Excerpt from the instructional book, Casting Practice: The Ten Rules of Casting.
Rule 5: Avoid core blows
5.1 BackgroundWhen sand cores are surrounded by liquid metal, the heating of the sand and its binder causes large volumes of gas to be generated in the core. Normally, the core will be designed so that the gas can escape through the core prints and so be dissipated in the mold. In this way, we hope that the pressure inside the core is prevented from rising to high levels. In some circumstances, however, the pressure of gas in the cores may rise to such a level that a bubble is forced out into the melt. It is blown into existence. Blow defect is therefore a good name for this type of gas pore. Bubbles formed in this way are of large size, and so, highly buoyant. They rise through the metal leaving oxidized bubble trails in their wakes.
This is, of course, another form of bubble damage as has been discussed under Rule 4. However, it is sufficiently distinct that it benefits from separate consideration.
For instance, bubble damage arising from surface turbulence in the filling system is generated by the high velocities in the front end of the system (in the basin, sprue or runner). The high shear stresses in the melt ensure that the bubbles are chopped mainly into small sizes, in the range of 1 to 10 mm diameter. Sof of the smaller bubbles have been observed in video radiographic studies to coalesce in the gate. These coalesced bubbles float quickly, before any significant solidification has taken place, and so burst at the liquid surface and escape. Bubbles smaller than about 5 mm diameter have only a tenth of the buoyancy of the 10 mm bubbles, and cannot split the oxides that bar their escape (figure 4.1c). If they succeed to reach the top of the casting they therefore remain trapped at a distance only a double oxide skin depth beneath the surface of the casting.
Turning now to the quite different type of bubble given off by the outgassing of a core, these bubbles are large. In irons and steels, the single core blow bubble is about 13 mm diameter. In light alloys, the effective bubble diameter is approximately 20 mm (Figure 6.22, Castings 2003). Although these large bubbles have high buoyancy, they are not produced immediately. The timing of their eruption into the melt determines the kind of defect that is formed in the casting. If, in relatively thick sections, the bubble detaches prior to any freezing, the repeated arrival of bubbles at the surface of the casting can result in repeated build-up of bubble skins, forming a puff-pastry of the multiple levels of oxide, know as an exfoliation defect (Figure 5.1b). More usually, the core takes time to warm up, and takes further time to build up its internal pressure, thus allowing time for some freezing to occur. Thus, by the time the bubble is finally forced into existance, it rises to sit under the frozen layer of solid (Figure 5.1a).
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