning design stage of a component is all –important in obtaining a cast of uniform section , and avoiding localized thicker areas which would lead to Hot –spot formation with the possibility of consequent cracking . It must also be borne in mind that sharp corners, where stress concentrations localize , are frequent areas where cracking can take place and that an edge should always be provided with a small radius. It has also been noted that slightly roughened die surfaces diminish the tendency crack formation.

Die Casting Systems for Magnesium Alloys: -

Both hot chamber and cold chamber processes are used for the production of magnesium alloy die casting. The essential requirements are:-

1) A fast cycling time to take advantage of the high production rates arising from the low heat content of the alloy.

2) A fast-acting second stage of the injection system of the order of 3-7 m/s, to fill the cavity before the inset of solidification.

2829




28

Copper Alloys: -

There is only pressure die casting alloy in the United Kingdom covered by BS 1400, and this is essentially a duplex brass. Figure 2.3 shows the copper -zinc thermal equilibrium diagram on which it is based.

The presence of lead, which is essentially insoluble in copper, is to aid mach inability whilst aluminum prevents zinc oxidation and loss and improves fluidity.

Since the end use of many pressure die cast components is in the form of plumber’s fittings for use in water, corrosion resistance is important .This problem can be solved in several ways. At elevated temperature the alloy has a duplex structure of alpha and beta phases, the percentage of beta becoming less as the temperature the alloy has a duplex structure of alpha and beta phases, the percentage of beta becoming less as the temperature is lowered. Beta-phase is necessary at high temperature to give resistance to hot cracking, but its presence at room temperature and at room temperature th e beta phase at high temperature but little at room temperature the beta phase will be in an isolated condition and corrosion will not proceed to any great extent . Corrosion of the alpha phase can be inhibited by small amount of arsenic.

Another approach to the problem is to give the castings a low temperature heat treatment to achieve the same result as just described , whilst a further answer is to introduce alloying elements. There are several proprietary alloys which achieve this, generally by the addition of manganese, nickel or tin.

The high casting temperature of brass means that die life is drastically reduced as compared to the other metals and this has lead to the development of die materials based on molybdenum and tungsten.

B2.2.8 COLD CHAMBER DIE CASTING DIE

A die casting die is basically a heat exchanger, where the heat from the molten metal is absorbed by the die cavity surfaces and the die body by the process of heat t ransfer, to solidify the molten metal and from the component as per the shape of the impression. This process is repeated and there is regular absorption of heat from the molten metal by the die casting die which has water –cooling system to maintain constant die temperature.

The die casting die is split into two sections so that the casting can be removed after it has been formed. These two sections are called the cover die and ejector die, or fixed half and moving half etc. The two sections, which meet at the parting line, contain the hardened cavities and cores. The cover die goes on the stationary platen of the casting machine to which the shot chamber is connected, and does not move during the casting cycle. The ejector half is fastened to the movable platen of the machine where die locking and ejector systems are provided.

The parting line formed by the two surface of the die must be smooth and finished so that they fit closely to gather. Straight parting lines are always preferable. The two halves

2929




29

of the die should be in exact register when the die is closed. This is accomplished by providing guide pins and bushes in the stationary or cover die members.

The main parts of the Cold Chamber die casting die include: -

1. Cavity plate + Back plate (Clamp plate)2. Core plate + Back plate (support plate)3. Guide pins and bushes4. Parallels of rails5. Ejector plate6. Ejector retainer plates7. Ejector pins8. Surface pins or return pins9. Die gates and runners10. Cores and slides11. Mounting holes and slots for clamping12. Stop blocks or stop pins etc.13. Injection cylinder sleeve.

The fig shows a typical die casting die layout suitable for the Horizontal cold chamber die casting machine. The die assembly section wit h the plans of ejector dies and cover die halves could be seen with its details. The cavity and core inserts, sprue bush and sprue insert.

- 1 -10



CHAPTER: B3 PRESSURE DIE CASTING (HOT CHAMBER)

- 1 -


PRESSURE DIE CASTING (HOT CHAMBER)

- 2 -10




- 2 -

Chapter Outline

B.3.1.1 Introduction

B.3.1.2 Classification of Die Casting Machines

B.3.1.3 Hot Chamber Machines

B.3.1.4 Hot Chamber Machines and Parts

- 3 -10




- 3 -

B.3.1.1 INTRODUCTION

DIE CASTINGS are produced by forcing molten metal under pressure into moulds called dies.

B3.1.2 CLASSIFICATION OF MACHINES

All die castings machines have one of two different metal pumping system: A hot or cold chamber system.

If the metal being cast melts at a low temperature and thus does not attack the injection pump material, the pump can be placed directly in the molten metal bath ( hot chamber machines). If the molten metal attacks the pump mat erial at casting temperature, the pump must not be placed in the metal bath, and a cold chamber machine must be used.

- 4 -10




- 4 -

B3.1.3 HOT CHAMBER MACHINES

The metal pumping system is shown in fig. 1, which consist essentially of pressure and power cylinders plunger, gooseneck and nozzle is typical of hot chamber injection (shot ) systems. The gooseneck containing the pressure cylinder and plunger is submerged in the molten casting metal and is thus at the temperature of the metal bath. This arrangement allows the metal to be injected into the die cavities in minimum time and with minimum decrease in temperature.

With the plunger in the up position as shown in fig. 1, molten metal flows from the pot into the pressure cylinder through the intake ports. With the die closed and locked, the power cylinder is energized to move the plunger downward. This seals off the intake ports. With further downward movement of the plunger, of the plunger the molten metal is forced through the gooseneck channel and the nozzle into the die cavity. After a present time to allow the metal to solidify in the die cavity, the power cylinder is activated in the reverse direction, thus pulling the plunger up. This uncovers intake ports, and metal flows from the port into the pressure cylinder the machine are ready for the next cycle. The power cylinder can be actuated either by air or oil.

Castings weighing from a fraction of an ounce to about 50lb can be produced in hot chamber machines of metal cast with one stroke can be varied by using different sizes of gooseneck assemblies. The weight of castings that can be made depends on the alloy being cast, the projected area of the shot, and the locking pressure.

Depending on degree of mechanization, process variables and the part being cast, hot chamber machines are generally at rates of 50 to 500 shots per hour. Special machines greatly exceed this rates, ranging from 2000 to 5000 shots per hour upto 18000 shots per hour for a zipper- casting machines.

The gooseneck is made of gray, alloy or ductile iron, or of cast steel. The choice of material is dictated by operating pressure, casting metal, and cast. Common practice is to insert a replaceable liner in the bore of the pressure cylinder (Fig. 1). The material for the liner should have good wear resistance and resistance to softening at operating temperature. The goosenecks usually have a nozzle seat (Fig. 1), which can be replaced when the spherical sealing surface becomes damage. A good seal must be maintained at the junction of the components (pressure cylinder, gooseneck channel, nozzle seat, and nozzle) through which the molten metal flows to the die.

Liners and nozzle seats usually are made from H13 or a high speed tool steel, nitrided alloy steel, or stainless steel. Nozzles must be able to resist the washing action of the molten metal and the scaling action of externally applied heat, and must be strong enough at operating temperature to resist the pressure of molten metal. Alloy cast iron, H13 tool steel, and stainless steel are among the materials used for nozzles.

The plunger is generally of alloy cast iron and may or may not be heat treated. Often, the plunger includes one or more rings (like piston rings ), which aid in maintaining pressure.

- 5 -10




- 5 -

Life of the pressure cylinder may be extended by honing the inside wall of the cylinder (often as much as 0.030 in. oversize in inside diameter ). A proportionally larger plunger is then used regardless of whether rings are included. A plunger-to-cylinder clearance of 0.002 to 0.003 in. per inch of bore diameter is considered optimum.

B.3.1.2 HOT CHAMBER MACHINES AND PARTS

Regardless of types of machines used, it is essential that die halves, cores and other movable sections be securely locked in place during the casting cycle. Generally, the lamping force is governed by (a) the projected area of the casting (measured at the die parting line) and (b) the pressure used to inject metal into the die. Most mechanisms actuated by hydraulic cylinders (sometimes air pressure) to achieve locking. Others use direct acting hydraulic pressure. Safety interlock systems are used to prevent the die during casting cycles. Die casting machines.

Hot chamber machine: - A type of die casting machine in which the pumping chamber submerged in molten metal to inject metal into the die. Plunger or air pressure is applied in the chamber of the machine. Which are known respectively plunger die casting machines and air operated die casting machines.

Gooseneck machine: - The hot chamber machine is also called as gooseneck machines as the pressure vessel or metal injection pump has the metal channel in the shape of gooseneck.

Holding furnace: - Furnace used for holding molten metal preparatory to pouring castings. This provides molten alloy at the desired temperature.

Locking force or clamping force : - The capacity of die casting machine to resist metal injection force which tend to separate the die halves.

Platens: - A member in die casting machine that supports die halves.

Movable: - This holds and supports the moving die half. The movement of this platen is used in the actuation of the ejector system in the die.

Stationary platen: - This holds and supports the cover die half which generally contains the cavity.

Tie rod: - A bar used in a casting machine to hold dies against pressure; also serves as a way along which the movable die platen slides.

Toggle: - Linkage in a casting machine employed to multiply pressure mechanically in locking the dies. Also, linkage used for core locking and withdrawing in a die.Injection cylinder: A hydraulic cylinder which is used for actuating the injection plunger.Injecting force: The force exerted by the injection plunger on the molten alloy in the shot sleeve.

- 6 -10




- 6 -

Injection stroke: - It is the total travel of the injection plunger.

Induction furnace: - Furnace heated by resistance of metal to flow of flux lines induced by alternative electric current.

Injection: - The act or process of forcing molten metal into a die.

Plunger machines: - Die casting machines having a plunger in continuous contact with molten metal (Hot chamber machines).

Reverberatory furnace: - A furnace having a vaulted ceiling that throws back the flame and heat toward the hearth of the upper surface of the charge to be melted.

Shot: - That portion of the casting cycle in which molten metal is forced into the die.

Plunger: - That part of the die casting machine which forces the metal alloy into the die.Distance between tie bars: The distance between tie bars that is available for clamping the die halves. The center distance between tie bars minus the size of one diameter of the tie bar gives this distance. (Fig .1)

Maximum clamp stroke: - It is the maximum amount of stroke the moving platen of the machine can travel from the open position to the close position. (Fig.1)

Minimum die thickness or minimum closed daylight: When the machine is in the fully closed position the minimum distance that is available between the fixed and moving platen for fixing the die. The moving platen will not travel further to meet the fixed platen. (Fig. 1).

Maximum die thickness or maximum closed daylight: It is the maximum distance between the fixed platen and moving platen in the closed position, which can accommodate a die of this thickness.

CLAMPING DIE SPACE NOMENCLATURE.

- 7 -10




- 7 -

FIGURE 1

Shot capacity: - It is the capacity of the molten alloy, which the cylinder or shot sleeve can inject. The total volume or weight of the alloy which can be injected from the shot sleeve is given generally in terms of aluminum alloys. Shot sleeves having different volumes are available with the machines, which gives various shot capacities.

Shot sleeves: - These are the cylinders, which are used for various capacities supplied along with the machines including the plungers. These are of different, internal diameters and external diameters remain the same.

Projected area: - It is the area of any shape of casting projected on a plane normal to the direction of the draw. This also includes the runners, gates & overflows and the sprue. This also indicates the total area of the casting machines. Maximum theoretical projected area A=F/p where F=locking force, p=injection pressure.

Cycle: - The complete elapsed time required to go through all the operations to mould the casting.

Number of shots per hour: - The number of shots per hour in the machines depends upon the type of the alloy used, value of metal per casting wall thickness, mould, temperature etc. the number of shots per hour indicates the production rates of the machine. Higher melting point alloys reduce the number of shot per hour due to the longer cycle time.

- 8 -10




- 8 -

Oil pressure: - It is the pressure of the oil, which actuates different hydraulic cylinders in the machine, made available from the accumulator.

Accumulator: - The pressure used in injection system and the general operation of the hydraulic cylinders in the die casting machines is provided from a gas oil vessel called accumulator. The gas used is generally nitrogen. This supplies the require volume of oil at high pressure and rates necessary.

Intensifier: - It is a pressure multiplier which at the end of the injection piston travel increases the pressure on the molten metal in the shot chamber and thus on the casting. The intensifier prevents, voids and porosity in thick-wall casting. The die casting machine essentially consists of a frame on which the die and actuating equipment are mounted. The actuating device opens and closes the die which in all modern machines is a hydraulic cylinders and piston, or hydraulic cylinder and toggle arrangement. The injection system of the machines forces the molten into the die under pressure. The modern machines are equipped to give slow initial plunger movement, a variable speed filling stroke and an intensified final squeezing pressure before the solidification is complete. The ejection system consists of knock out rod or plates, hydraulic cylinder and rack and pinion. The power cylinders are actuated either hydraulically or pneumatically from a pressurized accumulator.

FIGURE -1: SMALL (21-IN) DIE CASTING MACHINE HAVING A HYDRAULICALLY OPERATED TOGGLE MECHANISM THAT CLOSES MACHINE AND LOCKS THE DIE IN PLACE.

- 9 -10




- 9 -

FIGURE : 1

CASTING CYCLE ON A GOOSENECK MACHINE (E. M. D. NO. 12).

1. Limit switch 2. cylinder plunger rod3. power cylinder4. cover platen 5. nozzle6. nozzles seat 7. channel 8. gooseneck 9. plunger ( in up position)10. coupling 11. intake port(1 to 3)12. pressure cylinder 13. liner14. burner (1 of 2)15. fire brick 16. pot17. steel shell

- 10 -10




- 10 -

FIG 10.HOT CHAMBER MACHINE

When the plunger is in up position the molten metal flows from the pot into the pressure cylinder through the inlet pot. With the die closed and locked, the shot cylinder is actuated to move the plunger downward. As the plunger moves down, it seals off the inlet pot and molten the metal is force through the gooseneck channel and the nozzle in to the die casting die. After the metal solidifies in the die cavity, the power cylinder is actuated in reverse direction, pulling the plunger up. This uncovers the inlet pot allowing metal to flow from the pot into the pressure cylinder. The machine is now ready for the next cycle as the two halves of the die are closed.

The term “gooseneck” sometimes is also applied to this type of machines because of its hollow “U” shape. These machines are also now as submerged plunger machines.

Casting weighting up to 25kg (50 lbs) could be produced in the hot chamber machines. These machines generally operated at 50 to 500 shot/hour. A good seal must be maintained at

115


DIE CASTING TG-2

THEORY

CHAPTER: B4 DEFECTS AND REMEDIES ON DIE

CASTING DIE

1

UNIT-2

CHAPTER - B4

DEFECTES AND REMEDIES ON DIE CASTING DIES

215


DIE CASTING TG-2

THEORY


CASTING DIE

2

CHAPTER OUTLINE

B4.1.1 Introduction to Various Defects on Die Casting

Components.

B4.1.2 Identification of Defects.

B4.2.1 Classification of Defects.

B4.2.2 Description for Different Type of Defects.

B4.2.3 Horizontal cold chamber machine

B4.2.4 Vertical cold chamber machine

B4.2.5 Comparisons of hot and cold chamber process

B4.2.6 Process parameter & control

B4.2.7 Cold chamber die casting processing metals and alloys

B4.2.8 Cold chamber die casting die

315


DIE CASTING TG-2

THEORY


CASTING DIE

3

B4.1.1 INTRODUCTION TO VARIOUS DEFECTS ON DIE

CASTING COMPONENTS

With the exception of mechanical damage, such as distortion, drag marks,

burnishing or cracking, all casting defects are related to the interaction of heat flow and

metal flow with in the cavity and with process parameters and processing condition.

The effects of poor casting design can be minimized by good die design but where

obvious bad features exist the casting specifier should be informed and alternative designs

proposed.

Casting defects are undesirable in critical structural areas, particular by those subject

to fatigue. Minor defects on decorative surfaces can become very obvious after surface

treatment. Defects are generally eliminated by the die caster through control of the casting process. However the product and Tool Designer should be conversant with casting defects,

particularly those that are affected by design practice.

B4.1.2 IDENTIFICATIONS OF DEFECTS In General most of defects can be seen visually and some of the defects can be

found with special testing like Ultrasonic , UV testing etc., some of the following listed

procedure to finding the casting defects (See the given table).

SR.NO. CRITERIA DESCRIPTION

1 Casting Handling

To avoid damage to the casting they should be handled smoothly and care fully.

2 Bubbles If any swelling appear on the casting surface due to air

trapping , then they are know as bubbles or blisters.

3 Crack Inspect the casting and find whether any crack or damage

has appeared in the casting. If it is found then the casting is a rejected one.

4 Cold shut Check whether there are stream to merge (join) properly in

the casting. If not so, then the casting is defective due to cold

shut.

5 Flow defect In the casting surface , if any lines or small pits are found

, then they are called as flow defect

6 Gate broken Look at the gating area of the casting for any damage. If it is

found the casting is a defective

7 Lack of fill See the casting , if material is not properly filled in the

cavity area ( mould) then reject the casting . The defect is

due to lack of fill.

8 Pin bend Check diametrically the wall thickness of the hole by measuring at two or more suitable places. If they vary more

than the customer specified tolerance then the pin is said to

have bend.

9 Shrinkage Usually appears in the casting surface opposite to any heavy

415


DIE CASTING TG-2

THEORY


CASTING DIE

4

section as a rib or boss. The are known as shrinkage defects.

10 Surface finish Check whether the casting is smooth and stain free. If it is

not so it said to have a bad surface finish.

11 Weld Inspect the casting If there is any torn skin free. If it is not

so it said to have a bad surface finish

12 Ejector pin project or

depression

MECHANICAL DEFECT Inspect the surface of the casting in ejector face. If suppose

there are any projections or depression in the ejector pin

location area within the specified tolerance then the casting

is said to be rejected.

B4.2.1 CLASSIFICATION OF DEFECTS

The various Defects are commonly encountered in die-casting can be classified in to

three groups.

1. Cold type defects: -

Lack of fill

Cold shut

Severe chill

Chill Flow lines.

2. Hot type defects: -

Soldering

Cracks

Broken part

Bent part Heat marks or shrinkage pits

3. Miscellaneous defects: -

Scale Blisters

Porosity

Excessive Flash

Mechanical defects Ejector Pin marks.

When correcting faults during a casting run, it is important that the right corrective

action is taken as soon as possible A Fault correcting procedure, intended for Shop Floor

user is given below.

B4.2.2 DISCRIPTION FOR DIFFERENT TYPES OF DEFECTS

515


DIE CASTING TG-2

THEORY


CASTING DIE

5

1.) Cold Type Defects: -

Lack of Fill: - This condition has three basic causes the first cause is inadequate

metal in the Gooseneck or Cold Chamber. the metal level in the holding furnace of a hot chamber machines, the correct size idle must be used and care must be taken

to insure that the ladle is full for each shot When an automatic ladle is being used,

it must be properly adjusted to ladle exactly the correct amount of metal into the

cold chamber secondly this defects may be caused by cold metal, cold die or both The temperatures Should be Checked and adjusted as necessary. Finally lack of

Fill may be the result of Slow shot speed. The Shot control hydraulic valves should

be opened the proper amount.

Cold Shut: - It is like lack of fill is caused by cold metal slow shot or low die

temperatures if air vents and / or over flows are C logged with flash they may also

contribute to the problem All these factors must be checked and corrected as

necessary.

Severe Chill: - It is similar to cold Shut, but it will Cover a large surface of the

casting of being a Single line excessive release material as well as cold metal, Slow Shot low die temperature ,or clogged air –vents may be cause severe Chill

usually appears when shots are made in to a cold die but Will rarely occur during

normal operation.

Chill: - This defect has the same appearance and is caused by the same

conditions as Severe Chill but is less noticeable Slight or faint Chill lines on the

Surface may not be cause for rejection of certain types of castings However, castings used for ornamental parts will usually require a chill free Surface Low

die temperature, low metal temperature, slow Shot Speed, or excessive die

release material are all causes of chill.

