< CERAMIC SHELL
CERAMIC SHELL – AN OVERVIEW
As discussed in the introduction to this section, the ceramic shell investment system was developed to meet the needs of precision, high volume industries [ref 1 ]. In this kind of production environment, time consuming preparation and finishing work can mean that a cast quickly becomes uneconomic to produce. By accurately casting components from a high quality mould, the commercial founder can reduce the amount of skilled machining and finishing undertaken later in the tool shop, and therefore minimise costs. With the exception of DIE CASTING, ceramic shell investment casting is perhaps the most accurate means of producing a metal cast. The materials used in forming the mould are exceptionally fine and able to reproduce near microscopic detail; ceramic is also a stable non-reactive material, with minimal expansion and contraction characteristics even when super-heated. A ceramic mould is structurally strong, and able to endure both rapid and repeated cycles of heating and cooling.
Unlike die casting, a shell mould enables cost effective production of complex, heavily undercut or ‘short production run’ designs, making ceramic shell among the most flexible of the precision casting processes. On top of all this, ceramic investment moulds can be constructed and processed over a short period of time simply, not possible to achieve when using traditional investment refractories.
Although the art founder is obviously not producing precision industrial components, their artist clients are no less demanding in the standard of reproduction they expect. Ceramic shell investments offer a degree of faithful translation that can be more difficult to achieve in plaster and grog without lengthy experience and a great deal of skill. The founder is also able to simplify the runner system for filling shell moulds, and so save on metal, casting and finishing costs. These benefits and savings have to be set against the increased costs involved in purchasing and maintaining ceramic shell investments when compared to traditional investments; but clearly on balance, many founders feel that the gain to their clients (and their own bottom line profit), makes the extra outlay worthwhile.
As well as developing new materials for ceramic shell investment, new methods of mould construction have also been evolved. An extreme example of this is the robotic processing of shell moulds – although relatively common in large scale industrial facilities, few if any art founders are able to take advantage of advanced automation. Despite a continued reliance on manually building up an investment mould, the construction techniques used in the modern ceramic shell art foundry are quite distinct from the mould construction techniques used by PLASTER & GROG founders [ref 2].
Instead of building a BLOCK INVESTMENT mould in one or two broad stages, the ceramic shell founder uses a multi-stage process. This requires the founder to build up a shell mould one individual layer at a time until an adequate thickness is reached. To apply each layer, the founder first wets the wax assembly in a fluid slurry, then dusts on a dry refractory grit which adheres to the wet surface of the assembly. The founder allows the deposited investment layer to dry before repeating the process multiple times to build up a suitably strong mould wall.
Because the founder applies one thin layer of investment at a time, the shell mould closely follows the underlying form of the wax pattern and assembly attachments. Unlike the traditional block mould, which is more or less indistinguishable in shape from it’s neighbour, each shell mould retains the general form characteristics of the underlying wax assembly. This typically allows each shell mould to remain easily recognisable to the founder, even when fully built to its maximum thickness.
One of the most revolutionary features of the ceramic shell investment system, is that it allows for an OPEN CORE mould. The preceding section WAXWORKING describes in detail the role of the solid core mass. This is the method used in traditional plaster and grog investments to create a hollow cast. Ceramic shell investments also allow the founder to create a hollow cast, but without first inserting a solid core mass into the wax pattern. This is done by adapting the wax pattern in a fashion that allows slurry and grits to enter the wax’s internal cavity, adhere to internal surfaces and then evacuate any excess. Allowing the investment to flow into the wax’s interior space enables the mould wall to be built up internally at the same rate as the external mould. The INVESTMENT WALL is continuous inside and out, eventually leading to a hollow ceramic shell mould ready for pouring.
THE CERAMIC SHELL SYSTEM >
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