The pressure in the injection molding process includes plasticizing pressure and injection pressure, which directly affects the plasticization of plastics and the quality of products.
⒈ Plasticizing pressure: (back pressure) When using a screw injection machine, the pressure on the top of the screw when the screw rotates backward is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted by the overflow valve in the hydraulic system. In injection, the size of the plasticizing pressure needs to be changed according to the design of the screw, the requirements of product quality and the type of plastic. If these conditions and the speed of the screw remain unchanged, increasing the plasticizing pressure will strengthen the shearing effect, that is, it will increase the temperature of the melt, but it will reduce the efficiency of plasticization, increase backflow and leakage, and increase the driving power.
In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the colorant mixes evenly and discharges the gas in the melt. In general operation, the plasticizing pressure should be determined as low as possible while ensuring the quality of the product. Its specific value varies with the type of plastic used, but it usually rarely exceeds 20 kg/cm2.
⒉ Injection pressure: In current production, the injection pressure of almost all injection machines is based on the pressure applied by the plunger or screw top to the plastic (converted from the oil circuit pressure). The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, increase the speed of molten material filling the mold, and compact the molten material.
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