The process of metal casting consists of liquid metal being poured into a mould, which is essentially a hollow the shape of the desired product. The metal is allowed to cool and solidify and thus take on the shape of the mould. The hardened metal, which is referred to as a casting, is then broken out of or gently removed from the mould. This process can be used to make intricate shapes which would be difficult or expensive to make using another method such as hand forging.
Two of the main processes used in metal casting are expendable moulding and permanent moulding.
Expendable moulding is used for the more intricate geometric shapes. With this process the mould can only be used once as the mould has to be broken to get the casting out. These moulds are generally made out of plaster, sand or similar, comparatively soft and cost effective materials.
The interior cavity of the expendable moulds, in which the liquid metal will solidify, is created using a pattern. These patterns are made from wood or plastic and the moulding material is compacted around the pattern to create the casting. The pattern is made bigger than what the casting needs to be to compensate for the fact that the metal will shrink in the cooling process.
Permanent moulds can be used more than once. Metal or a strong ceramic is usually the material of choice for these moulds. Sometimes the permanent moulds have sections which can open to allow the casting to be removed.
Patterns, depending on the material and size of the casting to be made, can be made out of a soft wood like pine, a hardwood like mahogany, metals like cast iron, steel or aluminium, or even out of various plastics.
Cores are used for metal castings with internal geometry. Cores are inverted replicas of the internal features of the piece which need to be cast. These, like the patterns, also take into consideration the shrinking involved in the cooling process. Cores can only be used once because they are broken up after the casting has cooled down.