Injection Molding Tolerance Guide
Plastics shrink, warp, and flex — plan accordingly
Injection molding tolerances are fundamentally different from metal machining. Plastics shrink as they cool (0.4–2.0% depending on material), absorb moisture, and warp from uneven wall thickness. This guide helps you set realistic expectations and design parts that are producible at the quality you need.
Overview
Commercial-grade injection molding holds ±0.1–0.2 mm on most features. Fine-grade molding with controlled conditions can achieve ±0.05 mm. Material choice has a huge effect — glass-filled nylon is 3× more dimensionally stable than unfilled polypropylene.
Tolerance Specifications
| Feature | Standard | Precision | Notes |
|---|---|---|---|
| Linear Dimensions (< 50 mm) | ±0.10 mm | ±0.05 mm | Precision requires steel molds, controlled process parameters, and dimensionally stable resin. |
| Linear Dimensions (50–150 mm) | ±0.20 mm | ±0.10 mm | Shrinkage variation accumulates with part size. Glass-filled resins hold tighter. |
| Across Parting Line | Add ±0.05–0.10 mm | Add ±0.03 mm | Flash and mismatch at parting line. Precision requires well-maintained molds. |
| Wall Thickness | ±10% of nominal | ±5% of nominal | Core shift in deep parts causes thickness variation. Short cores are more stable. |
| Flatness | 0.2 mm per 25 mm | 0.1 mm per 25 mm | Warpage is the #1 issue. Uniform wall thickness and proper gate location are critical. |
| Hole Diameter | ±0.05 mm | ±0.025 mm | Core pin holes. Precision requires polished, hardened core pins with proper draft. |
| Surface Finish (SPI) | SPI B-2 to C-1 | SPI A-1 to A-2 (mirror) | Mirror finish requires hardened steel mold with diamond polishing. Adds significant mold cost. |
| Sink Marks | < 0.1 mm depth | < 0.05 mm (cosmetic) | Ribs should be 50–60% of wall thickness. Boss walls same rule. |
| Draft Angle | 1°–2° per side | 0.5° minimum | Texture adds draft requirement: add 1° per 0.025 mm texture depth. |
Key Considerations
Material Shrinkage Matters Most
ABS shrinks 0.4–0.7%, Nylon 66 shrinks 1.0–2.0%, PP shrinks 1.0–2.5%. Choose dimensionally stable resins (POM, PC, glass-filled) for tight tolerances.
Uniform Wall Thickness
The single most important design rule. Varying wall thickness causes differential cooling → warpage → tolerance problems. Target ±10% wall variation.
Gate Location Affects Everything
Gate position determines fill pattern, weld line location, and dimensional stability. Discuss gate location with your molder early.
Mold Temperature Control
Proper mold cooling (conformal cooling for complex parts) is essential for repeatable tolerances. Cheap molds with poor cooling produce inconsistent parts.
💰 Cost Impact of Tolerances
Precision injection molding requires hardened steel molds (P20 → H13), scientific molding process validation, and often 100% CMM inspection — mold cost jumps 30–50%, piece price 15–25% higher.
Standard tolerances are achievable with pre-hardened steel molds (P20, 718) and basic process control.
For parts that need tight tolerances in a few areas, design those features on the same mold half (cavity or core) and use standard tolerances everywhere else.
⚠️ Common Tolerance Mistakes
- ⚠ Specifying ±0.05 mm on a large polypropylene part — PP's high shrinkage rate (1.0–2.5%) makes this impossible without glass-fill.
- ⚠ Designing thick walls to add strength — thick sections shrink more, causing sink marks and warpage. Use ribs instead.
- ⚠ Ignoring gate vestige location on cosmetic surfaces — always specify gate location in your requirements.
- ⚠ Tolerancing snap-fit features tighter than needed — snap fits are inherently forgiving by design.
- ⚠ Not specifying which surfaces are cosmetic vs. non-cosmetic — leads to expensive over-finishing.
💡 Pro Tips
- ▸ Glass-filled resins (GF30 Nylon, GF20 PBT) are the go-to for tight tolerances — they shrink less and resist warpage.
- ▸ For transparent parts requiring tight flatness, consider PMMA (acrylic) over PC — lower internal stress.
- ▸ Our Vietnamese mold makers build production-grade P20 and 718 steel molds at 40–60% of US/European cost with the same tolerances.
- ▸ Request T1 samples (first trial) with CMM measurements before approving production tooling.
- ▸ For multi-cavity molds, expect slightly wider tolerances than single-cavity — cavity-to-cavity variation adds ±0.02–0.05 mm.
Ready to get started with Injection Molding?
Upload your design and get a quote with DFM feedback from our engineering team.