Plastic injection molding tolerances should be set from part function, the selected material and the inspection method—not copied from a metal drawing or treated as a single number that fits every feature. Buyers and molders need to agree which dimensions matter, how the part will be measured and what evidence will determine acceptance.
ISO 20457:2026 is the current edition of the international standard for tolerances and acceptance conditions for plastic moulded parts. ISO published the second edition in August 2026; it replaces the withdrawn 2018 edition. Use the purchased or authorized standard and the project drawing for exact tolerance values and clauses. This guide explains how to prepare a buyer-side review without reproducing the standard’s tables.
What ISO 20457:2026 covers
ISO describes the standard as a framework for geometrical and dimensional tolerances, acceptance conditions and technically achievable tolerances for plastic moulded parts. Its stated scope includes non-porous parts made from thermoplastics, thermoplastic elastomers and thermosets using processes such as injection moulding. Check the official scope and the standard itself for the process, material and part covered by a specific project.
The standard does not replace the product drawing, purchase specification or any product-specific requirement. It also does not mean that a part is automatically acceptable simply because a drawing mentions ISO 20457. The buyer and supplier still need to identify the applicable edition, features, measurement conditions and acceptance rules in their contract documents.
Separate functional dimensions from general dimensions
Mark the dimensions and geometrical relationships that control assembly, sealing, motion, appearance or another defined product function. For each critical feature, state what it interfaces with and the consequence of a deviation. This gives the toolmaker and molder context for reviewing the drawing.
- Identify mating parts, datum references and the assembly condition used for evaluation.
- Call out only the features that need a direct tolerance or a specific geometrical requirement; define how remaining features are controlled.
- Include the material grade and any relevant reinforcement, colorant or regrind requirement in the project specification.
- Describe the part condition for measurement where time, temperature, moisture or assembly state can affect the result.
For a design review, pair the drawing with a plastic injection molding DFM checklist so wall transitions, draft, ribs, gates and other design choices can be considered alongside the tolerance callouts.
RFQ checklist for a tolerance review
| Topic | Buyer provides | Supplier confirms |
|---|---|---|
| Standard and edition | Applicable standard, edition, drawing notes and any customer-specific rules | Which requirements are understood as applicable and any requested exceptions |
| Part function | Critical dimensions, datum scheme, mating parts and assembly conditions | Potential conflicts between the requested tolerance and the design or process |
| Material and part definition | Resin grade, reinforcement, color and controlled model or drawing revision | Material assumptions used for tooling review and how substitutions will be approved |
| Measurement method | Inspection locations, part condition and required report format | Proposed instrument, datum setup, sampling and measurement limitations |
| Acceptance | Sample stage, quantity, acceptance criteria and treatment of deviations | Evidence available from the tool trial or production-intent samples |
How to review a supplier’s tolerance proposal
- Send a controlled drawing. Identify the revision, material, intended function and any referenced standard. Avoid sending an unlabeled screenshot as the only source of dimensional requirements.
- Ask for feature-specific feedback. Have the toolmaker or molder mark dimensions that need clarification, a different datum strategy or a design review. Ask for the reason behind each proposed change.
- Resolve the measurement setup. Agree where and how each critical characteristic will be measured, what equipment is appropriate and how the part will be conditioned or supported during inspection.
- Record agreement on the controlled documents. Update the drawing, inspection plan or purchase specification so that the buyer, toolmaker, molder and inspector are working from the same requirements.
- Review production-intent samples. Compare dimensional results with the agreed criteria and retain the sample identity, material lot, tool or cavity, measurement method and report revision.
- Control changes. Repeat the relevant review if the material, tool, process, drawing or measurement method changes. Record who approved any deviation and for which lot or period it applies.
A tolerance proposal is specific to the part, material, tool and process under review. A general capability claim or a result from a different product does not establish acceptance for the current drawing.
Agree sample inspection and acceptance before tooling release
Before ordering a tool or approving a production-intent sample, define the inspection stage and records required for approval. The project team can decide whether to use first-article inspection, repeated measurements, process data or an agreed capability study for particular characteristics. Do not assume a single capability threshold applies to every part or contract.
- List the critical characteristics and the drawing revision they belong to.
- Define the sample quantity, part identification and tool or cavity traceability.
- Specify the measurement equipment, datum setup, part preparation and conditioning state.
- State how results, nonconforming samples and approved deviations will be recorded.
- Agree the approval authority and what must be rechecked after a change.
For related quality planning, see DAYIN’s guide to quality and precision in plastic injection molding. Confirm that each requirement in the final drawing and inspection plan is project-specific and approved by the relevant parties.
Common tolerance specification gaps
- A drawing says “tight tolerance” without identifying the functional feature, limit or inspection method.
- The standard is named without an edition or without stating how customer-specific requirements take precedence.
- A critical dimension has no clear datum, part condition or agreed way to hold the part during measurement.
- The drawing, 3D model, purchase specification and inspection plan use different revisions or conflicting values.
- A material substitution, tool change or process adjustment is made without an agreed review of affected dimensions.
Discuss a plastic injection molding project
For an OEM/ODM tolerance review, share the controlled drawing or 3D model, resin specification, target market, annual volume range, critical interfaces and sample acceptance requirements. DAYIN can review the project inputs and confirm what information is needed for a quotation. Contact DAYIN to start the discussion.