Analysis of injection process of PC plastics
PC plastics have excellent properties, high transparency, good impact toughness, creep resistance, wide temperature range. PC plastics have the following process characteristics
The sensitivity of melt viscosity to shear rate is small, but to temperature is large, there is no obvious melting point, the viscosity of melt is high, the resin is easy to hydrolyze at high temperature, and the product is easy to crack. In view of these characteristics, special attention should be paid to differential treatment: to increase the fluidity of the melt, it is not achieved by increasing the injection pressure but by increasing the injection temperature. The runner and gate of the mould are required to be short and thick, so as to reduce the pressure loss of the fluid and have a higher injection pressure. The resin shall be fully dried before forming and processing to control its moisture content below 0.02%. In addition, insulation measures shall be taken for the resin during processing to prevent moisture absorption again. It is not only necessary to design the products reasonably, but also to master the forming process correctly, such as increasing the mold temperature, post-treatment of the products and so on, which can reduce or eliminate the internal stress. Adjust the process parameters in time according to the different conditions of the products.
Process characteristics of PC: PC viscosity is large, melt temperature is high, and fluidity is poor. In this case, it must be injection molded at a higher temperature (270 ~ 320 ℃), relatively narrow range of material temperature adjustment, and the processibility is inferior to PMMA. The injection pressure has little effect on the fluidity, but because of the high viscosity, the injection pressure still needs to be higher. In order to prevent the internal stress, the pressure holding time should be as short as possible. The shrinkage rate is large and the size is stable, but the internal stress of the product is large and easy to crack, so it is better to improve the fluidity by increasing the temperature rather than the pressure, and reduce the possibility of cracking by increasing the temperature of the mold, improving the structure of the mold and post-treatment. When the injection speed is low, the gate is easy to produce corrugations and other defects. The temperature of the radial nozzle should be controlled separately, the mold temperature should be high, and the resistance of the runner and gate should be small.
Injection molding performance of PC: PC is a crystalline plastic with obvious melting point. It melts at 220 ℃ and decomposes at 350 ℃; the general processing temperature is 250-320 ℃; it absorbs water and a small amount of water can cause it to decompose at high temperature. During injection molding, it must be dried, and the drying temperature can be used at 120 ℃ for 4-5h; the melting viscosity of PC material is large, so it needs a large pressure during injection molding; In the processing of PC plastics, if the conditions permit, the mold temperature can be increased by the mold temperature machine to reduce the residual stress of the product; if the product has inserts, the diameter of the inserts is less than 1-2mm, preheating can be ignored, and the larger inserts should be considered preheating, otherwise, stress cracking will occur; The shrinkage of PC has nothing to do with the processing conditions and the wall thickness of the products. Its longitudinal and transverse shrinkage are relatively close, so it can be processed into products with high precision. Its shrinkage is 5 ‰ and the overflow value is 0.05mm.
It can only be made after considering the shape, size, mold structure, product performance, requirements and other aspects of the product. Generally, when the temperature is between 270 ℃ and 320 ℃, if the material temperature is higher than 340 ℃, PC will decompose, the color of products will become darker, and defects such as silver wire, dark strip, black spot, bubble will appear on the surface, and the physical and mechanical properties will also decline significantly.
It has a certain influence on the physical and mechanical properties, internal stress, molding shrinkage of PC plastic products, and has a great influence on the appearance and demoulding performance of products. Too low or too high injection pressure will cause some defects of products. Generally, the injection pressure is controlled between 80 ~ 120MPa for thin-walled, long process, complex shape and small gate products, in order to overcome the melt flow Resistance, so that even if the mold cavity is full, a higher injection pressure (120-145mpa) can be selected to obtain a complete and smooth surface of the product.
Pressure maintaining pressure and time
The magnitude and duration of holding pressure have great influence on the internal stress of PC products. Too small holding pressure, small feeding effect, easy to appear vacuum bubble or surface shrinkage; too large holding pressure, easy to produce large internal stress around the gate. In actual processing, the method of high material temperature and low holding pressure is often used to solve this problem. The selection of holding time depends on the thickness of products, gate size, mold temperature, etc. generally, small and thin products do not need a long holding time; on the contrary, large and thick products should have a long holding time. The length of holding time can be determined by the test of gate sealing time.
There is no obvious effect on the performance of PC products. Except for thin-walled, small gate, deep hole and long process products, medium speed or slow processing is generally adopted, multi-level injection is preferred, and slow fast slow multi-level injection is generally adopted.
Generally, it can be controlled at 80-100 ℃, and it can also be increased to 100-120 ℃ for products with complex shape, thinner and higher requirements, but it can not exceed the hot deformation temperature of the mold.
Screw speed and back pressure
Due to the high viscosity of PC melt, the requirement of screw speed should not be too high to prevent the screw from being overloaded, which is favorable for the maintenance of plasticizing, exhausting and injection molding machine. It is generally suitable to control the screw speed at 30 ~ 60R / min, while the back pressure should be controlled at 10% ~ 15% of the injection pressure.
In the process of injection molding, the use of release agent should be strictly controlled, and the use of recycled material should not exceed three times, and the use amount should be about 20%.
It is required that the maximum injection volume (including flow channel, gate, etc.) of PC products should be no more than 70% ~ 80% of the nominal injection volume
The length diameter ratio (L / D) of the screw is 15 ~ 20, and the set compression ratio is 15 ~ 20