Flow line: - Flow line defects are similar to chill and cold shut. Flow lines can

usually be reduced or eliminated by increasing the die temperature, metal

temperature or both Like chill the defect may be corrected by higher Shot Speed

or Less die release material.

2.) Hot Type Defect: - Soldering: - This condition is the result of the cast metal bonding to the die surface.

Upon ejection the casting teams away, leaving a layer that has banded to the die

surface upon ejection the casting teams away leaving a layer that has banded to the

die when soldering occurs It may create additional Problems such as cracking or bending of the casting, out of tolerance dimensions, depressed ejector pin parks in

the casting and low porosity with in the casting.

615


DIE CASTING TG-2

THEORY


CASTING DIE

6

Soldering may be caused by excessive metal temperature incorrect die

temperature (too hot or too cold) or insufficient die release material. If the condition is Severe or when other methods fails to remove the Solder, It must be cleaned

from the die caustic solutions provided for this purpose may be used or the

material may be polished out of the die it is recommended that a die maker, or

Other Person With Special training , Polish the die cavity if the polishing is not done properly, a rough die Surface may be created that will increase the Soldering

condition. Care must also be exercised When increasing the amount of die release

material applied to the Soldering area. Excessive amounts of this materials may

create other defects Such as porosity Chill, or blisters, When adjustments to the above conditions do not eliminate the Soldering condition, It is likely that the

problem may have been caused by the metal alloy being cast or it may result from

the die construction.

Cracks: - Casting may cracks from internal Stress or from abnormal pressure

during ejection The first cause , internal stress, is created from excessive metal or

die temperatures If the condition persists after Several temperature adjustments

have been made it may be necessary to increase the Shot or machine timer Settings the timers Should be adjusted only after every thing else has failed.

Cracks from abnormal ejection pressure may be indirectly the result or

Soldering The operator Should Carefully inspect the Casting For signs or Soldering. If Soldering Exists the appropriate corrective measure Should be taken

In Sufficient draft or a rough cavity finish in the die can also cause abnormal

ejection pressure Additional die release material may help , but will not correct this

Situation In severe instances , die release material, or Soldering

Broken Part: - A portion of the casting may stick in the die during ejection and

the rest or the casting break away eject normally. The cause is the same condition as

described for cracks: excessive metal or die temperature, in sufficient die release material, or soldering.

Bent Part: - Casting may bend instead of breaking when part or the casting sticks in

the die. This is a different result from the same conditions that cause a broken part.

Heat Marks: - Have the appearance of surface pits and are caused by excessive die

temperature or excessive metal temperature. Depressed areas in the casting and sharp

inside defect. Some times die temperature balance can be adjusted to eliminate heat marks. For example the flow of cooling Water to the area affected may be increased,

and decreased to the other die half in the same area.

Clogged vents or excessive die release may cause large volumes of gas to become entrapped in the die Such gases will increases the size of heat marks and may

cause the pits to become rounded and Smooth on the inside.

715


DIE CASTING TG-2

THEORY


CASTING DIE

7

Wave or Lake: - Irregular lines or slight steps on other Wise Smooth casting

Surfaces are usually caused by excessive die temperature in the area of the defect. Increasing the water (or Solvent) dilution of the die release material will reduce such

defects.

Sink Marks: - Shallow smooth depressions on the castings surface are called Sink

Marks Such marks usually appear on the casting Surface opposite any heavy Section

Such as a rib or boss, and are caused by uneven Shrinkage of the casting. Reduced die

temperature in the area of the sink mark reduced metal temperature, and Some times

increasing the temperature of the other die half will minimize these defects. Sometimes increased injection (Shot) pressure coupled with higher die temperatures

between the defect and the gate will help reduce Sink marks.

3.) Miscellaneous Defects: -

Scale: - Build up of die release materials (or the oxides) on the die results in an

irregular rough Surface on the casting. The material must be removed with a Caustic Solution or by Polishing the die. The same care is needed as described for soldering.

After cleaning the die, the amount of release material applied for each shot must be

reduced. Sometimes it may be desirable to change type of release material and /or the

ratio of release material to solvent (or water).

Blisters: - Bubble-like bumps on the casting surface are caused by air or other gases

trapped inside the casting, slower shot speed, clean vents, reduced die temperature, or

less die release material will usually eliminate blisters.

Porosity: - The holes in the casting are called porosity. Low die temperature

(Particularly in the runner and gate areas), Low shot pressure, clogged vents, or

excessive release material can cause porosity. Porosity is also often related to lack of fill, cold shut, heat marks, and blisters. When such a relation ship exists the correction

for the related defect will often improve or eliminate the porosity.

Excessive Flash: - Excessive flash results from material such as flash sticking to the die faces and holding the die open, excessive injection pressure or speed, or in

sufficient clamping force. The first problem is corrected by cleaning the die faces.

Flash that has become embedded into the die face must be scraped off corrections to

injection speed and pressure must be made adjusting the appropriate hydraulic valves. The clamping force is increased by adjusting the tie-bar nuts, very slight adjustments

to these nuts, are usually all that is required flash indicates that an extra thick casting

is being made extra thickness causes extra heat input to the die, and may result in

additional problems.

Mechanical Defects: - The operator should be aware of all moving and fragile parts

of the die cavity which are subject to wear, breakage, or other failures that could cause

defective castings small cores or thin blades of the die forming deep narrow holes or slots in the castings can be easily broken or bent. Ejector pins and moving cores can

wear, break, or not seat properly. In any of these situations, the die will not make the

part to the correct shape.

815


DIE CASTING TG-2

THEORY


CASTING DIE

8

Ejector Pin Marks: - High and low ejector pin marks: Ejector pins may push into the

casting when solder and / or a rough cavity surface result in the casting sticking in the cavity. High casting temperature at the time of ejection also can let the pins push in to

the parts. Increased flow of cooling water or more liberal application of release

material will some times reduce low ejector pin marks.

SUMMARY: -

A.) Some Cold Type Defects

CO

LD

M

ET

AL

CO

LD

D

IE

SL

OW

S

HO

T

SP

EE

D

EX

CE

S

SIV

E

DIE

RE

LE

A

SE

CL

OG

GE

D

AIR

VE

NT

S

INS

UF

I

CIE

NT

M

ET

AL

LACK OF FILL

COLD SHUT

SEVERE CHILL

FLOW LINES

B) Some Hot Type Defects: -

HO

T M

AIL

HO

T D

IE

INS

UF

FIC

IEN

T

DIE

RE

LE

AS

E

RO

UG

H C

AV

ITY

PO

OR

GA

TIN

G

PO

OR

PA

RT

DE

SIG

N

SOLDERING

CRACKS

BROKEN PARTS

HEAT MARKS

WAVE OR LAKE

Variation in Mechanical Properties: -

The microstructure of castings is affected by the rate of shear at the gate. High gate

velocities and shear rates produce dense, finely structured casting which exhibit a higher

resistance to crack propagation than castings produce with low gate velocities.

Variation in Dimension: -

Dimensional variation between castings is due to variations in the temperature at which castings are ejected. This is a most important consideration when large accurate

castings are to be produced. It is (almost) impossible to produce castings within a narrow

tolerance range under manual, or semi automatic conditions, and indeed it is difficult to

produce accurate castings even under automatic operating conditions unless control is

915


DIE CASTING TG-2

THEORY


CASTING DIE

9

achieved in die opening temperatures. The microstructure of castings is also affected by the

rate at which heat is removed from the metal.

In thick castings it is important that the die cooling system be designed to provide

rapid chilling of any thick areas if gross dendrite structure and shrinkage voids are to be

avoided.

B4.2.3 DIFFERENT TYPES OF DEFECTES & THEIR CAUSES

& REMEDIES

A.) Blisters: -

Formation: - Blisters are generally the result of the expansion of air or gas under the skin

of the part.

Cause:-

Blister is generally the result of the expansion of air or gas under the skin of the casting Casting removal : - This defect is caused by the presence of pockets of air gas under

the skin of the part when the temperature is too high these expand and form small

bubbles or blisters on the surface of casting.

Air pocket

- Turbulent flow

- Jet filling

- Retention of air

Gas pocket from die spray

- Die spray - Retention of gas

Temperature

Air Pocket: - The process of air Pockets can be used by turbulent Flow (Phase 1) by

Jet Fitting (Phase 2) or by problems in evacuating air from the cavity during Filling.

Turbulent Flow: The presence of air can result from turbulence cause by too high a

phase 1 speed. This turbulence creates pockets of air in the runner system which is

carried in to the cavity.

Jet Filling: The presence of air can also result from Jet filling which is Slower than spray filling leads to flow which impacts on the wall opposite the gate and thus

to reverse filling of the cavity

1015


DIE CASTING TG-2

THEORY


CASTING DIE

10

Retention of Air: The surface defects caused by blisters may be aggravated if in

addition the difficulties in evacuating air from the cavity prevent complete cavity filling.

Gas Pocket from die spray: - The pressure of excess gas from die spraying can

either be because too much was sprayed on to the die surface or because of gas evacuation problems during filling.

Die Spray: The presence of gas under the skin of the part is due to excessive die

spraying. A high plunger speed in phase 2 leads to a short filling time which impedes the evacuation of this gas.

Retention of Gas: The surface defect caused by this blister may be aggravated if

in addition the difficulties in evacuating air from the cavity prevent complete cavity filling.

Temperature: - During ejection the air or gas trapped under the skin of the casting

tends to increase in volume because of the high temperature The skin of the Casting is not strong enough to stop this expansion

Remedies: - They are essentially two means of eliminating this defect Reduce the

ejection temperature ( Palliative Reduce the amount of air or gas beneath the skin of

the part

B.) Flow Marks: -

Formation: - Flow marks can form when part of the liquid metal in the Die cavity

Solidifies before the metal flow marks are therefore linked to premature and localized solidification of the metal.

Cause: - The appearance of flow marks is linked to a problem of localized premature solidification of the metal Premature Solidification Premature

Remedies: - Prevention of flow marks requires the elimination of their cause that is premature Solidification

C.) EJECTION DAMAGE: -

Formation: - For various reasons (see the heading origins of defeat’) the part may be

damaged during ejection

This damage can occurred at different times

1. during die opening

2. during ejection

3. as the casting drops

1115


DIE CASTING TG-2

THEORY


CASTING DIE

11

Cause: - To diagnose the origin of ejection damage it is important to identity the point in

time when it occurred.

Remedies: - To diagnose the cause of ejection damage it is important to identity the point

in time at which it occurred.

D.) DEFECT CHECKLIST: -

Formation: - Under the pressure applied by the cylinder during injection metal can be

squeezed into the parts of the die which are not pressure –tight and get outside the cavity.

Cause: - The formation of flash in the result of a lack of sealing between the parts of the die.

Remedies: - To eliminate flash, the problem of the pressure –tightness of the die must be

dealt with

E.) SOLDERING: -

Formation: - With certain conditions of metal flow speed & temperature , the stock of the liquid metal on the cavity wall can cause an electrochemical reaction Cause : Soldering

is favored by two factors : -

Abnormally high gate velocity presence of hot spots these conditions are complementary

Remedies : - preventing soldering means eliminating electrochemical reactions taking place at the interface both the metal & the cavity surface.

F.) SINKS: -

Formation: - During solidifications, liquid metal contracts this inevitable phenometion is

called shrinkage.

Cause : - The appearance of sinks is essentially linked to the presence in the cavity of hot

spots ( thicker Sections ) Which flavor shrinkage at those points.

Remedies: - Preventing sinks means counteracting the effects of differential shrinkage.

G.) DEFECT CHECKLIST: -

Formation: - Presolidified inclusions can be caused either by the current shot or by a

previous shot.

Cause: - The origin of a cold lap can vary according to whether it is caused by a previous

shot ( previous misrun) or by the same shot ( deep flow marks)

1215


DIE CASTING TG-2

THEORY


CASTING DIE

12

Remedies: - Presolidified inclusions are eliminated by means Similar to those used for

flow marks & misruns) or whether the inclusions are severe flow marks or whether they result from earlier misruns.

H.) DEFECT CHECKLIST: -

Formation: - Misrun is formed when the metal Solidifies before the cavity is completely full. In Other wards there is primitive freezing of the metal.

Cause: - Misrun generally occur as a result of premature freezing of the metal in the cavity,

Remedies: - The remedies will depends on the origin of the defect If a misruns is

associated with defects Such as flow marks or presolidified inclusions, it is likely that premature solidification is the cause . In general the solutions are those used for defects that

a rise from Solidification problems.

I.) DEFECT CHECKLIST: -

Formation: - Scaling generally appears after mechanical treatment of the casting surface.

Cause: - The appearance of scaling is linked to poor adherence between certain Parts of the

skin of the casting Remedies Scales are serve from of flow lines, So the appearance of flow lines should from the out set be eliminated or at least reduce.

a.) Defect: Cold shot, Laps Lack of fill: -

CAUSE SUGGESTED REMEDY

1. Metal temperature too low.

2. Plunger speed too low. 3. Nitrogen pressure in accumulator bottle

too low.

4. Die temperature too low.

5. Inadequate venting.

6. Excessive die lubricant.

7. Worn plunger tip or shot sleeve.

8. Inadequate gating.

9. Excessive gating.

1. Maintain correct metal temperature:

check heat

instrument control. 2. Determine and maintain correct plunger

speed.

3. Check oil level in sight gauge, watch

bottle pressure gauge during shot for successive pressure drop and nitrogen if

necessary.

4. Maintain correct die temperature. 5. Add or reworks vents overflows.

6. Use correct amount and concentration of

Die lubricant.

7. Replace if necessary. 8. Add gates to weak spots if problem is

chronic.

9. Plug gates individually if required.

b.) Defect: Cracks: -

1315


DIE CASTING TG-2

THEORY


CASTING DIE

13


1. Solder

2. Bent cores.

3. Die or metal too hot. 4. Casting or runners not solidified.

5. Contraction stresses.

1. If condition is severe and other methods fail to remove solder, have die room remove.

2. Straighten or replace cores.

3. Maintain correct temperature.

4. Add cooling water to die or add waterlines in trouble areas.

5. If any cracks are occurring in shaft corners,

increase the fillet radii. Reduce hold time.

c.) Defect: Porosity: -


1. Excessive die lubricant. 2. Metal too hot.

3. Entrapped air or gas: vents blocked.

4. Die flashed too much or running too loose.

5. Insufficient metal in shot.

6. Overflow broken off of a part and stuck in

die.

7. Insufficient vents overflow wells. 8. Shrinkage.

1. Reduce lubrication. 2. Maintain correct metal temperature.

3. Do not use excessive die or plunger

lubricant. Clean vents. Vents should be no

deeper than 8.08 mm. To prevent metal from passing into them. Use minimum plunger

speed. Use hydraulic pressure intensifier.

4. Clean die faces or lock die tight.

5. Gradually increase metal in shot. Always maintain a 20~25 mm minimum biscuit

length.

6. Keep overflows cleaned out.

7. Add vents and overflows.

8. Add gate to problem area. Use hydraulic

pressure intensifier. Put shot chiller behind

problem area; Improve feeding in problem area by increasing wall thickness or adding

ribs.

1415


DIE CASTING TG-2

THEORY


CASTING DIE

14

1515


DIE CASTING TG-2

THEORY


CASTING DIE

15

- 1 -46


DIE CASTING TG-2

THEORY

CHAPTER : C1 Feed System

- 1 -

UNIT-3

CHAPTER – C1

FEED SYSTEM

- 2 -46


DIE CASTING TG-2

THEORY


- 2 -

CHAPTER OUTLINE

C1.1 INTRODUCTION.

C1.2 DIE CASTING FEED SYSTEM ELEMENT.

C1.3(a) GATING SYSTEMS.

C1.3(b) FEED SYSTEM FOR HOT CHAMBER CASTING DIES.

C1.3(c) FEED SYSTEM FOR COLD CHAMBER CASTING DIES.

C1.4 CLASSIFICATION OF GATE SYSTEM.

C1.5 PRINCIPLE OF FEED SYSTEM.

C1.6 BALANCING OF FEED SYSTEM.

- 3 -46


DIE CASTING TG-2

THEORY


- 3 -

C1.1 INTRODUCTION

The casting quality is affected by a number of variables one of which is the molten

flow rate. The material flow rate depends on variables such as runner geometry gate area

and the machine variables such as shot sleeve diameter.

It is necessary to provide a flow way in the Die casting dies to connect the machine nozzle in case of hot chamber machine or to connect plunger sleeve in case of cold

chamber machine to the impression. This flow-way is termed as the feed system. Normally

the feed system comprises spure runner and gate.

The channel to convey the molten metal from the machine to the Impression or

cavity is called Feed System.

When lying out the gating system and the cooling channel configuration

of a die: -

1. Choose the position where the metal will Enter the die cavity through the gate

2. Decide on the best direction of metal Entry into the cavity. 3. Decide on the required cavity fill time and Gate speed.

4. Decide on the required die temperature.

5. Calculate the gate and runner dimensions to achieve the set the temperature and

casting 6. Calculate the Cooling channel positions to achieve the set die Temperature and

casting Speed.

7. Deside the provision of vents and overflows, the slow plunger approach length and

speed and check the waking force requirement.

Runner: - The metal Entering the Sprue is directed into one or more passages called

“Runners” Near the Die Cavity, the Cross-Sectional area of the Runner decreases to from a

“gate” designed to the metal into the die cavity

C.1.2 DIE CASTING FEED SYSTEM ELEMENT : -

he Die casting feed System contain following Elements.

1.) Sprue

2.) Sprue Spreader

3.) Shot sleeve and biscuit

- 4 -46


DIE CASTING TG-2

THEORY


- 4 -

4.) Runner

5.) Gate 6.) Shock absorb bears

7.) Overflows

1.) Sprue : - The Spure is tapered hole extending from the mounting surface through the cover die to the dies parting surface The molten metal flows through that hole to the

runners.

2.) Sprue Spreader : - This is used for spreading the material equally in all direction and act as metal saver It is having the same taper as that of sprue bush.

3.) Shot Sleeve and Biscuit: - Shot sleeve is used for cold chamber machines. The shot sleeve extends through the cover half and ends at the parting plane of the die.

There is a circular biscuit cavity in the ejector die at the end of the shot sleeve,

That cavity is the diameter of the shot sleeve I.D. and usually the depth of the runner leading from it The biscuit must Drove draft usually 5 deg allowed for ejection.

4.) Overflow Wells: - A small cavity or impression connected to the casting cavity with the shallow gate is called overflow well. They serve as receptacle for the first metal for

each shot.

5.) Gate: - The gate is the most restrictive orifice in the total flow in a die casting die Most of the casting defects such as bad surface, improper filling, flow martis cold Shuts,

Soldering , thermal imbalance etc can be attributed to improper gating.

6.) Shock Absorbers : - They control the very high transient velocities which may

occur at the small ends of the runners Metal flow into and around the shock absorber

cavity traps air which is compressed causing metal to decelerate.

C1 .3 (a) GATING SYSTEMS

Permanent mold castings can be gated from the top, side or bottom and single or

multiple gates can be used.

Top Gating: - In this gating arrangement. the Spure and the riser are usually the same Thin sections are placed farthest from the gate, so that directional solidification is

toward the gate. After pouring, the gate functions as a riser. The metal in the riser solidifies

last thus ensuring sound metal throughout the casting.

Side gating is frequently used particularly for aluminum castings. In this gating

system, the riser is at top of the casting. The gate extends up the side of the casting to nearly

75% of its height, which ensure that the metal at the top of the casting and in the riser is hotter than the first metal to enter the mold. Thin sections should be placed remote from the

- 5 -46


DIE CASTING TG-2

THEORY


- 5 -

gate and riser. The direction of solidification is from the mold cavity toward the gate and

riser. The direction of solidification is from the mold cavity toward the gate and riser, so that porosity caused by shrink a minimized.

