Pre-molded riser sleeve made of insulating compounds (Fig. 1, ASK Chemicals) to be inserted into the casting mold. Fig. 2, Insulating riser sleeve, (Foseco Foundry Division Vesuvius GmbH)
When filling the mold, insulating risers first absorb heat from the liquid metal until reaching equilibrium temperature; from this moment on, they protect the liquid casting metal against further heat losses for a certain time. This permits risers of smaller dimensions, resulting in increased yields. Therefore, insulating risers delay the beginning of solidification and promote seal-feeding. They are different from exothermic risers made of exothermic compounds which self-heat as a result of an aluminothermic reaction (exothermic reaction).
It is advantageous to fit the riser sleeves with a cover that is insulating as well in order to also prevent heat losses at the riser top. Moreover, closed insulating riser sleeves are used which are provided with a wedge molded into the top that acts as an air core (see Atmospheric riser). These insulating riser sleeves are also fitted with a breaker or knock-off core.