Meaning
Molecular chain ordering occurs as a semi-crystalline thermoplastic cools from its molten state to form structured crystalline regions. This phase transition is polymer crystallization, and it directly affects the shrinkage, density, and optical clarity of the moulded part. The rate at which this occurs depends on the cooling rate imposed by the mould.
Shrinkage Effects
Semi-crystalline polymers shrink more than amorphous ones because the ordered molecular chains pack closer together. Rapid cooling limits polymer crystallization, resulting in lower shrinkage but also lower mechanical and chemical resistance. Mould temperature must be high enough to allow the chains to organize and achieve the desired density.
Optical Properties
Crystalline regions scatter light, which makes semi-crystalline polymers appear translucent or opaque. In contrast, amorphous polymers are transparent because their disordered chains do not scatter light. Controlling polymer crystallization through mould temperature manipulation allows processors to optimize either mechanical strength or clarity depending on part requirements.
Tooling Considerations
Mould cooling design must be highly uniform to ensure that crystallization occurs at the same rate throughout the part. Non-uniform polymer crystallization creates localized differences in shrinkage, which leads to warpage and dimensional instability. Incorporating high-conductivity inserts can help maintain even temperatures across the cavity.
Uniform cooling is especially important for complex geometries with varying wall thicknesses.
