POM hygroscopicity is small, the resin generally does not need to dry before processing. If necessary, can be in 90 ~ 100 ℃, drying 2 ~ 4h.
POM's melt viscosity is sensitive to the shear rate. Therefore, to improve the melt fluidity, can not be used alone to increase the temperature, but also from improving the injection rate and injection pressure.
POM is a heat-sensitive plastic, 240 ℃ will be severely decomposed. At 210 ℃. Dwell time can not exceed 20min; even at 190 ℃, dwell time is best not to exceed 1h. injection molding, under the premise of ensuring the fluidity of the material, should try to choose a lower molding temperature and shorter heating time.
POM has a relatively obvious melting point, *** poly POM for 165 ℃, homopolymer POM for 175 ℃. Molding, the barrel temperature distribution: the front section 190 ~ 200 ℃, the middle section 180 ~ 190 ℃, the back section 150 ~ 180 ℃, the nozzle temperature of 170 ~ 180 ℃. For thin-walled products, the barrel temperature can be increased appropriately, but not more than 210 ℃.
The mold temperature is usually controlled at 80 ~ 100 ℃, for thin-walled long distance and complex shape products, mold temperature can be increased to 120 ℃. Increase the mold temperature is conducive to melt flow work, to avoid the cooling rate is too fast to make the product defects, and can also improve the impact strength of the product, but also improve the molding shrinkage.
Injection pressure on the mechanical properties of POM products has a small impact, but the melt fluidity and surface quality of the product has a great impact. The size of the injection pressure, mainly by the shape of the product, wall thickness, mold circulation, gate size and mold temperature. For small gate, thin wall, long flow distance, large area products, injection pressure is higher, 120 ~ 140MPa; and large gate, thick wall, short flow distance, small area products, injection pressure of 40 ~ 80 MPa; general products for about 100MPa. Appropriate increase in injection pressure, favorable to improve the melt fluidity and product surface quality, but the pressure is too high will cause product overflow.
Due to the high crystallinity of POM, volume shrinkage, in order to prevent the products appear hollow, dents and other defects, there must be enough holding time to make up for shrinkage. In general, the thicker the product, the longer the holding time.
The injection rate depends on the wall thickness of the product. Thin-walled products should be injected quickly to avoid premature solidification of the melt; thick-walled products should be injected slowly to avoid jetting, affecting the appearance and internal quality of the product.
In order to eliminate the residual internal stresses in the product and reduce shrinkage, heat treatment is usually required. Heat treatment is to air or oil as a medium, the temperature of 120 ~ 130 ℃, the length of time by the wall thickness of the product; in general, the wall thickness of each increase of 1mm, annealing time increased by about 10min. Heat treatment effect can be used to determine the polar solvent impregnation method; will be heat-treated products, into the 30% hydrochloric acid solution impregnated for 30min, if there is no crack, indicating that the residual internal stresses in the product is small, to achieve the purpose of the treatment.
POM plastic more than a certain temperature or processing temperature for a long time after the heat, will occur after the degradation, the release of a large number of harmful formaldehyde gas, not only affects the quality of the product, corrosion molds, hazardous to human health, in serious cases, it will cause gas expansion in the cylinder to produce an explosion and other production accidents. Therefore, in addition to strict control of the operation of the molding process conditions, should also pay attention to the following:
① strict control of the molding temperature of the POM and the material in the barrel of the residence time;
② driving before the temperature, the first preheating nozzle, after heating the barrel;
③ processing of the POM, if the barrel of the memory of the material processing temperature exceeds the POM, the first to be PE as a cleaning material to clean the barrel, and the first to be used as a cleaning material. Cleaning material will be cleaned, to be the temperature down to the processing temperature of POM, and then use PE to clean the barrel once before the material can be put into the molding operation;
③ in the molding process, such as the discovery of serious pungent formaldehyde flavor, yellowish-brown streaks on the product, indicating that the material has been degraded, at this time should be immediately used in the empty injection method, the barrel of the material will be emptied, and PE cleaning barrel, to be normal and then processed;
③ when processing POM, if there are materials in the barrel processing temperature exceeds the POM, to first use PE as a cleaning agent will be cleaned, until the temperature is reduced to the processing temperature of POM, and then PE cleaning barrel. To be normal before processing;
⑤ Some materials or additives (such as PVC, halogenated flame retardants, etc.), POM has a role in promoting degradation, must be strictly separated, not allowed to mix with each other.
