Crystallinity

Some thermoplastics polymers are called semi-crystalline polymers. Examples of semi-crystalline polymers include polyethylene, polypropylene, nylon, acetal, polyester, and most of the specialty resins. Semi-crystalline polymer chains develop rigid crystalline structures during cooling.
Semi-crystalline polymers crystallise as they cool. If a semi-crystalline polymer cools slowly, it develops more crystallinity. Therefore, increased mould or melt temperatures yield higher crystallinity.
The advantage of higher crystallinity is the increase in rigid crystalline sites which result in higher mechanical strength. However, crystallinity reduces part clarity, and increases shrinkage during cooling.
Thermoplastics polymers which do not crystallise are called amorphous polymers. Examples of amorphous polymers include polystyrene, ABS, PVC, acrylic, and polycarbonate.