Hard bentonite

Bentonites which are burnt onto or bonded to the sand grain by fireclay in the mold material cycle due to the casting heat above 500°C (Oolitization).

The extent of oolitization is determined by the bentonite content in the mold material mixture, the thermal load of the mold material, and ultimately by the degree of refreshment of the circulating system (see Recirculating system sand). If not enough new sand is added (see Addition of new sand) or if the quantity of coresand addition is too small, the individual mold material grain is constantly enveloped with binder coatings during conditioning which are then virtually ceramically bonded to the grain. The consequence is a decreasing refractoriness of the mold material system and thus surface defects in the casting.

Also during thermal reclaiming of mixed systems (core sands and bentonite-bonded sand) the remaining active bentonite is transformed into hard bentonite. Subsequent mechanical treatment of the reclaimed material to remove these oolitic shells is not always successful and generally a handicap when reclaiming wet casting sands.

Additional references:
Bentonite sand
Green sand
Calcium bentonite
Sodium bentonite
Western bentonite
Overburned bentonite
Sand reclamation
Reclamation rate
Reclaimed material yield   

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