What are the POM/PMMA injection molding processes?

POM (also known as Saigang and Trane). It is polymerized with formaldehyde and other raw materials. POM-H (polyoxymethylene homopolymer) and POM-K (polyoxymethylene copolymer) are thermoplastic engineering plastics with high density and high crystallinity.
It 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 decreases rapidly. When the temperature exceeds a certain limit or the melt is heated for a long time, it will cause decomposition. Copper is a catalyst for POM degradation, and copper or copper materials should be avoided at the part in contact with POM melt.
1. Plastic treatment
POM has low water absorption, generally 0.2% - 0.5%. Generally, POM can be processed without drying, but wet raw materials must be dried. The drying temperature is 80 ℃ and the time is more than 2 hours. The details shall be carried out according to the supplier's data.
The proportion of recycled materials is generally not more than 20-30%. However, it depends on the type and end use of the product, sometimes up to 100%.
2. Selection of plastic machine
POM has no special requirements for injection molding machine, except that the screw is required to have no material stagnation area. Generally, injection molding is enough.
4. Melt temperature
It can be measured by air injection method
POM-H can be set to 215 ℃ (190 ℃ - 230 ℃)
POM-K can be set to 205 ℃ (190 ℃ - 210 ℃)
5. Back pressure
The lower the better, generally not more than 200bar
POM-H can stay at 215 ℃ for 35 minutes
POM-K can remain at 205 ℃ for 20 minutes without serious decomposition
The melt cannot remain in the barrel for more than 20 minutes at the injection temperature. POM-K can remain for 7 minutes at 240 ℃. In case of shutdown, the barrel temperature can be reduced to 150 ℃. In case of long-term shutdown, the barrel must be cleaned and the heater must be turned off.
6. Shutdown
Use PE or PP to clean the barrel, turn off the electric heat and push the screw to the front position. The barrel and screw must be kept clean. Impurities or dirt can change the overheating stability of POM (especially POM-H). Therefore, when halogenated polymer or other acidic polymer is used up, the POM material can be used only after PE cleaning, otherwise explosion will occur. Improperly applied pigments, lubricants or materials containing GF nylon will lead to plastic degradation.
PMMA injection molding process
PMMA is commonly known as plexiglass, acrylic, etc. The chemical name is polymethylmethacrylate. Due to the shortcomings of PMMA such as low surface hardness, easy to scrub, low impact resistance and poor molding flow performance, the modification of PMMA has emerged one after another. For example, the copolymerization of methyl methacrylate with styrene and butadiene, and the blending of PMMA and PC. 372 plexiglass is obtained by copolymerization of methyl methacrylate and styrene. If a small amount of nitrile rubber (about 50%) is added to 372 plexiglass, 373 plexiglass can be made.
The flow of PMMA is worse than that of PS and ABS, and the melt viscosity is sensitive to the change of temperature. During the molding process, the melt viscosity is mainly changed from the injection temperature. PMMA is amorphous polymer with melting temperature greater than 160 ℃ and decomposition temperature up to 270 ℃.
1. Treatment of plastics
PMMA has certain water absorption, its water absorption is up to 0.3-0.4%, while the injection temperature must be below 0.1%, usually 0.04%. The existence of water makes the melt appear bubbles, air lines, transparency reduction, etc. Therefore, drying treatment shall be carried out. The drying temperature is 80-90 ℃ and the time is more than 3 hours.
Recycled materials can be used 100% in some cases, and the actual weight depends on the quality requirements, usually more than 30%. Recycled materials should avoid pollution, otherwise it will affect the transparency and the nature of the finished product.
2. Melt temperature
It can be measured by air injection method: from 210 ℃ to 270 ℃, depending on the information provided by the supplier.
3. Injection temperature
Rapid injection can be used, but to avoid generating high internal stress, multi-stage injection should be used, such as slow-fast-slow, etc. When injecting thick parts, slow injection should be used.