Black spots in granulate are one of the fastest ways to reject a lot — and one of the most over-applied. 'There are black spots' is treated as a verdict, when it should be the start of a question: how many, how big, and in what part?
The problem
Contamination specks (black spots) are often judged as a blanket quality defect: present means bad, absent means good. That binary rejects usable material and accepts unsuitable material in equal measure, because it ignores where the material is going.
Why it happens
Whether a given level of black spots matters depends heavily on the end product, not the pellet in isolation:
- End product & function — a hidden technical part tolerates far more than a visible consumer surface.
- Colour — spots that disappear in a black compound can be unacceptable in natural or light colours.
- Wall thickness & part size — a speck is negligible in a thick part and glaring in a thin film.
- Visible surface — cosmetic A-surfaces have a different threshold than internal structures.
- Process — some processes disperse or hide specks; others concentrate them.
How to judge it properly
Replace 'spots: yes or no' with a real specification:
- Count / ppm — define an acceptable level, not just presence.
- Size & distribution — a few large specks and many fine ones are different problems.
- End application — set the threshold from the part's requirement, visible or not.
- Sampling — judge from a representative sample, not one bag or one scoop.
Key takeaway
Black spots are not automatically a defect. Define count/ppm, size, distribution and the end application — then decide which parts a lot is still right for, instead of rejecting or accepting it blind.
The question is never just 'are there spots?' — it's 'spots, how many, and for which part?'
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