Pattern (casting)

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The top and bottom halves of a sand casting mould showing the cavity prepared by patterns. Cores to accommodate holes can be seen in the bottom mould, the drag.

In the process of casting, a pattern is a replica of the object to be cast, used to prepare the cavity into which molten material will be poured during the casting process. The pattern needs to incorporate suitable shrinkage allowances, this is called contraction allowance, depending on the alloy being cast and the exact sand casting method being used. Some alloys will have overall linear shrinkage of up to 2.5%, whereas other alloys may actually experience no shrinkage or a slight "positive" shrinkage or increase in size in the casting process (notably certain cast irons)/ The shrinkage amount is also dependent on the sand casting process employed, for example clay-bonded sand, chemical bonded sands, or other bonding materials used within the sand.

Pattern making is a skilled trade. Patternmakers learn their skills through apprenticeships and trade schools over many years of experience..

Patterns used in sand casting may be made of wood, metal, plastics or other materials. Pattern are made to exacting standards of construction, so that they can last for a reasonable length of time, according to the quality grade of the pattern being built, and so that they will provide a repeatable dimensionally acceptable casting.

The patternmaker or foundry engineer decides where the sprues, gating systems, and risers are placed with respect to the pattern. Where a hole is desired in a casting, a cores may be used which defines a volume or location in a casting where metal will not flow into. Sometimes chills may be located on a pattern surface, which are then formed into the sand mold. Chills are heat sinks which enable localized rapid cooling. The rapid cooling may be desired to refine the grain structure or determine the freezing sequence of the molten metal which is poured into the mold.

Patterns continue to be needed for sand casting of metals. For the production of gray iron, ductile iron and steel castings, sand casting remains the most widely used process. For aluminum castings, sand casting represents about 12% of the total tonnage by weight (surpassed only by die casting at 57%, and semi-permanent and permanent mold at 19%; based on 2006 shipments). The exact process and pattern equipment is always determined by the order quantities and the casting design. Sand casting can produce as little as one part, or as many as a million copies.