< CENTRIFUGAL CASTING
KILN & CASTING – A SUMMARY
The BURNOUT (de-wax) procedure removes the wax pattern and wax casting attachment assembly from the interior of the investment mould. Evacuation leaves a negative impression of the wax image in the cavity of the investment mould, which is later filled with metal. FIRING also alters the investment mould’s chemical structure, changing it from a GREEN condition, to a fully prepared investment with optimum refractory properties.
Burnout and firing can either be done as a single continuous operation, or in two distinct steps by first removing the wax via a steam autoclave or by chemical means, then later firing the evacuated mould in a standard kiln.
The kiln temperature and firing cycle (which may be extended over a period of some days), is controlled to prevent an under or over firing of the investment mould. Maximum kiln temperatures for firing the investments used in fine art founding do not usually exceed 2000°F (1100°C).
Ceramic shell investments can be FLASH de-waxed and fired simultaneously. The highly efficient thermal properties found in the materials used in this investment system, allows for the immediate filling of a very hot mould as soon as the firing cycle has been completed.
If minor faults are detected in the fabric of a (ceramic) investment mould after firing, these can often be repaired simply by applying a refractory cement patch and briefly re-firing the mould. Patches may be held securely in place on the investment by wrapping with wire for extra security.
Most art foundries melt metals in either LIFT OUT or TILT type furnaces. These may be GAS, OIL, or occasionally ELECTRICALLY fuelled. The capacity of the largest crucibles can be as much as 1000lb+ (450kg)+ of copper alloy.
A metal CHARGE is melted as rapidly as possible, avoiding the uptake of atmospheric gases and other impurities. The charge is then DEGASSED with an alloy specific tablet or cartridge before pouring commences. Care is taken throughout the melt not to burn off light alloying elements with low boiling points.
Most of the COPPER ALLOYS used in art founding are poured at an optimum temperature of between 2000-2200°F (1100-1200°C). Pouring is carried out as a rapid, but controlled sequence of actions avoiding excessive spillage, turbulence and the entry of foreign material (INCLUSIONS) into the refractory mould.
VACUUM and CENTRIFUGAL casting is carried out with the aid of specialist equipment. Either process allows intricate and detailed works to be accurately reproduced in metal by artificially ‘forcing’ the molten charge into the investment cavity.
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