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:
1. Wall thickness: Thicker walls shrink more
2. Injection pressure: Higher pressure reduces shrinkage
3. Mold temperature: Higher mold temp increases crystallization and shrinkage
4. Hold time: Longer hold time reduces shrinkage
5. 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.