Mold insert made of particularly durable and resistive mold materials (e.g. fireclay, zircon sand) located at the transition zone from the sprue to the runner (at the bottom of the sprue) (Fig. 1).
It prevents damage of the mold by the inflowing metal. In this way, sand inclusion due to erosion of the mold and mold parts breaking loose. These types of cores are of particular importance with large casting heights (see casting method).