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  • Summarize the most practical POM injection molding process

Summarize the most practical POM injection molding process

POM (also known as SAI steel, Trim). It is obtained by polymerization of formaldehyde, etc. POM-H (polyacetal homopolymer), POM-K (polyacetal copolymer) is high-density, high crystallinity thermoplastic engineering plastics. They have good physical, mechanical, and chemical properties, especially excellent friction resistance.

POM is a crystalline plastic with an obvious melting point. Once the melting point is reached, 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 POM degradation catalyst, and POM melt contact parts should avoid the use of copper or copper materials.

1. Plastic processing

POM water absorption is small, generally 0.2% -0.5%. Under normal circumstances, POM can be processed without drying, but the wet raw materials must be dried. Drying temperature above 80 ℃, time more than 2 hours, the specific should be carried out according to the supplier information.

The percentage of recycled material used is usually not more than 20-30%. However, depending on the type of product and end-use, sometimes up to 100%.

2. The choice of plastic machine

POM has no special requirements for injection molding machines, except for the requirement of the screw without stagnant area, the general injection can be.

3. Mold and gate design

Common mold temperature control is 80-90℃, runner diameter is 3-6mm, the gate length is 0.5mm, gate size depends on the thickness of the rubber wall, round gate diameter should be at least 0.5-0.6 times the thickness of the product, rectangular gate width is usually 2 times or more than the thickness, depth is 0.6 times the wall thickness, demold slope is between 40′-130′.

Venting system.

POM-H thickness 0.01-0.02m, width 3mm

POM-K thickness 0.04mm, width 3mm

4. Melting speed

Measurable by air injection method.

POM-H can be set to 215℃ (190℃-230℃) POM-K can be set to 205℃ (190℃-210℃)

5. Injection speed

It is common for medium speed to be fast, too slow to produce ripples, too fast to produce shot lines, and shear overheating.

6. Back pressure

The lower the better, generally not more than 200bar.

7. Stagnation time

If the equipment does not have a melt retention point POM-H can be retained at 215℃ for 35 minutes, POM-K can be retained at 205℃ for 20 minutes without serious decomposition, the melt cannot be retained in the barrel for more than 20 minutes at the injection temperature.

POM-K can be retained at 240°C for 7 minutes. If stop, the barrel temperature can be reduced to 150℃, if you want to stop for a long time, you must clean the barrel and turn off the heater.

8. Shutdown

Clean the barrel must use PE or PP, turn off the electric heater, and push the screw in the front position. The barrel and screw must be kept clean. Impurities or dirt will change the superheat stability of POM (especially POM-H). So when finished with halogen-containing polymers or other acidic polymers, apply PE to clean up before playing POM material, otherwise, an explosion will occur. If the role of improper pigments, lubricants, or materials containing GF nylon, will lead to plastic degradation.

9. Post-processing

For the very temperature use of the parts and high-quality requirements, the heat treatment shall be carried out.

Annealing treatment effect, you can put the product into the concentration of 30% hydrochloric acid solution for 30 minutes to check, and then use the naked eye to determine whether there are residual stress cracks.
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