< INVESTING A WAX PATTERN IN CERAMIC SHELL 1
INVESTING A WAX PATTERN IN CERAMIC SHELL 2
3 FIRST DIP
If still revolving, the slurry tank is switched off. The degreased and dried wax assembly is gradually lowered into the tank containing premium grade slurry. The fluid ceramic is carefully worked into the wax pattern’s surface (often with the aid of a soft brush), studiously avoiding the entrapment of air pockets. The wax is lifted clear, then held above the tank to drain excess fluid from the assembly.
If a wax assembly is oversize and therefore too large to be tank dipped, the slurry may be poured, brushed or even sprayed over the wax surfaces to apply a coating. Whatever method is used, the founder aims to deposit a fine, dense and uniform layer of slurry over the entire wax assembly. The layer should be free of air pockets, runs (similar in appearance to gloss paint CURTAINS), finger marks, and any other defects that might transfer to the wax, and later become evident on the surface of the metal cast.
The wax assembly has now received it’s first slurry coating, which is allowed to air dry. Failure to allow this first deposit to dry before applying a second of slurry may cause the initial layer to lose adhesion with the underlying wax, leading to the deposit’s slippage and a poor quality cast surface. To help prevent wet slippage and promote rapid drying, many workshops have dedicated drying racks. Drying racks allow the assembly to be suspended without damaging the delicate layers of applied investment.
Once hung on racks, air is forced into the interior of the assembly via flexible tubes connected to blowers. FORCED AIR DRYING may be repeated for each additional layer of investment added, though it is vital that heated air is never used – this would cause the underlying wax pattern to expand and break up the relatively weak investment wall.
By allowing the slurry to run into the hollow interior of an OPEN SHELL pattern, a continuous layer of investment can be deposited over both the internal and external wax surfaces. The internal wall replaces the usual core mass found in traditional investments. The hollow shell technique enables the interior of the wax pattern to be built up with investment at the same rate and to the same thickness as the external surface.
4 SECOND DIP
Once the first layer of slurry has fully dried, the assembly is re-dipped, again in the first fine grade slurry to ensure any missed areas of wax are fully covered. In some cases, immediately prior to dipping, the assembly is washed over with pure colloidal sol, as this can assist the adhesion of the second layer to the first. Work requiring a very high quality of surface finish is sometimes allowed to dry after this second dip, effectively forming a ‘double thickness’ first coat. More commonly, the founder will proceed immediately to the next step of STUCCO application, whilst the dipped assembly is still wet.
INVESTING A WAX PATTERN IN CERAMIC SHELL 3 >
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