PVC polymer chains break easily when they encounter shear. Any processing or equipment changes you can make to reduce the shear on the material will significantly improve your process.
Question: We have tried increasing and decreasing the mold temperature. We have also tried to reduce the barrel temperature with the use of a blower fan but we cannot eliminate the silverstreaking by the gate.
Additional information provided:
Defect: Silver Streak on the surface
Material: Rigid PVC (black)
Screw: GP screw (not for PVC)
My Response: The defect appears to be the result excessive shear on the material throughout the process. This assumption is reinforced by the need for the additional cooling fan on the machine barrel.
PVC polymer chains break easily when they encounter shear. The best approach to this would be to minimize shear and shear heating as much as possible. We will address this is two parts 1) processing and 2) equipment. Although the process may improve the situation, your assumption about the equipment is likely the root of the problem and may or may not have to be addressed.
1) Using a lower screw speed, lower back pressure, or lower injection speed may reduce the shear. In some cases, a higher rear zone temperature may provide more conductive heating, but this must be done very carefully with PVC.
2) A PVC screw has some of these characteristics... low compression ratio, long transition zone, and a smear tip.
If you already have another screw or tip available with any of these characteristics, you should consider trying it in that machine.