In the injection production of plastic products, there are plasticizing pressures of raw materials and injection pressures of melts. The injection pressure also includes the holding pressure.

1. Plasticizing pressure

Plasticizing pressure refers to the rotation of the screw, which pushes the plasticizing liquid to the head of the screw, and the melt at the head of the screw simultaneously generates back pressure on the head of the screw. When the back pressure is greater than the sum of the frictional force between the screw and the inner wall of the cylinder and the oil return resistance in the opposite direction of the cylinder piston, the screw begins to reverse. The back pressure that causes the screw to reverse is the rotational dynamic plasticizing pressure of the screw on the raw material, also known as the screw back pressure. By adjusting the return oil pressure of the oil cylinder, different screw plasticizing pressures can be obtained.

The adjustment of plasticizing pressure has a significant impact on the plasticizing quality and production efficiency of raw materials for injection molded products. With the increase of plasticizing pressure, the mixing and plasticizing quality of raw materials are improved, and volatile substances in the raw materials can also be discharged, thereby ensuring the quality of injection molded products. The disadvantage is that increasing the plasticization pressure will prolong the plasticization time of the raw material in the barrel, reduce the plasticization rate, and the extension of the plasticization time will increase the possibility of raw material decomposition. Therefore, the adjustment of raw material plasticization pressure should be based on the quality of injection molded products. On this basis, the smaller the plasticizing pressure, the better.

The plasticizing pressure of plastics with different performance should also be appropriately controlled. For heat sensitive raw materials (PVC, POM), to prevent their decomposition due to prolonged residence time in the barrel, lower plasticizing pressure should be used; For plastics with low melt viscosity of raw materials (PET, PA), lower plasticizing pressure should be used to reduce leakage flow during the plasticizing process. For plastics with good thermal stability (PE, PP, PS), higher plasticizing pressure can be obtained, which can improve production and improve the plasticizing and mixing quality of raw materials.

2. Injection pressure

Injection pressure refers to the pressure exerted by the screw head on the plasticized melt. During the injection process, the pressure exerted by the screw head on the molten material overcomes the frictional resistance of the molten material flowing into the mold cavity within the cylinder, and enters the mold cavity at a certain speed to compact the molten material. The choice of injection pressure depends on the type and specifications of the injection molding machine, the shape of the product, the structure of the mold, the performance of the raw materials, and related process parameters.

The selection criteria for injection force are as follows.

2.1. When injection molded parts are missing due to large size errors and uneven surfaces, the injection pressure should be increased.

2.2. It is difficult to demould the product, and the injection pressure should be reduced during overflow flash evaporation.

2.3. For products with large external dimensions, complex shapes, and small wall thickness, high injection pressure should be used.

2.4. When the viscosity of the raw material melt is high and the glass transition temperature is high (such as PC material), a higher injection pressure should be used.

2.5. High pressure injection molded products are prone to generating significant internal stress. For products that are prone to deformation, annealing treatment should be carried out after injection molding.

2.6. If the temperature of the plasticizing melt is high, the injection pressure should be appropriately reduced, otherwise the injection pressure should be appropriately increased. The injection pressure should be selected based on the performance of the raw materials used in the injection molded product and the complexity of the product shape. The empirical data in Tables 6-9 is for reference.

Maintaining pressure refers to the melt pressure (cooling time of the molten material and condensation time of the external material) for a period of time after the injection pressure completes the filling of the molten material. The purpose of maintaining pressure is to provide sufficient melt replenishment during melt condensation and contraction, in order to ensure the molding density and dimensional accuracy of injection molded products. The holding pressure can be the same as or slightly lower than the injection pressure. Maintaining the same holding pressure as the injection pressure can completely supplement the product. After the product is formed, the shrinkage rate is small and the dimensional accuracy is high. The length of insulation time depends on the diameter of the melt flow channel, mold temperature, and melt temperature.