Die casting injects molten metal into a hardened steel die under high pressure, producing thin walled, detailed parts at very short cycle times. It is the metal analogue of injection moulding and shares its economics: an expensive tool amortised across a large volume of near net shape parts that need very little finishing.
Hot chamber and cold chamber
Hot chamber machines keep the injection mechanism in the molten metal and suit low melting alloys, above all zinc, giving very fast cycles. Cold chamber machines ladle metal into the shot sleeve for each cycle and are used for aluminium and magnesium, which would attack a submerged mechanism. The alloy therefore chooses the machine, and the machine sets the cycle time.
Alloys and what they are for
Zinc alloys cast the thinnest walls and the finest detail, take plating well and are dense. Aluminium alloys give the best strength to weight and dominate structural and thermal parts. Magnesium is lighter still and is used where mass matters most. Each has its own shrinkage, its own tool life implications and its own finishing behaviour.
Tooling and what it costs
Dies are hardened steel and see thermal cycling, erosion and soldering from the molten metal, so they wear. Tool life is quoted in shots and is much shorter for aluminium than for zinc because aluminium is more aggressive. Multi cavity dies, slides for undercuts, and the trim tooling that removes flash and overflows are all separate parts of the same investment.
Porosity and what it forbids
High velocity filling entrains gas, so die castings usually contain some porosity. That limits heat treatment, because trapped gas expands and blisters, and it limits welding. It also means a machined surface can open a subsurface void. Where a part must be pressure tight, either specify a low porosity process, plan for impregnation, or design so machining stays away from the risky sections.
Questions people ask about alloy die casting
What is die casting?
Forcing molten metal into a hardened steel die under high pressure, producing thin walled detailed parts at very fast cycle times.
Which alloys are die cast?
Mostly zinc, aluminium and magnesium. Zinc gives the finest detail and longest tool life; aluminium gives the best strength to weight.
Can die castings be heat treated?
Usually not conventionally, because entrained gas blisters at solution treatment temperatures. Low porosity variants of the process exist where heat treatment is required.
How accurate are die castings?
Tighter than sand casting and looser than machining. Critical features are machined afterwards, and the casting is designed with allowance for that.