Casting
- Casting pours molten metal into a mould. In sand casting, a pattern makes a cavity in sand; the sand is broken to release the part. It suits one-off or low-volume, large parts, but has a rough finish.
- Die casting injects molten metal (such as aluminium or zinc) into a reusable steel die. It gives accurate parts with a good finish, suited to high volumes, but dies are expensive.
Forming metals
- Forging shapes hot metal by hammering or pressing it. The grain structure follows the shape, making parts strong, such as spanners and crankshafts.
- Pressing (stamping) shapes sheet metal between dies, such as car body panels.
- Bending folds sheet metal, often on a press brake. Extrusion forces material through a shaped die to make long parts with a constant cross-section, such as aluminium window frames.
Forming polymers
- Injection moulding: thermoplastic granules are melted and injected into a mould under pressure. It makes complex parts at very high volume and low unit cost, but moulds are very expensive.
- Vacuum forming: a heated thermoplastic sheet is sucked over a mould, such as for packaging trays.
- Blow moulding: air inflates a heated tube of plastic inside a mould, making hollow items such as bottles.
- Extrusion also makes plastic pipes and window frames.
Scale of production
- One-off: single items, often by hand. Batch: a set quantity. Mass: large numbers, often on production lines. High tooling costs are only worthwhile for high volumes.
Key terms
- Sand casting
- Pouring molten metal into a sand mould made using a pattern.
- Die casting
- Injecting molten metal into a reusable steel die.
- Forging
- Shaping hot metal by hammering or pressing.
- Extrusion
- Forcing material through a die to make a constant cross-section.
- Injection moulding
- Injecting melted thermoplastic into a mould under pressure.
- Vacuum forming
- Sucking a heated plastic sheet over a mould.
- Blow moulding
- Inflating heated plastic inside a mould to make hollow items.