POM(Polyoxymethylene Resin) Definition: Polyoxymethylene is a linear polymer with no side chain, high density and high crystallinity.

POM types and differences

According to the difference of chemical structure in its molecular chain, it can be divided into two kinds of homopolymeric formaldehyde and copolymeric formaldehyde.

The important differences between the two are: density, crystallinity and melting point of homopolymer are high, but the thermal stability is poor, the processing temperature range is narrow (about 10 ℃), and the stability of acid and alkali is slightly lower;

Co-polymer formaldehyde density, crystallinity, melting point, strength are lower, but good thermal stability, not easy to decompose, processing temperature range (about 50 ℃), the stability of acid and alkali is better. It is an engineering plastic with excellent overall performance.

It has good physical, mechanical, and chemical properties, especially excellent friction resistance. Commonly known as race steel or seize steel, the third-largest general engineering plastic. Suitable for the production of wear-resistant parts, transmission parts, and chemical, instrumentation, and other parts.

What are the process conditions for POM injection molding?

1. The moisture absorption of POM is low, generally 0.20%-0.25%, usually without drying can be injection molded, but the wet raw materials must be dried. Drying can also improve the gloss and mechanical strength of the product.

2. The melting temperature range of POM is very narrow, and the melting and solidification speed is fast, and the crystallization speed is also fast. When the material temperature is slightly below the melting temperature, crystallization will occur and the fluidity will drop rapidly.

3. POM is very heat sensitive and easy to decompose. The decomposition temperature is 240°C. However, it also decomposes when it is left at 200°C for more than 30 mm. When decomposing, it will give off irritating and corrosive gas.

4. POM has high crystallinity. When the amorphous melt becomes a crystalline solid, the shrinkage rate is close to 20%. Therefore, pressure filling must be performed to prevent excessive shrinkage to ensure the dimensions of the plastic injection molded part.

5. POM has a low coefficient of friction and high elasticity. When the side grooves of plastic parts are shallow, they can be demolded without damaging the plastic parts.

6. The nozzle diameter of POM should be larger and a straight through nozzle should be used. In order to prevent the flow of mouth water, the nozzle hole can be flared.

7. POM injection molding products are very hard and rigid, and the plastic parts have good demoldability and are easy to demold quickly.

8. The resistance of the pouring system to the molten material should be small, so the inlet should be increased appropriately, and dead-end stagnation in the flow channel should be avoided as much as possible.

9. POM should be molded by screw injection molding machine, the excess material should not be too much, and the residence time in the barrel should not be too long.

10. In the case of high material temperature of POM and low temperature of the nozzle, high pressure air injection can easily cause explosion and injury.

11. The fluidity of POM melt is medium. The change of flowability is not sensitive to temperature, but more sensitive to the change of injection molding pressure.

12. For POM injection molding products with metal inserts, the inserts must be preheated at a preheating temperature of 10 0 ~ 150 ℃, which can reduce the internal stress around the inserts in the plastic parts to avoid cracking.

13. The ratio of shear stress to shear rate of POM melt is almost constant, i.e. Newtonian melt. Melt viscosity is largely influenced by the shear rate, but not the temperature.

14. Copper is the catalyst of POM degradation, and the parts in contact with POM should avoid using copper and its alloy materials.

15. In the same POM product, molding shrinkage is not uniform, the thick wall part is cooled slower than the thin wall part, resulting in density difference, making the molten material in the thick wall part flow to the thin wall part. Therefore, the wall thickness of the product should be as uniform as possible.

16. At higher temperatures, POM will decompose into formaldehyde and even formic acid, which can easily corrode the mold, so the mold should be made of corrosion-resistant materials.

Characteristics of POM materials

1.POM can not be dried before processing, it is best to preheat in the process of processing (about 80℃), is good for the stability of the product size.

2. The processing temperature of POM is very narrow (0~215℃), and it will decompose when the residence time in the barrel is slightly longer or the temperature exceeds 220℃, producing strong irritant formaldehyde gas.

3. When POM material is injected, the holding pressure should be larger (similar to the injection pressure) to reduce the pressure drop. Screw speed can not be too high, the residual amount should be less;

4. POM product shrinkage rate is large, easy to produce shrinkage or deformation. POM specific heat, high mold temperature (80~100℃), the product is very hot when demoulding, need to prevent burns fingers.

