In the production of castings, moulds are used to form the shape of the finished product. Moulding technologies can be placed into one of three categories: sand moulds, dies and investment moulds.
Sand moulds are formed using sand in combination with a binder. This binder may be clay and water (greensand) or a chemical (resin bonded). The sand mixture is placed in the pattern box and compacted so that it precisely fits the pattern. The hardness of greensand moulds depends on the amount of compaction applied to the mould. Where chemical binders are used, moulds are chemically hardened after light compaction.
Sand moulds are used when large castings need to be made.
In the lost foam process, expanded polystyrene patterns are imbedded in sand, but not removed before the liquid metal is poured. The pattern is decomposed by the molten metal, which replaces the polystyrene pattern and exactly duplicates its features.
Dies are made from durable materials that are heat and wear resistant, and can be used many times. Typical die materials are grey iron, steel and graphite. The mould cavity is generally machined out using CNC machining. The process has several advantages: a high degree of accuracy is reproducible over a large number of parts; parts have excellent surface finish; many parts can be produced in a short space of time. However, there are limitations to the process: only alloys with relatively low melting temperatures can be cast (e.g. aluminium, magnesium, zinc); cost of the tooling is high (although this is offset by the large number of castings produced from one mould); complex shapes are difficult to produce, and the process is usually limited to small components.