As plasticization has become a major development trend in today's international automobile manufacturing industry, most of the interior and exterior of automobiles are made of injection molded parts. The process flow and related important parameters of injection molded parts have also become a concern for many manufacturers of injection molded products. First of all, let's talk about the decoration used inside and outside the car. What is made of plastic? Interior injection parts generally include instrument panel parts, seat parts, floor parts, roof parts, steering wheel parts, door interior parts, rearview mirrors and various buckles and fixing parts; appearance injection parts include front and rear lights, air intake grille, fenders, mirrors.
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. Under the pressure of the plastic melt, it enters the vertical flow channel of the mold through the nozzle of the injection molding machine (it is also the main flow channel for some molds), the main flow channel, and the shunt. 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 progress of the filling process. During the injection molding process, the pressure at the injection molding machine nozzle is the highest to overcome the flow resistance of the melt throughout the process. After that, the pressure gradually decreases along the flow length towards the front of the melt. If the cavity is well vented, the final pressure at the front of the melt is atmospheric pressure.
The injection time 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 very short and has little impact on the molding cycle, the adjustment of the injection time has a great effect on the pressure control of the gate, runner and cavity. Reasonable injection time helps to fill the melt ideally, and is of great significance for improving the surface quality of the product and reducing dimensional tolerances.
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 the plastic part. In the mold flow analysis, only when the melt is completely driven by the screw 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.
Automotive 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 (for the 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 plasticization of the melt will be poor, 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 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 the properties of the material, up to 30 °C. This is caused by 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 try to measure the temperature of the melt when the melt is injected into the air, and the other is to include the nozzle in the modeling.
Due to the development requirements of automobile lightweight and low energy consumption, the material composition of automobile parts has undergone obvious changes in replacing steel with plastic. From the application of automotive injection parts, the amount of automotive injection parts has become an important indicator to measure the level of automotive production technology.