The screw rotates in the barrel, and the friction between the material and the two causes the working surface of the screw and barrel to gradually wear: the diameter of the screw will be reduced by friction, and the diameter of the inner bole of the barrel will increase due to friction.
Therefore, the matching diameter gap between the screw and barrel will become larger and larger (the screws have been subjected to surface nitriding treatment during work to improve the surface hardness). Since the resistance of the head and the manifold in front of the barrel has not changed, the leakage flow rate of the extruded material will increase when it advances, that is, the amount of material flowing from the diameter gap to the feeding direction increases.
As a result, the output of the extruder will decrease. This causes the material to stay longer in the barrel and causes the material to decompose. If the material is polyethylene, the hydrogen chloride gas produced by decomposition strengthens the corrosion of the screw and barrel, forming a vicious circle.
The materials produced by the crusher include fillers such as calcium carbonate and glass fiber. The friction of these materials on metal materials is often much greater than that of molten plastics. When injecting these plastics, if the high rotation speed increases the shearing force to the plastic, it also produces more shredded fibers. The shredded fibers contain sharp ends, which will cause more wear on the screw and barrel. Therefore, the speed of the extruder should not be set too high when it is working.
This situation causes a sudden increase in the torque of the screw. This torque exceeds the strength limit of the screw of the screw extruder, causing the screw to break. This situation is an unconventional damage of the screw and barrel.
Each plastic has an ideal plasticization temperature range. Granular plastic enters the barrel from the hopper and will first reach the feeding section. Dry friction will inevitably occur in the feeding section. When these plastics are not heated enough and melted unevenly, it is easy to cause increased wear on the inner wall of the barrel and the surface of the screw. Similarly, in the compression section and homogenization section, if the melting state of the plastic is not uniform, it will also cause increased wear of the screw and barrel.