Design Nomenclature / Physical Model Making

Shrinkage allowance

Also called shrink rule, contraction allowance, pattern maker's rule

This entry has not been translated yet and is shown in English.

Extra size built into a pattern because the casting contracts as it cools or cures. Aluminium runs about 1.3%; urethane resin a few tenths; silicone moulds shrink themselves and then keep shrinking. Copy a part by moulding it and the copy is measurably smaller. Do that twice and it is out of tolerance.

A cavity in section with three outlines nested inside it. The mould wall is the outermost; the pattern that made it sits just inside; the cooled casting sits inside that again, standing clear of the wall on every side. The gap is dimensioned as a percentage, and a note marks the second-generation copy shrinking again by the same amount.

In practice

Patternmakers used a shrink rule — a ruler physically cut long by the alloy's percentage, so a pattern measured "to size" on it came out deliberately oversize.

Not to be confused with

How this term connects

Investment castingMasterNominal dimensionPatternResinRiserRTV siliconeSand castingSLSToleranceShrinkage allowance
Related Confused with