Polyformaldehyde POM injection molding process:

1, plastic treatment

POM plastic 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 more than 2 hours, which should be carried out according to the supplier's information.

The proportion of recycled materials is generally not more than 20-30%. But depending on the type of product and the end use, sometimes up to 100%.

(PS: In order to ensure product quality during processing, it is recommended to do dry treatment, which can improve the quality of materials)

2, the selection of plastic machine

POM In addition to requiring the screw to have no hysteresis area, there is no special requirement for the injection molding machine, and the general injection molding can be done.

3. Mold and gate design

Common mold temperature control is 80-90℃, the diameter of the runner is 3-6mm, the length of the gate is 0.5mm, the size of the gate depends on the thickness of the rubber wall, the diameter of the circular gate should be at least 0.5-0.6 times the thickness of the product, the width of the rectangular gate is usually 2 times the thickness or more, and the depth is 0.6 times the wall thickness. Release inclination between 40 '-1°30'.

Exhaust system

POM-H thickness 0.01-0.02mm width 3mm

POM-K thickness 0.04mm wide 3mm

4, melting temperature

It can be measured by air emission

POM-H can be set to 215℃ (190℃-230℃)

POM-K can be set to 205℃ (190℃-210℃)

Injection molding influence condition

Barrel temperature:

Because polyformaldehyde (POM plastic raw materials) is a crystalline plastic, the residence time of the molten material in the cylinder will affect the number and size of its crystal nuclei, at the same temperature above its melting point, the longer the residence time of the melt, the number of crystal nuclei will decrease. The cylinder temperature should usually be controlled at 150 ~ 180 ° C, the temperature should not be too high, too high will lead to discoloration decomposition, too low will plasticize uneven, affecting fluidity. Therefore, on the premise of ensuring fluidity, try to use a lower processing temperature and a shorter heating time. In addition, the nozzle temperature should be slightly lower than the barrel temperature.

Mold temperature:

Usually the mold is the main factor affecting the strength of polyformaldehyde (POM), the mold temperature is high, the crystallization time is long, which is conducive to the growth of the crystal, the crystallization is more complete, and the fluidity will be better. Under normal circumstances, the mold temperature should be controlled at 75 ~ 120℃.

Injection pressure:

The size of the injection pressure mainly depends on the melting flow of polyformaldehyde (POM), the thickness and width of the runner, the gate, and the thickness of the plastic products. Usually 40 ~ 130Mpa, for thick wall products, the injection pressure should be a small value, whereas thin wall products should be a large value.

5. Injection speed

The common speed is medium fast, too slow is easy to produce ripples, too fast is easy to produce jet and shear overheating.

6. Back pressure

The lower the better, generally no more than 200bar

7. Retention time

If the device has no glue retention point

POM-H can be retained at 215 ° C for 35 minutes

POM-K can be retained at 205 ° C for 20 minutes without serious decomposition

The melt should not remain in the barrel for more than 20 minutes at injection molding temperature. POM-K can be retained for 7 minutes at 240℃. If the machine is stopped, the barrel temperature can be reduced to 150 ° C, if you want to stop for a long time, you must clean the machine cylinder and turn off the heater.

8. Stop

Cleaning the barrel must use PE or PP, turn off the electric heat, and push the screw in the front position. Barrel and screw must be kept clean. Impurities or dirt can change the overheating stability of POM (especially POM-H).

Therefore, when the halogen-containing polymer or other acidic polymers are used, the PE can be cleaned up before the POM plastic raw materials can be played, otherwise there will be an explosion. Improper use of pigments, lubricants or materials containing GF nylon can lead to plastic degradation.

9. Post-processing

For very warm parts and high quality requirements, heat treatment must be carried out.

Annealing treatment effect, the product can be immersed in 30% hydrochloric acid solution for 30 minutes to check, and then use the naked eye to determine whether there is a residual stress crack.

There are several factors for the appearance of bubbles:

1. Check whether there is a problem with mold exhaust (tried after inspection, not this reason)

2. Appropriately increase the mold temperature, especially the mold temperature of the piece that produces bubbles (also not this reason)

3. Increase injection energy: pressure, speed, time and quantity, and increase back pressure to make the filling mold full (much less bubble, but there is still)

4. Set the gate on the thick part of the part to improve the flow condition of the nozzle, runner and gate and reduce the consumption of pressure (at that time, I thought that this step would not succeed, but I did not expect that there would be no bubble in the end)

The appearance of white products, the main solutions are as follows:

(1) To eliminate the interference of gas, it means to prevent the pollution of impurities.

(2) Increase the material temperature and mold temperature, adjust the cylinder temperature in stages, but to prevent the temperature from being too high and decomposition.

(3) Increase the injection pressure, extend the holding time, and improve the back pressure