Design and manufacturing of Die Casting Dies

INTRODUCTION

Die-cast components are being used increasingly in the automobile, aerospace, electronic and other industries after Doehler manufactured diecasting product by using Al alloys in 1915.Diecasting is not suitable for a small quantity production because of the high cost. But it has various advantages such as manufacturing products of complex geometry and thin-wall sections, high productivity, smooth surface of cast and excellent dimensional accuracy. Therefore, diecasting process is developing sharply with establish thousands of diecasting machines.

Diecasting die design consists of the selection of materials for diecasting alloys, the application of shrinkage, and the casting plan including designs of cast, gate, runner and overflow. While manufacturing die design is highly demanded for high precision and shorts the date of delivery, in most of the case, it is designed by determining product geometry. So it is needed experienced know-how and experts who have a skill for manufacturing die. In result, such diecasting die design has much economical losses and wastes of time by trial and error method. Therefore, designs of automatic shape of die and to makes a 3D modeling for diecasting die is done by CAD/CAM system.

Diecasting die design includes a process of determining geometrical figure of the product and dies and selecting condition for forming products. Mechanical and external quality of the ultimate die casting product is determined by interaction of each variables of the design. Therefore the die designer has to design after due consideration of the problems that can be caused at the time of production. The traditional die design has been carried out a designer who experienced for many years and followed a process of trial and error that happens in the time from designing product and die to producing the ultimate product. Such processes cause the term of production to extend and have the prime cost rise. As a result, there have been attempts to reduce them in various ways.

One of them is construction of system that assists initial step developing diecasting product and die design CAD system. The other is finding formability of product and mechanical defects before manufacturing process and considering the countermeasure in advance by simulating diecasting process.

 Generally speaking, die design still depends on experience, due to lack of analytical ability in die and melting metal flow and heat transfer. Current shop floor practice uses the trial-and-error method to determine die design, when new molds 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 gating system to simulate the molten metal flow and to analyze the pressure and metal movement during the casting process.

Although some finite element analysis software is capable of analyzing the melting process and flow conditions of the products (work piece) under various injection conditions, they are only giving some limited suggestions and information to die design.

Diecasters usually carry out the diecasting experiments before producing new casts. At the diecasting stages, the runner-gate part is always repeatedly corrected, which leads to a lengthened processing time and increased processing cost. The diecasting die design should consider component system factors, such as runner, gate, over flow and air vent. A large amount of experience is essential in manual assessment and if the design is defective, much time and a great deal of efforts will be wasted in the modification of the die. Thus human negligence should be minimized.

DESIGN OF DIE CASTING DIE

Design is done in three stages i.e. cast design, Die layout design and Die generation

1) CAST DESIGN

The cast must be designed because the dies can be generated from the cast in diecasting die design. The cast design consists of three parts; cast input, material selection and application shrinkage

a)      Cast Input

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 for diecasting.

b)     Material Selection

After inputting the cast in this system, the material of the cast should be selected. Most of the diecasting processes are used to shape or form parts made from both ferrous and nonferrous metals, principally aluminum, magnesium, and zinc.

c)      Application Shrinkage

Next, the cast should be applied to shrinkage. In establishing dimensions for cavities, an allowance must be added to the dimensions specified for the part to be cast, for shrinkage of the casting metal. The shrinkage allowances normally used are: 0.005in. per inch for zinc alloys, 0.006in. per inch for aluminum alloys, and 0.007in. per inch for magnesium alloys. Shrinkage allowances for copper alloys vary from 0.008 to 0.018 in. per inch, the allowance used depending largely on foundry experience with the type of alloy being cast. The above values are influenced by several variables, primarily size and shape of the casting. For castings that have irregular surface contours, die sections and cores are designed to prevent free shrinkage in specific areas. Die sections or cores so designed are often called “shrink resistors”.

For close-tolerance castings, it may be necessary to make an allowance for the expansion of the die cavity caused by the difference in the temperature at which it was made and the operating temperature. In general, the calculation of shrinkage allowances at room temperature is illustrated below equation.

