‘What tolerance can you hold?’ is only useful when tied to a particular feature. A small hole, a long flat housing and a dimension crossing the mold parting line do not present the same manufacturing problem. Begin with the function the part must perform, then agree an achievable specification.
General procurement reference. Final specifications, manufacturing feasibility and any compliance evidence must be confirmed for the particular product and intended use.
Translate the function into dimensions
A tolerance is the permitted variation in a specified characteristic. For example, a 50.00 mm dimension with ±0.10 mm limits permits 49.90–50.10 mm. This is an explanation of notation, not a statement that any particular molded part can achieve that range.
Identify what fails if a feature changes: an assembly may bind, a seal may leak or an external gap may become unacceptable. Review mating parts and tolerance accumulation together. Cosmetic appearance, texture and color need their own acceptance criteria rather than being hidden in a dimensional note.
Why material and geometry change the answer
Cooling, shrinkage, moisture and the molding process can affect the measured result. Use the selected resin grade and its supplier data; a generic polymer name is not enough to set a shrinkage allowance. Fillers, geometry and processing conditions also require review.
| Feature or condition | Review question |
|---|---|
| Long span or thin, flat wall | How will warpage affect the function and the measurement setup? |
| Dimension crossing a parting line | Which mold components control it, and can a critical fit be moved to a more stable reference? |
| Slider, lifter or insert feature | How will movement, alignment and wear be controlled? |
| Hygroscopic or filled resin | What conditioning, material direction and supplier data apply? |
| Machining or assembly after molding | At which stage is the feature accepted, and which operation sets its final tolerance? |
Build a drawing that can be inspected
ISO 20457 addresses tolerances and acceptance conditions for plastic molded parts. If used on a drawing or purchase order, agree the applicable edition and tolerance choices. Referencing a standard does not establish a supplier’s process capability or replace a part-specific review.
- Specify the resin grade, units, drawing revision and the state of the part being measured.
- Identify reference features and datums where needed, and show how the part is supported or constrained during inspection.
- Apply an agreed general tolerance scheme and call out functional exceptions individually; do not copy an unrelated metal-part tolerance block.
- Specify temperature and conditioning when they affect the result; agree when after molding measurements are taken.
- Agree the measuring instrument, fixture and method, especially for flexible parts or tight limits.
Review the cost of precision without a blanket premium
Tighter limits may require changes to tooling, materials, process control, inspection or secondary operations. Their cost depends on the feature and volume; there is no reliable universal percentage premium for moving from one tolerance to another.
Ask the supplier to separate functional requirements from preferences. Compare an as-molded solution with a design change or secondary operation when appropriate. Any proposed relaxation should be approved by the party responsible for the design, with the drawing updated before manufacture.
Agree evidence and acceptance before production
A conforming first sample shows the result for that sample. It does not, by itself, establish repeatable capability across cavities, shifts or future batches. Decide what measurement records and sampling coverage are needed for the actual risk and order.
- List the critical features, limits, inspection method and sample identification.
- Agree the sampling plan, conditioning and treatment of borderline or out-of-limit results.
- Separate visual master-sample approval from dimensional acceptance.
- Resolve differences between a sample and drawing in writing; neither should silently override the agreed specification.
- Record any approved change and reassess its effect on mating parts and inspection.
Frequently asked questions
Can you guarantee ±0.05 mm?
A commitment requires the specific feature, nominal dimension, resin grade, geometry and acceptance method. Submit these for review instead of applying one figure to the whole part.
Should I apply shrink compensation to the CAD model?
State whether the model represents the required finished part. Agree who applies tooling compensation and which material data is used, so compensation is not applied twice.
What should accompany a tolerance inquiry?
Provide the current drawing and model, material grade, critical features, mating-part information, quantity and measurement conditions. Include an existing inspection method if the customer requires it.
Bring the requirements to your project
Send your drawing, resin grade, critical dimensions and inspection requirements for a project-specific feasibility review.