Atom gating is always used for pouring gray iron castings and may be used for the

other metals. A typical gating system as shown in Fig 49 consists of a pouring basin, a chocked Spure. A spure well. a horizontal bottom runner a runner extension , and vertical

gates For the mold shown in Fig 49. The two bottom die cavities lead into breakers and

intermediate gates that lead the two breaker and intermediate gates that feed the two top die

cavities. These lead into risers Rise ring is a minor consideration in gating system for pouring gray iron, because a hypereutectic cast iron does not shrink during solidification.

Gating systems for permanent molds are less flexible than those for sand molds. and

are nearly always located in parting planes Gating must supply metal fast enough to

fill all sections of the casting. With a minimum of turbulence. Gating systems that permit minimum turbulence are especially important especially important in casting of aluminum

and magnesium alloy are poured in the vertical position or tilted from vertical .By this

method, air is displaced and vented off at the mold

Besides supplying liquid metal to compensate for casting shrinkage, risers reduce

the velocity of the metal before it enters the cavity. and help sustain the mold temperature

The number of points at which metal is admitted to the mold cavity depends on section,

thickness and the distance the metal must flow. Excessive flow through an insufficient number of inlets may result in hot spot and consequent shrinkage. Sprues are usually

restricted in area to choke of f and prevent dross and air from entering the cavity

Because gating systems are often easier to enlarge than to reduce , and because oversize

gating can slow up the cycle, it is common practice to start with small gates, and to enlarge them if necessary.

Misrums: - There are several possible causes for musruns, including entrapped air. Insufficient mold temperature or insufficient pouring temperature often. misruns are caused

by a combination of two or more such conditions Adequate venting is the simplest way

to eluminate entrapped an Increasing the mold temperature or the pouring

In redesigned mold for the new gating provision was made for application of

external heat to prevent misruns In original mold , misruns area was impractical to heat

and an insulating mold coating was used to prevent misruns.

Temperature or both may eliminate misruns. But is likely to cause other defects

such as porosity One common approach is to increase the mold temperature at the critical

locations by applying heat from an external source ( antic hill) or by using an insulating

type of mold to prevent the liquid metal from chilling in a specific area. Cause and the corrective steps taken to eliminate misruns the apply in some cases are illustrated in Fig

50.alyleuous dies closely resemble all gravity cast- dies in cavity layout and runner

/Visen design the truid and prouen ae gravity die designs Typically wide narrow gates

as opposed to narrow deep gates ( thene can designed for yellow bran 60% Cn 40%

- 6 -46


DIE CASTING TG-2

THEORY


- 6 -

Zn ) Enter cavities as low as is practically possible Runner should be approx twice the

height of the cavity.

C1.3 (b) BASIC CONCEPTS OF METAL FLOW IN THE DIE –

CASTING PROCESS

Both the size and configuration of the metal feed system influence the flow of metal

into and through the cavity. One simple method of controlling flow in the feed system is to

design the system as shown in Figure 4.1. In this figure, the sections labeled “cavity” could

be either separate cavities or different sections of a large single cavity. In a system designed in this manner, the pressure will be high and uniform throughout the complete runner

system, and a large pressure drop will exist across the relatively small ingates. This

system can be considered as a constant –pressure system. The metal flow through any part

of the ingate is easily calculated for this system by means of simple orifice equations. Orifice equation apply in a typical metal die-casting operation because Reynolds number

are very high, or stated another way, flow velocities are high enough so that viscous

effects are negligible and almost all the pressure drops in the system occur as a result of accelerating the zinc to high kinetic energy levels at the orifices ( gates ) and then

dissipating those high kinetic energies in turbulence as the zinc leaves the “ ingate” –

the orifice in the system..

With a system designed as shown in Figure 4.1 the metal flow rate through any

ingate or through any portion of a long ingate will be proportional to the cross-sectional

area of that gate and to the square root of the runner pressures . The fact that the feed

system must be much larger than metal the gates has several inherent disadvantages, such as:

(1) The use of large runners increases the volume of air entrapped in the system and the

probability of blister defects. (2) The use of large runners increases the volume of air entrapped in the system and the

probability of blister defects.

(3) The maximum runner size that can be used practically is limited by availability of

standard components such as nozzles and sprees. (4) The use of small gates area increases the time required to fill the die cavity and the

probability of producing cold lap defects.

(5) The use of large runners increases remelting costs.

(6) A high pressure drop across the gate will produce high gate velocities which may contributes to soldering or die washing problems.

When the metal is following at a high velocity, sudden enlargements or sharp

turns in the runner will cause flow separations, and the resulting turbulence in the following stream of metal causes the kinetic energy in the system to be converted to

thermal energy . Turbulence increases the flow resistance in that particular flow path and

decreases the flow rate. To minimize this reduction in flow path and decreases the flow

rate. To minimize this reduction in flow rate, it is important that the runners be smooth that

- 7 -46


DIE CASTING TG-2

THEORY


- 7 -

turns have large radii, and that the size of the runners not to change suddenly (a constant

cross- sectional area should be maintained). It should be noted that by changing the flow direction with a large- radius turn, as shown in a runner with constant area, the absolute

value of the velocity does not change , V1 =V2 and no kinetic energy is lost. If a sharp

90-degree turn were used, the absolute velocity of much of the metal in the stream

would go to zero at the turn, and increased energy, supplied by the shot system, would be required to reaccelerate the metal in the y direction.

In order to achieve the ideal goal of using large radii for all turns, a runner-gate

system might be constructed as is shown in Figure 4.4. In this version the main runner is split into several secondary runners, and each of these secondary runners feeds a fan

get which distributes the metal into the cavity. The concept shown in Figure 4.4

Represents a theoretically ideal method of controlling flow distribution into the cavity. Unfortunately however, it also represents a relatively complex and expensive

distribution system especially for castings with a long gates. A compromise version of

this design which offers lower cost and an effective control of flow is shown in Figure

4.5. Although that design does require a change in flow direction without the assistance of turning vanes at the points where the metal leaves the secondary runners and centre s

the cavity, experience to date has shown that satisfactory control of flow is maintained .

- 8 -46


DIE CASTING TG-2

THEORY


- 8 -

Because of this satisfactory field experience, the general gating concept shown in Figure

4.5 is recommended for large castings where secondary (side) runners are used.

Poorly designed runner system found in many die casting dies: - Because the side runners are not correctly proportioned large quantities of

zinc flow to the ends of the side runners and enter the cavity at those points. Sides

flows then occur in the cavity and where these side flows meet, large quantities of gas are trapped and Blisters form (as shown by the circled areas in Figure 4.6). The regions of the

cavity where zinc enters at a high flow rate fill rapidly, and normally, only negligible

cold –lap ( cold –shut) defects occur in such areas. However, it would be excepted that

the regions between the parts of the casting that filled rapidly would have severe cold-lap defects ( marked with wavy lined ), because those areas filled more slowly In this

example , the overflows are small so there is little chance of moving the cold metal

and gases out of the cavity and into the overflows.

An ideal runner –and –gating configuration is shown in Figure 4.7. In this design the

runners and the Spure have the same cross –sectional area, so that the zinc velocity is

the same in the runners as it is in the sprue. Thus there is little tendency for stream of

metal to shoot ahead of the main flow and trap pockets of gas. Most of the gas is carried out of the runner ahead of the following stream of zinc or in the first zinc that flows through the

system and into the overflows. Smooth turns are provided to minimize turbulence metal

enters the centre of the cavity through fan gates which distribute metal evenly in those

areas, and turning vanes are used to turn the flow of zinc smoothly into the side runners parallel to the gates.

- 9 -46


DIE CASTING TG-2

THEORY


- 9 -

Fig 4.7

- 10 -46


DIE CASTING TG-2

THEORY


- 10 -

The side runners are proportioned to maintain a constant –velocity flow of

molten zinc in the runners are proportioned to maintain a constant –velocity flow of molten zinc in the runner and to continually feed metal into the ingates. The ingates are

proportioned to provide a uniform flow of metal cross the cavity and to cause metal to

reach all the overflow gates simultaneously. In the ideal deign, large overflow are used,

so that most of the gas can be flushed out of the runners and the cavity and into the overflows. These overflows, of course, also help prevent cold laps. Thus, most of the cold-

lap and blister defects on the casting have been forced from the casting cavity and out into

the overflows.

Selection of the Runner and Gating System: -

The first step required to achieve uniform flow of molten metal from the die inlet

to the overflows is to select a complete metal –feed –system layout which includes

runners, casting cavity, and overflows. Usually there will be serval possible alternative ways to deliver molten metal from the die inlet ( sprue area) to the casting cavity . One

decision that must be made is wheather the casting will be made in a single-cavity die or a

multicavity die. That question often will be decided by the shot capacity of the machine and

the size and configuration of the casting.

General rules that should be considered in selecting the metal feed system

are as follows: -

(1) This runner lengths should be minimized.

(2) Runners and ingates should be located to provide uniform flow across the cavity so

that all sections of the cavity are filled and the metal reaches all overflows simultaneously.

(3) Runners and ingates should be located on the side of the casting where the

surface finish is critical, and overflow should be located on the side of the casting

where a lower quality surface finish might be acceptable. (4) Flow distance across the casting cavity should be as short as is practical.

(5) Avoid designs where zinc must flow across large cutouts (openings) in the castings.

Where zinc must flow across cutouts, select a system which provides for free

flow across the opening by flashing ( if the openings are small) or for runners across the

opening. Gates should be provided at the inlets and outlets of the openings.

(1) Locating the spruc and runner system inside the opening, and locating the overflow

around the outside perimeter as shown in Figure 4.8a

(2) Locating the over flows in the centre of the casting, and locating the runners around

the outside of the casting as shown in Fig 4.8b (3) Feeding from one side of the casting placing runner across the opening the

runners around the outside of the casting as shown in Figure 4.8c.

Any of these configurations probably could be used to successfully produce this

casting size and configuration, but usually the preferred method of gating a casting of

- 11 -46


DIE CASTING TG-2

THEORY


- 11 -

this type is to use the centre –gating technique shown in Figure 4.8a This configuration

has the advantages that both runner lengths and flow distance across the mold cavity are minimized while its only disadvantage are that the velocity of the zinc flow tends to

decrease as it progresses across the cavity, and a multicavity die cannot be used. It has

been found in practice that these are not usually significant problems. In some cases, the

reduction in the volume of metal which must be delivered to the die and cooled makes it possible to achieve higher production rates with a centre –gated single –cavity die than

can be achieved with an outside gated two-cavity die.

- 12 -46


DIE CASTING TG-2

THEORY


- 12 -

If there are two or more large openings in a casting, the sprue usually should be

located in one of the openings to carry the metal across the opening with a minimum heat

loss, however, the other configurations ( peripheral or side gating) shown in Figure 4.8

also could be used for this part.

- 13 -46


DIE CASTING TG-2

THEORY


- 13 -

Calculation of Runner Dimensions: - After the gate areas have been calculated, the runners are designed to maintain a

constant velocity of zinc in all parts of the runner system. The general rule for calculating

the cross-sectional area of the runner at any point is that the runner area should be

equal to the total gate area of the portion of the die being fed by that runner. The sketch in Figure 4.14 shows the equations used in making the runner – area calculation for a

runner which is parallel to a gate and is continuously feeding metal through that gate.

Figure 4.14 also shows the locations of the areas used in the calculations.

After the runner areas have been calculated, runner cross-section configuration are

selected that : ( 1) will minimize heat loss from the runners ( 2) will cool rapidly enough so that high production rates can be obtained ( 3) can be easily machined, ( 4) are easily

designed and ( 5) have enough draft so that the casting can be easily ejected. It is usually

desirable to make the cross section of the long section of the runners round in order to

minimize heat losses. Where short main runners can be used , a rectangular runner section is usually desirable in order to maximize the heat transfer from the runner and maximize

production rate.

Cross section through different segments of the runner –and- gating

system now used with the TV-Bezel Dies: -

Metal Pressure: -

The pressure exerted on the metal is calculated by the following formula:

(P1 * A1) – (P2 * A2)

Pm = -------------------------------

AP

- 14 -46


DIE CASTING TG-2

THEORY


- 14 -

Where (see also Figure 2.4):

Pm = Pressure on Metal P1 = Inlet pressure

From the above formula , it can be seen that to calculate the pressure on the molten

metal, the area of both sides of the hydraulic piston and the area of the injection piston

must be know.

Fig 2.5 shows a typical pressure variation during injection The significant point to note about this trace is that there are two pressure peaks; one at the change over

point from the slow fast stage and the second is at the end of fill. These peaks represent

the pressure varation in the hydraulic system. From the designer‟s viewpoint the magnitude

of the largest pressure peak is important in that it may cause die flashing. Thus , lock force

- 15 -46


DIE CASTING TG-2

THEORY


- 15 -

calculation of the tool must take into account the magnitude of the pressure pulse at the

end of cavity fill injection . It is useful to note that the magnitude of the pressure peak depends on the mass and hydraulic oil flow speed the pipe feeding the injection

cylinder.

Plunger velocity is measured by using a suitable velocity Transducer. The velocity trace shows acceleration and deceleration in the die as it is being filled.

Having measured the plunger velocity, the corresponding molten metal flow rate can be

calculated by using the following formu1

The casting quality is affected by a number of variables one of which is the molten

metal flow rate. Thus in designing the metal flow system it is important to able to

evaluate the effect of such variables as runner geometry , gate area, and the machine

variables , such as shot sleeve diameter, on the molten metal flow rate. It is now common practice to use a PQ squared diagram ( Fig. 3.1 ) to show the inter-relationship

between a metal flow system and the injection system of the die casting machine. By

using this diagram , the molten metal flow rate through the runner and gate can be

predicted for a given set of machine conditions.

The concept of the PQ squared diagram derives from the fact that the hydraulic

pressure changes, in the both molten metal and oil flow system , are directly

proportional to the square of velocity of the injection plunger. Therefore, a plot of metal pressure against the square of speed will approximate to a straight line. This being the case

it is more convenient to plot metal pressure (MPa) against the square of volume flow rate

(liters/sec). The advantages of this approach are:-

- 16 -46


DIE CASTING TG-2

THEORY


- 16 -

1) The data is in a form that is directly usable for predications of cavity flow time or

velocity at any part of the feed system... 2) The effect of changes in the shot sleeve dimensions upon flow can be shown.

3) Die character which are best stated as pressure for a given flow rate can be

drawn directly onto the same diagram.

4) Machine power at any setting of the speed control valve or accumulator pressure can be shown.

5) Comparison of the metal pumping capabilities of machines is simplified

6) The useful working range of the machine can be defined (see Figure 3.1).

Metal pressure „p‟ is plotted on a linear scale (Fig. 3.2). Metal flow rate „Q‟ is

plotted on a square scale. This enables all plots to be drawn as a straight line not

necessarily passing through zero.

The point Q on line - Q represents the dry shot flow rate - that is the flow rate

which would occur if there were no resistance to metal flow- ie. in the cold chamber that speed which would be achieved if there were no metal in the sleeve.

The point P on line -P represents the pressure exerted on the metal at the given gas

pressure in the accumulator. This pressure is the maximum static metal pressure exerted on the metal at the end of the injection.

By joining the points Q and P in the Figure 3.2, the machine full injection characteristic has

been described.

This characteristic shown how the available metal pressure decreases as the metal flow rate increases due to the increase of the pressure losses in the valves and

pipes. The slope of the line P-Q is proportional to the resistance in the hydraulic system of

the die casting machine and hence line P-Q represent the machine characteristic.

The Gooseneck Characteristic: - In Figure 3.2 the point „ Q‟ represent the dry shot flow rate of the machine .In

this case of the cold chamber machine, this is the maximum flow rate that can be

achieved through the runner system. However, in the hot chamber machine , the resistance to metal flow due to the channel in the gooseneck and nozzle causes the plunger to

- 17 -46


DIE CASTING TG-2

THEORY


- 17 -

decelerate before the molten front reaches the exit of the nozzle. The deceleration is

dependent upon the design of the channel of the gooseneck , other words, on the pressure losses. As the pressure losses in the gooseneck and nozzle could be significant

, the metal flow rate out of the nozzle should be obtained . In practice , this is done by

measuring the plunger speed and the hydraulic pressure at the instant the metal front

reaches the exit of the nozzle . The corresponding metal pressure and the flow rate can be calculated and plotted on the PQ squared diagram. Figure 3.3 show a plot. The

experimental points on this plot represent the pressure and flow of metal at the instant

front reached the exit of the nozzle at various shot valve settings. The best fit line passing

through these points and the origin (zero point) point represent the Gooseneck /Nozzle characteristic.

Fig 3.3

Because the die casting machine has a limited source of energy , it is

necessary to utilize this energy in a manner that minimizes pressure losses ( in the metal

flow system. The main factors that influence pressure losses ( hence energy losses ) in

the metal flow system are :

1) Surface Roughness: -

To appreciate the effect of surface roughness on the pressure loss, the pressure

loss due to the flow of molten zinc Alloy 3 in a straight runner having cross-sectional

are 100 square millimeter and a length of 1000 millimeter is calculated. The result is

shown in figure 4.1.

- 18 -46


DIE CASTING TG-2

THEORY


- 18 -

This shows that the rough surface increases the pressure loss several times.

A step change in the runner cross-section gives rise to a pressure drop across the step. This

is because , as shown in Figure 4.2, eddies are created at the corners and immediately

downstream from the step which reduce the effective cross-sectional area ( commonly

called vena contracta).

Step Changes In Runner Cross-Sectional Areas: -

- 19 -46


DIE CASTING TG-2

THEORY


- 19 -

A step change in the runner cross-section gives rise to pressure drop across the step.

This is because , as show in Figure 4.2 eddies are created at the corners and immediately downstream from the step which reduce the effective cross sectional area (

commonly called vena contracta).

Change In Flow Direction: - It is inevitable that changes in direction will occur in the metal flow system. For

example a 90 degrees turn is required between the shot sleeve and the runner. In Figure 4.3

the pressure loss is plotted against the flow rate of molten Aluminium through a sharp bend and a radiused bend. It demonstrates that the bend geometry has significant influence

on the pressure loss.

Protrusion Into The Flow Stream: - Any protrusion or depression in the runner system should be avoided as it

becomes the source of eddies and hence pressure loss in the runner . Figures 4.4 and 4.5

show the flow problems caused by poorly fitted ejector pins.

- 20 -46


DIE CASTING TG-2

THEORY


- 20 -

- 21 -46


DIE CASTING TG-2

THEORY


- 21 -

Air Entrainment: - Poor runner geometry may also result in air being trapped with in the flowing

metal which may cause porosity in the casting Figures 4.6 to 4.8 demonstrate the effect

or runner bend geometry on the metal front.

Sharp corners have a high resistance to flow and should be avoided in both the runner and the cavity. Figure 4.6 demonstrates the impact of the metal and subsequent shattering the

metal front.

- 22 -46


DIE CASTING TG-2

THEORY


- 22 -

Bends with small radius and constant cross-sectional area do not give constant velocity as shown in Figure 4.7. Because of the effect of centrifugal action the metal

on the outer wall will accelerate at a greater rate than the metal on the outer wall will

accelerate at a greater rate than the metal on the inner wall. This produces a separation

effect on the inner wall and consequent production of eddies and cavitation bubbles. At some point downstream of the bend the metal front, which has accelerated but has no

pressure to sustain the acceleration, now shatter and trap air. It may also cause partial gate

block due to premature solidification of the molten metal front droplets.

The above problems can be mimised by having a larger radius the band and/or making the bend inlet area much larger than i outlet area as shown in Figure 4.8.

The flow system comprises a series of passages of varying lengths and cross sections

which carry molten metal at varying velocities from the injection plunger to the gate. In

Addition to length and cross sectional changes in direction occur. At each change energy is consumed in overcoming the resistance to flow. It is most important therefore to

ensure that the total flow. It is most important therefore to ensure that the total flow It is

most important therefore to ensure that the total flow system is designed and

manufactured to minimize such losses. In general the cross sectional area of the flow system should converge from plunger to gate, sharp corners should be avoided and the

surfaces should be smooth and well finished.

