Materials Processing
Observation of Air Entrapment during Mold Filling of Die Casting Using Water Model Experiment for Mold Filling Simulation
2020 Volume 61 Issue 7 Pages 1364-1368
Details
2020 Volume 61 Issue 7 Pages 1364-1368
The mold filling simulation in the casting CAE is used to investigate the filling pattern of molten metal and to predict the casting defect. However, the prediction method of defects caused by unsuitable flow has not been established because the verification of analysis accuracy is insufficient. Especially, the air entrapment during mold filling is very important problem to obtain the sound casting. In this study, the direct observation of mold filling behavior using a water model equipment is carried out. The movement of free surface and the volume change of water are measured from observed filling behavior. The air entrapment is analyzed by quantification method using image processing. It is clear the location of thickness direction and entrainment timing of air varied with the experimental conditions. Further, the mold filling simulation is done using the casting CAE software TopCAST, which has the two-phase flow analysis MARS method. As a result of the analysis, the mold filling behavior of water and the volume of air entrapped almost agreed with experimental result. Although the calculated results have almost the same tendency with experiment, it has some unsuitable results. Therefore, it is necessary to make simulation closer to the real phenomenon by the validation of calculation conditions, and by introducing new algorithm of air movement.
The casting CAE (Computer Aided Engineering)1–3) is used to simulate various casting phenomena and to optimize4,5) the casting process. The mold filling simulation is used to investigate the filling pattern of molten metal and to predict the casting defect. However, the prediction method of defects caused by unsuitable flow has not been established because the verification of analysis accuracy is insufficient. Especially, the air entrapment during mold filling1) is very important problem to obtain the sound casting, but the quantitative estimation of air entrapment is insufficient. In order to simulate melt flow behavior with enough accuracy, it is necessary to verify the simulated results with experimental ones.
In this study, we assume that water and aluminum alloys have similar free surface behavior due to almost the same value of Reynolds numbers and the direct observation of mold filling behavior using a water model equipment, which is similar to cold chamber type die casting process, is carried out. The movement of free surface and the volume change of water are measured from observed filling behavior, and trying analysis of the air entrapment by quantification method using image processing. Further, the mold filling simulation using casting CAE software TopCAST6–9) is performed to estimate air entrapment.
The schematic illustration of experimental apparatus and injection conditions are shown in Fig. 1 and Table 1. This experimental apparatus using water consists of plunger, sleeve, sprue bush, gate, cavity and overflow, which are like cold chamber type die casting process. The sleeve and mold cavity are made of acrylic for visualization. The object of visualization is a thin cavity with width of 100 mm, height of 150 mm, and thickness of 5 mm, shown by red lines in Fig. 1. The injection amount is 259 g of water, and the sleeve filling rate is 42.8%.
Schematic illustration of experimental apparatus and injection conditions.
Two kinds of switch timing of plunger speed as P1 and P2 are set in the experiment. In the case of P1, plunger speed is changed from low-speed to high-speed before ingate the cavity, and P2 is changed after ingate the cavity. The velocity VL of the low-speed section is the constant of 20 mm/s. The velocity VH of the high-speed section is set to 100, 150 and 200 mm/s. The total velocity conditions are six as shown in Table 1.
The cavity filling behavior and the flow behavior in the sleeve are recorded from the front and sides of the device by using 60 fps video camera. The experiment is performed three or more times with the same condition in consideration of reproducibility. Figure 2 shows the plunger speed change in the case of VH = 100 m/s at P1 and P2. The plunger movement is like a step function. As shown in Fig. 2, while there is an experimental error of about 1 mm at the plunger speed switching position, switching to high speed requires about 0.5 mm. Since the response delay of the experimental device was within the range of the experimental error, it was determined that there was no need to consider it. The reproducibility has been confirmed from the viewpoint of the movement of the plunger, the cavity filling behavior, and the air entrapped volume.
The plunger speed change in the case of VH = 100 mm/s at P1 and P2.
It is necessary to compare the quantity of air entrapment in order to investigate how the injection conditions effect on the air entrapment during cavity filling. Since it is difficult to directly measure the volume of air entrapped, the method of indirectly measuring the volume of air entrapped from picture is used as shown in Fig. 3. The cavity filling behavior are recorded by using 60 fps video camera and then 60 images per a second are obtained. The sample image is shown in left of Fig. 3. The volume of air and water, as named Volume-1, is defined as the volume without air entrapment in the area surrounded by the wall surface and the free surface, as shown in center of Fig. 3. The Volume-2 is defined as the volume of water actually filled in the cavity measured in consideration of the color tone of water, as shown in right of Fig. 3. The volume of air entrapped at that time is calculated as volume obtained by subtracting Volume-2 from Volume-1. Where, the Volume-2 is measured using “Classification by multiple thresholds”9,10) in the visualized cavity region. “Classification by multiple thresholds” is a process of representing a target image by gray scale values of 0 to 255, and then determining the target image by tone of color by multiple thresholds. By this processing, it is possible to distinguish the tone of water color in five stages. In other words, it can be determined by the water filling rate of five stages in the cavity thickness direction, such as 0, 25, 50, 75, 100%. After that, the filling volume of water [mm3] is calculated by multiplying the filling rate [%] (= occupied thickness of water [mm]) and the occupied area [mm2].
