The injection molding machine screw works for a long time under high temperature, high pressure, high mechanical torque and high friction environment. The process conditions of the first few factors are required, while the friction causes loss. Generally, the screw is subjected to nitriding treatment to increase the surface hardness, that is, to improve the abrasion resistance. However, if the cause of wear is neglected, and the failure to try to reduce the wear as much as possible will inevitably greatly reduce the life of the screw.
The following are the causes of screw wear and the method of reducing wear.
(1) Each plastic has an ideal plasticizing processing temperature range. The processing temperature of the barrel should be controlled to be close to this temperature range. The granular plastic enters the barrel from the hopper, first reaching the feeding section and then adding the section. Dry friction is inevitable. When these plastics are insufficiently heated and melted unevenly, it is easy to cause the inner wall of the cylinder and the surface of the screw to wear more. Similarly, in the compression section and the homogenization section, if the molten state of the plastic is uneven It also causes an increase in wear.
(2) The speed should be adjusted properly. Due to some plastic refueling enhancers, such as fiberglass, minerals or other fillers. The resistance of these materials to metal is often much greater than that of molten plastic. When molding these plastics, if high speed is used, the shearing force on the plastic will be increased, and the fortifier will also produce more shredded fibers correspondingly. The shredded fibers contain sharp ends. The wear force is greatly increased. When the non-polar minerals slide at high speed on the metal surface, the scraping effect is also quite a lot. Therefore, the speed should not be adjusted too high.
(3) To check for debris in plastics. In general, the original plastics purchased from the original are not miscellaneous, but after transportation, weighing, drying, color mixing, especially the addition of recycled materials, it is possible to mix debris. Small as metal scraps, as large as hot-roll nut paper clips, and even a string of warehouse keys, there are precedents that have been mixed into the barrel, the damage to the screw is inexplicable (the barrel is also damaged at the same time), so Management and monitoring must be strictly applied.
(4) The moisture contained in the plastic has a certain influence on the wear of the screw surface. If the plastic does not completely remove the moisture before the injection, the residual moisture enters the compression section of the screw, and becomes a "steam particle" with high temperature and high pressure melted and mixed in the molten plastic. With the advancement of the screw in the injection process, the homogenization section Up to the head of the screw, these "steam particles" are relieved and expanded during the course of the shot, like a fine impurity of hard particles, which causes friction damage to the wall surface. In addition, for certain types of plastics, at high temperatures and pressures, moisture may become a catalyst for cracking the plastic, producing harmful impurities that can attack the metal surface. Therefore, the drying work before plastic injection molding not only has a direct relationship with the quality of the parts, but also affects the working life of the screw of the injection molding machine.
As mentioned above, in order to extend the working life of the screw, flame hardening and nitriding treatment are usually employed. However, due to the widespread use of filled plastics and reinforced (glass reinforced) plastics, and the corrosive or strong wear of individual plastics, the life of these treated screws is still too short, and some new ones have been developed in recent years. The high-tech processing method has greatly changed the situation. The introduction is as follows:
Using a plasma transfer method, a hard material such as nickel and a cobalt alloy is injected into a plasma arc in a powder form, and then melted and coated on the surface layer of the screw to form a metal-bonded state, which is more solid than ordinary spraying. .
Nitrogen nitridation is used instead of gas nitriding to chemically react nitrogen with the metal on the screw to form a thin, uniform, hard, multi-layer coating. Since the ion nitriding treatment is relatively inexpensive, and its wear life is 2 to 3 times that of a conventional hardening tool, it is often used for a screw having a smaller diameter.
In addition, there is an ion insertion treatment method, and the obtained screw has a higher hardness and a longer life.