Calculating Part Shrinkage…

This question overshadows a common mistake made by many molders and designers…

What is the correct way to calculate shrinkage? One engineer insists that you divide the mold dimension by the part dimension and eliminate the one, but this sounds weird.

My Response
Shrinkage is a measurement based on change, and is a represented as either a ratio or as a percentage. The correct method is to determine the difference between the mold and part dimension and then divide that number by the mold dimensions.
ratio = (mold – part) / mold
% = (mold – part) (100%) / mold
Additional Thoughts
The method used by your engineer is close for small shrinkage values, but the disparity becomes increasingly larger as shrinkage values and dimensions increase.

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