Plastic Shrinkage Calculator

Injection-molded plastic parts shrink as they cool from melt temperature to room temperature, so a mold cavity machined to the exact finished part dimensions would produce an undersized part. Toolmakers compensate by machining the cavity larger than the target part by the material's shrinkage rate -- this calculator applies that compensation to all three dimensions (length, width, depth) using cavity size = part size / (1 - shrinkage rate), which matches the ASTM D955 definition of mold shrinkage as (mold dimension - part dimension) / mold dimension, i.e. shrinkage is measured relative to the cavity, not the finished part (a common shop shorthand instead multiplies by (1 + shrinkage rate), which is close at low shrinkage rates but understates the needed cavity size as the rate climbs). Because each linear dimension grows by the same percentage, the resin's shrinkage rate is the single factor controlling how oversized the cavity needs to be; the part's raw dimensions only determine how much absolute growth that percentage translates into.

Different resins shrink at very different rates -- semi-crystalline materials like polyethylene and polypropylene typically shrink more (roughly 1-3%) than amorphous materials like ABS or polycarbonate (roughly 0.4-0.7%) because their molecular chains pack more tightly as they cool and crystallize, while amorphous polymers cool into a more disordered, dimensionally stable structure. The calculator also estimates volumetric shrinkage, which compounds across all three dimensions and is therefore noticeably larger than the linear shrinkage rate alone -- a 1.5% linear shrinkage rate produces roughly three times that in volume, since volume scales with the cube of each linear dimension. Always verify against the resin manufacturer's datasheet, since actual shrinkage varies with wall thickness, cooling rate, and mold temperature, not just resin family.