Injection molding is to inject the plastic melt into the closed mold cavity at high speed under high pressure, and get the plastic product with the shape of the mold cavity exactly after cooling and shaping. Injection molding must meet two necessary conditions: first, the plastic must be injected into the mold cavity in a molten state; The second is that the injected plastic melt must have sufficient pressure and flow speed to completely fill the mold cavity. Therefore, injection molding must have the basic functions of plastic plasticizing, melt injection and pressure holding molding. 
(1) Plasticization process In the plasticization process of injection molding, the solid plastic through the transmission of the rotating screw, constantly moving forward along the direction of the screw groove, after heating, compaction, shear mixing of the screw thread and other effects of heating into a uniform density, viscosity and composition and temperature distribution of viscous plastic fluid. The heat required for plasticizing solid plastics mainly comes from the heating of the external cylinder to the plastic and the friction shear heat of the injection screw to the plastic. In the plasticizing process, whether the temperature of the plastic melt meets the injection requirements and whether the temperature distribution is uniform is an important parameter to measure the plasticizing function of the injection molding machine, and the plasticizing function refers to the amount of molten plastic that the injection molding machine can provide in a unit time. After the solid plastic is plasticized into melt, it is pushed to the head of the screw by the constantly rotating screw and stored in the storage area at the front end of the barrel. The plastic melt in the storage area has a certain pressure, and the pressure of the melt acts on the screw to push the screw to overcome various resistance and retreat. The screw stops rotating after retreating to a certain distance, the plastic melt volume (called the injection amount) in the storage area is determined, the plasticization process is ended, and the injection process is entered. (2) The plasticized plastic melt in the injection process is stored in the storage area of the barrel. When injected, the screw moves axially. Under the action of the injection pressure of the screw, the plastic melt flows through the nozzle installed in the front end of the barrel and is injected into the mold cavity at a certain rate. 
(3) The plastic melt injected into the mold cavity during the cooling and setting process overcomes various flow resistance and fills the mold cavity. The plastic melt filled with the mold cavity is subjected to huge pressure from the mold cavity, which is driven by the plastic melt flow back to the barrel; Moreover, due to the cooling effect of the mold cavity, the plastic melt produces cooling shrinkage, at this time, the injection screw continues to provide pressure to keep the plastic melt full of the mold cavity without backflow, and the plastic melt is appropriately added to the mold cavity to fill the shrinking space in the mold cavity until the plastic melt gradually cools and solidifies into the product.
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