What are the uses of polyoxymethylene?

Polyoxymethylene is mainly intended for storage and transportation

Polyoxymethylene (POM) is a linear polymer without side chains, characterized by high density and high crystallinity. It possesses excellent mechanical properties, outstanding creep resistance and stress relaxation capabilities; its fatigue resistance is among the highest among thermoplastics. It also features remarkable self-lubricating properties, wear resistance, and chemical resistance, making it a widely used engineering plastic. POM has low moisture absorption, and the resin generally does not need to be dried before processing. If necessary, it can be dried at 90–100°C for 2–4 hours. The melt viscosity of POM is sensitive to shear rate. Therefore, to improve melt flowability, it is not sufficient to simply raise the temperature; increasing the injection rate and injection pressure can also be effective approaches. POM is a thermosensitive plastic that decomposes severely at 240°C. At 210°C. The residence time must not exceed 20 minutes ； Even at 190°C, the residence time should preferably not exceed 1 hour. During injection molding, while ensuring the fluidity of the material, it is advisable to use a lower molding temperature and a shorter heating time. POM has a relatively distinct melting point, at 165°C for copolymerized POM and 175°C for homopolymerized POM. During molding, the temperature distribution in the barrel is as follows: 190–200°C in the front section, 180–190°C in the middle section, and 150–180°C in the rear section; the nozzle temperature is 170–180°C. For thin-walled products, the barrel temperature can be increased slightly, but it must not exceed 210°C. The mold temperature is usually controlled between 80 and 100°C; for products with thin walls, long flow distances, and complex shapes, the mold temperature can be increased to 120°C. Increasing the mold temperature is beneficial for the flow work of the melt, helps to prevent defects in the product caused by too rapid cooling, and also improves the impact strength of the product; however, it increases the molding shrinkage as well. Injection pressure has little effect on the mechanical properties of POM products, but it has a significant impact on the fluidity of the melt and the surface quality of the products. The level of injection pressure is primarily determined by the shape of the product, wall thickness, flow in the mold, gate size, and mold temperature. For products with small gates, thin walls, long flow distances, and large areas, the injection pressure is high, ranging from 120 to 140 MPa ； For products with large gates, thick walls and short flow distances, as well as small surface areas, the injection pressure is 40–80 MPa ； The value for ordinary products is around 100 MPa. Appropriately increasing the injection pressure helps improve melt flow and the surface quality of the product, but too high a pressure can cause overflow during production. Due to the high crystallinity and large volume shrinkage of POM, sufficient holding time is necessary to ensure adequate feeding and prevent defects such as voids and indentations in the finished products. Generally, the thicker the product, the longer the holding time. The speed of injection depends on the wall thickness of the product. Thin-walled products should be injected quickly to prevent the melt from solidifying too early ； For thick-walled products, slow injection is recommended to avoid spraying, which could affect the appearance and internal quality of the product. To eliminate residual internal stresses in the product and reduce post-shrinking, heat treatment is usually required. Heat treatment is carried out using air or oil as a medium, at a temperature of 120–130°C, with the duration depending on the wall thickness of the product ； Generally, for every 1mm increase in wall thickness, the annealing time increases by about 10 minutes. The effect of heat treatment can be determined using the polar solvent impregnation method ； The heat-treated product is immersed in a 30% hydrochloric acid solution for 30 minutes; if no cracks appear, it indicates that the residual internal stress in the product is low, achieving the purpose of the treatment. When POM plastic is exposed to temperatures above a certain threshold or is heated for extended periods at processing temperatures, it degrades and releases large amounts of harmful formaldehyde gas. This not only affects the quality of the products, corrodes molds, and endangers human health, but in severe cases it can also cause gas expansion inside the barrel, leading to production accidents such as ※※. Therefore, in addition to strictly controlling the molding process conditions during operation, the following points should also be taken into account: ① Strictly control the molding temperature of POM and the residence time of the material in the barrel ； ②When heating up before driving, preheat the nozzle first, then heat the barrel ； ③When processing POM, if the barrel contains material at a processing temperature higher than that suitable for POM, it is necessary to first clean the