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PC material molding process and the selection of screw

by:FUHONG     2020-10-13
PC material molding process and the selection of screw PC performance, high transparency, good impact toughness, creep resistance, using a wide temperature range, the technical trait of the PC: the sensitivity of melt viscosity on the shear rate is small, and their sensitivity to temperature, no obvious melting point, high viscosity, melting body easy to hydrolysis, high temperature resin products easy to crack. According to these characteristics, we pay close attention to discriminate: to increase the liquidity of the melt, not by increasing the injection pressure and should be used to increase the temperature of the injection method to achieve. Mold flow channel, the sprue short, thick, in order to reduce the pressure of the fluid loss, at the same time the high injection pressure. Resin before processing should be fully dry processing, make its moisture content control in 0. Below 02%, in addition, in the process of the resin also heat preservation measures should be taken to prevent moisture absorption. Needs not only reasonable product design, but also should correctly grasp the injection molding process, such as increasing the mold temperature, can reduce or eliminate internal stress of products post-treatment. Depending on the situation of each product in a timely manner is adjusting process parameters. Molding process: 1, injection temperature: must be integrated products shape, size, the die structure. Product performance, requirements and so on various aspects of the situation into consideration to make. Usually chooses in the molding temperature is between 270 ~ 320 ℃, material temperature, such as more than 340 ℃, the PC will be decomposed, product color is darker, surface defects such as silver, dark and black spot, air bubble, also a significant reduction in physical and mechanical properties. 2, injection pressure, physical and mechanical properties of PC products, molding shrinkage internal stress has a certain influence on products such as appearance and demoulding of sex has great influence, too low or too high injection pressure can make products appear some defects, the injection pressure control between 80 ~ 120 mpa, the thin wall, a long process, complex shape, the gate of the smaller products, in order to overcome the resistance of the melt flow, so that full of cavity in time, to choose the high injection pressure ( 120~175MPa) 。 To obtain complete and smooth surface of the products. 3, the holding pressure and holding time: the size of the holding pressure and holding time length has a larger effect on the internal stress of PC products, the holding pressure is too small, the feeding effect of small vacuum bubble is seen or surface concave, the holding pressure is too large, the internal stress of gate around easily generate large, in the actual processing, often in high temperature, low pressure of the ways to solve. The holding time should be selected depending on the products, to the thickness of gate size, die temperature, such as the case may be, generally small and thin products don't need a long holding time, on the contrary, big and thick product the holding time should be longer. The length of the holding time can be confirmed by gate sealing time. 4, the injection speed. No obvious influence on the performance of the PC products, in addition to the thin wall, small gate, deep hole, long process products, generally adopts medium or slow processing, it is best to multi-stage injection, generally USES the slow & ndash; — Fast & ndash; — The slow way of multi-stage injection. 5, mold temperature: general control in 80 ~ 100 ℃, the complex shape, thin, the products of the high requirements, also can increase to 100 & ndash; — 120 ℃, but no more than the mould thermal deformation temperature. 6, the screw rotation speed and back pressure: due to large PC melt viscosity, from favorable plasticizing, favorable exhaust, favorable processing molding, to prevent the screw load is too large, requirements for the speed of the screw is too high, general control in 30 ~ 70 r/min, and back pressure control between 10 to 15% of the injection pressure is advisable. 7, PC in the injection molding process to strictly control the use of release agent, reworked material use no more than three times at the same time, consumption should be around 20%. Molding machine requirements for production PC products: the maximum number of injections required products ( Including the runner and gate, etc. ) Should not be greater than the nominal amount of injection 70 ~ 80% advisable, choose the single thread pitch screw, with loopback gradient compression screw, the screw length to diameter ratio L/D for 18 ~ 24, compression ratio C/R is 1. 8 ~ 2. 2.
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