First, mold
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Multi-gate mold to adjust the mold filling speed of each gate, it is best to open the gate symmetrically.
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The key parts of the mold should be effectively provided with cooling waterways to ensure that the cooling of the mold has a good effect on eliminating or reducing shrinkage.
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The whole mold should be free of burrs and have mold sealing, and can withstand the filling mold of high pressure, high speed and low viscosity melt.
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Second, equipment
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Insufficient supply of materials. The screw or plunger wear seriously, the molten material leakage occurs during injection and pressure holding, which reduces the mold filling pressure and the amount of material, resulting in insufficient molten material.
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The nozzle hole is too large or too small. If it is too small, it is easy to block the feed channel, and if it is too large, the shooting force will be small and the mold filling will be difficult.
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Third, the process
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Increase injection pressure, maintain pressure, prolong injection time. For plastics with large fluidity, high pressure will produce flash and cause pit collapse, the material temperature should be properly reduced, the temperature of the front section of the barrel and the nozzle should be reduced, so that the volume change of the molten material entering the cavity is reduced, and it is easy to cold solid; For high viscosity plastics, the barrel temperature should be increased to make the mold easy to fill. When shrinkage occurs in the gate area, the pressure holding time should be extended.
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Increasing the injection speed makes it easier to fill the part and eliminate most of the shrinkage.
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Thin-walled parts should increase the mold temperature to ensure smooth material flow; The mold temperature of thick-walled parts should be reduced to accelerate the curing of the skin.
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Extending the cooling residence time of the parts in the mold, maintaining a uniform production cycle, increasing the back pressure, and retaining a certain cushion in the front section of the screw are conducive to reducing the shrinkage phenomenon.
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Low-precision products should be molded as early as possible to cool slowly in air or hot water, which can make the shrinkage sag gentle and does not affect the use.