A RUNNER-GATE DESIGN SYSTEM FOR DIE CASTING DIES

Chang-Ho Kim *, Taek Hwan Kwon**

* Dept. of Mechanical Engineering, Dong-Eui University, Kaya-Dong 24, Pusanjin-Ku, Pusan, Korea, 614-714

** Graduate School, Dept. of Mechanical Design Engineering, Pusan Nat'l University, #30 Jangjeon-Dong, Kumjeong-Ku,

Pusan, Korea, 609-735

Abstract

Various analytical FEM, FDM tools for flow process including die casting have presented, they are only giving information

to predetermined die design whether it is proper or not. Current shop floor practice uses the trial-and-error methods to

determine new die design.

This paper describes a research work of developing computer-aided die design system for die casting. Approach to the

CAD system has been written in circumstance Auto LISP with personal computer. This system has been developed to present

algorithms for automation of die design, especially runner-gate system using 3-D geometry. This system quantifies practical

knowledge and experiences in die design as formulating procedure. It is possible for engineers to make automatic and

efficient design of and it will result in reduction of required expenses and time. It is composed of selection of cast alloy,

product design, runner-gate design etc. In addition, specific rules and equations for the system have been presented. An

example is applied to cap-shaped cast using the proposed system.

Key words : Die casting, Die design system, Rule base, Runner, Gate

NOMENCLATURE

Qa volume of cavity to be filled, cm3

Vg main gate velocity, m/sec

tg filling time, sec

K heat capacity per unit volume, cal

q' the rate heat evolved per unit time during solidification, cal/sec

L latent heat during solidification, cal/g

Cp specific heat of molten metal, cal/g¡ ¤¡ É

Tm temperature of molten metal, ¡ É

Ts solidus temperature, ¡ É

Td die temperature, ¡ É

¥ ñ density of alloy, g/cm3

S radiation area, cm2

X a half thickness of cast, cm

¥ ö thermal conductivity of alloy, cal/cm¡ ¤sec¡ ¤¡ É

1. INTRODUCTION

Die casting is one of the most economical casting processes for manufacturing precision shaped parts in mass

production. It is a precise casting method in which molten metal is injected at high pressure into a die cavity. As soon as the

molten metal has filled the cavity, it solidifies by fast cooling. Die-cast components are being used increasingly in the

automobile, aerospace, electronic and other industries because of their premium quality, low cost and low weight. Because of

high pressure and short cycle times involved, thin-wall sections are possible.

In die-design system of various forming processes, Shaffer developed progressive die design by computer (PDDC)

system and J.P. Kruth made efforts to develop an integrated CAD/CAM system for mould design and manufacture. J.C. Choi

developed a compact and practical CAD system for blanking or piercing of irregular shaped- sheet metal products and stator-

rotor parts.

In studies of die casting process, C.C Thai used runner-optimization design method and the abductive network in

modeling the die casting process according to the experimental data. W. Zhang researched the CAD/ CAE system that die

designer can determine the shape and dimension of runner-gating system under different casting condition. Generally

speaking, die designer still have to depend on engineer’s experiences and know-how due to lack of proper analytical ability

metal flow and velocity and heat transfer in die. Current shop floor practice uses the trial-and-error method to determine die

design, when new moulds are used. This method is costly and results in a lot of wasted casting. To solve this problem a study

was done on the runner and gate system to simulate the molten metal flow and to analyze the pressure and metal movement

during casting process. Although some finite element analysis software is capable of analyzing the melting process and flow

conditions of the products (workpiece) under various injection conditions, they are only giving some limited suggestions and

information to die design.

In this study, die design system for die casting process has been developed to present flow chart for automation of die

design, especially runner-gate system. In addition, specific rules and equations for runner-gate system have been presented to

avoid too many trials and errors with expensive equipment. It is possible for engineers to make an automatic and efficient die

design and it will result in reduction of expense and time.

2. STRUCTURE OF CASTING PRODUCT AND DIE

An example of cast product is seen in Fig.1. It is generally composed of cast cavity, gate, runner, biscuit, overflow and

to be formed out from die. Irrespective of the shape of the cast, the quality is always determined by the runner-gate system.

Generally, the system contains their arrangements and shapes to design properly. The effects of metal velocity and die

temperature on the metal flow distance and quality are discussed.

