Excellent
PC performance, high transparency, good impact toughness, creep resistance, and wide temperature range. Process characteristics of PC: melt viscosity has little sensitivity to shear rate, but has high sensitivity to temperature, no obvious melting point, high viscosity of melt, easy hydrolysis of resin at high temperature, and easy cracking of products.
For these characteristics, we have to treat them differently:
1. To increase the fluidity of the melt, it is not necessary to increase the injection pressure but to increase the injection temperature.
2. The flow path and gate of the mold are required to be short and thick to reduce the pressure loss of the fluid, and at the same time, a higher injection pressure;
3. The resin needs to be fully dried before molding, so that the water content is controlled below 0.02%;
4. In addition, the resin should also be insulated during the processing to prevent re-absorption;
5, not only need a reasonable product design, but also should correctly master the molding process, such as increasing the mold temperature, post-processing the product, etc. can reduce or eliminate internal stress, adjust the process parameters in time according to different conditions of the product.
Molding
Drying of raw materials
PC plastics, even in the case of very low moisture, can cause hydrolysis, breakage, molecular weight reduction and physical strength reduction. Therefore, before the molding process, the moisture content of the polycarbonate should be strictly controlled to be 0.02% or less to avoid a decrease in the mechanical strength of the molded article or an abnormal appearance of bubbles, silver streaks or the like on the surface.
PC is extremely sensitive to water, so it must be fully dried before injection to reduce its water content to below 0.02%. PC general drying conditions: 100~120 °C for at least 4 hours;
2. Injection temperature
The injection temperature must be integrated into the shape, size, and mold structure of the product. Only after considering the performance and requirements of the product can be made.
Generally, the temperature is between 270 and 320 °C in the molding. When the material temperature is too high, such as more than 340 °C, the PC will decompose, the color of the product will become darker, and defects such as silver, dark stripes, black spots and bubbles will appear on the surface. At the same time, physical and mechanical properties also dropped significantly. PC is also sensitive to temperature. The melt viscosity decreases significantly with increasing temperature. The barrel temperature is 250-320 ° C (preferably not more than 350 ° C). Appropriately increasing the temperature of the barrel is beneficial to PC plasticization. If necessary, the internal stress is annealed. The oven temperature is 125~135°C for 2 hours and naturally cooled to normal temperature.
3. Injection pressure
It has a certain influence on the physical and mechanical properties, internal stress and molding shrinkage of PC products, and has a great influence on the appearance and release property of the products. Too low or too high injection pressure will cause some defects in the products. The injection pressure is controlled between 80-120 MPa. For thin-walled, long-flowing, complex-shaped products with small gates, in order to overcome the resistance of the melt flow, in order to fill the cavity in time, a higher injection pressure is selected. -145 MPa). Thereby a complete and smooth surface product is obtained.
Poor fluidity, high pressure injection is required, but it is necessary to take into account the large internal stress of the rubber parts (may cause cracking). Injection speed: the wall thickness is taken at medium speed, and the wall is thin.
4. Holding pressure and holding time
The size of the holding pressure and the length of the holding time have a great influence on the internal stress of the PC product. The holding pressure is too small, the feeding effect is small, and the vacuum bubble or the surface is concave and concave, and the holding pressure is too large. Large internal stress is easy to be generated around the mouth. In actual processing, it is often solved by high material temperature and low pressure holding method. The choice of holding time should be determined according to the thickness of the product, the size of the gate, the temperature of the mold, etc. Generally, the small and thin products do not require a long holding time. On the contrary, the thick and thick products should have a longer holding time. The length of the holding time can be determined by the test of the gate sealing time.
8. Injection speed
There is no obvious influence on the performance of PC products. Except for thin wall, small gate, deep hole and long process products, it is generally processed at medium or slow speed, preferably multi-stage injection, generally adopting slow-fast-slow Multi-stage injection method.
6. Mold temperature
Mold temperature control: 85~120 °C, generally controlled at 80-100 °C, for complex shapes, thinner, higher requirements, can also be increased to 100-120 ° C, but can not exceed the mold heat distortion temperature. The mold temperature is high to reduce the difference between the mold temperature and the PC material temperature, which can reduce the internal stress of the part.
7. Screw speed and back pressure
Due to the high viscosity of PC melt, it is beneficial to plasticize and facilitate exhaust. It is beneficial to the maintenance of the plastic machine and prevent the screw load from being too large. The rotation speed of the screw should not be too high. Generally, it is suitable for 30-60r/min. The back pressure control is preferably between 10-15% of the injection pressure.
8. Use of additives
PC should strictly control the use of release agent in the injection molding process, and the use of recycled materials should not exceed three times, and the usage should be about 20%.
9.PC injection molding, high requirements on the mold
1) Design a flow path that is as thick and short as possible, with a small number of bends, and that uses a circular cross-section runner and a flow path to polish and reduce the flow resistance of the melt.
2) The injection gate can be any type of gate, but the inlet water level is not less than 1.5mm.
10. Requirements for presses for the production of PC products
The maximum injection volume (including runners, gates, etc.) of the product is required to be no more than 70-80% of the nominal injection volume;
Mode-locking pressure: 0.47 to 0.78 tons per square centimeter of finished product projected area (or 3 to 5 tons per square inch);
Machine size: The weight of the finished product is about 40 to 60% of the capacity of the injection molding machine. If the machine is expressed in polystyrene (Australia), it needs to be reduced by 10%. The capacity of GUANG DA is used. 1 ounce = 28.3 grams.
Screw: The length of the screw should be at least 15 diameters long and the L/D is 20:1. The compression ratio is preferably from 1.5:1 to 30:1. The stop valve at the front end of the screw should adopt a sliding ring type, and the resin flowable gap should be at least 3.2 MM.
11.PC injection processing
Use PE material to pass the machine.
