The pressure in the injection molding process includes two kinds of plasticizing pressure and injection pressure, and directly affects the plasticization of plastic and product quality.

    1. Plasticizing pressure: (Back pressure) When a screw injection machine is used, the pressure at the top of the screw when the screw rotates backwards is called the plasticizing pressure, also known as back pressure. The magnitude of this pressure can be adjusted through the relief valve in the hydraulic system. In injection, the plasticizing pressure needs to be changed according to the screw design, product quality requirements, and the type of plastic. If these conditions and the screw speed are unchanged, increasing the plasticizing pressure will strengthen the shear. The effect is to increase the temperature of the melt, but it will reduce the efficiency of plasticization, increase the counterflow and leakage, and increase the driving power.

In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the mixing of the color material uniform, and discharge the gas in the melt. In general operation, the decision of plasticizing pressure should be as low as possible under the premise of ensuring good product quality. The specific value varies with the type of plastic used, but usually rarely exceeds 20 kg / cm.

2. Injection pressure: In current production, the injection pressure of almost all injection machines is based on the pressure exerted on the plastic by the plunger or screw top (converted by the oil pressure). The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, give the molten material a filling rate and compact the molten material.