5. POM should be formed under the conditions of “medium pressure, medium speed, low material temperature, and high mold temperature”, and the mold temperature should be controlled when precision injection molding products are formed

6. High mechanical strength and rigidity

7. Highest fatigue strength

8. Environmental resistance, good resistance to organic solvents

9. Good resistance to repeated impact, good electrical properties, good recovery, self lubrication, good wear resistance, good dimensional stability.

Properties of POM materials

The long-term heat resistance of POM is not high, but it can reach 160℃ in the short term. The short-term heat resistance of homopolymer POM is more than 10℃ higher than that of copolymer POM, but the long-term heat resistance of copolymer POM is about 10℃ higher than that of homopolymer POM.

It can be used in the temperature range of -40℃~100℃ for a long time.

POM is easily decomposed at 240 ° C. Irritant and corrosive gases occur when decomposition, so the mold steel should be made of corrosion-resistant materials.

Paraformaldehyde is a smooth, shiny hard and dense material, light yellow or white, thin wall part is translucent. 

Combustion characteristics are easy to burn, from the fire to continue to burn, the upper end of the flame is yellow, the lower end is blue, melting dripping, there is a strong irritant formaldehyde taste, fishy smell.

Polyformaldehyde is white powder, generally opaque, good coloring, specific gravity 1.41-1.43 grams/cubic centimeter, molding shrinkage rate 1.2-3.0%, molding temperature 170-200℃, drying conditions 80-90℃ 2 hours.

1. POM is a crystalline plastic with a density of 1.42g/cm3. It has good rigidity and is commonly known as “Match steel “.

2. It has excellent performance of fatigue resistance, creep resistance, wear resistance, heat resistance, impact resistance, and small friction coefficient, good self-lubrication.

3 POM is not easy to hygroscopic, water absorption rate of 0.22~0.25%, in a humid environment with good dimensional stability, its shrinkage rate of 2.1%(larger), injection size is difficult to control, hot deformation temperature of 172℃, POM has two kinds of homopolymer, different performance (homopolymer temperature resistance is better).

4. One of POM synthetic resin, also known as polyformaldehyde resin, POM plastic, steel, etc.; Is a kind of white or black plastic particles, with high hardness, high rigidity, high wear resistance characteristics. Mainly used for gears, bearings, auto parts, machine tools, instrument parts and other skeleton products.

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

6. POM is a crystalline plastic with obvious melting point. Once the melting point is reached, the viscosity of the melt decreases rapidly. 

Decomposition occurs when the temperature exceeds a certain limit or when the melt is heated for too long. Copper is a POM degradation catalyst. Copper or copper materials should be avoided in contact with POM melt.

What are the notes for POM injection molding?

Injection temperature

In this case, when the injection temperature is 20~30℃ above the melting point, not only the comprehensive physical and mechanical properties are good, but also the apparent smooth and smooth product can be obtained at this temperature, so the injection temperature control between 190~ 200℃ is the most ideal.

For thin-walled POM injection molding products can be raised to 210℃ for processing, exceeding this temperature can not improve the fluidity of the material, but may lead to the decomposition of the material. Pom-h can be set to 215℃ (190℃-230℃) POM-K can be set to 205℃ (190℃-210℃)

Barrel temperature

POM is a crystalline plastic, and the length of time the melt stays in the barrel will affect the number and size of crystal nuclei contained. At the same temperature above the melting point, the number of crystal nuclei decreases with the increase of residence time of the melt.

The temperature of the barrel is generally controlled at 150 ~ 180℃, which is easy to cause discoloration and decomposition. Too low will lead to uneven plasticizing and poor fluidity, so try to use a lower processing temperature and a shorter heating time on the premise of ensuring fluidity. Nozzle temperature is usually slightly lower than barrel temperature.

Mold temperature

Mold temperature is the main factor affecting POM strength. Higher mold temperature and longer crystallization time are favorable for crystal growth. Crystal integrity and good fluidity. Usually the mold temperature is controlled at 75-120℃. If plastic parts are required to be in good surface condition, the mold temperature may be taken to a larger value.

Injection pressure

The injection injection molded pressure depends on the melt fluidity of the POM, the flow channel, the thickness and width of the gate and the thickness of the product. Injection pressure range is wide, usually 40 ~ 130MPa, for thick wall products, injection pressure can be set to a smaller value; Thin-walled products should be of great value.