Holding Block

Cavity Block

Shot Chamber

Ejector pins

Ejector plate

Guide pins Surface pin

Ejector die holding block

Biscuit

Runner

Gate

Cast

Overf low

Fig. 1 Casting product Fig. 2 Essential components of simple die casting die

Essential components of simple casting die are illustrated in Fig.2. A die consists of two sections at the die parting plane.

It 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 (moving die) half and ejector die (fixed die) half. The cover die half may be machined in the solid cover die half, or

it may be inserted. The cover die half is fastened to the stationary platen on the casting machine and does not move during the

casting cycle. The ejector half is mounted on the movable platen of the machine. A cavity block that is a reproduction of a

section of the part that is to be a casting is formed or machined into two halves of the die block.

Cast design Die Generation

Rule Base for diecasting die design

Die Layout Design

Cast Input

(3D Wire-frame )

Material Selection

Apply Shrinkage

Gate Design

Runner Design

Runner-Gate

system

Overflow Design

Cavity Block

Design

Die type

-One side

-Both side

Die Generation

Fig. 3 Flow chart of die design system

3. ALGORITHM FOR DIE DESIGN SYSTEM

In this study, die design system means automated generation system that master mould and other components are

generated automatically when three-dimensional drawing of product is prepared. If a drawing of product is input into an

AutoCAD by output drawing file on a screen, product design will be carried out and the size of die block will be determined.

This system is roughly composed of cast (product to be formed) design, and die layout design and die generation. Fig.3

shows the flow chart of the system. The shapes of each component are determined by rule-base and are connected at

specified position by assembly flow chart. After components and casting are connected, the shapes of two die halves and

layout are determined. And die design is finished after user's confirmation. This might be analyzed by commercial CAE

software if the design is proper or not.

3.1 Cast design

The cast design consists of three parts; cast input, material selection and application shrinkage. In cast input part, the cast

modeling in commercial modeler as IGES file format is input. The input cast is located fitting viewpoint from desirable

direction. And the parting surface should be determined for detailed die design. But the algorithm that determines the parting

surface is not constructed, and in this system it is supposed that user recognizes the location of parting surface in advance.

The material of the cast should be selected. Next, it may be necessary to consider a shrinkage allowance of the cast caused by

the temperature difference between in and out of die.

3.2 Die Layout Design

When the cast design is completed, the die layout design for constructing master mold should be followed it on. In

the process of die layout design, the gate, runner and overflow are designed for constructing dies. In this system, the die

layout design is divided into four parts; gate design, runner design, runner-gate design and overflow design. In gate design,

its material properties are input and the cross-sectional area of gate is determined by filling speed and time. The runner area is

determined by gate area in runner design. The connecting part of gate and runner can be designed and assembled with cast in

runner-gate system. And the overflow can be designed in the same way of runner design. Fig. 4 shows the flowchart of this

system.

Gate Design

Runner-Gate System

Runner Design

Calculate Filling

Speed

( by minimum

thickness)

Calculation of runner area

( by gate area)

Calculate Filling Time

Determination of

gate area

Selection runner-gate type

Determination of specific

dimension

( user )

Overflow Design

Input Value for gate design

Selection of gate thickness

Determination of

gate width

Selection normal line of

parting surface

Selection overflow type

Determination of specific

dimension

( rule base )

Selection normal line of

parting surface

Selection

parting surface

Selection

parting surface

Fig. 4 Flowchart for die layout design

3.2.1. Gate Design

The metal entering the sprue is directed into one or more passages, or runners. Near the die cavity, the cross-sectional

area of the runner decreases to form a gate designed to direct the metal into the die cavity. The main function of the runner

and gating system is to deliver molten metal passed into the mould into all section of the molten cavity. Runner and gate are

major components in this design system. Fig. 5 shows the flowchart for gate design. Cast material is selected and then cavity

volume is calculated. Once mechanical properties of cast are input and the filling speed of molten material into dies is

selected, the gate area is calculated.

Calculation of filling time

Determination of gate area

Determination of gate width

Selection gate shape

Determination of gate thickness

Gate

Confirm?

Yes

No

Input Value for gate design

Mechanical Property of cast

Part generation by rule base

Fig. 5 Flowchart for gate design Fig. 6 Flowchart for runner design

The cross-sectional area of gate, Ag is given by (1)

gg

ag

tV

QA

⋅= (1)

The filling time of die cavity, tg is assigned to be that a fraction of solidus comes up to 70 %.

