Injection molding is the high-speed injection of plastic melt into a closed mold cavity under high pressure, followed by cooling and shaping to obtain a plastic product that is completely consistent with the shape of the mold cavity.

Injection molding must meet two necessary conditions: firstly, 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 rate to fully fill the mold cavity. Therefore, injection molding must have the basic functions of plastic plasticization, melt injection, and pressure retaining molding.

(1) Plasticizing process During the plasticizing process of injection molding, solid plastics continuously move forward along the direction of the screw groove through the conveying function of the rotating screw. After heating, compaction, shearing and mixing of screw threads, they are heated and transformed into a viscous plastic fluid with uniform density, viscosity, composition and temperature distribution. The heat required for solid plastic plasticization mainly comes from the heating of the plastic by the external barrel and the friction and shear heat of the injection screw on the plastic. During the plasticizing process, whether the temperature of the plastic melt meets the injection requirements and whether the temperature distribution is uniform are important parameters 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 solid plastic is plasticized into a melt, it is pushed to the head of the screw by the continuously 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 back against various resistance. After the screw retreats to a certain distance and stops rotating, the volume of plastic melt in the storage area (called injection volume) is determined, and the plasticization process ends and enters the injection process.

(2) The plasticized plastic melt during the injection process is stored in the storage area of the barrel. During injection, the screw moves axially. Under the injection pressure of the screw, 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 that fills the mold cavity is subjected to tremendous pressure from the mold cavity, which drives the plastic melt 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 continues to provide pressure to maintain the plastic melt filling the mold cavity without backflow, and appropriately replenishes the plastic melt to fill the shrinkage space in the mold cavity until the plastic melt gradually cools and solidifies into a product.