Die-Casting

Die-casting is a metal casting process characterized by forcing molten metal under high pressure into a mould cavity. The mould (also called a "die" due to its cubic shape) is created using two hardened tool steel halves which have been machined into shape and work similarly to an injection mould during the process. Most die-castings are made from non-ferrous metals, namely zinc, copper, aluminium, magnesium, lead, pewter and tin based alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.

The casting equipment and the metal dies represent large capital costs and this tends to limit the process to high volume production. Manufacture of parts through die-casting is relatively simple, involving only four main steps, which keeps the incremental cost per item low. It is especially suited for a large quantity of small to medium sized castings, which is why die-casting produces more castings than any other casting process. Die-cast parts are characterized by a very good surface finish (by casting standards) and dimensional consistency.

Two variants are pore-free die-casting, which is used to eliminate gas porosity defects; and direct injection die-casting, which is used with zinc castings to reduce scrap and increase yield.

Hot-chamber machines

Schematic of a hot-chamber machine: Hot-chamber machines, also known as "gooseneck" machines, rely upon a pool of molten metal to feed the die. At the beginning of the cycle the plunger of the machine is retracted, which allows the molten metal to fill the gooseneck. The pneumatic or hydraulic powered plunger then forces this metal out of the gooseneck into the die. The advantages of this system include fast cycle times and the convenience of melting the metal within the casting machine itself. The disadvantages of this system are that high-melting-point metals cannot be utilized and aluminium cannot be used because it picks up some of the iron while in the molten pool. Due to this, hot-chamber machines are primarily used with zinc, tin, and lead based alloys.

Cold-chamber machines

A schematic of a cold-chamber die-casting machine: These are used when the casting alloy cannot be used in hot-chamber machines; included are aluminium, zinc alloys with a large composition of aluminium, magnesium and copper. The process for these machines consists of melting the metal in a separate furnace. Then a precise amount of molten metal is transported to the cold-chamber machine where it is fed into an unheated shot chamber (or injection cylinder). This shot is then driven into the die by a hydraulic or mechanical plunger. This biggest disadvantage of this system is the slower cycle time due to the need to transfer the molten metal from the furnace to the cold-chamber machine.