POM has low hygroscopicity, so the resin generally does not need to be dried before processing. If necessary, it can be dried at 90 ~ 100℃ for 2 ~ 4h hours. The melt viscosity of polyoxymethylene is sensitive to shear rate. Therefore, in order to improve the melt fluidity, not only the temperature can be increased, but also the injection rate and pressure can be increased. POM is a kind of heat-sensitive plastic, which will decompose seriously at 240℃. At 2 10℃. Stay time can't exceed 20min；; Even at 190℃, the residence time should not exceed1h. In injection molding, the lower molding temperature and shorter heating time should be selected as far as possible on the premise of ensuring the fluidity of materials. POM has obvious melting point, * * poly POM is 165℃, and homopolymer POM is 175℃. During molding, the barrel temperature distribution is: front section 190 ~ 200℃, middle section 180 ~ 190℃, rear section 150 ~ 180℃, and nozzle temperature170. For thin-walled products, the cylinder temperature can be appropriately increased, but it cannot exceed 2 10℃. The mold temperature is usually controlled at 80 ~ 100℃, and it can be increased to 120℃ for thin-walled products with long flowing distance and complex shape. Increasing the mold temperature is beneficial to melt flow, avoiding the defects of products caused by too fast cooling rate, improving the impact strength of products, and improving the molding shrinkage. Injection pressure has little effect on the mechanical properties of POM products, but it has great influence on the fluidity of melt and the surface quality of products. Injection pressure mainly depends on product shape, wall thickness, mold cycle, gate size and mold temperature. For products with small gate, thin wall, long flow distance and large area, the injection pressure is high, between 120 ~ 140 MPa; For products with large gate, thick wall, short flow distance and small area, the injection pressure is 40 ~ 80 MPa and the average product is about 100MPa. Properly increasing injection pressure is beneficial to improve melt fluidity and product surface quality, but too high pressure will cause product overflow. Due to the high crystallinity and large volume shrinkage of POM, in order to prevent defects such as cavities and dents, it is necessary to have enough heat preservation time for feeding. Generally speaking, the thicker the product, the longer the heat preservation time. The injection rate depends on the wall thickness of the product. Thin-walled products should be injected quickly to avoid premature solidification of melt; Thick-walled products should be injected slowly to avoid splashing and affecting the appearance and internal quality of the products. In order to eliminate residual internal stress and reduce later shrinkage, heat treatment is usually needed. Heat treatment takes air or oil as medium, the temperature is 120 ~ 130℃, and the length of time depends on the wall thickness of the product; Generally, when the wall thickness increases by 1mm, the annealing time increases by about 10min. The effect of heat treatment can be judged by polar solvent impregnation method; Soak the heat-treated product in 30% hydrochloric acid solution for 30 minutes. If there is no crack, it shows that the residual internal stress in the product is small and the treatment purpose is achieved. POM plastics will degrade when heated above a certain temperature or at the processing temperature for a long time, releasing a lot of harmful formaldehyde gas, which will not only affect the product quality, corrode the mold and endanger human health, but also cause production accidents such as explosion due to gas expansion in the barrel. Therefore, in addition to strictly controlling the molding process conditions, we should also pay attention to the following points: ① strictly controlling the molding temperature of POM and the residence time of materials in the barrel; (2) When the temperature rises before driving, preheat the nozzle first, and then heat the barrel; (3) When processing POM, if there are materials with processing temperature exceeding POM in the barrel, PE should be used as the cleaning machine barrel for cleaning materials first, and then the PE cleaning machine barrel should be used once when the temperature drops to the processing temperature of POM, and then the material is fed for molding operation; (4) In the molding process, if the product is found to have severe pungent formaldehyde smell and yellow-brown stripes, it means that the material has been degraded. At this time, the materials in the barrel should be emptied immediately by injecting into the air, and the barrel should be cleaned with PE, and then processed after it is normal; ⑤ Some materials or additives (such as PVC, halogen-containing flame retardant, etc.). ) can promote the degradation of POM, so it must be strictly separated and not allowed to mix with each other. Polyoxymethylene, also known as polyoxymethylene, is called POM in English. Regular molecular structure and crystallinity make it have excellent physical and mechanical properties, and it is called metallic plastic. POM is a kind of milky white crystalline linear thermoplastic resin, which has good comprehensive properties and colorability, high elastic modulus, high rigidity and hardness, and its specific strength and stiffness are close to those of metal. Excellent tensile strength, bending strength, creep resistance and fatigue resistance, repeated impact resistance and unloading recovery; Low friction coefficient, wear resistance, good