CERAMIC SHELL SYSTEM - PROCESS

One of the most revolutionary features of the ceramic shell refractory moulding system, is that it allows the founder to create an OPEN CORE or SHELL mould. The article on FEEDING & VENTING briefly describes the function of the solid core mass, the method used in traditional plaster and grog refractories to create a hollow cast. Ceramic shell refractories also allow the founder to create a hollow cast, but without first inserting a solid core mass into the wax pattern. This is made possible by adapting the wax pattern to allow both slurry and grits to enter a wax patterns cavity, adhere to the internal surfaces and then evacuate any excess. Allowing the refractory to flow into the wax’s interior spaces allows the mould's wall thickness to be built up internally at the same rate as the outer walls. The REFRACTORY MOULD WALL is continuous inside and out, eventually leading to the construction of hollow ceramic shell mould for metal pouring.

The process of constructing a ceramic shell mould can be briefly surmised as follows:

1. The WAX PATTERN ASSEMBLY (the wax model and added metal running attachments), is specifically adapted for the process. Patches may be cut from the wax pattern to allow ceramic materials to more easily enter and exit the internal cavities of the design and the running system may be reinforced with metal bars to support the assembly during mould construction and drying cycles.

2. The wax pattern assembly is de-greased using a proprietary alcohol solution, this effectively 'etches' the wax's surfaces allowing the SLURRY to more easily adhere to the wax. The alcohol is drained and the assembly fully dried before proceeding.

3. The wax pattern assembly is dipped in a premium CERAMIC SLURRY. The slurry is usually held in vats, small hand held waxes can be manually dipped, larger waxes may be manipulated via lifting equipment attached to temporary steel reinforcing cages built into or around the wax assembly. Some foundries will also either spray or paint the slurry over the wax pattern. As with PLASTER & GROG moulds, trapped air and other undesirable features like 'curtains' are carefully removed from this first coating, thus ensuring as good as possible a reproduction of the waxes surface.

4. The pattern is drained and allowed to fully dry, forced air may be drawn over the drying assembly and tubing is often used to ensure all the internal cavities dry evenly. Some systems have a dye added to the slurry to provide the founder with a visual indicator of wet and dry areas of applied slurry.

4 The wax pattern assembly is re-dipped in the premium slurry. In some cases the founder will 'pre-wet' the assembly with fluid colloidal silica sol to assist adhesion. Some founders also allow this coating to dry as above (4), however in most cases the assembly proceeds to the next stage and has a STUCCO grit applied.

5. While wet, the assembly is removed from the dipping area to a dry zone where STUCCO grits are applied. For this first stucco application, a very fine grit is dusted over the wetted assembly, either manually using a MESH SCREEN, or with a RAIN MACHINE. Both methods distribute the stucco grit evenly whilst excluding oversize particles that might disrupt the surface quality of the mould. Excess stucco is removed from the assembly which is then dried as before.

6. After the initial premium layers of refractory have been built up around the wax assembly, the founder is able to switch to a secondary slurry and coarser grits. These allow the remaining structure of the mould to be constructed rapidly using with cost effective materials. Again, the wax assembley/first coat shell is 'wetted' by dipping/painting on/spraying (in a secondary slurry mix).

7. The wetted assembly/shell mould is removed from the vat, drained and has an intermediate (typically a 30-80 mesh fused silica), stucco grit applied (see stage 5). After drying this cycle may be repeated several times before moving on to an even coarser grade grit (typically 16-40 mesh). This cycle of dip - drain - stucco - dry is repeated until the founder has built up the mould's internal/external walls to a desirable thickness (small moulds circa 1/4" [6mm], large moulds 1" [24mm]+).

8. To finish the ceramic shell mould the founder finally applies a sealing coat. This is applied by simply dipping the mould in ceramic slurry and allowing it to drain and dry. This consolidates the mould's surface and prevents any loose stucco from detaching from the mould during handling or firing.

9. At various stages of a ceramic shell's mould construction the founder may apply a range of additions, usually to help strengthen the mould's structure. These additions typically include stainless steel wire or mesh (often sandwiched between intermediate layers), or shredded glass fibre which is usually applied with the sealing coat. Once the mould has been finished the founder cleans any entrances and vent outlets of stray material then stands the GREEN mould for a period (if possible), as this aids strengthening of the mould's structure.

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Click here to download ceramic shell process flow chart pdf (34kb)

WARNING: Refractory products may contain hazardous substances, read MSDS sheets, product data and follow OHS guidelines/safe working practices.

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