Shrinkage, it’s no laughing matter

Published On: May 13th, 2014|Categories: Blog|

Shrinkage, it’s no laughing matter (especially if you have cut your steel.)

Shrinkage, (with all due respect to the Seinfeld skit), happens to your plastic on the injection molding production floor. In regards to the plastic injection molding process, shrinkage is 1) a given and 2) a factor to account for. The change in the plastic’s density between the temperature during molding, and the ambient room temperature, results in the plastic shrinking. The shrinkage of molded plastic parts can be as much as 20 percent by volume, when measured between the ranges of processing and ambient temperature. Crystalline and semi-crystalline materials are more susceptible to thermal shrinkage; amorphous materials to a lesser extent. On top of just the stresses imparted by temperature, the other injection process settings can induce shrinkage. Excessive shrinkage, beyond the acceptable level, can be caused by low injection pressure, short pack-hold time or cooling time, high melt temperature, high mold temperature, and low holding pressure.


Problems caused by part shrinkage


If not countered through processing, volumetric contraction (shrinkage) leads to dimensional non-conformity as well as sink marks in the component. Controlling shrinkage must be addressed during part, mold, and process design, most importantly in applications where components have tight tolerances. Aside from processing, the mold design must plan for shrinkage to meet part dimensions and tolerances.


Talk to your molder about shrinkage, seriously. Include them in on your part designs. It will save you time and money in accommodating the shrinkage that we will all face when designing in plastics.


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