Key takeaways
- A moulded part moves after moulding. Measure it in the agreed conditioned state, not straight from the press.
- Fixturing can hide a warp or create one. Support the part at the datums and avoid over-clamping.
- GD&T on a plastic part should reflect the function, especially at a sealing face or a snap fit.
- Record the condition, the fixture and the method with the result. A number without that context is not evidence.
Why plastic measurement is different
A machined metal part is stable when it comes off the machine. A moulded plastic part is not. It shrinks as it cools, absorbs moisture, creeps under its own weight and can flex under the smallest clamp force. The same part in two states can give two different answers.
The difference is not a reason to accept loose measurement. It is a reason to define the measurement condition. The drawing, the inspection plan and the first article report should agree on the temperature, the humidity, the conditioning time, the fixture and the datum scheme. Without that agreement, a dimensional dispute has no common basis.
Plastic has a lower modulus than metal, so the measurement force matters. A touch probe that is fine on steel can deflect a thin plastic wall, and a clamp that holds the part flat can remove the warp the drawing controls. The method has to match the material.
Condition the part before measuring
Most plastics are measured after conditioning at 23°C and 50% relative humidity. The conditioning time depends on the material and the wall thickness. A thin part may reach equilibrium in a day; a thick PA66 part may take longer. ISO 291 sets out the standard atmosphere and the conditioning procedure.
PA66 is the clearest example. A dry moulded part grows as it absorbs moisture, and the growth can be enough to change a fit. If the drawing calls for a conditioned dimension, the inspector needs to know the conditioning state and the time. If the drawing calls for a dry dimension, the part must be measured before it picks up moisture, which is difficult to guarantee in a normal inspection room.
Temperature matters as well. A part measured at 18°C is not the same as one measured at 23°C, especially for a large housing with a long dimension. Let the part stabilise in the inspection room before measuring, and record the room condition with the result.
The condition is part of the dimension
A measured value without the conditioning state is incomplete. Put the condition on the drawing and in the report.
Fixture the part without changing its shape
A fixture has two jobs: locate the part repeatably and hold it without distorting it. Those jobs can conflict. A clamp that is strong enough to hold a flexible housing can also flatten a warp or pull a datum away from its true position.
The usual approach is to support the part at the datum features and use the minimum force needed to keep it stable. For a part with a flexible base, a support at the mounting points is more representative than a clamp across the middle. For a part that is assembled under load, a restrained fixture may be appropriate, but the report must say so.
Free state and restrained state are different measurements. A cover that is flat only when it is screwed down is not flat in the free state. If the function depends on the assembled shape, measure it in a fixture that simulates the assembly and state the restraint in the report. Do not mix the two states in one table.
Use GD&T that reflects the function
GD&T gives a plastic part a way to control form, orientation, location and profile without stacking linear tolerances. A profile of a surface control can hold a sealing face or a mating contour. A position control can locate a hole or a boss relative to the datums. A flatness control can hold a cover interface.
The datum scheme is the foundation. Choose the features that locate the part in the assembly, not the features that are easiest to measure. A housing may locate on two mounting holes and a face; those are the datums. A boss that is only cosmetic should not be a datum.
Plastic parts need careful use of modifiers. A maximum material condition modifier gives bonus tolerance on a hole, but only if the function is clearance. A projected tolerance zone suits a boss or stud that mates with another part. Do not add modifiers the inspection method cannot verify.
Profile is often the most useful control for a moulded surface, because it limits the form and the location together. It also suits a CMM or an optical scan. A profile tolerance that is tighter than the process can hold will add cost without improving the fit.
Choose the method for the feature
A coordinate measuring machine with a touch probe is the default for dimensional work, but it needs a low probing force and a stable fixture. A thin wall or a soft surface can be marked or deflected by a probe. An optical system or a structured light scanner avoids contact and can capture a full surface, but it needs a matte surface and a suitable reference.
