UNIT I METAL CASTING

PROCESSES

Sand casting – Sand moulds - Type of patterns – Pattern materials – Pattern

allowances – Types of Moulding sand – Properties – Core making – Methods of

Sand testing – Moulding machines – Types of moulding machines - Melting

furnaces –Working principle of Special casting processes – Shell, investment

casting – Ceramic mould – Lost Wax process – Pressure die casting –

Centrifugal casting – CO2 process – Defects in Casting – Inspection methods

Introduction to Manufacturing Processes

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Definition of Manufacturing

The word manufacturing is derived from Latin:

“manus = hand”, “factus = made”

Manufacturing is the economic term for making goods and services available to satisfy

human wants.

Men’s Material Welfare (MMW) in all respects covering housing, clothing, medicine,

education, transport, communication and also entertainment.

– The successful creation of men’s material welfare (MMW) depends mainly on

– Availability of natural resources (NR)

– Exertion of human effort (HE); both physical and mental

– Development and use of power tools and machines (Tools),

The materials are then shaped and formed into different useful components through

different manufacturing processes to fulfil the needs of day-to-day work. Manufacturing

implies creating value to a raw material by applying useful mental and physical labour.

Manufacturing ProcessesManufacturing processes is a very fundamental subject since it is of interest not only to

mechanical engineers but also to engineers from other discipline of engineering.

There are various manufacturing processes by which a product can be made.

Each process however has its own limitation and restriction and due to this reason a

particular process is adopted to certain specific applications.

Thus while a product can be manufactured by two or more processes, the real problem

is to select the most economical out of them.

A detailed understanding of various manufacturing processes is thus very essential for

every engineer. This helps in designing the proper product required for him.

He would be able to assess the feasibility of manufacturing from his designs.

Proper choice of the process which would require lowest manufacturing cost.

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CLASSIFICATION OF MANUFACTURING PROCESSES

Manufacturing processes can be grouped as:

Casting, foundry or moulding processes.

Forming or metal working processes.

Machining (metal removal) processes.

Joining and assembly

Surface treatments (finishing).

Heat treating

These groups are not mutually exclusive. For example, some finishing

processes involve a small amount of metal removal or metal forming. A

laser can be used for joining/metal removal/heat treating.

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5

CLASSIFICATION OF MANUFACTURING PROCESSES

Casting, foundry or moulding processes

• Sand casting

• Investment casting

• Die casting

• Centrifugal Casting

• Continuous Casting

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Forming or metal working processes

• Rolling

• Forging

• Extrusion

• Drawing

• Sheet metal works

Joining processes

• Welding (SMAW, TIG, MIG, PLASMA, LBW, EBW etc.)

• Soldering

• Brazing

• Adhesive bonding

• Riveting

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Conventional Machining processes

• Turning

• Milling

• Drilling

• Shaping

• Grinding

• Broaching

Nonconventional Machining processes

• Electro chemical Machining (ECM)

• Electro Discharge Machining (EDM)

• Wire Electro Discharge Machining(WEDM)

• Abrasive Jet Machining (AJM)

• Ultrasonic Machining (USM)

• Liquid Jet Machining (LJM)

• Electron Beam Machining (EBM)

• Laser Beam Machining (LBM)

• Ion Beam Machining (IBM)

• Plasma Arc Machining (PAM)

Manufacturing Processes and

Manufacturing system• Manufacturing system:

• A collection of operations and processes used to obtain a desired

product(s) or component(s) is called a manufacturing system.

• The manufacturing system is therefore the design or arrangement

of the manufacturing processes..

• Production system:

• A production system includes people, money, equipment, materials

and supplies, markets, management and the manufacturing system

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Production System

9

Raw materials Manufacturing Process

Manufacturing Process

Finished product

Manufacturing System

People, Money, Equipment, Materials and Supplies, Markets,

Management

DEFINITION OF MANUFACTURINIG

INPUT OUTPUTCONVERSION

TECHNOLOGY - Means

• Application of science to provide society those things

that are needed or desired

• Technology help to live better

• In the broadest sense it is a of converting raw material

into useful product

BASIC CLASSIFICATION OF MANUFACTURING PROCESS

BASIC MANUFACTURING

PROCESS

METAL

CASTING

METAL

FORMING

METAL

MACHINING

FABRICATION

TECHNOLOGY

TYPICAL COMPONENTS MADE BY VARIOUS

MANUFACTURING

METAL MACHININGMETAL CASTING

METAL FORMING FABRICATION TECHNOLOGY

HISTORICAL DEVELOPMENT OF CASTING

• Earliest casting : high bronze statue

– Mohango daro Civilizaation -3000-3500 BC

• Harappan Civilization contained- Kiln (oven ) for melting , copper , gold

• Fine art , hand craft-idols of made by chola dynasty – of 9th century-

technology for making -intricate casting

• Casting tech transferred from India to middle east to Europe through

Portuguese in 14th century

• First cast iron tripot object weighing 270kg by Chinese technology-600BC

• Iron pillar in Delhi -QUTIPMINAR weighs 6 tones speaks about the

Casting Technology in the earliest Indian Civilization during Chandragupta

dynasty that dates back 5AD

• Colossal statue of Buddha – 1252AD in Jaipur speaks about Marvels of

Casting Technology existed in India

Casting

Refractory mold pour liquid metal solidify, remove finish

It is the process of making metal parts, by pouring molten metal into

the cavity of the required shape and allowing it to solidify.

OR

Casting may be defined as ‘metal shaping obtained by the

solidification of molten alloy in the mould cavity of a required

shape’.

Why casting preferred?

