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the second gate. This results in lower fluid velocity at the second gate.


• Vena contracta due to the back pressure is less in the second gate, which results in lower velocity. In non-pressurized systems, the


velocity through the second gate is slightly greater than the first gate. This might be associated with an inappropriate tapering of the runner. For controlling velocity, stepping may be better than straight tapering. Effects of Gating Ratios—By increas-


ing the gate-to-sprue area ratio (G/S), or runner-to-sprue area ratio (R/S) for the systems with similar G/S values, fluid velocity decreases. Accordingly, CdV increases. This is predictable, as fluid is less turbulent through increas- ing cross-sectional areas. This trend is more apparent in pressurized systems where turbulence and friction are greater (Fig. 3). Effect of Area to Perimeter of Gate—


By increasing the G/S ratio, the area/ perimeter ratio (A/P) of the gate also is increased. In systems with similar G/S ratios (3 vs. 3A and 5 vs. 5A), A/P is another important parameter. For instance, Systems 3 and 3A have similar G/S and R/S values and similar velocities. However, the area/ perimeter ratio of the gate is lower for 3A, which means the system has a greater gate periphery and conse- quently higher friction and reduced CdV (Fig. 4). The ratio of filled area over the


total cross-sectional area of the gate is called areal coefficient of discharge and is less than or equal to one. The areal coefficient of discharge is shown in Table 3. As indicated, all the gates except those in Systems 3A and 5A were not completely filled, which could be attributed to vena contracta in pressur- ized systems and vortex formation in non-pressurized systems. Parameters affecting the areal coefficient of dis- charge are: Gate Distance from Sprue—In each


system, the areal coefficient of dis- charge is greater for the second gate than the first (except Systems 3A and 5A, with CdA =1), due to back pressure from filling the runner. Effects of the Gating Ratios (G/S and


R/S) and Gate Velocities—Generally, by increasing the G/S ratio, gate velocity decreases. As a result, the area filled by the fluid increases. This translates into a higher CdA (Fig.5). However,


MODERN CASTING / September 2010


pressurized systems have lower filled areas because of vena contracta. This effect reduces the filled area more significantly than vortex formation in non-pressurized systems. Effect of Thickness-to-Width Ratio


of the Gate—In non-pressurized sys- tems of equal G/S values, reducing the gate thickness/width ratios will result in greater filling of the gate area and consequently an increased CdA (Fig. 6). System 4 had a higher CdA value compared to System 3. This was attributed to the larger A/P of the runner and thus lower velocity and higher filled area. The overall coefficient of discharge


is a function of filled area and gate velocity and is calculated by mul- tiplying the velocity by the areal coefficients of discharge (Cd= CdV x CdA). The closer the Cd value is


to 1, the lower the turbulence will be in the system and more predictable mold filling will be. In an attempt to rationalize the


total coefficient of discharge, two points should be emphasized. In ac- tual casting processes, the optimized flow conditions should give rise to the maximum flow rate at a minimum ve- locity. As shown in Table 4 and Fig. 7, non-pressurized systems have higher flow rates. Non-pressurized systems also have closer flow rate values to theoretical flow rates, and systems designed based on the critical gate velocity could represent enhanced gating systems.


MC Acknowledgements


The authors would like to thank Professor J. Camp- bell of Birmingham University for making valuable comments and suggestions and also J. Jordan for reading the manuscript.


Fig. 6. The variation of CdA vs. T/W for systems with the same G/S values is shown.


Fig. 7. The correlation between velocity, actual and theoretical flow rates is shown. 39

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