POM has good physical, mechanical and chemical properties, especially with excellent resistance to friction. POM is a crystalline plastic with a sharp melting point. Once the melting point is reached, the melt viscosity drops rapidly. Decomposition occurs when the temperature exceeds a certain limit or the melt is heated for too long. Copper is a POM degradation catalyst and copper or copper materials should be avoided where it is in contact with the POM melt.
1, plastic processing
POM has a small water absorption, generally 0.2%-0.5%. Under normal circumstances, POM can be processed without drying, but it must be dried for wet materials. The drying temperature is above 80 °C and the time is more than 2 hours. The specific conditions should be based on the supplier's data.
The proportion of recycled materials used is generally not more than 20-30%. However, depending on the type of product and the end use, it can sometimes be up to 100%.
2, the selection of plastic machine
3, mold and gate design
The common mold temperature is controlled to 80-90 ° C, the runner diameter is 3-6 mm, the gate length is 0.5 mm, the gate size depends on the thickness of the rubber wall, and the circular 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 of the thickness, the depth is 0.6 times the wall thickness, and the draft angle is between 40' and 130'.
Exhaust system:
POM-H thickness 0.01-0.02m, width 3mm
POM-K thickness 0.04mm, width 3mm.
4, melting speed
Available by airshot method:
POM-H can be set to 215 ° C (190 ° C -230 ° C)
POM-K can be set to 205 ° C (190 ° C -210 ° C).
5, injection speed
It is common for the medium speed to be too fast, too slow to produce ripples, too fast and easy to produce grain and shear overheating.
6, back pressure
The lower the better, generally no more than 200bar.
7, staying time
If the equipment does not have a glue retention point POM-H can stay at 215 ° C for 35 minutes, POM-K can stay at 205 ° C for 20 minutes, there will be no serious decomposition, the melt can not stay in the barrel more than 20 at the injection temperature minute.
POM-K can be retained for 7 minutes at 240 °C. If the machine is shut down, the barrel temperature can be lowered to 150 ° C. If you want to stop for a long time, you must clean the machine and turn off the heater.
8, downtime
The cleaning cylinder must use PE or PP to 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 change the thermal stability of the POM (especially POM-H). Therefore, when the halogen-containing polymer or other acidic polymer is used up, the POM material can be used after cleaning with PE, otherwise an explosion will occur. Inappropriate pigments, lubricants or materials containing GF nylon can cause deterioration of the plastic.
9, post processing
For very warm parts and high quality requirements, heat treatment is required.
The annealing treatment effect can be carried out by immersing the product in a hydrochloric acid solution having a concentration of 30% for 30 minutes, and then visually discriminating whether or not cracks have residual stress.