Polyformaldehyde, also known as polyoxymethylene, the English name poly0xymethylene (referred to as POM). Molecular structure regularity and crystallinity make its physical and mechanical properties are very excellent, there is a metal plastic called. POM for milky white opaque crystalline linear thermoplastic resin, with good overall performance and coloring, has a high modulus of elasticity, high rigidity and hardness, than the strength and rigidity close to the metal; tensile strength, bending strength, creep resistance and fatigue resistance, resistance to repeated impacts, de-loaded back to the superiority of the coefficient of friction is small, abrasion resistance, good dimensional stability, good surface luster, have a higher Viscoelasticity, excellent electrical insulation, and not affected by temperature;, good insulation resistance and not affected by humidity; excellent chemical resistance: in addition to strong acids, phenols and organic halides, stable to other chemicals, oil resistance; mechanical properties are less affected by temperature, with a high heat distortion temperature. Disadvantages are poor flame retardant, fire Xu Xu burning, oxygen index is small, even if the addition of flame retardant can not get satisfactory requirements, in addition to weathering is not ideal, outdoor applications to add stabilizers.
Physical and chemical properties
High specific gravity
Non-isotropic shrinkage
Molding is more difficult
Polyacetal (POM) hard, resistant to latent change and strong. It has a low coefficient of friction. They also remain stable when used at high temperatures and provide superior stability in hot water environments.
They are virtually unaffected by strong bases, but can be attacked by strong mineral acids.
Polyacetal (POM) with 40% glass fiber reinforcement has a flexural modulus of 1.6 psi x 10E6 (11024 MPa) and a heat deflection temperature of 330 degrees F (166 degrees C)
Homoacetal is highly crystalline, and has better mechanical strength, rigidity, and heat deflection temperatures than ****polyacetal, which has a low melting point, thermal stability, chemical resistance, flow characteristics, and processing. Low melting point, thermal stability, chemical resistance, flow characteristics, processability are better than the polyformaldehyde, newly developed products for ultra-high flow (rapid prototyping), impact resistance and reduce mold deposition grades, but also inorganic filler, enhanced grades.
POM water absorption is greater than 0.2%, should be pre-dried before molding, POM melting temperature and decomposition temperature is similar to the poor molding, can be injection molding, extrusion, blow molding, rotomolding, welding, bonding, coating, printing, plating, machining, injection molding is the most important processing methods, molding shrinkage is large, the mold temperature is appropriate to be higher or annealed, or add reinforcing materials (such as alkali-free) glass fiber).
POM high strength, light weight, commonly used to replace copper, zinc, tin, lead and other non-ferrous metals, widely used in industrial machinery, automotive, electronics, daily necessities, pipes and fittings, precision instruments and building materials and other sectors.
