Injection Molding Tolerances: A Complete Guide to Dimensional Accuracy
Injection molding tolerances determine how precisely a plastic part matches its design specifications. For engineers and procurement professionals sourcing custom plastic parts, understanding tolerance standards is critical to balancing quality, cost, and manufacturability.
What Are Injection Molding Tolerances?
Injection molding tolerances define the allowable variation in a part’s dimensions. These are specified in the 2D drawing using standard tolerance classes (fine, medium, coarse) or feature-specific tolerance callouts (e.g., ±0.10mm on a critical hole diameter).
The three main standards used in injection molding are:
- ISO 20457:2018 — Plastic moulded parts — Tolerances and acceptance conditions
- DIN 16901 — German standard for plastic moulded parts tolerances
- SPI/ANSI — US standard from the Society of the Plastics Industry
Standard Tolerance Classes
| Class | Typical Tolerance | Application |
|---|---|---|
| Fine (A) | ±0.05mm to ±0.15mm | Precision mechanical parts, medical devices, gear systems |
| Medium (B) | ±0.15mm to ±0.40mm | Consumer products, automotive interior, general industrial parts |
| Coarse (C) | ±0.40mm to ±1.00mm | Non-critical parts, handles, cosmetic covers |
| Very Coarse (D) | ±1.00mm to ±3.00mm | Unseen structural parts, large rough components |
Key Factors Affecting Tolerances
1. Material Shrinkage — Every plastic material shrinks as it cools: ABS (0.4-0.7%), PC (0.5-0.7%), PA66 (1.0-1.5%), PP (1.5-2.5%), POM (1.8-2.5%).
2. Mold Design — Proper gate placement, cooling channel layout, and ejection system design directly affect dimensional stability.
3. Processing Conditions — Injection pressure, holding pressure, melt temperature, mold temperature, and cooling time all influence final part dimensions.
4. Part Geometry — Uniform wall thickness (typically 2.0mm-3.0mm) is recommended for consistent tolerances.
5. Mold Steel — Hardened tool steel (NAK80, S136) maintains dimensional stability over millions of cycles.
GD&T Basics for Plastic Parts
Key GD&T controls for injection molded parts include: Flatness (controls warpage), Position (controls hole-to-hole location), Profile (controls surface contours), and Concentricity (controls round part alignment).
How to Specify Tolerances
- Identify critical dimensions that affect fit, function, or assembly
- Assign tolerance classes — Fine only where absolutely necessary
- Note the standard on your drawing: “Tolerances per ISO 20457:2018 Class B”
- Call out tight features directly (e.g., ⌀10.00 ±0.05)
- Include material and expected shrinkage rate
RCH Plastic routinely holds Medium (B) class tolerances on production molds. Contact our engineering team to discuss your tolerance requirements.