HYBRID PLASTER & GROG/CERAMIC SHELL SYSTEMS

The CERAMIC SHELL system described in the previous articles is well suited to casting small scale designs that are easily handled. Larger scale wax patterns can also be invested using this hollow core/shell moulding technique, though inevitably the handling and manipulation of larger volume moulds becomes more problematic as the scale of the design increases. Founders working with large scale open shell moulds usually prepare their wax assemblies to enable the attachment of lifting aids; this usually requires the founder to insert steel supports and lifting points through wax runners (these supports are later removed after wax is burnt out). Whilst effective, adding these reinforcements can significantly increase the amount of time and expense involved in constructing a large refractory mould.

Rather than repeatedly handle large wax assemblies for slurry dipping, some founders instead employ a HYBRID ceramic shell/grog and plaster refractory system which allows the assembly to be constructed in a static position. Hybrid refractory moulding combines the consistant surface reproduction qualities offered by CERAMIC SHELL, with the relative economy and ease of use provided by PLASTER & GROG. A hybrid refractory mould can be built up over a WAX ASSEMBLY without having to manipulate or move delicate wax structures until KILN FIRING. This allows the founder to invest very large sections of wax without heavy lifting or risk of damage. A brief summary of the basic hybrid technique is as follows:

1 WAX ASSEMBLY PREPARATION

A hollow wax pattern for hybrid refractory moulding contains a traditional plaster and grog refractory CORE. The core is extended beyond the wax pattern cavity, this extension later acts as a stand. As with traditional plaster and grog refractories, core pins, core vents and core reinforcements are applied to the assembley as needed. The insertion of a core into the hollow wax allows the founder to construct the mould without having to run slurry and stucco grits into an interior cavity.

Runner and riser attachments are fixed to the wax pattern and drains are applied to the base of the pattern to allow the wax content to evacuate the mould in the KILN. Once the wax is burnt out, these now hollow drains are blocked with a flexible plastic vent material with the loose end pullled up to the cup. The refractory mould and vents are then encased in a set sand (like soduim silicate) for casting.

2 FIRST COATS (CERAMIC SHELL)

The wax pattern assembly is prepared by DEGREASING with alcohol, then allowed to dry before the application of premium grade CERAMIC SLURRY. The refractory moulding procedures followed here are essentially the same as those for CERAMIC SHELL except here the assembly usually remains static at all times. Small waxes can be dipped in the SLURRY tank; however, in most cases the sheer scale of the work requires the slurry to be painted or sprayed over the wax surface instead.

STUCCOS can be applied to the wet slurry by either cascading the fine grits through a fine wire mesh sieve, or by throwing on by hand (the latter is especially effective when using later coarse grade grits). Because the void inside the hollow wax has been filled earlier by a refractory core, neither slurry nor stucco need be applied to the interior of the assembly.

3 PLASTER AND GROG REINFORCEMENT LAYER

Once the final application of slurry and stucco has dried, a plaster and [coarse] grog layer can be applied over the top of the ceramic shell ‘facing’ coats. The ceramic layer is usually pre-weted with pure COLLOIDAL SOL before the applying PLASTER & GROG mix to aid adhesion between the two refractories and minimise the risk of delamination. The founder usually applies the wet plaster and grog mix directly over the ceramic coating by hand, manually building up a mould wall thickness.

The thickness of plaster and grog built up over the ceramic layers varies according to the size of the wax assembly, though it rarely exceeds a depth of three to four inches (75 - 100mm). This relatively thin wall thickness allows the applied heat of the kiln to rapidly penetrate through a mould wall and melt the contents wax out, minimising the pressure on the mould wall from expanding wax. Where very large scale hybrid refractory moulds are constructed, the founder may additionally place light-gauge steel reinforcement bars in the plaster and grog layer to help support the dead weight of the surrounding refractory.

4 SEALING COATS (CERAMIC SHELL)

The plaster and grog layer is normally applied in a single session – this allows large scale moulds to be built up to suitable thickness both quickly and with a reasonable economy of labour. Once the plaster and grog layer has dried out, one or two further layers of back-up slurry/coarse stucco are applied over the plaster and grog coating, before a final slurry only SEALING coat is applied. The sealing coat consolidates the outer surface of the completed refractory mould, and helps protect the relatively fragile layer of underlying plaster and grog refractory from any direct heat within the kiln. The GREEN hybrid mould can now be left to stand and thoroughly dry if time permits, as hybrid moulds tend to ‘improve’ by standing for between two to seven days before kiln firing. Any drain/riser stub or cup entrance fouled with refractory is cleared this debris before the mould’s progression to the kiln and burn out stages.

See also: 'METAL FEEDING & VENTING SYSTEMS' & 'SODIUM SILICATE BONDED PROCESS'

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