< PERMANENT PATTERNS FOR SAND CASTNG
MATERIALS FOR PATTERN CONSTRUCTION
One of the more significant differences between master patterns designed for LOST WAX casting and those for sand casting, is that the sand pattern must be well enough constructed to withstand both a substantial amount of handling and the pressure of compacted sand during moulding operations. For this reason, soft or fragile patternmaking materials that are sometimes used for a lost wax process (clays, plasticine, ceramic, and so forth), should never be considered suitable for sand casting.
One of the better and most popular materials for constructing fine art sand patterns is PLASTER OF PARIS. Direct plaster techniques usually involve working wet material over a polystyrene core which is reinforced with a scrim layer (although a wire mesh framework can be used as an alternative sub-structure). Artists who prefer modelling in a soft medium such as clay, will normally translate the soft modelled form into plaster by means of a WASTE MOULD or other similar process. Plaster patterns of all varieties are usually easily cut into more manageable pieces in the event of a complete pattern design being too large for casting or transportation in a single piece.
Although well constructed plaster patterns are excellent for sand casting in short-run EDITIONS (ie 3-5 copies), patterns intended for longer production runs, or else use over a prolonged period of time, may be better formed from a more robust material such as wood, metal or resin. All of these materials usually offer better long term stability, in use or in storage, than standard gypsum products.
Where practical, the use of a strong lightweight material such as RESIN for constructing large scale master patterns is highly recommended. Although the cost of originating resin and similar patterns is usually higher than the plaster equivalent, a good part of this expense can be effectively recovered through easier transportation, handling and storage longevity. Robust patterns suffer little from incidental handling damage and distortion and this helps limit the potential for founding errors to arise. In some cases a foundry will offer some level of discount if supplied with an appropriately made, lightweight pattern. If cutting a resin or plywood pattern for casting is necessary, the design should be cut with care to minimise released STRESS DISTORTION. Additional stiffening may also be necessary to prevent lightweight patterns from flexing under the force of compacted sand in the moulding flask.
Sand casting processes are well suited to the reproduction of large scale, regular forms and volumes which have relatively simple surfaces and textures. Straightforward designs compliment both the techniques of pattern construction just described as well as the methods used by the founder to create sand moulds. The suitability of sand for reproducing simple forms and surfaces does not totally prohibit the artist including of some degree of complexity in a design or surface texture, provided care is taken not to introduce excessive UNDERCUTTING.
MOULDING SURFACES & UNDERCUTS >
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