Under the pressure of the screw, th

Under the pressure of the screw, the melted plastic flows through the nozzle at a high shear rate and enters the mold cavity. When the melt flows through the nozzle with narrow diameter at high speed, it will be sheared greatly. Some pressure will be changed into heat energy through resistance loss, which will increase the temperature of the melt. At the same time, part of the pressure can be converted into velocity energy, so that the molten material can be injected into the mold cavity. In the pressure maintaining stage, a small amount of molten material is needed to be fed into the cavity through the nozzle. Therefore, whether the nozzle design is perfect or not will affect the pressure loss of the injection melt, the amount of shear heat, the size of the feeding effect and the distance of the range. （1） The structural form of nozzle nozzle is mainly determined by the characteristics and use of processing plastics. For UPVC with high viscosity and poor thermal stability, it is suitable to use the open nozzle with small flow resistance and small shearing effect, and for low viscosity crystalline plastic, it is better to use the locking nozzle with heating. The nozzle with small diameter should be used for thin-walled products with complex shape; on the contrary, the nozzle with large diameter and good feeding performance should be used for thick-walled products. The commonly used nozzles can be basically divided into three types: open nozzle, closed nozzle and special-purpose nozzle. The structure of open nozzle is simple, the pressure loss is small, the feeding effect is large, and it is not easy to produce the phenomenon of material decomposition. Therefore, it is mainly used to process high viscosity plastics, such as UPVC, polycarbonate, plexiglass, polyphenylene ether, etc.