Quality · Note 06
Which tolerances are worth paying for
A tight tolerance on a moulded feature is not free — it buys tool work, slower cycles, and more inspection. Spend it where the part actually has to fit.
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- Polydyne LLC
Moulding tolerance is the sum of several things that move independently: how precisely the steel was cut, how consistently the process runs shot to shot, how much the resin shrinks, and whether the feature crosses a parting line or a side action. A dimension held entirely within one half of the tool is far easier to hold than the same dimension measured across the split.
| Class | Typical band | What it costs |
|---|---|---|
| Standard commercial | ± 0.005 – 0.010 in | Normal tooling and process |
| Fine | ± 0.002 – 0.005 in | Tighter steel, closer process control, more inspection |
| Precision | ± 0.001 – 0.002 in | Steel-safe tooling, tool tuning at sampling, tight process window |
Broad guidance on achievable tolerance for a 1 inch feature. Actual capability depends on the resin, the geometry, and the tool.
Rules that keep the price honest
- Tolerance the features that mate with something. Leave the rest at a general note.
- Avoid tight tolerances across the parting line or across a side action where you can.
- Semi-crystalline resins move more than amorphous ones; a tolerance that is routine in ABS may be a precision job in acetal.
- Say which datum matters. A stack of tight dimensions from different datums is harder than one clear reference.
- Ask for steel-safe tooling on the critical features, so the first article can be corrected by removing steel rather than welding it.