Table 1. Design Details of Gating Systems (mm) System
Sprue:Runner:Gate* (S:R:G)
1 2 3
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
1:2.3:4.5 1:2.3:4.5
* Area ratio of sprue:runner(s):gate(s) ** Runner thickness at end
the construction limitations, most of the corners were sharp.) Open-ended models were set up and leveled, then colored water was used to simulate the pouring operation. For pouring, the wa- ter tank was kept closed with respect to the pouring basin so it was filled slowly and with caution. To avoid premature entry of water through the sprue en- trance, a conical piece of polystyrene was used as a stopper. (Throughout each trial, the basin head was kept at a 50-mm height.) In a number of experi- ments, the water was mixed with tea leaves to aid in observing streamlines and inner turbulence. All of the gating systems were open-ended, and thus the back pressure during the filling of the cavity was not considered. The flow behavior was recorded by a video camera and analyzed by computer software capable of capturing 50 frames per second. Preliminary results verified that the gates were not completely filled, and direct velocity measure- ment did not yield accurate flow rate information. It was therefore decided to measure the flow rate by evaluat- ing the weight and volume of water discharged during a time interval (it is worth mentioning that accumulation measurements were taken after reach- ing a steady state condition). For actual tests, pure commercial aluminum was melted and poured at 750C. The molds
3A 4 5
5A * VG
A: area (m2 VG Ave
36
1: 0.9: 0.8 1: 0.7: 0.5 1.6: 2: 4 1.6: 2: 4 1.6: 4: 4
R/S 0.9 0.7
were made of silica sand (AFS 90) and sodium silicate binder, hardened by carbon dioxide gas. To permit obser- vation of the filling process, different parts of the gating systems were fitted with a double layer of normal silicate glass. The gating systems, which had a constant sprue height of 250 mm, are shown in Fig. 1, and the design details are listed in Table 1. Pressurized (System 1) 1:0.9:0.8—Tur-
bulent flow, incomplete runner filling, vena contracta (effective choke) at the runner/gate junctions, and splashing of liquid were apparent throughout the filling process. In addition, a molten aluminum test verified the vena con- tracta in sand casting. Pressurized (System 2) 1:0.7:0.5—Sys-
tem 2 exhibited a similar flow pattern to System 1. Due to smaller ratios in comparison to the first system, the gate velocity was higher. Vena contracta and premature fluid exit from the gates were observed. Fluid motion in pressurized systems
has a particular pattern. After the flow changes direction from vertical to horizontal (sprue to runner), the fluid is turbulent and contains bubbles. Since the ratio gradually decreases in a pressurized system, the velocity in- creases progressively and flow becomes increasingly agitated. By designing the choke at the gate(s), the system is filled backwards from the gate(s) and, due to the high velocity that results from locat- ing the gate(s) below the runners, fluid enters the mold before completely filling the system. The turbulence and high velocity facilitate inclusions and air pocket entry into the mold cavity. The vena contracta, forming as a
Fig. 2. A schematic representation of the vortex formation in System 3 is shown.
Table 2. Gate Velocity Measurement and Velocity Coefficient of Discharge System No. Sprue:Runner:Gate 1 2 3
G/S (A/P)G 0.8 0.5
1: 2.3: 4.5 1: 2.3: 4.5
: average of gate velocities (m/s) MODERN CASTING / September 2010
1.28 2.56 1.28 2.56 2.56 2.56 2.27 2.27
4.5 4.5
: theoretical gate velocity (m/s), VG
2.8 2.2 6.2 5.5 6.2 7.2 3.8
values for the remaining gates were 0.45, 0.4, 0.35, 0.36 and 0.31 ), P: perimeter (m), VGt
(A/P)R 5
4.7 6 6
8.3 8 8
VGt VG1
result of the larger cross-section of the runner compared to the gates and sharp changes of direction (which is intensified in the water model), is ob- served where the fluid exits the gates. Both the water modeling and alumi- num casting showed the second gate yielded lower vena contracta, which is associated with back pressure. In ad- dition, at the point of vena contracta, the contact of the fluid stream with
VG2 1.7 CdV1 CdV2 VG ave 0.44 0.37 1.84 CdV
2.85 1.83 1.66 0.64 0.58 1.75 0.61 4.54 1.98
0.4
0.88 0.59 0.65 0.67 0.74 0.62 0.71 0.88 0.58
0.6 0.45 0.48
0.88 0.61 0.69 0.69 0.78 0.65 0.74 0.5 0.5
0.9 0.45 0.46* 0.9 0.4 0.8 : real gate velocity (m/s), CdV: velocity coefficient of discharge (average of CdV1 0.66 0.68 0.59 0.67
0.96 0.47 0.93 0.92
and CdV2 ),
Top 38 38 38 38 38 38 38
Sprue Diameter Bottom
25 25 25 25 25 25 25
Well
Height Depth 60 50 52 50 36 50 36 50 50 50 48 50 48 50
Runner
Length Width Width** 15 30 9 13 26 9 36 18 6 36 18 6 50 25 6 47 24 12 47 24 6
Gate
No. Length Width 2 28 7 2 22 5.6 2 30 21 2 42 15 2 30 21 2 56 20 7 32 10
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