due to the core slant and due to the opening of the flask, it takes longer to build up a given metal head over the up- per core portion. This results in an overall higher core tem- perature at seal and therefore a higher gas pressure. This is further illustrated in the left panel of Figure 4, where we plot simulated metal height history in the mold. While the metal flow rate into the flask is near constant, the rate of head change drops from 3.5 cm/s to 0.8 cm/s during fill.
A method was devised to eliminate gas blow into the metal by providing vents drilled into the water jacket core from the prints. The vent channels were 90 mm long and 1.5 mm in diameter. Two were provided per upper jacket leg and one per lower (see Figure 2 for vent locations). The absence of gas blow from the vented jacket implies that the gas pressure at core peaks right after submersion is at, or below the metal surface tension threshold (second term on the right hand side of Equation 2)
Numerical Computation of Core Gas Pressure
The numerical computation of core gas pressure in the water jacket-slab core assembly was carried out in the symmetry reduced portion of the geometry as indicated in Figure 2. The physical model described above was discretized on a finite-volume mesh in the core interior. The fluid flow equa- tions were concurrently solved in the casting cavity, so that the metal filling pattern was computed simultaneously with binder pyrolysis and gas transport. The thermo-physical val- ues used in the computations are summarized in Table 3.
A conservative first-order numerical method was used to compute temperatures in the mold and the metal, SOLA- VOF algorithm was used to solve metal momentum and advection equations,5
while the compressible gas transport
Equation (6) coupled with the Equation of state (7) and the Darcy law, Equation (5), were solved with a variant of the
Figure 3. Bottom Row: Computed core gas gauge pressure at 17.3 and 30 seconds. Top Row: Computed locations of gas blow to metal at 17.3 and 30 s. The core submerges at 15 s and at 30 s no more gas blows to metal. The gas seals in the lower portion of the jacket at 17.5 s. As in Figure 2 red circles mark approximate locations where bubbles were blown to metal in foundry tests.
62 International Journal of Metalcasting/Summer 2011
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