Material crystallinity is the process whereby liquid material solidifies into a transparent crystal-like state.
The molecules in plastics are long chains of atoms, as shown in the following diagram. These long molecular chains may be regularly aligned, which create a crystalline structure; or randomly arranged chains, which create an amorphous structure; or a combination of crystalline and amorphous sections which creates a semi-crystalline structure.
Crystalline materials have a naturally higher shrinkage than amorphous materials. This means that a part with variations in crystallinity will also have variations in shrinkage, and therefore will probably warp.
Semi-crystalline materials have a tendency to crystallize, but their degree of crystallinity present is affected by the cooling rates of the melt. The faster the melt freezes, the less time there is available for the crystalline sections of the plastic to form.