Screw extruder is the core equipment of plastic forming and modification. Its working principle is generally to generate pressure and shear force through the rotation of screw, so that the materials will undergo physical or chemical changes after mixing, extrusion and shear, and finally extrude the finished products at the end of the equipment. In the production process of various industries, the screw of extruder is in a harsh high-pressure and high-temperature environment, and bears huge friction and shear force. Therefore, the screw has an important impact on the plasticization of materials, product quality and production efficiency.
Causes of extruder screw damage
1. The preheating time before startup is insufficient, the raw materials accumulated in the screw and barrel are not softened completely, the startup torque is too large, or there are large pieces of nonsoftened materials, resulting in sprain or collapse of the screw barrel and damage to the reduction gearbox. Each plastic has an ideal plasticizing processing temperature range. The processing temperature of the barrel should be controlled to make it close to this temperature range. When granular plastics enter the barrel from the hopper, they will first reach the feeding section. Dry friction is bound to occur in the feeding section. When these plastics are not heated enough and melt unevenly, it is easy to increase the wear of 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 disordered and uneven, it will also cause faster wear.
2. The screw rotates in the barrel, and the friction between the material and the two makes the working surface of the screw and the barrel wear gradually. The screw rotates in the barrel, and the friction between the material and the two makes the working surface of the screw and the barrel wear gradually: the diameter of the screw decreases gradually, and the diameter of the inner hole of the barrel increases gradually. In this way, the fit diameter gap between the screw and the barrel increases little by little with the gradual wear of the two. However, since the resistance of the head and splitter plate in front of the barrel has not changed, this increases the leakage flow of the extruded material when it moves forward, that is, the flow of the material from the diameter gap to the feeding direction increases. As a result, the production of plastic machinery decreased. This phenomenon also increases the residence time of materials in the barrel, resulting in material decomposition. In the case of polyethylene, the hydrogen chloride gas produced by decomposition strengthens the corrosion of the screw and barrel.
3. The diameter of the screw decreases gradually and the diameter of the inner hole of the barrel increases gradually. In this way, the fit diameter gap between the screw and the barrel increases little by little with the gradual wear of the two.
4. Since the resistance of the head and splitter plate in front of the barrel has not changed, this increases the leakage flow of the extruded material when it moves forward, that is, the flow of the material from the diameter gap to the feeding direction increases.
5. The production of the extruder decreases, which increases the residence time of materials in the barrel, resulting in material decomposition. If there is calcium carbonate, glass fiber, minerals or other filling materials in the materials, the wear of the screw and barrel will be accelerated. The friction force of these substances on metal materials is often much greater than that of molten plastics. When these plastics are injected, if they are made at a high speed, while increasing the shear force on the plastics, the reinforcement will correspondingly produce more torn fibers. The torn fibers contain sharp ends, which greatly increases the wear force. When inorganic minerals slide at high speed on the metal surface, their scraping effect is not small.
6. Because the material is not plasticized evenly, or metal foreign matters are mixed into the material, the rotating torque of the screw suddenly increases, which exceeds the specified strength of the screw and breaks the screw.
7. During the installation of screw and barrel, the level, straightness and clearance are not adjusted in place, and the fastening screws are not reinforced. When starting up, the screw barrel has friction, which accelerates the damage of the screw barrel.
Maintenance of screw extruder
- If there is slight wear or scratch on the working surface of the screw, grind and polish the damaged part with oilstone or fine abrasive cloth.
- In case of serious wear or scar groove on the working surface of the screw, the wear and scratch causes of the screw shall be checked and analyzed to eliminate the fault, so as to avoid similar phenomena again. Then repair the scratch and groove of the screw by welding.
- If there are serious damage marks on both the screw and the barrel, and the fit gap between them is large, the screw should be replaced. The outer diameter of the screw thread should be prepared according to the inner hole diameter of the barrel after repair to ensure that the fit gap between the barrel and the screw is within the specified range.
- It is not allowed to start the screw rotation at will. Before start-up, the empty vehicle shall be checked and the test run shall be started at a low speed. The shorter the rotation time, the better (not more than 2 ~ 3min). If everything is normal, stop immediately.
- The heating and constant temperature time before start-up must be guaranteed, and the heat preservation and constant temperature time after reaching the process temperature shall not be less than 1 ~ 2H. In this way, the working rotation torque of the screw is prevented from increasing due to uneven temperature during plasticization.
- During shift handover, carefully listen to whether the working rotation sound of the screw is normal. In case of abnormal sound, stop the machine immediately and report to relevant personnel for inspection and troubleshooting.
- When removing the screw, use special tools to remove it, and do not knock it with a heavy hammer.
- When the parts on the screw are assembled together, the threaded connection parts shall be coated with molybdenum disulfide heat-resistant grease to facilitate the next disassembly.
- The surface of the screw not used for the time being shall be coated with protective oil, wrapped and hung in a ventilated place.