Injection Process
Special Composite Materials
Typical Injection
Molding Conditions
General Injection Molding Conditions
English
English
Inch
SI Metric
Temperature
Rear zone Back zone 350 - 390 °F 177 - 199 °C
Center zone Middle zone 360 - 400 °F 182 - 204 °C
Front zone Front zone 370 - 410 °F 188 - 210 °C
M C
Melt 360 - 425 °F 182 - 218 °C
Mold 175 - 225 °F 79 - 107 °C
Pressures
Injection 10,000 - 15,000 psi 69 - 103 MPa
< p> Hold Pressure 5000 - 12000 psi 34 - 83 MPaBack Pressure 50 - 100 psi 0.34 - 0.69 MPa
Speeds
Fill Speed 1 - 2 in/sec 25 - 51 mm/sec
Screw RPM 60 - 90 rpm 60 - 90 rpm
Drying Conditions
Time & Temperature
Time & Temperature 2 Hrs @ 250 °F 2 Hrs @ 121 °C
Dew Point Dew Point -25 °F - 32 °C
Moisture Content
Moisture Content 0.15 % 0.15 %
Permanent Antistatic Composites
Typical Injection
Molding Conditions
General Injection Molding Conditions
English
English
English
SI Metric
Metric
Temperature
Rear zone Back section 310 - 350 °F 154 - 177 °C
Center zone Middle section 330 - 370 °F 166 - 188 °C
Front zone Front section 350 - 390 °F 177 - 199 °C
Melt Melt temperature 350 - 400 °F 177 - 204 °C
Mold Mold temperature 180 - 250 °F 82 - 121 °C
Pressures
Injection Injection pressures 10000 - 15000 psi 69 - 103 MPa
Hold 5000 - 10000 psi 34 - 69 MPa
Back 50 - 100 psi 0.34 - 0.69 MPa
Speeds
Fill 1 - 2 in/sec. 25 - 51 mm/sec
Screw Speed 60 - 90 rpm 60 - 90 rpm
Drying Conditions
Time & Temperature
Time & Temperature 2 Hrs @ 250 °F 2 Hrs @ 121 °C
Dew Point Dew Point -25 °F -32 °C
Moisture Content
Moisture Content 0.15 % 0.15 %
EMI Masking Composite Materials
Typical Injection
Molding Conditions
General Injection Molding Conditions
English
English
SI Metric
Metric
Temperature
Rear zone Back zone 370 - 410 °F 188 - 210 °C
Center zone Center zone 360 - 400 °F 182 - 204 °C
Front zone Front section 350 - 390 °F 177 - 199 °C
Melt 360 - 425 °F 182 - 218 °C
Mold 180 - 225 °F 82 - 107 °C
Pressures
Injection 10,000 - 15,000 psi 69 - 103 MPa
Hold 5,000 - 10,000 psi 34 - 69 MPa
Back 50 - 100 psi 0.34 - 0.69 MPa
Speeds
Fill Speed 1 - 2 in/sec 25 - 51 mm/sec
Screw Speed 30 - 60 rpm 30 - 60 rpm
Drying Conditions
Time & Temperature
Time & Temperature 2 Hrs. amp; Temperature 2 Hrs @ 250 °F 2 Hrs @ 121 °C
Dew Point Dew Point -20 °F -29 °C
Moisture Content
Moisture Content 0.15 % 0.15 %
Applications
POM is widely used in the manufacture of a variety of sliding, Rotating machinery parts, do all kinds of gears, levers, pulleys, sprockets, especially suitable for bearings, hot water valves, precision metering valves, conveyor chain rings and rollers, flow meters, automotive interior and exterior handles, cranks and other window rotating machinery, oil pump bearing housing and impeller gas switch valve, electronic switch parts, tight solid, lug mirror cover, fan parts, heating plate, instrument knobs; audio-visual tape bearings; all kinds of pipelines and agricultural sprinkler systems and valves; POM is widely used in the manufacture of various sliding, turning mechanical parts. Pipeline and agricultural sprinkler systems and valves, sprinkler heads, faucets, bath tub parts; switch keyboards, buttons, audio-visual tape reel; temperature control timer; power tools, garden finishing tools parts; in addition can be used as surfboards, sailboats and a variety of sled parts, watch micro gears, sports equipment, frame accessories and backpacks with a variety of loops, fasteners, cigarette lighters, zippers, grommets; medical equipment heart pacemaker; artificial heart valves, vertebrae, prosthetic limbs, etc.