Plastic Shrinkage in Injection Molding: Guide to Shrinkage Rates and Compensation
Understanding Plastic Shrinkage
Shrinkage is the reduction in volume of a plastic part as it cools from melt temperature to room temperature. In injection molding, understanding and compensating for shrinkage is critical to producing parts that meet dimensional specifications.
Shrinkage occurs in two phases: mold shrinkage (while the part is still in the mold cavity under pressure) and post-mold shrinkage (after ejection, as the part continues to cool to ambient temperature).
Shrinkage Rates by Material
| Material | Mold Shrinkage (%) | Post-Mold Shrinkage | Notes |
|---|---|---|---|
| ABS | 0.4-0.7 | Minimal | Amorphous, predictable |
| HDPE | 1.5-4.0 | 0.1-0.3% | Crystalline, varies with cooling |
| LDPE | 1.5-3.5 | 0.1-0.3% | Similar to HDPE |
| PP | 1.0-2.5 | 0.1-0.2% | Highly crystalline |
| HIPS | 0.4-0.7 | Minimal | Amorphous |
| Nylon 6 | 0.5-1.5 | 0.1-0.5% | Moisture-sensitive |
| Nylon 6/6 | 0.8-2.0 | 0.2-0.6% | Higher shrinkage |
| PC | 0.5-0.7 | Minimal | Amorphous, stable |
| POM (Acetal) | 1.8-2.5 | 0.1-0.3% | Very crystalline |
| PMMA | 0.2-0.8 | Minimal | Amorphous |
| PBT | 1.5-2.5 | 0.1-0.2% | Crystalline |
| LCP | 0.0-0.3 | Minimal | Very low, anisotropic |
Factors Affecting Shrinkage
Material Type
Crystalline materials (PP, Nylon, POM) shrink 2-4x more than amorphous materials (ABS, PC, PS). The crystalline structure packs molecules more densely during cooling.
Wall Thickness
Thicker walls shrink more than thin walls due to longer cooling times and greater volumetric change. A 4mm PP section may shrink 2.5%, while a 1mm section of the same part may shrink 1.5%.
Processing Parameters
- Higher melt temperature increases shrinkage
- Higher mold temperature reduces shrinkage rate
- Higher hold pressure reduces shrinkage
- Longer hold time reduces shrinkage (until gate freezes)
- Faster injection speed increases molecular orientation
Mold Dimension Compensation
Cavity Dimension = Part Dimension x (1 + Shrinkage Rate / 100)
Example: For a 100mm part in ABS (0.6% shrinkage): Cavity = 100 x 1.006 = 100.6mm
For critical dimensions, experienced mold makers use a 0.5-0.7% tolerance band and may cut a test cavity first for new materials.
Shrinkage Compensation Tools
- Mold flow simulation (Moldflow, Moldex3D) provides directional shrinkage prediction
- Shrinkage test: Mold a test plaque, measure at 24h and 48h after molding
- Steel-safe approach: Machine cavity undersized by 0.2%, then EDM to final if needed