Injection molding is the process of injecting plastic melt at high speed into a closed mold cavity under high pressure, and after cooling and shaping, obtaining plastic products that are completely identical to the shape of the mold cavity.  
Injection molding must meet two necessary conditions: first, the plastic must be injected into the mold cavity in a molten state; Secondly, the injected plastic melt must have sufficient pressure and flow velocity to completely fill the mold cavity. Therefore, injection molding must have the basic functions of plastic plasticization, melt injection, and compression molding.  
(1) During the plasticization process of injection molding, solid plastic continuously moves forward along the direction of the screw groove through the conveying effect of the rotating screw. Through heating, compaction, and shear mixing of the screw thread, it is heated and transformed into a viscous plastic fluid with uniform density, viscosity, composition, and temperature distribution. The heat required for plasticizing solid plastics mainly comes from the heating of the plastic by the external barrel and the frictional shear heat of the injection screw on the plastic. During the plasticization process, whether the temperature of the plastic melt meets the injection requirements and whether the temperature distribution is uniform are important parameters for measuring the plasticization function of the injection molding machine. The plasticization function refers to the amount of molten plastic that the injection molding machine can provide per unit time.  
After the solid plastic is plasticized into a melt, it is continuously pushed to the head of the screw by the 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 it back against various resistance. After the screw retreats to a certain distance and stops rotating, the volume of plastic melt in the storage area (known as the injection volume) is determined, and the plasticization process ends, entering the injection process.
(2) The plastic melt that has been plasticized during the injection process is stored in the storage area of the barrel. During injection, the screw moves axially, and under the action of the screw injection pressure, the plastic melt flows at a certain rate through the nozzle installed at the front end of the barrel, the mold pouring system, etc., and is injected into the mold cavity.  
(3) The plastic melt injected into the mold cavity during the cooling and shaping process overcomes various flow resistance and fills the mold cavity. The plastic melt filling the mold cavity is subjected to tremendous pressure from the mold cavity, which drives the plastic melt to flow back to the barrel; Moreover, due to the cooling effect of the mold cavity, the plastic melt undergoes cooling shrinkage. At this time, the injection screw continuously provides pressure to keep the plastic melt filled in the mold cavity without backflow, and appropriately supplements the plastic melt into the mold cavity to fill the shrinkage space until the plastic melt gradually cools and solidifies into the product.