Usually oblong, silvery surface streaks which are open in the shapre of a U counter to the direction of flow. The material pumped out of the cylinder is foamed and shows blisters on the injection molding parts surface.
Moisture content too high; water vapor is produced during melting and this results in the surface of the molding being torn open.
1. Use material adequately predried to a residual moisture content of < 0.1 %.
2. Increase back pressure.