POM (also known as Saigang, Trane). It is obtained by polymerization of formaldehyde as raw materials. POM-H (polyoxymethylene homopolymer) and POM-K (polyoxymethylene copolymer) are thermoplastic engineering plastics with high density and high crystallinity. Has good physical, mechanical and chemical properties, especially excellent friction resistance.
POM is a crystalline plastic with obvious melting point. Once it reaches melting point, melt viscosity drops rapidly. When temperature exceeds a certain limit or melt is heated for too long, it will cause decomposition.
Copper is a catalyst for POM degradation, copper or copper materials should be avoided in parts in contact with POM melt.

 

POM has low water absorption, generally 0.2%-0.5%. Under normal circumstances, POM can be processed without drying, but wet raw materials must be dried. Drying temperature is above 80℃, time is above 2 hours. Details should be carried out according to supplier's information.
Proportion of recycled materials used generally does not exceed 20-30%. But it depends on type of product and end use, and sometimes it can reach 100%.

POM has no special requirements for injection molding machines except that screw has no stagnant area. Generally, injection molding is sufficient.

Common mold temperature control is 80-90℃, runner diameter is 3-6mm, gate length is 0.5mm, gate size depends on thickness of rubber wall, round gate diameter should be at least 0.5-0.6 times thickness of product. Width of rectangular gate is usually 2 times or more than thickness, depth is 0.6 times wall thickness, and demolding slope is between 40'-130'.
Exhaust system
POM-H thickness 0.01-0.02mm width 3mm
POM-K thickness 0.04mm width 3mm

Can be measured by air shot method
POM-H can be set to 215℃ (190℃-230℃)
POM-K can be set to 205℃ (190℃-210℃)

It is common that speed is too fast, too slow to produce ripples, too fast to produce ripples and shear overheating.

The lower the better, generally not more than 200bar
POM injection molding process characteristics and process parameter setting
1. POM is also a typical heat-sensitive plastic, which will decompose severely at 240℃. At 210℃, residence time cannot exceed 20min; even at 190℃, residence time should preferably not exceed 1h. Therefore, during injection molding, under premise of ensuring fluidity of material, a lower molding temperature and a shorter heating time should be used as much as possible.
2. POM has obvious melting point, homopolymer POM is 175℃, copolymer POM is 165℃. During molding, temperature distribution of barrel: 190-200℃ in front section, 180-190℃ in middle section, 150-180℃ in rear section, and 170-180℃ in nozzle temperature. For thin-walled products, barrel temperature can be appropriately increased, but it should not exceed 210℃.
3. POM has low hygroscopicity, resin does not need to be dried before processing. If necessary, it can be dried at 90～100℃ for 2～4h.
4. Melt viscosity of POM is sensitive to shear rate. Therefore, to improve fluidity of melt, it is not possible to increase temperature alone, but also to increase injection rate and injection pressure. For products with large gates, thick walls, short processes, and small areas, injection pressure is 40-80MPa; general products are about 100MPa. For products with small gates, thin-walled long processes, and large areas, injection pressure is relatively high, ranging from 120 to 140 MPa.
5. Mold temperature is usually controlled at 80-100℃. For products with thin walls, long processes and complex shapes, mold temperature can be increased to 120℃.
6. High crystallinity results in high molding shrinkage (2～2.8%), defects such as dents and shrinkage are prone to occur in thick wall parts.

Description: Injection molded parts have correct color tone but occasionally spots or streaks are visible.

A. Degraded plastic from last production run is solidified in gun screw and rubber ring. ——Use cleaning mixture or high molecular weight acrylic material to clean injection device.
B. Plastic is trapped in "corner" or non-flowing area of injection device, making it stay at high temperature for too long. ——Remove barrel and screw to thoroughly clean surfaces in contact with molten polymer; check whether nozzle is correctly located in barrel; replace open-close type nozzle with an open or through-type nozzle; check whether rubber head has been damaged.
C. Speed at which plastic enters cavity is too fast to cause excessive shearing of assembly. ——Reduce injection speed.
D. Melt temperature is too high. ——Reduce barrel temperature; check whether flow rate of cooling body is sufficient for hopper locking device, adjust flow rate if necessary; reduce cycle time to increase plastic passing through injection device.
E. Using incorrect screw surface speed and back pressure, causing excessive shearing of molten plastic. ——Use minimal back pressure and correct screw surface speed

A. Oil stains and oil appear in the mold. ——Remove and clean mold, especially nozzle.
B. Too few gates. ——Increase size of gate.
C. Wall thickness of injection molded part is too thin, so that plastic can not flow fully without degradation-check correctness and consistency of wall thickness.
D. Impurities are generated in hot runner mold-remove hot nozzle for cleaning