The basic mechanism of extrusion is simple - a screw rotates in the cylinder and pushes the plastic forward. The screw is actually a bevel or slope, wound on the center layer. The aim is to increase the pressure in order to overcome the greater resistance. In the case of an extruder, there are three kinds of resistance to overcome: the friction between the solid particles (feed) on the cylinder wall and the friction between them when the screw rotates a few laps (feed zone); The adhesion on the cylinder wall; the flow resistance of the melt when it is pushed forward.
Newton has explained that if an object does not move in a given direction, then the force on the object is balanced in this direction. The screw does not move axially, although it may rotate horizontally around the circumference. Thus, the axial force on the screw is balanced, and if it exerts a large forward thrust on the plastic melt, it also applies an equal backward thrust to an object. Here, the thrust it exerts is on the bearing-thrust bearing acting on the back of the inlet.
Most single screws are right-hand threads, like screw and bolts used in woodworking and machines. If they are from the back, they are reverse, because they have to try to pull out the cylinder. In some twin-screw extruders, the two screws in the two cylinder reverse rotation and cross each other, so one must be right, the other must be left. In other bite double screws, the two screws are rotated in the same direction and must therefore have the same orientation. However, no matter what the situation has to absorb the backward force of the thrust bearing, Newton's principle is still applicable.