Injection pressure is generated by the injection molding machine's hydraulic system. Pressure from the hydraulic cylinder is transmitted via the injection screw to the polymer melt; driven by this pressure, the melt flows from the nozzle through the sprue (which serves as the primary vertical channel in some mold designs), the main runner, and the branch runners, finally entering the mold cavity through the gate. This sequence constitutes the injection-or filling-process. Pressure is required to overcome the resistance encountered during melt flow; conversely, the machine's pressure must counteract this flow resistance to ensure the filling process proceeds smoothly.
During injection, the pressure is highest at the machine nozzle to overcome the flow resistance encountered along the entire flow path. Subsequently, the pressure gradually decreases along the flow path toward the melt's leading edge (flow front); provided the mold cavity is adequately vented, the pressure at the flow front eventually drops to atmospheric pressure.
Many factors influence melt filling pressure; these can be broadly categorized into three groups: material factors, such as plastic type and viscosity; structural factors, such as the type, number, and location of the gating system components, the shape of the mold cavity, and the wall thickness of the part; and molding process parameters.
