POM injection molding process: you must pay attention to these nine problems in the injection molding process.

Polyoxymethylene (POM) is a very important engineering plastic. We use it on the pinion of our printer. Today we will thoroughly understand the key points of the Injection Molding process of POM. It is obtained by polymerization of formaldehyde or the like as a raw material. POM-H (polyoxymethylene homopolymer), POM-K (polyoxymethylene copolymer) is a high density, high crystallinity thermoplastic engineering plastic. Has good physical, mechanical and chemical properties, especially with excellent friction resistance.

Polyoxymethylene (POM) injection molding products POM is a crystalline plastic with a sharp melting point. Once the melting point is reached, the melt viscosity drops rapidly. Decomposition occurs when the temperature exceeds a certain limit or the melt is heated for too long. Pay attention to the following points in injection molding

1, drying treatment

POM has a small water absorption, generally 0.2%-0.5%. Under normal circumstances, POM can be processed without drying, but it must be dried for wet materials. The drying temperature is above 80 °C and the time is more than 2 hours. The specific conditions should be based on the supplier's data.

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

2, mold and gate design

The common mold temperature is controlled to 80-90 ° C, the runner diameter is 3-6 mm, the gate length is 0.5 mm, the gate size depends on the thickness of the rubber wall, and the circular gate diameter should be at least 0.5-0.6 times the thickness of the product. The width of the rectangular gate is usually 2 times or more of the thickness, the depth is 0.6 times the wall thickness, and the draft angle is between 40' and 130'.

Exhaust system

POM-H thickness 0.01-0.02mm width 3mm

POM-K thickness 0.04mm width 3mm

3, the selection of plastic machine

In addition to the requirement that the screw has no stagnant area, the POM has no special requirements for the injection molding machine, and generally can be injection molded.

4, the melting temperature

Available by airshot method

POM-H can be set to 215 ° C (190 ° C -230 ° C)

POM-K can be set to 205 ° C (190 ° C -210 ° C)

5, injection speed

It is common for the medium speed to be too fast, too slow to produce ripples, too fast and easy to produce grain and shear overheating.

6, back pressure

The lower the better, generally no more than 200bar

7, staying time

If the device does not have a glue retention point

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

POM-K can stay in 205 ° C for 20 minutes without serious decomposition

The melt cannot remain in the barrel for more than 20 minutes at the injection temperature. POM-K can be retained for 7 minutes at 240 °C. If the machine is shut down, the barrel temperature can be lowered to 150 ° C. If you want to stop for a long time, you must clean the machine and turn off the heater.

8, stop

The cleaning cylinder must use PE or PP to turn off the electric heating and push the screw in the front position. The barrel and screw must be kept clean. Impurities or dirt can change the thermal stability of the POM (especially POM-H). Therefore, when the halogen-containing polymer or other acidic polymer is used up, the POM material can be used after cleaning with PE, otherwise an explosion will occur. If the improperly applied pigment lubricant or GF nylon-containing material will cause the plastic to deteriorate.

9, post processing

For very warm parts and high quality requirements, heat treatment is required. The annealing treatment effect can be carried out by immersing the product in a hydrochloric acid solution having a concentration of 30% for 30 minutes, and then visually discriminating whether or not cracks have residual stress.