WAX EVACUATION & FIRING THE MOULD
Once the complete, a GREEN [unfired] refractory mould is usually allowed to 'stand' for a period of time in ambient conditions. Standing allows the mould to to air dry and can improve it's structural qualities. The period of time allowed for standing varies according to the refractory system used by the founder (and the volume of refractory in the mould). CERAMIC SHELL refractories tend to acquire a satisfactory green strength more rapidly than PLASTER & GROG or HYBRID refractories. Indeed, ceramic shell moulds can proceed to the wax burnout stage almost immediately after the the final sealing coat has dried (though some standing time is nearly always preferable).
Once the refractory mould has air dried, the WAX ASSEMBLY (the design and casting attachments in wax), must be removed from the mould before a metal charge can be poured in to the evacuated air gap. In addition to removing wax contents, the mould materials must also be fired to attain their full REFRACTORY properties.
During the early stages of a combined WAX BURNOUT and FIRING process, the wax pattern and running system within the refractory mould are flushed out. Continued firing then modifies the chemical structure of the refractory materials used for the mould’s construction allowing the mould to achieve maximum resistance to thermal exposure. The heat applied to the refractory mould in the kiln also eliminates ambient moisture, allowing the founder to pour molten metal into the mould without risking an explosion or damage to the cast due to a violent steam reaction.
The wax burnout and mould firing process can be carried out either as a single, or a two stage procedure. Most art foundries employ a single stage procedure, removing the wax assembly and firing the refractory mould in a continuous chain of events.
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