effect of gating design on mold filling - american foundry society

Effect of Gating Design on Mold Filling

M. Masoumi, H. Hu

Department of Mechanical, Automotive & Materials Engineering University of Windsor, Windsor, Ontario, Canada

J. Hedjazi, M. A. Boutorabi Department of Materials and Metallurgy

Iran University of Science and Technology, Tehran, Iran

Copyright 2005 American Foundry Society ABSTRACT During light metal casting processes, the quality of a casting can be considerably affected by the pattern of mold filling. In this study, the effect of gating design including gate geometry and size on the flow pattern was investigated by pouring molten metal of aluminium alloy A413 into a sand mold. The direct observation method was used by which various flow patterns resulting from different gating designs was recorded by a video camera, and further analyzed by a computerized system. The experimental results indicate that the geometry and size of the gate and the ratio of the gating system has a great influence on the pattern of mold filling. INTRODUCTION Throughout a casting operation, mold filling plays a very significant role in casting quality control [1-12]. Due to the importance of mold filling, extensive research effort has been made in attempt to study the effect of gating design on the flow pattern of melt entering the mold [7]. It has been shown that an optimum gating system design could reduce the turbulent extent of the melt flow, minimize gas and entrap inclusion and dross [11]. The formation of various casting defects could be directly related to fluid flow phenomena involved in the stage of mold filling [7, 8]. For instance, rigorous streams could cause mold erosion; highly turbulent flows could result in air and inclusions entrapments; and relatively slow filling might generate cold shuts. Thus, the design of gating/runner as well as venting/overflow systems has to take into consideration for the proper control of filling pattern. The first investigation in this field was done in early of 1930s, in high-pressure die-casting, and still the investigation in this field is continuing [3]. Runyoro et al [9] studied the effect of gate sizes on the entry velocity of molten metal into the vertically-cast plate mold which was bottom gated. Their results indicate that a critical entry velocity is present during mold filling, under which oxide entrapment is minimized. Xue et al [10] reported that the gating system ratio plays a more important role than gate sizes on mold filling. They observed the highest surface wave when the ratio of the gate over sprue (G: S) reached 0.5. The lowest surface wave was obtained as the ratio of G: S approached 2, which minimized the entrapment of inclusions in the casting. Furthermore, the work by Fulli [5], Yeh [12] and Xue [10] shows that the geometry of gating system is another very important factor influencing mold filling patterns. For horizontally-cast plate molds, the numerical simulation by Hwang and Stoehr [4] demonstrates that mold filling patterns are related to the entrance velocity of melt and size of casting. However, experimental work on the direct observation of filling flow taking place in a horizontally-cast plate is limited in the open literature. In this study, the effect of gating design including gate geometry and size on the flow pattern in the horizontally-cast plate mold is investigated. The direct observation method was employed in which a real-time video camera was used to record various flow patterns resulting from different gating designs, and further analyzed by a computerized system. The present paper discusses informative results, which were obtained from experiments carried out in horizontally casting plates. EXPERIMENTAL PROCEDURES The cast metal was aluminum alloy A413, and the pouring temperature was set at 750C. The molds were made of silica sand with AFS 90, which was mixed with 5% silicate sodium as a binder. After molding of a plate casting with a dimension of 18×18×2 cm, the mixture of sand and binder were hardened by blowing the CO2 gas. Note that all dimensions used in the remaining content of this paper are in centimeter. A grid glass was fixed in the cope as the top surface of casting for

Paper 05-152(02).pdf, of 12AFS Transactions 2005 © American Foundry Society, Schaumburg, IL USA

visualizing the movement of the molten metal front in real time. An overflow pouring cup was used to maintain liquid metal at a constant level within the cup as shown in Figure 1. The 3-dimensional full casting is schematically illustrated in Figure 2. It can be seen that the casting plate was filled by a horizontal parting gating system. The pouring basin was obstructed by a stopper and filled with the superheated molten metal. When the stopper was removed, the melt flowed

Fig. 1. Cross-section of mold.

Fig. 2. Schematic diagram showing the casting and gating system.

