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.
The Patternless™ Process, invented and developed by Cti, uses CNC machining to cut the mould directly into a block of resin bonded sand, based on a computer generated 3D model. This does away for the need for expensive pattern equipment, which makes the process particularly suitable for one-off or small production quantities.
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.
Investment moulds are ceramic shells formed around an expendable pattern. The patterns are made from either from wax or polystyrene, to which a series of ceramic coatings are applied. After several coats have been built up and have fully dried, the pattern is removed by melting (wax) or burn-out (polystyrene).
The benefits of the process include its ability to produce precision engineerd components with minimum wastage of materials and energy and subsequent machining directly from the poured metal. The process is very versatile, since the mould is formed from a single piece and there are no issues with draft angles and undercut. In addition to this, the surface finish of the casting is excellent: the first ceramic coat is formed with particularly fine ceramic particles.
The Replicast® and Repliwax® processes build on the benefits of investment casting, opening up a whole wealth of other opportunities.