Solution:

Shrinkage Allowance: As metal solidifies and cools, it shrinks and contracts in size. To compensate for this, a pattern is made larger than the finished casting by means of a shrinkage or contraction allowance. To provide an allowance, a patternmaker uses shrink or contraction rule which is slightly longer than the ordinary rule of the same length. Different metals have different shrinkages; therefore, there is a shrink rule for each type of metal used in a casting. It is also called as contraction allowance

When liquid metal starts to cool , metal gets shrink & reduces size of the component To reduce above problem, allowance are provided on the pattern

Patterns are made larger than actual size

Different metal have different shrinkage

It has three forms (a) Liquid Contraction (b)Solidifying Contraction (c)Solid Contraction

1) First two are reduced by gates & risers

2) Solid contraction can be reduced by providing more allowance on pattern