During steel solidification volumetric contraction occurs inside of an ingot mold, resulting in a cavity know as pipe. To minimize a depth of the pipe insulating materials are applied at the top of the ingot. To form hot top feeder head, insulating or exothermic coverings are used, in the form of plates, rings, segmented plates or pre-shaped form. Further are used exothermic insulating agents at the top of the ingot to protect an open surface of the steel and to keep the ingot head hot. Agents are in the form of powder or plates. The hot top feeder head acts as a reservoir of liquid steel that feeds the lower portion of the ingot, preventing thou the occurrence of the pipe. The left side of the animation shows the effect of applied insulating materials at the top of the mold/ingot, the right side shows the occurrence of the pipe if the ingot head is not isolated.
Source: Livarski vestnik – https://www.drustvo-livarjev.si/index.php?module=vestnik&op=show&vestnikId=72