Color variation and color streaks
The color is uneven from part to part or within a part.
Symptoms
- Colored streaks or marbled appearance
- Color difference between parts in the same shift
- Different shade in the gate area
Tip: Do not raise the back pressure too much; shear heat can spoil the color and lengthens the cycle.
Order of solutions
- Check the masterbatch dosing and the feeding arrangement
- Increase the back pressure somewhat, reduce the screw speed
- Reduce the melt temperature and residence time
- Consider a static mixer or a mixing screw
- Apply a color check (reference plaque) for every batch
Likely causes
Process setting
- Screw mixing insufficient (low back pressure, low speed)
- Melt temperature too high (pigment degradation)
- Residence time too long
Raw material
- Masterbatch ratio fluctuates
- Pigment dispersion insufficient
- Moisture or carrier incompatibility
Machine
- Dosing unit calibration
- Screw design has no mixing element
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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.