dimensional stability, good surface gloss, high viscoelasticity and excellent electrical insulation, and is not affected by temperature; Good insulation resistance, not affected by humidity; Excellent chemical resistance: stable to other chemicals except strong acids, phenols and organic halides, and good oil resistance; The mechanical properties are less affected by temperature and have higher thermal deformation temperature. Disadvantages are poor flame retardancy, slow combustion in case of fire, low oxygen index, and even adding flame retardant can not meet the requirements. In addition, the weather resistance is not ideal, and stabilizers should be added for outdoor applications. Polyacetal (POM) is hard, creep-resistant and high in strength, because it has high physical and chemical properties and difficult anisotropic shrinkage formability. Has a low friction coefficient. It can also maintain stability at high temperature and provide excellent stability in hot water environment. They are not actually affected by strong alkali, but will be eroded by strong inorganic acids. 40% glass fiber reinforced polyacetal (POM) has a flexural modulus of1.6 psi×10e6 (11024 MPa) and a thermal deformation temperature of 330 degrees Fahrenheit (166 degrees Celsius), and POM has high crystallinity. Chemical corrosion resistance, flow characteristics and processability are better than paraformaldehyde. The newly developed products have ultra-high fluidity (rapid prototyping), impact resistance and reduced mold deposition grade, as well as inorganic filling and reinforcement grade. The water absorption of POM is more than 0.2%, and it should be pre-dried before molding. The melting temperature of POM is close to the decomposition temperature, and its formability is poor. Injection molding, extrusion, blow molding, rotational molding, welding, bonding, coating, printing, electroplating, machining and injection molding are the most important processing methods. The molding shrinkage is large, so the mold temperature should be higher, or annealing should be carried out, or reinforcing materials (such as alkali-free glass fiber) should be added. POM has high strength and light weight, and is often used to replace non-ferrous metals such as copper, zinc, tin and lead. Widely used in industrial machinery, automobiles, electronic appliances, daily necessities, pipelines and accessories, precision instruments, building materials and so on. Injection molding process special composite material? Typical injection
Forming condition
General injection molding conditions? English
English? international system of units (SI)
Metric temperature back zone 350-390 f 177- 199 c central zone 360-400f182-204 cfrontzone front zone 370-4 10 F 18. 8-210 c melt melting temperature 360-425 f182-218 c mold temperature175-225 f 79-107 c pressure injection pressure 60. 8+05000psi69- 103MPa holding pressure 5000- 12000 psi 34-83 MPa back pressure 50- 100 psi 0.34-0.69 MPa filling speed1-2in/sec25-5/. temperature
Time and temperature 2 hours @ 250 F2 hours @ 12 1 dew point dew point-25 f-32 c moisture content.
Permanent antistatic composite with moisture content of 0. 15%0. 15%? Typical injection
Forming condition
General injection molding conditions? English
English? international system of units (SI)
After metric temperature, the rear section is 310-350f154-177c, the middle section is 330-370f166-188cfrontzone is 350-390 F65. 438+077- 199 c melt temperature 350-400 f 177-204 c mold temperature180-250f82-/kloc pressure injection. -15000psi 69-103mpa holding pressure 5000- 10000 psi 34-69 MPa back pressure 50- 100 psi 0.34-0.69 MPa filling speed1-2in/sec20. temperature
Time and temperature 2 hours @ 250 F2 hours @ 12 1 dew point dew point-25 f-32 c moisture content.
Humidity content 0. 15%0. 15%EMI shielding composite? Typical injection
Forming condition
General injection molding conditions? English
English? international system of units (SI)
Metric temperature back zone 370-410f188-210c central zone 360-400 F 182-204 C front zone 350-390 F 17. 7-199 c melt melting temperature 360-425 f182-218 c mold temperature180-225 f 82-107 c pressure injection pressure 650. 8+05000psi69- 103MPa holding pressure 5000- 10000 psi 34-69 MPa back pressure 50- 100 psi 0.34-0.69 MPa speed filling speed 1-2 inches/second 25-. temperature
Time and temperature 2 hours @ 250 F2 hours @ 12 1 dew point dew point-20 f-29 c humidity.
Humidity content 0. 15%0. 15%? Scope of application POM is widely used for manufacturing various sliding and rotating mechanical parts, such as gears, levers, pulleys and sprockets, especially for bearings, hot water valves, precision metering valves, chain rings and rollers of conveyors, flow meters, internal and external handles of automobiles, cranks and other window rotating machinery, oil pump bearing seats and impeller gas switch valves, electronic switch parts, compact solids, rearview mirror covers, electric fan parts, heating plates and instrument buttons; Carrying of audio and video tapes; All kinds of pipes and agricultural sprinkler irrigation systems, as well as valves, nozzles, faucets and bath parts; Switch keyboards, buttons, audio and video reels; Temperature control timer; Parts of electric tools and garden finishing tools; In addition, it can also be used as parts of surfboards, sailboats and all kinds of sledges, micro gears of watches, frame accessories of sports equipment and various ring buckles, fasteners, lighters, zippers and buckles of backpacks; Pacemakers in medical equipment; Artificial heart valve, spine, prosthesis, etc.