PLASTIC MOULDING COMPOUNDS
One of the advantages of using a PVC compound for reproduction moulding is the ability to recycle the moulding compound once an existing mould has been finished with. This feature is of limited use in the professional foundry because the majority of moulds are either stored indefinitely or returned to the sculptor 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 especially when there is a high turnover of short term moulds. There is however, an initial investment required to purchase suitable melting equipment.
PVC moulds can be used to produce copies of a design in materials such as WAX, PLASTER, RESIN, CONCRETE, CIMENT FONDUE and so on. The plasticiser in these compounds 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/hardness desired in the end product.
Plasticity is measured using the SHORE SCALE, low numerical values indicate a soft compound. In practice, most manufacturers simply colour code their PVC products to indicate various hardnesses over an optimum range of the scale.
PVC is supplied in blocks, which must be heated and fluidised in a vat for moulding purposes. Specifically designed vats with multiple elements can be obtained from art suppliers, these safely 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 (see photographs), constructed to retain the decanted plastic medium in position as it cools and soldifys.
WARNING: Whilst safe when used in accordance with manufacturers' recommendations, PVC moulding compounds may become toxic if overheated. Take care when handling heated plastics to avoid burns and other injuries, read MSDS & supplied instructions.
RESTRAINED CASE MOULDING
(FOR PLASTICS & RUBBERS)
1. The pattern is divided via
mould case parts with a clay thickness
to create an air gap allowance.
(Note: keys & air vents).
2. Cases replaced with
pouring cup for compound entry.
3.Complete mould (stripped).
impression in case.
Left: impression removed to show keys.
(All photographs © ANPP).
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