                                         DL = b (T - 20 ) - a (t - 20 )

2) DIE LAYOUT

 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 four parts; gate design, runner design, runner-gate design and overflow design.

    (A) Gate Design

In gate design part, the properties are input for gate design and the gate sectional area is determined by filling speed and time. The main function of the runner and gating system is to deliver molten metal passed into the mold into all section of the molten cavity. First, casting material is selected and cavity volume is calculated. Once

mechanical properties of cast are input and filling speed is selected, the gate area is generated.

The cross-sectional area of thegate Ag is shown by  below equation

             A_g Q_a /V_g*t_g   ……………………………………………………………..(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

K  = [L + C_p  × ( T_m-T_s )] × p × S ×X                 ……………..(2)

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

q=x ×S (T_m-T_a) /X                       ……………….(3)   

From the equation (1) and (2), filling time, tg can be obtained.

t_{g =} (K/q) ×0.7

Generally, the gate thickness, t is selected properly, which is between 0.5 and 3.0 mm, considering rimming etc. The width of gate L is determined by following equation from gate area calculated by equation (3).

L= A_g/t

   (B) Runner Design

Runners should be designed with a stepped increase in cross-sectional area from the gate via branch runners to main runners, and on to sprue or biscuit, to promote uniform metal velocities and uniform ratios of cross section to perimeter. The cross-sectional area of a feed runner is equal to, or less than, the sum of the cross sectional areas of the branch runners. On runners of different lengths feeding identical parts, the longest runner should be given a slightly larger cross section. A runner that converges into a long gate should increase in cross section toward the feed runner, to keep metal velocities as uniform as possible. Theoretically, these runners should taper out at the ends to the thickness of the gate, but practical considerations require a compromise. Turns and leading edges should have generous radii and should be smoothly blended where thickness or width changes occur. Runners should have a reasonably smooth surface finish. A thick runner will not solidify fast enough for the cycling rates generally used. A thin, flat runner will cause the metal to lose too much heat before it enters the gate. As a compromise, a standard width-to-depth ratio of 1.6:1 to 1.8:1 , side angle is 10~20 inch  each corner radius is over 6mm. has been adopted. This ratio provides for reasonably fast cooling without excessive heat loss during cavity filling. 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.

    (C) Runner Gate Design

The part of connecting gate and runner can be designed and assembled with cast in runner-gate system. To obtain “gate-controlled fill” of the die cavity, the cross-sectional area of a runner must be larger than of the gate. However, for minimum heat loss, metal velocity in the runner feeding a gate must be as high as possible. For these reasons, a runner-to-gate area ratio of 1.15:1 to 1.5:1 is generally used. Oversize runners will increase metal losses and remelting costs.

   (D) Overflow design

The placing of overflows is generally predictable, and their location and size are designed into the gating system of a die. However, the addition or relocation of overflows is the most frequent cause of failure in the 15% of dies for which first-shot success is not achieved. The weight of metal in overflows should be added to the part weight in calculating the total weight of metal flowing through the gate.

Airvent on the die faces usually lead out of overflows. The total of the cross-sectional areas of vents should be at least 50% of the gate area. Self-cleaning of vents can be ensured by making vents 20 – 30mm thick, 0.1 – 0.15mm length. 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 shape of the finished component determines the design of a diecasting die. But there are a number of aspects involved in the design and sizing of a die, which can have an influence and important bearing on die life.

3) DIE GENERATION

The cavity block can be generated by geometry recognition and rule base. After generating the cavity block, the type of dies is determined according to the geometry of the cast. In this system, the types of dies are set up in two types. Thus, One of them is the case that the cast is located at one side of dies and the other is the case that the product is divided by parting surface. Here, because of difficulty of detailed geometry recognition user can determine the selection of die. Consequently, the cavity block is generated and the type of dies is selected, and ultimately the dies can be generated.

DIE MANUFACTURING AND PREPARATION

Dies are typically machined from tool steel. Dies last between 15,000 and 500,000 castings, depending on the casting temperature of the alloy. Dies for aluminum, a moderate-temperature alloy, have an expected lifetime of 100,000 castings.

