Injection Pressure

Injection pressure is provided by the hydraulic system of the injection molding machine. The pressure from the hydraulic cylinder is transferred to the plastic melt through the injection molding screw, and under the push of the pressure, the plastic melt enters the mold's vertical runner (or main runner for some molds) and分流道 via the nozzle, then flows into the mold cavity through the gate. This process is known as the injection process, or more commonly referred to as the filling process. The presence of pressure is to overcome the resistance during the melt flow, or conversely, the resistance existing during the flow process requires the pressure of the injection molding machine to offset it, ensuring the smooth progress of the filling process.

During the injection molding process, the pressure at the nozzle of the injection molding machine is used to overcome the flow resistance throughout the molten material. Subsequently, the pressure gradually decreases along the flow length towards the front wavefront of the molten material. If the mold cavity is well vented, the pressure at the front of the molten material is atmospheric pressure.

Many factors influence the melt filling pressure, which can be summarized into three categories: (1) material factors, such as the type and viscosity of the plastic; (2) structural factors, including the type, number, and position of the casting system, the shape of the mold cavity, and the thickness of the product; (3) process elements of the molding.

Injection Molding Time

The injection time refers to the duration required for the plastic melt to fill the mold cavity, excluding auxiliary times such as mold opening and closing. Although the injection time is short and has a minimal impact on the molding cycle, adjusting the injection time plays a significant role in controlling the pressure at the gate, runner, and mold 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.

Injection time is significantly less than cooling time, approximately 1/10 to 1/15 of the cooling time, which can serve as a basis for predicting the total molding time of the plastic part. During mold flow analysis, the injection time in the analysis results only equals the set injection time in the process conditions when the melt is completely filled into the mold cavity by the screw rotation. If the pressure holding switch of the screw occurs before the mold cavity is filled, the analysis results will exceed the set process conditions.

Injection Molding Temperature

Injection temperature is a significant factor affecting injection pressure. The injection molding machine barrel typically has 5 to 6 heating zones, and each material has an appropriate processing temperature (for detailed processing temperatures, refer to the data provided by the material supplier). Injection temperature must be controlled within a certain range. Too low a temperature can lead to poor plasticization of the melt, affecting the quality of the molded parts and increasing process difficulty; too high a temperature can cause the material to decompose easily. During the actual injection molding process, the injection temperature is often higher than the barrel temperature, with the difference being related to the injection rate and material properties, up to 30°C. This is due to the high heat generated by the melt as it passes through the nozzle. During mold flow analysis, this difference can be compensated 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 modeling.

4. Pressure Retention and Time

At the near end of the injection molding process, the screw stops rotating and only advances forward, at which point the injection enters the holding pressure stage. During the holding pressure process, the nozzle of the injection molding machine continuously adds material to the mold cavity to fill the volume that becomes empty due to the shrinkage of the part. If the mold is not held at pressure after it is filled, the part will shrink by approximately 25%, especially at the rib areas where excessive shrinkage can cause contraction marks. The holding pressure is generally around 85% of the filling pressure, of course, the actual situation should be determined.

Back pressure

Back pressure refers to the force required to retract the screw and remove the material. High back pressure is beneficial for colorant dispersion and plastic melting, but it also extends the screw retraction time, reduces the length of plastic fibers, increases the pressure on the injection molding machine, so the back pressure should be lower, generally not exceeding 20% of the injection pressure. When molding foam plastic, the back pressure should be higher than the pressure of gas formation; otherwise, the screw may be pushed out of the barrel. Some injection molding machines can program the back pressure to compensate for the reduction in screw length during melting, which will decrease the input heat and lower the temperature. However, due to the difficulty in estimating the results of this change, it is not easy to make corresponding adjustments to the machine.