A RUNNER-GATE DESIGN SYSTEM FOR DIE CASTING DIES Chang-Ho Kim *, Taek Hwan Kwon** * Dept. of Mechanical Engineering, Dong-Eui University, Kaya-Dong 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, 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, cm 3
V g
main gate velocity, m/sec
t g
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
C p
specific heat of molten metal, cal/g ¡
T m
temperature of molten metal,
T s
solidus temperature,
T d d
die temperature,
¥ ñ
density of alloy, g/cm 3
S
radiation area, cm2
X
a half thickness of cast, cm
¡ É
¡ É
¡ É
¡¤ É
¥ 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 statorrotor 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.
Gu i d e pi n s
Su rf ac e p i n
Holding Block Overflow
Ejector pins
Cavity Block Ejector plate Cast Ejector die holding block
Gate Runner Biscuit Shot Chamber
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 Layout Design
Die Generation
Gate Design
Cavity Block Design
Cast Input (3D Wire frame ) Material Selection Apply Shrinkage
Runner Design RunnerGate system Overflow Design
Die type
- One side - Both side Die Generation
Rule Base for diecasting die design
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 Design
R u n ne r - G a t e System
Overflow Design
Selection r u n ne r - g a t e t y p e
Selection overflow type
Determination o f specific dimension
Determination o f specific dimension
( user )
( rule bas e )
Selection normal line of parting surface
Selection normal line of parting surface
Selection parting surface
Selection parting surface
Input Value for gate design
Calculate Filling Speed ( by minimum thickne ss) Calculate Filling Time
Calculation of runner a rea ( by gate area)
Determination o f gate area
Selection of gate thickness
Determination o f gate width
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. Mechanical Property of cast
Calculation of filling time
Input Value for gate design
Determination of gate area D e t e r m i na t i o n o f gate thickness
Determination of gate width
Selection gate shape
No
Confirm?
Ye s
P a r t g e n e r a t io n b y r u le b a s e
Gate
Fig. 5 Flowchart for gate design
Fig. 6 Flowchart for runner design
The cross-sectional area of gate, Ag is given by (1)
A g
=
Qa
(1)
V g ⋅ t g
The filling time of die cavity, t g is assigned to be that a fraction of solidus comes up to 70 %. Heat capacity per unit volume, K is given by (2)
K = [ L
+
C p ⋅ (T m
−
T s )] ⋅ ρ ⋅ S ⋅ X
(2)
The flow rate heat per unit time, q' is given by (3)
q′
= χ ⋅ S ⋅ (T m −
T d ) / X
(3)
From the equation (2) and (3), filling time, t g can be obtained.
t g
=
K q′
×
0 .7
(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).
L
=
A g
(5)
t
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¡ ,Æand corner radius longer than 6mm. Side Angle
Depth(D)
Radius(R) Width(W)
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.
Connecting datum plane
Parting surface
Determination of conne ction location
Gate
Runner
Overflow
Feature c reation at the determined loca tion
Confirm?
Airvent
No
Yes Ass embly Complete !
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.
A C K N O W L E D G E ME N T S
The authors wish to acknowledge Brain Korea 21 of Department of Mechanical Engineering, Dong-Eui University, Pusan, Korea.
REFERENCES
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