Molded part shrinkage from thermal expansion

In injection molding, the pol5uner is cooled from the melt temperature to a temperature below the solidification, which is accompanied by a reduction of pressure in a mold before the molded item is ejected from the mold. Since the ejected molded part is still hot, its cooling to room temperature occurs under atmospheric pressure. As a result of temperature and pressure changes, the cooled polymer shrinks according to the PVT (pressure-specific volume-temperature) relationship or the thermal expansion coefficient of the polymer. The linear mold shrinkage is 0.1-0.8% for amorphous polymers and 1.0-2.0% for crystalline polymers. For polypropylene, it is about 1.7%. Shrinkage affects the dimensional accuracy and dimensional stability of molded items. 

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