I believe that almost everyone knows plastic products, because in modern life, we often use plastic products, such as plastic washbasins, plastic buckets, plastic cups, plastic stools, and so on.It is believed that many people also know that most daily plastic products are produced by plastic injection molding process.But how many people know the five most important parameters in the injection molding process? I don't think many people know.Let's have a look at the five important parameters set by the injection molding machine during the injection molding process.

         1. Injection pressure

The injection pressure is provided by the hydraulic system of the injection system. The pressure of the hydraulic cylinder is transferred 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 (also the main runner for some molds), the main runner and the diversion 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 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 filling process.

In the process of injection molding, the pressure at the nozzle of the injection molding machine is the highest to overcome the flow resistance of the melt in the whole process. Then, the pressure gradually decreases along the flow length to the front end of the melt. If the internal exhaust of the mold cavity is good, the final pressure at the front end of the melt is atmospheric pressure.

There are many factors affecting the melt filling pressure, which can be summarized into three categories: ⑴ material factors, such as plastic type, viscosity, etc.; ⑵ Structural factors, such as the type, number and position of the gating system, the shape of the mold cavity and the thickness of the product; ⑶ Forming process elements.

          2. Injection time

The injection time mentioned here refers to the time required for the plastic melt to fill the cavity, excluding the auxiliary time for mold opening and closing. Although the injection time is very short and has little impact on the molding cycle, the adjustment of injection time plays a great role in the pressure control of the gate, runner and cavity. Reasonable injection time is conducive to the ideal filling of melt, and is of great significance for improving the surface quality of products and reducing dimensional tolerance.

The injection time 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 plastic parts. During mold flow analysis, the injection time in the analysis result is equal to the injection time set in the process conditions only when the melt is completely driven by the screw rotation to fill the cavity. If the pressure maintaining switch of the screw occurs before the cavity is filled, 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 to 6 heating sections, and each raw material has its appropriate processing temperature (see the data provided by the material supplier for detailed processing temperature). The injection temperature must be controlled within a certain range. If the temperature is too low, the melt will not plasticize well, which will affect the quality of the molded parts and increase the process difficulty; If the temperature is too high, the raw material is 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 material properties, up to 30 ℃. This is caused by the high heat generated by the shearing of the molten material when it 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 injection molding into the air, and the other is to include the nozzle in modeling.

           4. 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 maintaining process, the nozzle of the injection molding machine continuously replenishes the mold cavity to fill the empty volume due to the shrinkage of the parts. If the cavity is filled and the pressure is not maintained, the part will shrink by about 25%, especially the shrinkage trace will be formed at the rib due to excessive shrinkage. The pressure retaining pressure is generally about 85% of the maximum filling pressure, which should be determined according to the actual situation.

           5. Back pressure

Back pressure refers to the pressure to be overcome when the screw reverses and reverses to store material. The use of high back pressure is conducive to the dispersion of pigments and the melting of plastics, but it also prolongs the screw retraction time, reduces the length of plastic fibers, and increases the pressure of the injection molding machine. Therefore, the back pressure should be lower, generally not more than 20% of the injection molding pressure. When injecting foamed plastics, 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 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 make corresponding adjustments to the machine.