Injection-moulded parts are rarely uniform optical samples. Curvature, ribs, bosses, flow lines, weld lines, texture and wall-thickness changes can alter both the material appearance and the way an instrument sees it. A reliable method begins by defining measurement zones on the actual component.
Select representative measurement locations
Choose flat areas large enough to fully cover the instrument’s viewed area. Record the location with a drawing or photograph and identify the measurement orientation. Avoid gates, ejector marks, edges and obvious local defects unless those features are part of the acceptance requirement. For large parts, use several fixed zones rather than one convenient point.
Control the moulding variables
- Use the same resin grade, masterbatch let-down ratio and regrind policy.
- Record melt and mould temperatures, residence time and drying conditions.
- Hold wall thickness, texture and cooling conditions consistent.
- Condition parts to an agreed temperature and time after moulding.
- Keep the same face, flow orientation and backing at every reading.
Use plaques carefully
A moulded plaque is useful for supplier approval and process trials because it provides a flat, controlled surface. It does not automatically predict a finished component with different thickness, texture or cooling. Establish the relationship between plaque and part, then state which specimen controls release.
Measurement workflow
- Standardise the instrument and confirm its verification check.
- Inspect and clean the measurement zones without polishing them.
- Place the part in a fixture or against a stable support without deforming it.
- Measure the specified locations and orientations.
- Review individual results as well as the average; a good average can hide a bad zone.
- Compare to the approved working standard using the stated color-difference formula and component limits.
Instrument fit
The SD-500 offers camera-assisted positioning and aperture options for many moulded parts; the SD-700 UV adds UV control where fluorescent or optically brightened materials require it. Benchtop models can help when fixtures and repeatable presentation are central to the method.
See the complete plastics guide, textured-plastics method and plastic-part tolerance guide for the rest of the workflow.
Last reviewed 21/09/2026 · Level: Intermediate
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