Size- Few grams to several ton

Shape – Simple to complex

Production Quantity – One number to

multiple

Accuracy – Close dimensional accuracy

with special casting features

Finish- Rough to fine

REQUIREMENTS OF CASTING PROCESS

CASTING PROCESS SEQUENCE

– PATTERN DEVELOPMENT

– MOULD PREPARTION

– MELTING & POURING

– FETLING & CLEANING

Sequence of mould making

Sequence of Casting operation

SEQUENCE OF MAKING CASTING

Cleaning

Machining

Major applications

Transport- Automobile, aerospace, railways, shipping,

space application

Heavy Equipment – Farming, mining, construction

Defence- vehicles, artillery

Electrical machine- Motor casing, pump base,

compressor

Municipal castings- pipe fittings, manhole cover,

sewage pipe, sanitary fittings

Home appliance- kitchen ware, furniture

Handi craft- sculptures, jewelry , idols, lamp stand,

decorative items.

TYPICAL CASTING

COMPONENTS

TYPICAL CAST COMPONENTS

COMMON CAST METALS

METAL USAGE TYPICAL APPLICATION

Grey Iron 54% Engine Block, brake drum,

Machine tool bed, housing

Ductile Iron 20% Crank shaft, Camshaft, Valves,

brackets

Aluminium 12% Piston, Oil& fuel pump, Clutch,

housing, Carburetor

Steel 9% Machine parts, Gears

Copper base 2% Hydraulic Pump parts, Impellers

Zinc base 1% Handle, Grills, Toys

ADVANTAGES & LIMITATIONS OF CASTINGS

ADVANTAGES LIMITATIONS

Intricate shape both external

internal can be cast on single

piece

Poor finish of sand cast parts

Hollow & large parts be made Sand casting labour intensive

Any metals which can be melted

– Ferrous & Non Ferrous metal

Cast parts bulky & brittle

Uniform cooling, better

properties & damping capacity

Some casting require machining

to get fine finish

No limitation of size, shape &

product complexity

Surface defects, porosity, cracks

on casting,

SAND CASTING OR SAND MOULD CASTING

Permanent or Removable pattern process

• The pattern is removed from the mould cavity,

before the molten metal is poured into the

mould cavity.

Expendable or Disposable pattern process

• The pattern is not removed from the mould

cavity, before the molten metal is poured into

the mould cavity.

Pattern : It is the replica or facsimile of the product to be

made in respect of shape.

Generally used in making Sand mould

Number of casting to be produced

Size and complexity of the shape of casting

Type of moulding method.

PATTERN MATERIAL

Wood

Metal

Plastic

Wax

Thermocole

ADVANTAGES & LIMITATIONS OF DIFFERENT PATTERN MATERIALS

MATERIAL ADVANTAGES LIMITATIONS

WOOD Light weight & inexpensive

Good workability

Easy to repair

Inherently non uniform structure

Poor wear & abrasive resistance

Absorbs moisture tends to warp

METAL More durable & accurate

Provide smooth finish

Do not deform on storage

Wear, abrasion, corrosion

resistance

Relatively expensive than wood

Not easily repaired

Heavier than wooden pattern

Ferrous metal gets rusted

PLASTICS More economical in cost

High resistance to

corrosion

Lighter than wood/metal

Moulding sand stick less in

plastics than wood

No moisture absorption

strong & dimensionally stable

Tends to bend on heavy

ramming force

Additional equipment is needed

for making pattern

Difficult to repair or rework

EXHAUST MANIFOLD

WOODERN PATTERN

ALUMINIUM PATTERN

TYPES OF PATTERN

TYPES OF PATTERN

TYPE ADVANTAGES DISADVANTAGES SKETCH

One piece

or Solid

Pattern or

loose

Pattern made of one

piece

Simple in shape

Generally made of

wood

In expensive

Low quantity of

production

Determining the

location of parting line

between the two halves

of the mould

Split or

Parted

pattern

Two piece pattern

Complicated shape

can be produced

TYPES OF PATTERN

TYPE ADVANTAGES DISADVANTAGES SKETCH

Match

plateHigher production

Split pattern having

cope and drag

Holes in the plate

allow the top and

bottom sections of the

mould to be aligned

accurately

The gate and runners

are also mounted on the

match plate but small

manual work is required

TYPES OF PATTERN

TYPE ADVANTAGES SKETCH

Cope and

drag

pattern

Separate match plates

One operator working

on the cope part of the

mould and the other

operator working on the

drag part.

TYPES OF PATTERN

TYPE DEFINITION ADVANTAGES

Sweep

pattern or Pit

or Floor

moulding

Sweep is a section of proper

contour that is rotated about one

edge to shape mould cavities

having shapes of rotational

symmetry.

large circular moulds of

symmetric kind by revolving a

sweep attached to a spindle

Casting of large size is to be

produced in short time.

SKETCH

TYPES OF PATTERN

TYPE ADVANTAGES

Skeleton

PatternLarge casting having simple

geometrical shape.

SKETCH

TYPES OF PATTERN

TYPE ADVANTAGES

Gated PatternOne or more loose pattern

attached gates and runners

Pattern are used for producing

small casting in mass production

system and on moulding machine

SKETCH

TYPES OF PATTERN

TYPE PRINCIPLE

Follow board

pattern:A follow board is not a pattern

but is a device (wooden board)

