Optimization of Plunger Injection Input for Die Casting Process

High-pressure die casting (HPDC) has been used for molding various products in some mass-producing industries. And there are a great number of advantages in HPDC, such as high dimensional accuracy as well as the ability to mold products into complex shapes. However, HPDC also has an unignorable disadvantage which is casting defects. Blowholes may often occur because of the air involvement while the molten metal in the sleeve is being injected into the mold at a high speed by the plunger and molded.  When this casting defect occurs, the problem is, the mechanical properties of the casting product drops significantly. Therefore, it is necessary to prevent the occurrence of this casting defect from HPDC.

Many studies have been made so far on the prevention of die casting defects. However, previous studies have some tasks to be solved that the formation of defects caused by the behavior of molten metal in the sleeve has not been made clear. As a specific task, no solution has been yet found out to prevent casting defects such as blowholes using plunger control. Therefore, it is very important to analyze the mechanism of defect generation caused by the flow of molten metal in the sleeve and control the speed of the plunger to avoid the occurrence of casting defects.

As a cause of the occurrence of the casting defect, an air confinement phenomenon, an air entrapment phenomenon, and a forerunning phenomenon of the molten metal at the time of injection of the plunger are considered. The first is a phenomenon in which the molten metal is discharged before all the air in the sleeve is discharged so that the air is not discharged and remains in the product. The second is a phenomenon in which the melted metal is disturbed and remains in products to be aerated. The third is a phenomenon in which a part of the molten metal flows into the die hollow first and solidifies first.

In this paper is proposed an evaluation function to quantitatively evaluate these phenomena and optimized the plunger injection input to prevent the occurrence of casting defects by using computational fluid dynamics (CFD) simulator. A genetic algorithm that incorporates a CFD simulator for optimization was needed for the study. It was defined that the target injection input as a two-step speed switch that is common in die casting, where the variables were ①low speed, ②high speed, and ③ speed switch position, and the acceleration time as constant.

As a result of CFD analysis, the injection input derived by the optimization showed that the obtained optimum injection input was less likely to involve air and was able to prevent the lead of the molten metal. Also, we performed actual machine experiments using the derived optimal injection input and confirmed the effectiveness of the experimental results.