When injecting the melt, in order to overcome the flow resistance when the melt flows through the nozzle, runner and mold cavity, the screw (or plunger) must apply sufficient pressure to the melt, which is called injection pressure. The formula for calculating the injection pressure is as follows:
In the formula: P-injection pressure, MPa; D0-inside diameter of injection cylinder, cm;
D-outside diameter of screw (or plunger), cm;
P0 ― Oil circuit pressure during injection molding machine operation, MPa.
Excessive injection pressure may cause flashing, difficulty in demolding, affect the apparent quality of the product, generate large internal stress, and affect the life of the machine. If the injection pressure is too low, the melt will not fill the mold cavity. The injection pressure used must be greater than the injection pressure required for the product. Factors such as melt viscosity, product shape, plasticization, mold temperature, and product accuracy must be considered. For example, products with good melt fluidity, simple shapes of plastic parts, and thick walls have injection pressures of less than 70 MPa; products with low melt viscosity and general requirements for shape and accuracy require injection pressures of 70 to 100 MPa; the melt viscosity is medium, For products with certain accuracy requirements, the injection pressure is selected to be 100 to 140 MPa; for products with higher melt viscosity, high accuracy requirements, thin plastic parts and the largest size, the injection pressure is about 140 to 180 MPa; for some shapes with various precisions.The injection molding of high-precision plastic products requires injection pressures up to 230 ~ 250MPa.