Choice of alloy screw barrel

Update:04 Jun 2019

The material of alloy screw barrel is generally 38CrMOALA or SACM645, the specific selection please see: 1, PC process characteristics are: melt viscosity to shear rate sensitivity is small, and the temperature sensitivity is large, no obvious melting point, melt viscosity is high, high temperature resin hydrolysis, products easy to crack. In view of these characteristics, we should pay special attention to the distinction: to increase the melt flow, not by increasing the injection pressure should be achieved by raising the injection temperature. The requirements of the mold runner, gate short and thick, in order to reduce the pressure loss of fluid, at the same time to higher injection pressure. Resin in the molding process before the need for adequate drying treatment, so that the water content of the control of 0.02%, in addition, in the process of resin should also take insulation measures to prevent re-absorption of moisture. Not only need reasonable product design, but also should correctly grasp the molding process, such as raising the temperature of the mold, post-processing of products can reduce or eliminate the internal stress. Adjust and correct process parameters in time according to different conditions of products. 2, PC molding process is: 1, injection temperature must be integrated products shape, size, mold structure. Product performance, requirements and other aspects of the situation to be considered before making. Generally choose the temperature between 270~320℃ in the molding, high material temperature such as more than 340℃, PC will appear decomposition, product color darker, surface silver wire, dark strip, black spots, bubbles and other defects, while the physical and mechanical properties also significantly decreased. 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 high injection pressure (mpa), 120-145. Thus obtaining complete and smooth surface products. 3, the holding pressure and holding time of 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 selection of pressure holding time should depend on the thickness of products, gate size, mold temperature and other conditions, generally small and thin products do not need a long time of pressure holding time, on the contrary, large and thick products should be long time of pressure holding time. The length of pressure holding time can be determined by the test of gate sealing time. 4. The injection speed has no obvious influence on the performance of PC products. In addition to thin wall, small gate, deep hole and long process products, generally adopt medium or slow speed processing. 5. The mold temperature is generally controlled at 80-100℃, and it can be raised to 100-120℃ for the products with complex shapes, thinner and higher requirements, but it cannot exceed the thermal deformation temperature of the mold. 6, screw speed and back pressure due to PC melt viscosity, from the favorable plasticizing, favorable exhaust, favorable maintenance of plastic machine, to prevent the screw load is too large, the screw speed requirement is not too high, generally control in 30-60r/min is appropriate, and back pressure control in the injection pressure between 10-15% is appropriate. 7, PC in the injection molding process to strictly control the use of mold release agent, at the same time, the use of raw materials can not be more than three times, the use should be about 20%. Requirements for PC products molding machine: the maximum injection amount (including runner, gate, etc.) of the required products should not be more than 70-80% of the nominal injection amount. The screw should use single-head thread and other pitch, gradual compression screw with check ring, and the length-diameter ratio of the screw should be 15-20.

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