with the gating system wall is lost, which can be a source of air entrainment. The air could enter from permeable sand or from joints in metal molds. Also, cohesion of the fluid to the bottom surface was greater than that at the top. This was attributed to the force of gravity. In addition, the equivalent level of the runner and gate facilitated the cohesion condition. Non-Pressurized (System


3) 1.6:2:4—The metal flow was less turbulent in Sys- tem 3, and the runner filled completely before the gates. A new phenomenon was ob- served in the system—vortex formation during mold filling. The vortex was studied in the aluminum casting from both the front and back views. As schematically presented in Fig. 2, the liquid contacted the gate walls, surpassed the gate’s peripheral area, formed a vortex, and finally entered the gate through a stirring motion. This phe- nomenon was likely the result of the high velocity in the runner and high ratio of gate width to length. The video recordings of


Fig. 3. The effect of the gate/sprue ratio on CdV is shown.


In System 4, the fluid demon- strated the same behavior as in the other non-pressurized systems. However, the sys- tem presented the advantage of inducing less turbulence. Non-Pressurized (System


5) 1:2.3:4.5—This system was designed to yield the critical gate velocity of 0.5 m/s, which allows mold filling with the acceptable turbulence. A vortex also was created in this system; however, due to the ex- tended length of the gates, the vortex was unstable and eliminated instantly. Non-Pressurized (System


Fig. 4. A variation of CdV was shown with respect to A/P for systems with equivalent G/S values.


the water modeling and mol- ten aluminum experiments showed that the liquid initially contacted the runner walls at high velocity and was guided past the entrances of the gates. When it finally entered the gates, the flow surpassed the gate’s peripheral area and formed streams due to the grav- ity and velocity reduction. Incomplete filling of the gates and formation of a low-pressure zone could result in air aspiration and more double oxide film formation. The probability of vortex


3A 4 5


5A


1: 0.9: 0.8 1: 0.7: 0.5 1.6: 2: 4 1.6: 2: 4 1.6: 4: 4


G/S 0.8 0.5


formation can be minimized by de- creasing the gate-width-to-length ratio. The fluid velocity in the runner also can be reduced by appropriate design and installation of other features, such as a filter. Non-Pressurized (System 3A) 1.6:2:4—


This system was designed to overcome the vortex formation. It was shown that reduction of the length-to-thickness ratio can eliminate the vortex. Non-Pressurized (System 4) 1.6:4:4—


Table 3. Filled Area of Gates and Areal Coefficient of Discharge System No. Sprue:Runner:Gate 1 2 3


1: 2.3: 4.5 1: 2.3: 4.5


T: gate thickness, W: gate width, AGt of discharge


2.56 2.56 2.56 4.5 4.5


(T/W)G 0.25 0.25 0.7


0.36 0.7


0.36 0.31


AGt


196 123 630 630 630


: theoretical gate area (one gate, mm2 MODERN CASTING / September 2010 ), AG tot AG1


126 77


450 630 590


320 AG2


154 102 573 630 600


1120 1086 1098 320


320


5A) 1:2.3:4.5—In this alter- native to System 5, reducing thickness and increasing width were shown to be effective in eliminating the vortex (due to the improper tapering of the runner, the fluid exit from the gates was not simultaneous). The system also raised questions about the number of gates used, which depends on the geometry of the casting and design preferences. For in- stance, numerous gates lead to better directional solidifi- cation, while long runners lead to lower velocities and


higher discharge coefficient values that can result in incomplete mold filling. In aluminum gating systems, the


fluid velocity at the bottom of the sprue results in the runner being initially filled with turbulent metal. However, in non-pressurized systems, the runner cross-section is larger, which allows the fluid to become more stable and tranquil. By positioning the gates over the runner and in the cope, the runner is filled initially before fluid enters the


CdA1 0.64 0.63 0.72 1


0.94 0.97 1


: average of calculated area (mm2


CdA2 0.79 0.83 0.91 1


0.95 0.98 1


), CdAave


AG tot 280 179


1023 1260 1190 2184 2240


CdAave 0.71 0.73 0.81 1


0.94 0.98 1


: average of areal coefficient 37


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