Injection molding tolerances
Tolerance is not a single number. It depends on the material, the feature, the tool, the process and how the part is measured. A drawing that specifies ±0.05 mm everywhere is usually over-specified and more expensive than it needs to be.
What drives tolerance
- Material — shrinkage and moisture uptake vary by resin.
- Feature — a dimension across a cavity is not the same as a wall thickness or a boss height.
- Tool — steel, cooling, gate and ejection affect repeatability.
- Process — hold pressure, melt temperature and cycle time.
- Measurement — datum, fixture and gauge decide what “in tolerance” means.
Typical values
| Feature | Typical | Note |
|---|---|---|
| General plastic dimensions | ±0.1 mm | Confirm against the drawing and material. |
| Precision features | ±0.05 mm | Needs a stable process and a defined measurement method. |
| Wall thickness | Material and flow dependent | Avoid thick sections that sink or warp. |
| Boss height / diameter | Feature dependent | Draft and ejection matter. |
| Flatness / warp | Part and material dependent | Cooling and gate location drive this. |
How to specify
- Mark only the dimensions that affect fit, function or assembly.
- Use general tolerance for the rest; do not tighten everything.
- Define the datum and the measurement method for critical features.
- Tell us which dimensions are checked at first article.
- If a tolerance is not achievable, discuss a design change before tooling.
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