Detailed explanation of injection molding process parameters

1. 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 pressure, the plastic melt enters the vertical flow channel (which is also the main flow channel for some molds), main flow channel, and diversion channel of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate. This process is called the injection molding process, or the filling process. The existence of pressure is to overcome the resistance during the melt flow process, or conversely, the resistance during 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 entire process. Afterwards, the pressure gradually decreases along the flow length towards the front end of the melt. If the internal exhaust of 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 filling pressure of the melt, 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 position of the pouring system, the shape of the mold cavity, and the thickness of the product; (3) Forming process elements.
2. Injection time
The injection time mentioned here refers to the time required for the plastic melt to fill the mold cavity, excluding auxiliary time such as mold opening and closing. Although the injection molding time is very short and has little impact on the molding cycle, adjusting the injection molding time plays a significant role in controlling the pressure of the gate, runner, and cavity. Reasonable injection molding time helps to achieve ideal melt filling, and is of great significance for improving the surface quality of products and reducing dimensional tolerances.
The injection molding time is much lower than the cooling time, approximately 1/10-1/15 of the cooling time, which can be used as a basis for predicting the entire molding time of plastic parts. When conducting 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 driven by the rotation of the screw to fill the cavity. If the pressure maintaining switch of the screw occurs before the cavity is filled, the analysis result will be greater than the set process conditions.
3. Injection temperature
Injection temperature is an important factor affecting injection pressure. The injection molding machine barrel has 5-6 heating sections, and each raw material has its appropriate processing temperature (detailed processing temperature can be found in the data provided by the material supplier). The injection temperature must be controlled within a certain range. The temperature is too low, resulting in poor plasticization of the molten material, which affects the quality of the formed parts and increases the difficulty of the process; If the temperature is too high, the raw materials are prone to decomposition. 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 material performance, up to 30 ℃. This is caused by the high heat generated by the shear of the molten material passing through the injection port. There are two ways to compensate for this difference in mold flow analysis: one is to measure the temperature of the melt during injection molding into the air, and the other is to include the nozzle during modeling.
4. Holding pressure and time
At the end of the injection molding process, the screw stops rotating and only advances forward, at which point the injection molding enters the pressure maintaining stage. During the pressure maintaining process, the nozzle of the injection molding machine continuously fills the cavity with material to fill the volume that has been vacated due to the shrinkage of the workpiece. If the cavity is filled without pressure maintaining, the workpiece will shrink by about 25%, especially at the ribs where shrinkage marks are formed due to excessive shrinkage. The pressure maintaining pressure is generally around 85% of the maximum filling pressure, which should be determined based on the actual situation.
5. Back pressure
Back pressure refers to the pressure that needs to be overcome when the screw rotates backwards to store material. Adopting high back pressure is beneficial for the dispersion of pigments 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 molding pressure. When injecting foamed 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 back pressure to compensate for the reduction in screw length during melting, which reduces input heat and causes a temperature drop. However, due to the difficulty in estimating the results of such changes, it is not easy to make corresponding adjustments to the machine.