used for various purposes

SKETCH

PATTERN ALLOWANCE

• To compensate for any dimensional and structural changes

which will happen during the casting or patterning process,

allowances are usually made in the pattern

TYPES OF

ALLOWANCE

PURPOSE

1.SHRINKAGE

ALLOWANCE

To compensate for volumetric contraction

during freezing of metal, this allowance is added

Shrinkage though volumetric, allowance

linearly expressed as % of mass or length

Pattern made larger applying shrink rule

depending metal / product

TYPES OF ALLOWANCE PURPOSE

2. DRAFT ALLOWANCE The allowance which facilitate easy withdrawal

of pattern from mould walls

Slight taper or draft is provided to vertical side

of mould walls all around

Drat allowance is additive: 1 – 1.5 % of length

TYPES OF

ALLOWANCE

PURPOSE Diagram

3.MACHINING

ALLOWANCE

To remove rough surface exhibited in

casting especially sand casting by

subsequent machining

Allowance is added to pattern to

provide compensation during machining

Depends on finish requirement,

product complexity and type of material

to be cast

4.DISTORATION Thin section or abnormal change in

size of the product restrained in mould

which tend to distort upon cooling

U or V shaped section tends to distort

more while cooling in mould

1-2º is given as allowance inwardly

TYPES OF

ALLOWANCE

PURPOSE

5.SHAKE or

RAPPING

Cavity enlarge while pattern is withdrawn

due to tapping

Cavity made smaller. tapping of pattern

results original dimension – It is a subtractive

allowance

Added to parting surface or parallel to

parting surface

Size of the final pattern

MOULDING SAND

Aggregate of sand (Moist) , clay and additives

Moulding Sand Consists of

1.Sand – Silicon oxide, Zirconium, olivine, chromites

Silica sand – upto 2% clay

Weak sand – upto 10% clay

Strong sand – upto 30% clay

2. Clay

Bentonite (Formed on ignous rock – creamy white powder – Temp 1300oC)

Kalonite (Decomposition and slow weathering of granite – China fire clay – Temp

1700oC)

3. Additives- organic additives

Sea coal – By add 8% to increase strength of mould

Cereals – By add 2% to increase the green and dry strength mould

Saw dust – By add 6% to increases the gas permeability and deformability

Silica Flour – Improve the surface finish of the casting

MOULDING SAND REQUIREMENTS

Good refractiveness - Ability to with stand high temperature without

fusing or charing

Good permeability- Ability of the sand to permit gases to escape through

sand mould. Hard ramming - poor permeability

Chosiveness- (good bonding of grain size of silica – ability of mould

sand to retain the shape when packed into mould without crack

Collapsibility- Ability of moulding sand to disintegrate from casting

easily . Improved by the addition of material like cellulose

Strength- Retain the mould shape under molten metal pressure (Hydro

stastic) - Must resist penetration of molten metal into mould . –

(Green strength (Moist) ; Dry Strength)

Types of Moulding sand

Green sand

Dry sand

Loam sand

CO2 mould sand

TYPES OF MOULDING SAND

TYPE ADVANTAGES DISADVANTAGES APPLICATION

Green

sand

mould

Silica

Sand 18-

30% Clay

and 6-8%

moisture

Least expensive

Less distortion than

in dry sand mould

It is weak sand and

can't store for a longer

period

Erosion of mould is

more common –

production of large

castings

Manufacturing of

large and heavy

casting

Dry sand

mouldStronger than green

sand mould

Surface finish of

casting is better

Overall dimensional

accuracy is better

Distortion is greater

than for green sand

moulds because of

baking

Production rate is

slower than for green

sans mould

TYPE ADVANTAGES DISADVANTAGES APPLICATION

Loam sand mould

(Sand + fine

refractories +

graphite + clay

+fibrous

reinforcement)

Clay 30-50% and

18% water

Making heavy

and large

components

Engine body,

Machine bed

etc.

large grey iron

castings.

CO2 sand mould

(Sodium silicate

+water + binder +

CO2 (instead of

clay) CO2

mould sand (with

in 1 min)

Sands are free

flowing ramming is

eliminated.

Tensile strength

of mould is

increased

Production rate is

high

Sand must be used

immediately

Aluminum

cylinder

Impeller

Core

A core is a body made of refractory materialwhich is set in to the prepared mould beforeclosing and pouring it, for forming through holes,recesses, projections and internal cavities.

1. Conventional MethodHand curing method Core blowing machine Extrusion machine

2. Hot core mould box method3. Synthetic resin cold curing core sand method4.Cold curing CO2 process

MOULD & CORE ASSEMBLY

CASTING MOULD &

CORE

ASSEMBLY

CHAPLET

Purpose of core

To produce hallow casting with the use of core/ core print

MOULD SAND TESTING

Purpose:

To ensure consistent quality of moulding sand in respect of their properties

TYPES OF TEST

Grain size: Seiving dry sand downward through a set of parallel

standard seives of decreasing mesh size - Sand in weighed after seving .

Grain size is expressed in grain fines number.

Moisture content: 50g sample sand specimen is heated to 100ºC and

difference in weight is estimated and expressed as % moisture.

Clay constant: 50g moulding sand is washed with sodium hydroxide

solution. & weighed. Difference is expressed as percentage of clay.

Permeability test: Standard test specimen placed in permeability meter

and flow of air is maintained. Pressure drop is determined. Pressure

drop indicate measure of permeability.

Hardness strength: Using standard specimen of moulding sand in

Hardness tester , strength is evaluated in wet & dry stage to evaluate

green and dry strength

TYPES OF MOULDING PROCESS

Hand moulding

Machine moulding

1. Jolt machine

2. Jolt & square

3. Slinger

TYPES OF MOULDING PROCESS

Hand moulding

Machine Moulding

1. Jolt machine

2. Jolt & squeeze

3. Slinger

ADVANTAGES OF MACHINE MOULDING

Offers high production rates in making mould

Produces uniform and quality casting

Lesser labour cost & less skilled labour can be employed

Establishes most suitable gating system- runner , riser etc mounted on match

plate pattern itself

Pattern offers longer life

Lesser machining allowance are required

Greater accuracy is secured in core assembly

LIMITATIONS

Moulding large and complex and intricate shape are difficult

Moulding Machine is not flexible as sand moulding

Expensive equipment

Not suitable for small production

JOLT MACHINE and JOLT & SQUEEZE

ADVANTAGE

Jolting offers better consolidation of moulding sand with uniform properties

Higher production rate in making sand moulds

Large size casting economically produce by this method

Lesser cost of mould making

• LIMITATION

• Variation of sand density from bottom to top the moulding box

• Jolting produces noise pollution

• Doesn't ram properly at deep pocket and narrow places between vertical surfaces

JOLT & SQUEEZE MACHINE

SAND SLINGER

• Sand slinger consists of impeller rotating at desire rotation speed.

