The chills can then be reclaimed and reused. Because they are at a much cooler temperature, and often a different metal from what is being poured, they do not attach to the casting when the casting cools. 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 mould. Chills are heat sinks which enable localized rapid cooling. Sometimes chills may be placed on a pattern surface prior to molding, which are then formed into the sand mould. Where a hole is desired in a casting, a core may be used which defines a volume or location in a casting where metal will not flow into. The patternmaker or foundry engineer decides where the sprues, gating systems, and risers are placed with respect to the pattern. Design Sprues, gates, risers, cores, and chills or no Plaster of Paris is usually used in making master dies and molds, as it gains hardness quickly, with a lot of flexibility when in the setting stage. A combination of paraffin wax, bees wax and carnauba wax is used for this purpose. Wax patterns are used in a casting process called investment casting. Metal patterns are long lasting and do not succumb to moisture, but they are heavier, more expensive and difficult to repair once damaged. Fiberglass and plastic patterns have gained popularity in recent years because they are water proof and very durable. True Honduras mahogany is harder to find now because of the decimation of the rain forests, so now there is a variety of woods marketed as mahogany. Once the pattern is built, the foundry does not want it changing shape. Once properly cured, it is about as stable as any wood available, not subject to warping or curling.
Honduras mahogany was used for more production parts because it is harder and would last longer than pine. Sugar pine wood is the most commonly used material for patterns, primarily because it is soft, light, and easy to work.
Wax and Plaster of Paris are also used, but only for specialized applications. Typically, materials used for pattern making are wood, metal or plastics. Although an engineer may help to design the pattern, it is usually a patternmaker who executes the design. Patternmakers (sometimes styled pattern-makers or pattern makers) learn their skills through apprenticeships and trade schools over many years of experience. The making of patterns, called patternmaking (sometimes styled pattern-making or pattern making), is a skilled trade that is related to the trades of tool and die making and moldmaking, but also often incorporates elements of fine woodworking.
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