Vacuum casting is a process commonly used to cast intricate small scale designs. This process requires specialist equipment and the tooling can vary greatly in sophistication and cost. The need for specialist equipment means that vacuum casting facilities are not available in many fine art foundries, though most will be able to sub-contract suitable work out to a good specialist company if required.
Vacuum casting involves the founder placing a prepared FLASK (containing a refractory mould), either into a vacuum table, or within a sealed chamber, which is itself part of an integrated casting mechanism. A vacuum is applied to the flask/refractory as the charge enters the mould, thus providing an additional force to gravity for filling the mould cavity. Where an integrated vacuum casting machine is used, the charge is melted in a separate chamber which is normally flooded with an inert gas (argon), to create a ‘clean’ melting atmosphere. The melted CHARGE is then released, into the refractory mould at a predetermined temperature. As the charge is released a sensor activates a valve between the casting chamber and a remote vacuum reservoir to apply the maximum vacuum effect at the moment at which the charge enters the mould.
Some art foundries have adapted the basic principles of this specialist process for casting sculpture. Most of the art foundry systems involve placing an unmodified ceramic shell mould into a dry particle (sand) bed. The positioning of the shell mould may be assisted by pumping air up through the particulate, causing the loose grains to behave like a fluid. Once the mould is correctly set, the air flow is shut off, compacting the sand grains around the mould. By reversing the pump during metal pouring, air is pulled back down through the mould, assisting the inward flow of the molten charge. This system of PARTIAL VACUUM assisted casting functions especially well with small scale ceramic shell moulds due to their light weight and exceptionally thin/porous refractory walls.
More sophisticated vacuum systems involve the construction of specially modified ceramic shell moulds which can placed within a sealed chamber for casting. Unlike gravity fed moulds which have a pouring cup, these vacuum assisted moulds use an extended ceramic tube as an 'INGATE' entrance. The chamber and it's contained shell mould is then positioned over the molten charge, with the ceramic tube extended through the chamber casing. The upper part of the chamber is attached to a vacuum pump and when the ceramic tube is lowered into the charge, the applied suction is sufficient to draw the molten charge up into the mould's cavity. This process causes minimum disturbance to the metal's flow and consequently leads to fewer casting faults. This system is limited to use on moulds of a volume no larger than 3ft3 (1m3), though a mould of this volume is substantially larger than the average jewellery mould [ref].
Cross-sectional diagram of a
partial vacuum assisted shell.
Cross-sectional diagram of a
vacuum casting chamber, melting
area above mould chamber.
(diagrams cc ANPP).
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