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 the opposite is also true. Degrees of crystallinity affect the level of shrinkage. Higher crystallinity results in greater shrinkage.
Thick regions of injection molded parts tend to cool slowly relative to thinner sections, and so thick sections have higher crystalline content and greater shrinkage.
During cooling, some polymers form ordered molecular structures called crystalites.