Right

LeftCasting

Gate

Runner

Sprue

Pouring basin

Overflow

Grid glass plate

Upper

Lower

10

20

18


through a sprue and runner, and then entered a horizontal cast plate mold cavity via a gate. The mold filling processes were recorded by a video camera, and analyzed by a computer software which is capable of clipping 50 frames per second out of the recorded video. During casting trials, ten different designs of the gating systems were experimented since the objective of this work (except one case) was to investigate the effect of size and geometry of gate (size and dimension of runner) on mold filling pattern. For all the tested designs, the dimension of pouring basin, sprue and runner (except in one case) was kept constant. The pouring basin and overflow dimensions were as 18×6.5×8.5 and 8×6.5×5. the cross section of bottom and top of sprue were as 3.46 cm2 and 5.4cm2. sprue base was cylindrical with diameter of twice of the diameter of bottom sprue and with the height of 1.5 times of runner thickness. Table 1 lists the dimensions of the gating systems designs in details. In the first series of experiments with a constant runner dimension of 4.5 ×2, the gate thickness remained at a constant of 1 cm. But, different gate widths of 3, 5, 7, 9 and 11 were trialed. In the second series, the gate and runner dimensions of 11×1 and 5×2.2 were used, respectively. In the third series, the gate dimensions of 9×1, 4.5×2 and 6×1.5 were employed with the constant gate cross section area of 9 cm2. In the fourth series, the gate thickness was varied with a constant gate width of 7 Cm. Three different gate thicknesses of 1, 1.57 and 2 was experimented. Overall, each experiment was duplicated for multiple times to ensure their repeatability.

Table 1. Gate Sizes and Gating System Ratio

Test No Choke surface area

(cm2)

Runner surface area

(cm2)

Runner dimensions

(cm)

Gate surface area

(cm2)

Gate dimensions

(cm)

Gating system ratio

1 3.46 9 4.5×2 3 3×1 1:2.6:0.86

2 3.46 9 4.5×2 5 5×1 1:2.6:1.45

3 3.46 9 4.5×2 7 7×1 1:2.6:2

4 3.46 9 4.5×2 9 9×1 1:2.6:2.6

5 3.46 9 4.5×2 11 11×1 1:2.6:3.18

6 3.46 11 5×2.2 11 11×1 1:3.18:3.18

7 3.46 9 4.5×2 9 4.5×2 1:2.6:2.6

8 3.46 9 4.5×2 9 6×1.5 1:2.6:2.6

9 3.46 9 4.5×2 11 7×1.57 1:2.6:3.18

10 3.46 9 4.5×2 14 7×2 1:2.6:4.55 RESULTS AND DISCUSSION EFFEECT OF GATE WIDTHS ON MOLD FILLING Typical mold filling patterns, for a fixed ratio of sprue over runner (1:2.6) during the casting trials, are illustrated in Figures 3-7, for the gate dimensions of 3×1, 5×1, 7×1, 9×1 and 11×1 respectively, through a sequence of real-time video clips. For gates 3×1, 5×1 and 7×1, it can be seen that molten metal first flows straightforward into the mold due to its high inertia pressure (ρV2, where ρ and V are the density and velocity of the melt, respectively). After advancing a certain distance, the molten metal tends to spread in a direction perpendicular to the path of melt flow. Then, the narrowing of the molten metal front appears. As the melt reaching the opposite wall, the presence of the back pressure force causes the expansion of the entire melt flow before splitting in two streams. With reference to Figure 8, the expansion and narrowing of melt stream may be explained by considering the surface tension of melt front, which can be expressed:

⎥⎦

⎤⎢⎣

⎡⎟⎠⎞

⎜⎝⎛+⎟

⎠⎞

⎜⎝⎛=∆

rRp 11γ (1)


where γ is surface tension, r and R are the two radii which define the curvature of the meniscus in two planes at right angles, and ∆p is restrain pressure due to the presence of surface tension and the curvature change of melt front. As the inertia

0.2 sec 0.3 sec 0.6 sec

1.0 sec 1.2 sec 1.4 sec

Fig. 3. Mold filling pattern of gate 3×1.

