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.
Shrinkage, warpage, and crystallinity
A part will not warp if it shrinks uniformly in all directions and in all areas of the mold.
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.
How crystallinity develops
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.
If parts of the molding cool at a slower rate, these areas will have a higher crystalline content, hence higher shrinkage.
Two main factors affect how fast the melt freezes:
- Mold temperature
- The higher the mold temperature, the longer the temperature will be maintained, which delays the cooling of the melt.
- Mold geometry
- Thick regions tend to cool slower than thinner sections and therefore have higher crystalline content and higher volumetric shrinkage than thin sections that cool quickly and so are more amorphous.Thin regions can have a lower volumetric shrinkage than that predicted from equilibrium pvT data.