Tags: Injection Molding 101
Amorphous resins such as ABS, Polycarbonate, and Polystyrene have much lower shrinkage values than the polyolefins. The higher shrinkage of polyolefins is due to the fact that, in their molten state, they take up more volume than in the solid state because polyolefin resins are semi-crystalline. When the resin solidifies, the chains in the crystalline regions pack tightly together resulting in a reduction in volume. In general, the polyolefins can be ranked for shrinkage:
HDPE ≥LLDPE ≥LDPE ≥PPOnce a resin has been selected, shrinkage can be controlled, to some extent, through mold design and processing conditions. Studies on a test mold in which the thickness and gate area of a flat plaque can be varied, indicate the following:
- Shrinkage is reduced as part thickness decreases. The response to a thickness change is more pronounced with HDPE than PP.
- Shrinkage is reduced when the gate area is reduced.