Injection molding tolerances are the allowable dimensional variations in molded plastic parts. Unlike metal machining, plastics shrink and warp during cooling, making tight tolerances harder to achieve.
Two main standards define injection molding tolerances:
- ISO 20457 (formerly DIN 16901) — International standard for plastic parts
- GB/T 14486 — Chinese national standard, widely used by domestic manufacturers
ISO 20457 defines 5 tolerance grades based on material type and mold complexity:
Grade A (Fine) — ±0.05% of dimension, for precision components with engineered plastics
Grade B (Medium) — ±0.10%, suitable for general engineering parts
Grade C (Coarse) — ±0.15%, for consumer products
Grade D (Very Coarse) — ±0.25%, for non-critical parts
Grade E (Extremely Coarse) — ±0.40%, for large structural parts
Note: These percentages are approximate. Actual achievable tolerances depend on part geometry and gate location.
Material | Shrinkage Rate | Expected Tolerance
--- | --- | ---
ABS | 0.4-0.7% | ±0.08-0.15mm
Nylon PA66 | 1.0-2.0% | ±0.15-0.30mm
Polycarbonate PC | 0.5-0.7% | ±0.08-0.15mm
Polypropylene PP | 1.5-2.5% | ±0.20-0.40mm
POM (Delrin) | 1.8-2.5% | ±0.20-0.35mm
Acrylic PMMA | 0.3-0.8% | ±0.08-0.18mm
1. Design mold cavities with shrinkage compensation — add compensation values to cavity dimensions
2. Control melt temperature within ±5°C for consistent shrinkage
3. Use mold temperature controllers (±2°C accuracy) for cavity temperature uniformity
4. Apply uniform wall thickness (±0.5mm variation max) to minimize differential shrinkage
5. Run sampling trials to validate actual shrinkage before full production
Chinese mold manufacturers typically achieve ISO Grade B for standard molds and Grade A for precision mold applications.