During plasticising in injection molding process, the melt is pushed forward by the rotating screw.
The pressure that is build up in front of the screw (P1) is responsible for its backward movement.
Backpressure (P2) is needed to withstand this backwards movement of the screw for a certain time.
This is to ensure a better mixing of color and additive, better melt uniformity, and controlling shot volume.
In the process shear heat is applied to the melt as well.
A recommended range of backpressure for most plastic materials range from 20 bar to 200 bar.
PVC on the other side uses backpressures of up to 400 bar.