PC plastic injection molding process analysis, including the entire process
PC plastic has excellent performance, high transparency, good impact toughness, creep resistance, wide temperature range, and PC plastics have the following process characteristics.
The melt viscosity has little sensitivity to shear rate, and has high sensitivity to temperature, no obvious melting point, high viscosity of the melt, easy hydrolysis of the resin at high temperature, and easy cracking of the product. In view of these characteristics, special attention should be paid to the difference: to increase the fluidity of the melt, it is not achieved by increasing the injection pressure and increasing the injection temperature. The flow path and gate of the mold are required to be short and thick to reduce the pressure loss of the fluid and to have a high injection pressure. The resin should be thoroughly dried before molding to keep the water content below 0.02%. In addition, the resin should be insulated during processing to prevent re-absorption. Not only reasonable product design is required, but also the molding process should be properly mastered. For example, increasing the mold temperature and post-processing the product can reduce or eliminate internal stress. Process parameters are adjusted in a timely manner depending on the product's different conditions.
Process characteristics of PC: PC has high viscosity, high melt temperature and poor fluidity. It must be injection molded at a higher temperature (270-320 °C). Relatively speaking, the temperature adjustment range is narrower and the process is not as good as PMMA. Injection pressure has little effect on fluidity, but due to its high viscosity, it still requires a large injection pressure. To prevent internal stress, the holding time should be as short as possible. The shrinkage is large and the size is stable, but the internal stress of the product is large and easy to crack. Therefore, it is better to increase the temperature instead of the pressure to improve the fluidity, and to reduce the possibility of cracking by increasing the mold temperature, improving the mold structure and post-treatment. When the injection speed is low, the gate is prone to ripples and other defects, the temperature of the nozzle is controlled separately, the mold temperature is high, and the flow path and gate resistance are small.
PC injection molding performance: PC is a crystalline plastic with obvious melting point, melting at 220 ° C, decomposition at 350 ° C; general processing temperature is 250 ~ 320 ° C; it absorbs water, a small amount of moisture can cause it to decompose at high temperatures, When injection molding, it must be dried. The drying temperature can be dried at 120 °C for 4 to 5 hours. The melt viscosity of the PC material is large. When injection molding, a large pressure is required. When the PC plastic is processed, if conditions permit. The mold temperature can be used to increase the mold temperature to reduce the residual stress of the product; if the product has inserts, the diameter of the insert is less than 1~2mm, the preheating can be ignored, and the larger insert should consider preheating, otherwise Stress cracking occurs; the shrinkage rate of PC is independent of the processing conditions and the wall thickness of the product. Its longitudinal and transverse shrinkage ratios are relatively close, so it can be processed into a highly accurate product; its shrinkage rate is 5 ‰, overflow The side value is 0.05 mm.
PC plastic injection molding process.
1, injection temperature
It must be considered after considering the shape, size, mold structure, product performance, requirements and other aspects of the product. 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 be decomposed, the color of the product will become darker, and defects such as silver wire, dark stripes, black spots and bubbles will appear on the surface. At the same time, the physical and mechanical properties also decreased significantly.
2, injection pressure
It has certain influence on the physical and mechanical properties, internal stress and molding shrinkage of PC plastic 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. Generally, the injection pressure is controlled between 80 and 120 MPa for thin-walled, long-flow, complex-shaped, small gates, in order to overcome the resistance of the melt flow, so that even if the cavity is filled, the higher injection pressure is used. 120 to 145 MPa) to obtain a complete and smooth surface.
3. 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; the holding pressure is too large. Large internal stresses are easily generated around the gate. 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, gate size, mold temperature, etc. Generally, small and thin products do not require a long holding time; on the contrary, large 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.
4, 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 with medium or slow speed, preferably multi-stage injection, generally adopting slow-fast-slow Multi-stage injection method.
5, mold temperature
Generally, it can be controlled at 80-100 °C. For products with complicated shape, thinner and higher requirements, it can also be raised to 100-120 °C, but it can't exceed the heat distortion temperature of the mold.
6, screw speed and back pressure
Due to the high viscosity of PC melt, from the maintenance of plasticization, exhaust and injection molding machine, to prevent excessive screw load, the screw speed requirement should not be too high, generally controlled at 30 ~ 60r / min, and the back The pressure control is preferably between 10% and 15% of the injection pressure.
7, release agent
The PC should strictly control the use of the release agent during the injection molding process, and the use of the recycled material should not exceed three times, and the usage should be about 20%.
8. 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% to 80% of the nominal injection volume. The pitch of the single-head thread, the gradient compression type screw with the check ring, and the aspect ratio of the screw are required. The L/D is 15 to 20, and the set compression ratio C/R is 2 to 3.
These are PC plastic injection molding process analysis, including the entire process.