0.2 sec 0.4 sec 0.6 sec

0.8 sec 1.0 sec 1.2 sec



0.2 sec 0.3 sec 0.5 sec

0.7 sec 0.9 sec 1.1 sec


0.2 sec 0.4 sec 0.6 sec

0.9 sec 1.2 sec 1.4 sec



0.2 sec 0.4 sec 0.6 sec

1.0 sec 1.3 sec 1.5sec

Fig. 7. Mold filling pattern of gate 11×1(1:2.6:3.18).

Fig. 8. Schematic diagram showing the geometry of melt front in a thin section casting.

X

Y Melt front

Gate


pressure is parallel to the direction of melt flow (Y axis), the surface tension of the melt is locally tangential to the curved melt front. At each point of the melt front, a resultant pressure exerts due to the combination of the inertia pressure and the restraining pressure. The resultant pressure could be split locally into two components in X and Y directions. The pressure in Y direction tends to move the melt straightforward while the one in X direction expands it. At the very first moment of mold filling, because of the predominance of the inertia pressure, the melt just moves in a straight direction. When flowing further in mold, however, the melt loses its momentum, and its inertia pressure decreases as a result. Therefore, the effect of the restraining pressure caused by surface tension becomes dominant, which leads to an expansion of melt shape in the X direction. But, the expansion results an increase in the radius (R) of the curvature of the melt front. According to equation (1), it is evident that the restrain pressure decreases as the R arises. The reduction of the restrain pressure causes the occurrence of melt flow narrowing. As the gate width increases to 9 cm as shown in Figure 6, however, no expansion of molten metal after entering the mold is observed, which is contrary to the above-mentioned cases. This is because an increase in the gate width increases the R and consequently decreases its restraining pressure. Insufficient restraining pressure of molten metal becomes incapable of keeping melt expanding.. Once the gate being further enlarged to 11, the metal flow deviates from the centerline of the mold, and moves toward the side close to the sprue after entering the mold. Upon striking the opposite wall of the mold, for the case with the gate dimensions of 3×1, the primary melt stream moves to the two side surface of the mold, and splits into two secondary streams along both sides of the mold. As racing back toward the entering sidewall of the mold, two secondary streams encounter the primary one. As a result, two vortices form, of which one is on each side of the gate. The vortex location is the last region to be filled, into which most of available gases and inclusions in the mold are squeezed to this area near the end of the filling process. For the gate dimension of 5×1 and 7×1, the primary stream spreads outward to both sides of the mold after hitting the wall opposite from the gate. As the spread stream flows back to the entering sidewall of the mold, the last filled areas forms at the two lower corners along the gate wall of the mold. In the case of 9 ×1 , the entire primary stream is expanded by the back pressure once reaching the opposite wall. As the filling proceeds, the two lower corners along the gate wall of the mold become the last filled regions, which is similar to that of the cases with the gate of 5×1 and 7×1. However, no secondary streams form during the entire period of mold filling in these three cases of 5×1, 7×1 and 9×1. For the gate of 11 ×1, the expansion of the deviated primary melt stream occurs once it approaches the opposite wall of the mold. As the primary melt stream further expands, the filling of the right side of the mold follows the pattern similar to that of the cases of 5×1 and 7×1. However, the left side of the mold is filled with a pattern analogous to that of the gate of 3×1. EFFECT OF GATING SYSTEM RATIO ON MOLD FILLING PATTERN As discussed above, the deviation of the metal flow occurs from the centerline of the mold for the gate of 11 ×1 as it enters the mold. The flow deviation may be primarily attributed to the presence of the melt pressure gradient within the widened gate, which results from the original design of the nonpressurized gating system. Based on such a consideration, the cross section area of the runner was increased to 5×2.2 from 4.5×2, which changed the gating system ratio from 1:2.6:3.18 to 1:3.18:3.18. This change of the gating system ratio was attempted to change the gating system type from nonpressurized to nonpressurized-pressurized system. Figure 9 illustrates the sequence of the mold filling for the gating system with a ratio of 1:3.18:3.18 and gate dimension of 11 ×1. It can be evidently seen that the change of the gating system ratio eliminates the deviation of the metal flow after entering the mold, which creates a flow pattern similar to that of the case of the gate of 9 ×1. The phenomenon of the metal flow deviation from the centerline of the mold in the nonpressurized gating system may be further elucidated by the fundamentals of fluid mechanics. From Figure 7, it is evident for the gating system ratio of 1:2.6:3.18 that the velocity of molten metal on the left side of the gate is higher than that of the right side. The Bernoulli’s equation can be expressed

