JOINTEFLON COMPANY LIMITED

Summarize the most practical POM injection molding process

30-11-2020

POM (also known as Acetal). It is obtained by polymerization of formaldehyde as raw materials. POM-H (polyoxymethylene homopolymer) and POM-K (polyoxymethylene copolymer) are thermoplastic engineering plastics with high density and high crystallinity. It has good physical, mechanical and chemical properties, especially excellent friction resistance.


POM is a crystalline plastic with obvious melting point. Once it reaches the melting point, the melt viscosity drops 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 parts in contact with POM melt.

 

1. Plastic treatment

POM has low water absorption, generally 0.2%-0.5%. Under normal circumstances, POM can be processed without drying, but wet raw materials must be dried. The drying temperature is above 80℃, and the time is above 2 hours. The details should be carried out according to the supplier's information.

The proportion of recycled materials used generally does not exceed 20-30%. But it depends on the type of product and the end use, sometimes it can reach 100%.

 

2. Selection of plastic machine

POM has no special requirements for injection molding machines except that the screw has no stagnant area. 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 rubber wall, round gate diameter should be at least 0.5-0.6 times the product thickness , The width of the rectangular gate is usually 2 times or more than the thickness, the depth is 0.6 times the wall thickness, and the demolding slope is between 40'-130'.

Exhaust system:

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

POM-K thickness 0.04mm, width 3mm

 

4. Melting speed

It 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℃)

 

5. Injection speed

It is common that the speed is too fast, too slow to produce ripples, and too fast to produce ripples and shear overheating.

 

6. Back pressure

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

 

7. Detention time

If the equipment does not have a melt retention point, POM-H can stay at 215°C for 35 minutes, and POM-K can stay at 205°C for 20 minutes without serious decomposition. The melt cannot stay more than 20 in the barrel at the injection temperature minute.


POM-K can stay for 7 minutes at 240°C. If the machine is shut down, the temperature of the barrel can be reduced to 150°C. If it is to be shut down for a long time, the barrel must be cleaned and the heater turned off.


8. Shutdown

You must use PE or PP to clean the barrel, turn off the electric heating, and push the screw to the front position. The barrel and screw must be kept clean. Impurities or dirt will change 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 be used after the PE is cleaned up, otherwise it will explode. Improper use of pigments, lubricants or materials containing GF nylon will cause degradation of the plastic.

 

9. Post-processing

For parts that are used at very warm temperatures and require high quality, heat treatment is required.


For the effect of annealing treatment, the product can be immersed in a 30% hydrochloric acid solution for 30 minutes for inspection, and then visually observed to determine whether there are residual stress cracks.


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