What is the principle of injection molding process?

(1) In the pre-plasticization stage, the screw starts to rotate, and then the plastic conveyed from the hopper is conveyed to the front end of the screw. Under the action of high temperature and shear force, the plastic is plasticized evenly and gradually gathered at the front end of the barrel. As the molten plastic gathers, the pressure becomes greater and greater, and finally overcomes the screw back pressure and pushes the screw back gradually. When the plastic at the front of the barrel reaches the required injection volume, the screw stops retreating and rotating, and the pre-plasticization stage ends.

(2) In the injection stage, the screw moves forward under the action of the injection cylinder, pushing the plastic stored in the front of the barrel forward at multiple speeds and pressures, and injecting it into the closed mold cavity through the runner and gate.

(3) The injection pressure during the holding stage of the pressure-holding process is called the holding pressure. Under this pressure, the melt in the mold cavity is compensated for cooling, and the product is compressed and densified. In this process, the holding pressure plays a leading role.

Principle of Plastic Injection Mold

Injection molding is the process of injecting plastic melt into a closed mold cavity at high speed under high pressure, and then 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: first, the plastic must be injected into the mold cavity in a molten state; second, the injected plastic melt must have sufficient pressure and flow rate to completely fill the mold cavity. Therefore, injection molding must have the basic functions of plasticizing, melt injection and pressure holding molding. (1) Plasticizing process During the plasticizing process of injection molding, solid plastic is continuously moved forward along the screw groove through the conveying action of the rotating screw. After heating, compaction, and shear mixing of the screw thread, it is heated and converted into a viscous plastic fluid with uniform density, viscosity, composition and temperature distribution. The heat required for plasticizing solid plastic mainly comes from the heating of the plastic by the external barrel and the friction 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. The plasticizing 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 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. The pressure of the melt acts on the screw to push the screw to overcome various resistances and retreat. After the screw retreats to a certain distance, it stops rotating. The volume of the plastic melt in the storage area (called the injection volume) is determined, the plasticizing process ends, and the injection process begins.

(2) Injection process The plasticized plastic melt is stored in the storage area of the barrel. During injection, the screw moves axially. Under the action of the screw injection pressure, the plastic melt flows through the nozzle installed at the front end of the barrel, the mold pouring system, etc. at a certain rate and is injected into the mold cavity.

(3) Cooling and shaping process: The plastic melt injected into the mold cavity overcomes various flow resistances and fills the mold cavity. The plastic melt filling the mold cavity is subjected to huge 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 shrinks during cooling. At this time, the injection screw continues to provide pressure to keep the plastic melt filling the mold cavity without backflow, and appropriately replenishes the plastic melt into the mold cavity to fill the shrinkage space in the mold cavity until the plastic melt gradually cools and solidifies into a product.