POM (also known as Saigang, Trane). It is obtained by polymerizing formaldehyde and other raw materials. POM-H (polyoxymethylene homopolymer) and POM-K (polyoxymethylene copolymer) are high density, high crystallinity thermoplastic engineering plastics. Has good physical, mechanical and chemical properties, especially excellent friction resistance.
POM is a crystalline plastic with a clear 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 too long, it will cause decomposition. Copper is a catalyst for POM degradation, and copper or copper materials should be avoided in the area in contact with POM melt.
1. Plastic processing
POM has low water absorption, generally 0.2% -0.5%. Under normal circumstances, POM can be processed without drying, but wet materials must be dried. The drying temperature is above 80 ℃, and the time is more than 2 hours. It should be carried out according to the supplier's information.
The proportion of recycled materials generally does not exceed 20-30%. But depending on the type of product and end use, sometimes it can reach 100%.
2.Selection of plastic machine
In addition to the requirement that the screw has no stagnation area, POM has no special requirements on the injection molding machine, and generally injection molding is sufficient.
3. Mould 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, and the diameter of the circular gate 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 the thickness, the depth is 0.6 times the wall thickness, and the demolding slope is between 40′-130 ′.
POM-H thickness 0.01-0.02m, width 3mm
POM-K thickness 0.04mm, width 3mm
4. Melt speed
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 usually too fast at medium speed. If it is too slow, it is easy to produce ripples. If it is too fast, it is easy to produce jetting and shearing overheating.
The lower the better, generally not exceeding 200 bar.
7. Detention time
If the equipment does not have a glue retention point, POM-H can be held at 215 ° C for 35 minutes, and POM-K can be held at 205 ° C for 20 minutes without serious decomposition. The melt cannot stay in the barrel at more than 20 at the injection temperature. minute.
POM-K can be retained for 7 minutes at 240 ° C. If the machine is stopped, the barrel temperature can be reduced to 150 ° C. If the machine is to be stopped for a long time, the barrel must be cleaned and the heater turned off.
To clean the barrel, you must use PE or PP, turn off the electric heating, and push the screw in the front position. The barrel and screw must be kept clean. Impurities or dirt can alter the overheating stability of POM (especially POM-H). Therefore, when the halogen-containing polymer or other acidic polymer is used up, the POM material can only be used after cleaning with PE, otherwise an explosion will occur. Improper use of pigments, lubricants or materials containing GF nylon can lead to plastic degradation.
For very warm parts and high quality requirements, heat treatment is required.
For the effect of the annealing treatment, the product can be immersed in a 30% hydrochloric acid solution for 30 minutes to check, and then visually judge whether there are cracks caused by residual stress.