Mold thermal data from two thin slab casters was investigated. Both facilities employ funnel molds that were instrumented with thermocouple arrays that extend into the lower part of the mold. Under specific operating conditions, regular saw-tooth-like temperature fluctuations were observed in the lower part of the funnel area at both facilities. Flux film sampling and analysis of the mold temperature data suggest that the fluctuations are caused by the fracture, removal and regrowth of the mold flux crystalline layer in the convergent area of the funnel. The effects of the film fracture on the heat flux profile in the mold are examined.