Die casting

Die casting (high-pressure die casting, HPDC; pressure die casting in British English) is a casting process in which molten metal is injected at high velocity and under high pressure into a split steel die. The metal fills the die in a fraction of a second, solidifies under intensification pressure and the finished casting is ejected. A cycle usually takes tens of seconds, and even less for small parts, so the process is intended for high volumes. The principle resembles plastic injection molding, but the die and the machine are built for molten metal.

Machines are either hot-chamber or cold-chamber. In a hot-chamber machine, the injection mechanism is immersed directly in the molten metal, which allows fast cycles but suits only metals with lower melting points, mainly zinc alloys (Zamak) and magnesium alloys. In a cold-chamber machine, the metal is metered into a shot sleeve separately for each cycle. This is how aluminum alloys, larger magnesium parts and copper alloys are cast.

A common aluminum die casting alloy is AlSi9Cu3(Fe) (EN AC-46000), comparable to A380, the most widely used die casting alloy in the US. Thin-walled parts use AlSi12(Fe), which flows well, and structural parts that need ductility use alloys of the AlSi10MnMg type. Die castings have a smooth surface and good dimensional accuracy and allow thin walls, ribs, holes and screw bosses without further machining. Tolerances and machining allowances are specified according to ISO 8062-3; in North America, the NADCA product specification standards are the usual reference.

The main limitation is porosity. The metal flows through the die turbulently and entrains air and gases from the die release agents. Conventional die castings are therefore usually not welded or heat treated to high strength, because the gas in the pores would form blisters on the surface when heated. For structural parts, vacuum-assisted die casting is used, which evacuates the die and significantly reduces porosity.

Die casting pays off for metal parts with complex shapes in volumes from thousands of pieces per year upward: gearbox and electronics housings, heat sinks, pump bodies, automotive brackets, power tool bodies or door handles. The steel die is a major investment. For aluminum, its service life is usually in the range of tens to hundreds of thousands of cycles; for zinc, which is cast at a lower temperature, it tends to be longer.

For low volumes, gravity casting into permanent molds, sand casting or machining from solid stock are better options. Prototypes of future die castings are often machined or sand cast, but they have different mechanical properties and different porosity than the production part.

Adapt the design to the process: uniform wall thickness (roughly 1.5 to 4 mm for aluminum), sufficient draft angles, rounded corners and ribs instead of massive sections, where shrinkage cavities form. Undercuts require slides and make the die significantly more expensive. Expect ejector pin marks and flash at the parting line, which is removed by trimming.

Machine functional surfaces with the smallest possible allowance, because the densest part of a casting is its surface skin, and deeper machining exposes pores. For parts that must seal a pressurized medium, agree on a leak test and, if needed, impregnation. Check with the die caster on achievable tolerances, price, ownership and service life of the die, the cost of sampling and how internal defects are inspected (X-ray, pressure testing). Review the design with the die caster before you freeze it, and for more complex parts ask for a mold filling simulation.