Black specks (contamination)
Black or dark-colored specks and streaks in the part.
Symptoms
- Random black specks
- Contamination after a color change
- Specks that increase over time
Tip: If the machine stood still for a long time and was restarted, purge out the degraded material first.
Order of solutions
- Empty the barrel with a suitable purging compound
- Reduce the melt temperature and residence time (check the shot/barrel ratio)
- Clean the raw material, the hoppers and the regrind
- Plan the cleaning of the inside of the hot runner
Likely causes
Process setting
- Excessive temperature, long residence time (carbonization)
- Fast screw rotation, shear heat
Raw material
- Contamination in the raw material, dusty regrind
Machine
- Degraded material accumulated in the barrel or hot runner
- Dead zones in the nozzle or manifold
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Other defects
Short shot (incomplete fill)
The cavity does not fill completely; the farthest or thinnest area of the part stays incomplete.
Flash
Plastic escapes through the parting line or around the ejector pins.
Sink mark
A dent on the surface opposite a thick section; the surface is drawn inwards.
Void (internal vacuum void)
Internal cavity in a thick section of the part; the outer surface may look perfect.