ConstHg

VgP =++

2

2

ρ (2)

where P is the pressure of melt, V is the melt flow velocity, H is the metal head, g is the gravity acceleration, and ρ is the melt density. As equation (2) being applied to the melt flow in the gate area, except for variables P and V, the other parameters are constant, which indicates that the higher velocity leads to the lower pressure of the melt flow. Figure 7 shows evidently that the melt entry velocities on two sides of the mold centerline are different. The difference in the melt entering velocities causes the variation of the pressure in the gate. The presence of the pressure gradient in the gate deviates the melt flow away from the centerline of the mold.


This pressure difference may result in a vortex flow in molten metal, which causes two forces (inertia force and vortex force) exerted on the melt. The vortex force is perpendicular to direction of molten metal flow. Therefore, the combination of these two forces forms a resultant force in a direction different from that of the inertia force. Consequently, the direction of the melt flow is changed by the emergence of the resultant force. For the experimental trials with the gate size of 3×1, 5×1, 7×1, and 9×1, the gating system is nonpressurized-pressurized, in which the gate is pressurized (The system of 11×1 (1:3.18:3.18), despite that the ratio of the runner to gate is one, could still be considered as an un-pressurized-pressurized due to the presence of friction force and viscous force resulting from the melt temperature drop). From Figures 3-6 and 9, almost no variation of the melt entry velocities in the gate on both sides of the

0.3 sec 0.6 sec 0.8 sec

1.0 sec 1.2 sec 1.4 sec

Fig. 9. Mold filling pattern of gate 11×1 (1:3.18:3.18). mold centerline is observed. Based on equation (2), the pressure distribution of the melt inside the gate should be symmetrical to the centerline of the mold. Hence, no deviation of the melt flow from the centerline is present in these cases. EFFECT OF GATE GEOMETRY ON MOLD FILLING Figures 10 and 11 display a sequence of real-time motion clips of mold filling for the gate dimensions of 6×1.5 and 4.5×2, respectively. It should be pointed out that these two designs have the same cross section area as the gate of 9×1. Examination of the video clips manifests the difference of mold filling patterns among the gate designs of 6×1.5, 4.5×2 and 9×1 (Figure 6). Apparently, the pattern of mold filling for the gate 6×1.5 is analogous to that of 7×1. The similarity of the filling patterns between 4.5×2 and 3×1 is observable. The difference in mold filling patterns by the change of gate is mainly due to the head pressure increase of the molten metal at the gate exit resulting from an increase in the gate thickness for the cases of 6×1.5 and 4.5×2. As the metal flow proceeds further in the mold, the thickness of the metal flow is reduced, and consequently its head pressure tends to decrease. The reduction in the head pressure of the molten metal as melt moves further in mold leads to an increase in the velocity of the metal flow, which can be determined by Equation (2). The velocity increase, which is experimentally evident in Figures 10 and 11, should be responsible for the resulted flow patterns. EFFECT OF GATE THICKNESS ON MOLD FILLING In an effort to understand the effect of the gate thicknesses on mold filling, three different gate thicknesses of 1, 1.57 and 2 with the same width of 7 were experimented. As shown in Figures 12 and 13, the mold filling pattern of the gate 7×2 is similar in high extent to that of the gate 7×1.57. For these two cases, molten metal moves straightforward without having flow expansion once entering the mold. The feature of no flow expansion differentiates the mold filling pattern of the gates 7×1.57 and 7×2 from that of the gate 7×1. The difference in the mold filling pattern between the gate7×1, and the gates


7×1.57 and 7×2 results from the variation of entering melt thicknesses due to the change in the gate thickness, which was discussed in the previous section.

0.2 sec 0.5 sec 0.8 sec

1.0 sec 1.2 sec 1.4 sec

Fig. 10. Mold filling pattern of gate 6×1.5.

0.3 sec 0.5 sec 0.8 sec

0.9 sec 1.1 sec 1.2 sec

Fig. 11. Mold filling pattern of gate 4.5×2.


