Precision casting differs from sand casting and shell moulding in that the moulds they employ consist of only one part, while the pattern itself is expendable each time a casting is made. Precision casting processes offer much more freedom to the designer, and produce castings of a superior surface finish and a high degree of dimensional accuracy. Among other purposes, they are used for the casting of metals and alloys that are difficult to machine, since the castings usually require very little or no finishing treatment. These castings are used in precision engineering, clockmaking, and other fields of industrial production
Sand casting can be used for all common metals, and ther are many different sand-casting processes and special processes derived from this method. These are known by various names such as open sand moulding, pit moulding, box moulding and template moulding etc. The most commonly used method for making small castings is box moulding. With this method the pattern is embedded in the sand or other mould material, within a moulding box which is usually made up of an upper and lower part, the sand being compacted by ramming, vibration or pressure. The box is then opened and the pattern is removed, the cores inserted, the box closed again and the casting is carried out.
For casting very large, heavy or intricate parts the pit moulding process is employed. The pit method is where the mould is built up in a casting pit. To give the sand greater strength when used as a mould material for large castings, cement can be added. For symmetrically shaped castings the mould is sometimes formed by means of a template, a metal plate cut to the required profile for producing a certain shape when it is moved along a guide track or rotated on a pivot.
In dry sand moulding the mould is baked ; in green sand moulding and the mould is used with sand in the damp or 'green' condition. The metal is poured from above into an open mould. The more usual kind of moulds are the closed moulds which are filled through a special system of channels known as gate runners, and are usually so contrived that the metal enters at a low point and rises in the mould. Once the metal has solidified and cooled, the casting is removed from the mould and the runners and risers are detached from the casting. The casting is then cleaned up by abrasive tumbling, blasting, cutting or grinding.
When casting with expendable moulds, the individual pattern parts are first made by hand or by mechanical means and then assembled. The moulding materials are those used for constructing the actual moulds in which the metal will be cast, are usually mineral substances such as cement, fireclay, plaster etc., in conjunction with bonding agents such as water glass, synthetic resins, oil, sulphite solution etc., which give the moulds the necessary strength and dimensional accuracy. The bond action is either achieved by drying or by chemical consolidation.
Precision non-ferrrous die casting, is the casting of metals such as zinc, aluminium and brass.