A pattern is a replica of the desired casting and is used for making the mould cavity. Various types of patterns are used in casting - the choice depending upon the production quality and the casting configuration. Single-piece patterns are obviously least expensive but have limited applications. Split patterns or two piece patterns are most widely used since it can be easily withdrawn from the mould and can be used for intricate shapes.
Patterns are always made somewhat larger than the final casing to allow for shrinkage, distortion and finishing. These are taken care by shrinkage, draft, finish, distortion and shake allowances.
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