Thermoforming Tolerances Explained: What Accuracy to Expect

Thermoforming is a near-net-shape process, not machining — so the way you specify tolerances matters. Ask for injection-mould tolerances on every dimension and you’ll overpay or get a “no-quote”; specify them where they actually matter and you get accurate, affordable parts. Here’s how to think about it.

Two kinds of features, two levels of accuracy

The key idea: a thermoformed part has formed features and trimmed features, and they hold different tolerances.

• Formed features — surfaces and geometry created as the sheet draws over the tool. These follow the tool but are affected by material shrinkage and the draw, so they carry looser, general tolerances.
• Trimmed features — edges, cut-outs, holes and interfaces created by 5-axis CNC trimming after forming. Because these are machined against datums, they can be held much tighter.

So the practical rule is: put your tight tolerances on the CNC-trimmed, datum-referenced features (mounting holes, mating edges, interface points), and accept general tolerances on the broad formed surfaces.

What “ISO 2768 coarse” means in practice

We make parts to ISO 2768 — coarse class (c) as standard for general formed dimensions, with tighter tolerances on critical, CNC-trimmed and datum-referenced features. ISO 2768 is a general-tolerance standard: it assigns permitted deviations by feature size and class (fine f, medium m, coarse c, very coarse v) so you don’t have to tolerance every dimension individually. Coarse class is realistic for the formed geometry of a thermoformed part; the features that must fit an assembly are then called out tighter and trimmed to datum.

Why shrinkage matters

All thermoformed parts shrink as they cool, and the amount depends on the grade. Tooling is sized to compensate for the material’s known shrinkage — which is one reason consistent, in-house-controlled sheet helps: if the material varies lot to lot, so does the shrinkage, and so do your dimensions. Controlled cooling and a stable cycle keep shrinkage repeatable.

How to specify tolerances that work

1. Datum your drawing. Define the datums the part must locate to in the assembly. Trimmed features are held to these.
2. Tolerance the critical features individually — the holes, edges and interfaces that must fit. Make these tight.
3. Let the rest fall under a general class (ISO 2768 coarse) for the broad formed surfaces.
4. Flag flatness / warp requirements on large flat panels, so we design in stiffening geometry and controlled cooling.
5. Don’t over-tolerance. Every unnecessary tight tolerance adds cost or forces a no-quote; reserve them for where fit genuinely depends on it.

The takeaway

Specify tight where it’s trimmed and it matters; general everywhere else. That gets you a part that drops into your assembly, quotes cleanly and doesn’t cost more than it should. Not sure how to tolerance a feature? Send the drawing and we’ll advise what’s achievable and where to hold it — or read the full design guide.