The hot chamber machine differs from the cold chamber machine in that a

gooseneck, nozzle and Spure are required. The gooseneck and nozzle and Spure are required the gooseneck and nozzle and Spure are required. The gooseneck and nozzle

design is important as it affects the flow performance of the whole metal flow system.

Gooseneck: - Because the hot chamber process has an immersed gooseneck it can be seen (Fig.

5.1) that the flow path is quite long with a number of bends, steps and joints. A poorly

designed geometry or one which creates unnecessarily high velocities ( bores too small)

will produce an inefficient system which may create the conditions for hydraulic cavitations to occur with consequent damage to the die.

- 23 -46


DIE CASTING TG-2

THEORY


- 23 -

V The nozzle is basically a heated pipe connecting the gooseneck to the sprue. In

general the bore of the nozzle should be only slightly smaller than the bore of the

gooseneck so that molten metal velocity is kept down below 30 meters/second ( Fig. 5.2).

Low gooseneck and nozzle velocities are desirable to ensure that the molten front is kept intact up to the sprue to avoid air entrainment.

The Sprue: -

- 24 -46


DIE CASTING TG-2

THEORY


- 24 -

Figure 5.3 shows a commonly used sprue bush configuration which is poorly

designed and does not benefit from modern flow technology . Whilst it is very

economical to make, it is grossly inefficient and results in molten alloy and air being

thoroughly mixed. The accompanying area chart ( Fig. 5.4 ) Shows graphically that the

cause is the massive increases in cross sectional area encountered in the flow path by the molten alloy and until there is a sudden reduction at the runner entry. One other

problem encountered with this design is that the included angle of the sprue is too small and

this limits the provision of cooling water within the male form.

Attempts to overcome this deficiency are made by the provision of an annular

channel in the sprue bush. The surface area is so large that extra heat is required by the

nozzle to prevent freezing. As the nozzle cannot conduct

Sufficient heat to its extremities the centre becomes overheated with consequent reduction in its effective life.

The effect of sprue design on casting quality has in the past been largely

unrecognized. Because it was designed as a low cost standardized replacement part, little consideration was given to the fluid dynamics and heat flow aspects. In the

METLFLOW design concepts, the sprue serves two basic functions: -

(1) To provide a smooth flow path between the nozzle and the main runner (s).

- 25 -46


DIE CASTING TG-2

THEORY


- 25 -

(2) To provide a means of rapidly solidifying molten metal at the entrance to the die

after the cavity has filled and flow has stopped . This requires a large temperature difference between sprue and nozzle.

Runner type Sprues ( Fig. 5.5 ) have been developed to achieve a smooth flow

path with uniform or reducing cross sections and smooth transition from nozzle to sprue

and runner . The use of such a shape also enables the development of a more effective cooling system , particularly where large runners are required, reduces the need for

sprue bush cooling.Examination of many runner designs seems to indicate that in the

past greater emphasis was placed upon easing the toolmaker‟s difficulties than was

given to the flow behavior of the molten metal either in the runner or in the cavity . This resulted in the designers main concern being the determination of the gate area, with

very little thought being the determination of the gate area, with very little thought being

given to the shape and size of the runner leading to the gate.. The METLFLOW design

concept is based on the fact that the flowing metal will not change its flow direction unless it is channel1ed to do so. The designer must bear in mind that it is the design of the runner

that directs the metal to the appropriate die cavities. Thus the runner system must

incorporate features that are designed specifically to turn the flow direction and /or

distribute the flow to predetermined parts of the cavity or cavities . An understanding of

- 26 -46


DIE CASTING TG-2

THEORY


- 26 -

runner fill behavior is important in appreciating the significance of the design of the

runner geometry on its own filling behavior and ultimately that of the cavity fill pattern. The following example highlight some of the problem related to poorly designed runner

geometry.

Figure 6.1 shows a once popular runner design for a multi-cavity die. If one takes a

series of cross sections in the direction of flow and expresses them in a progressive area chart the result shows the large increase in runner cross-sectional area downstream (

Fig. 6.2). This results in air entrainment in the runner and consequent inability to predict

the flow direction and runner fill characteristic.

Examination of this situation in some detail shows that the entry to the main

runner from the shot sleeve has the smallest cross-sectional area therefore , after this point , the metal front will break up giving rise to an unpredictable flow pattern. The

traditional method of attempting to achieve improvements by increasing the area of the

gate will have no effect whatsoever until the cavity is nearly full when all the problems

created by the poor design are already present in the casting. Attempts to overcome the defects in the casting, using overflows of sufficient size to prolong the overall flow

- 27 -46


DIE CASTING TG-2

THEORY


- 27 -

time in the hope that the air trapped in the runner and casting will eventually be flushed

out into the overflows, are seldom successful.

Figure 6.3 illustrates a runner system which has been designed to fill three

identical cavities through gates of equal size. The expectation is that the fill time for

each cavity would be identical and would also commence at the same time. However this is not the case. Because of the high kinetic energy in the flowing metal the flow

direction will not change until there is more physical constraint , such as a bend , or

there is a change in the resistance to flow such as reduction in channel size . In the

illustration the side runners will not fill until the molten metal reaches the gate of the centre runner when the resistance to flow will increase.

The most significant effect caused by a poorly designed junction is that the

flow system produces unbalanced cavity filling even though the gates may be the same size.Figure 6.4, 6.5 and 6.6 illustrate one of the most common problems associated

with the use of a T-junction runner feeding a long gate. With a T-junction runner most

die caster had an expectation that metal would flow first through the centre as shown

in figure 6.4 This however is not the case and the real flow pattern is quite different.

- 28 -46


DIE CASTING TG-2

THEORY


- 28 -

When metal traveling in direction of the arrow V hits the gate ( Fig 6.5) it meets

a resistance to flow but, since there is a greater area and less resistance to each side of

the point of contact, molten metal will flow into these areas. A little metal will pass

through the gate due to kinetic energy but the pressure will not be maintained and flow through the gate will cease until the runner is full. Since this is happening at guide high

velocity the separation of the metal at the gate into two streams V1- V2 traveling in

opposite directions will create zero velocity in the centre of the gate during transient

runner fill. This will produce a cavity fill pattern as shown in Figure 6.6 This shows that the flow from the ends of the runners is a potential source of casting defects. There is a

likelihood of air being trapped in the area above the centre of the T-junction.

- 29 -46


DIE CASTING TG-2

THEORY


- 29 -

Metal Flow System Features: -

The foregoing discussion on runner fill behavior indicates that, to overcome

filling problem, the metal flow system should be designed with the following basic

features in mind :

(a) The cross-sectional area of the runner should be greater than the gate area. (b) To facilitate a change in flow direction, a radiuses bend should be used.

(c) To direct the molten metal into separate runner branches, a y-junction runner should

be used.

It is noted that the definition of „ metal flow system‟, here , includes the design of the sprue runners, gooseneck and nozzle.

Implementation: - The MELTFLOW program implements these basic features. Specifically, the

program has the following standardized runner elements (Fig. 6.7):

- Taper runner - Y-junction

- Radiuses bend

- Joint Runner

The runner standardization is analogous to plumbing systems where standard pipe

fittings, such as T pieces and valves, are used.

- 30 -46


DIE CASTING TG-2

THEORY


- 30 -

C1.3 (C) FEED SYSTEM FOR COLD CHAMBER CAVITY DIES

METAL PRESSURE

The pressure exerted on the metal is calculated by the follow formula: -

(P1 * A1) - (P2*A2)

Pm = ------------------------ AP

Where (see also Figure 2.4): -

Pm = Pressure on Metal P1 = Inlet pressure

P2 = Exhaust pressure

A1 = Hydraulic piston area at inlet

A2 = Hydraulic piston (less shaft) area Ap = Area of injection plunger.

From the above formula, it can be seen that to calculate the pressure on the

molten metal, the area of both sides of the hydraulic piston and the area of the injection piston must be know.

Figure 2.5 shows a typical pressure variation during injection. The significant point

to note about this trace is that there are two pressure peaks; one at the change over

- 31 -46


DIE CASTING TG-2

THEORY


- 31 -

point from the slow fast stage and the second is at the end of fill. These peaks represent

the pressure variation in the hydraulic system. From the designer‟s viewpoint the magnitude of the largest pressure peak is important in that it may cause die flashing .

Thus , lock force calculation of the tool must take into account the magnitude of the

pressure pulse at the end of cavity fill injection. It is useful to note that the magnitude of

the pressure peak depends on the mass and hydraulic oil flow speed the pipe feeding the injection cylinder.

Plunger velocity is measured by using a suitable velocity Transducer. The velocity

trace shows acceleration and deceleration corresponding to the change in the flow resistance in the die as it is being filled..

Having measured the plunger velocity, the corresponding molten metal flow rate

can be calculated by using the following forms

Q = Vp * AP

Where

Q = Metal flow rate Vp = plunger velocity

Ap = plunger cross-sectional area

In the case of the cold chamber machine, it is useful to measure the speed of the plunger operating without the metal in the sleeve. This speed is commonly called the „dry

shot‟ speed of machine. Such measurements must be taken at various settings of the plunger

speed control valve. For a hot chamber machine, the plunger velocity at the instant the

metal front reaches the ex of the nozzle must be determine from the plunger velocity The metal front positions at various times during injection can be estimated from the

plunger is proportional to the distance traveled by the plunger.

Shot Sleeve and Biscuit: -

Aluminum magnesium and copper alloys are usually cast in cold chamber machines.

Dies for casting those alloys must accommodate the cold chamber ( i.e. shot ) sleeve as it is usually called). The shot sleeve extends through the cover half, as shown in Fig 9-2

and ends at the parting plane of the die.

- 32 -46


DIE CASTING TG-2

THEORY


- 32 -

There is a circular biscuit cavity in the ejector die at the end of the shot sleeve. That cavity is the

diameter of the sleeve I.D. and usually the depth of the runner (s) leading from it. The

biscuit must have draft, usually 5 deg., all around for ejection. The runners should lead off the top third of the biscuit cavity so that metal ladled into the shot sleeve will not run

into the runners by gravity and begin to solidify prematurely.

The biscuit cavity is usually made in a separate piece of steel , sometimes called the shot plate or butt plate, which is inserted into the ejector die block The shot plate

has drilled waterlines to accommodate the high heat transfer rates in the biscuit area.

By using an insert , the sharp thermal gradients are isolated in the biscuit area without

affecting adjacent regions of the die . The insert can also be replaced or repaired easily.

It is sometimes desirable to place the biscuit forward of the parting surface as

shown in fig 9.3 This so-called raised biscuit construction has two major advantages. It

allows the designer wide freedom of cover die thickness while still accommodating standard ( or existing ) shot sleeves, and it shortens the shot sleeves , and it shortens the

shot sleeve. Except for extremely large and thick castings, it is usually an advantage to

have the shot sleeve as short as possible so that it will be more completely filled by the

molten ladled. The shot sleeve length and diameter relationships are developed more thoroughly in the SDCE courses and textbooks on Gating and process Control.

- 33 -46


DIE CASTING TG-2

THEORY


- 33 -

The raised biscuit shown in Fig 9-3 has some disadvantage but these will rarely out

weight the advantages. The first of these is that runner must be longer and flow down

the steep side wall of the raised shot plate . That is a minor problem if at all. Second , if

the plate is not a piece inserted into the die block, a large amount of the die block face must be machined away to leave the necessary protrusion The raised biscuit should

probably be inserted into the die block unless the large amount of machining is required

for other reasons.

A third disadvantages is that the vertical surfaces around the raised biscuit are

subject to die shift. Unless the raised portion is on the centerlines established by the die‟s

alignment system, it must have clearance for die shift. That clearance will allow a cone of

flash to develop As in other flash areas it is best to allow sufficient clearance for a solid flash to from on very shot so it will eject completely with each casting.

When considering the cold chamber System which has a much simplified

geometry the transition from sleeve to runner is the only cause for concern the Tradition method was to into the runner in moving Half and create a pocket of diameter

larger them that of the piston.

This produced sharp corners at the points indicated which are detrimental to good flow conditions. Examination of the main runners often shows a condition where there is

an apparent vena contract thus reducing the effective area in the runner with consequent

increase of pressure losses and air entrainment due to the break – up of the metal front.

A better geometry created by a modified sprue design reduces the potential for this problem ( Fig . 5.7 ). Note that the inlet area to sprue runner often shows a condition

where there is an apparent with consequent increase of pressure losses and air

entrainment due to the break –up of the metal front..

A better geometry created by a modified sprue design reduces the potential for

this problem ( Fig . 5.7). Note that the inlet area to the sprue runner is made much larger

than its outlet. This ensures that the metal front remains in contact with the die wall.

- 34 -46


DIE CASTING TG-2

THEORY


- 34 -

C1.4 CLASSIFICATION OF GATE SYSTEM

The transition between the runner and the casting is called the gate. The gate. The

gate acts as an entrance into the cavity through which the molten metal passes. It also

facilitates the „Trimming process which separates the casting from the runner. The flow

pattern in the cavity is affected by the runner geometry and the position of the gate. It is necessary to describe the types and gates that are commonly used in

practice so that their relative benefits from the cavity fill view point can be discussed.

There are currently three main gating approaches used in pressure die-casting: i. The Chisel gate

ii. The Fan gate

iii. The „Taper Runner‟ gated along its side.

a) CHISEL GATE: -

The simplest system is the chisel shaped gate (Figure 7.1). The chisel gate is very

easy to design and very easy for the toolmaker to cut. However , the flow from it is so

directional that the resulting fill pattern becomes a concentrated stream not much

larger than the gate itself . For the majority of casting , such fill patterns may lead to

porosity problems in the castings, such fill patterns may lead to porosity problems in the casting due to vortices created in the cavity which trap air as shown in Figure 7.2.

However the chisel gate is useful for small castings where vortices can not occur. Figure

7.2 shows metal flowing around the edges of the impression in accordance with the

now generally accepted Former theory of cavity fill.

Former conjectured that at high velocity metal would pass through the test

impression striking the wall on the far side flow back around the outside of the cavity

to the gate and would thus rejoin the metal flowing through the gate . In the final stage, he conjectured two pockets of air would be formed - one each side of the main stream

from the gate. Examination of casting indicates that this is so.

b) FAN GATE: -

- 35 -46


DIE CASTING TG-2

THEORY


- 35 -

Development of the fan gate (Fig. 7.3) in an effort to improve the flow in the cavity

is a satisfactory solution for small castings. However, for large castings requiring long

gates, the resulting surface area of the gate may become too large which m cause

premature solidification at the gate due to excessive head loss. Hence, the use of a fan

gate is not recommended for feed a large casting.

c) Gated Taper Runner: -

A gated taper (Fig. 7.4) runner is a runner that has a progressively reducing across

sectional area in the direction of flow and is gated along one of its sides. The rate of

reduction is such that the area of the runner at any point along its length is the same or larger than the area of the gate downstream

The development of a mathematical model to analyze the flow behavior of molten

metal in taper runners gated along their length has given a better understanding of how the gate and runner should be designed to achieve the most desirable cavity fill

pattern. It has been found that the ratio of the runner inlet to the gate area has been found

that the ratio of the runner inlet to the gate area has the most significant influence on

the flow direction out of the taper runner . For example when the runner cross-sectional area is the same as the gate area, the molten metal will flow through the gate

in a direction at approximately 45 degrees to the normal of the gate . By changing the

size of the runner cross-sectional area with respect to the gate area, flow direction (

called flow angle) is also changed.

This model has now been incorporated within the METLFLOW program to assist

the design of gated taper runners. It should be noted that the motion of „taper runner‟

includes both straight and curved runners that follow the casting periphery. In METLFLOW „ the „Radiuses Bend‟ runner element can function as a curved taper runner

provided its inlet area is made larger than its outlet area and it can be gated on either

the inner or outer side of bend.

The benefits of using a taper runner gating system stem from the fact the gate

speed and flow angle at any point of the gate is the resultant of the metal velocity

components along the runner and 90 degree to it . In Figure 7.4 the following terms are

defined:

(1) Normal Gate Velocity is the velocity in the direction normal to the gate – line OA.

(2) Runner Velocity is the velocity along the main axis of the taper runner (arrow C).

(3) True Gate Velocity is the resultant velocity between the normal gate velocity and runner velocity – line ( OB).

(4) The Flow Angle ( Af) is the angle between the true gate velocity and the

velocity normal to the gate.

- 36 -46


DIE CASTING TG-2

THEORY


- 36 -

Figure 7.5 shows a plot of the magnitude of the flow angle an the true gate

velocity relative to the runner velocity against the ratio of the taper runner inlet area ( Ain) and the gate a ( Ag). This diagram shows that for small angles of flow large runners

are required and that for large angles of flow very slender runners will result.

This diagram is the result of computer analysis of a particle taper runner. However, it can be used to estimate the flow and that will be achieved for other sizes of taper runners.

METLF calculates the flow angles of flow very slender runners will result.

This diagram is the result of computer analysis of a particular taper runner.

However, it can be used to estimate the flow and that will be achieved for other sizes of

taper runners. METLF calculates the flow angle automatically and displays this in flow Analysis.

A significant observation that can be made from Figure 7.5 is that the ratio of the

magnitude of the true gate velocity to the runner velocity varies from 1.42 to more than

2.5 This suggests that runner and gate design procedures which ignore the flow angle

tend to underestimate the magnitude of gate velocity. Thus additional energy is required to accelerate the metal, and the machine slows down more than anticipated.

For example, when taper runner inlet area to gate area ratio equals 1, the magnitude

of the true gate velocity is approximately 40% higher than normal gate velocity and

the pressure required to achieve this speed is roughly double.

Zones of Fill: - When attempting to visualize flow pattern in the cavity it is useful to consider

that the casting is being filled by molten metal streams that may have different flow directions. The use of the taper runner gated along its length allows the flow angle

of the metal stream to be varied by changing the runner inlet area to gate area ratio.

This suggests that the cavity can be considered to be made up of a number of „Zones of

Fill‟ the boundaries of which are determined by selecting appropriate flow angles from the runners. The flow angle is a fluid dynamic phenomenon in the pressure die casting

process which can be used to advantage by the die designer if he knows how to select

the boundaries of the zone of fill which give the most desirable fill pattern.

Figure 7.6 shows the use of single taper runner feeding a flat plate casting along its longest side. It shows that the metal flow direction is unidirectional and that

the choice of a flow angle of 20 degrees results in only a small area of the casting that

is not filled directly from the gate. If this area is less than 10% of the surface area of

the casting it may be ignored and the whole casting could be considered as one zone. If for some reason the casting can not be gated along all of the longest side and the

flow angle is 45 degrees , there will be two Zones of fill. The choice of flow angle is

crucial, in this case, as it affects the size of the zone that is not fed directly from the

gate ( from now on called an „ ungated zone‟ as shown in Figure 7.7 One could expect casting defects in Zone 2. To minimize the problem, a small flow angle should be chosen.

This can be achieved by making the size of the runner inlet to be at least two and half

times larger than the gate area. This will produce a flow angle of 20 degrees.

- 37 -46


DIE CASTING TG-2

THEORY


- 37 -

The use of paired taper runners to fill a long casting, shown in Figure 7.8 creates a

different fill pattern. In this case, it has three distinct zones of fill:

(1) The centre zone (Zone 1),

(2) The left zone (Zone 2), and

(3) The right zone (Zone 3).

Zone 2 and Zone 3 may be adequate as the volume in Zone 1 is relatively small

and sufficient gate area in the delta can be achieved in practice to feed Zone 1. Such is

not the case when the casting is wider and the flow angle is 45 degrees as shown in Figure 7.9. Here, the volume of Zone 1 is much larger than Zone 2 or Zone 3 and yet the gate

length in Zone 1 is much shorter. Hence problem may be encountered in the casting

unless steps are taken to design the taper runners to reduce the flow angle to, say, 25

degrees.

