< INVESTMENT MATERIALS
COMMON FEATURES OF INVESTMENT MOULDS
The internal feeder network within a lost wax investment mould (the runners, risers and pouring cup to deliver a molten metal CHARGE), is imprinted by the wax assembly which is in turn attached to the wax pattern. The wax assembly needs no further intervention from the founder during the construction of the refractory mould, other than to keep any openings (drains, riser tops and cup entrance) free of fouling by stray investment.
The majority of investment systems use at least two refractory grades at various stages of the investment 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 even ‘flour’ like consistency. Some refractory flours contain particulate which can be as small as 75µm (1µm [micron]= 0.001mm or 0.00003937inch).
The fine grade powder material is suspended in a fluid BINDER – either plaster of Paris/water for traditional investments, or COLLOIDAL SILICA for CERAMIC SHELL investments. It is this first layer that is applied directly to the surface of a wax pattern assembly and ultimately reproduces all the fine pattern detail in the metal cast.
The first coat of any investment system is the most critical, both in preparation and application, so the refractory used for this is typically made up of 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 wet investment. 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 the first layer refractory is highly sensitive to surface undulation and textures, it 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 investment 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 investment, it would be both excessively expensive, and inefficient at dispersing casting gases.
To counter these 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 investment wall, and vent harmlessly out of the mould.
The refractory body or ‘grits’ used in investment systems to build up mould walls are normally graded by mesh size, though some investment 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 investment 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 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 investment 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), and then have the dry grit (STUCCO) applied immediately afterwards by dusting on.
The 'adhesive' binders 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 reistant 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.
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