The injection pressure is provided by the hydraulic system of the injection molding system. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine. Under the pressure, the plastic melt enters the vertical flow channel (which is also the main flow channel for some molds), main flow channel, and diversion channel of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate. This process is called the injection molding process, or the filling process. The existence of pressure is to overcome the resistance during the melt flow process, or conversely, the resistance during the flow process needs to be offset by the pressure of the injection molding machine to ensure the smooth filling process.

During the injection molding process, the pressure at the nozzle of the injection molding machine is the highest to overcome the flow resistance of the melt throughout the entire process. Afterwards, the pressure gradually decreases along the flow length towards the front end of the melt. If the internal exhaust of the mold cavity is good, the final pressure at the front end of the melt is atmospheric pressure.

There are many factors that affect the filling pressure of the melt, which can be summarized into three categories: ⑴ material factors, such as the type of plastic, viscosity, etc.; ⑵ Structural factors, such as the type, number, and position of the pouring system, the shape of the mold cavity, and the thickness of the product; ⑶ Forming process elements.