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For semi-crystalline like PET, Polyolefins, Nylon, and Acetal, product properties are affected by the amount of crystalline vs amorphous regions within the polymer and by the orientation that occurs during the molding process.
In the melt state, crystalline materials are amorphous and as cooling starts crystal growth begins at the nucleus (one or more nucleus). Nuclei are initiated in three different ways or in combination, i.e., random motion of the polymer chain, referred to as spontaneous or homogenous nucleation, nonhomogenous melts, (unmelt), can form nuclei, and foreign substances, (nucleating agents, color, etc). The predominant player is spontaneous nucleation. Crystal formation will continue until the temperature of the material drops to a point where chain mobility inhibits further growth. The rate at which crystallinity develops is highly temperature dependent and referred to as the cooling rate.
Effects of Crystallinity on Properties:
As crystallinity increases:
Impact strength -- Decrease;
Stiffness -- Increase;
Density -- Increase;
Thermal Properties -- Increase;
Permeability -- Decrease;
Shrinkage -- Increase.
How to control Crystallinity?
Faster cooling rate, lower pressure, or faster post molding cooling will decrease Crystallinity.
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