Differences in colour or colour streaks on the surface of the moulding.
The cause is usually due to non-uniform distribution of the pigments during plasticisation but can also be due to non-compatibility of the polymer and master batch.
1. Check compatibility of the master batch with the base polymer.
2. Check the screw and barrel for defects.
3. Increase the speed of the injection.
4. Reduce and increase the screw speed. Evaluate the effects on streak formation.
5. Increase back pressure during plasticisation to improve mixing.
6. Confirm the temperature of the melt is within specification.
7. Confirm the temperature is within limits of the master batch.
8. Check the hot runner temperature control and operation of thermocouples.
9. Check whether the shot weight, barrel volume and residence time is within guidelines.
10. Check suitability of the screw L/D ratio, shear and/or mixing section.