THE CERAMIC SHELL SYSTEM (MATERIALS)
1 THE BINDER
There are two basic binder options use in ceramic shell moulding systems – water based SILICA SOL, or alcohol based ETHYL SOL. The alcohol version is less common, mainly due to the difficulties of safely maintaining this volatile material. Some art founders use both silica and ethyl sols for refractory moulding, this combined use can dramatically reduce production times. Where both types of binder are used, the water based silica sol is normally reserved for wetting the first few (primary) layers and the more volatile ethyl sol base for secondary ‘back up’ coats. Ethyl sol promotes a rapid drying of the secondary layers, especially in the presence of ammonia vapour.
Both silica and ethyl sol binders are COLLOIDS, a term originating from the Greek word for glue – ‘kolla’. A colloid is a state of matter associated with electrically charged particles in dispersion. The particulate dispersed within ceramic shell sols is a refractory mineral SILICA The average particle size of any colloid is extraordinarily small, typically between 1 and 500 nanometers (1nm = 0.000001mm), this is a measurement only slightly larger than that of a basic molecule. The silica particles suspended within refractory moulding colloids usually fall within the range of 10-25nm. These minute particles are so densely dispersed throughout the sol that a single gram (1g) of colloid contains sufficient particulate to (in theory), cover a surface area of some 600ft2 (200m2) [ref 1]. The electrical charge carried by the silica particles prevents the colloid from setting to a viscous gel, allowing the sol to maintain fluidity until air dried. It is this colloid which forms the basis of the ceramic slurry, which in turn is used to wet the surface of the wax assembly.
2A SLURRY REFRACTORY
Whilst silica particulate is already present in the sol, some additional refractory material is always added to the colloid before application to the wax assembly. This additional refractory is usually a high quality, finely graded mineral FLOUR. The flour disperses throughout the sol to create the wet refractory material known as CERAMIC SLURRY.
Some founders use a combined fine/coarse grade ceramic slurry, though most professionals prefer to maintain two distinct grades of slurry in separate holding tanks. The first slurry grade consists of a colloid with added zircon silicate flour - or a similar fine non-reactive refractory flour capable of depositing a uniformly dense coating over the wax pattern assembly. This fine deposit is essential to the formation of a premium quality facing layer. The second (coarser) grade of slurry later used to wet the assembly before applications of back up grits, usually contains a refractory flour produced to less exacting specifications.
In contrast to expensive first coat zircon flours, the second coat slurry refractory is usually a finely milled flour of China clay or alumina silicate. In both the first and second coat slurries, the refractory flour is added to the colloidal sol in very precise proportions, thickening the basic sol to a ‘creamy’ consistency at optimum viscosity. The slurry refractory is maintained in suspension by rotating the holding tank with the rotating slurry agitated by built-in paddles designed to prevent the flour from separating out from the sol and settling in a mass at the bottom of the tank.
Once mixed, the slurry is regularly monitored and adjusted to maintain optimum quality and consistency. Viscosity is checked using a flow cup and the slurry’s ability to adequately coat the wax pattern is tested by dipping a glass plate into the tank, drying the deposit, then inspecting the glass surface against a strong light source for pin holes and other faults. Ambient room temperatures, humidity and the pH levels within the slurry are also usually monitored.
To test the pH level, a universal indicator paper is dipped into the slurry and the result compared to a chart. Monitoring ensures the slurry is kept within acceptable limits (slurry is an alkaline based material, most slurry products have an optimum pH level of between 8.5 - 9.5). Contamination by a foreign substance such as run off alcohol from the de-greasing of a wax assembly, or metals (which may have been used to form a part of the holding tank or wax attachment assembly), can significantly reduce the useful working life of a slurry. In practice though, the degradation of slurry is marginal in a foundry turning over large numbers of refractory moulds.
2B STUCCO REFRACTORY
As well as the refractory materials present within sols and mineral flours (which combine to form the slurry), a second ‘dry’ refractory grit is applied separately to the mould when building up the shell’s wall thickness. These dry grits are composed of increasingly coarse grades of ceramic. Early on in the mould’s construction, the applied grits are in the form of a fine free running powder (similar in consistency to a table salt).
Later, back up dry refractory applications use a much larger and coarser grit (reminiscent in size and angular shape to cat litter). This dry refractory serves a broadly similar function to that of coarse GROG grits – allowing the founder to rapidly build up a wall thickness that is both structurally strong and semi-porous to evolved casting gases. Like fireclay grogs, dry ceramic grits have no ability to self-adhere, relying instead on a wet slurry coating to ‘glue’ the loose grit to the wax assembly.
Ceramic refractory grit (more correctly referred to as STUCCO), is derived from a dried, fired and processed clay product known as FUSED SILICA. Fused silica (particle size apart), shares many of it’s refractory properties with the fine China clay flours added into coarse grade slurries.
3 OTHER ADDITIVES
As well as a binder (silica/ethyl sol) and supplemental refractory (zircon silicate/China clay flour), a wet ceramic slurry requires two further additions to function effectively. The first of these ingredients is a wetting agent - a detergent like substance which is dispersed in the slurry to assist coverage and adhesion to the wax assembly surface. The second additive is an anti-foaming agent which counters the tendency of the wetting agent to produce a froth on the slurry’s surface. This froth can potentially lead to the transfer of air bubbles to the dipped wax assembly, this in turn would diminish the quality of the cast’s surface. Both wetting agent and anti-foam are carefully matched to the manufacturers colloid – incompatible wetting agents can cause the sol to set or GEL, conversely a poor quality anti-foam will inhibit the efficiency of the wetting agent. Slurry may also contain a chemical indicator, this additive is formulated to change colour or tone as the ceramic coating dries over the wax assembly. The changes in colour provides the founder with a visual cue that the refractory moulding process can safely proceed to the next stage.
WARNING: Refractory products may contain hazardous substances, read MSDS sheets, product data and follow OHS guidelines/safe working practices.
Click here to download ceramic shell process flow chart pdf (34kb)
Part completed shell mould.
Note the outer framework reinforced
with steel bars which are removed
prior to casting.
(Click on image to view wax assembly)
Completed shell mould.
This mould has a glass
backed finishing coat added.
(Bronze Age Ltd)
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