PVC injection molding plastic resin -3
PVC injection molded product
The backpressure should be kept at about 1Mpa, because excessively high backpressure will produce excessive shear force, which will cause the PVC to decompose. At the same time, the multi-stage backpressure control should be adopted in the melting process. In the beginning when the effective length of the screw is long, the backpressure should be kept low. As the effective length of the screw decreases, the backpressure should be appropriately increased to compensate for the heat loss. Shortly before material recycling ends, the backpressure should be lowered to prevent leakage.
The screw speed should vary according to the diameter. Usually, when the diameter is < 60mm, the speed is 50-70rpm; when the diameter is > 70mm, the speed is 20-50rpm, so as to prevent excessive shearing and PVC degradation.
Usually, to inject the plastic into the cavity with a uniform speed throughout the process, the multi-stage injection speed control is adopted, of which the principle is to start with a low speed, and as the molding area increases, the injection speed must be increased to avoid cracks and shrinkage marks. The injection speed cannot be so fast, or the product surface will be denuded.
During the plastic injection molding process, the barrel temperature should be kept between 170-190°C. To prevent the PVC from overheating and degrading, when the temperature of the barrel reaches the set temperature, the blower should be started to cool the temperature down. On the contrary, when the temperature needs to be raised, the air blower should be shut down.
(To be continued...)