Injection screw works under high temperature, high pres […]
Injection screw works under high temperature, high pressure, high mechanical torque and high friction environment for a long time. The first several factors are required by the process conditions, and the loss caused by friction is inevitable. Generally, the screw has been nitrided to improve the surface hardness, that is, to improve the wear resistance. However, if the cause of wear is ignored and the wear is not reduced as much as possible, the working life of the screw will be greatly reduced.
1、 Causes of screw barrel wear
1. Every kind of plastic has an ideal processing temperature range. The processing temperature of the barrel should be controlled to make it close to this temperature range. When the granular plastic enters the barrel from the hopper, it will first arrive at the feeding section, and there will inevitably be dry friction in the feeding section. When the plastic is not heated enough and the melting is uneven, 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 the homogenization section, if the melting state of the plastic is disordered and uneven, the wear will also increase.
2. The speed should be adjusted properly. Because some plastics are added with strengthening agents, such as glass fiber, minerals or other fillers. The friction of these materials on metal materials is often much greater than that of molten plastics. When injection molding these plastics, if the speed is high, the shear force of the plastics will be increased, and more torn fibers will be produced correspondingly. 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. So the speed should not be set too high.
3. When the screw rotates in the barrel, the friction between the material and the two causes the working surface of the screw and the barrel to 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 clearance between the screw and the barrel increases with the gradual wear of the two. However, because the resistance of the head and the splitter plate in front of the barrel has not changed, the leakage flow of the extruded material is increased, that is, the flow of the material from the diameter gap to the feed direction is increased. As a result, the production of plastic machinery decreased. This phenomenon also increases the retention time of materials in the barrel, resulting in material decomposition. In the case of polyethylene, the hydrogen chloride produced by decomposition strengthens the corrosion of screw and barrel.
4. If there are fillers such as calcium carbonate and glass fiber in the material, it can accelerate the wear of screw and barrel.
5. Because the material is not plasticized evenly, or there are metal foreign matters mixed into the material, the torque force of the screw suddenly increases, which exceeds the strength limit of the screw, making the screw break. This is an unconventional accident damage.
2、 The way to reduce wear: pay attention to the daily maintenance of screw barrel
1. Do not start the machine when the barrel screw does not reach the preset temperature.
2. Prevent metal chips and sundries from falling into the hopper. If the recycled materials are processed, a magnetic hopper shall be added to prevent iron chips from entering the barrel.
3. When using anti salivation, make sure that the plastic in the barrel is completely melted, so as not to damage the parts of the transmission system when the screw retreats.
4. When using the new plastic, the surplus material of the barrel shall be cleaned.
5. When the temperature of the melt plastic is normal but black spots or discoloration of the melt plastic are found continuously, check the rubber screw
6. In the processing, try to make the material plasticized evenly, do not let the metal foreign matter mix into the material, reduce the screw rotation torque force.