What are the differences in processing shrinkage between injection-molded plastics?

Shrinkage in injection-molded plastics varies significantly depending on material composition, processing conditions, and part design. Thermoplastics like polyethylene (PE) and polypropylene (PP) typically exhibit higher shrinkage rates (1.5-3.5%) due to their semi-crystalline structure, which contracts more during cooling. Amorphous plastics such as polystyrene (PS) and polycarbonate (PC) show lower shrinkage (0.5-0.7%) because their molecular chains are less ordered.

Processing factors like mold temperature, injection pressure, and cooling time also influence shrinkage. Higher mold temperatures reduce internal stresses but may increase shrinkage, while faster cooling can minimize dimensional changes. Additives like fillers or reinforcements (e.g., glass fibers) further alter shrinkage by restricting polymer chain movement.

Understanding these differences is critical for manufacturers to optimize tooling designs, anticipate dimensional tolerances, and ensure consistent product quality across various plastic materials.