• Moulding sand from the hopper is feed into the gravity

• Rotating Impeller throw the moulding sand downward into moulding cost

as contiously

• The Rotating impeller to the moulding sand create necessary force that fills

the moulding box with sand.

• Produces uniform density packing through out the mould

ADVANTAGES OF SAND SLINGER

Mould filling and ramming are done at the same time

Mould hardness can be controlled by regulating by impeller speed

Delivers large quantity of sand that is suitable for ramming big moulds

Irrespective of the size and shape of the mould , mould produce are of uniform

density.

Total automation of sand handling can be done

LIMITATIONS

Expensive equipment

Skilled operator to regulate the machine

Not suitable for small volume production

RUNNER

Ingate for feeding molten metal into cavity

Suitably designed to admit the metal easily & permit filling of mould

cavity

For large size casting , more ingate may be provided

RISER

Passage through which metal rises to ensure the molten metal in filled

fully

Provides feeder head to compensate for shrink

PURPOSE

To feed the molten metal to solidifying casting so that shrink cavities

are avoid

To ensure proper mould filling

To permit gas, vapour to scrange to atmosphere

MELTING PRACTICES

• Melting is an equally important parameter for obtaining a quality castings.

• A number of furnaces can be used for melting the metal, to be used, to

make a metal casting.

• The choice of furnace depends on the type of metal to be melted.

TYPES OF FURNACES

• Crucible furnaces

• Cupola

• Induction furnace

• Reverberatory furnace

CRUCIBLE FURNACE

• Small capacity furnace typically used for small melting applications.

• Suitable for the batch type intermittent production and suited for small

foundaries

• The metal is placed in a crucible which is made of clay and graphite.

• The energy is applied indirectly to the metal by heating the crucible by fuel

like coke, oil or gas.

• (i) Pit type furnace.

• (ii) Tilting type furnace.

COKE FIRED CRUCIBLE

FURNACE

Pit type crucible furnace:

• These are fixed wholly or party in the ground from which the crucible must

be lifted when the metal is ready.

• The furnace is usually fired with sufficient coke being packed round and

above the crucible pots to melt and superheat the charge without re-coking.

Tilting type crucible furnace

• This type of furnace is built above the ground level, and contains a firmly fixed crucible.

• The furnace is fired with coke, oil or gas and the forced draught is used.

• When the metal charge is ready for pouring, the whole furnace is tilted and the crucible

emptied by operating a geared trunnion.

• For the metals of high melting points, clay or plumbago crucibles are used; for the low-

melting-point metals, such as zinc-base or aluminium, cast iron or steel crucibles are suitable

Reverberatory or Air Furnace

• This is used for melting in one heat large quantities of metal, those most suited being all

grades of cast-iron and the alloys of brasses and bronzes.

• It may have either a sloping roof, or a double arched roof which forms a dip in the centre.

• A chimney is provided at one end and a fire grate or burners at the other end. A hearth

or well is provided in the centre for holding the metal.

• It employs natural draught, which is controlled by dampers.

• The fuel can be either small lumpy coal, which is used on the fire grate, or powdered

fuel, which is supplied through burners. The object is to create a long flame which

reverberates or strikes back from the furnace roof on to the metal to be melted in the

hearth.

Electric Furnace

• This is used especially where rigid control over temperature and analysis is required. It is

suitable for all types of metals and alloys.

(i) Direct-arc furnace :

• It consists of a round, bowl-shaped carbon hearth with a domeshaped roof supporting one or more

carbon electrodes through which passes the current which strikes arcs with the metal in the

hearth, thus giving heat direct to the metal.

• This type of furnace can be either stationary or made to tilt. The roof is usually so made that it

can be removed for charging purposes.

• The capacity of these furnaces for production work varies from 3 to 10 tonnes. These are best

suited for laboratory work where very small quantity of a few kg is needed for research work.

• These furnaces give high melting rate, high pouring temperature and excellent-control of metal

analysis and temperature.

(ii) Indirect-arc furnace:

• This furnace is used for melting all types of metallic alloys but especially useful in

production of copper-base alloys.

• It consists of a horizontal cylinder lined with a refractory material with two electrode on the

horizontal axis. An arc is struck between the electrodes in the centre of the furnace. The arc does

not come in contact with the metal to be melted, the heat given to the charge by radiation from the

arc and reflection from the walls of the furnace. The furnace is designed to give a rocking motion

as the melting proceeds, that quickening up the melt by distributing the heat more rapidly. The

charging, tapping and slagging are done through an opening in the side of the furnace.

Induction furnace

Induction furnace:

• An induction furnace is a tilting furnace used chiefly for the melting of

nonferrous metals. Heat is generated by the resistance offered to an induced

current set up with the metal in the furnace.

• The design of the furnace is such that a small channel is formed inside at its

base; this channel is filled with metal, which should never be allowed to

solidify owing to the amount of damage it would do to the lining.

• When working, an alternating current is supplied to the primary coil of a

transformer , which is built with the furnace.

CUPOLA• Cupola furnace is employed for melting scrap metal

or pig iron for production of various cast irons

• Tall, cylindrical furnace used to melt iron and ferrous

alloys.

• Alternating layers of metal and ferrous alloys, coke,

and limestone are fed into the furnace from the top.

• Cupola is lined with refractory bricks

• The molten metal flows out of a spout at the bottom

of the cupola. .

SCHEMATIC DIAGRAM OF CUPOLA

DESCRIPTION OF CUPOLA

• The cupola consists of a vertical cylindrical steel sheet and lined inside with

acid refractory bricks.

