<THE METAL FEEDING & VENTING SYSTEM
WAXWORKING – A SUMMARY
Foundry waxes are obtained from animal, insect, vegetable, mineral or synthetic sources. Mineral waxes, especially the microcrystalline type, are of central importance in the art foundry.
Foundry waxes usually have an artificial colour added and are also blended to enhance specific qualities, including plasticity, hardness and melting range. Blending can involve a variety of additives, including natural resins, plastics, dyes and pigments as well as different wax types. Specific blends can be further modified to take into account changing conditions such as seasonal temperature variations.
The wax pattern produced from a reproduction mould is a combustible copy of the sculptor’s original studio model (the master pattern).
The painting on and 'slush casting' of a wax thickness in the reproduction mould allows a hollow wax pattern to be created. The wall thickness of the wax pattern dictates the wall thickness of the finished cast, so the building up of a correct wall thickness is a critical procedure.
The insertion of a refractory core helps maintain a constant air gap inside the evacuated investment mould cavity when using traditional investment casting methods. A liquid core is poured into the interior of the hollow wax pattern and allowed to set hard. The reproduction mould temporarily supports the weight of liquid core material in the fragile wax pattern as it sets, thus preventing the wax from bursting under pressure.
After release from the reproduction mould, any obvious discrepancies and faults in the wax pattern are repaired and ‘made good’. Faults evident in the wax will be translated into the metal cast unless removed at this stage. The sculptor normally visits the foundry and approves the wax before it progresses on to the investment process.
Detail such as signatures, dates, reference numbers and foundry marks may be added to the wax pattern. Waxes with no core can be weighed to give an accurate indication of the metal charge required to fill the investment mould (approximate wax to copper alloy [bronze] weight ratio 10:1).
Attachments are added to the wax pattern to create a wax assembly. The pouring cup, runners and drain attachments variously allow wax to escape the investment mould during the kin burnout, and permit the introduction of a metal charge into the evacuated air gap between the core and outer mould. Waste gases and displaced air created by the charge are vented out of the mould cavity via risers. Core pins and core prints fix the core mass in position, and core vents allow any gases evolved within the core mass to escape without damaging the fabric of the cast.
WAXWORKING KEYWORDS >
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