Calculation method of volume of air entrapped.
The behavior of cavity filling and air entrapment is simulated using casting CAE software TopCAST. Figure 4 shows the analysis model. In order to imitate the experimental device, the end of the overflow is connected to the bucket through the discharge hose, as shown in Fig. 4. In an initial condition, water of 259 g (sleeve filling rate of 42.8%) is set in the sleeve. After that, the plunger is operated with the same conditions of the experiment.
Outline of analysis model.
Table 2, Table 3 and Table 4 show the element, the calculation and the materials conditions, respectively. The object of present calculation is the plate type cavity with thin thickness of 5 mm. To obtain the results with sufficient accuracy, the element size of 0.5 mm is used in the x, y, z directions, which means the mesh number of 10. The Porous Media method (PM) grid6,7) is also used to approximate the curve or complex shape. Further, the Multi-interface Advection and Reconstruction Solver (MARS) method8) is used for analysis of free surface boundary. Then, the MARS method has taken consideration of the surface tension by Continuum Surface Force (CSF) model.11) It is possible to highly capture accuracy of the interface shape and it has also an accurate estimation of the quantity of advection.
Therefore, the MARS method can perform flow analysis with higher accuracy than the conventionally used VOF method.
The cavity filling and air entrapment behavior obtained by simulation are compared to the experimental ones. The quantitative evaluation is performed about air entrapment using the same method as visualization experiment.
Figure 5 shows the influence of velocity switching position on cavity filling behavior in the case of injection velocity VH = 100 mm/s. From Fig. 5, there is a difference in the color tone of water between switching position P1 and P2. If the air exists in the thickness direction of the cavity, the color tone of water will be pale color, as shown in the switching position P1. When the injection velocity switches from low-speed to high-speed at position P1, the flow front straightly goes to the top of cavity with the unfilled region in the thickness direction. However, the case of P2, the water goes from bottom to top with filling the cavity region.
Influence of velocity switching position on cavity filling behavior in the case of injection velocity VH = 100 mm/s.
Figure 6 shows the volume change of air entrapped in accordance with inlet velocities and switch timings of velocity. The horizontal axis shows the time since the flow front reaches ingate position. The volume of air entrapped indicated vertical axis show the calculated value by the scheme of Fig. 3. Because the water is biased toward the wall of front side in the case of P1, the volume of air entrapped increases with time. It is clarified that the volume of air entrapped tends to increase slightly with the increase of velocity VH. With the increase of velocity VH, the unfilled region in the thickness direction will be generated in the later stage of filling.
Volume change of air entrapped in accordance with inlet velocities and switch timings of velocity.
On the other hand, the cavity filling progresses while occupying the thickness direction in the case of P2, so the volume of air entrapped at the early stage of filling is reduced comparing of P1. The volume of air entrapped in the early stage of filling until 0.3 s is almost zero. After that, the volume of air entrapped increases because the unfilled region is generated at the left and right side of cavity. The tendency is larger than the case of P1, and the volume increases in accordance with the increase in velocity VH.
3.2 Comparison of experimental results and computational resultsFigure 7 shows the comparison of cavity filling behavior between experiment and simulation in the case of P1 and, VH = 150 mm/s. The indicator shows the filling ratio of cell. In order to observe the air entrapment in the thickness direction of the cavity, the front side view and the sleeve side view are shown in Fig. 7. From Fig. 7, the injection behavior obtained by simulation, which the water is biased toward the wall of front side of cavity and filling progresses, is the same with the experimental ones. The air entrapment will be occurred in the thickness direction of cavity as well as the experiment. The volume of air entrapped obtained by simulation in the case of P1 is compared to the experiment as shown in Fig. 8. The solid line and dotted line shows the experiment and simulation, respectively. The relative error between the experiment and the calculation was maximum value of 9.4%, and it is considered that the experimental value was sufficiently estimated. However, it was not enough to obtain a detail tendency of the experiments.
Comparison of cavity filling behavior between experiment and simulation in the case of P1 and VH = 150 mm/s.
Comparison of volume of air entrapped between experiment and simulation in the case of P1.
The result of the case of P2 and VH = 150 mm/s is shown in Fig. 9. The figure shows the calm cavity filling behavior from bottom to top. On the other hands, the calm filling before the speed switching is reproduced, but after the speed switching it exhibits a disturbed free surface shape which is slightly different from the experiment. Also, unlike in the experiment, air entrapment in the thickness direction of the cavity is observed. Therefore, the volume of air entrapped indicates the higher values than the experiments in whole conditions, as shown in Fig. 10. However, the tendency of the increasing volume of air entrapment in accordance with the increase of velocity VH is the same. From this result, the present simulation is not perfect to predict the filling behavior and the volume of air entrapment. Therefore, it is necessary to make simulation closer to the real phenomenon.
Comparison of cavity filling behavior between experiment and simulation in the case of P2 and VH = 150 mm/s.
Comparison of experiment and analysis volume of air entrapped under P2 conditions.
The direct observation of cavity filling behavior using water model equipment, which is like cold chamber type die casting process, is carried out. The movement of free surface and the volume change of water are measured from observed filling behavior. Further, the mold filling simulation is done using the casting CAE software TopCAST. The following results obtained;
