Observation of Air Entrapment during Mold Filling of Die Casting Using WaterModel Experiment for Mold Filling Simulation

Atsushi Niida+1 and Yasuhiro Maeda+2

Department of Mechanical Engineering, Daido University, Nagoya 457-8530, Japan

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 isinsufficient. Especially, the air entrapment during mold filling is very important problem to obtain the sound casting. In this study, the directobservation of mold filling behavior using a water model equipment is carried out. The movement of free surface and the volume change ofwater are measured from observed filling behavior. The air entrapment is analyzed by quantification method using image processing. It is clearthe location of thickness direction and entrainment timing of air varied with the experimental conditions. Further, the mold filling simulation isdone using the casting CAE software TopCAST, which has the two-phase flow analysis MARS method. As a result of the analysis, the moldfilling behavior of water and the volume of air entrapped almost agreed with experimental result. Although the calculated results have almost thesame tendency with experiment, it has some unsuitable results. Therefore, it is necessary to make simulation closer to the real phenomenon bythe validation of calculation conditions, and by introducing new algorithm of air movement. [doi:10.2320/matertrans.F-M2020821]

(Received December 9, 2019; Accepted April 3, 2020; Published May 22, 2020)

Keywords: direct observation, simulation, air entrapment, plunger speed, image processing

1. Introduction

The casting CAE (Computer Aided Engineering)1­3) isused to simulate various casting phenomena and tooptimize4,5) the casting process. The mold filling simulationis used to investigate the filling pattern of molten metal and topredict the casting defect. However, the prediction method ofdefects caused by unsuitable flow has not been establishedbecause the verification of analysis accuracy is insufficient.Especially, the air entrapment during mold filling1) is veryimportant problem to obtain the sound casting, but thequantitative estimation of air entrapment is insufficient. Inorder to simulate melt flow behavior with enough accuracy, itis necessary to verify the simulated results with experimentalones.

In this study, we assume that water and aluminum alloyshave similar free surface behavior due to almost the samevalue of Reynolds numbers and the direct observation ofmold filling behavior using a water model equipment, whichis similar to cold chamber type die casting process, is carriedout. The movement of free surface and the volume changeof water are measured from observed filling behavior, andtrying analysis of the air entrapment by quantification methodusing image processing. Further, the mold filling simulationusing casting CAE software TopCAST6­9) is performed toestimate air entrapment.

2. Experimental and Computational Analysis

2.1 Experimental apparatus and conditionsThe schematic illustration of experimental apparatus and

injection conditions are shown in Fig. 1 and Table 1. Thisexperimental apparatus using water consists of plunger,sleeve, sprue bush, gate, cavity and overflow, which are likecold chamber type die casting process. The sleeve and mold

cavity are made of acrylic for visualization. The object ofvisualization is a thin cavity with width of 100mm, height of150mm, and thickness of 5mm, shown by red lines in Fig. 1.The injection amount is 259 g of water, and the sleeve fillingrate is 42.8%.

Two kinds of switch timing of plunger speed as P1 and P2are set in the experiment. In the case of P1, plunger speed ischanged from low-speed to high-speed before ingate thecavity, and P2 is changed after ingate the cavity. The velocityVL of the low-speed section is the constant of 20mm/s. Thevelocity VH of the high-speed section is set to 100, 150 and200mm/s. The total velocity conditions are six as shown inTable 1.

The cavity filling behavior and the flow behavior in thesleeve are recorded from the front and sides of the device byusing 60 fps video camera. The experiment is performed threeor more times with the same condition in considerationof reproducibility. Figure 2 shows the plunger speed changein the case of VH = 100m/s at P1 and P2. The plungermovement is like a step function. As shown in Fig. 2, whilethere is an experimental error of about 1mm at the plungerspeed switching position, switching to high speed requiresabout 0.5mm. Since the response delay of the experimentaldevice was within the range of the experimental error, itwas determined that there was no need to consider it. Thereproducibility has been confirmed from the viewpoint of themovement of the plunger, the cavity filling behavior, and theair entrapped volume.

2.2 Evaluation method of experimentIt is necessary to compare the quantity of air entrapment

in order to investigate how the injection conditions effect onthe air entrapment during cavity filling. Since it is difficult todirectly measure the volume of air entrapped, the method ofindirectly measuring the volume of air entrapped from pictureis used as shown in Fig. 3. The cavity filling behavior arerecorded by using 60 fps video camera and then 60 imagesper a second are obtained. The sample image is shown in left

+1Graduate Student, Daido University+2Corresponding author, E-mail: y-maeda@daido-it.ac.jp