• The lining is generally thicker in the lower portion of the cupola as the

temperature are higher than in upper portion

• There is a charging door through which coke, pig iron, steel scrap and flux

is charged

• The air blast is blown through the tuyeres

• These tuyeres are arranged in one or more row around the periphery of

cupola

• Hot gases which ascends from the bottom (combustion zone) preheats the

iron in the preheating zone

• Cupolas are provided with a drop bottom door through which waste

materials consisting of coke, slag etc. can be taken out after melt

• A slag hole is provided to remove the slag from the melt

• Through the tap hole molten metal is poured into the ladle

• At the top conical cap called the spark arrest is provided to prevent the spark

emerging to outside

1. Well

• The space between the bottom of the tuyeres and the sand bed inside the cylindrical

shell of the cupola is called as well of the cupola. As the melting occurs, the molten

metal is get collected in this portion before tapping out.

2. Combustion zone (also called as oxidizing zone)

• The total height of this zone is normally from 15 cm. to 30 cm

• Consuming the free oxygen completely from the air blast and generating tremendous

heat.

• A temperature of about 1540°C to 1870°C is achieved in this zone

• 3. Reducing zone

• CO2 is changed to CO through an endothermic reaction, as a result of which the

temperature falls from combustion zone temperature to about 1200°C at the top of this

zone

• C + O2 CO2 + Heat

• 2Mn + O2 MnO2 + Heat

• Si+ O2 SiO2 + Heat

• 3Fe + 2CO Fe3C + CO2

• C O2 + C (coke) 2CO + Heat

4. Melting zone

• The metal charge starts melting in this zone and trickles down through coke bed and

gets collected in the well.

5. Preheating zone

• Preheating zone starts from the upper end of the melting zone and continues up to the

bottom level of the charging door

• The main objective of this zone is to preheat the charges from room temperature to

about 1090°C before entering the metal charge to the melting zone

• the metal charge in solid form picks up some sulphur content in this zone

6. Stack

• The empty portion of cupola above the preheating zone is called as stack. It provides

the

• passage to hot gases to go to atmosphere from the cupola furnace.

• 3Fe + 2CO Fe3C + CO2

ADVANTAGES OF CUPOLA

Simple design and easier construction

Low initial cost as compared to other furnaces of same capacity

Simple to operate and maintain in good condition

Economy in operation and maintenance

Less floor space requirements as compared to those of other furnaces of

capacity

Cupola can be continuously operated for many hours.

LIMITATIONS

Close temperature is difficult to maintain.

Since molten iron and coke come in contact with each other, certain

elements (like Si, Mn) are lost which others (like sulphur) are picked

up. This changes analysis of molten iron.

Special Casting Processes

Types:

• Gravity / Permanent mold casting

• Die casting

– Low pressure die casting

– High pressure die casting

• Hot chamber

• Cold chamber

• Shell molding

• Investment casting

• CO2 process

• Centrifugal casting

– True centrifugal casting

– Semi centrifugal casting

– Centrifuge casting

PERMANENT MOLD OR GRAVITY DIE CASTING

• A permanent mold casting makes use of a mold or metallic die which is

permanent

• Molten metal is poured into the mold under gravity only and no external

pressure is applied to force the liquid metal into the mold cavity.

• However, the liquid metal solidifies under pressure of metal in the risers, etc.

The metallic mold can be reused many times before it is discarded or rebuilt.

These molds are made of dense, fine grained, heat resistant cast iron, steel,

bronze, anodized aluminum, graphite or other suitable refractoriness.

• The mold is made in two halves in order to facilitate the removal of casting

from the mold. It may be designed with a vertical parting line or with a

horizontal parting line as in conventional sand molds.

• The mold walls of a permanent mold have thickness from 15 mm to 50 mm.

The thicker mold walls can remove greater amount of heat from the casting. For

faster cooling, fins or projections may be provided on the outside of the

permanent mold. This provides the desirable chilling effect.

PERMANENT MOLD CASTING AND TYPICAL

PRODUCTS

Advantages

(i) Fine and dense grained structure is achieved in the casting.

(ii) No blow holes exist in castings produced by this method.

(iii) The process is economical for mass production.

(iv) Because of rapid rate of cooling, the castings possess fine grain structure.

(v) Close dimensional tolerance and Good surface finish and surface details are

obtained.

(vi) Casting defects observed in sand castings are eliminated.

(vii) Fast rate of production and less labor.

Disadvantages

(i) The cost of metallic mold is higher than the sand mold. The process is impractical

for large castings.

(ii) The surface of casting becomes hard due to chilling effect.

(iii) Refractoriness of the high melting point alloys.

Applications

(i) This method is suitable for small and medium sized casting such as carburetor

bodies, oil pump bodies, connecting rods, pistons etc.

(ii) It is widely suitable for non-ferrous casting.

DIE CASTING

Gravity die casting or permanent mould casting

FEATURES

Uses preshaped cavity called dies

Dies are split type with runner, riser, gates to feed metal

Split dies enables easy extraction of casting from dies

Dies made of cast iron

Metal dies are better heat conditions & faster rate of cooling

Metal mould resist high temp & can be used repeatedly

METAL SUITABLE

Aluminium alloys, copper alloys, magnesium & low melting point

methods

Typical parts made

Automobile piston, cylinder heads, gear blank for appliance and

kitchen ware

ADVANTAGES LIMITATION

1. Good surface finish, close dimensional

tolerance

1. Metal dies are expensive

2. Better mechanical properties

3. High production rate

2. Not economical for low production runs

3. Intricate shapes cannot be cast

Die Casting Machines

Designed to hold and accurately close two

mold halves and keep them closed while

liquid metal is forced into cavity

Molten metal is forced into metallic mold or

die under pressure in pressure die casting

• The pressure varies from 70 to 5000 kg/cm2

and is maintained while the casting solidifies

• Two main types:

