It is powered by the hydraulic system of the injection molding system. During the injection molding process, the pressure of the hydraulic cylinder is transmitted to the plastic melt through the injection molding machine screw, pushing it through the injection molding machine nozzle into the sprue, runner, and branch channels of the mold, and into the mold cavity through the gate. This process aims to overcome the resistance in the melt flow and ensure the smooth progress of the filling process.

During the injection molding process, the pressure at the nozzle is the highest to overcome the flow resistance throughout the process. Subsequently, the pressure gradually decreases along the flow path. If the mold cavity is well-vented, the final pressure at the melt front will be close to atmospheric pressure.

Many factors affect the melt filling pressure, including material factors such as plastic type and viscosity, structural factors such as gating system design, mold cavity shape, and product thickness, and process elements of molding. In addition, injection time is also one of the important process parameters. It refers to the time required for the plastic melt to fill the cavity. Although short, it is of great significance. Reasonable control of the injection time helps the ideal filling of the melt, improves the surface quality of the product, and reduces dimensional tolerances. In practice, the injection time is usually much shorter than the cooling time, about 1/10 to 1/15 of the cooling time. This rule can be used to predict the overall molding time of the plastic part.
Injection temperature is a parameter that cannot be ignored in the injection molding process. The injection molding machine barrel usually has 5 to 6 heating sections, and each raw material has its specific processing temperature range. Temperature control is crucial. Too low a temperature will result in poor melt plasticization, affecting product quality and increasing process difficulty; while too high a temperature may cause the raw material to decompose. In practice, the injection temperature is often slightly higher than the barrel temperature. This difference is mainly due to the heat generated by the shear of the melt at the nozzle. To compensate for this temperature difference, two methods can be used in mold flow analysis: one is to measure the temperature of the melt during a dry shot, and the other is to incorporate the nozzle into the modeling process.
When the injection molding process is nearing completion, the screw will stop rotating and only move forward, at which point the holding pressure stage begins. During this stage, the injection molding machine nozzle continuously replenishes the melt to the cavity to fill the voids created by the shrinkage of the part. Without holding pressure, the part may shrink by about 25%, especially in the ribs, and excessive shrinkage may result in noticeable shrinkage marks. The holding pressure is usually set to about 85% of the maximum filling pressure, but the specific value needs to be adjusted according to the actual situation.

Back pressure refers to the pressure encountered when the screw reverses and retreats to store material. Appropriate back pressure helps to uniformly disperse the colorant and fully melt the plastic. However, excessive back pressure will prolong the screw retraction time, reduce the length of the plastic fibers, and increase the injection molding machine pressure. Therefore, in practice, the back pressure is usually controlled to be no more than 20% of the injection pressure. For injection molding foam plastics, the back pressure should be slightly higher than the pressure of gas formation to prevent the screw from being pushed out of the barrel.

It is worth noting that some injection molding machines are equipped with back pressure programming functions, which are designed to compensate for the reduction in screw length during melting. This function can reduce the input heat and slightly lower the temperature. However, it should be used cautiously because the results of this change are often difficult to predict and may make machine adjustments difficult.