Entrainment defects include bifilms, bubbles and exogenous (particularly sand) inclusions. All are associated with the unwetted side of the oxide film on the surface of the liquid. Attempts to model two of these features, bifilms and bubbles, are reviewed here. The concepts of the entrainment of bifilms as a process consisting of the folding-in of the surface is described and modeled with some success by a number of authors, despite admitted difficulties and limitations. The entrainment of bubbles is tackled rather less satisfactorily, but adequate techniques for such modeling appear to be developed, but merely not actioned at this time. The opportunities for optimizing the filling system designs for casting process is the most exciting challenge, promising properties, at minimum cost, that have so far been unattainable.
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