Stop

PE or PP must be used to clean the barrel, turn off the electric heating, and push the screw in front. The barrel and screw must be kept clean. Impurities or dirt can alter the overheating stability of POM (especially POM-H).

So when the use of halogenated polymer or other acidic polymer, the application of PE clean after POM material, otherwise there will be an explosion. If the improper action of pigments, lubricants or materials containing GF nylon, will lead to plastic degradation.

Plastic treatment

POM has low water absorption, generally 0.2%-0.5%. In general, POM can be processed without drying, but the wet materials must be dried. The drying temperature shall be above 80℃ and the drying time shall be above 2 hours 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 end use, it can be as much as 100 percent.

Selection of plastic machine

POM has no special requirements for injection molding machine except for the screw without hysteresis area. General injection molding can be done.

Mold and gate design

The common mold temperature control is 80-90℃, the runner diameter is 3-6mm, the gate length is 0.5mm, the gate size depends on the thickness of the glue 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 or more of the thickness, and the depth is 0.6 times the wall thickness. Demoulding Angle between 40 ‘-1°30’.

Exhaust system POM-H thickness 0.01-0.02mm width 3mm POM-K thickness 0.04mm width 3mm

Injection speed

Common for medium speed is fast, too slow easy to produce ripples, too fast easy to produce lines and shear overheating.

The back pressure

The lower the better, generally no more than 200bar

Length of detention

If the equipment has no glue retention point

Pom-h can remain at 215℃ for 35 minutes

Pom-k can be retained at 205℃ for 20 minutes without serious decomposition. At injection molding temperature, the melt cannot remain in the barrel for more than 20 minutes.

Pom-k can be retained for 7 minutes at 240℃. If stop, barrel temperature can be reduced to 150℃, if you want to stop for a long time, you must clean the barrel, close the heater.

Post-treatment of 12

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

Annealing effect, the product can be immersed in the concentration of 30% hydrochloric acid solution for 30 minutes to check, and then observe with the naked eye to determine whether there is residual stress crack generation.

Advantages and disadvantages of POM materials

Properties: Crystalline thermoplastic plastic, with obvious melting point 165~175℃, the properties of the most similar to metal, generally known as plastic steel.

Advantages of POM materials

1. High mechanical strength and rigidity

2. Maximum fatigue strength

3. Good environmental resistance and organic solvent resistance

4. Strong resistance to repeated impact

5. Wide operating temperature range (-40℃~120℃)

6. Good electrical properties

7. Good recovery

8. With self lubrication and good wear resistance

9. Excellent size stability

Disadvantages of POM materials

1. If the processing process is easy to decompose under high temperature for a long time

2. No self-extinguishing

3. Poor acid resistance

4. Forming shrinkage rate is large

Application field of POM materials

POM raw materials are of high strength and light weight, and are commonly used to replace copper, zinc, tin, lead and other non-ferrous metals.

They are widely used in industrial machinery, automobile, electronic appliances, daily necessities, pipes and accessories, precision instruments and building materials and other sectors.

POM raw materials are widely used in manufacturing all kinds of sliding and rotating machinery parts, making all kinds of gears, levers, pulleys, sprockets, especially suitable for bearings, hot water valves, precision metering valves, conveyor chains and rollers, flow meters, car internal and external handles, cranks and other window rotating machinery. 

Oil pump bearing seat and impeller gas switch valve, electronic switch parts, tight solid, terminal mirror mask, electric fan parts, heating plate, instrument button;

Recording and videotape bearing; All kinds of pipes and agricultural irrigation systems and valves, sprinkler heads, faucets, bath tub parts; Switch keyboard, button, audio and video reel; Temperature control timer; Power tools, garden finishing tools parts; 

In addition, it can be used as surfboard, sailboat and various sled parts, watch micro-gear, sports equipment frame accessories and backpacks with all kinds of buckles, fasteners, lighters, zippers, buckles; Cardiac pacemaker in medical instruments; Artificial heart valves, apex vertebrae, prostheses, etc.

Conclusion

This blog introduces the types, properties and injection molding process of engineering plastic POM, the selection of equipment in actual production, the considerations for product modeling and mold design, and the solutions to common defects.

In the actual selection of POM materials and POM injection molding manufacturing, it is recommended to choose an experienced injection mold manufacturer and injection molding product supplier to comprehensively recommend a suitable POM material from the use, functional requirements, appearance and other aspects of the product to ensure the smooth implementation of the project.