Measure and Inspect - 8 of 13Updated 13 November 2026
Measure Surface Roughness with Contact
A contact roughness tester can measure Ra on a Rigid Plastiform impression. For Rt or Rz, use a non-contact instrument.
The usual product for this application is P80 Ra, a Rigid Pasty product made for indirect roughness measurement.
When to Use P80 Ra
- The roughness tester cannot reach the surface to inspect, inside a straight bore for example.
- You need a contact roughness measurement and the shape has zero removal constraint.
P80 Ra is a Rigid product. It cannot be extracted from undercuts, threads, or grooves without breaking. If the shape has any removal constraint, use a Flexible product and a non-contact method instead.
Step-by-Step
- Degrease the surface with a validated degreaser and wipe it dry with a clean microfiber.
- Mount the cartridge with a standard nozzle. Do not use a tip.
- Extrude a test drop and check that its color is uniform.
- Apply the product while keeping the nozzle in permanent contact with the surface. Use slight pressure so the material penetrates the surface properly.
- Compress with a flat card. A business card or the extraction card works well, as does any flat object. This improves contact with the surface and gives the impression a flat back, which helps the roughness tester.
- Wait for polymerization. P80 Ra can be removed after 6 minutes at 23°C.
- Extract straight out. Because there is no removal constraint, the impression should slide out easily.
- Measure with a roughness tester. Wait 30 minutes after polymerization before you measure, so the surface details are rigid enough for the stylus. Fix the impression to a flat base with a thin layer of adhesive wax or putty so it cannot move. Measure at a temperature close to the one at which you took the impression.
Verification
Independent tests by CETIM Laboratories have shown that P80 Ra impressions match the original surfaces within microns.
For your own validation, compare the impression result with a measurement taken directly on a known reference surface.
Troubleshooting
| Problem | Likely Cause | Solution |
|---|---|---|
| Ra value is too low | Surface not fully polymerized or air trapped at the surface | Apply more pressure, compress with a flat card, and degrease the part again. |
| Impression cracks on extraction | Removal constraint was underestimated | Recalculate the constraint. If P80 Ra cannot be extracted, switch to a Flexible product and a non-contact method. |
| The stylus leaves scratches on the impression | Impression is not fully polymerized | Wait 30 minutes before you measure, until the product reaches its full hardness. |
| Measurement repeats poorly | Impression moves on the fixture | Fix it with a stronger adhesive wax or a dedicated holder, so the stylus cannot make the impression slide. |
| Surface looks smeared or soft in spots | Grease or oil residue | Degrease again with a validated degreaser and take a new impression. |
Related Pages
- Final Hardness
- Removal Constraint
- Measuring Impressions
- Measure Surface Roughness Without Contact
- Measure with a Contact Instrument
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