A drawing can look complete while still leaving consequential dimensional decisions unresolved. Which surfaces locate the part in its assembly? Which dimension controls sealing, motion or appearance? At what state and fixture condition will it be checked? Injection molding GD&T can communicate design intent, but it needs a moulding-aware plan for datums, profiles and functional interfaces. Start by identifying each injection molding CTQ—critical-to-quality characteristic—as a function to protect, rather than treating every displayed dimension as equally important. The same discipline applies to injection molding CMM inspection: agree what is measured, how the part is constrained and what result is recorded before a report is used for acceptance. This work is part-specific; polymer, geometry, cooling and the mating assembly all influence the useful tolerance plan.
ASTM D955 measures shrinkage at 24 and 48 hours, but says standard-specimen data cannot predict absolute values for actual parts.[2]
01
1. Begin with function, then separate critical from general dimensions
The first buyer decision is not a blanket tolerance grade. Distinguish features that govern fit, force, sealing, alignment, travel, visible flushness or safety from those that merely describe envelope. For each functional feature, identify the mating condition, allowable variation, datum scheme and whether size, position, profile or a functional check is appropriate. Features with no defined assembly or use relationship may be suitable for a general tolerance. A snap, insert, long cosmetic surface or flexible arm should not inherit the assumptions of a simple cover.
ASME Y14.5 sets out the symbols, rules, definitions and practices used to state and interpret GD&T on drawings and digital product definition.[3] Its purpose is clearer design communication, not a promise that every geometric control is equally economical or appropriate for a moulded part. ISO’s moulded-parts tolerance standard likewise allows additional dimensional and geometrical specifications for functional needs, while noting that it does not replace requirements in a product standard.[1] Use a standard as a shared language; write the product-specific functional requirement and acceptance route separately.
02
2. Treat shrinkage and warpage as design inputs, not after-the-fact explanations
Plastic cools and changes dimension after it leaves the cavity. The amount and direction are affected by resin behaviour and the actual part and process: flow path, wall transitions, local packing, cooling, fibre orientation, gate placement and temperature gradients. ASTM D955 says mold and melt temperature, fill time and packing conditions significantly affect shrinkage; it also cautions that its standard specimens cannot predict absolute values in parts with different flow paths, wall thicknesses, pressure and temperature gradients.[2] Resin data and supplier processing guidance are inputs, not a guaranteed finished-part outcome.
Make dimensional risks visible before hardening a requirement. A symmetric, evenly cooled part may permit a straightforward approach; a long, thin, asymmetric or filled-material part may need deliberate feature placement, simulation, trial evidence or a functional gauge. Simulation can help explore fill, pack, cooling and deflection, and Autodesk identifies warpage, cooling and material selection among the issues its molding simulation addresses.[4] It is a decision aid, not a substitute for part measurement or assembly evaluation.
| Feature situation | Buyer question to resolve | Practical evidence to agree | Why the answer is part-specific |
|---|---|---|---|
| Mating bore, clip or locator | What functional relationship must still work? | Datum scheme, feature result and, where useful, an assembly or functional check. | Mating materials, loads and assembly method set the real need. |
| Broad or contoured visible surface | Is local shape, gap or appearance the governing concern? | Profile or gap-check approach, fixture and visual reference if applicable. | A size dimension alone may not describe perceived flushness. |
| Long, thin or uneven-wall geometry | Where could differential cooling change form? | Risk review and an agreed evaluation state or fixture; simulation when justified. | Geometry, gate location and material response interact. |
| Threaded, inserted or overmoulded interface | Which interface controls assembly and load transfer? | Interface-specific dimensions plus a representative mating check where needed. | Insert, mating part and use condition may dominate the result. |
03
3. Put the measurement condition into the requirement
A reported dimension has meaning only with its measurement definition. Specify the controlled drawing or model revision, feature and datum interpretation, relevant points or scan strategy, part condition, fixturing and report format. For an easily distorted moulding, unconstrained, gently supported and functionally constrained checks can produce different but legitimate values. Define which condition represents the intended decision rather than accepting the most convenient setup on the day.