The delta area between paired taper runners is a complex area of flow in that

metal must flow across the delta during transient runner fill; i.e. When there is low

pressure on the metal. This means that the shape of the runner in relation to the delta is most important. To ensure a satisfactory pressure velocity relationship with in runner

system at the delta ( see Figure 7.10 , the area of the runner at the delta ( see Figure

7.10), the area of the runner at the inlet to the y –junction must be greater than the

combined total of the area at the junction outlets and the gate in the delta.

Shock Absorber: -

Gate velocity can be excessive if the design of the flow system is poor and may

result in a situation where metal leaves the end of the runner at a transient velocity ( due

to kinetic energy ) that is greater than the steady state velocity. This will result in a

spurt of metal ( see Figure 7.11) racing around the edge of the cavity sealing off air vents and losing heat. The resultant semi-solidified metal will now be moved around

the cavity by the rest of the flowing material until the cavity is full. Since the spurt does

not remelt in the cavity it shows up as a casting defect called „cold shut‟ .

To overcome this situation shock absorbing disc ( Figure 7.12) are designed at

the end of the tapered runners. The disc which is placed tangentially to a small parallel section at the end of the runner, directs the flow in a circular mode trapping air in the

centre of the disc and thus decelerating the metal in the runner and minimizing the spurt.

Discontinuous Gate: -

The discussion on runners and gates so far has been confined to continuous gates.

Whilst this satisfies most situations the use of discontinuous gates or comb-gates can

- 38 -46


DIE CASTING TG-2

THEORY


- 38 -

be a satisfactory method when the zones of fill are created to suit the shape of the

casting ( Fig 7.13).

C.1 5 PRINCIPLE AND FEED SYSTEM: -

The gate is the most restrictive orifice in the total fluid flow in a d ie casting.

Most of the casting defects such as bad Surface improper filling, flow marts , cold

shuts soldering thermal should be directed along the time of least resistance .

The successful design of gating system requires the following factors into

consideration: -

a) Die Cast part design b) The quality level of the casting

c) Die parting line

d) Gating layout

e) Fill time f) Temperature looser

g) Metal through gate

h) Gate velocity

i) Fill rate j) Gate Area

k) Gate Thickness

l) Runner Design

m) Injection pressure and machine capacity n) Metal Plunger

o) Metal piston Velocity

p) Overflows

q) Die rents

- 39 -46


DIE CASTING TG-2

THEORY


- 39 -

a) Die Cast Part Design: -

Turbulence is a always associated with the filling of die cavities under very high

injection pressure so provided generous radius around the sleep corners which helps in

better flow of metal. Castings designed with both very thick and very thin sections is very difficult to get, so use uniform section thickness.

b) Quality level of Casting: -

It is relatively easy to design a gating system if the casting is not of hardware

type ( good finish) decorative type of finish ) Normally thing gates with high injection

pressure are required to obtain hard ware finish.

With thick gates it is possible to avoid jet type of filling but the casting surface

finish may not be very good There is also possibility of pin hole porosity near the gate Thick gates are difficult to trimmed edge Then gates assure good , clean trimmed

edge For the same fill time the thin gats requires longer length of gate.

In some cases the surface finish can be improved by adding overflows and proper

air venting.

c) Die Parting Line: -

Sufficient thought should be given for gating and runner system and to make

sure that the proposed gating system will assure a good flow pattern. When parting

lines are stepped there should be enough space so that, if necessary the gating length can be extended and there should be enough space for the overflows.

d) Gating layout: -

On single cavity dies, the configuration of the component all determines the type of

gate i.e. side or centre gating.

With centre gating the metal has to travel the minimum distance to fill the

cavity Side gating is specially suited for multi-cavity work. For multicavity dies, it is

necessary that all the cavities are filled at the same fill time.

If castings of different weights are cast the cavity which fills first is bound to have

bad finish. The terminal pressure which is applied at the end of the stroke (i.e. final

squeezing pressure) is transmitted only if the metal in the gate is not solidified. Once the cavity is filled , the gate in that particular cavity is bound to solidify first and the

terminal pressure has no effect.

iv. Fill time: -

- 40 -46


DIE CASTING TG-2

THEORY


- 40 -

The time during which cavity is filled with molten metal is called fill time .

When the fill time is longer, there is more time the air in the cavity to escape through the

die vents.

Optimum venting is desirable to obtain a good quality casting. However the cavity must fill before the solidification of the metal in any part of the casting which

requires faster filling time. Thus two opposing requirements for cavity fill relative to the

time have to be reconciled.

a) The longest possible fill time must be used for optimum venting and

b) Cavity fill must be fast enough to be completed before the compression face sets

in and before solidification of metal can take place in the most remote and the

thinnest section of a given casting.

Parameters which effect the fill time are: -

i. Type of alloy (flow characteristics, specific gravity etc.,differ)

ii. Volume of the casting iii. Average wall thickness

iv. Ratio of weight to surface area of the casting (thin + more surface area means less fill

time should be allowed).

v. Temperature of the metal at the gate vi. Cavity surface temperature (Die temperature)

vii. Condition of the cavity surface

viii. Amount of super heat of metal (the flow ability depends on the pouring temperature)

ix. The thinnest wall section anywhere in the casting.

The first six points are taken into accounts for the calculations made in this paper ( Refer figs. 1, 2, 3 and 4) . with the fig.1 and the DCRF gating work sheets one

arrives at similar cavity fill times for average castings.

Temperature losses: -

In cold chamber machines the super heated metal losses its heat to the ladle,

shot cylinder sleeve, and the runner. Before finally solidifying it loses its heat to the die

cavity. To determine the fill time it is necessary to determine the losses that occur in the

ladle, the shot cylinder sleeve and the runner In a hot chamber machine, the super heat losses occur in the runner and the die cavity.

Heat loss in the ladle is considered unimportant because when the ladle is

immersed in the metal bath it attain the temperature to that of the molten metal in the bath. The temperature to that of the molten metal in the bath. The temperature loss in the

ladle , the shot cylinder sleeve and the runner In a hot chamber machine, the super heat

losses occur in the runner and the die cavity.

- 41 -46


DIE CASTING TG-2

THEORY


- 41 -

Heat loss in the ladle is considered unimportant because when the ladle is immersed in the metal bath it attain the temperature to that of the molten metal in the

bath . The temperature less in the ladle is minimum and does not exceed more than 40 F.

Heat losses in runner are similar to that a shot sleeves limited by the thin oxide

lubricating layer between the molten metal and the die. When the runner is long , more heat loss will occur. This will be aggravated if the cross section of the runner is very thin .

The metal temperature loss varies directly with the ratio of thermal temperature loss

varies directly with the ratio of thermal conductivity of the lubricant layer over the

thickness of the lubricant layer and also the ratio of length over the thickness of the runner. The metal temperature losses depend also on the temperature difference between

the die cavity surface and the molten metal.

Metal through gate: -

The volume of metal that will flow across the gate opening to fill the cavity and the

overflows is called the metal through gate. This is the volume of casting and the volume of

overflows.

Gate velocity: - The velocity of metal through gate area is called the gate velocity. It is to be

remembered whatever be the gate velocity, the fill time remains the same. This means the

gate area will be increased when the metal velocity through the gate is reduced.

A lower gate velocity can reduce the washout problem and will help reduce the deposits on cores and cavity walls. However when thin walled castings are encountered a

higher gate velocity is a must to avoid premature solidification.

When velocities are cropped below 100 ft/sec. the surface quality at the far end of the gate area may not be suitable for decorative purposes. Relatively very thick

parts have been more with higher injection pressure with gate velocities as low as 70

ft/sec ., and this indicates the use of very thick gates.

For the purpose of the gate calculation it must be sufficient to refer table – I

showing a normal range of gate velocities from 100- 160 ft/sec. for Aluminum and

Magnesium alloys and 30-120 ft/sec. for Zinc alloys.

1. Fill rate: -

- 42 -46


DIE CASTING TG-2

THEORY


- 42 -

The fill rate Q can be defined as the rate at which the cavities and the overflows are

filled with molten metal.

Q = V/T

Where:

Q = fill rate in in3/sec.

V = volume of metal are filled

T = fill time in sec.

Gate Area: -

Once the fill rate Q is calculated, the gate area can be calculated as the velocity is

already known.

Gate area x velocity = fill rate

Ag = Q /12 Vg

Where:

Ag = area of gate in in2

Q = fill rate in in3/sec., and

Vg = velocity of metal through gate in ft/sec

Gate Thickness: - --------------------------------------------------------------------------------------------

Thickness of wall Gate Thickness

Aluminum Zinc

-------------------------------------------------------------------------------------------

Very thin wall parts

(0.75 – 1.5 mm) 0.032 in (0, 5-0, 8mm) 0, 028in (0, 5-0, 6mm)

Thin wall parts

(1, 5 – 2, 5) 0.050 in ( 0, 8-1,25mm) 0,043in ( 0, 6-0,85)

Medium wall parts (2,5 -4mm) 0.063 in ( 1,25 -1,60mm) 0,054in ( 0,85 -1 ,35)

Thick wall parts

- 43 -46


DIE CASTING TG-2

THEORY


- 43 -

(4-6mm) 0.080 in (1, 60-2.00mm) 0,066 in (1,35-1 65)

The maximum of 6mm thick walls are used, but rarely upto 10mm wall thickness is

allowed.

--------------------------------------------------------------------------------------------

The proportion of the area of runner to area of gate must be 1.25: 1 to 1.6. 1 (some times upto 2: 1).

The proportion of the area of runner to area of gate must be 1.25 : 1 to 1.6 : 1 ( Some

times upto 2 : 1).

Thin gates are preferable for trimming operations. It is very unlikely to achieve solid front fill with thin gates. With very thick gates, there is a possibility of pin hole porosity at the

gate area coupled with trimming problems. However, with thick gates, it is possible to

achieve solid fill.

Runner Area: -

The fill rate should be controlled by the gate. To obtain gate controlled fill the cross

section of the runner feeding a cavity must be larger than the gate area. For minimum heat

losses metal velocity in the runners feeding a gate must be as high as possible . For this

reason, a gate to runner area ratio of approx. 1: 1, 25 - 1 : 1, 6 is generally used. This

would ensure the estimated gate velocity. On different lengths of runners feeding identical parts, the longer runner is given a slightly larger cross section. Turns and leading edges

should have generous radii and be smoothly blended where thickness or width changes

occur (in case of family dies). A reasonable quality of surface finish is required in the

channels. The cross sectional area of main or feed runner should be equal to the sum of

cross section areas of all the branch runner The runners can be in either half of the die

according to the dictates of the circumstances U e.g. runners can be carried over slides.

M) Injection pressure and Machine capacity: -

The zinc-die casting alloys are generally cast in the rang of 1500-2000 psi ( 105 -

140 kg/cm2) while Aluminum and Magnesium alloys are cast in the range of 4000 psi to

10,000 psi ( 280-700g/cm2 ). So higher injection pressures demand higher capacity die casting machine. If projected area of casting is A which includes cavity area, overflows,

runners, biscuit or sprue U and metal pressure is M, the total die opening force T is

given by

T = M x A

The locking force should exceed this by 15-20 percent For change in injection

pressure we may change the sleeves considering the proper fill rates.

- 44 -46


DIE CASTING TG-2

THEORY


- 44 -

The die casting machine must be able to withstand ( M x A) tons of die

opening force.

As a thumb rule, Total projected area = Projected area of cavity + 20 percent of

cavity projected area for runner gates overflow etc., is roughly adopted

Metal Plunger: -

The injection cylinder sleeve can be changed based upon the volume of metal

required for producing the die casting. Once the shot cylinder is fixed the only variable

that can be altered is hydraulic oil pressure in the hydraulic cylinder . This can be

controlled by hydraulic pump. If the diameter of oil piston is D ( hydraulic cylinder

piston inches ) and if oil pressure is P then injection force is given by

F = D2 P

4

Where:

F is in lbs

P is in psi D is piston dia of metal cylinder in inches

So

D2 x P = D2 x M 4 4

Where:

M is metal pressure required ( psi ) Metal plunger dia ( in )

Now d2 = d2P , d = D2

M M

The nearest standard size of metal piston die available can be selected.

Metal Piston Velocity: -

Piston velocity of injection cylinder is given by

Q = AP x Vp VP = Q/Ap = ___Q__

d2 /4

Where:

VP = Metal piston velocity in inches /sec.

Q = fill rate in in3 /sec.

- 45 -46


DIE CASTING TG-2

THEORY


- 45 -

d = die of metal piston = sleeve die

Ap = area of cross section of metal piston in sq. in

Overflows: -

Placing of Overflow wells is generally predictable, and their location and size

are designed into the gating system of a die The addition of overflows and their

relocation are the most frequent causes for correction. Overflow is generally added to

improve casting quality by drawing excess flow of metal over the trouble area. In the calculation of gate the weight of overflow is added to the weight of casting to arrive at the

metal flow through gate. Over flows should be provided in their predetermined

locations but the feed should be made after the first die trail The feeds for the

overflow. Should be made nearest to troubled spot in casting. Overflows should never be gated their full length the because metal may flow back from overflow to cavity.

Overflows curve following additional purpose : -

(1) Assist venting

(2) Increase die heating

(3) Act in a poritory for partly chilled metal

(4) Protect the edges of the Die or castings (5) Assist Ejection

Overflows should be short. With long overflow there is a possibility of feeding

back of metal into the cavity if we gate it at two places. It is preferable that the overflows are not interconnected Overflow are also added to increases the die

temperature far away from the gate area. In some family dies, more overflows are

added to the smallest casting then are required for optimum filling in order to bring

the weight nearer to that of larger members of the rate cluster.

Air Vents: -

Vents air passages normally lead of overflow in certain casting or directly

from the edge of the casting. The metal flow pattern should be such that the die vents

should be the last area to be filled with molten metal during the fill time. Total vent cross section area should be at least 50 percent of the gate area.

The effective of the die vents is lost once the vents are closed during the initial

portion of the fielding time. The depth of vents at die inserts should be 0.010-0.015 inches

(0.25 to 0.3mm). It should be between 0.002 -0.006 inches ( 0.05-0.15) at end of the die half or after a distance of 1.5 to 2 inch from the cavity edge.

Ejector pins can be used for venting by grinding flats on slides. The depth of flats

should not be more than 0,002 inches

- 46 -46


DIE CASTING TG-2

THEORY


- 46 -

C1.6 BALANCING OF FEED SYSTEM

Runner Layout: -

The layout of the runner system will depend upon the following factor : (I) the number of impressions, (ii) The shape of the components , ( iii) The type of die ( iv)

The type of gate. There are two main considerations when designing a runner layout.

The runner lengths should always be kept to a minimum to reduce pressure losses, and the runner system should be balanced.

Runner balancing means that the distance the metal travel from the Sprue to

the gate should be the same for each molding . This interruption providing the gate lands and the gate areas are identical.

Single - Impression Dies: -

Single - impression Dies are usually by a side feed into the impressions. In which

case a short runner may be used But note that by gating a single impression in this way

the impression itself must be offset This is undesirable, Particularly with a large impression as the injection pressure will exert an unbalanced force which will tend to

open the Die one side and may result in flashed casting

Two – Impression Dies: -

The simplest case is where the runner takes the shortest path between the two

impressions is seen that solely from the viewpoint of Die layout it is desirable to have the impressions positioned as shown with short runners to the side of the impressions.

Thus enabling the size of the Die to be kept to a minimum. However there are other

considerations, such as that of correct gating , and it may be desirable to gate at one

end of the impression.

Posting of Gate: -

Ideally the position of the gate should be such that there is an even flow of

melt in the impression, so that it fills uniformly and the advancing melt front a spreads

out and reaches the various impression extremities as the same time. In this way two or more advancing fronts would rarely meet to from a weld line with consequent

mechanical weakness.

- 1 -16


DIE CASTING TG-2

THEORY

CHAPTER : C2 Cooling System

- 1 -

UNIT-3

CHAPTER – C2

COOLING SYSTEM

- 2 -16


DIE CASTING TG-2

THEORY


- 2 -

CHAPTER OUTLINE

C2.1 INTRODUCTION TO COOLING OF THE DIE CASTING DIES.

C2.2 DEFINATION OF ELEMENTS AND FUNCTIONS OF

COOLING SYSTEM

C2.3 PRINCIPLE OF COOLING OF DIE CASTING DIES.

C2.4 CLASSIFICATION OF COOLING SYSTEM.

- 3 -16


DIE CASTING TG-2

THEORY


- 3 -

C 2. 1 INTRODUCTION TO COOLING OF THE DIE CASTING

DIES

Surplus heat has to be artificially removed from the die by the die cooling systems.

Cooling requirements depend on the alloy and the size of the casting. Cooling channels , spot cooling ‘fountains’ and spiral cooling cores are the most often used methods

Compressed air is used at times, mainly to cool slender moving cores and very small

dies areas, long slender cores and locations, where conventional cooling channels

would be too close to the die surface The heat pipe is a hermetically sealed tube of a self - contained evaporative heat exchanger system, which will transfer considerable

heat from its heated end to the cooled area. An intimate metal – to- metal contact with

the die steel is essential for efficient heat transfer . The maximum performance is

reached when its condenser end is vertically above the evaporator. Heat pipes have been in use since the mind –seventies , but have not gained universal acceptance.

Thermal oil-operated die cooling /heating units are used increasingly by die

caster in spite of the relatively high purchase price. The popularity of the units is due

to their superior performance against conventional die cooling by recalculated water .

The upper temperature limit of these system is around 350C if screwed pipe joints are

used and 250C if quick connect coupling are used to join the pipes to the die.

Heat flow and cooling /heating calculations: - Die designers could not carry out accurate thermal calculation in the past due to the

lack of available data. The rather basic equations recommended by various sources use

some empirical constants to simplify the involved calculating process but we need more accurate and predictable cooling design to reduce the numerous process variables . The

complex nature of the heat flow calculations in a die makes these ideally suited for

computer Fig 18.7 illustrates the conventional heat transfer model, and Fig. 18.8

downs the temperature variations in the die during on casting cycle.There is also a temperature drop in the die steel caused by the effect of the thermal resistance . The

thermal losses at the boundaries are rather small compared to the losses with in the

steel . One has to know the heat transfer coefficients and the thermal conductivity of

the steel to calculate the temperature drops. During the design stage, the following thermal parameter must be determined : -

- 4 -16


DIE CASTING TG-2

THEORY


- 4 -

(1) The net thermal input to the die ( heat content of the metal poured heat content

of the ejected casting). (2) The thermal losses of the die (heat dissipated to the environment, absorbed by the

machine patents, and taken away by the die spray).

(3) The die heat-up energy requirements.

(4) 4.The die heating energy requirements during production ( this is essential if the calculated thermal losses are greater or near equal to the heat input to the

die).

(5) The die cooling channels ( this is best accomplished by dividing the die into

zones and each zone considered separately). (6) Heat flow analysis (this calculation will determine the optimum die and metal

temperature).

C2.2 DEFINATION OF ELEMENT AND FUNCTION OF

COOLING SYSTEM

The Cooling System in Die Casting Contain Following Elements: -

a) Water Line: - To maintain the required temperature difference between the die

and metal, Water, ( or other fluid) is circulated through holes or channels is termed

as water ways.

b) Orings : - To prevent leakage of coolant orings are fitted around the care and cavity inserts.

c) Baffle: - To ensure that the coolant circulates down each individual hole, baffles

must be fitted into each The baffle is usually made of brass.

d) Connecting Plates: - ( Main folds ) Instead of using an internal cross drilling a

milled slot may be used in conjunction with a connecting plate.

e) Plugs: - The end of certain holes are necessary to be plugged to form a continuous

circuit The plugs have a squares or hexagonal projection ( or depression so that a

suitable Key can be used to ensure a leak free joint when the plug is scuwed into the

die.

f) Nipples: - For inlet and outlet of water into the die nipples are used.