Heat capacity per unit volume, K is given by (2)

XSTTCLK smp ⋅⋅⋅−⋅+= ρ)]([ (2)

The flow rate heat per unit time, q' is given by (3)

XTTSq dm /)( −⋅⋅=′ χ (3)

From the equation (2) and (3), filling time, tg can be obtained.

7.0×′

=q

Kt g (4)

Generally, the gate thickness, t is selected properly, which is between 0.5 and 3.0mm, considering trimming etc. The

width of gate L is determined by following equation from gate area calculated by equation (1).

t

AL

g= (5)

3.2.2. Runner Design

The runner is machined entirely in the ejector half and the cover half forms only the flat side of the runner. Fig. 6 shows

the flow chart of runner design. After the cross- sectional area of gate is determined from equation (1), that of runner can be

calculated based on volume constancy point of view. And then the shape of runner is selected from database. The width and

depth of runner varies with the volume of metal to be injected into the cavity. Finally, the shape and numerical data are

generated. Various shapes of runners are illustrated in Fig. 7. Cross-sectional shape of runner is inverted trapezoidal as shown

in Fig. 8. Generally, the area of runner is 4~5 times of that of gate, the fraction of depth to width 1:1.5~3.0, side angle

10~20¡ Æ, and corner radius longer than 6mm.

Width(W)

Radius(R)

Depth(D)

Side Angle

Fig. 7 Shapes of runner Fig. 8 Cross-section of runner

3.2.3. Overflow Design

Overflows provide exits for the air from the casting cavity and serve as receptacles for the first metal entering the die

cavity during each shot. They provide additional mass to small casting, thereby helping to maintain a satisfactory and stable

die temperature. Overflows are closely spaced in the thin sections of the die cavity and around the areas of die cavity that are

farthest from the source of hot metal. Venting may also be provided by small grooves cut across the parting plane of the die,

or by the clearance around the ejector pins or movable cores and slides. The volume of overflow is determined by equations

as above and then the geometry of overflow (the shape of cross-section, width and depth) is selected from database. Finally,

overflow is generated.

3.3. Die generation

After designs of all parts are finished, the designed parts are assembled with each other and are determined their

connection position from user's confirmation and then die cavity is generated. Fig. 9 shows flow chart to assemble generated

parts: gate-cavity, gate-runner, cavity-overflow and overflow-airvent. The cavity is subtracted from die block, and then die is

split into two dies, moving and fixed dies. Finally die is generated on the determined parting plane.

Confirm?

Yes

No

Connecting datum plane

Determination of connection location

Feature creation at the determined location

Gate

Assembly Complete !

Parting surface

Runner

Overflow

Airvent

Fig. 9 Flow chart for assembly

4. APPLICATION TO CAP-SHAPED CASTING

4.1. Casting Modeling

An example is given to apply the proposed system for cap-shaped casting. Fig.10 demonstrates cast product resulting

from commercial modeler (Pro/Engineer 2000i), and Fig. 11 shows the cavity block where the cast is inserted in die block,

which is separated with moving and fixed die at parting plane.

Fig. 10 3D modeling of cap-shaped cast Fig. 11 Die generation

4.2. Runner-Gate-Overflow Design

Fig. 12 shows a casting that represents a designed runner-gate-overflow system. A parting line is confirmed by a user.

Fig. 12 Wire-frame of assembled parts Fig. 13 Generation of fixed and moving die

4.3. Die Generation

By subtracting casting from generated cavity block, die is generated. Die is divided into moving and fixed die after

generation cavity block. Fig. 13 shows the final moving and fixed die after die division.

5. CONCLUSIONS

The primary conclusions of this study are as follows.

1. This study proposed an easy and effective die design system for die casting product . It is constructed with die design

algorithm and database in the circumstance AutoCAD. A novice who may not have any experience of die design can perform

die design, especially runner-gate design only if he has a little knowledge about die casting.

2. A novice who may not have any experience of die design can perform it only if he has a little knowledge about die

casting. But this system cannot be applied for solidification, flow analysis, and NC machining data because it is constructed

with simple wire frames. And the used lines for geometries of object are only straight.

3. This system is basic at present and was applied to the simple cap-shaped cast. Next, this system will be applied more

complex shape with more developed system.

ACKNOWLEDGEMENTS

The authors wish to acknowledge Brain Korea 21 of Department of Mechanical Engineering, Dong-Eui University, Pusan,

Korea.

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