Introduction of PC plastic injection molding process

PC has excellent performance, high transparency, good impact toughness, creep resistance, and a wide temperature range. The melt viscosity is high, the resin is easy to hydrolyze at high temperatures, and the product is easy to crack. In view of these characteristics, special attention should be paid to different treatments: Increasing the fluidity of the melt is not achieved by increasing the injection pressure but by increasing the injection temperature. The runner 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 to have a high injection pressure. The resin needs to be fully dried before molding so that its moisture content is controlled below 0.02 percent . In addition, insulation measures should be taken for the resin during processing to prevent the re-absorption of moisture. Not only a reasonable product design is required, but also a correct grasp of the molding process, such as increasing the mold temperature and post-processing the product, can reduce or eliminate internal stress. Adjust the process parameters in time depending on the different conditions of the product.

Process characteristics of PC: PC has a high viscosity, high melt temperature, and poor fluidity. In this case, it must be injected at a higher temperature (270-320 degree C). Relatively speaking, the material temperature adjustment range is narrow, and the processability is not as good as PMMA. The injection pressure has little effect on the fluidity, but due to the large viscosity, the injection pressure is still large. Correspondingly, in order to prevent 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 it is easy to crack. Therefore, it is advisable to increase the temperature instead of the pressure to improve the fluidity, and increase the mold temperature, improve the mold structure, and post-treatment to reduce the possibility of cracking. When the injection speed is low, defects such as ripples are likely to occur at the gate. The temperature of the nozzle should be controlled separately, the mold temperature should be high, and the resistance of the runner and gate should be small.
PC injection molding performance: PC is a crystalline plastic with an obvious melting point. It melts at 220 degree and decomposes at 350 degree ; the general processing temperature is 250-320 degree ; it absorbs water, and a small amount of water can cause it to decompose at high temperatures. During injection molding, it must be dried, and the drying temperature can be used at 120 degree for 4 to 5 hours; the melt viscosity of PC material is large, and greater pressure is required during injection molding; during PC processing, if conditions permit, The mold temperature can be increased with a mold temperature machine to reduce the product
If it is a product with inserts, the diameter of the inserts is less than 1-2mm, and preheating may not be considered. For larger inserts, preheating should be considered, otherwise, stress cracking will occur; PC shrinkage rate and processing conditions Regardless of the wall thickness of the product, its longitudinal and transverse shrinkage rates are relatively close, so it can be processed into products with high precision; its shrinkage rate is 5‰, and the overflow value is 0.05mm.


(1) Injection temperature
The shape, size, mold structure, product performance, requirements, and other aspects of the product must be considered after consideration.
to make. Generally, the temperature used in molding is between 270 and 320 degree . When the material temperature is too high, such as over 340 degree , PC
Decomposition will occur, the color of the product will become darker, and defects such as silver threads, dark stripes, black spots, and bubbles will appear on the surface.
The performance also dropped significantly.


(2) Injection pressure
It has a certain influence on the physical and mechanical properties, internal stress, and molding shrinkage rate of PC products, and has a greater impact on the appearance and mold release of the product. Too low or too high injection pressure will cause some defects in the product. Generally, the injection pressure is controlled between 80 and 120MPa. For products with thin walls, long processes, complex shapes, and small gates, in order to overcome the resistance of the melt flow, so that even if the cavity is filled, a higher injection pressure (120 145MPa) to obtain a complete and smooth surface product.


(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. It is easy to generate large internal stress around the gate. In actual processing, it is often solved by the method of high material temperature and low pressure. The selection of the holding time should depend on the thickness,
It depends on the gate size and mold temperature. Generally, small and thin products do not need a long pressure holding time; on the contrary, large and thick products should have a long pressure holding time. The length of the holding time can be determined by the test of the gate sealing time.


(4) Injection speed
It has no obvious effect on the performance of PC products. Except for thin-walled, small gate, deep-hole, and long-process products, a
Generally use medium speed or slow speed processing, preferably multi-stage injection, generally use slow-fast-slow multi-stage injection method.

(5) Mold temperature
Generally, it can be controlled at 80 to 100 degree . For products with complex shapes, thinner products, and higher requirements, it can also be increased to 100 to 120 degree , but it cannot exceed the heat deformation temperature of the mold.

(6) Screw speed and backpressure
Due to the high viscosity of PC melt, it is beneficial to the maintenance of plasticization, exhaust, and injection molding machine, to prevent the screw from being overloaded, and the speed of the screw should not be too high. It is advisable to control the pressure between 10 percent and 15 percent of the injection pressure.

(7) Release agent
In the injection molding process of PC, the use of mold release agents should be strictly controlled, and the use of recycled material should not exceed three times, and the amount used should be about 20 percent .

(8) Requirements for plastic machines for the production of PC products
It is required that the maximum injection volume of the product (including runners, gates, etc.) should not be greater than 70 percent to 80 percent of the nominal injection volume, and a gradient compression screw with a single-head thread with equal pitch and a non-return ring is used. The length-diameter ratio of the screw L/D is 15-20, and the aggregate compression ratio C/R is 2-3.