Sheet & material defect
Black spots & specks in thermoforming
Also called: Contamination, black specks, gels
Black spots (or specks) are small dark contamination particles embedded in the sheet and visible on the formed part — most obvious on light colours. They come from degraded or foreign material getting into the melt during extrusion, or from dirty regrind.
Why it happens
- Degraded polymer burnt onto the extruder screw or die, then flaking off
- Contaminated or dirty regrind mixed into the sheet
- Foreign material or dust entering the material feed
- Extruder running too hot or material dwelling too long
How to fix & prevent it
- Purge and clean the extruder screw and die on schedule
- Control regrind — clean, dry and correctly ratioed, or use virgin sheet for critical cosmetics
- Keep the material feed sealed and contamination-free
- Run the correct melt temperature and avoid long dwell/stagnation
- Filter the melt with a screen pack to catch particles
How Indu prevents it: Because we extrude our own sheet, we control purging, regrind quality, melt temperature and screen filtration in-house — so cosmetic parts stay speck-free instead of depending on an outside sheet vendor’s housekeeping.
Troubleshooting help
Fighting black spots & specks on a part?
Send your drawing, sample or a photo of the issue — our engineers will diagnose the cause and reply with the tooling, material and process fix, usually within 24 hours.
ISO 9001:2015 certified · Most quotes back within 24 hours