C2.2 PRINCIPLE OF COOLING DIE CASTING DIES

Waterlines: -

The drilled hole through which cooling water is pumped is the primary thermal

control feature in the die Locating the Waterline The amount of heat that must enter any particular thermal section of the cavity surface can be computed The values qj of

heat flow for vary greatly from place in the cavity surface, the interior of the die must

be colder than the average surface temperature of the cavity . The concept is illustrated

- 5 -16


DIE CASTING TG-2

THEORY


- 5 -

in Fig 10-5 The relationship between the interior die temperature Td, average cavity

surface temperature Tc and the distance between them d’ is given by equation 10-1

Tc – Td = Qj

d’ KA

Where: Tc = Average cavity surface temperature, F deg ( C deg.)

Td = Interior die temperature, F deg ( C deg.)

d’ = Distance from cavity surface to Td, in. ( mm)

Qj = Rate at which heat must flow through the j zone of the die, Btu/ hr (watts)

K = Thermal conductivity of the die material Btu/hr. – in -F ( Watts/mm-C

A = Cavity surface area of the j zone, sq. in. (sq. mm.)

Equation 10-1 is only valid for large flat castings. However, its results offer a first approximation that can be modified where necessary for curved cavity

surfaces.

Waterline Size and Flow Rate: -

If the waterlines shown in Fig 10-7 have too much total length , are the wrong

diameter or have too much water flowing through them, they will remove too much

heat. The die temperature , Tm, around the lines will then drop below the 175F required , causing too much heat to flow the cavity surface. The average cavity surface

temperature Tc, will drop Eventually the heat flow will balance, but the colder die may

cause defective castings. Short lines or insufficient water flow will have the opposite effect.

- 6 -16


DIE CASTING TG-2

THEORY


- 6 -

The diameter of the waterline is usually the tap drill size for the threading tap of

the pipes used to connect the cooling water supply to the die . To simplify set-ups in

the die casting machines , a single pipe size is usually used for all dies in the plant.

Holes of 7/16 in. (11 mm.) diameter are the most efficient in the normal range of heat transfer for die casting dies. A 5/ 16 in. (7.9 mm.) dia. hole for a 1/8-in pipe thread

may be more controllable if the heat removal rate is less than 800 Btu/hr-in ( 9.23

watts/mm.), of line.

The heat absorbed by a waterline is: -

Qj = Rate at which heat must flow through the die in zone j, Btu/hr ( Watts)

A = Area of water line, sq. ft. (Sq.m.) Td = Temperature of die around waterline, deg. F (deg. C)

Tw = Temperature o f the water flowing through the waterline, deg. F (deg. C)

hc = Heat transfer coefficient , Btu /hr - sq . ft . - F) (Watts /Sq.m. - C)

And:

A = DL

Where:

D = Diameter of the waterline, ft ( m.)

L = Length of waterline, ft. ( m)

The die temperature is one of the most important process parameters besides the specific pressure, metal velocity and the alloy type.

- 7 -16


DIE CASTING TG-2

THEORY


- 7 -

The recommended working temperature, which may vary slightly –

according to type of casting are as follows: -

ALLOYS KELVIN OC

PB, SN 333- 393 K 60 - 120

ZN 423 -473 K 150 - 200

AL 453 - 573K 180 - 300

MG 473 - 523K 200 - 250

CU 573 - 623K 300 - 350

Calculation of the Cooling Capacity: -

PK = n. mS. q

PK = Cooling capacity (Kw)

n = Number of shot per second (n/sec) mS = Shot weight ( kg )

q = Total heat factor (KJ /kg )

A1 , Mg = 607 KJ /Kg

Cu = 275 kg /kg Zn = 201 KJ /kg

T 200 K

See E. Brunhuber: “Praxis der Druckgussfertigung’’

Example

Calculation Of The Cooling Capacity

Solution:

GIVEN: NUMBER OF SHOTS PER SECOND n = 0.11/SEC

TOTAL HEAT FACTOR q = 607 Kj/KG SHOT WEIGHT mS = 2.5 KG

PK = n. q. mS = 0.011 .607. 2.5 = 16.7 kW

Where:

PK = Cooling capacity (kW)

mt = n. MG = ( kg /sec) mG = Shot weight without runner ( kg)

n = Number of shots per second (n/sec)

q = Total heat factor kJ/kg

Zn - alloys q = 17

- 8 -16


DIE CASTING TG-2

THEORY


- 8 -

AlSi - alloys q = 887

AlMg- alloys q = 795 Mg - alloys q = 712

Ms60 – alloys q = 452

Pk = mt. q . S f Sf = Safety factor: 1.2 … 1.4 for the determination of the heating units

Example

Calculation Of The Cooling Capacity

Given: ALLOY ALSiCu

TOTAL HEAT FACTOR q = 887 KJ/K

SHOT WEIGHT WITHOUT RUNNER mG = 2.5

NUMBER OF SHOTS PER SECOND n = 0.11/SEC mG . n = mt = 0.025 KG/SEC

SAFETY FACTOR Sf = 1.2

Solution: PK = q. m t. Sf = 887. 0.025. 1.2 = 26.6 kW

The next step is the calculation of the throughput of the cooling or heating media.

We use the following equation:

Flow Rate: -

V = Flow rate (1/min) Q = Rate of heat kWh

C = Spec. heat of fluid kJ/kgk

Calorific oil C = 0.5

V = Q________

C . T . . 60

T = Admissible temperature drop (K) on the die

(difference between inlet and outlet temperature of fluid)

p = Spec. weight of fluid kg/dm

Calorific oil 0.8

E X A M P L E

Calculation Of The Flow Quantity

GIVEN: COOLING CAPACITY

SPECIFIC HEAT CAPACITY

PERMISSIBLE DIFFERENCE IN TEMPERATURE OVER DIE

SPECIFIC DENSITY OF OIL

Solution:

Vt = PK .60 = 13.3.60____ = 23.8 L/min

C.T. 2.093.20. 0.8

- 9 -16


DIE CASTING TG-2

THEORY


- 9 -

One should have regard to the following points when die cooling or heating

channels are designed. (1) The channels should be divided into zones ( as already discussed sprue /biscuit

runner zone, gate area, casting area and die frame).

(2) Cores and areas adjacent to thick walled casting sections have to be well cooled.

(3) One should take good care to design uniform die cooling to avoid stress cracks.

(4) A number of smaller diameter cooling channels are better than a few of large

diameter.

(5) Fixed cores forming large hollow castings may be cooled. with spiral fountains. (6) The cooling channels have to be placed so far below the surface that there is no

changer of any through cracks.

The minimum distance for water cooled channels is 20-25 mm. Minimum distance for oil cooled channels in 10-20 mm.

CLASSIFICATION OF COOLING SYSTEMS

Table 10-1 Minimum Distances Between Waterlines and Other Features of the Die as

Illustrated in Fig 10-20

_________________________________________________________________________

Dimension in ( mm

Feature

Identification in Fig 10-20 1/8" Pipe 1/4" Pipe 3/8" Pipe

Tap Drill Size D 5/17 (7.9) 7/16(

11.1) 37/64 (14.7)

- 10 -16


DIE CASTING TG-2

THEORY


- 10 -

Length of thread or pipe plug D 5/16 ( 8.0) 7/16(

12.0) 19/32 (15.0)

Pipe clearance hole. Size C 7/16( 12.0)

9/16( 15.0 23/32( 11.0)

Centre distance between waterlines

X 0.56 (14.0)

0.68 ( 17 0)

0.875 ( 22.0)

Clearance to cavity surface when casting

S

( Zinc ) 0.75(19.0) 0.75 ( 19.0)

0.75 ( 19.0)

( Aluminum 0.75(19.0) 0.75 ( 19.0)

0.75 ( 19.0)

(Magnesium) 0.75(19.0) 0.75 ( 19.0)

0.75 ( 19.0)

( Brass) 1.00 ( 25.0)

1.00 ( 25.0)

1.00 ( 25.0)

Clearance to parting Surface P 0.62 (

16.0) 0.62 ( 16.0)

0.62 ( 16.0)

Clearance to edge of insert or to ejector pin

E & R 0.25 ( 6.5) 0.25 ( 6.5) 0.25 ( 6.5)

( * Dies for Zinc)

0.50 ( 13.0)

0.50 ( 13.0)

0.50 ( 13.0)

A correctly – sized baffled waterline will remove more heat than a straight hole.

Waterlines in die casting dies usually form many individual circuits. Pipe nipple are screwed into the melts and outlets and these are connected to control valves or drains

with hoses clamps . by adjusting the control valves . There are instances where a series of

lines may operate with the same flow rate and a single inlet would suffice These may be

connected by appropriate drilled holes in the die. When drilling is not convenient, the holes may be connected permanently with copper or steel tubing as shown in Fig. 10-23

Feature Size, In. ( mm)

Normal ( Pipe) size of Waterline, D 1/8 1/4 3/8

Actual Water line Diameter, D

0.312 ( 7.9)

0.439 ( 11.1)

0.578 ( 14.7)

Pipe P' ud, P 3/8-18 1/2-14 3/4-14

TAP Drill , T 0.578 (14.7)

0.703 ( 17.9)

0.921 ( 23.4)

Diameter of Raffled Hole, 1

0.500 ( 12.7)

0.688 ( 17.5)

0.875 ( 22.2)

Area Ratio 1.4 1.4 1.4

- 11 -16


DIE CASTING TG-2

THEORY


- 11 -

Controlling the Heat Flow Path: - Heat flow paths can be controlled by the arrangement of features in the die. Such

features must create heat flow paths that will result in the required gradients at the cavity

surface.

Heat Sinks & Heat Dams: - A cavity such as that illustrated in Fig 10-25 however, does not require uniform

heat flow paths. The heavy sections and the areas of converging heat flow must be compensated

for by shortening the heat flow paths and /or by terminating the paths in colder heat

sinks such as waterlines.

The measure are designed to control the heat flow paths through the interior of

the die material so that the required gradients will be achieved all over the cavity surface

- 12 -16


DIE CASTING TG-2

THEORY


- 12 -

Cores and Insert: -

It is often tempting to design the die to facilitate its construction instead of to

obtain an ideal heat flow, Cores and other inserted pieces are useful and other inserted pieces are useful and often necessary but the heat flow

The amount of resistance to heat flow is dependent upon the actual fit obtained

and therefore may be nearly total in some instance s Thermal barriers at insert seams can sometimes be used to advantage. The thermal problem illustrated in Fig 10-26 is

solved by the design modifications shown in Fig 10-27.

- 13 -16


DIE CASTING TG-2

THEORY


- 13 -

Die Materials: -

Heat flow can also be controlled through the proper selection and use of die

materials. If a constricted heat flow condition cannot be designed out of the die, the

use of a die material with a high thermal conductivity such as molybdenum or tungsten

alloy might reduce the calculated gradient sufficiently . When water cooling is too severe, or when water cooling cannot be built into die, air cooled high thermal

conductivity materials may be used . These materials may be inserted into the area to be

cooled as shown.

Table 10-2 Typical Thermal Conductivity k Values for Various Die Material

- 14 -16


DIE CASTING TG-2

THEORY


- 14 -

Thermal Conductivity

Material

Btu Btu Btu Watts KFactor

H- 13 ( Steel) 16.5 1.3 8 0.384x10'-3 28.7 x 10-3 1.00

P-20 ( Steel ) 19.2 1.6 0 0.444x10'-3 33.2 x 10-3 1.16

Titanium ( 6% A1, 4% V) 6.5 0.5 4

0.150 x10-3 11.2 x 10-4 0.39

Tungsten 4

6.0 3. 83 1.065 x10-3 79.6 x 10-3 2.78

TZM 72.

0 6. 00 1.667 x10-3

124.6 x10-

3 4.35

Copper Beryllium 62.

0 5. 17 1.435 x10-3

107.3 x10-

3 3.75

Air 0.0154 1.28 x 10-3

0.35 x 10-6 26.2 x 10 0.00

Runner and Overflows: -

Runner and overflows are also heat sources. They must be considered as integral

parts of the casting when analyzing heat flow paths. However , the physical size, shape,

and often the location of a runner of overflow can be modified to increase or reduce

the heat input to a certain area of the die. Logical evaluation of the thermal patterns should determine the initial design of runners and

overflows.

Waterline Diagrams: -

For the system of waterlines in the die to be effective the water flow rate

through each must be effective, the water flow rate through each must be accurately controlled . Such control is beyond the scope of the die designer. The control of the water

flow is the responsibility of the die casting machine operator a set-up person, or a process

technician.

(1) The path of the waterline through the die. (This is not always obvious from the

outside of a completed die.)

(2) The effective cooling area of the line ( Clearance holes around pipe nipples

block heat flow to the water flowing through the pipe) (3) Which control valve is connected to which waterline? (When a die has several

waterlines, the mass of connecting hoses makes it difficult to trace the connection.)

- 15 -16


DIE CASTING TG-2

THEORY


- 15 -

- 16 -16


DIE CASTING TG-2

THEORY


- 16 -

- 1 -20


DIE CASTING TG-2

THEORY

CHAPTER : C3 Ejection Systems/Techniques.

- 1 -

UNIT-3

CHAPTER – C3

EJECTION SYSTEM

/TECHNIQUES

- 2 -20


DIE CASTING TG-2

THEORY


- 2 -

CHAPTER OUTLINE

C3.1 INTRODUCTION.

C3.2 VARIOUS ELEMENTS OF EJECTION SYSTEMS &

FUNCTIONS.

C3.3 VARIOUS EJECTION TECHNIQUES.

C3.4 PRINCIPLE OF EJECTION SYSTEMS.

- 3 -20


DIE CASTING TG-2

THEORY


- 3 -

C3.1 INTRODUCTION

The ejector pin is a pin passing through the ejector block and its end forms part

of the cavity surface on which the molten metal is cast. After the casting has solidified and

the die is opened, the ejector pins are moved forward to push the casting out of the

ejector die half. The end of the ejector pin opposite the cavity has a head which is sandwiched between two plates known as the ejector plate and the ejector retaining

plate. These plates are moved by the machine’s ejector system to actuate the ejector

pins and push the casting out of the ejector die half.

The ejector die block “bridges” across the space within the ejector box rails.

However, the ejector die block must still withstand the forces imposed upon it without

deforming enough to exceed the dimensional criteria So, it is common practice to place

steel supporting pillars through clearance holes cut through the ejector and ejector retaining plates Four round support pillars and their clearance holes are shown near

the centre of the die.

If the ejector plate is not hydraulically returned to its “ casting” position it will be

returned with the return pins as the die closes . The return pin is secured between the

ejector plate and retainer plate like the ejector pins However, the end of the return

pin at the die parting surface is not in any cavity , runner or overflow area . It bears directly against the cover die. As the die closes , the return pins will contact Then as the

die closes , the return pins will contact Then as the die closes. The return runs will push

back the ejector assembly.

C3.2 VARIOUS ELEMENTS FO EJECTION SYSTEM AND

FUNCTION

Stripper Plate: - A steel plate insulated between the core and the cavity plate, generally

in a multi-part design for the purpose of ejection.

Stripper Ring: - A hollow steel disk incorporated in a die design to serve as a local

stripper plate.

Ejector System: - The casting as it cools shrinks on the core and this means that

some sort of ejector system is necessary to get the component pushed out from the

core.

Ejector Grid: - The ejector grid is that part of the mould to which ejector elements

are attached The assembly is contained in a pocket, formed by the ejector grid , directly behind the core plate. The assembly consists of an ejector plate on to which are mounted a

number of conveniently shaped support blocks.

Ejector plate assembly : - This is that part of the mould to which ejector elements are attached The assembly is contained in a pocket, formed by the ejector grid , directly

behind the core plate.

- 4 -20


DIE CASTING TG-2

THEORY


- 4 -

Ejector Retainer Plate: - This is the plate, in which the counter bores for the ejector

pin collars are provided.

Ejector plate: - Plate used to actuate ejector pins. This is the block plate for the ejector

retaining plate which is fixed to retaining plate with cap screws.

Ejector Pins: - Pins used to eject castings from a die.

Retainer pin: - The function of Retainer pins is to return the ejector assembly when the

die is closed.

- 5 -20


DIE CASTING TG-2

THEORY


- 5 -

C3.3 VARIOUS EJECTION TECHNIQUES

Most die casting dies utilize the basic pin and plate ejection system described

above. Some castings. However, require variations of that system. If the two sides of a

casting have die pull directions at an angle to each other, the ejection system may need

to be set up at an angle to the die pull opening travel as shown in Fig 15-2 It is not always necessary to mount the ejector plate at a right angle to its travel The example in

Fig 15-3 shows the ejector plate travel, however, is at an angle to the die opening

travel.

- 6 -20


DIE CASTING TG-2

THEORY


- 6 -

Sleeve Ejection: -

Another variation to the ejection system is the use of sleeve or ring ejectors. The

most common sleeve ejector type is shown in Fig 15-4 The core extends through the

ejector plate and is mounted to the die mounting plate. An alternative sleeve

construction is shown in Fig 15.5. There, the sleeve is cut away to from legs which slide in slots of the core shank. The core is then mounted to the ejector die block..

- 7 -20


DIE CASTING TG-2

THEORY


- 7 -

Ring Ejection: - Similar to sleeve ejection is ring ejection. An example of ring ejection is shown in

Fig 15-6 The ring ejection is used when a simple sleeve or a ring of pins could leave

unacceptable flash lines on the casting surface. The ring will form a substantial portion of

the cavity.

Blade Ejection: -

For ease of construction, round ejector pins should be used whenever possible. In

some instances. However, the use of non-circular forms is essential. This situation arises

when ejection must be on the edge of thin walled sections and circular ejectors would be much too fragile to be effective. The ejectors used must conform to the shape of the

recess.

Two forms of non-circular are satisfactory , and then only when it is possible

to mount them at a joint in the die . The first is of plain rectangular section and can be made of any convenient width. It is housed in a slot in the shouldered core or die insert ,

as seen in Fig 15-7

- 8 -20


DIE CASTING TG-2

THEORY


- 8 -

The second type is flat on three sides but conforms to the curvature of the

casting wall on the fourth . Both these ejectors can be accurately fitted , whereas other non-circular forms require much laborious bench work. Acceptable and non-acceptable

forms are shown in Fig 15-8.

Two Stage Ejections: -

Many dies require only the simple single action ejection systems described

above. However, there are occasions when the ejector pins should be divided into two

groups that have different action, strokes or sequencing . When only a few ejector

pins are required for the secondary motion , they may be operated by a “slip” action through the first ejector plate. In that instance s, the second ejector plate may be omitted.

A possible construction is shown in Fig 15-11.

- 9 -20


DIE CASTING TG-2

THEORY


- 9 -

Slender Pins: -

Situations will arise where the ejector pin diameters must be small but their lengths

great. Such long slender pins are subject to buckling. Any pin smaller in diameter than recommended in the section. Ejector Pins and Sleeves, of should be considered a long

slender pin. Similarly, if the pin is the recommended diameter, but considerably longer

than normal for the machine size, it would be considered a long slender pin.

There are several ways to minimize the length of the pins. First is to have the

ejector plates as close to the ejector die block as possible. The ejector dies block as

possible. The ejector die block can be recessed to provide clearance for features of the

ejector plate. And, the ejection stroke can be kept to a minimum. Another technique to stiffen long slender pins is to use a steeped diameter as shown in Fig- 15-13 The

stepped pin can be made by reducing the diameter of the end of a larger pin, or by

placing a sleeve over the long small diameter pin.

- 10 -20


DIE CASTING TG-2

THEORY


- 10 -

Few examples are below for different ejection systems.

- 11 -20


DIE CASTING TG-2

THEORY


- 11 -

- 12 -20


DIE CASTING TG-2

THEORY


- 12 -

- 13 -20


DIE CASTING TG-2

THEORY


- 13 -

- 14 -20


DIE CASTING TG-2

THEORY


- 14 -

C3.4 PRINCIPLE OF EJECTION SYSTEM

In the preceding discussions several hints were given for positioning the cavity

to facilitate the ejection of the casting. Usually the cavity is placed so that it penetrates

into the die steel a minimum distance. The primary rule is to place the most restrictive

situation in the ejector side of the die. The casting tends to shrink onto the die members which form the casting’s internal surfaces and away from these die members whereby

external surface are shaped.

