dynamics is not modeled and a fixed surface tension threshold is used as an input quantity. When the equality is achieved, the boundary gas pressure is fixed and the amount of gas blown follows from Equation (5). At the core print boundaries, the pressure is fixed corresponding to perfect venting. In all cases the boundary gas density is set consistently from the boundary pressure and boundary gas temperature using Equation (7).
To model the pressure in core vents an energy balance is written for the whole vent cavity which includes advected energy and flow work done on/by the gas in the vent cavity. Unlike in the bulk of the sand, conductive heat transfer from vent gas to the surrounding core is relatively small and is neglected. Because the gas is taken as ideal the total energy of the gas in the vent (Ev and vent volume (Pv
) is simply related to vent pressure , Vv ): Equation 8
The rate of change of vent pressure can then be written as:
Equation 9
where the integration is performed over the vent surface ∂Av
.
Foundry Tests and Observations
A 730C, A319 alloy was pumped from below into an open flask, submerging the jacket core and allowing the obser- vation of bubble formation. The ge- ometry and its CAD representation are
shown in Figure 2. The PUCB bonded core was a two-piece assembly of the slab, directly printed into the mold wall, and the water jacket core, printed into the slab. The jacket wall thickness ranged from 8 to 14 mm. Because of the V-engine geometry, the jacket core was at a 45 degree slant which resulted in the lower portion of the jacket submerging at 7.5 seconds and upper at 15 seconds after the beginning of flask fill. Bubbles were observed forming from lower and upper portions of the 48 cm long water jacket core as soon as the local peaks were submerged. In Figure 2 these core peaks are marked with open red circles. In a separate set of mea- surements the jacket was X-rayed during fill and a typical bubble size at detachment was seen to be 0.75 cm.
When sufficient metal height was built up over the core the bubbling stopped. To suppress the bubbling from the lower and upper core portions, 7.5 cm and 12.5 cm of metal head was required (measured from the respective core peaks). Though the water jacket core is geometrically symmetric,
Table 3. Thermophysical Data Used to Compute Gas Pressures
Figure 2. Left panel: Details of core geometry. The standard jacket core is not vented. For the vented jacket, the locations of vent drills on the core leg are marked with black circles (solid black for the two vents per leg in the upper jacket and open circle for one vent per leg in the lower jacket). Right panel: CAD model of the flask (light grey), slab/water-jacket assembly (dark grey) and the simulation domain (light green). Five metal gates into the flask are visible just below the slab core. The lower and upper core peaks where gas bubbles were seen to detach are marked with open red circles.
International Journal of Metalcasting/Summer 2011 61
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