Patternmaker’s shrink (thermal contraction) Shrinkage after solidification can be dealt with by using an oversized pattern designed for the relevant alloy. Pattern makers use special “contraction rulers” (also called “shrink rules”) to make the patterns used by the foundry to make castings to the design size required. These rulers are 1 – 3% oversize, depending on the material to be cast. These rulers are mainly referred to by their actual changes to the size. For example, a 1% ruler would add 1 mm to 100 mm if measured by a “standard ruler” (hence being called a 1% contraction ruler).

Using such a ruler during pattern making will ensure an oversize pattern. Thus, the mold is larger also, and when the molten metal solidifies it will shrink and the casting will be the size required by the design, if measured by a standard ruler. A pattern made to match an existing part would be made as follows: First, the existing part would be measured using a standard ruler, then when constructing the pattern, the pattern maker would use a contraction ruler, ensuring that the casting would contract to the correct size.

Contractions not listed can be supplied upon request. Listed above are contractions that apply to most metals in use today. They must not be taken too literally but used as a guide only. Each user must judge for himself.

Graduation:

• Topside: Metric 1 mm
• Backside: Contraction 1% to 2%

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