Generally, to satisfy this primary rule, cavities will be sunk in the cover die and protrude from the ejector side as shown in fig 7-18.

As the die opens, the casting will stick to these ejector die surface and be pulled out

of the recessed cover die cavity. The ejector pins then push the casting off the ejector die surface. In some instances the general orientation shown in Fig 7-18 will be reversed

because there are no surface on which to place ejector pins. If for example , ejector

pin marks could not be tolerated on the side of the casting formed by the ejector die in Fig 7-18 it might be necessary to reverse the cavity as shown in Fig 7-19 It is

sometimes possible to provide sufficient ejection from the gate runner and overflow

encircling the part. If such outboard ejection is possible with the arrangement in Fig 7-18

the “No Ejector Pin Marks” requirement could be met. Similarly, ejector pins outside the cavity may be required to “pull” the casting clear of the cover die in Fig 7-19. It is

sometimes possible to use moving submarine cores which travel parallel to the die pull

direction to facilitate ejection . The part shown in Fig 7-20 has restricted draft

conditions on both the cover and ejector surfaces. The requirement for no ejector pin marks is on surfaces. that would normally be made in the ejector side. A movable core as

shown in Fig 7-21 is a solution to such a problem. As the casting solidifies and cools before

the die opens, it shrinks onto the moving core and away from the cover die cavity. When

the die begins to open , the core maintains its position in respect to the ejector die as it would if it were a fixed core. The core pulls the casting from the cover die. As soon as

the casting is free of the cover die, the moving core is retracted into the ejector die.

The casting is then free of all restricted draft surfaces. It is contacting the die cavity only with the surfaces. It is contacting the die cavity only with the

surface . It is contacting the die cavity only with the surface marked “A” in Fig 7-20

The ejector pins behind the peripheral gate runners and overflow can eject the casting

readily. Sometimes a casting can be ( or in some instances must be) Positioned to use angular ejection It is not necessary to have the ejector pins move in a direction parallel

to the die opening Fig 7-22 illustrates a cavity feature that can be formed with a moving

core ( left), or angular ejection ( right ). If the product shown has close dimensional

requirements they might determine which construction is used The features formed by the moving core in Fig 7-22 ( Left), are close to the cover die as shown. The moving core

arrangement could still be used if the cavity were reversed as to the cover /ejector

orientation.

- 15 -20


DIE CASTING TG-2

THEORY


- 15 -

The angular ejection construction shown at the right in Fig 7-22 offers a simplified

cavity construction but more complex ejection system. The casting features that would have been formed by the moving core are now formed by the ejector die cavity and are

therefore in dimensional relationship to the ejector instead of to the cover die.

The angular ejection construction is more limiting as to the choice of parting location. Also, the cavity can be reversed in its cover/ejector orientation only if a severely –

stepped parting plane is adopted.

- 16 -20


DIE CASTING TG-2

THEORY


- 16 -

- 17 -20


DIE CASTING TG-2

THEORY


- 17 -

- 18 -20


DIE CASTING TG-2

THEORY


- 18 -

- 19 -20


DIE CASTING TG-2

THEORY


- 19 -

- 20 -20


DIE CASTING TG-2

THEORY


- 20 -

- 1 -10


DIE CASTING TG-2

THEORY

CHAPTER: D1 PRE CASTING

- 1 -

UNIT-4

CHAPTER – D1

PRE CASTING

- 2 -10


DIE CASTING TG-2

THEORY


- 2 -

CHAPTER OUTLINE

D1.1 INTRODUCTION.

D1.2 PRINCIPLE OF METAL CASTING TECHNIQUE.

D1.3 CLASSIFICATION OF METAL CASTING TECHNIQUE.

D1.4 PRE CASTING TECHNIQUE.

D1.4 (a) PRE CASTING TECHNIQUE RELATED EQUIPMENTS.

- 3 -10


DIE CASTING TG-2

THEORY


- 3 -

D1.1 INTRODUCTION

The purpose of this chapter is to describe the types of furnaces and metal handling

practice, so that the diecaster will be able to provide his machines or permanent mold

casting stations efficiently with metal of good, controlled quality. To produce castings of a

satisfactory standard economically a supply of molten metal in the correct condition for

casting must be available. The diecaster can choose from a variety of furnace types, depending on the volume of production, the energy and fuels available and the

environmental conditions, which must be maintained. The arrangements for melting and

the distribution of the metal require a number of decision and choices to be made.

D1.2 PRINCIPLE OF METAL CASTING TECHNIQUE

Metal casting is a process in which molten metal is feed into a precisely

dimensioned steel molds, within which pressure is maintained untill the solidification has

been completed.

D1.3 CLASSIFICATION OF METAL CASTING TECHNIQUE

Metal casting process can be classified into following method based on its

precasting condition and metals used for casting.

a) Sand casting.

b) Pressure diecasting (cold and hot chamber). c) Gravity diecasting.

d) Investment casting.

e) Low-pressure casting.

D1.4 PRE-CASTING TECHNIQUE.

To process casting we required molten metal, this can be achieved with pre-casting technique.

The melting and handling of zinc alloy can entail five operations: -

1. Melting prealloyed ingot.

2. Alloying virgin ingot.

3. Remelting scrap.

4. Handling molten metal. 5. Transferring molten metal.

Some shops find it economically advantageous to manufacture their own alloys,

and to perform melting and holding as separate operations. The furnaces generally used for melting and alloying are pot and immersion-tube furnaces. Some zinc suppliers for

alloying use large reverberatory furnaces. In practice a melting furnace should have a bath

capacity of approximately six times the amount of metal required per hour. Most holding

furnaces are the open-pot type and immersion-tube.

- 4 -10


DIE CASTING TG-2

THEORY


- 4 -

The amount of molten metal required: -

A small foundry making limited quantities of casting in several alloys might use

one, or two side-by-side melting/holding furnaces at each machine. Medium-sized and large plants will have one or more bulk-melting stations to provide large amounts of

molten alloy, which is distributed to the holding furnaces at diecasting machines while

aiming to achieve maximum quality, fuel economy and minimum metal loss.

Choice of fuel: -

Oil of a wide range of viscosity and quality is used in many parts of the world; town gar or, more likely natural gas is widely used when such supplies are available and

economic. Electric melting is efficient and clean and has been used for many years in

countries with great resources of hydro-electric furnace, for bulk melting and holding,

have been installed world-wide. Some companies take advantage of the lower tariffs for night-time electricity; they install a medium-sized coreless induction furnace, melting

about 2 tonnes per hour through the night, regularly transferring molten metal into a large,

very economical electric radiant furnace, which holds the molten metal ready electric and

fossil fuel furnaces has been complied by an Australian author efficiencies, energy consumption and annual fuel costs of different types of furnaces, but individual

calculations have to be made for each country, taking all aspects into account.

Metallurgical efficiency and metal quality: -

Metal is so expensive that furnaces which convert too much metal into oxide are to

be avoided, no matter what speed of melting is offered.

Diecastings, which contain inclusions or practicals of oxide, will cause damage to

cutting tools, so will not be acceptable for the automated machining operations that will

be performed on them.

Ecology: -

Employees, factory inspectors and neighboring residents expect that working

conditions will be of a high standard, with emissions of fume and abtrusive noise being

brought to a minimum.

Molten metal deliveries: -

The heat required to raise the temperature of one tone of aluminium from room temperature to melting point, then to change it from solid to liquid and then to raise the

temperature of the molten metal to 700 degree C is theoretically 253000 K (a K Cal is

1000 calories). Thus under ideal conditions it would require 27 liters of oil, 294 KWh of

electricity, 1 003 125 BTUs or 10 therms of gas to bring one tonne of aluminium up to casting temperature. These figures might be compared with the actual requirements of any

diecasting plant.

- 5 -10


DIE CASTING TG-2

THEORY


- 5 -

D1.4 (a) PRE-CASTING TECHNIQUE RELATED EQUIPMENTS.

Central melting: Reverberatory furnaces: -

Furnaces such as those shown in Fig. 22.1 are popular in the US for the central

melting of large quantities of aluminium alloy required, allowing for additional capacity

as production expands. The reverberatory furnace shown in the illustrations is used for receiving molten metal deliveries or for melting large, solid materials. Materials scrap

would increase melt loss in this furnace.

To obtain the best possible performance of theses furnaces the wise diecaster will

perform at least the following furnace maintenance routine.

1. Check the gas analysis regularly. 2. Adjust burners for the most efficient combustion.

3. Clean the burners regularly.

4. Check and calibrate the pyrometers regularly.

5. Clean air filters regularly. 6. Keep the furnace refractory lining clean, an din good repair.

- 6 -10


DIE CASTING TG-2

THEORY


- 6 -

In addition to routine maintenance, attention should be focused on the following fuel conservation measures:

1. Open charging doors only when necessary 2. Avoid prolonged holding periods.

3. Cover exposed wells, to reduce heat loss.

4. The use of recuperates.

- 7 -10


DIE CASTING TG-2

THEORY


- 7 -

Rotary furnaces: -

The rotary furnace is basically a refractory-lined drum, which rotates to bring the

metal into contact with hot refractory. It is fired centrally at one end of the drum and exhausts from the other. The constant movement of the metal bath disturbs and breaks the

oxide inclusion in the metal. Rotary furnaces are more often used to extract free

aluminium from scrap skims and drosses. Large additions of a wet type flux are charged

with the scrap to aid better separation of the oxides from the metallic aluminium. The metal can then be ingoted of transferred molten to another reverberatory furnace for

chemical adjustment and cleanliness treatment.

Electric induction furnaces: -

In areas where electric power is competitive with fossil fuels, or in plants, which

require a high standard of metal quality, serious thought should be given to central melting in induction furnaces. They are clean, efficient and give a lower melting loss than

any other type of furnace. Although they are not yet used extensively in the US, they are

available, and widely used elsewhere. In contrast to other furnaces, in which heat is

transferred by radiation, the power is from the normal 50 Hz for very large units, 500 to 1000 Hz for diecasting bulk melting, to 10000 Hz for small furnaces in laboratories.

- 8 -10


DIE CASTING TG-2

THEORY


- 8 -

There are three types of induction furnace: channel furnace, coreless furnaces and, a recent development, channel less detachable inductor furnaces. The first two are used in

diecasting, the channel furnace mainly for holding and the coreless furnace for metal

melting.

The channel furnace shown in Fig. 22.7a consists of a large upper bath with a

small channel at the bottom of the furnace. Heat is induced in the channel through a coil

surrounded by ceramic refractory, which permanently contains molten metal. It is the most energy efficient induction furnace but is only economic if run continuously so is not

suitable for plants, which operate, only single or double shift working. In the past the

cleaning of this holding furnace presented a further problem: particles of aluminium oxide

tend to accumulate at the side of the inductor channel, causing clogging which had to be cleared by rodding and fluxing or be the use of a nitrogen lance.

For induction melting, the following components are essential: -

1. A power source, such as a line transformer, or a 50 to 60 cycle, main line voltage

supply.

2. Capacitor banking for correction the one power factor to near unity. 3. Power switching gear, temperature recording instruments and safety circuits.

4. A cooling system for the power coils leads and control components.

Some advantages of the induction furnace are -

1. Close temperature control.

2. Low melting loss.

3. In the coreless type, charges can be 100% fines while still achieving a comparatively low melting loss.

4. Close composition control, with maximum recoveries when alloying.

5. Very little operation skill required.

6. A more pleasant working environment than obtained with other types of furnaces.

The disadvantages are -

1. The initial investment is much higher than a fuel-fired furnace of the same

capacity. However the capital cost of induction melting and holding equipment has

declined in real terms in recent years. Most important, this has been coupled with

an improvement in performance, so the fewer kilowatt-hours are now needed to melt or hold a fixed amount of metal.

2. Installation costs are higher compared with a fossil fuel furnace.

- 9 -10


DIE CASTING TG-2

THEORY


- 9 -

Tilting crucible furnaces: -

The ole-fashioned tilting crucible furnace is not really suitable as a bulk melting

init in a diecasting plant. It is hardly ever used in America and only to a limited extent in small European foundries. A more recent development, the basin tilting crucible furnace

is more widely, used by some medium sized producers. The products of combustion are

taken over the top edge of the crucible, which improves thermal efficiency and results in a

fast melting rate. As the flame is not in contact with the molten metal, losses are kept to less than one percent for aluminium.

These furnaces are made in suitable sizes to accommodate basin type crucibles

from 65 to 900 kg aluminium capacity. The large open basin makes charging of bulky scrap easier than with tall crucibles used in the traditional tilting furnaces. Most basin

tilting furnaces are fitted with manually operated burners of the low pressure air type

which because of the nature of the flame, must be brought to full heat gradually to avid

damaging the crucible. To eliminate this warming cycle and to reduce burner operation to a minimum, automatic push button burners, similar to those fitted to bale- out furnaces,

can now be supplied with basin tilting furnaces. These burners can be arranged to switch

off automatically once the metal has reached the preset temperature.

Holding furnaces: -

In the years before energy costs escalated, furnaces in diecasting plants were inefficient and were often used to melt and to hold it at the diecasting machined. A typical

crucible furnace would consist of an ouster steel shell, lined with refractory bricks; into

which consist of clay bonded graphite or silicon carbide was placed. The combustion

gases circulated around the crucible. Such equipment, when used for melting and holding, would require 2½ million K calls for 1 tonne of aluminium alloy, a figure ten times the

theoretical amount of heat that should be needed.

Melting and fluxing: -

Zinc melting furnaces can be made for both alloying and remelting, with in

difference. New alloyed metal is seldom fluxed, while remelting zinc is usually fluxed. When alloying, special high-grade zinc ingots or sows (99.99% pure) are used. The

aluminium and magnesium are added as hardener, usually purchased as alloy shot which

is either melted in a separate furnace and added to zinc in a molten condition, or it is

added as slid shot. Alternatively power transmission line clippings such as wire, which is relatively pure aluminium and to which the magnesium is added as ingot clippings or

scrap, may be used.

In some cases, scrap is melted along with zinc to be alloyed, followed by hardener and special high-grade ingots. This practice holds the hardener in the melting and mixing

of the hardener can occur to homogenize the alloy. During the treatment care must be

taken to avoid entrapped air by excessive stirring.

- 10 -10


DIE CASTING TG-2

THEORY


- 10 -

Depending in the flux used, over fluxing may cause the flux layer in the bath to thicken or because of the exothermic nature of the relation, the dross may become

superheated (750 degree C or higher) causing the zinc to vaporize. Any superheated flux

should be skimmed off as soon as possible and the melt composition checked. Careful control of melt temperature and flux usage will result in the least amount of dross

formation and a clean melt. Equipment is available to reclaim zinc from the hot dross, but

normally it is sold to a smelter for recovery. Fluxes, commonly used in the past.

Products of copious fume create pollution problems, although in recent years,

nonfuming proprietary fluxes containing magnesium chloride have been developed which

also retard the rate at which magnesium is removed from the alloy.

The choice of whether to flux or not depends on the standard procedures of each

foundry, but such practice can only result in cleaner metal for casting. When handling

metal: -

1. Store ingots in dry, preferably heated areas.

2. Preheat metal to be charged, when economically advantageous.

3. Use minimum pouring height to transfer molten metal.

4. A continuous charging method is recommended. 5. Keep pot type furnaces as full as possible.

Good furnace maintenance increases melting efficiency and this can be improved

by the following precautions.

1. Furnace covers should be used t reduce heat losses from the metal surface.

2. Furnace linings should be checked intervals.

3. Burners should be kept clean, and the gas air mixture adjusted regularly. 4. Dross should not be allowed to build up on the furnace walls, pots, or immersion

tubes, thereby decreeing heat transfer.

The magic word in handling zinc alloys is temperature. Overheating above 850 degree F (455 degree C) should be avoided. Careful temperature control and judicious use

of flux will help to ensure a good clean supply of metal for casting. A program of

preventive maintenance is advantageous for furnace covers. Linings, burner combustion

ratios, temperature controllers, and other similar areas.

- 1 -19


DIE CASTING TG-2

THEORY

CHAPTER: D2 POST CASTING

- 1 -

UNIT-4

CHAPTER – D2

POST CASTING

- 2 -19


DIE CASTING TG-2

THEORY


- 2 -

CHAPTER OUTLINE

D2.1 INTRODUCTION.

D2.2 PRINCIPLE OF METAL CASTING.

D2.3 (a) CLASSIFICATION OF POST CASTING TECHNIQUES.

D2.3 (b) TRIMMING.

D2.3(c) POST MOLDING.

D2.3 (d) SURFACE DECORATION.

D2.3 (e) COATING.

- 3 -19


DIE CASTING TG-2

THEORY


- 3 -

D2.1 INTRODUCTION

Often, it is possible, and even desirable, to design features into the die casting die

that will be reproduced on the cast shot to help a subsequent operation that must be

performed on the casting. These features must be planned early in the development of

the die design .Once a die design is hardened , or even nearly complete, there is a reluctance to make major changes to incorporate these post casting processing aids.

Yet, when planned early, these features are usually simple and can greatly improve the

efficiency of secondary Cooperation.

D2.2 PRINCIPLE OF METAL CASTING

Parting Line and Flash: -

The most obvious and is the control of flash to help the trimming operation. In many

instances, it is desirable to build the die so some flash will from along the trim line. Some flash will form along the trim line. Some flash will invariably occur even when dies are

built to shut-off the flash completely. At first, such flash will form as thin feathers. These

do not trim well. Usually thin feathery flash will be folded down by the trimming die. It

will not cut off cleanly. If the die is built to allow 0.010 to 0.020 in. ( 0.25 to 0.50 mm.) of flash, the casting can be trimmed efficiently with a trimming die . Controlled flash

thickness conditions are shown in Fig 11-1.

Tie- Ins: -

Tie –ins are small runners between overflows, cavities, and runners, These small runners are used to hold the cast shot together . They can be used to minimize bending

- 4 -19


DIE CASTING TG-2

THEORY


- 4 -

during ejection. Sometimes they are used in multiple cavity dies to brace one casting to

another to minimize warpage of the shot for accurate trimming . The four –cavity cast shot 18 in Fig 11-2 shows both controlled flash ( around each casting),and tie-ins

between each pair of castings.

The small runner like Tie-Ins is usually cut into die face with a ball end cutter. The depth is cut to less then half the width as shown in Fig 11-3a to insure draft. The half-round

shape is most common although other shapes can be used. The width of the Tie-In will

usually be about 0.25 in. (6.5mm).

Locators: -

Locating pins called locators can be cast into the die cast shot to help register

the casting in the tools of secondary operations. This author prefers the primary –secondary locating system shown in Fig. 11-4 When these locators are built properly

and used properly in the secondary tooling ( i.e trimming dies drilling fixtures ,

tapping fixtures, etc.), the secondary operation will be dimensionally precise and parts

loading and unloading will be easy.

The relationships between the various dimensions of the locator are shown in

Table 11-1 The locator engages sockets in the secondary tool as shown in Fig 11-5

for trimming die. Exact location is established in the trimming die through the sequence shown in Fig 11-6. The casting is placed on the lower part ( i.e. nest) of the trimming die

as shown in Fig 11-6. The shot can be placed on the die mechanically, but it is usually

done by hand. Normally, there are three or four locators on the cast shot. The locators are

placed in the funnel –shaped sockets as shown.

- 5 -19


DIE CASTING TG-2

THEORY


- 5 -

- 6 -19


DIE CASTING TG-2

THEORY


- 6 -

Dimension Typical Sizes Relationships

X 0.75 to 1.50 in. (20.0 to 40.0 mm).

Design for easy loading into DD in Fig11-5 of tool for secondary operation.

D 0.25 to 0.375 in. (6.0 to 10.0 mm.)

Select size to give suitable ejector pin diameter.

DD 0.625 to 1.125 in DD = d + X/2

( Fig 11-5 (16.0 to 30.0 mm.)

L 0.50 to 1.75 in. (12.0 to 45.0 mm.)

