Injection molding involves two types of pressure-plasticizing pressure and injection pressure-both of which directly affect plastic plasticization and product quality.
Plasticizing Pressure (Back Pressure): When using a screw-type injection molding machine, the pressure exerted on the melt at the screw tip as the screw rotates and retracts is known as plasticizing pressure, or back pressure. This pressure can be adjusted via the relief valve in the hydraulic system. The required level of plasticizing pressure varies depending on the screw design, product quality requirements, and the type of plastic used. Assuming these factors and the screw speed remain constant, increasing the plasticizing pressure intensifies shear action-thereby raising the melt temperature-but reduces plasticization efficiency, increases backflow and leakage, and raises the required drive power.
Injection Pressure: In current production practice, the injection pressure for virtually all injection molding machines is defined based on the pressure applied to the plastic by the plunger or the screw tip
The role of injection pressure in the molding process is to overcome the flow resistance of the plastic as it moves from the barrel to the mold cavity, to determine the rate at which the melt fills the mold, and to compact the melt.
