Plastic Shrinkage Guide: Common Materials Compared

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Understanding plastic shrinkage is critical for mold design. Incorrect shrinkage assumptions lead to out-of-tolerance parts and expensive mold rework.

Shrinkage rates for common injection molding materials:

Crystalline materials:

POM (Acetal): 1.8-2.5 percent
PA6 (Nylon): 1.2-2.5 percent
PA66 (Nylon 66): 1.5-2.5 percent (glass-filled reduces to 0.3-0.7 percent)
PP (Polypropylene): 1.3-2.5 percent
HDPE: 1.5-3.0 percent (highest among common materials)
PBT: 1.5-2.5 percent

Amorphous materials:

ABS: 0.4-0.7 percent
PC (Polycarbonate): 0.5-0.7 percent
PMMA (Acrylic): 0.3-0.6 percent
PS (Polystyrene): 0.3-0.6 percent
PVC rigid: 0.2-0.5 percent
SAN: 0.4-0.7 percent

Semi-crystalline materials:

PET: 1.2-2.0 percent
PPS: 0.5-1.2 percent
LCP: 0.1-0.5 percent (very low)

Key factors affecting actual shrinkage:

Wall thickness: Thicker walls shrink more
Injection pressure: Higher pressure reduces shrinkage
Mold temperature: Higher mold temp increases crystallization and shrinkage
Hold time: Longer hold time reduces shrinkage
Glass fiber content: Each 10 percent glass fiber reduces shrinkage by roughly 0.2 percent

Rule of thumb: Design the cavity 0.5-1.0 percent larger than the nominal part dimension for amorphous materials, and 1.5-2.5 percent for crystalline materials.

Always validate shrinkage with mold flow simulation or test shots before cutting steel.

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