L > L' if X /2 does not fall on casting's side wall > L" at point X /2 on least draft portion ( A') of casting's

I 0.125 to 0.25 in. Designer 's choice

A ( 3.0 to 6.0 mm. A < A'

Sph. R 0.06 to 0.125 in. Designer 's choice (1.5 to 3.0 mm). D 0.375 to 0.625 in. D =2 [ Sph .R. (10.0 to 16.0

mm.) + tan A ( L - l)]

X' 0.125 to 0.250 in.

Designer 's choice

(3.0 to 6.0 mm.)

Fig 11.5 the locator cast onto an overflow as Fig 11.4, is hear shown (phantom), in

locating socket of the trimming die. The light die spring is moderately pre-loaded to insure

ejection of the locator as the die opens. The heavy die spring should be pre-loaded in excess of the compressed force (i.e. in the position shown) of the light die spring.

- 7 -19


DIE CASTING TG-2

THEORY


- 7 -

Fig 11-6. The locator shown in Fig 11-4 reaches its final located position as in Fig.

11.5 through the sequence shown here The ejector pin side ( diameter d.) is placed into

the funnel shaped pre-locating socket of the tool for the secondary operation ( a). In this example, the tool is a trimming die. Closes, the trimming steel contacts the spherical radius

( b) of the upper locator and pushes the lower locator ( diameter d) down the conical

surface of the lower percolating socket ( c). The dimensional relationships specified in

Table 11-1 insure that the cutting edge of the trimming steel will not touch the casting. Once the overflow contacts the top rim of the funnel – shaped pre-locating socket , the

upper locator will compares the light spring in the trimming steel. The hole in the

trimming steel engages the D diameter of the locator to insure exact positioning of the

casting to the trimming steel ( d). The final closing of the trimming die compresses the heavy spring below the funnel-shaped pre-locating socket to facilitate trimming as

shown in Fig 11-5.

- 8 -19


DIE CASTING TG-2

THEORY


- 8 -

Wing Nut casting: -

The small straightforward wing-nut die-casting shown at the top in Fig 11-9

illustrates some trimming considerations. If the surface finish is unimportant, the cheapest

treatment is barrel tumbling ( i.e. burnishing The as-cast hole can then be tapped without

clearing to size. The draft on the core will not allow a full-from thread at the outer end of the bore, but often that is not a disadvantage.

Assuming the component is to tumbled, the flash around the core should not be

close to the hole . Fragments of flash are likely to be bent over and impacted rather than broken away cleanly. A position on the flat end –face remote from the cored hole is also

unsatisfactory. If exceptionally heavy flash should form in that position, it may not be

bear upon a flat surface when in use . The most favorable position for the flash-line is

that indicated at ( a) in Fig 11-9. The flash is around the largest diameter, as shown in ( a) , it is more likely to be broken away cleanly by tumbling. The end-face formed by the core is

perfectly clean and flat.

If a better quality thread were required, clearing or reaming would be necessary to

size the bore. It would then be advantage to have the flash around the edge of the hole, as

at (b) in Fig 11-9 A shouldered cutting tool could be used to clear the bore and simultaneously shear away the flash. A bevel or radius may be added to the edge of the hole

at the same time. The cutter could extend to face the whole area of the base.

If the wing-nut were to be polished and plated die-trimming would be preferable to tumbling for flash removal. A trimming die will remove, at one pass. both the flash

around the main parting and that are the core intersection. If the flash around the core is

at the edge of the hole there is nothing to shear against during the first half of the stroke ,

consequently the edge of the hole is likely to break away-raggedly To avoid that problem, it is necessary to shoulder the core, having the flash about half-way between the

cored hole and the outside diameter of the wing –nut as at ( c), Fig 11-9 Solid metal

then supports the flash through out the trimming stoke.

- 9 -19


DIE CASTING TG-2

THEORY


- 9 -

Splined Cap: -

The splined cap shown in Fig 11-10a is another example of how finishing

operations can affect the die ( and even the part) design As shown at ( a) , the splines

end the same at both top and bottom. That necessitates a parting line at one end of the

splines. End the same at both top and bottom. That necessitates a parting line at one end of the splines. A modified design shown in ( b) is much more suitable for die casting . In (

b) of Fig 11-10 the outside diameter below the serrated portion is increased by more

that the depth of the serrations or splines.

For the first design , the radiuses portion of the wall would be formed in the

ejector die, and the parting surface would be at the bottom of the splined portion as

shown at ( a). Subsequent trimming is accomplished with a serrated shaving die. There can

be some difficulty preserving an accurate register between the two halves of the cavity.

The modified design, (b), also a radius along the outside bottom corner,

necessitating a raised parting line. However, since the part is around , it could be cast with a sharp square corner at the bottom as shown at ( c) The radius would then be cut on

the part and the flash removed with a turning operation as depicted in ( d). Machin ing

marks can be masked by wire brushing the surface immediately after machining.

Cast in Features VS. Secondary Operations: -

It is frequently necessary for the die caster to decide wheather some particular feature of a component shall be formed in the die or produced by subsequent machining.

The problem arises where small holes are required through thin sections. A choice must be

made between coring (which entails flash clearance), piercing or drilling. Sometimes the

issue is more obscure the provision of some feature in a die may be difficult , while its omission , necessitating subsequent machining, may be equally unsatisfactory . An

example is provided by the clip shown in Fig 11-16 which is required to have a 0.032- in.

(0.8-mm.) Slot between the two cored lugs when the faces are parallel. The internal bore

is also required to be reamed to remove the coring draft.

- 10 -19


DIE CASTING TG-2

THEORY


- 10 -

If the part was to be cast solid and the slot cut later with a slitting saw, the large mass of metal through which the saw must cut is likely to contain excessive

porosity However the lugs could be cast slightly open as shown in the right –hand

illustration of Fig 11-16.

The examples of cast or machine problem could go on indefinitely. These, however,

suffice to show the importance of a correct balance. The die casting die should form as

much of the part as possible. But, when the die becomes too complicated , or when its

operating cycle becomes uneconomically long, restrictive features can be formed by inexpensive operations on the casting.

Trimming Handles : -

Often the trimming operation can be made much more efficient and safe by

casting a handle onto each shot. The handle is usually a flat runner-like feature extending

outward from the sprue or biscuit. The trimming operator can grasp this handle and use it to place the shot into the trimming die without exposing his hand to pinch areas in the die.

Summary: -

The design of the die casting die can be made to help operations that are performed to the casting after it is die cast. Placement of the parting line and flash, tie-ins,

locators, cavity and run identification, base or work lines and insert detail can effect the

secondary operations.

D2.3 CLASSIFICATION OF POST CASTING TECHNIQUES

The post casting operation of die casting dies can be classified as follows: - (a)Trimming

(b)Post Machining

(c)Surface Decoration

(d)Coating

- 11 -19


DIE CASTING TG-2

THEORY


- 11 -

These operations are done to give the final shape to die casting components.

D2.3 (a) TRIMMING

Present day automatic diecasting equipment provides a product, which more than ever before, can be described as the shortest distance between raw material and finished

product. Ten year ago, the runner sprue and flash of a diecasting were often removed

manually or by separate clipping processes. Nowadays these are included in the machine

cycle time, where a robot or extractor device removes the casting and sprue from the die. Places it in a quench tank, removes it after a predetermined cooling period and then locates

it in a clipping press., after clipping the casting is deburred or placed in a transfer line or

machining centre for drilling, tapping or other secondary operation and is ready for

consignment to the customer or for surface treatment processes.

During the post decade there has been an ever-increasing insistence by the

diecasting company undertakes all the machining operations, so that the product goes straight to the assembly line.

Runner and overflow removal: -

The scientific developments in runner and overflow design which have been

introduced in the past decade have led to runner systems being substantially thinner and

lighter than those used before. The shape and thickness of the gate and the section thickness

from casting to overflow often determines how the surplus metal must be removed and whether it will be a fast process or a troublesome one.

Breaking off needs to be done carefully, whether by robot, extractor or manually.

Simple fixtures can assist breaking off and often there is only one way in which this can be done. Where the gate mouth is in one half of the casting, as shown in Fig 26-1, the

operation should be carried out in one direction only to avoid ‘Breaking-in’, which

frequently creates an extra fettling or banding operation to restore the proper contours of the

casting, consequently the gate mouth is often shaped to avoid this effect. With equal areas of the gate mouth in each die half, as shown in Fig 26-2, ‘breaking-in’ is not normally

encountered, because the draft angles exercises control over both broken edges. Where the

gate mouth need to follow the contour of a circular casting, as shown in Fig 26-3, it is often beneficial to design the gate mouth and runner as shown in Fig 26-4, to keep the witness at

either end as small as possible. Failure to appreciate removes the gate witness. When a

casting extractor of the robot type can be programmed to carry out the breaking-off

operation, large time and labour savings can be made, provided there is effective separation of castings from sprues, so that damage is avoided and the sprues can then be controlled

and carried away for re-melting.

- 12 -19


DIE CASTING TG-2

THEORY


- 12 -

Deburring operations: -

The oldest and simplest form of deburring plant consists of a steel barrel mounted

on a drive shaft. Many of these, still in operations, are used for the separation of small

castings from sprues. Several versatile machines, developed from the original barrels,

remove sharp edges or flash from diecasting or prepare surfaces for subsequent painting or plating.

During deburring controlled metal removal is required, usually along the parting

line of the casting, where flash has occurred or where small upstanding parts of the gate require removal. Vibratory barrels are wither circular or straight but the principles involved

in both types are the same. A container, lined with rubber or neoprene, contains ‘media’;

- 13 -19


DIE CASTING TG-2

THEORY


- 13 -

the castings to be duburred are loaded into the container; water, containing a detergent and

other compounds is circulated thorough the mass of castings and media, then the whole is subjected to vibration. Light deburring can be done on diecasting for furniture and cabinet

hardware by vibratory finishing with steel media; enormous tonnages of zinc alloy parts are

treated by such techniques. Ceramic products of natural and synthetic stone, fused

aluminum oxide and related materials are often used when a strong cutting action is required.

D2.3 (b) POST MACHININIG The alloy used for pressure die casting die are free machining and comparatively

soft. For turning, milling spot, facing and boring, tungsten carbide tools are used to grate a advantage, the especially with aluminium-silicon alloy which have associate problems of

wear. High speed, high rake angles and low feed give the best results. Since castings are

produced closed to finish size, they amount of metal to be removed is generally very small.

In the machining of zinc and aluminium alloys there is a tendency fore the metal being cut to be build up on the cutting action. This may be minimized by using the correct cutting

angles, setting the tool correctly and lapping the cutting edges to fine finish to e4ncourage

the sheared metal to flow away from the cutting edge. The choice of lubricant will also play a major role in this problem.

Drilling: - The possibility of drilling holes instead of coring them should be kept regularly

under review.The increasing cost of labour, the need to obtain maximum utilization of diecasting machines, the coast of die maintenance compared with the extra raw material

cost of a die casting which requires machining, all must be taken into account. Drilling

holes, therefore, is an important secondary operation in any die-casting plan and the

collection of example shown in Fig 26-16 is only a small cross-section of the types of castings that require drilling.

Satisfactory results in drilling both zinc and aluminium alloy castings are easily

achieved using high-speed steel drills.

In drilling aluminium alloys high rates of speed and penetration can be used, hence disposal of swarf is very important.

- 14 -19


DIE CASTING TG-2

THEORY


- 14 -

Zinc alloy readily clogs the drill flutes and, because of the relatively low melting point, the heat generated by cutting operation can result in zinc alloy softening sufficiently

to welt to the flutes. To avoid these problems, drills with wide polished flutes should be

used and the drill should be break out of the hole to relive the swarf. High lip relief, longer

drill point angles, very thin webs at the point and narrow margins should be used, particularly on deep holes.

Magnesium alloys require fairly high drilling speeds and heavy feed rates. Because

of the high penetration rates, drills are required with ample chip space and equipment must have provision for efficient chip disposal.

The recent development in CNC equipment have given a new look to a vide range of industries for automated drilling, tapping and light milling. Fig 26.19 shows a CNC drill

developed in Britain. It has a unique cubic table with a facility to provide component

unloading, loading and swarf debris elimination. The three axis drill head allow the

component to remain stationary during machining and eliminates the need for any additional tool changing system. With transfer speeds of about 20 meter per minute, the

cubotic drill head is able to service a stationary and accessible 15-state tool loft. After

component loading, work changeover is speedy, with the cube indexing thorough 90

degrees in about four seconds.

- 15 -19


DIE CASTING TG-2

THEORY


- 15 -

D2.3 (c) SURFACE DECORATION

When zinc alloy diecastings require a decorative corrosion –resistant or

functional finish it is usual to electroplate the surface with a metal coating giving the

required a decorative corrosion-resistant or functional finish it is usual to electroplate the surface with a metal coating giving the required properties to withstand the

conditions under which it is to be used in service. Electroplating systems developed

over the past decade are available to satisfy the requirements of all present –day

standards Satisfactory performances are, however are, However, only obtainable when attention is paid to the following factors.

1. The diecasting should be of sound design with sympathetic consideration for the

case of polishing /vibrating and electroplating. 2. The diecasting should be produced using only high purity alloy. They should be

metallurgic ally sound .free from surface porosity and general imperfections, and

process the so-called hard wear surface finish. 3. The die-casting should be prepaid and electro plated w2ith an approved process, the

choice of thickness being adequate for the enviournment in which it is to be used.

D2.3 (d) COATING

Electroplating: -

When zinc alloy diecasting require a decorative corrosion-resistant or functional

finish it is usual to electroplate the surface with a metal coating giving the required

properties to withstand the conditions under which it is to be used in service. Electroplating

system developed over the past decade is available to satisfy the requirements of all present-day standards. Satisfactory performances are, however, only obtainable when

attention is paid to the following factors.

1. The die-castings should be of sound design with sympathetic consideration for the ease of polishing/vibrating and electroplating.

2. The die-castings should be produced using only high purity alloy. They should be

metallurgical sound, free from surface porosity and general imperfection, and

process the so-called ‘hard wear’ surface finish. 3. The die-castings should be prepared and electroplated with an approved process, the


Relevant specifications are British standard 1224 1970. International standard 1456

1974 and US specification ASTM B456-1979.

Design for polishing/vibrating: -

Prolonged experience in design requirement has now eliminated the uneasy working

relationship between die casters and the metal finishing industry that was due to poor

communication and misunderstanding of each other’s problems and requirements. It is not

the intention of the metal finisher to stifle the creative ability of any stylist or designer, as

- 16 -19


DIE CASTING TG-2

THEORY


- 16 -

without their contributions to industry products would soon become drab and uninteresting.

However, good design enables trimming, pretreatment and electroplating to be carried out with minimum time, labour and materials. Poor designing can make a component difficult

to polish to a suitable standard, and virtually impossible to electroplate except at a

prohibitive cost. It is therefore recommended that the designer should have a working

knowledge of the finishing processes he specifies, thus avoiding time consuming and costly errors, minimizing production costs, maximizing output and ensuring good service life for

the finished product.

After clipping, parting lines are usually given and initial smoothing operation, usually by mechanical polishing, with a lightly abrasive-coated mop or belt, or an initial

finishing operation in a tumbling or vibratory barrel using abrasive media. Articles should

be of as simple shape and contour as is practical; intricate designs tend to become, indistinct

and blurred, with sharp edges and projections becoming rounded.

Large flat areas should be lightly broken up with a simple design, or the surface

made slightly concave or convex. Flat surfaces are difficult to polish or vibrate to achieve a

uniform finish, and are vulnerable to scratching during subsequent operations, which may well result in the rejection on final inspection. Components for vibratory finishing should

be robust enough to withstand the severe mechanical action, and designed not to interlock.

Design for electroplating: -

After a suitable cleaning operation, the components to be electroplated are connected to the cathode or negative electrode of a low voltage de supply. The positive

electrode or anode is usually of the metal to be deposited, or in special circumstances, an

insoluble metal may be used. When the current is switched on, the metal ions flow to the

cathode and are deposited as metal.

Current flows between anode and cathode and, as the ions prefer to take the shortest

path, far more are attracted to edges and projections, proportionally less into recessed. In

most electroplating systems, therefore (these include nickel and chromium), an uneven thickness of deposit is produced, the distribution being dependent on the plating solution,

the method of jigging and the current density. The protective value of any coating depends

on the minimum thickness, as the strength of chain depends on the articles being

electroplated (the more complicated the shape), the greater the thickness of electro deposit which needs to be applied in order to obtain minimum thickness requirements. Sharp

changes in contour should be avoided, as projections will attract excess thickness may well

be impossible to achieve adequate coating thickness and premature failure will occur in

service. Fig 29.1, reproduced by permission of the American Zinc Institute, illustrates how designs can be improved to achieve better palatability.

- 17 -19


DIE CASTING TG-2

THEORY


- 17 -

- 18 -19


DIE CASTING TG-2

THEORY


- 18 -

Surface finishing prior to electroplating: -

Well-designed dies, regularly maintained with a mirror-bright surface, will result in

substantial savings during finishing by reducing the time required for th initial mechanical

operation and significantly improve the finish achieved after electroplating. The gate and

vents should be as thin as practical and the trimming operations carried out as close to the main casting as possible. Due to the relative ease by which zinc alloy diecastings can be

prepared for electroplating, the overall cost of this surface finishing operation prior to

electroplating was estimated to be around 30% of the whole finishing operation when done

by automatic machine polishing. By adopting the now widely-used Vibratory Mass Finishing Techniques, the above figure can be substantially reduced. Components can be

handled in bulk and neither individual handling nor fixing of the parts in require. Consistent

results can be achieved and, although there is some loss of reflectivity, the work is

acceptable for electroplating.

Chromate conversion coatings on aluminum diecastings: - Where a coating of lower protective value is required or where it is necessary to

increase the durability of subsequent organic coating, a simple immersion chromate coating

can be applied. Most chromate conversion coatings are of the proprietary type, generally

based on two formulations:

Type 1. – based on Acid Chromate

Type 2. – based on Chromate-Phosphate

ASTM B440-1972 gives the following information on class, colour, and use film

weight.

Class Color Use Coating weight

1 Yellow to brown For maximum corrosion

resistance, usually unpainted.

3.2-1.1 mg/dm2

2 Iridescent yellow General purpose good base for paint.

1.1-3.8 mg/dm2

3 Colorless to yellow Decorative applications, low

electrical contact resistance, can be used as base for paint.

Below 1.1 mg/dm2

Acid chromate.

Proprietary mixtures are based on acid chromates, fluorides, boric acid and ferricyanide or other activators. Application by immersion spray, concentration,

temperature and immersion times can b varied to achieve coatings from colourless through

yellow to golden brown. The more intense the colour the better the corrosion-resistance.

- 19 -19


DIE CASTING TG-2

THEORY


- 19 -

Coating weights vary from 1.0-1.2 mg/dm2. Films can be dried at relatively high

temperature around 90 Centigrade.

Chromate phosphate.

Also of proprietary nature, the process is usually based on a complex mixture of phosphate and chromic acid together with fluorides, applications is by spray or immersion.

Concentration temperature and immersion times can be varied to achieve films ranging

from light iridescent to a heavy opaque green; coating weights vary from 5-30 mg/dm2. A

number of process recommend a final swill with a weak chromic acid dip to improve the corrosion performance prior to drying at a temperature which should not exceed 70

centigrade.

Electroplating on magnesium alloy die-castings.

Due to their chemical activity and their affinity for oxygen, which results in rapid

formation of tenacious oxide film on surface, magnesium alloys are not easy to electroplate. Like aluminium, their strength-weight ratio is attractive and electroplating is used to

improve surface appearance, increase corrosion resistance, wear resistance and electrical

conductivity. The general procedure for pretreatment is similar to the employed for the

electroplating of aluminum, ie. To remove the oxide film and prevent it from reforming by the deposition of an immersion deposit prior to conventional electroplating.

- 1 -19


DIE CASTING TG-2

THEORY


- 1 -

UNIT-4

CHAPTER – D2

POST CASTING