POM (also called acetal) is polymerized by formaldehyde and other raw materials, etc. POM-H (Polyoxymethylene homopolymer), POM-K(Polyoxymethylene copolymer) are thermoplastic engineering plastic with high density and crystallinity. It has good physical, mechanical and chemical properties, especially excellent rub resistance.
POM injection molding Technology is a crystalline plastic with an obvious melting point. Once it reaches the melting point, the melt viscosity will rapidly decrease. When the temperature exceeds a certain limit or the melt is heated too long, it will cause decomposition. Copper is POM’s degradation catalyst, the areas contacting with POM melt should avoid the use of copper or copper materials.
Processing of plastic
POM has low water absorption which generally is 0.2%-0.5%. Under normal circumstances, POM can be processed without drying, but the wet raw material must be dried. The drying temperature is above 80 Celsius degree, the time is above 2-4 hours which should be conducted according to the supplier’s datasheet. The utilization of recycled materials is generally within 20-30%. However, it depends on the species and final use of products; sometimes it can reach 100%.
Selection of injection molding machine
In addition to the requirement on the screw with no material banking zone, there is no special requirement on the injection molding machine; the general injection molding is available.
Design of mold and gate
The common mold temperature is controlled 80-90 Celsius degree, the diameter of flow channel is 3-6mm, the length of gate is 0.5mm, the size of gate depends on the thickness of plastic wall, the diameter of circular gate should be at least 0.5-0.6 times of the thickness of the products,the width of the rectangular gate is usually 2 times or more of its thickness,depth is 0.6 times of the wall thickness, stripping slope is between40′ and 1-30.
POM-H the thickness is 0.01-0.02mm and the width is 3mm.
POM-K the thickness is 0.04 mm and the width is 3mm
You can use air injection method to measure the temperature.
POM-H it can be set to 215 Celsius degree(190 -230 Celsius degree)POM-Kit can be set to 205 Celsius degree(190 -210 Celsius degree).
The common is middle speed which is a little fast, too slow speed is easy to produce corrugation, too fast speed is easy to produce ray lines and shear overheating.
It’s better to be lower for packing. Generally, it does not exceed 200bar.
If the device doesn’t have melt retention point,
POM-H In the temperature of 215 Celsius degree, the retention time is 35 minutes.
POM-K In the temperature of 205 Celsius degree, the retention time of 20 minutes will not have serious decomposition.
In the temperature of injection molding, the melt cannot be stranded in the barrel for more than 20 minutes. POM-K can strand 7minutes in the 240 Celsius degree. If in the downtime, the temperature can decrease to 150 Celsius degree, if the downtime is for a long time, you must clean the barrel, turn off the heater.
You must use PE or PP to clean the barrel, turn off the heater and push the screw frontier area, keep the barrel and screw clean. Impurities or dirt will change the superheating stability of POM (particularly POM-H). So after exhausting the halogen-containing polymer or another acidic polymer, you should use PE to clean up and then Play PM material, or it will cause an explosion. If you use improper pigments, lubricants or GF nylon materials, it will cause the plastic degradation.
For the products used in non-normal temperature, there are higher requirements on quality; it needs to make heat treatment.
The annealing treatment effect is checked by immersing products in a hydrochloric acid solution of concentration 30% for 30 minutes, and then uses eyes observing and determining whether there are residual stress cracks.
The post-processing technology of POM
Methods, Temperature, and Time
Air bath annealing 140-150 Celsius degree, If the thickness of wall increases 5mm, it increases 40-60 minutes.
Oil bath annealing140-150 Celsius degree, If the thickness of the wall increases 5mm, it increases 20-30 minutes.
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