What are the specific parameters of the injection molding process? Let’s introduce them in detail below.
Injection pressure
The 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 push of the pressure, the plastic melt enters the vertical channel of the mold through the nozzle of the injection molding machine (for some molds, it is also the main channel), the main channel, and the shunt. Road, and through the gate into the mold cavity, this process is the injection molding process, or called 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 progress of the 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 throughout the melt. Afterwards, the pressure gradually decreases along the flow length to the wave front of the front of the melt. If the cavity is well exhausted, the final pressure at the front of the melt is atmospheric pressure.
There are many factors affecting the melt filling pressure, which can be summarized into three categories:
(1) Material factors, such as the type of plastic, viscosity, etc.;
(2) Structural factors, such as the type, number and location of the gating system, the cavity shape of the mold and the thickness of the product, etc.;
(3) Forming process elements.
Injection time
The injection molding time mentioned here refers to the time required for the plastic melt to fill the cavity, excluding auxiliary time such as mold opening and closing. Although the injection time is very short and has little impact on the molding cycle, the adjustment of the injection time has a great effect on the pressure control of the gate, runner and cavity. A reasonable injection time is helpful for the ideal filling of the melt, and it is very important for improving the surface quality of the product and reducing the dimensional tolerance.
The injection molding time is much lower than the cooling time, about 1/10 to 1/15 of the cooling time. This rule can be used as the basis for predicting the entire molding time of plastic parts. When doing mold flow analysis, only when the melt is completely driven by the screw rotation to fill the cavity, the injection time in the analysis results is equal to the injection time set in the process conditions. If the pressure switch of the screw occurs before the cavity is full, the analysis result will be greater than the setting of the process condition.
What is injection molding process parameters
Injection temperature
Injection temperature is an important factor affecting injection pressure. The barrel of the injection molding machine has 5 to 6 heating sections, and each raw material has its suitable processing temperature (for detailed processing temperature, 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 plasticization of the molten material will be poor, 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 temperature is often higher than the barrel temperature, and the higher value is related to the injection rate and the performance of the material, up to 30°C. This is due to the high heat generated by the shearing of the melt through the gate. There are two ways to compensate for this difference when doing mold flow analysis. One is to try to measure the temperature of the melt when it is injected into the air, and the other is to include the nozzle when modeling.
Holding pressure and time
At the end of the injection molding process, the screw stops rotating and just advances, and the injection molding enters the pressure holding stage. During the pressure holding process, the nozzle of the injection molding machine continuously replenishes the cavity to fill the volume vacated due to the shrinkage of the part. If the pressure is not maintained after the cavity is filled, the part will shrink by about 25%, especially the ribs will form shrinkage marks due to excessive shrinkage. 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 needs to be overcome when the screw reverses and backs up the storage. The use of high back pressure is conducive to the dispersion of colorants and the melting of plastics, but at the same time it prolongs the retraction time of the screw, reduces the length of plastic fibers, and increases the pressure of the injection molding machine. Therefore, the back pressure should be lower, generally no more than injection molding 20% of the pressure. When injecting foam, 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 reduces the heat input and lowers the temperature. However, because the result of this change is difficult to estimate, it is not easy to make corresponding adjustments to the machine.
