Under the action of the screw, the granular plastic stored in the hopper continuously moves along the screw groove. Due to the combined effect of heating outside the barrel and shearing of the screw, the plastic is continuously softened by heating and eventually becomes a molten viscous state. At the same time, the force of the melt at the head of the screw pushes the screw back. By changing the screw back pressure (return resistance of working oil) to adjust the screw retraction speed and change the plastic flow condition in the screw groove, the purpose of controlling the plasticizing performance of the plastic is finally achieved.