Die cast tooling employs cast iron molds which allow aluminum & zinc castings to be produced more accurately and cheaply than with sand castings. Tooling costs of gravity die casting are a fraction of those needed for high pressure die castings. The rapid chilling gives excellent mechanical properties whilst non-turbulent filling ensures production of heat treatable gravity castings with minimal porosity.
The molten metal is injected into the mold under pressure. This results in a more homogeneous part, generally good surface finish and good dimensional accuracy, compared to that achieved using conventional casting methods. A result of this is the significant reduction in the requirement for post casting processing to achieve the required final configuration.
Here are the common alloys used in die cast tooling:
Is cast at a temperature of 650 Celsius. It is often alloyed with Silicon and Copper. Silicon increases the melt fluidity whilst it reduces machinability, Copper increases hardness and reduces the ductility. A low amount of Copper (less than 0.6%) gives improved chemical resistance, and is often used in marine applications. A high silicon content (improved wear resistance), is often used in automotive applications where wear resistance is important.
Alloys used for closer tolerances and thinner walls than is possible with Aluminum, due to its high melt fluidity. Zinc is usually alloyed with Aluminum to give improved strength and hardness. The casting is done at a fairly low temperature of 425 Celsius allowing the parts to be ejected from the dies earlier resulting in shorter cycle times. Zinc alloys are typically used to produce precision parts such as sprockets, gears, and connector housings.
This alloy possess high hardness and high corrosion resistance. It offers excellent wear resistance and dimensional stability, with strength approaching that of steel parts. Typically used in plumbing, electrical and marine applications where corrosion and wear resistance is important.
The easiest alloy to machine, magnesium has an excellent strength-to-weight ratio and is the lightest alloy commonly die cast.
Lead and Tin
These alloys offer high density and are capable of producing parts with extremely close dimensions. They are also used for special forms of corrosion resistance.
From a design point of view, it is best to engineer parts with uniform wall thicknesses to ensure uniform cooling, and to have cores of simple shapes. It is preferable to have thin sections, as heavy sections can result in cooling problems, trapping gases causing porosity. All corners should be radiused with internal radii at least equal to the wall thickness to avoid stress concentration. Draft allowance should be provided to all sides to ease the releasing the parts from die cast tooling, these are typically from 0.25 to 0.75 degree per side depending on the material.