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INVESTING A WAX PATTERN IN CERAMIC SHELL 4
7 BACK-UP STUCCO APPLICATIONS
After each ‘dip and drain’ in a second coat slurry, a dry refractory stucco is applied to the still wet assembly surface. The founder usually selects an intermediate sized stucco grit for the first two or three back up layers (30–80 is typical), before moving on to a very coarse grade stucco for the outer layers (a 16–40 mesh size is typical). This cycle of ‘dip – drain – stucco – dry’ is repeated until the founder builds up the shell wall, inside and out, to a desirable thickness.
The finished wall thickness of a ceramic shell investment mould can vary considerably – some foundries reinforce shell moulds in sand backed flasks for casting. This safely allows for a much thinner mould wall than possible in moulds intended for an unsupported, free standing metal pour. Naturally enough, the scale and volume of the underlying wax assembly is also a defining factor in determining an investment shell thickness. Small moulds may be as thin-walled as 1/4 inch (6mm), larger moulds may have a shell wall thickness well in excess of 1 inch (25mm). In any event the shell mould wall must be of a sufficient thickness and strength to be capable of withstanding the pressures of expanding wax in the kiln, the force of an entering metal charge, and manual handling in a workshop environment.
8 SEALING COAT
On reaching the appropriate mould wall thickness, it is normal to apply one last coating of secondary slurry to the mould without adding a further stucco dusting. This slurry application is known as the ‘sealing coat’, and it effectively binds any residual refractory stucco to the mould’s surface, preventing loose debris from later falling away from the finished investment. The smooth surface of a sealed mould makes the work easier to handle, and additionally helps the founder identify which investment moulds have been completed, and which are work in progress.
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