INVESTMENT – A SUMMARY
The words INVESTMENT & REFRACTORY can be used as a reference to both the materials and the techniques used to construct refractory moulds for lost wax casting processes.
A WAX PATTERN that has had various casting attachments added, including cup, runners and risers, is known as a WAX ASSEMBLY.
Refractory moulds are designed to resist thermal shock of applied heat and the pressure of expanding wax in the kiln as well as poured metal. Refractory moulds are also constructed in a way that permits gasses from the metal pour to partially vent through the mould’s walls. Refractory moulds are sometimes reinforced with metal clips or mesh, chopped fibres, set sand and other materials.
All art foundry refractory moulds consist of three basic components: a BINDER, a REFRACTORY and modifying ADDATIVES. The binder ‘glues’ together non-adhesive refractory body. Modifying additives are added to improve the properties of the binder and/or the refractory material. The use of additives varies according to the refractory moulding system employed by the founder.
The four most common lost wax refractory systems used in art founding are:
1 PLASTER & GROG (traditional). Uses a gypsum based binder in combination with a fireclay grog body, this is formed into a BLOCK MOULD around the wax assembly. Progressively coarser grades of grog are applied as the mould's wall thickness is built up. Plaster & grog refractory materials are generally less expensive to buy and easier to prepare than those used in ceramic shell systems; however, moulds of this type are bulky, take longer to process and can be prone to damage and breakdown during kiln firing and casting.
2 CERAMIC SHELL moulds are built up through a repeated immersion of an underlying wax assembly in a ceramic SLURRY. Fused silica STUCCOS of progressively larger grit sizes are applied to the wetted assembly in successive layers until the mould wall is built up to an optimum thickness. A solid refractory core is usually omitted, instead the founder constucts up a continuous shell jacket covering both the internal and external surfaces of the wax pattern. Ceramic refractory systems produce moulds with excellent strength to weight ratios that capable of faithfully reproducing fine design detail; however, the system does require the use of specialist equipment, together with frequent slurry monitoring and maintenance schedules. COLLOIDAL SOL and stucco materials are relatively expensive to purchase in comparison to plaster and grog refractories. Ceramic shell moulds are also suitable for use for casting non-metallic materials such as glass.
3 The HYBRID refractory mould is constructed using both ceramic shell (facing layers) and plaster & grog (backing layers), systems. Combined with SAND VENTING techniques, hybrid moulds are especially effective for the production of large scale lost wax casts. Using both refractory moulding systems allows the founder to combine the high quality of surface reproduction and stability offered by ceramic shell, with the economy provided by less costly plaster and grog refractories. Use of a hybrid system does require the founder to stock a wide range of materials and to have access to specialist equipment – the plaster and grog component of the system also lengthens kiln firing times.
4 JEWELLERY refractories for use in either VACUUM or CENTRIFUGAL casting processes, are usually composed of a gypsum binder and a crystobalite (silica) refractory body. The refractory, binder and any added ingredients, are normally supplied to the founder in the form of a single pack combined powder preparation. Unlike other refractory moulding systems, jewellery and dental type refractories are not graded (with coarser grits towards the outer wall of the mould), instead they are made up of a single fine grade. The refractory/binder powder is mixed with water or silica sol, degassed, then applied in a single ‘shot’ either to a single wac design or cluster of wax patterns mounted on a SPRUE TREE. The tree is contained within a FLASK to contain the fluid refractory. The resulting block mould is normally cast with vacuum or centrifugal assistance. Investment powders for this process are can be very expensive; likewise, the sophisticated machinery required for degassing and casting can require considerable financial outlay. Suitable for the production of small scale works only, relatively few art foundries offer direct access to these refractory moulding systems.
WARNING: Refractory products may contain hazardous substances, read MSDS sheets, product data and follow OHS guidelines/safe working practices.
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