1. Hot-chamber machine

2. Cold-chamber machine

Die Casting

A permanent mold casting process in which

molten metal is injected into mold cavity under

high pressure

• Pressure is maintained during solidification,

then mold is opened and part is removed

• Molds in this casting operation are called dies;

hence the name die casting

• Use of high pressure to force metal into die

cavity is what distinguishes this from other

permanent mold processes

ADVANTAGES OF PRESSURE DIE CASTING

Used for mass production resulting low unit cost

Thin and dense structure of metals with finer details as the

solidification is under external pressure

Extremely fine finishes can be obtained elementary subsequent

machining operation

Components can be produced to very close dimensional tolerance

and with precise reproduction

Die casting operations permit automation of the process

Mechanical properties are usually superior to sand cavity

More economic use of metal- minimum waste

Inserts can be incorporated

Hot-Chamber Die Casting

Metal is melted in a container, and a piston

injects liquid metal under high pressure into

the die

• High production rates - 500 parts per hour

• Applications limited to low melting-point

metals that do not chemically attack plunger

and other mechanical components

• Casting metals: zinc, tin, lead, and magnesium

Hot-Chamber Die Casting

Figure 11.13 Cycle in hot-chamber casting: (1) with die closed

and plunger withdrawn, molten metal flows into the chamber

Hot-Chamber Die Casting

Figure 11.13 Cycle in hot-chamber casting: (2) plunger

forces metal in chamber to flow into die, maintaining

pressure during cooling and solidification.

Cold-Chamber Die Casting Machine

Molten metal is poured into unheated chamber from

external melting container, and a piston injects metal

under high pressure into die cavity

Mold material : Tool steel and Mold steel.

Tungsten and molybdenum with good refractory

qualities

• Casting metals: aluminum, brass, and magnesium

alloys

• Advantages of hot-chamber process favor its use on

low melting-point alloys (zinc, tin, lead)

Cold-Chamber Die Casting

Figure 11.14 Cycle in cold-chamber casting: (1) with die

closed and ram withdrawn, molten metal is poured into

the chamber

Figure 11.14 Cycle in cold-chamber casting: (2) ram forces metal to

flow into die, maintaining pressure during cooling and solidification.

Cold-Chamber Die Casting

Advantages and Limitations

Advantages

– Economical for large production quantities

– Good accuracy and surface finish

– Thin sections are possible

– Rapid cooling provides small grain size and

good strength to casting

Disadvantages

– Generally limited to metals with low metal

points

– Part geometry must allow removal from die

HOT CHAMBER DIE CASTING & COLD CHAMBER DIE CASTING

HOT CHAMBER METHOD COLD CHAMBER METHOD

Melting furnace integral part of machine Melting furnace in away from die

casting machine

Metered quantity of melt is injected by a plunger Required melt quantity is ladled to

machine and pushed to die cavity

Generally suited low melting non ferrous metals Metals used in non ferrous with

relatively high melting point

Faster production cycle Low volume production

Shell Moulding Recent invention casting techniques for mass production and smooth surface

finish at low cost.

It is also called as Carning or C process.

Casting process in which the mold is a thin shell of sand held together by

thermosetting resin binder.

Step 1 : A match-plate or cope-and-drag metal pattern is heated (170 to 370o C) and placed over a box containing sand mixed with thermosetting resin.

Step2: Box is inverted so that sand and resin fall onto the hot pattern, causing

a layer of the mixture to partially cure on the surface to form a hard shell;

Step3: Box is repositioned so that loose uncured particles drop away.

Step 4: Sand shell is heated in oven for several minutes to complete curing.

Step 5: Shell mold is stripped from the pattern;

Step 6: Two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished.

Step 7: The finished casting with sprue removed.

Advantages :

•A very smooth surface is generally obtained.

•The shell cast parts can be produced with dimensional tolerance of ± 0.2 mm.

•Reduced cleaning and machining costs.

•Gives rapid production rate.

•Uniform grain structure.

•Minimum finishing operations.

•The moulds can be stored until required.

•Complex shapes can be produced with less labour.

•The process can be automated fairly easily. Less skilled labour is required.

Disadvantages :

•The resin binder is more expensive than other binders.

•The initial cost of metal patterns and other specialised equipment is high.

•Dimensional limitations.

•Limited to some specific metals.

•The minimum thickness of section that can be cast is 4 nun.

Applications:

•Shell-mould casting applications include small mechanical parts that require high

levels of precision, such as gear housings, cylinder heads, and connecting rods.

•Shell moulding is also widely used in producing high-precision moulding cores,

such engine block water jackets.

CERAMIC MOULD CASTING

• The ceramic-mold casting process (also called cope-and-drag investment

casting) is similar to the plaster-mold process, except that it uses refractory mold

materials suitable for high-temperature applications.

• Typical parts made are impellers, cutters for machining operations, dies for

metalworking, and molds for making plastic and rubber Components. Parts

weighing as much as 700 kg have been cast by this process.

• The slurry is a mixture of fine-grained zircon (ZrSiO4), aluminum oxide,

and fused silica, which are mixed with bonding agents and poured over the

pattern, which has been placed in a flask.

• The pattern may be made of wood or metal. After setting, the molds

(ceramic facings) are removed, dried, ignited to burn off volatile matter,

and baked. The molds are clamped firmly and used as all-ceramic molds.

In the Shaw process, the ceramic facings are backed by fireclay (which

resists high temperatures) to give strength to the mold. The facings then

are assembled into a complete mold, ready to be poured.

• The high-temperature resistance of the refractory molding materials allows

these molds to be used for casting ferrous and other high-temperature

alloys, stainless steels, and tool steels. Although the process is somewhat

expensive, the castings have good dimensional accuracy and surface finish

over a wide range of sizes and intricate shapes.

