Polyoxymethylene is divided into two types: homopolymerization and copolymerization. Copolyoxymethylene has good strength in the short term and homopolyoxymethylene has good flexibility. The copolymer has a softening point 10°C higher than that of a homopolymer, has a high thermal deformation temperature under load, has good thermal stability, and has a wide molding temperature range. Commonly used in industry is copolyformaldehyde. Homopolyoxymethylene is mostly used in luggage products and making zippers. There are differences in the molding process. What we are discussing here is copolyoxymethylene.
Although the hygroscopicity of POM is smaller than that of PA, it must be dried before processing due to the consideration of size and surface quality. Gears and other precision gear parts: 100～120℃, 4～8 hours; general parts: 80～90℃, 3～4 hours.
The melting point of POM is 163°C, above which it will melt. Generally speaking, the processing range of most POM is 170～230℃, and the temperature usually used is 170～200℃. POM is a heat-sensitive plastic part, which will decompose when exposed to heat for a long time. The decomposed formaldehyde is easily oxidized into formic acid, which further accelerates the decomposition of polyformaldehyde. If such a situation is found, it should be cleaned in time to eliminate the decomposition products. PE can be used for cleaning.
Since formaldehyde is easily soluble in water, the cleared POM material can be poured into a bucket of water to reduce smoke. Therefore, the following points should be made:
1. Do not heat POM above 250°C to avoid pyrolysis;
2. When the barrel temperature exceeds 210℃, the residence time of POM melt in the barrel shall not exceed 30 minutes. The higher the barrel temperature, the shorter the residence time.
3. As long as the machine is turned on, the nozzle temperature must be kept above the melting point of POM (163°C). This way, when overheating occurs, the nozzle will not be clogged.
4. If the machine has just processed PVC, the barrel must be cleaned strictly, and the PVC particles must not be mixed into the POM particles, otherwise a strong reaction will occur.
5. If it is necessary to stop: stop for a short time, adjust the temperature of the material to close to the melting point, remove the plastic in the barrel, the screw should always be in the forward position, and turn off the heater. If the machine needs to be shut down for a long time, the PE should be cleaned according to the normal procedure.