Die-casting is normally done using two different processes:

In a cold chamber process, molten metal is ladled into the cold chamber for each shot. There is less time exposure of the melt to the plunger walls or the plunger. This is particularly useful for metals such as aluminum and copper that alloy easily with Iron at  higher temperatures.

In a hot chamber process the pressure chamber is connected to the die cavity and immersed permanently in the molten metal. The inlet port of the pressurizing cylinder is uncovered as the plunger moves to the open position. This allows a new charge of molten metal to fill the cavity and thus can fill the cavity faster than the cold chamber process. The hot chamber process is used for metals of low melting point and high fluidity such as tin, zinc, and lead that tend not to alloy easily with steel at their melt temperatures. Aluminum, zinc and copper alloys are the materials predominantly used in die-casting. Pure aluminum, however, is rarely cast due to high shrinkage and susceptibility to hot cracking.

Every metal alloy available for die casting offers different advantages:

• Aluminum has good corrosion resistance and mechanical properties, high thermal and electrical conductivity, as well as strength at high temperatures. Aluminum is often cast at a temperature of 650 C. It can be alloyed with silicon and copper. Silicon increases the melt fluidity, whereas copper increases hardness and reduces ductility. 
• Zinc is the easiest alloy to cast. Zinc is economical for small parts and has a low melting point. The casting is done at a fairly low temperature of 425 C so the part does not have to cool much before it can be ejected from the die. This, in combination with the fact that Zinc can be run using a hot chamber process allows for a fast fill, fast cooling.
• Magnesium is the lightest alloy in common use.
• Copper possesses high hardness, high corrosion resistance and the some of the best mechanical properties out of all alloys. Copper alloys are used in plumbing, electrical and marine applications where corrosion resistance is important.

Some Questions and Answers

Q. What are die casting's advantages over producing a design as a sand casting?

A. Compared with sand castings, die castings require little or no machining to meet specifications, can be made with thinner walls, can have all or nearly all holes cored to size, can be held within much closer dimensional limits, and are produced more rapidly in dies which can make many thousands of castings without replacement, rather than requiring new cores for each casting. Die castings have smoother surfaces and involve much less labor cost per casting. Sand castings, on the other hand, can be made from ferrous metals and from many nonferrous alloys not suitable for die casting which provide higher strength and wear resistance. Certain shapes not producible by die casting are available in sand castings, maximum size can be greater, tool cost is usually less and small quantities can be produced more economically but may require extensive machining.

Q. What is the economic quantity level for conversion to a die casting from a sand casting, or other gravity casting process?

A. The key to determining the lowest economic production quantity level for a conversion from sand casting to die casting, or from many other lower volume production processes, depends largely on the configuration, size and complexity of the part. While the die casting process is most economic at higher volumes, die casting can achieve comparative savings at quantities at or below 2,000 pieces if extensive post-casting machining or surface finishing can be eliminated. Please consult us early on in your design process so that a proposal or existing part can be evaluated for die casting, and the design optimized for lowest-cost China die cast production.

Q. What are die casting's advantages over producing a design as an investment casting?

A. Investment casting is a high-precision process that employs alloys with properties similar to foundry alloys. Tooling cost is substantially lower than for die casting, but production costs are higher. Investment casting is competitive with die casting only at very low production volumes.

Q. When can die casting offer advantages over permanent mold castings?

A. Die castings can be made to closer dimensional limits and with thinner sections than permanent mold castings. Holes can be cored in die castings, and they are produced at higher rates with less manual labor. They have smoother surfaces and usually cost less per part. Permanent mold casting, however, involves somewhat lower tooling costs and can be made with sand cores yielding shapes not available in die casting.

Q. What are die casting's advantages over producing a design as a plastic molding?

A. Compared with the most widely specified plastic injection moldings, die castings can be  stronger, stiffer, more stable dimensionally, more heat resistant, and have superior mechanical properties per unit of cost. Die castings have a high degree of permanence under load when compared to plastics and are far more resistant to ultra-violet irradiation, weathering and stress cracking in the presence of various reagents. Castings offer built-in EMI/RFI shielding, which is often a problematic and costly post-casting operation with plastic housings. However, plastic raw material costs less on a unit volume basis, and has color- inherent properties which tend to eliminate finishing. Plastic, while more temperature sensitive, also has a high degree of electrical resistance.

Q. If you require a surface color on a component, and it is possible to produce the part as a plastic molding, won't the optimum choice be to produce the part as integrally colored plastic?

A. While plastic moldings offer integral color properties, the die casting process may be selected based on rigidity, impact strength, heat resistance, dimensional stability, and built-in EMI/RFI shielding characteristics.

Q. When can die casting offer advantages over producing a design as a stamping?

A. Stamping from sheet steel offers economy that is difficult to equal when a component can be made from one relatively simple stamping. Steel stamping dies that perform a single operation are less costly than die casting dies. The relative costs for tooling and processing depend on the number and types of dies and presses needed. When a highly complex stamping or several stampings are required, die casting can be a cost-effective alternative, and can achieve complex shapes impossible with a stamping. In the case of multiple stampings, costs of fixturing and welding added to the costs of fabricating the additional parts, can make die casting very competitive. Material costs for stamping may be substantially higher than indicated by published per pound costs due to high scrap rates. Stampings invariably consume more material than is contained in the end product, sometimes substantially more.

Q. What are die casting's advantages over producing a design as a powdered metal part?

A. Die casting and powdered metal processes are highly competitive with respect to dimensional precision and part machining requirements. The advantage usually hinges on the orientation of features and desired wall thicknesses. The choice between die casting and powdered metal frequently depends on product size, weight or performance requirements rather than economics. Light-weight die castings can be made in sizes that exceed the capabilities of powdered metal. Powdered metal can be the choice when metals such as ferrous, stainless steel, and copper alloys are required to achieve strength, wear resistance, or high operating temperatures.

Q. When can die casting offer advantages over screw machining?

A. Automatic screw machining entails the lowest tooling cost of any production method. Highly automated screw machines are not labor intensive. The process, which uses bar stock as raw material, may sometimes offer poor material utilization, possibly less than 50%. The choice versus die casting will usually depend on production quantities, with the die casting advantage increasing as production rates increase. Unusually complex design shapes are routinely produced as die castings, while they would be costly or impossible as machined parts.

Q. When can die casting offer advantages over producing a design as an extrusion?

A. Extrusions made from aluminum, magnesium, and zinc alloys exhibit strength and rigidity similar to die castings; however, the ductility is generally higher. Tooling and production costs are comparatively low, making the process very competitive. Extruded designs that require changes in cross section, or features such as holes and slots, can often be converted to die castings, eliminating the machining operations required for extrusions. The choice is usually governed by the number of machining operations, but occasionally by minor differences in material properties, such as strength or surface treatment characteristics.

Q. When can die casting offer advantages over forgings?

A. Where a die casting alloy can satisfy the design requirements for strength and density, die casting will offer complex shapes not possible in forged parts, with thinner sections held to closer tolerances. A new generation of metal matrix composites, squeeze cast, and semisolid cast parts can offer significant cost savings over forgings at substantial weight reductions.

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