Pressure during injection molding includes two types: plasticizing pressure and injection pressure, which directly affect the plasticization of the plastic and the quality of the finished product.
1. Plasticizing Pressure (Back Pressure):
When using a screw-type injection molding machine, the pressure exerted on the molten material at the top of the screw as the screw rotates and retracts is called plasticizing pressure, also known as back pressure. This pressure can be adjusted using an overflow valve in the hydraulic system. During injection, the plasticizing pressure needs to be varied depending on the screw design, the required product quality, and the type of plastic. If these factors and the screw speed remain constant, increasing the plasticizing pressure will enhance shearing action, thus increasing the melt temperature, but will reduce plasticizing efficiency, increase backflow and leakage, and increase drive power.
Furthermore, increasing the plasticizing pressure often results in more uniform melt temperature, more uniform colorant mixing, and better removal of gases from the melt. In general operation, the plasticizing pressure should be as low as possible while ensuring excellent product quality. The specific value varies depending on the type of plastic used, but it rarely exceeds 20 kg/cm².
2. Injection Pressure:
In current production, almost all injection molding machines use the pressure applied to the plastic by the plunger or screw top (converted from the hydraulic pressure) as the standard. The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, to provide the rate of molten material filling the mold, and to compact the molten material.