POM is polymerized from formaldehyde, etc. POM-H (polyoxymethylene homopolymer) and POM-K (polyoxymethylene copolymer) are high-density and high-crystallinity thermoplastic engineering plastics with good physical and mechanical properties And chemical properties, especially with excellent friction properties.
POM is a crystalline plastic, the melting point is obvious, once reached the melting point, the melt viscosity decreased rapidly when the temperature exceeds a certain limit or the melt is heated for too long, will cause decomposition. Copper is POM degradation catalyst, and POM melt contact parts should Avoid using 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 for wet raw materials must be dried. Drying temperature above 80 ℃, more than 2 hours, according to the specific suppliers Data is carried out.
The proportion of recycled materials generally does not exceed 20-30%, but depending on the type of product and end use, sometimes up to 100%.
2, the selection of presses
POM addition to the screw without hysteresis area, no special requirements on the injection molding machine, the general injection can be.
3, mold and gate design
Common mold temperature control of 80-90 ℃, the flow channel diameter 3-6mm, the gate length of 0.5mm, the gate size depends on the thickness of the plastic 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, the depth of 0.6 times the wall thickness, the stripping slope between 40'-130 '.
Exhaust system:
POM-H thickness 0.01-0.02m, width 3mm
POM-K thickness 0.04mm, width 3mm
4, melt speed
Free-air method available measurement:
POM-H can be set to 215 ℃ (190 ℃ -230 ℃)
POM-K can be set at 205 ° C (190 ° C - 210 ° C)
5, injection speed
Common for the speed too fast, too slow and easy to produce ripples, too fast and easy to produce patterns and shear overheating.
6, back pressure
The lower the better, generally not more than 200bar.
7, residence time
If the equipment does not have a retention point of POM-H at 215 ° C for 35 minutes, POM-K remains at 205 ° C for 20 minutes without severe decomposition, and the melt can not remain in the barrel at injection temperature for more than 20 minute.
POM-K can hold for 7 minutes at 240 ° C. If the machine is down, the barrel temperature can drop to 150 ° C and for long periods of time the machine must be cleaned and the heater switched off.
8, downtime
Clean the barrel must use PE or PP, turn off the electric heat, push the screw in front of the barrel and the screw must be kept clean impurities or dirt will change the POM overheating stability (especially POM-H) Halogenated polymers or other acidic polymers should be cleaned with PEL before they can be pommerized or exploded. Improperly applied pigments, lubricants or GF nylon containing materials can result in plastic degradation.
9, post-processing
For non-temperature parts and the quality requirements of higher heat treatment to be carried out.
Annealing effect, the product can be placed in a concentration of 30% hydrochloric acid solution soak for 30 minutes to check, and then with the naked eye to determine whether there is a residual stress cracks.