Dies are a large capital investment, especially for small firms, and their cost must be distributed over a long use phase. Similarly, the environmental investment in die-making can be amortized over the 100,000 casting lifetime. A die for 170 cm3 of casting requires a shot size of 370 cm3, including overflow wells and feed system. Removing that much metal from 800 cm3 of stock requires 4300 kJ.

Lubricants are used both in making the die and preparing the die for each casting. Oil-based cutting fluids are the most popular for machining, such as when making steel dies for

casting. They frequently include naphtha, and, despite being diluted to 95% v/v with water, release more volatile organic compounds than their water-based counterparts. To make the representative die considered above would require 0.04 L soluble oil cutting fluid and 0.8 L water diluents. Both oil-based and water-based lubricants are commonly applied to the die and plunger tip before casting. On the die, lubricants act as releasing agents.

Despite the seemingly small volumes, oil-based lubricants are a major source of air releases from die casting facilities, as reflected in the Environmental Protection Agency’s Toxics Release Inventory (EPA TRI) for aluminum die casting, standard industrial code (SIC) 3363. Actual emissions vary with the composition of the lubricant, but typically volatile organic compound (VOC) emissions are associated with oil-based lubricants.

Products containing alkylbenzene sulfonate, 1,2-epoxypropane, alkylether, and poly(oxyethylene) nonyl phenyl ether are commonly used (SCE, 2001). Water-based lubricants have lower VOC emissions, but may be associated with increased hazardous airborne particle (HAP) emissions. Cumulative VOC emissions are around 1 kg per tonne of produced casting (Roberts, 2003). Throughout the die casting process, because of the proprietary nature of the input compounds and the wide variety of reactions that can occur, the exact composition of VOCs is not as closely monitored or regulated as the total emission of VOCs. VOCs include any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions (US GPO, 2003).

In some foundries, dies are preheated to reduce thermal stress and extend die life. This is most common in dealing with high-temperature copper and magnesium alloys (US DOE, 1999).

ADVANTAGES OF USING DIE CASTING

Some of advantages of using die casting are as follows

1.   Excellent dimensional accuracy (dependent on casting material, but typically 0.1 mm for the first 2.5 cm  (0.005 inch for the first inch) and 0.02 mm for each additional centimeter (0.002 inch for each additional inch).
2.  Smooth cast surfaces (Ra 1–2.5 micrometres or 0.04–0.10 thou rms).
3.   Thinner walls can be cast as compared to sand and permanent mold casting (approximately 0.75 mm or 0.030 in).
4.  Reduces or eliminates secondary machining operations.
5.   Rapid production rates.
6.   Casting tensile strength as high as 415 MPa.
7.   Casting of low fluidity metals.

APPLICATION OF DIE CASTING

Some of the application die castings are as follows

1.  Automotive parts
2.  Lighting
3.  Electronics
4.  Aircrafts
5.  Boats
6.  Hardware
7.  Speakers
8. Appliances

REFERENCES:

1)                  J.C. Choi*, T.H. Kwon**, J.H. Park**, J.H. Kim**, C.H. Kim***

Dept. of Mechanical Design Engineering, ERC for NSDM at Pusan Nat'l University Graduate School, Dept. of Precision Mechanical Engineering at Pusan Nat'l University Dept. of Mechanical Engineering, Dong-eui University

2)                  Life cycle analysis of conventional manufacturing techniques: DIE CASTING

By Stephanie Dalquist and Timothy Gutowski Massachusetts Institute of Technology

3)                  Casting product–process–producer compatibility evaluation and improvement  

M. M. AKARTEy and B. RAVI*z

International Journal of Production Research, Vol. 45, No. 21, 1 November 2007, 4917–4936

4)                  Simulation-enabled casting product defect prediction in die casting process M.W. Fua* and M.S. Yongb  ,International Journal of Production Research , Vol. 47, No. 18, 15 September 2009, 5203–5216