Materials Transactions, Vol. 61, No. 7 (2020) pp. 1364 to 1368©2020 Japan Foundry Engineering Society

of Fig. 3. The volume of air and water, as named Volume-1,is defined as the volume without air entrapment in the areasurrounded by the wall surface and the free surface, as shownin center of Fig. 3. The Volume-2 is defined as the volume ofwater actually filled in the cavity measured in considerationof the color tone of water, as shown in right of Fig. 3. Thevolume of air entrapped at that time is calculated as volumeobtained by subtracting Volume-2 from Volume-1. Where,the Volume-2 is measured using “Classification by multiplethresholds”9,10) in the visualized cavity region. “Classificationby multiple thresholds” is a process of representing a targetimage by gray scale values of 0 to 255, and then determiningthe target image by tone of color by multiple thresholds. Bythis processing, it is possible to distinguish the tone of watercolor in five stages. In other words, it can be determined bythe water filling rate of five stages in the cavity thicknessdirection, such as 0, 25, 50, 75, 100%. After that, the fillingvolume of water [mm3] is calculated by multiplying thefilling rate [%] (= occupied thickness of water [mm]) and theoccupied area [mm2].

Fig. 1 Schematic illustration of experimental apparatus and injection conditions.

Table 1 Experimental Conditions.

0 100 200 3000

50

100

150

200

250

Distance, x/mm

Plun

gerS

spee

d, VP/

mm

/s

P1VH=100mm/s-N1VH=100mm/s-N2VH=100mm/s-N3

P2VH=100mm/s-N1VH=100mm/s-N2VH=100mm/s-N3

Fig. 2 The plunger speed change in the case of VH = 100mm/s at P1 andP2.

Fig. 3 Calculation method of volume of air entrapped.

Observation of Air Entrapment during Mold Filling of Die Casting Using Water Model Experiment for Mold Filling Simulation 1365

2.3 Computational procedureThe behavior of cavity filling and air entrapment is

simulated using casting CAE software TopCAST. Figure 4shows the analysis model. In order to imitate theexperimental device, the end of the overflow is connectedto the bucket through the discharge hose, as shown in Fig. 4.In an initial condition, water of 259 g (sleeve filling rate of42.8%) is set in the sleeve. After that, the plunger is operatedwith the same conditions of the experiment.

Table 2, Table 3 and Table 4 show the element, thecalculation and the materials conditions, respectively. Theobject of present calculation is the plate type cavity with thinthickness of 5mm. To obtain the results with sufficientaccuracy, the element size of 0.5mm is used in the x, y, zdirections, which means the mesh number of 10. The PorousMedia method (PM) grid6,7) is also used to approximatethe curve or complex shape. Further, the Multi-interfaceAdvection and Reconstruction Solver (MARS) method8) is

used for analysis of free surface boundary. Then, the MARSmethod has taken consideration of the surface tension byContinuum Surface Force (CSF) model.11) It is possible tohighly capture accuracy of the interface shape and it has alsoan accurate estimation of the quantity of advection.

Therefore, the MARS method can perform flow analysiswith higher accuracy than the conventionally used VOFmethod.

The cavity filling and air entrapment behavior obtainedby simulation are compared to the experimental ones. Thequantitative evaluation is performed about air entrapmentusing the same method as visualization experiment.

3. Results and Discussion

3.1 Experimental resultsFigure 5 shows the influence of velocity switching

position on cavity filling behavior in the case of injectionvelocity VH = 100mm/s. From Fig. 5, there is a differencein the color tone of water between switching position P1 andP2. If the air exists in the thickness direction of the cavity, thecolor tone of water will be pale color, as shown in theswitching position P1. When the injection velocity switchesfrom low-speed to high-speed at position P1, the flow frontstraightly goes to the top of cavity with the unfilled region inthe thickness direction. However, the case of P2, the watergoes from bottom to top with filling the cavity region.

Fig. 4 Outline of analysis model.

Table 2 Element conditions.

Table 3 Calculation conditions.

Table 4 Material properties.

P1, V

H=1

00m

m/s

Tim

e

8.37s 8.42s 8.46s 8.51s 8.56s 8.61s

P2, V

H=1

00m

m/s

Tim

e

8.69s 8.77s 8.85s 8.93s 9.01s 9.09s

Fig. 5 Influence of velocity switching position on cavity filling behavior inthe case of injection velocity VH = 100mm/s.

A. Niida and Y. Maeda1366

Figure 6 shows the volume change of air entrapped inaccordance with inlet velocities and switch timings ofvelocity. The horizontal axis shows the time since the flowfront reaches ingate position. The volume of air entrappedindicated vertical axis show the calculated value by thescheme of Fig. 3. Because the water is biased toward the wallof front side in the case of P1, the volume of air entrappedincreases with time. It is clarified that the volume of airentrapped tends to increase slightly with the increase ofvelocity VH. With the increase of velocity VH, the unfilledregion in the thickness direction will be generated in the laterstage of filling.

On the other hand, the cavity filling progresses whileoccupying the thickness direction in the case of P2, so thevolume of air entrapped at the early stage of filling is reducedcomparing of P1. The volume of air entrapped in the earlystage of filling until 0.3 s is almost zero. After that, thevolume of air entrapped increases because the unfilled regionis generated at the left and right side of cavity. The tendencyis larger than the case of P1, and the volume increases inaccordance with the increase in velocity VH.

