During the injection process, the screw is pushed forward in the injection unit.
The pressure applied to the screw, will cause melted material flowing back into the screw.
To prevent this effect, the tip of a screw is equipped with a movable ring called Non-Return Valve.
During the dosing process, this ring open a gap between screw and screw tip. This allows the melt to enter the area in front of the tip.
During injection stroke, the ring is pushed back and it closes the gap.
This prevents material to flow back into the screw.