Injection molding is the rapid injection of plastic melt into a closed mold cavity under high pressure, followed by cooling and shaping to obtain plastic products with exactly the same shape as the mold cavity.

Injection molding needs to meet two essential conditions: firstly, plastic needs to be poured into the mold cavity in a molten state; The second is that the injected plastic melt needs to have sufficient pressure and flow rate to fully fill the mold cavity. Therefore, injection molding needs to have the basic functions of plastic plasticization, melt injection, and pressure retaining molding.

(1) In the plasticization process of injection molding, solid plastic continuously moves forward along the direction of the screw groove through the conveying action of the rotating screw. It is heated and transformed into a viscous plastic fluid with uniform density, viscosity, composition, and temperature distribution through heating, compaction, and shear mixing of the screw threads. The heat required for solid-state plastic plasticization mainly comes from the heating of the plastic by the external barrel and the friction and shear heat of the plastic by the injection screw. 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. Plasticizing function refers to the amount of molten plastic that the injection molding machine can provide per 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 corresponding pressure, and the pressure of the melt acts on the screw to push the screw back against various resistance. After the screw recedes to the corresponding distance and stops rotating, the volume of plastic melt in the storage area (referred to as the injection amount) is determined, and the plasticization process ends and enters the injection process.

(2) The plasticized plastic melt in the injection process is stored in the storage area of the machine barrel. During injection, the screw moves axially. Under the pressure of the screw injection, the plastic melt flows through the nozzle and mold pouring system installed at the front end of the machine barrel at a corresponding rate and enters 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 continuously provides pressure to keep 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.