This work presents a novel modeling methodology which allows one to predict slag infiltration in a 3D slab model by coupling the slag behavior with the metal flow, heat transfer, oscillation and solidification within the mold. The model is able to describe the dynamic behavior of liquid/solid slag in the shell mold gap and the bed, as well as its effects on heat extraction and shell formation. This paper also illustrates both the application of the model to industrial practice and the challenges faced during its implementation. Furthermore, the model allows one to predict the conditions leading to irregular shell growth, which is likely to generate defects such as cracking and bulging. It also provides, at the same time, a more fundamental understanding of the mechanisms responsible for defect formation.