⒈Injection 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 vertical flow channel (also the main flow channel for some molds), the main flow channel, the branch flow channel of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate. This process is the injection molding process, or the filling process. The existence of pressure is to overcome the resistance in the flow process of the melt, or conversely, the resistance in the flow process needs to be offset by the pressure of the injection molding machine to ensure the smooth filling process. During the injection molding process, the pressure at the nozzle of the injection molding machine is the highest to overcome the flow resistance of the melt throughout the whole process. Afterwards, the pressure gradually decreases along the flow length to the front end of the melt wave front. If the exhaust inside the mold cavity is good, the final pressure at the front end of the melt is atmospheric pressure. There are many factors that affect the melt filling pressure, which can be summarized into three categories: ⑴Material factors, such as the type and viscosity of plastic; ⑵Structural factors, such as the type, number and position of the gating system, the cavity shape of the mold and the thickness of the product; ⑶Process elements of molding.
⒊ Injection temperature
The injection temperature is an important factor affecting the injection pressure. The barrel of the injection molding machine has 5 to 6 heating sections, and each raw material has its appropriate processing temperature (for detailed processing temperatures, please refer to the data provided by the material supplier). The injection molding temperature must be controlled within a certain range. If the temperature is too low, the molten material will not be plasticized well, which will affect the quality of the molded parts and increase the difficulty of the process; if the temperature is too high, the raw materials will easily decompose. In the actual injection molding process, the injection molding temperature is often higher than the barrel temperature. The higher value is related to the injection molding rate and the performance of the material, and can be up to 30°C. This is caused by the high heat generated by the shearing of the molten material when passing through the injection port. There are two ways to compensate for this difference when performing mold flow analysis. One is to try to measure the temperature of the molten material when injecting into the air, and the other is to include the nozzle when modeling.
⒋ Holding pressure and time
When the injection molding process is about to end, the screw stops rotating and only moves forward. At this time, the injection molding enters the holding stage. During the holding process, the nozzle of the injection molding machine continuously adds material to the cavity to fill the volume vacated due to the shrinkage of the part. If the cavity is filled and the pressure is not maintained, the part will shrink by about 25%, especially the ribs will shrink too much and form shrinkage marks. The holding pressure is generally about 85% of the maximum filling pressure, which should be determined according to the actual situation.
⒌ Back pressure
Back pressure refers to the pressure that needs to be overcome when the screw reverses and retracts to store materials. Using high back pressure is conducive to the dispersion of colorants and the melting of plastics, 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 injecting foam plastics, the back pressure should be higher than the pressure formed by the gas, otherwise the screw will 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 reduce the input heat and reduce the temperature. However, since the results of this change are difficult to estimate, it is not easy to make corresponding adjustments to the machine.
