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 the melting point, the melt viscosity drops rapidly. When the temperature exceeds a certain limit or the melt is heated for too long, it will cause decomposition.
Copper is a catalyst for POM degradation, and copper or copper materials should be avoided in the parts in contact with POM melt.
1. Plastic treatment
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. The drying temperature is above 80℃, and the time is above 2 hours. The details should be carried out according to the supplier’s information.
The proportion of recycled materials used generally does not exceed 20-30%. But it depends on the type of product and the end use, and sometimes it can reach 100%.
2. Selection of plastic machine
POM has no special requirements for injection molding machines except that the screw has no stagnant area. Generally, injection molding is sufficient.
3. Mold and gate design
Common mold temperature control is 80-90℃, runner diameter is 3-6mm, gate length is 0.5mm, gate size depends on the thickness of the rubber wall, round gate diameter should be at least 0.5-0.6 times the thickness of the product , The width of the rectangular gate is usually 2 times or more than the thickness, the depth is 0.6 times the wall thickness, and the demolding slope is between 40′-130′.
POM-H thickness 0.01-0.02mm width 3mm
POM-K thickness 0.04mm width 3mm
4. Melting temperature
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℃)
5. Injection speed
It is common that the speed is too fast, too slow to produce ripples, and too fast to produce ripples and shear overheating.
6. Back pressure
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°C. At 210°C, the residence time cannot exceed 20min; even at 190°C, the residence time should preferably not exceed 1h. Therefore, during injection molding, under the premise of ensuring the fluidity of the 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, the temperature distribution of the barrel is: 190-200℃ in the front section, 180-190℃ in the middle section, 150-180℃ in the rear section, and the temperature of the nozzle is 170-180℃. For thin-walled products, the barrel temperature can be appropriately increased, but it should not exceed 210°C.
3. POM has low hygroscopicity, and the resin does not need to be dried before processing. If necessary, it can be dried at 90～100℃ for 2～4h.
4. The melt viscosity of POM is sensitive to the shear rate. Therefore, to improve the fluidity of the melt, it is not possible to increase the temperature alone, but also to increase the injection rate and injection pressure. For products with large gates, thick walls, short processes, and small areas, the injection pressure is 40-80MPa; general products are about 100MPa. For products with small gates, thin-walled long processes, and large areas, the injection pressure is relatively high, ranging from 120 to 140 MPa.
5. The mold temperature is usually controlled at 80-100°C. For products with thin walls and long processes and complex shapes, the mold temperature can be increased to 120°C.
6. High crystallinity results in high molding shrinkage (2～2.8%), and defects such as dents and shrinkage are prone to occur in thick wall parts.
Dark brown spots
Description: Injection molded parts have the correct color tone but occasionally spots or streaks are visible.
Injection molding machine:
A. The degraded plastic from the last production run is solidified in the gun screw and rubber ring. ——Use cleaning mixture or high molecular weight acrylic material to clean the injection device.
B. The plastic is trapped in the “corner” or non-flowing area of the injection device, making it stay at high temperature for too long. ——Remove the barrel and screw to thoroughly clean the surfaces in contact with the molten polymer; check whether the nozzle is correctly located in the barrel; replace the open-close nozzle with an open or straight-through nozzle; check whether the rubber head has been damaged.
C. The speed at which the plastic enters the cavity is too fast to cause excessive shearing of the assembly. ——Reduce the injection speed.
D. The melt temperature is too high. ——Reduce the barrel temperature; check whether the flow rate of the cooling body is sufficient for the hopper locking device, and adjust the flow rate if necessary; reduce the cycle time to increase the plastic passing through the injection device.
E. Using incorrect screw surface speed and back pressure, causing excessive shearing of the molten plastic. ——Use minimal back pressure and correct screw surface speed
A. Oil stains and oil appear in the mold. ——Remove and clean the mold, especially the mouth.
B. Too few gates. ——Increase the size of the gate.
C. The wall thickness of the injection molded part is too thin, so that the plastic can not flow fully without degradation-check the correctness and consistency of the wall thickness.
D. Impurities are generated in the hot runner mold-remove the hot nozzle for cleaning