3.2 Comparison of experimental results and computa-tional results

Figure 7 shows the comparison of cavity filling behaviorbetween experiment and simulation in the case of P1 and,VH = 150mm/s. The indicator shows the filling ratio of cell.In order to observe the air entrapment in the thicknessdirection of the cavity, the front side view and the sleeve sideview are shown in Fig. 7. From Fig. 7, the injection behaviorobtained by simulation, which the water is biased towardthe wall of front side of cavity and filling progresses, is thesame with the experimental ones. The air entrapment willbe occurred in the thickness direction of cavity as well asthe experiment. The volume of air entrapped obtained bysimulation in the case of P1 is compared to the experimentas shown in Fig. 8. The solid line and dotted line shows theexperiment and simulation, respectively. The relative errorbetween the experiment and the calculation was maximumvalue of 9.4%, and it is considered that the experimentalvalue was sufficiently estimated. However, it was not enoughto obtain a detail tendency of the experiments.

The result of the case of P2 and VH = 150mm/s is shownin Fig. 9. The figure shows the calm cavity filling behaviorfrom bottom to top. On the other hands, the calm fillingbefore the speed switching is reproduced, but after the speedswitching it exhibits a disturbed free surface shape which isslightly different from the experiment. Also, unlike in theexperiment, air entrapment in the thickness direction of thecavity is observed. Therefore, the volume of air entrappedindicates the higher values than the experiments in wholeconditions, as shown in Fig. 10. However, the tendency ofthe increasing volume of air entrapment in accordance withthe increase of velocity VH is the same. From this result, thepresent simulation is not perfect to predict the filling behaviorand the volume of air entrapment. Therefore, it is necessaryto make simulation closer to the real phenomenon.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

10000

20000

30000

40000

50000

60000

70000

80000

Time, t/s

Vol

ume

of A

ir En

trapp

ed, V

/mm

3 Experiment P1,VH=100mm/s P1,VH=150mm/s P1,VH=200mm/s P2,VH=100mm/s P2,VH=150mm/s P2,VH=200mm/s

Fig. 6 Volume change of air entrapped in accordance with inlet velocitiesand switch timings of velocity.

P1, V

H=1

50m

m/s

Sim

ulat

ion

(fro

nt si

de)

Sim

ulat

ion

(sle

eve

side

)Ti

me

8.68s 8.70s 8.72s 8.73s 8.75s

Fig. 7 Comparison of cavity filling behavior between experiment andsimulation in the case of P1 and VH = 150mm/s.

0 0.05 0.1 0.150

10000

20000

30000

40000

50000

60000

70000

80000

Time, t/s

Vol

ume

of A

ir En

trapp

ed, V

/mm

3

Experiment P1,VH=100mm/s P1,VH=150mm/s P1,VH=200mm/s

Simulation P1,VH=100mm/s P1,VH=150mm/s P1,VH=200mm/s

Fig. 8 Comparison of volume of air entrapped between experiment andsimulation in the case of P1.

Observation of Air Entrapment during Mold Filling of Die Casting Using Water Model Experiment for Mold Filling Simulation 1367

4. Conclusion

The direct observation of cavity filling behavior usingwater model equipment, which is like cold chamber type diecasting process, is carried out. The movement of free surfaceand the volume change of water are measured from observedfilling behavior. Further, the mold filling simulation is doneusing the casting CAE software TopCAST. The followingresults obtained;(1) The air entrapment is analyzed by quantification

method using image processing.(2) The plunger speed and the switch timing of plunger

speed are large influenced on the air entrapment, whichis the location of thickness direction and the volumeof it.

(3) It indicates it is possible to estimate the volume of airentrapped using the casting CAE software. The locationand the volume of air entrapped obtained by simulationare agreed with experimental ones.

(4) The calculated results have some unsuitable results.Therefore, it is necessary to make simulation closer tothe real phenomenon by the validation of calculationconditions, and by introducing new algorithm of airmovement.12,13)

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P2, V

H=1

50m

m/s

Sim

ulat

ion

(fro

nt si

de)

Sim

ulat

ion

(sle

eve

side

)Ti

me

9.00s 9.03s 9.07s 9.10s 9.13s

Fig. 9 Comparison of cavity filling behavior between experiment andsimulation in the case of P2 and VH = 150mm/s.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

5000

10000

15000

20000

25000

30000

35000

40000

45000

Time, t/s

Vol

ume

of A

ir En

trapp

ed, V

/mm

3

Experiment P2,VH=100mm/s P2,VH=150mm/s P2,VH=200mm/s

Simulation P2,VH=100mm/s P2,VH=150mm/s P2,VH=200mm/s

Fig. 10 Comparison of experiment and analysis volume of air entrappedunder P2 conditions.

A. Niida and Y. Maeda1368