How to calculate mould shrinkage

Thermoplastics shrink as they cool and solidify during the plastic injection molding process and after part ejection.


The plastic injection mold cavity is machined larger than the final desired molded plastic part size in order to compensate for part shrinkage as the part cools.


Mold shrinkage data for the specific plastic material can be used to calculate the amount of mold expansion needed.



Shrinkage in mould units are expressed as length-per unit-length, or as a percentage, and is calculated using the following mould shrinkage calculation formula :



(mold dimension – part size) / mold dimension  = Mold Making Shrink Factor


We could alse use a simplied formula as below.




  • Shrinkage; D-Mold Size; M-Molded part size


However, since the actual shrinkage is influenced by many factors, only the approximate value can be used,When building the mould, the cavity should be machined according to the lower deviation while the core be machined according to the upper deviation to leave more stock room for necessory adjustment.

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