A small amount of melt solidifies at the gate point before the start of injection and is pushed into the mould cavity via the melt stream.
Evidence of a cold slug normally occurs close to the injection point.
1. Increase the temperature of the nozzle slightly.
2. Reduce the back pressure.
3. Increase the retraction of the screw - decompression.
4. Add a "cold slug well" to the cold runner feed system.
5. Consider increasing the nozzle / gate diameter.
6. Consider use of a shut-off nozzle.
7. Reduce the time that the nozzle is contact with the cold mould.