Advantages:Intricate detail, close tolerances, smooth finish

Limitations:Mold material is expensive and not reusable

Common metals:Ferrous and high-temperature nonferrous metals are most common; can be used with alloys of aluminum, copper, magnesium, titanium and zinc

Ceramic Mould Casting

Investment casting (lost wax casting)

(a) Wax pattern

(injection molding)

(b) Multiple patterns

assembled to wax

sprue

(c) Shell built

immerse into ceramic

slurry

immerse into fine sand

(few layers)

(d) dry ceramic

melt out the wax

fire ceramic (burn

wax)

(e) Pour molten metal (gravity)

cool, solidify

[Hollow casting:

pouring excess metal before

solidification

(f) Break ceramic shell

(vibration or water

blasting)

(g) Cut off parts

(high-speed friction

saw)

finishing (polish)

Investment Casting

Figure 11.8 Steps in investment casting: (3) the pattern tree is coated with a

thin layer of refractory material, (4) the full mold is formed by covering

the coated tree with sufficient refractory material to make it rigid

Figure 11.8 Steps in investment casting: (5) the mold is held in an inverted

position and heated to melt the wax and permit it to drip out of the cavity,

(6) the mold is preheated to a high temperature, the molten metal is

poured, and it solidifies

Investment Casting

Figure 11.8 Steps in investment casting: (7) the mold is broken

away from the finished casting and the parts are separated

from the sprue

Investment Casting

Figure 11 9 A one-piece compressor stator with 108 separate airfoils made by investment casting (photo courtesy of Howmet Corp.).

Investment Casting

Advantages and Disadvantages

• Advantages of investment casting:

– Parts of great complexity and intricacy can be cast

– Close dimensional control and good surface finish

– Wax can usually be recovered for reuse

– Additional machining is not normally required - this is a net shape process

• Disadvantages

– Many processing steps are required

– Relatively expensive process

Vacuum casting Similar to investment casting, except: fill mold by reverse gravity

Easier to make hollow casting: early pour out

• Vacuum molding, also called the V-process, counter gravity low pressure (CL)

process

• In the vacuum-casting process, a mixture of fine sand and urethane is molded

over metal dies and cured with amine vapor. The mold is then held with a robot

arm and immersed partially into molten metal held in an induction furnace.

• The metal may be melted in air (CLA process) or in a vacuum (CLV process).

The vacuum reduces the air pressure inside the mold to about two-thirds of

atmospheric pressure, thus drawing the molten metal into the mold cavities

through a gate in the bottom of the mold.

• The metal in the furnace is usually at a temperature of 55°C above the liquidus

temperature of the alloy. Consequently, it begins to solidify within a very short

time. After the mold is filled, it is withdrawn from the molten metal.

• The process can be automated, and production costs are similar to those for

green-sand casting. Carbon, low- and high-alloy steel, and stainless steel parts

weighing as much as 70 kg have been vacuum cast by this method.

• CLA parts are made easily at high volume and relatively low cost. CLV parts

usually involve reactive metals, such as aluminum, titanium, zirconium, and

hafnium.

Centrifugal Casting

A family of casting processes in which the

mould is rotated at high speed so centrifugal

force distributes molten metal to outer regions

of die cavity

• The group includes:

– True centrifugal casting

– Semicentrifugal casting

– Centrifuge casting

True Centrifugal Casting

• In true centrifugal casting, molten metal is poured into a rotating mold to

produce a tubular part

• Examples of parts made by this process include pipes, tubes, bushings, and

rings.

• The high-speed rotation results in centrifugal forces that cause themetal to

take the shape of the mold cavity. Thus, the outside shape of the casting

can be round, octagonal, hexagonal, and so on.

Setup for true centrifugal casting.

Semicentrifugal Casting

Centrifugal force is used to produce solid castings rather than tubular parts

• Molds are designed with risers at center to supply feed metal

• Density of metal in final casting is greater in outer sections than at center of

rotation

• Often used on parts in which center of casting is machined away, thus

eliminating the portion where quality is lowest

• Examples: wheels and pulleys

Centrifuge Casting

Mold is designed with part cavities located

away from axis of rotation, so that molten

metal poured into mold is distributed to these

cavities by centrifugal force

• Used for smaller parts

• Radial symmetry of part is not required as in

other centrifugal casting methods

Centrifuge Casting

CO2 Process

Basically a hardening process

CO2 passed through the sand mix containing

sodium silicate, the sand becomes strong by

sodium silicate gel

Na2O, (x)SiO2 + (x)H2O + CO2

Na2CO3 + SiO2.

(x)H2O

Comparison of Casting Processes

Cleaning and Finishing

1. Casting is taken out of the mould by shaking and the Moulding

sand is recycled often with suitable additions.

2. The remaining sand, some of which may be embedded in the

casting, is removed by means of Shot blasting.

3. The excess material in the form of sprue, runners, gates etc.,

along with the flashes formed due to flow of molten metal into

the gaps is broken manually in case of brittle casting or

removed by sawing and grinding in case of ductile grinding.

4. The entire casting is then cleaned by either shot blasting or

chemical pickling.

5. Sometimes castings are heat treated to achieve better

mechanical properties.

CASTING DEFECTS

• Any unwanted deviation from the desired requirements

in a cast product results in a defect.

• Some defects in the cast products are tolerable while

others can be rectified.

