< INVESTMENT MATERIALS
COMMON FEATURES OF INVESTMENT MOULDS
The tubular network within an investment mould for the delivery of the molten metal CHARGE is imprinted by the wax assembly (runners, risers and pouring cup). 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 founder then selects a suitable method for the construction of the refractory mould, deciding which type of investment materials to use, what thickness the mould should be up built to and if any reinforcement or other special features will be needed.
Most founders use a minimum of two refractory grades during the construction of an investment mould. The initial refractory layer 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 within 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 any fine pattern detail in the metal cast.
The first coat of any investment system is the most critical, both in preparation it’s and application, so the refractory body used at this stage is composed of relatively 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 the first investment layers are cost and the greater resistance offered to gas escaping 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 secondary refractory layer is less critical, both in preparation and application, than the first layers. The ease of applying coarser layers also helps to reduce overall labour costs and time scales.
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 are dry and have no potential for self-adhesion. 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 a binder such as plaster of Paris before application to the wax assembly. Modern ceramic shell systems require the the wax assembly to be pre-wetted with a refractory binder (COLLOIDAL SLURRY), and then have a dry grit (STUCCO) applied immediately afterwards by dusting on.
The 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 such as 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 were degraded during KILN FIRING or CASTING, then the refractory body could breakdown either contaminating the metal cast with loose INCLUSIONS, or causing the mould to burst under the pressure of entering metal.
PLASTER & GROG > |
