1. Parameters of injection molding: injection pressure
The injection pressure is provided by the hydraulic system of the injection system. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine, and the plastic melt is driven by the pressure and enters the vertical runner, main runner and branch runner of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate, which is called the plastic injection molding process, or the filling process. The existence of pressure is to overcome the resistance in the melt flow process, or conversely, the resistance in the flow process needs to be offset by the injection molding pressure to ensure the smooth filling process.
During the injection molding process, the pressure at the injection molding machine nozzle is the highest to overcome the flow resistance of the melt throughout the process. After that, the pressure gradually decreases along the flow length towards the front of the melt. If the cavity is well vented, the final pressure at the front of the melt is atmospheric pressure.
2. Parameters of injection molding: injection 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 to fill the melt ideally, and is of great significance for improving the surface quality of the product and reducing dimensional tolerances. The injection time of injection mold tooling is much lower than the cooling time, which is about 1/10~1/15 of the cooling time. This rule can be used as the basis for predicting the total molding time of the plastic part.
3. Parameters of injection molding: 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 appropriate processing temperature. The injection molding temperature must be controlled within a certain range. If the temperature is too low, the plasticization of the melt 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 are easily decomposed. 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 properties of the material, up to 30 °C. This is due to the high heat generated by the shearing of the melt through the injection port. There are two ways to compensate for this difference in mold flow analysis, one is to try to measure the temperature of the melt when the melt is injected into the air, and the other is to include the nozzle in the modeling.
4. Parameters of injection molding: holding pressure and time
At the end of the injection molding process, the screw stops rotating and just pushes forward. At this time, 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 by the shrinkage of the part. If the cavity is full and the pressure is not maintained, 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.
5. Parameters of injection molding: back pressure
Back pressure refers to the pressure that the screw needs to overcome when the screw is reversed and withdrawn to store the material. The use of high back pressure is conducive to the dispersion of the pigment and the melting of the plastic, but at the same time, it prolongs the retraction time of the screw, reduces the length of the plastic fiber, and increases the pressure of injection molding. Therefore, the back pressure should be lower, generally not more than the injection molding pressure. 20% of the pressure. When injection molding foam, the back pressure should be higher than the pressure created 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, but reduces heat input and reduces temperature. However, the result of this change is difficult to estimate, so it is not appropriate to make corresponding adjustments to the machine.