Parameters of Plastic Injection Molding

1. 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. The plastic melt is pushed by the pressure and enters the vertical runner of the mold (or the main runner for some molds), the main runner, and the splitter through the nozzle of the injection molding machine. And enter the mold cavity through the gate. This process is the 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 pressure of the injection molding machine to ensure the smooth progress of the filling process.

In 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 process. After that, the pressure gradually decreases along the flow length to the wavefront of the front end of the melt. If the inside of the cavity is well vented, the final pressure at the front end of the melt is atmospheric pressure.

There are many factors that affect the melt filling pressure, which can be summarized into three categories: ⑴Material factors, such as the type and viscosity of plastics, etc.; ⑵Structural factors, such as the type, number and position of the pouring system, the cavity shape of the mold, and the size of the product Thickness, etc.; ⑶ process elements of molding.

2. 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 short and the impact on the molding cycle is small, the adjustment of the injection time has a great effect on the pressure control of the gate, runner and cavity. Reasonable injection time is helpful for the ideal filling of the melt, and it is very important for improving the surface quality of the product and reducing the dimensional tolerance.

The injection time is much lower than the cooling time, about 1/10 to 1/15 of the cooling time. This rule can be used as a basis for predicting the total 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 result is equal to the injection time set in the process conditions. If the holding pressure switch of the screw occurs before the cavity is full, the analysis result will be greater than the setting of the process conditions.

3. Injection temperature

Injection temperature is an important factor affecting injection pressure. The barrel of the injection molding machine has 5-6 heating sections, and each raw material has its appropriate 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. If the temperature is too low, the melt will be poorly plasticized, 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 easy to decompose. 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 performance of the material, up to 30°C. This is due to the high heat generated by the shear when the melt passes 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 during air injection, and the other is to include the nozzle when modeling.

4. Holding pressure and time

At the end of the injection molding process, the screw stops rotating and only advances 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 feeds the cavity to fill the volume vacated by the shrinkage of the part. If the pressure is not maintained after the cavity is filled, the part will shrink by about 25%, especially the shrinkage marks are formed at the ribs due to excessive shrinkage. The holding pressure is generally about 85% of the maximum filling pressure, of course, it must be determined according to the actual situation.

5. Back pressure

Back pressure refers to the pressure that the screw needs to overcome when reversing and retreating the material. The use of high back pressure is conducive to the dispersion of the color material and the melting of the plastic, but it also prolongs the screw retraction time, reduces the length of the plastic fiber, and increases the pressure of the injection molding machine. Therefore, the back pressure should be lower, generally not more than injection molding. 20% of stress. When injecting 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 in screw length during melting, which reduces the heat input and lowers the temperature. However, as the result of this change is difficult to estimate, it is not easy to adjust the machine accordingly.