0.3 sec 0.6 sec 1.0 sec

1.2 sec 1.4 sec 1.6 sec

Fig. 12. Mold filling pattern of gate 7×1.57.

0.3 sec 0.6 sec 0.8 sec

1.0 sec 1.2 sec 1.4 sec



Fig. 14. The molten metal in outlet of gate 7×2.

To further understand the filling pattern, a mold with the gate 7×2 was sectioned for direct visualization of flow motion in the gate. Figure 14 reveals the moment of the molten metal exiting the gate 7×2. It appears that only a portion of the gate was filled with the molten metal. The uncompleted gate filling should be attributed, at least in part, to the use of the nonpressurized gating system for this case. Since the filling patterns between the gates 7×1.57 and 7×2 are very similar, the uncompleted gate filling also expectedly took place in the gate 7×1.57. It is worthwhile mentioning that, for further analysis of filling flow; an effective cross-section area, which is completely filled with molten metal, should be defined during the design of a gating system. CONCLUSION In this study, direct observation was employed to experimentally observe the flow pattern for the mold filling. The results show that an increase in the width of the gate with a constant thickness results in the variation of mold filling, of which three different patterns were observed. They are the narrowing, expansion and deviation of molten metal fronts. It is also evident that the change of the gating system type deviates flow away from the centerline of the mold which leads to a great influence on mold filling pattern. A considerable effect of the geometry of a gate with a constant cross section area on flow pattern has been observed, which primarily results from the change of the metal head pressure at the entrance of the mold. In nonpressurized gating systems, the deviation of melt flow from the centerline of the mold or incomplete gate filling tends to appear. To avoid the problem of melt flow deviation or incomplete gate filling, a gating system ratio of G: R=1 is recommended for the design of an effective gating system. ACKNOWLEDGMENTS The authors would like to express their appreciations to the Natural Sciences and Engineering Research Council of Canada and Ministry of Science, Research and Technology of Iran and Iran University of Science Technology for financial support. REFERENCES 1. Campbell J, Casting, Butterwoth Heinemann, Oxford, (1991). 2. Hedjazi J, Vesali R, Fluid Flow Pattern in Mold, International Journal of Engineering, pp 75-88, (1991). 3. Hedjazi J, Solidification and Metallurgical Principles of Founding,, Iran University of Science and Technology, (1989). 4. Hwang W. S., Stoher R. A., Fluid Flow Modeling for Computer-Aided Design of Castings, Journal of Metals, pp 22-30,

(1983). 5. Fuli Z, Zhaohao, Hydraulic Simulation Study on Three-Step Gating Systems, AFS Transactions, vol.99, pp 57-62,

(1991). 6. Jong S. H., Hwang W. S., Measurement and Visualization of the Filling Pattern of Molten Metal in Actual Industrial

Casting, AFS Transactions, vol. 99, pp 489-497, (1991). 7. Jong S.H, Hwang W. S., Three-Dimensional Mold Filling Simulation for Casting and Its Experimental Verification,

AFS Transactions, vol. 99, pp 117-124, (1991). 8. Lin H.J, Hwang W. S., Three-Dimensional Fluid Flow Simulation for Mold Filling, AFS Transactions, vol. 97, pp 855-

862, (1989).

Cope

Drag Melt Flow

Gate


9. Runyoro J, Boutorabi S.M.A., Campbell J, Critical Gate Velocity for Film Forming Casting Alloys, a Basis for Process Specification, AFS Transactions, vol. 100, pp 225-234, (1992).

10. Xue X, Hansen S.F, Hansen P.N, Water Analog Study of Effects Gating Designs on Inclusion Separation and mold filling control, AFS Transactions, vol. 101, pp.199-209, (1993).

11. Xue X, Hansen S.F, Hansen P.N, Numerical Simulation and Experimental Verification of Mold Filling Processes Through Depressurized and Less-depressurized Gating Systems, AFS Transactions, vol. 101, 549-558, (1993).

12. Yeh J.L, Jong S.H, Hwang W.S., Improved 3-D Mold Filling Model for Complex Casting and Experimental Verification, AFS Transactions, vol. 101, 1055-1063, (1993).


effect of gating design on mold filling - american foundry society

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