Ceramic shell moulding

Ceramic shell moulding is a patented process (Replicast ) in which a polystyrene model is covered with a ceramic shell (2-3 mm thickness), based on ethyl silicate and refractory sand. The shell is hardened using ammonia and sintered at 1000 °C. The sintering cures the shell and bums away the polystyrene model. The metal is then poured into the shell. 

The sintering furnace is equipped with post combustion to reduce emissions. 

The technique allows a casting design with no parting lines, cores and draft angles and reduced feed metal. This reduces the need for finishing and machining operations. 

The technique results in a minimisation of dust emissions from moulding and finishing, as compared to sand moulding. The emissions of VOC are eliminated as no gas is emitted for the inert ceramic mould. Additionally the amount of waste (dust, metal) is reduced. The reduction of feeder systems results in a higher 5deld of castings per melt. 

Operational data from the owner of the technique are reported in Table 4.28, for the casting of valves, as compared to sand casting. 

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Fig. 8.10 A typical ceramic shell incest mem casting mould.

The pattern is made by pouring or injecting wax into a metal mould. In some cases a simple pattern may be prodnced in one step with an integral gating system, while in other cases complex patterns may be assembled from a nnmber of separate components prepared individually. In the ceramic shell process, after the wax pattern is formed it is dip-coated with a primary slurry coat of very fine particles to give a smooth surface it is then stuccoed with coarser refractory and dried. These steps are repeated until the required mould thickness is achieved. The primary and secondary dip-coats contain binders such as ethyl silicate, and the refractories are principally zircon, sillimanite... 

The ceramic shell process is widely employed for producing large castings in steel, nickel and other high melting point materials. The, block mould process is mainly used for light alloys and in the production of small and medium-size castings. 

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