Loose pellets create an optically irregular surface with gaps, shadows and variable orientation. The method must control sampling and presentation before the color numbers can be compared. ASTM D6290 specifically limits useful comparison to pellets of similar shape, size, texture and translucency and warns that pellet color does not necessarily predict a moulded or extruded product.
Take a representative laboratory sample
Pellets can segregate during conveying and packaging. Build the test portion from several increments taken from the agreed lot or container locations. Mix without grinding or altering the surfaces. If contamination, fines or off-color pellets are a separate defect category, record them rather than silently removing them from the color sample.
Standardise the presentation
- Use one opaque, non-contaminating sample cup or approved cell.
- Specify the fill depth so the layer is optically stable for the material.
- Use the same levelling and packing action—do not compact one sample and loosely pour another.
- Control pellet temperature, moisture conditioning and cleanliness.
- Keep the aperture, geometry, SCI/SCE condition and cup position fixed.
Repeat independent preparations
Several readings on one untouched cup mainly test instrument repeatability. To represent the material, empty, remix and refill the cup for each preparation, then report the mean and spread. Rotate the cup between readings only if that action is part of the written method.
Choose the right result
For colored compounds, record L*, a*, b* and the agreed Delta E formula. For near-white or nearly colorless resins, yellowness index may provide a more process-relevant trend, but the formula and measurement condition must be stated. ISO 17223:2026 covers yellowness index for clear, translucent and opaque plastics; use the edition and specimen method required by your specification.
Relate pellets to production
Use pellet limits as an incoming-material screen and establish them from process history. Final approval should remain tied to the moulded plaque, film or finished part when processing can change appearance. The SD-800B and SD-900B are the relevant benchtop options to assess when a controlled sample holder and broader optical capability are required.
Continue with masterbatch color measurement, moulded-part measurement and the complete plastics guide.
Last reviewed 21/09/2026 · Level: Intermediate
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