What is Double shrinkage allowance, why it is allowed in pattern?
A double shrinkage allowance is provided for the dimension of pattern that is used to make a metal pattern for another product. Double shrinkage allowance takes care the shrinkage of the final product (product material) as well as shrinkage of the pattern (pattern material). For example, if a pattern for an object (metal A) is made of metal (metal B), then you have to cast the pattern (metal B) by using another pattern (say wooden pattern). Then the double shrinkage allowance is provided to wooden pattern to compensate shrinkage of metal A and metal B.
Why shrinkage allowance of outer surface is higher than inner surface?
The shrinkage allowance always adds to the linear dimension, even in the case of internal dimension. The material has a tendency to contract towards the center during cooling. Usually, shirking occurs uniformly unless restraints are provided. For example, the core centers in casting can restrain the metal from contracting at the inner surface. However, there is no restriction to contracting at outer surface or edges. So it is desired to give higher shrinkage allowance at outer dimension when compared to the inner surface.