< FLASH DE-WAXING SHELL MOULDS
TWO STAGE BURNOUT AND FIRING
Rather than employ the single stage burnout and firing cycles described previously, some foundries split this procedure into two distinct processes – isolating the wax removal stage from investment firing. A two stage process is usually appropriate when the founder wishes to remove wax from a mould by a means other than ‘burning off’ in a kiln. The most common alternative to a hot kiln evacuation is wax removal by a steam autoclave. The steam AUTOCLAVE subjects any contained mould to super-heated water vapour, the steam causes the wax assembly to collapse and evacuate the investment. The spent wax drains from the mould into a lower holding tank, from which it can later be retrieved and recycled. A second method of wax assembly removal is to expose a suitable LOST PATTERN material (expanded polystyrene for instance), to a strong solvent (styrene for instance), under controlled conditions. This method chemically flushes the contents of the investment mould, leaving behind a hollow cavity.
The advantages of an autoclave mould evacuation include a high retrieval rate of recyclable wax, and the environmental benefits of not burning off wax products under flame conditions (which inevitably releases noxious gases, even if moderated by efficient waste filtering). As with any method of wax evacuation that initially bypasses the use of a kiln, the vacated mould must still be fired at some later point to prepare it for casting This second stage of the procedure, firing, can be carried out in a standard ceramics kiln. The adoption of the two-stage process is not especially widespread in art foundries, though it is quite common in industrial precision and jewellery/dental casting facilities as autoclaves are especially well suited to the clean evacuation of small scale CERAMIC SHELL and JEWELLERY block moulds.
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