1、Injection molding pressure
The injection molding pressure is provided by the hydraulic system of the injection molding system. The pressure of the hydraulic cylinder is transferred to the plastic melt through the injection molding machine screw. The plastic melt is pushed by the pressure and enters the vertical runner (for some molds it is also the main flow channel), main flow channel and manifold of the mold, and enters the mold cavity through the gate, which is the injection molding process, or called the filling process. The pressure exists to overcome the resistance in the melt flow process, or conversely, the resistance in the flow process needs to be offset by the pressure of the injection molding machine to ensure the filling process goes smoothly.
During the precision injection molding process, the pressure is highest at the injection molding machine nozzle to overcome the resistance to flow throughout the melt. After that, the pressure gradually decreases along the flow length towards the front end of the melt at the wave front. If the internal exhaust of the mold cavity is good, the final pressure at the front end of the melt is the atmospheric pressure.
2、Injection molding time
The injection molding time mentioned here is the time required for the plastic melt to fill the cavity, excluding the auxiliary time for mold opening and closing. Although the injection molding time is very short and has little influence on the molding cycle, the adjustment of the injection molding time has a great role in controlling the pressure of the gate, runner and cavity. A proper injection time helps to fill the melt ideally and is very important to improve the surface quality of the product and reduce the dimensional tolerance.
The injection time is much lower than the cooling time, about 1/10 to 1/15 of the cooling time, and this rule can be used as the basis for predicting all the molding time of the plastic parts.
3、Injection molding temperature
Injection molding temperature is an important factor affecting the injection pressure. There are 5-6 heating sections in the barrel of injection molding machine, and each raw material has its own suitable processing temperature. The injection molding temperature must be controlled within a certain range. If the temperature is too low, the melt will be poorly plasticized, affecting the quality of the precision mold parts and making the process more difficult; if the temperature is too high, the raw material will be 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 material properties, up to 30 ℃. In the mold flow analysis can be done in two ways to compensate for this difference, one is to try to measure the temperature of the melt to empty injection, and the other is to model the injection nozzle also included.
4、Injection molding pressure and pressure holding time
Near the end of the injection molding process, the screw stops rotating and just pushes forward, at which time the injection molding enters the pressure-holding stage. The nozzle of the injection molding machine continuously replenishes the cavity during the pressure-holding process to fill the empty volume due to the shrinkage of the part. If the cavity is filled without holding pressure, the part will shrink about 25%, especially at the tendons due to excessive shrinkage and the formation of shrinkage marks. The holding pressure is generally about 85% of the maximum filling pressure, but of course, it should be determined according to the actual situation.
5. Injection molding back pressure
Back pressure refers to the pressure that needs to be overcome when the screw reverses and recedes to store the material. The use of high back pressure is conducive to the dispersion of color materials and plastic melting, but at the same time prolong the screw retraction time, reducing the length of plastic fibers and increasing the pressure of the injection molding machine, so the back pressure should be lower, generally not more than 20% of the injection pressure.