Die casting

Metal shot into a steel die at high pressure

Die casting injects molten metal at high speed and high pressure into a hardened steel mold, called a die, then opens it seconds later. Laptop and phone frames, car gearbox housings and door handles are made this way. It is the fastest way to make large numbers of precise, thin-walled metal parts.

Photo: Gabeincal, CC BY 3.0, via Wikimedia Commons

Die casting · cold-chamber machine at injectionThe figure shows the die closed at the moment of injection; circled numbers match the steps below.

How it works

Close and clamp the die. The moving die half is pushed against the fixed half and held by a hydraulic clamp. The clamp force is typically hundreds to thousands of tonnes, so the die stays shut under pressure.
Fill the shot sleeve. A measured dose of molten metal, usually aluminum, is ladled into the shot sleeve (a steel tube leading into the die). This is the cold-chamber machine; for zinc, a hot-chamber machine keeps its pump sunk in the molten metal.
Inject. The plunger (a hydraulic piston) shoots the metal into the cavity in a fraction of a second, then keeps pushing at high pressure, often several hundred bar, to pack the metal in as it shrinks.
Cool in the die. Water flowing through channels in the steel removes heat fast. Thin parts are solid within seconds.
Open, eject and trim. The moving half pulls back and ejector pins (steel rods) push the casting out, usually into a robot's grip. The runners, the leftover slug called the biscuit and thin flash (metal squeezed into the parting line) are trimmed off in a press.

Key facts

Materials: Aluminum, zinc and magnesium alloys; some brass. Not steel, which is too hot for the die
Typical parts: Engine and gearbox housings, laptop and phone frames, power-tool bodies, door handles, faucets, toy cars
Part size: From a few grams (zinc parts) to whole car underbodies of about 50 to 100 kg in the largest machines
Precision: About ±0.05 to ±0.2 mm on small features
Surface finish: Smooth: Ra (average roughness) about 1 to 2.5 µm, ready to paint or plate
Batch size it suits: Tens of thousands to millions of parts
Tooling and machine cost: Dies from tens of thousands to over a million dollars; machines from hundreds of thousands to millions
Speed: Seconds to about two minutes per shot, depending on part size

Strengths and limits

Strengths

Very fast cycles, ideal for mass production
Thin walls, often 1 to 3 mm, and fine detail
Precise and smooth, needing little machining
Parts can include bosses, ribs and lettering in one shot

Limits

Dies are very expensive, so small batches do not pay off
Limited to lower-melting metals like aluminum, zinc and magnesium
Trapped air leaves tiny pores, so most parts cannot be heat-treated or welded easily
Long lead time, often months, to design and build a die

Where it's used

AutomotiveGearbox and engine housings, brackets, large one-piece body castings
ElectronicsLaptop and phone frames, heat sinks, connector housings
Consumer goodsPower-tool bodies, kitchen appliance parts, toy cars
Building hardwareDoor handles, locks, faucets (often zinc, then plated)
LightingStreetlight housings, LED heat sinks

How to recognize it

Smooth, even surfaces and crisp detail, often painted or chrome-plated.
Small round marks, slightly raised or sunken, where the ejector pins pushed the part out.
A thin trimmed line at the parting line, sometimes with tiny leftover flash.
Thin walls stiffened by ribs, and round bosses (posts) for screws.