Injection pressure
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 main runner and the branch runner of the mold through the nozzle of the injection molding machine, and then enters mold cavity through gate. This process is the injection process, or the filling process. The existence of pressure is order to overcome the resistance during the melt flow, or conversely, the resistance present during the flow process needs to be offset by the pressure of the injection molding machine to ensure a smooth filling process.

During the injection molding process, the pressure at the nozzle of the injection molding machine is highest to overcome the overall melt flow resistance. Thereafter, the pressure gradually decreases. 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 affecting the melt filling pressure. There are 3 categories in general:

• Material factors such as the type and viscosity of the plastic
• Structural factors such as the type, number and location of the gating system, the cavity shape of the mold, and the product thickness
• Process elements of injection molding

Injection time
The injection time mentioned here refers to the time required for the plastic melt to fill the mold cavity, and does not include the auxiliary time such as mold opening and mold clamping. Although the injection time is short and the effect on the injection molding cycle is small, the adjustment of the injection time has a great effect on the pressure control of the gate, runner and cavity. Proper injection time contributes to the ideal filling of the melt and is of great importance for improving the surface quality of the plastic product and reducing dimensional tolerances.

The injection time is much lower than the cooling time, which is about 1/10 to 1/15 of the cooling time. This rule can be used as the basis for predicting the overall injection molding time of the plastic parts. When do mold flow analysis, the injection time in the analysis result is equal to the injection time set in the process conditions only when the melt is completely driven by the screw rotation to fill the cavity. If the holding pressure of the screw occurs change before the mold cavity is filled, the analysis result will be greater than the setting of the process conditions.

Injection temperature
Injection temperature is an important factor affecting injection pressure. The injection molding machine barrel has 5 to 6 heating sections, each of which has its own suitable processing temperature (for detailed processing temperatures, please refer to the data provided by the material supplier). The injection temperature must be controlled within a certain range. The temperature is too low, the melt plasticization is poor, affecting the quality of the injection molded parts, increasing the difficulty in injection molding process. The temperature is too high, and the raw materials are easily decomposed. In the actual injection molding process, the injection temperature is often higher than the barrel temperature, and the exceeded value is related to the injection rate and the material performance, up to 30 °C. This is caused by the high heat generated by the shearing of the melt as it passes through the injection port. In the mold flow analysis, the difference can be compensated in two ways, one is to try to measure the temperature of the melt-to-air injection molding, and the other is to include the nozzle in the modeling.

Holding pressure and time
At the end of the injection process, the screw stops rotating and only advances, at which point the injection molding enters the pressure holding phase. During the pressure holding process, the nozzle of the injection molding machine continuously feeds melt material to the mold cavity to fill the volume vacated due to the shrinkage of the plastic part. If there is no pressure holding operation after the cavity is filled, the plastic part will shrink by about 25%. The holding pressure is generally about 85% of the maximum filling pressure, of course, it should be determined according to the actual situation.

Back pressure
Back pressure refers to the pressure that the screw needs to overcome when reversing the backstock. The use of high back pressure facilitates the colorant dispersion and the plastic melting, but at the same time prolongs the screw retracting time, reduces the length of the plastic fiber, and increases the pressure of the injection molding machine, so the back pressure should be lower, generally not exceeding the 20% of the injection pressure. Some injection molding machines can program the back pressure to compensate for the reduction in screw length during melting, which reduces the heat input and slows the temperature. However, because this change is difficult to estimate, it is not easy to adjust the machine accordingly.