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VACUUM CASTING
Vacuum casting is a process commonly used to cast intricate small scale artworks. This is made possible with the aid of specialist equipment, though tooling can vary in sophistication and cost. The need for specialist equipment means that vacuum casting facilities are not available in all fine art foundries, though most will be able to sub-contract suitable work out to a good specialist company if needed.
Vacuum casting involves the founder placing a prepared FLASK (containing the investment 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 investment as the charge enters the mould, thus providing an additional force to gravity when filling of the mould’s 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 investment 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 investment.
Some art foundries have adapted the basic principles of this specialist process for use in sculpture casting. Most of the art foundry systems involve the placing of an unmodified ceramic shell mould into a dry sand bed. The positioning of the investment mould may be eased by pumping air up through the sand, 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 investment walls.
More sophisticated vacuum systems require the construction of a specially modified ceramic shell mould which is placed within a sealed chamber for casting. Unlike gravity fed moulds, which have a pouring cup, the vacuum assisted mould has only an extended ceramic tube for an entrance. The chamber and contained 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 draws the molten alloy up into the investment cavity. This is done with a minimum of 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) or so, though a mould of this volume is substantially larger than the average jewellery investment (see INFO note right) [ref].
CENTRIFUGAL CASTING >
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