PC material has the characteristics of high strength, good transparency, low molding shrinkage, and good machining performance. It is widely used in the field of electronic appliances and industrial parts. Today Daliang will talk with you about the injection molding process of PC materials and the choice of screw.
PC has excellent performance, high transparency, good impact toughness, creep resistance, and wide temperature range.
The machining characteristics of PC are: the sensitivity of the melt viscosity to the shear rate is small, but the temperature sensitivity is large, there is no obvious melting point, the melt viscosity is high, the resin is easy to hydrolyze at high temperature, and the product is easy to crack.
In view of these characteristics, we must pay special attention to different treatments: to increase the melt fluidity, it is not to increase the injection pressure but to increase the injection temperature. Short and thick runners and gates of the mold are required to reduce the pressure loss of the fluid, and higher injection pressure is required. The resin needs to be fully dried before the molding process to control the water content below 0.02%. In addition, heat preservation measures should be taken to the resin during machining to prevent reabsorption. Not only a reasonable product design is required, but also the molding process should be properly mastered, such as increasing the mold temperature and post-machining the product to reduce or eliminate internal stress. Correct the process parameters in time according to the different conditions of the product.
Injection molding process
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- The injection temperature must be integrated with the shape, size and mold structure of the product. Product performance, requirements and other aspects of the situation can be considered only after consideration. Generally, the temperature is selected between 270 ~ 320 ℃ in molding. If the temperature is too high, the PC will decompose, the color of the product will become darker, and the surface will have silver wires, dark stripes, black spots, bubbles and other defects. At the same time, physical and mechanical properties have also decreased significantly.
- The injection pressure has a certain effect on the physical and mechanical properties, internal stress, and molding shrinkage of the PC product. It has a greater impact on the appearance and mold release of the product. Too low or too high injection pressure will make the product appear a certain For these defects, the injection pressure is generally controlled between 80-120MPa. For products with thin walls, long processes, complicated shapes, and small gates, in order to overcome the resistance of the melt flow, in order to fill the mold cavity in time, only the higher Injection pressure (120 -145MPa). Thus, a complete and smooth surface product is obtained.
- Holding pressure and holding time The size of holding pressure and the length of holding pressure have a greater influence on the internal stress of PC products. If the holding pressure is too small, the shrinkage effect is small and vacuum bubbles or surface shrinkage are likely to occur. Concavity, the holding pressure is too large, and large internal stress is easy to be generated around the gate. In actual machining, it is often solved by high material temperature and low holding pressure. The selection of the dwell time should depend on the product thickness, gate size, mold temperature, etc. Generally, small and thin products do not require a long dwell time. On the contrary, large and thick products should have a longer dwell time. The length of the dwell time can be determined by testing the gate sealing time.
- The injection speed has no obvious effect on the performance of PC products. Except for thin-walled, small gates, deep holes, and long-flow products, medium or slow machining is generally used. Multi-stage injection is preferred, and slow machining is generally used. -Fast-slow multi-stage injection molding method.
- Mold temperature is generally controlled at 80-100 ° C. For products with complex shapes, thinner, and more demanding products, it can also be raised to 100-120 ° C, but it cannot exceed the mold’s thermal deformation temperature.
- Screw rotation speed and back pressure. Because PC melt viscosity is large, it is beneficial for plasticization, exhaust, and maintenance of the plastic machine to prevent the screw load from being too large. The screw rotation speed must not be too high, and is generally controlled at 30. -60r / min is suitable, and the back pressure is controlled between 10-15% of the injection pressure.
- PC in the injection molding process must strictly control the use of release agent, while the use of recycled materials can not be more than three times, the amount should be about 20%.Requirements for plastic presses for the production of PC products: The maximum injection quantity (including runners, gates, etc.) of the product is required to be not more than 70-80% of the nominal injection quantity. The screw is selected with a single-head thread and other pitches, with a gradient of check loops. Compression type screw. The L / D ratio of the screw is 15-20, and the geometric compression ratio C / R is 2-3.