Table 4: Flow Rate From Gates and Total Coefficient of Discharge System No 1 2 3
3A 4 5
5A
Sprue:Runner:Gate 1: 0.9: 0.8 1: 0.7: 0.5 1.6: 2: 4 1.6: 2: 4 1.6: 4: 4
1: 2.3: 4.5 1: 2.3: 4.5
: total flow rates from gates (mm3 /s x103
G/S 0.8 0.5
2.56 2.56 2.56 4.5 4.5
* average of coefficient of discharge for 7 gates Qtot
gate. Inclusions and oxide particles are mostly trapped at the end of the runner and remain attached to the upper surface. Hence, the chance of contaminated metal entering the mold cavity becomes reduced. By designing a properly tapered runner (not a stepped runner, since the melt tends to be affected turbulently, bouncing at each step), the metal enters the mold concurrently through all the gates.
Qualitative Results The measured gate veloci-
ties, actual filled areas and flow rates are presented in Tables 2-4. Theoretical gate velocity was determined assuming complete filling of the gate area, while the actual gate velocity was determined based on the flow rate and proportion of area filled, which was measured by
Fig. 5. CdA varied with respect to G/S in each of the systems.
reviewing the recorded frames in the measured time. Using the actual veloc- ity, the velocity coefficient of discharge was calculated. The same concept was used for measuring the areal coefficient
Rules of the Flow
almost no time is available for tranquilization. Most of the contaminated metal and inclusions enter the mold cavity. Vena contracta (effective choke) in junctions also leads to a smaller area and higher velocity. Non-pressurized systems are less turbulent and have a lower gate velocity. By locating the gates above the runners, the chance of contaminated liquid and inclusions entering in the mold is minimized. Vortexes often are observed at the gates, which is associated with the velocity and gate ratio (width-to-length ratio). This phenom- enon can be eliminated by reducing the aforementioned ratio and/or decreasing the velocity. 2. In pressurized systems, the velocity coefficient of dis-
R
charge for the first gate is more than the second. The inverse is true for non-pressurized systems. For non-pressurized sys-
emember the following rules of thumb when developing your next aluminum gating system. 1. In pressurized systems, fluid flow is turbulent and
tems, this small difference can be due to improper tapering. 3. By increasing the gate-to-sprue area (G/S) and
runner-to-sprue area (R/S) ratios, the velocity coefficient of discharge (CdV) in all systems is increased. In the case of similar values for G/S, another effective parameter is the ratio of A/P. The higher the A/P value, the higher the CdV. In the case of identical values for (A/P)G, the (A/P) R is a better substitute. 4. The overall coefficient of discharge (Cd) is calculated
from experimental data and is defined as the ratio of the actual flow rate to the theoretical flow rate by multiplying the velocity coefficient of discharge by the areal coefficient of discharge. Closer values to 1 provide less turbulence and more predictable mold filling. With increased G/S values and velocity reductions, Cd becomes closer to one. Considering the similar values for G/S, another helpful parameter is the thickness-to-width ratio of the gate.
MC ), Qt
(T/W)G - -
0.7
0.36 0.7
0.36 0.31
Qt
Q1
Q2
Qtot
1117 230.6 255.6 486.2 1117 152.5 173.4 325.9 1117 265.5 372.5 1117 365.5 1117 359.9 1117 488.7 1117
638
378 414
144 : theoretical flow rate form all the gate (mm3 147.2 /s x103
743.4 774
Cd1
0.41 0.27 0.48 0.66 0.64
527 1015.7 0.87 896
0.9 ), Cd: total coefficient of discharge
of discharge (CdA). Table 2 presents the gate
velocity and velocity coef- ficient of discharge. The measured gate velocities were lower than the theoreti- cal values. This was due to variations in flow direction and friction between the liquid and the walls. The conventional coef-
ficient of discharge (CdV), calculated via the ratio of measured to theoretical ve- locity, is influenced by the following parameters: Gate Distance to the
Sprue—In the pressurized gating systems, the velocity and CdV is greater for the first gate(s) than for the oth-
ers. Considering the flow patterns, two factors should be considered: • The fluid is bounced back upstream after complete filling of the runner, resuling in turbulence in front of
Cd2
0.46 0.31 0.67 0.68 0.74 0.94 0.92
Cd
0.43 0.29 0.57 0.67 0.69 0.91 0.8*
38
MODERN CASTING / September 2010
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