The screw of an injection molding machine works for a long time in high temperature, high pressure, high mechanical torque, and high friction environments. The first few factors are required by the process conditions, and the loss caused by friction is inevitable. Generally, screws have undergone surface nitriding treatment to improve surface hardness and wear resistance. However, if the causes of wear are ignored and efforts are not made to minimize wear, it will inevitably greatly reduce the working life of the screw.
The following will focus on explaining the causes of screw wear and methods to reduce wear:
Each type of plastic has an ideal plasticizing processing temperature range, and the barrel processing temperature should be controlled to approach this temperature range. Granular plastics enter the barrel from the hopper and first reach the feeding section. Dry friction is inevitable during the feeding section. When these plastics are not heated enough and melt 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 and homogenization stages, if the melting state of the plastic is disorderly and uneven, it can also cause increased wear and tear
The speed should be adjusted properly. Due to the addition of strengthening agents such as fiberglass, minerals, or other fillers to some plastics. These substances often have much greater friction against metal materials than molten plastics. When injecting these plastics, if a high rotational speed is used, it will not only increase the shear force on the plastic, but also strengthen the corresponding production of more torn fibers. The torn fibers contain sharp ends, greatly increasing the wear force. When inorganic minerals slide at high speeds on metal surfaces, their scraping effect is also significant. So the speed should not be set too high.
To remove debris from the plastic. Generally speaking, fresh plastic purchased in its original packaging does not contain any impurities, but after transportation, weighing, drying, color mixing, and especially adding reused materials, there is a possibility of impurities being mixed. Small as metal shavings, large as heating rings, nuts, paper clips, and even a string of warehouse keys have all experienced damage to the screw when mixed into the barrel (the barrel will also be damaged at the same time). Therefore, it is necessary to install a magnetic iron frame and strictly manage and monitor feeding.
The moisture content in plastic has a certain impact on the wear of the screw surface. If the plastic does not completely remove moisture before injection molding, residual moisture enters the screw compression section, forming "steam particles" with high temperature and pressure that are mixed in the molten plastic before injection. As the screw advances during the injection process, these "steam particles" expand from the homogenization section to the screw head, releasing pressure during the injection process, like tiny impurities and hard particles, causing friction and damage to the wall surface.
In addition, for certain types of plastics, under high temperature and pressure, water may become a catalyst for plastic cracking, producing harmful impurities that can corrode the metal surface. Therefore, the drying work before plastic injection molding is not only directly related to the production volume, but also affects the working life of the screw.