Overburned bentonite

The thermal load during casting heats up the mold material in the vicinity of the casting so much that not only the bonding properties of clay vanish but also inactive fine particles are formed which fuse (fix) the binder coatings to the quartz grains.

Depending on the binder and water content, the thickness of the binder coatings is 2 to 6 µm. The design of the binder coatings is determined by the dispersibility of the bentonite used and the intensity of the conditioning process (s. Oolitization).

An increasing ratio of overburned bonding clay reduces the sintering temperature. Up to an oolitization degree of 10%, the sintering temperature is only moderately reduced, i.e. the refractoriness (sintering resistance) of quartz sand is decisive for the behavior. Above an oolitization degree of 10%, the sintering temperature rapidly decreases. The surface quality of the castings demonstrates a similar behavior. The increasing ratio of inactive fine particles almost proportionally decreases the sintering temperature and increases roughness.

The increasing oolitization degree continuously decreases the absolute expansion value of the mold material; at the same time, the critical scabbing time increases. In contrast, inactive fine particles increase the absolute expansion value thus increasing the tendency towards expansion defects (s. Sand expansion, Sand expansion defects). The main reason is the increasing water demand of the mold materials and the higher packing density. As a result, the compressive stresses increase, the wet tensile strength is reduced.

Additional references:
Scab testing
Scab time diagram