< CERAMIC SHELL INVESTMENT 5
HYBRID PLASTER & GROG/CERAMIC SHELL
The ceramic shell process described in the previous pages is well suited to the casting of small scale works that are easily handled. Larger wax pattern sections can also be invested using this hollow core shell moulding technique, though inevitably the handling and manipulation of these larger moulds becomes more difficult as the scale of the artwork increases. Founders working with large scale open shell moulds usually prepare their wax assemblies to allow the use of lifting tackle as an aid to handling. As described earlier, 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 building up an investment mould.
Rather than repeatedly handle large wax assemblies for slurry dipping, some founders instead use a HYBRID ceramic shell/grog and plaster investment system which allows the assembly to be invested in a static position. Hybrid investment moulding combines the high standards of surface reproduction offered by CERAMIC SHELL, with the relative economy and ease of use provided by PLASTER & GROG. A hybrid investment mould can be built up around a wax assembly without having to manipulate or move the work until KILN FIRING. This allows the founder to invest very large sections of wax without heavy lifting, and risk of damage. A brief summary of the basic hybrid technique is as follows:
1 WAX ASSEMBLY PREPARATION
A hollow wax pattern for hybrid investment moulding has a traditional plaster and grog refractory CORE inserted first. The poured core is slightly extended beyond the entrance into the reproduction mould’s, this excess later acts as a stand. The stand allows the assembly to be worked on in a static position without undue disturbance to both the wax and the applied investment material. As with traditional plaster and grog investments, core pins, core vents and core reinforcements are applied to the assembley as needed. The insertion of a core into the wax allows the founder to invest the form without having to run slurry and stucco grits into an interior cavity. If the cored assembly is too large to be manually handled, it may be mounted on a heavy steel pallet, this allows a fork lift to be used for moving, and later positioning the completed mould in the kiln.
Runner and riser components are placed onto the wax pattern using either direct or indirect techniques, though most founders using the hybrid investment system prefer the direct method of metal running. Direct running is often combined with gas venting through a sand jacket (see WAXWORKING). The sand venting technique is ideal for use on hybrid moulds because it allows the contained wax assembly to be easily evacuated from the investment without having to invert the mould cup down, the wax simply drains from the mould cavity via the riser stubs/drains positioned at the base of the mould. Given that large scale moulds are normally sand reinforced as a matter of course, there is no additional difficulty of expense involved in the adoption of this SOFT SAND VENTING technique.
2 FIRST COATS (CERAMIC SHELL)
The wax pattern assembly is prepared by DEGREASING with alcohol, then permitted to dry before the application of premium grade slurry. The investment procedures followed here are essentially the same as those for ceramic investment described above (stages 1 - 4), 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). Excess stucco, which does not adhere to the wet slurry and falls away from the mould, can collected up and sieved for reuse. Because the void inside the hollow wax has been filled earlier by an investment core, neither slurry nor stucco need be applied to the interior of the assembly. The initial layers of ceramic investment are built up over the wax assembly to about the first or second back up layer, (as described in stage 6 of the ceramic shell investment process).
3 PLASTER AND GROG REINFORCEMENT LAYER
Once the stage ‘6’ 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 investment is usually first pre-wet with pure COLLOIDAL SOL before applying plaster and grog; wetting assists adhesion between the two investments, minimising 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 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 3 - 4 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 investment moulds are constructed, the founder may additionally set light-gauge steel reinforcement bars within the plaster and grog layer to help support the dead weight of the surrounding investment.
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 investment mould, and helps protect the relatively fragile layer of underlying plaster and grog investment from any direct heat within the kiln. The 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 investment is cleared this debris before the mould’s progression to the kiln and burn out stages.
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