barrel using PE as a cleaning agent. Once the temperature drops to the level appropriate for processing POM, the barrel should be cleaned again with PE before feeding the material in for molding ； ④During the molding process, if a strong pungent odor of formaldehyde is detected or yellow-brown streaks appear on the product, it indicates that the material has degraded. In such cases, the material in the barrel should be immediately emptied using an empty injection method, and the barrel should be cleaned with PE; processing can resume only after everything is back to normal ； ⑤Certain materials or additives (such as PVC, halogen-containing flame retardants, etc.) accelerate the degradation of POM; they must be strictly separated and must not be mixed with each other. Polyoxymethylene is also known as polyoxymethylene, and its English name is polyoxymethylene (abbreviated as POM). Its regular molecular structure and crystallinity result in excellent physical and mechanical properties, earning it the nickname of metal plastic. POM is a milky-white, opaque, crystalline linear thermoplastic resin with excellent overall properties and coloring capabilities. It features a high elastic modulus, as well as high stiffness and hardness; its specific strength and specific stiffness are comparable to those of metals ； It exhibits excellent tensile strength, bending strength, creep resistance, and fatigue resistance, as well as good resistance to repeated impacts and excellent recovery after unloading ； It has a low coefficient of friction, is wear-resistant, offers good dimensional stability, has a pleasant surface finish, exhibits high viscoelasticity, provides excellent electrical insulation, and is not affected by temperature ； It has good insulation resistance and is not affected by humidity ； Excellent chemical resistance: It remains stable against other chemicals except strong acids, phenols, and organic halides, as well as being oil-resistant ； Its mechanical properties are little affected by temperature, and it has a high heat deformation temperature. The disadvantages are poor flame retardancy, as it burns slowly when exposed to fire; its oxygen index is low, and even the addition of flame retardants does not yield satisfactory results. Additionally, its weather resistance is inadequate, requiring the use of stabilizers for outdoor applications. 　　Homopolymerized polyoxymethylene has a high degree of crystallinity, and its mechanical strength, rigidity, and heat deformation temperature are superior to those of copolymerized polyoxymethylene. Copolymerized polyoxymethylene has a lower melting point, and it offers better thermal stability, chemical resistance, flow properties, and processability compared to homopolymerized polyoxymethylene. Newly developed grades include those with ultra-high flow properties (for rapid molding), impact resistance, and reduced mold deposition; there are also grades enhanced with inorganic fillers. 　　The water absorption rate of POM is greater than 0.2%, so it should be pre-dried before molding. The melting temperature of POM is close to its decomposition temperature, resulting in poor formability; however, it can be processed by injection molding, extrusion, blow molding, rotational molding, welding, bonding, coating, printing, electroplating, and machining. Injection molding is the most important processing method. Due to the high shrinkage rate during molding, the mold temperature should be kept high, or annealing treatment should be applied, or reinforcing materials such as alkali-free glass fibers can be added. 　　POM has high strength and low weight, and is often used as a substitute for non-ferrous metals such as copper, zinc, tin, and lead. It is widely applied in industries such as industrial machinery, automobiles, electronics and electrical appliances, daily necessities, pipes and fittings, precision instruments, and building materials. 　　POM is widely used in the manufacture of various sliding and rotating mechanical parts, including different types of gears, levers, pulleys, and sprockets. It is particularly suitable for use in bearings, hot water valves, precision metering valves, chain links and rollers for conveyors, flow meters, interior and exterior handles for vehicles, crankshafts and other components related to window operation mechanisms, oil pump bearing housings and impellers, gas control valves, electronic switch components, fastening elements, terminal cover mirrors, fan components, heating plates, and instrument buttons ； Bearings for audio and video tapes ； Various pipes and agricultural sprinkler systems, as well as valves, nozzles, faucets, and bath tub parts ； Power on keyboard, buttons, audio/video tape reels ； Temperature control timer ； Power tool parts, garden landscaping tool parts ； It can also be used as parts for surfboards, sailboats, and various sleds, as well as miniature gears for watches, components for the frames of sports equipment, and various buckles, fasteners for backpacks, lighters, zippers, and clasps ； Cardiac pacemakers in medical devices ； Artificial heart valves, cervical vertebrae, prosthetics, etc