< CASTING WAXES FROM MOULDS
BUILDING UP A FULL WAX THICKNESS
Building up a wax wall thickness in a REPRODUCTION MOULD is one of the most critical procedures of the entire lost wax process. Too heavy a wax thickness and the resulting metal cast will be overweight, uneconomic, and prone to many of the FAULTS associated with solid casts (shrinkage, porosity, cracking and so on). Too thin a wax on the other hand, can restrict the flow of metal into the investment mould, this in turn can cause a mis-run leading to an incomplete metal cast.
The optimum wall thickness for a hollow wax pattern varies according to the investment system and investment moulding technique used by the founder. Other factors which determine the optimum wall thickness of a wax pattern include the scale and complexity of the design, the intended casting alloy, and any need for the completed cast to fulfil a structural function once installed. Wax wall thicknesses can range from 1/16 th inch (1.5mm) in some commercial precision ceramic shell processes, through to 3/8th inch (10mm) plus for exceptionally large scale plaster and grog investments. On average though, most lost wax casts produced in art foundries will have a wall thickness of between 3/16th - 1/4 inch (4.5 - 6mm). With skill and a good investment system, large scale copper alloy casts can be produced with a wall thickness not greatly exceeding 1/4 inch (6mm). Small scale hollow casts may have a wall thickness as thin as 1/8th inch (3mm). Although possible, thinner than 1/8th - 3/16th inch is neither desirable, nor especially beneficial under most circumstances.
The painting-on of a first wax layer into a mould to achieve a good quality surface is a more or less universal procedure. How the founder builds up the remaining thickness is more variable, the main methods used are described below.
THE ‘BRUSH ON’ METHOD
The brush on technique of building a wax pattern thickness is simply an extension of the first ‘painted on’ wax layer described on the last page. If the size and layout of the reproduction mould allows, the founder may elect to continue adding layers of brushed wax until the desired overall wall thickness is achieved. Continuing a painted thickness once all the mould sections are fully assembled and secured is normally only possible if the entrance into the closed mould is of a size that allows easy access to it’s moulded rubber surfaces. The moulds most usually painted with wax to a full thickness are typically of an ‘open’ type (flat relief forms etc), or else large scale, volumetric moulds that can be easily reached into by hand to seal the seams.
In order to maintain an even wall thickness throughout a brushed on wax, the founder will usually place a series of depth markers at strategic points on the mould’s internal rubber surfaces. Continuity of thickness can also be intermittently checked by cutting small samples from the deposited wax, these sample thicknesses can then be checked before being replaced in the mould and continuing.
The brushed on method is particularly useful for obtaining wax patterns from very large scale moulds. These unwieldy moulds can be exceptionally difficult to manipulate when building up a wax thickness through an alternative means such as SLUSHING.
SLUSHING >
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