MAJOR DEFECTS IN SAND CASTINGS:

• 1. Gas defects

• 2. Shrinkage cavities

• 3. Moulding material defects

• 4. Pouring metal defects

• 5. Metallurgical defects

CASTING DEFECTS

TYPE OF

DEFECT

CAUSES REMIDAL REMARKS SKETCH

Blow

holes

Spherical /

flattened cavities

present inside or

surface of casting

caused due to

entrapment of gas-

void space

Proper moisture

control in sand mould

Improve venting

systems

Mould hardness

Pin hole

porosity

Caused due to

dissolved H2 gas, in

molten metal when

metal gets solidified,

expels the gas leaving

long, small diameter

holes

Controlling the

pouring temp

Use of additives while

pouring

Shrinkage Volumetric

contraction of metal

Insufficient

allowance

Allowance to be

verified

CASTING DEFECTS

TYPE

OF

DEFECT

CAUSES REMIDAL

REMARKS

SKETCH

Hot tear External cracks

causing tearing due to

thermal stress.

Abrupt change in

section of casting

Fillet & rounding

corner, design of

modification

Hot tear

Could

shut

Differential rate of

flow cooling

Lack of complete

fusion of metal

Improve runner &

gate design

Rat tail Penetration of

molten metal into

mould walls

Proper ramming

Scab Rough, thin layer of

metal protruding

above cast surface

CASTING DEFECTS

TYPE

OF

DEFEC

T

CAUSES REMIDAL

REMARKS

SKETCH

Mould

shift

Due to misalignment

of cope and drag

Shift of the mould

while pouring molten

metal

Check the

alignment of the cope

and drag

Check the axis of

the mould cavity in

cope and drag and

the axis must be

aligned correctly

Misrun Metal start freezing

before reaching the

farthest point of the

mould cavity due to

insufficient superheat

Improve runner &

gate design

Check the fluidity

of the metal

Various Gas Defects

• These defects are due to lower gas passing tendency of the

mould.

Caused due to lower venting , lower permeability of the

mould and improper design of the casting.

The lower permeability of the mould is due to use of

finer size grains of sand, higher percentage of clay &

moisture and excessive ramming of the mould.

Pin hole porosity: As the molten metal gets solidified

it loses the temperature which decreases the solubility of

gases and thereby expelling the dissolved gases.

GAS DEFECTS

Moulding Material Defects• These defects are originated due to some specific characteristics of the

moulding materials like insufficient strength, improper ramming etc.

The various defects under this category are discussed in detail.

Cuts and Washes: These appear as rough spots and areas of excess

metal and are caused by the erosion of the moulding sand by the

flowing molten metal. This may be due to insufficient strength of

mould material or the high velocity of the molten metal.

Remedies

• The proper choice of moulding sand

• appropriate moulding method

• Improve the design of gating

• To reduces turbulence

• Provide multiple ingates.

Moulding Material DefectsMetal Penetration: When molten metal enters

the gaps between the sand grains, the result

would be a rough casting surface.Caused due to use of coarse sand grains in mould

material.

Caused due to higher pouring temperature.

Remedies

Choosing appropriate grain size of sand with proper mould wash

Moulding materials Defects

Moulding Material Defects Fusion: Fusion of sand grains with molten metal.

Caused due to lower refractoriness of the binder.

very high pouring temperature.

Remedies

The choice of an appropriate type and amount of binder(Bentonite)

Run out: Caused due to leak of molten metal in the sand mould.

Remedies

Verify the mould design and alignment

Buckles: Long, broad, vee-shaped depression occurring on the surface

of a flat casting.

Mould expansion due to high temperature

Remedies

Verify the mould design with proper venting

Proper testing of moulding sand

Moulding Material DefectsRat tail: Long shallow angular depression normally

found in a thin casting.

Caused due to compression of moulding sand.

Remedies

Check the constancy and properties of moulding sand

Scab: Thin layer of a metal, protruding above the

casting surface.

Due to removal of the moulding sand due to flow of the

mould cavity.

Sand erosion

Remedies

Verify the properties of the moulding sand

Check the gating design

Moulding Material Defects Swell:

Under the influence of metallostatic forces, the mould wall may move

back causing a swell in the dimensions of the casting.

Caused due to improper ramming of the mould.

Drop:

An irregularly shaped projection on the cope surface of a

casting is called a drop.

Caused due to dropping of sand from the cope or other

overhanging projections into the mould.

Remedies

Adequate strength and hardness of the moulding sand

Provide gaggers if necessary.

Other Defects

• Mould and Core shift: A misalignment

between two halves of a mould or of a core

may give rise to a defective casting

Pouring Metal Defects Misrun:

Metal start freezing before reaching the farthest point of the mould cavity due

to insufficient superheat.

Cold shut:

Caused due to non fusion of two streams of metal resulting in a discontinuity

or weak spot in casting.

Remedies

Due to lower fluidity of the molten metal or small thickness of the casting.

The fluidity of the metal can be improved by changing the composition of

molten metal or raising the pouring temperature.

The surface area to volume ratio of the mould can be reduced.

Improve the venting to minize the back pressure in the mould cavity

Pictorial Representation of Defects

Metallurgical Defects • Hot tears:

Caused due to differential rate of cooling causing rupture in the

casting when its hot

Remedies

Improve the casting design

Hard spots: Caused by chilling of the casting excessively at

particular spot.

Causes of Blow HolesBlow Holes Defect Causes

Blow

Holes

Blow holes and Open blows:

These are spherical, flattened or

elongated cavities present inside

the casting or on the surface. When

present inside the casting it is

called blow hole while it is termed

as open blow if it appears on the

surface of the casting. These

defects are caused by the moisture

left in the mould and the core. Due

to heat of the molten metal the

moisture is converted into steam,

part of which when entrapped in

the casting ends up as blow hole or

ends up as open blow when it

reaches the surface. Thus in green

sand mould it is very difficult to

get rid of the blow holes, unless

properly vented.

Casting Defects

Casting Defects

Casting Defects

Casting Defects

Casting Defects

Visual inspection

Dimensional Inspection

Mechanical testing

Flaw detection by NDT

Metallurgical inspection

Inspection Methods

White light inspection, pressure test, magnetic particle inspection,

radiographic test, ultrasonic inspection etc. are used