Plastic parts cannot achieve the same tolerances as metal — thermal expansion, moisture absorption, mold shrinkage variation, and material creep all compound to create dimensional uncertainty that steel parts simply don’t have. Designing with realistic plastic tolerances prevents costly rework and production delays.
Realistic Tolerances by Manufacturing Method
| Method | Standard Tolerance | Precision Tolerance |
|---|---|---|
| Injection Molding (unfilled) | ±0.15-0.30mm | ±0.08-0.12mm |
| Injection Molding (GF-filled) | ±0.10-0.20mm | ±0.05-0.10mm |
| CNC Machined (plastic) | ±0.05-0.15mm | ±0.025-0.05mm |
| 3D Printed (SLS nylon) | ±0.15-0.30mm | ±0.10-0.15mm |
The DIN 16742 Standard
DIN 16742 is the international standard for plastic part tolerances, defining tolerance groups by material and manufacturing precision class. For nylon 6 injection molding at standard precision (TG 6), a 50mm dimension carries ±0.30mm tolerance. At fine precision (TG 5), that tightens to ±0.18mm. Note that these are achievable tolerances — not “draw whatever tolerance you want” guidelines. Every tighter-than-standard callout on your drawing increases inspection cost and scrap rate.
Frequently Asked Questions
Why can’t plastic parts hold metal-level tolerances?
Plastic undergoes three separate dimensional changes after processing: (1) mold shrinkage during cooling (0.5-2.0%), (2) post-molding crystallization shrinkage over 24-48 hours, and (3) environmental moisture/swelling. Metal parts only have thermal expansion to manage, which is reversible. Plastic’s dimensional changes are partially irreversible.
How should I dimension my plastic part drawing?
Dimension from a single datum (not chain dimensioning), specify ISO 2768-m or DIN 16742 as the general tolerance, and call out only truly critical dimensions with tighter tolerances. Over-dimensioning a plastic part drawing signals inexperience to your manufacturer and adds unnecessary inspection costs.
What happens if I specify ±0.05mm on an injection molded nylon part?
The mold maker will quote 30-50% higher to compensate for the tighter mold tolerances required, the injection molder will need to run with tighter process controls (statistical process control, potentially more scrap), and you’ll pay more for every aspect. In practice, ±0.05mm on a molded nylon part is at the very limit of what the process can achieve.
Can CNC machining hold tighter tolerances than injection molding for plastics?
Yes — CNC machining avoids the shrinkage variables of molding and can hold ±0.025mm or better. For low volumes (under 100-500 parts) where tight tolerances are critical, CNC machining is often the right manufacturing choice despite higher per-part cost compared to molded volume pricing.
Need a tolerance review for your plastic part design?
Send us your drawing or 3D model — we’ll flag over-toleranced dimensions and suggest realistic alternatives before you send the design out for quote.
