< FLEXIBLE MOULDING MATERIALS
PLASTICS
One of the advantages of a PVC compound is that the moulding material can be reused once each reproduction mould becomes redundant. This is of limited help in the professional foundry because artist’s moulds are often stored indefinitely or returned to the artist after use. However schools and colleges rarely need to consider issues of long term storage, so this makes PVC a very cost effective moulding process where there is a high turnover of short use moulds. There is however, some initial investment needed to acquire suitable melting equipment.
PVC moulds can be used to produce hard copies in materials such as wax, plaster, resin, resin-plasters, concrete and ciment fondue and so on. The plasticiser in the compound eventually migrates leading to a hardening of the plastic, and PVC is prone to attack from solvents and spirits, otherwise with care, PVC moulds are suitable for long term storage and use in either the foundry or artist’s studio.
Advances in modern polymer technology has produced an enormous range of plastic products offering quite distinctive characteristics. Plastics can be formulated to be as hard as some metals, or as soft as a foam. The plastic most closely associated with sculpture making and foundry moulding is polyvinyl chloride, better known by the abbreviation: PVC. This plastic is derived from ethelyne and made flexible by adding in a semi solvent or oil to between 5% and 50% of volume, variable according to the elasticity desired in the end product.
Plasticity is measured using the SHORE SCALE, with low numerical values indicating a softer compound. In practice, most manufacturers of plastic moulding materials simply colour code their products to indicate various hardnesses over an optimum range of this scale.
PVC is supplied in blocks, which must be heated and made fluid in a vat before use. Specially manufactured vats can be obtained from art suppliers, these melt and then hold the compound in controlled thermostatic conditions. Once fluid, the PVC compound can be decanted into a container and poured into a prepared mould case.
Because the working temperature of PVC is about 300°F (circa 150°C), this moulding material cannot be handled manually (ie brushed on). Instead, molten PVC must be poured over a master pattern that is either contained within a shuttering, or else has a restraining plaster case built up to hold the decanted plastic in position as it cools and sets firm.
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