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RESIN & PLASTICS
Resin is an extremely useful patternmaking material which produces strong lightweight constructions, suitable for use in all the common art foundry casting processes. Like plaster of Paris, resins can either be cast or constructed, though most resin foundry patterns are of the cast variety. Resin can be used in waste moulds as an alternative filler material to cast plaster, but usually resin casts are best formed in rubber moulds.
The two basic types of resin product available are acrylic and polyurethane, with polyurethane being most suited to patternmaking. Most commercial polyurethane products are supplied as a two part pack containing a basic resin product which may be pigmented or clear, hard or flexible, plus a separate catalyst/curing agent. Mixing the resin base with the catlyst, initiates an exothermic chemical reaction which causes it to SET hard. The resin/catalyst mix has an effective working period which can vary from about one minute to half an hour or so, variable according to the product combination chosen. CURING times may also vary, usually from about half an hour, up to a day or so.
The exothermic reactions in setting resin can be very significant, some approach 300°F (circa 150°C). This has a number of implications for the user. Apart from the potential for burns (wear protective clothing, and never apply resin to the skin), resin can also damage incompatible materials it contacts. In particular, this means that resins cannot be applied to low melting point materials and polystyrene foams (the chemical structure of the foam degrades in contact). Some reproductive moulding materials (certain rubber products for example), are also unable to withstand the exothermic reaction of setting resin (which is sometimes used instead of plaster as mould case material).
Fluid resin products can easily be poured into existing moulds to create a cast copy. A ‘body’ component can be added in to the mix to create a more malleable and plastic medium. Glassfibre matting is used in the same way that scrim is used to laminate and strengthen plaster constructions. Fillers (powders) can be added in which weight and colour a resin cast, this includes stone, metal and other materials, some of which can give the resin copy the impression of being carved or hot cast. This is useful if the artist wants to create a series of COLD CAST sculptures. If well made, cold cast images can closely resemble their ‘hot cast’ metal counterparts at a distance. These casts are sometimes used to gauge demand for a potential edition of bronzes, or as a cost effective (if slightly unsatisfactory), alternative to a genuine metal cast.
Similar in many respects to cast resin materials, preformed plastics are suitable for use with most casting processes, including LOST PATTERN casting with lightweight plastics. Plastics can be cast, manipulated and vacuum formed. Vacuum forming is especially useful for creating low reliefs of found objects quickly and in an inexpensive material. A vacuum formed pattern can potentially be used to make a direct lost pattern cast.
RESIN & CEMENT PLASTERS
More recently a number of new commercial moulding and modelling products have come into use. Many of these products share characteristics with more familiar materials such as plaster of Paris, resin and clay. Marketed under trade names like Plasterform, Jesmoite, Hydrocal and Formglas, most of these products are invariably composed of glassfibre reinforced gypsum (or a similar aqueous acrylic resin formula), and a crystalline mineral body. These products can invariably be coloured or filled with stone or metallic powders, they can be used in a fluid form to create slush cast copies from moulds, or be thickened for use as a plastic modelling medium, which can also be machined after setting and curing.
The typical high surface density and low porosity of these products is such, that completed sculptures and designs can easily be painted and can usually be shown out of doors without too much concern for medium term deterioration. In general, resin and cement plasters offer the sculptor all the combined possibilities of traditional plaster and resin products. One of the principal differences in these products over traditional resins is their relatively low exothermic setting temperature, low volatility and low toxicity, making them easy and safe to use.
PREPARING PATTERNS FOR CASTING >
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