A. Pressure
Injection molding 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 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 of the melt throughout the whole process. Afterwards, the pressure gradually decreases along the flow length to the front wave front of the melt. 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:
(1) Material factors, such as the type and viscosity of plastic;
(2) Structural factors, such as the type, number and position of the gating system, the shape of the mold cavity and the thickness of the product;
(3) Process elements of molding.
B. Time
The injection 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. Reasonable injection time helps the melt to fill ideally, and is of great significance for improving the surface quality of the product and reducing dimensional tolerance.
The injection time should be much lower than the cooling time, about 1/10 to 1/15 of the cooling time. This rule can be used as a basis for predicting the total molding time of plastic parts.
When performing 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 pushed by the screw to fill the cavity. If the screw pressure holding switch occurs before the cavity is filled, the analysis result will be greater than the process condition setting.
C. 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 appropriate 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. If the temperature is too low, the melt is not plasticized well, which affects the quality of the molded part and increases the difficulty of the process; if the temperature is too high, the raw material is easy to decompose. In the actual injection molding process, the injection temperature is often higher than the barrel temperature. The higher value is related to the injection rate and the performance of the material, and can reach up to 30°C.
This is caused by the high heat generated by the shearing of the melt when it passes through the injection port. There are two ways to compensate for this difference when doing 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.
D. Holding pressure
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 pressure stage. During the holding pressure process, the nozzle of the injection molding machine continuously replenishes 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 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, which should be determined according to the actual situation.
E. Back pressure
Back pressure refers to the pressure that needs to be overcome when the screw reverses and retreats to store material. 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 adjust the machine accordingly.
