An injection molding tolerances guide is only useful if it tells the buyer what the supplier can actually hold. Metals have ISO 2768 general tolerances; plastics shrink anisotropically, creep under load and keep changing for hours after demoulding, so the industry moved to a dedicated standard. ISO 20457, first issued in 2018 as the international successor to DIN 16742 and updated in 2026, gives moulded parts their own tolerance language: nine tolerance grades, material-dependent shrinkage allowances and acceptance conditions. This guide explains how to read it and how to write tolerances into an RFQ.
Why an Injection Molding Tolerances Guide Starts With Shrinkage
Polymer shrinkage is the root cause. A 100 mm PP feature moulded at 2% shrinkage targets a 98 mm cavity, while the same feature in PC at 0.6% shrinkage targets almost 99.4 mm. Shrinkage also differs between flow and transverse directions, varies with wall thickness and keeps moving as the part crystallises. ISO 20457 accounts for this by tying tolerance grades to the moulding material, its stiffness and the production effort invested, rather than treating every polymer like machined steel.
Reading the ISO 20457 Tolerance Grades
The standard defines nine tolerance grades TG1 to TG9. TG1-TG3 cover precision engineering and demand tight tooling, engineering resins and servo presses; TG4-TG6 are the standard commercial grades used for enclosures, bezels and appliance parts; TG7-TG9 cover wide-tolerance parts in high-shrinkage resins, with rotational moulding classified at TG9. Typical bilateral tolerances for mould-bound dimensions:
| Nominal size (mm) | TG3 precision | TG4 technical | TG5 commercial | TG7 wide |
|---|---|---|---|---|
| 1-6 | ±0.04 | ±0.06 | ±0.10 | ±0.24 |
| 18-30 | ±0.06 | ±0.09 | ±0.15 | ±0.35 |
| 50-80 | ±0.11 | ±0.16 | ±0.24 | ±0.56 |
| 120-180 | ±0.18 | ±0.27 | ±0.40 | ±0.90 |
| 180-250 | ±0.23 | ±0.34 | ±0.50 | ±1.15 |
General Tolerances and the Title Block
A general tolerance ISO 20457 note in the drawing title block covers every untoleranced feature, and the 2026 edition states that general tolerances always use Series 1 and are therefore at least TG3. That single note is the cheapest tolerance a drawing can carry, but it only works if the drawing identifies which surfaces are subject to general tolerance verification. Directly toleranced angles and edges require agreement between customer and producer, and visible parting line locations must be agreed because they are unavoidable. Location and form features use a datum system per ISO 5459 with geometric tolerancing per ISO 1101, so a feature is judged against the datum origin, not just its nominal size.
Acceptance Conditions
ISO 20457 also tells you when to measure: parts are conditioned per ISO 291 and tested no earlier than 16 hours and no later than 72 hours after production, unless the contract says otherwise. A part that misses a general tolerance is not automatically rejected if functionality is unaffected. The same distinction applies to feature type: mould-bound dimensions follow the grade table, while non-mould-bound features such as parting line clearances carry wider allowances because the tool halves float relative to each other. For buyers, this clause ends the argument about whether to inspect a part the day it was moulded or three days later.
Writing Tolerances Into the RFQ
Tight tolerances are a cost decision, not a free upgrade: a tool built for TG1-TG3 costs 20-40% more than one for TG7-TG9, and tightening a critical feature from commercial to precision grade typically adds 15-30% to tooling plus ongoing CMM inspection. The productive move is to mark only the two to five dimensions that genuinely need precision, such as mating interfaces, snap fits and bearing bores, and leave the rest on the general tolerance. A drawing with ±0.05 mm on every dimension is a warning sign that the tolerances were copied, not engineered.
For more context, see the custom molding services hub, the part design guidelines, the injection molding machine precision guide and the verified manufacturer directory.