A CMM can be useful for complex geometry, but its report is not automatically self-explanatory. NIST identifies measurement uncertainty and factors including part temperature, sampling strategy and operational effects in CMM dimensional measurement.[5] A close-limit result therefore merits review of the agreed method, datum alignment and measurement capability. For simple functional features, a purpose-built gauge or mating check may be more direct; for profiles or spatial relationships, programmed coordinate measurement may be more informative. Choose the method that answers the part’s functional question.
04
4. Build an acceptance plan around risk, not maximum paperwork
An effective first-article or qualification package need not measure every noncritical feature on every piece. It should confirm named functional interfaces, key geometry, material direction and relevant appearance or assembly conditions. Create a short feature list linking each item to its function, requirement, datum/fixture, measurement method, sample stage and report owner. This lets technical reviewers see why a feature is present and what result it is intended to decide.
General tolerances remain useful for dimensions that do not control function, but they should not disguise an unresolved critical interface. ISO 20457:2018 describes possible manufacturing tolerances for plastic moulded parts and covers general tolerances within a datum system as well as additional specifications for functional requirements.[1] ISO lists that edition as withdrawn and directs users to a newer edition, so a purchase specification should identify the edition agreed for the programme. A standard reference does not remove the need to set an inspection state for the individual part.
05
5. Preserve the baseline as the tool, material and process mature
Dimensional stability is not just a first-sample topic. The approved part baseline should connect drawing or model revision, resin grade, tool revision, critical-feature plan, measurement programme or gauge, and accepted sample or assembly result. When a change is proposed—such as a resin source, gate, cooling detail, cavity repair, mold setting or measurement routine—ask which functional outputs might move and what evidence is proportionate before release. The response may be a document review, targeted dimension check, assembly check or broader re-evaluation depending on change and part risk.
Avoid assuming a published shrinkage value settles the outcome, or treating every normal adjustment as proof of failure. ASTM D955’s limitations remind us that test data are comparative under specified conditions, not a universal prediction for production geometry.[2] Retain records of the approval condition and change decision so later reports are comparable to the baseline. This gives procurement a clearer basis for discussing evidence while leaving engineering free to choose controls that fit the actual part.
Practical takeaway
Before you move
to the next review.
- Name the few features that determine fit, function, sealing, movement, assembly or visible alignment, and link each to a functional reason.
- Define the controlling CAD or drawing revision, datums, tolerance method and any general-tolerance standard edition used for the programme.
- Record the material grade direction and the geometry, cooling or flow-related risks that could affect important features.
- Agree the measurement state, fixture, datum alignment, sampling or scan approach, result format and the role of any functional gauge.
- Set a written route for assessing and approving changes to tool, material, process, measurement method or mating-part baseline.
FAQ
Common questions
Should every dimension on a plastic part have a tight tolerance?
No. Apply explicit control where a feature affects the stated function, assembly, appearance or downstream operation, and use a sensible general approach for genuinely noncritical dimensions. Unnecessarily tight requirements can create cost and inspection effort without improving the product. The correct split depends on the part, material, geometry and mating context.
Can a material datasheet provide the finished-part shrinkage allowance?
It can inform early design and tool-planning discussions, but it should not be treated as the final allowance for every geometry. ASTM D955 explains that mould and process conditions affect measured shrinkage and that standard specimens do not predict absolute values in actual parts with different geometry and process conditions.[2] Confirm important interfaces with part-specific evidence.
When is a CMM report the right acceptance record?
It is often useful where spatial relationships, profiles or multiple datum-based features need a repeatable report. It is not automatically the best evidence for every feature. Agree the alignment, fixture, sampling strategy, part condition and uncertainty considerations, then compare that method with a functional gauge or assembly check where those more directly represent product use.
References
Source material
- [1] International Organization for Standardization, “ISO 20457:2018 Plastics moulded parts — Tolerances and acceptance conditions.”
- [2] ASTM International, “ASTM D955-21: Standard Test Method of Measuring Shrinkage from Mold Dimensions of Thermoplastics.”
- [3] ASME, “Y14.5-2018 (R2024): Dimensioning and Tolerancing.”
- [4] Autodesk, “Moldflow Adviser Ultimate.”
- [5] National Institute of Standards and Technology, “NISTIR 5170: Measurement Uncertainty Considerations for Coordinate Measuring Machines.”
This guide is general information for early procurement and engineering discussion. It does not replace product-specific DFM, qualification, validation, regulatory assessment or legal advice.