Injection Mold Parameters

Injection pressure is provided by the hydraulic system of the injection molding system. The pressure of the hydraulic cylinder is transmitted to the molten plastic through the injection molding machine screw. Driven by the pressure, the molten plastic enters the mold's runner (for some molds, this is also the main runner), branch runner, and finally enters the mold cavity through the gate. This process is called injection molding, or filling. The pressure exists to overcome the resistance during the melt flow process; conversely, the resistance present in the flow process needs to be offset by the injection molding machine's pressure to ensure smooth filling.

During injection molding, the pressure is highest at the injection molding machine nozzle to overcome the flow resistance throughout the melt's path. Subsequently, the pressure gradually decreases along the flow length towards the melt's leading edge. If the mold cavity has good venting, the final pressure at the melt's leading edge is atmospheric pressure.

Many factors influence melt filling pressure, which can be broadly categorized into three types:

(1) Material factors, such as the type and viscosity of the plastic;

(2) Structural factors, such as the type, number, and location of the gating system, the shape of the mold cavity, and the thickness of the part;

(3) Molding process elements.

The injection time discussed here refers to the time required for the molten plastic to fill the mold cavity, excluding auxiliary times such as mold opening and closing. Although the injection time is very short and has a small impact on the molding cycle, adjusting the injection time plays a significant role in controlling the pressure of the gate, runner, and cavity. A reasonable injection time contributes to ideal melt filling and is crucial for improving the surface quality of the product and reducing dimensional tolerances.

The injection time should be significantly shorter than the cooling time, approximately 1/10 to 1/15 of the cooling time. This rule can be used as a basis for predicting the total molding time of the plastic part. In mold flow analysis, the injection time in the analysis results is equal to the injection time set in the process conditions only when the melt is completely filled by the rotation of the screw. If the screw switches to holding pressure before the cavity is fully filled, the analysis results will be greater than the process condition settings.

Injection temperature is a crucial factor affecting injection pressure. Injection molding machine barrels have 5-6 heating zones, and each material has its suitable processing temperature (detailed processing temperatures can be found in data provided by the material supplier). Injection temperature must be controlled within a certain range. Too low a temperature results in poor plasticization of the melt, affecting the quality of the molded part and increasing process difficulty; too high a temperature leads to material decomposition. In actual injection molding, the injection temperature is often higher than the barrel temperature, with the difference depending on the injection rate and material properties, reaching up to 30°C. This is due to the high heat generated by shearing as the melt passes through the injection nozzle. This difference can be compensated for in mold flow analysis in two ways: one is to measure the temperature of the melt during air injection, and the other is to include the nozzle in the model.

Near the end of the injection process, the screw stops rotating but continues to advance, at which point the injection enters the holding pressure stage. During the holding pressure process, the injection molding machine nozzle continuously feeds material into the cavity to fill the volume vacated by the shrinkage of the part. If the cavity is filled without holding pressure, the part will shrink by approximately 25%, especially at the ribs where excessive shrinkage will leave shrinkage marks. The holding pressure is generally about 85% of the maximum filling pressure, but this should be determined based on the actual situation.

Back pressure refers to the pressure that the screw needs to overcome when reversing and retracting to store material. High back pressure is beneficial for pigment dispersion and plastic melting, but it also prolongs the screw retraction time, reduces the length of plastic fibers, and increases the pressure of the injection molding machine. Therefore, the back pressure should be lower, generally not exceeding 20% ​​of the injection pressure. When injection molding foam plastics, the back pressure should be higher than the pressure of the gas formation; otherwise, the screw will be pushed out of the barrel. Some injection molding machines can be programmed with back pressure to compensate for the reduction in screw length during melting, which reduces the input heat and lowers the temperature. However, because the results of this change are difficult to predict, it is not easy to adjust the machine accordingly.