Pattern (casting)

In 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.

Patterns used in sand casting may be made of wood, metal, plastics or other materials. Patterns 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 repeatably provide a dimensionally acceptable casting.

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. Patternmakers (sometimes styled pattern-makers or pattern makers) learn their skills through apprenticeships and trade schools over many years of experience. Although an engineer may help to design the pattern, it is usually a patternmaker who executes the design

Typically, materials used for pattern making are wood, metal or plastics. Wax and Plaster of Paris are also used, but only for specialized applications. Mahogany is the most commonly used material for patterns, primarily because it is soft, light, and easy to work, but also once properly cured it is about as stable as any wood available, not subject to warping or curling. Once the pattern is built the foundry does not want it changing shape. The downside is that it wears out fast, and is prone to moisture attack. Metal patterns are more long lasting, and do not succumb to moisture, but they are heavier and difficult to repair once damaged.

Wax patterns are used in a casting process called investment casting. A combination of paraffin wax, bees wax and carnauba wax is used for this purpose.

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.

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 core may be used which defines a volume or location in a casting where metal will not flow into. Sometimes chills may be placed on a pattern surface prior to molding, which are then formed into the sand mould. 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 mould. Because they are at a much cooler temperature, and often a different metal than what is being poured, they do not attach to the casting when the casting cools. The chills can then be reclaimed and reused.

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Wikipedia