A level of crystallinity achieved is determined by cooling rates. The rate of crystallization is a function of both temperature and time.
Rapid cooling rates are associated with lower levels of crystalline content and vice versa. The level of shrinkage is also affected by degrees of crystallinity, with higher crystallinity resulting in greater shrinkage.
Thick regions of injection molded parts tend to cool slowly relative to thinner sections so thick sections have higher crystalline content and greater shrinkage. During cooling some polymers form ordered molecular structures called crystalites.