Factors affecting the filling pressure of melt
There are many factors that affect the filling pressure of melt。
Material factors, such as type and viscosity of plastics;
Structural factors, such as the type, number and location of the gating system, the shape of the mold cavity and the thickness of the product, etc;
Forming process elements.
The injection time here refers to the time required for the plastic melt to fill the cavity, excluding the auxiliary time such as mold opening and closing. Although the injection time is very short and has little influence on the molding cycle, the adjustment of injection time has a great effect on the pressure control of gate, runner and cavity. Reasonable injection time is helpful to the ideal filling of melt, and it is very important to improve the surface quality of products and reduce the dimensional tolerance.
The injection time is much lower than the cooling time, about 1 / 10 ~ 1 / 15 of the cooling time, which can be used as the basis for predicting the whole molding time of plastic parts. In the 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 maintaining switch of screw occurs before the cavity is filled, the analysis result will be greater than the setting of process conditions.
Injection temperature is an important factor affecting injection pressure. There are 5-6 heating sections in the barrel of injection molding machine, and each raw material has its proper processing temperature (for detailed processing temperature, please refer to the data provided by the material supplier). The injection temperature must be controlled within a certain range.
Too low temperature and poor plasticization of melted material affect the quality of formed parts and increase the process difficulty; too high temperature makes raw materials 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 material performance, up to 30 ℃. 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 measure the temperature of the melt when it is injected into the air, the other is to include the nozzle when modeling.
Pressure maintaining pressure and time
At the end of the injection molding process, the screw stops rotating and only pushes forward. At this time, the injection molding enters the pressure maintaining stage. In the process of pressure maintaining, the nozzle of the injection molding machine continuously feeds the cavity to fill the volume that is vacated due to the shrinkage of the part. If the cavity is filled without pressure maintaining, the part will contract about 25%, especially the shrinkage trace at the rib due to excessive shrinkage. Generally, the pressure maintaining pressure is about 85% of the maximum filling pressure, which should be determined according to the actual situation.
Back pressure refers to the pressure to be overcome when the screw reverses and retreats to store materials. High back pressure is conducive to the dispersion of pigment and the melting of plastic, but at the same time, it prolongs the screw retraction time, reduces the length of plastic fiber, and increases the pressure of injection molding machine. Therefore, the back pressure should be lower, generally no more than 20% of the injection pressure. When injection molded 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 of screw length during melting, which will reduce the input heat and make the temperature drop. However, because the result of this change is difficult to estimate, it is not easy to adjust the machine accordingly.