die casting

Die casting is like sand casting sped up and made permanent: instead of a sand mold you use up once, you force molten metal under high pressure into a precisely machined steel mold (the die) that you reuse thousands of times. Because the metal is rammed in fast and hard, it fills thin walls and fine detail that sand could never reproduce, and it pops out with a smooth surface needing little finishing. Think of the metal housing on a power tool, a zinc toy car, or an aluminium laptop chassis.

The metal is injected at pressures of tens of megapascals and fills the die in a fraction of a second, then freezes fast against the cold steel. That speed is the whole point: you can make one part every few seconds. It works best for metals that melt at moderate temperatures, chiefly zinc, aluminium, and magnesium alloys, because a steel die would erode quickly with higher-melting metals like steel itself. Fast cooling gives a fine-grained surface skin, which is good for strength.

The honest catch is trapped air. Filling so fast tends to whip air into the metal, leaving fine internal porosity. That porosity usually means a die casting cannot be heat treated (the trapped gas expands and blisters the surface) and is hard to weld, and it can hurt fatigue life. So die casting buys speed, precision, and a great surface, at the cost of internal soundness and a very expensive die that only pays off in high volume.