Shrinkage in plastic parts is an important property to quantify, especially when the tolerances for the part are tight. Highly crystalline polymers such as polyethylene (PE) shrink more than amorphous polymers as they cool. Shrinkage can be lessened by altering processing conditions of the material. Having shrinkage in plastic parts can cause warpage in the forms of wrinkling and distortion which often leads to a part outside of specs.
This standard is conducted in either injection molded or compression molded processes to determine the shrinkage of plastic parts relative to their original mold dimensions. Specimen dimensions are measured 24 and 48 hours after molding and the shrinkage of the parts are determined in both the direction parallel to flow and perpendicular to flow.