Reference

Injection Molding Tolerance Chart

Typical achievable molding tolerances by dimension range, how DIN 16742 tolerance groups work, and why resin shrinkage and wall thickness set the limit.

Injection molding tolerances are looser than machining because the part shrinks as it cools, and how much it shrinks depends on the resin, the wall thickness, and the geometry. Instead of a fixed number, molders budget a percentage of the nominal dimension: roughly 0.5% for commercial work and 0.2% for fine, with tighter values only on small features in low-shrink resins. This chart shows typical achievable tolerances by dimension range, how the DIN 16742 general-tolerance system is called out, and how resin shrink rate drives the whole thing.

Why molding tolerances differ from machining

A machined part is cut to size, so tolerance is set by the machine and inspection. A molded part is formed in a cavity and then shrinks 0.3% to 2.5% or more as it cools, so the cavity is cut oversize to compensate for the resin's expected shrink. Variation in melt temperature, pack pressure, cool time, moisture, and tool wear all move the final dimension, which is why tolerances scale with size and resin rather than being a single fixed value. Amorphous resins (ABS, PC) shrink less and hold tighter; semi-crystalline resins (PP, nylon, acetal) shrink more and hold looser.

Typical achievable molding tolerances by dimension

Commercial tolerances are what a molder holds without special effort. Fine or precision tolerances require a tuned tool, a low-shrink resin, and tighter process control, which adds cost. As a rule of thumb, budget about 0.5% of the nominal dimension for commercial and 0.2% for fine over roughly 25 mm.

Nominal dimension	Commercial (approx 0.5%)	Fine / precision (approx 0.2%)
Small feature (up to ~25 mm)	+/- 0.10 to 0.125 mm	+/- 0.05 to 0.075 mm
over 25 to 50 mm	+/- 0.20 mm	+/- 0.10 mm
over 50 to 100 mm	+/- 0.35 mm	+/- 0.15 mm
over 100 to 200 mm	+/- 0.50 mm	+/- 0.25 mm
over 200 mm	approx 0.5% of dimension	approx 0.2% of dimension

Typical values, resin and geometry dependent. Tight precision (down to about +/- 0.025 mm) is realistic only on small features in low-shrink amorphous resins with a tuned tool.

DIN 16742 tolerance groups

DIN 16742 (which superseded DIN 16901, alongside the international ISO 20457) is the general-tolerance standard for injection-molded plastics. Rather than tolerancing every dimension by hand, the designer calls out a tolerance group (TG) on the drawing, and the standard supplies the permissible deviation for each nominal dimension range within that group; a lower group number is tighter. The standard also separates mold-fixed dimensions (formed within one mold half) from non-mold-fixed dimensions (crossing the parting line or moving cores), which carry a larger allowance because tool movement adds variation. Pick a group your molder confirms is achievable in your resin before release.

Shrink rate by resin (a key DFM factor)

Shrinkage is designed into the tool, so a higher and less predictable shrink rate means a looser achievable tolerance. Uniform wall thickness is the other lever: non-uniform walls cool at different rates and warp, widening the effective tolerance.

Resin	Type	Typical mold shrinkage
ABS	Amorphous	0.4 to 0.7%
Polycarbonate (PC)	Amorphous	0.5 to 0.8%
Nylon (PA6 / PA66)	Semi-crystalline	0.7 to 1.5% (up to ~2%)
Polypropylene (PP)	Semi-crystalline	1.5 to 2.2%
Acetal (POM)	Semi-crystalline	1.8 to 2.5%
Glass-filled grades	Filled	Lower but directional (often ~0.2 to 0.5% in flow)

Typical ranges; actual shrink depends on grade, wall thickness, gate location, and process. Confirm the shrink value with your molder.

Values reflect DIN 16742 / ISO 20457 framing and common molding practice. Achievable tolerance depends on resin, wall thickness, tool design, and geometry. Confirm what a molder can hold on your part before release.

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Frequently Asked Questions

What is a typical injection molding tolerance?

For general commercial parts, roughly +/- 0.1 mm on small features, scaling to about 0.5% of the nominal dimension on larger ones. Fine or precision work runs near 0.2% (down to about +/- 0.025 mm on small features in low-shrink resins), which adds cost.

Why are molding tolerances looser than machining tolerances?

A molded part shrinks as it cools, typically 0.3% to 2.5% depending on the resin, and that shrink varies with process conditions and wall thickness. The tool is cut oversize to compensate, but the residual variation is what sets the achievable tolerance, so it scales with size and resin rather than being fixed.

What is DIN 16742?

DIN 16742 is the general-tolerance standard for injection-molded plastic parts (it replaced DIN 16901, alongside ISO 20457). The designer calls out a tolerance group on the drawing and the standard supplies permissible deviations by dimension range, with larger allowances for non-mold-fixed dimensions that cross the parting line or moving cores.

How does resin choice affect the tolerance I can hold?

Amorphous resins like ABS and PC shrink less (about 0.4 to 0.8%) and hold tighter tolerances. Semi-crystalline resins like PP, nylon, and acetal shrink more (roughly 1.5 to 2.5%) and less predictably, so they hold looser. Glass fill lowers shrink but makes it directional, which can warp parts if walls are uneven.

Tell us your resin, part size, and the few dimensions that must be tight, and set a budget. We source molders that can hold it and bring back a best-match quote.

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