COMMON FEATURES OF REFRACTORY (INVESTMENT) MOULDS
The internal feeder network contained within a lost wax refractory mould (the runners, risers and pouring cup designed to deliver a molten metal CHARGE), is imprinted by the WAX ASSEMBLY which is attached to the WAX PATTERN. The wax assembly needs no further intervention from the founder whilst the refractory mould is being constructed, other than keeping any openings (drains, riser tops and cup entrance), free of fouling by stray refractory.
The majority of refractory moulding systems use at least two refractory grit grades at various stages of the mould's construction. The first refractory layer applied to a wax assembly is variously referred to as the 'primary', 'facing', or 'first coat'. This first layer is composed of a very fine refractory material which is milled to a fine powder or FLOUR. Some refractory flours contain particulate which can be as small as 75µm (1µm [micron]= 0.001mm or 0.00003937inch).
This fine grade flour is suspended in a fluid BINDER – either plaster of Paris/water for traditional refractory moulding, or COLLOIDAL SILICA for CERAMIC SHELL. It is this first layer that is applied directly to the surface of a wax pattern assembly and ultimately reproduces all the fine wax pattern detail in the metal cast.
The first coat of any refractory moulding system is the most critical, both in preparation and application, so the refractories used for this stage are typically expensive premium grade materials. The application of this first layer to the wax assembly is carried out by carefully brushing, dipping or spraying on the refractory. Considerable care is taken to avoid the entrapment of air bubbles and to ensure that all detail in the wax pattern surface is adequately coated. The fine particulate of a first layer refractory is highly sensitive to surface undulation and textures, the coating should dry as a dense layer over the wax pattern, free of coarse lumps, air bubbles and other defects.
The two main disadvantages of primary refractory coatings are the high cost of premium materials and the greater resistance offered to gas release (through the mould’s wall - due to the densely distributed particulate). Were the entire mould constructed from a first layer refractory, it would be both excessively expensive and inefficient at dispersing casting gases.
To counter the above drawbacks, secondary layers of the mould are built up with an increasingly coarse and less costly refractory body. The coarse particles of the back up layers are not in direct contact with the wax assembly and consequently have no detrimental effect on the surface quality of the metal cast. In forming a less dense wall than the first layer, the secondary layers allow casting gases to more easily permeate through the refractory mould wall, and vent harmlessly to atmosphere.
The body or ‘grits’ used in refractory moulding systems to build up a mould's thickness are normally graded by mesh size, though some materials (especially fireclay grogs), are supplied simply as a coarse, medium or fine grade. The numerical values quoted for each mesh grade refer to the ability of a grit to pass through the mesh screen of the lower value, but not that of the higher value. A single value only indicates that a grit cannot pass through a mesh screen of this size – this mostly applies to very fine grits or flours. The (ASTM) mesh sizes for refractory grits as used for sculpture and design lost wax casting typically fall within the following ranges; –200 mesh (flour) for a first layer, with a 30-70 and 18-45 mesh size grit for secondary coatings. Unlike the flours used for first layers, the individual grains of secondary grits are easily visible to the naked eye, with particles here often approaching 1/32 inch (1mm) or so in diameter.
Refractory grits have no inherent adhesive qualities as such. This means the grits can only be applied to the wax assembly successfully with the aid of an added ‘glue’ (BINDER). Traditional refractory grits (GROGS) are wet mixed with plaster of Paris before application to the wax assembly. Ceramic shell systems require the the wax assembly to be pre-wetted with a refractory binder (COLLOIDAL SLURRY), then have the dry grit (STUCCO) applied immediately afterwards by dusting on.
The 'adhesive' binders (plaster or Paris/colloidal silica), used for fixing refractory grits to the surface of the wax assembly also have an inherent refractory content, for example plaster of Paris contains quantities of refractory minerals like quartzite. This thermally resistant mineral helps the binder to maintain mould adhesion whilst exposed to high temperatures in the kiln and during casting. If a binder becomes degraded during KILN FIRING or CASTING, then the refractory body could breakdown either contaminating the metal cast with loose INCLUSIONS, or cause the weakened mould to burst under the pressure of entering metal.
WARNING: Refractory products may contain hazardous substances, read MSDS sheets, product data and follow OHS guidelines/safe working practices.
Cross section of mould.
Core (grey), air gap & graded
refractory material (dense near face
coarse towards mould walls.
Wax pattern assembly as
typically prepared for
the ceramic shell process.
Cross sectional diagram of
typical ceramic shell mould
(All diagrams cc ANPP).
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