1. EVA carving injection pressure

Injection pressure is supplied by hydraulic system of injection system. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine. Driven by the pressure, the plastic melt enters the vertical channel (for some molds, it is also the dry channel), the dry channel and the shunt 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 defeat the resistance in the process of melt activity, or conversely, the resistance in the process of activity needs to be offset by the pressure of the injection molding machine to ensure the smooth filling process.

In the process of injection molding, the pressure at the nozzle of injection molding machine is the highest, so as to defeat the movement resistance in the whole process of melt. After that, the pressure decreases gradually along the active length to the front of the melt. Assuming that the cavity is well vented, the final pressure at the front of the melt is atmospheric pressure.

There are many factors that affect the filling pressure of melt, which can be summed up into three categories: (1) material factors, such as the type and viscosity of plastic; (2) structural factors, such as the type, number and orientation of the pouring system, the shape of the mold cavity and the thickness of the product; (3) molding skills.

2. EVA carving injection time

The injection molding 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 plays an important role in the pressure control of gate, runner and cavity. Reasonable injection time is helpful to melt filling, 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. This rule can be used as the basis for guessing the whole molding time of plastic parts. In mold flow analysis, only when the melt is completely driven by the screw rotation to fill the cavity, the injection time in the analysis effect is equal to the injection time set in the skill conditions. Assuming the pressure maintaining switch of screw before cavity filling, the analysis effect will be greater than the setting of skill conditions.

3. EVA carving injection temperature

Injection temperature is the main factor affecting injection pressure. The barrel of injection molding machine has 5-6 heating sections, and each material has its own suitable processing temperature (for detailed processing temperature, please refer to the data provided by the material supplier). It is necessary to control the injection temperature within a certain range. Temperature is too low, melting material plasticization is bad, affecting the quality of molding parts, adding skills is difficult; temperature is too high, the material is simply decomposed. In the process of practical injection molding, the injection temperature is generally higher than the barrel temperature, and the higher value is related to the injection rate and the function of the material, up to 30 ℃. This is due to the fact that the molten material is sheared when it passes through the injection port, resulting in high heat. 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.

4. Pressure and time of EVA engraving

At the end of the injection molding process, the screw rotates intermittently, only pushes forward, and the injection molding enters the pressure maintaining period. 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 shortening of the parts. If the cavity is full without pressure maintaining, the parts will be shortened by about 25%, especially the rib will be shortened too much to form a shortening trace. Generally, the pressure maintaining pressure is controlled by 85% of the maximum filling pressure, which should be determined according to the practice.

5. EVA carving back pressure

Back pressure refers to the pressure needed to be defeated when the screw reverses to evacuate the storage. Selecting high back pressure is conducive to the loosening of the pigment and the melting of the plastic, but it also extends the screw retraction time, reduces the length of the plastic fiber, and adds the pressure of the injection molding machine, so the back pressure should be lower, generally not more than 20% of the injection molding pressure. For injection molded foam, the back pressure should be higher than that of the gas, otherwise the screw will be pushed out of the cylinder. 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 effect of this change is difficult to estimate, it is not easy to adjust the machine accordingly.