The screw and barrel of the injection molding machine works for a long time under high temperature, high pressure, high mechanical torque and high friction environment. The first few factors are required by the process conditions, while the loss caused by friction is inevitable.
Generally, the screw and barrel is subjected to surface nitriding treatment 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 minimized, the working life of the screw will inevitably be greatly reduced.
The following will focus on the causes of screw and barrel wear and methods to reduce wear:
Each plastic has an ideal plasticizing processing temperature range, and the barrel processing temperature should be controlled to be close to this temperature range. Granular plastic enters the barrel from the hopper and first reaches the feeding section. Dry friction will inevitably occur in 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 section and homogenization section, if the melting state of the plastic is disordered and uneven, it will also cause increased wear.
The speed should be adjusted properly. Because some plastics are added with reinforcing agents, such as glass fiber, minerals or other fillers. The friction of these substances on metal materials is often much greater than that of molten plastics. When these plastics are injected, if a high speed is used, while the shear force on the plastic is increased, more torn fibers will be produced accordingly. The torn fibers contain sharp ends, which greatly increase the wear force.
When inorganic minerals slide on the metal surface at high speed, their scraping effect is also not small. Therefore, the speed should not be adjusted too high.
It is necessary to remove the debris in the plastic. Generally speaking, there is no debris in the original fresh plastic purchased, but after transportation, weighing, drying, color mixing, and especially adding recycled materials, there may be debris.
Small metal shavings, large heating coil nuts, paper clips, and even bunches of warehouse keys have been mixed into the barrel, which has happened before. The damage to the screw is self-evident (of course, the barrel will also be damaged at the same time), so it is necessary to install a magnet rack and strictly manage and monitor the feeding.
The moisture contained in the plastic has a certain effect on the wear of the screw surface. If the water in the plastic is not completely removed before injection molding, the residual water will enter the compression section of the screw, forming “steam particles” with high temperature and pressure that are previously melt-mixed in the molten plastic. As the screw advances during the injection process, from the homogenization section to the screw head, these “steam particles” expand during the injection process, like tiny impurity particles, which cause friction and damage to the wall surface.
In addition, for some types of plastics, under high temperature and high pressure, water may become a catalyst that promotes the cracking of plastics, producing harmful impurities that can corrode the metal surface. Therefore, the drying work before plastic injection molding is not only directly related to the quality of the product, but also affects the working life of the screw.
The maintenance of screw and barrel for injection molding machine is very important for your production, with good performance of injection machine, your production will be more easy and your injection mold will be have longer life.