A calliper or a micrometer is useful for a simple length or a wall thickness, but it is not a substitute for a datum-based measurement. A wall thickness measured with a calliper at the edge may not represent the wall at the centre of the part. An ultrasonic thickness gauge can measure a wall without cutting the part, but it needs a calibration standard.
For a critical internal feature, a section cut through the part is often the most direct check. It shows the wall, the rib root and the insert bond in one view. A section is a destructive test, so it belongs on a sample, not on the first-off part that has to be returned.
Feature, method and the trap
The table pairs common moulded features with a suitable method and the mistake that makes the result unreliable. It is a starting point for the inspection plan, not a substitute for the drawing.
| Feature | Suitable method | Common trap |
|---|---|---|
| Overall length | CMM or calliper on a conditioned part | Measuring dry PA66 and comparing with a conditioned drawing |
| Wall thickness | Ultrasonic gauge or section | Calliper at the edge instead of the wall centre |
| Flatness of a cover face | CMM or surface plate in the free state | Clamping the part flat before measuring |
| Hole position | CMM with the datum scheme | Using a convenient edge instead of the functional datum |
| Sealing face profile | CMM scan or optical scan | Measuring in the restrained state without saying so |
| Insert position | CMM or section | Checking the thread position but not the boss wall |
The condition and the fixture belong in the report for every row. A method that ignores either can give a repeatable but wrong answer.
Report the result with its context
A first article report should state the drawing revision, the material batch, the moulding condition, the conditioning state, the fixture, the measurement method and the result. The result should include the nominal, the tolerance and the deviation. A report that lists only the measured value cannot be reviewed.
For a plastic part, add the shrinkage check and the critical features. If the part is measured after a defined cooling time, say so. If the part is measured in a restrained fixture, say so. If the measurement is optical, state the scan resolution and the reference frame.
The report should also record any feature that could not be measured. A missing result is more honest than a result that was taken in the wrong state. The reviewer can then decide whether the feature needs a different method or a sample section.
What to put on the drawing
Put the general tolerance, the critical dimensions, the datum scheme and the surface requirements on the drawing. State the material, the colour and the flame rating. Add the conditioning requirement if the material is moisture sensitive. If the part is measured in a restrained state, show the restraint or reference the assembly drawing.
Keep the critical dimensions to the features that locate, seal or carry load. A drawing with every dimension tight tells the supplier that nothing is critical, and it makes the inspection plan expensive without improving the fit. A general tolerance covers the rest.
If the part has a snap fit, a sealing face or a terminal pocket, define the functional requirement as well as the dimension. A profile tolerance on a sealing face is more useful than a stack of linear dimensions, because it controls the surface that actually seals.
Frequently asked questions
At what temperature should a plastic part be measured?
Most plastics are measured at 23°C and 50% relative humidity after conditioning. The conditioning time depends on the material and the wall thickness. Let the part stabilise in the inspection room and record the room condition with the result.
Should a moulded part be measured free or restrained?
It depends on the function. A free-state measurement shows the part as moulded; a restrained measurement shows it as assembled. State which one the drawing requires and use the same state in the report. Do not mix the two in one table.
Why does a PA66 part measure differently after a few days?
PA66 absorbs moisture, and the absorbed water makes the part grow. A dry part and a conditioned part are different. Specify the conditioned dimension or the dry dimension, and measure in that state. Glass-filled grades move less, but they still change.
Can a CMM measure a thin plastic wall?
It can, but the probing force must be low and the part must be supported. A thin wall can deflect or be marked by a probe. An optical method or an ultrasonic thickness gauge may be better for a delicate feature. Confirm the method in the inspection plan.
Sources and further reading
- ISO 291:2008, Plastics, standard atmospheres for conditioning and testing
- ISO 20457:2018, Plastics moulded parts, tolerances and acceptance conditions
- ASME Y14.5-2018, Dimensioning and tolerancing
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