< POLYURETHENE RUBBERS
RIGID REPRODUCTION MOULDS
The rigid reproduction moulds used in art foundries are closely related to the piece moulds used in the ceramics industry for clay slip moulding. Most rigid foundry moulds are constructed from plaster of Paris, though it is also possible to create lightweight resin piece moulds. Prior to the common availability of rubber and plastic moulding compounds, plaster piece moulding was one of the few alternatives available to gelatine moulding. Unlike gelatine moulds, plaster moulds (if used carefully), would remain serviceable over a long period of time. A selection of plaster piece moulds for art foundry use can be viewed in the Museé d’ Orsay (Paris) [ref]. These moulds were made for the designer Carlo Bugatti (1856-1940) by craftsmen at the Hébrand foundry and would no doubt produce quality wax patterns if used today, some fifty years on .
The main difficulty with using RIDGID MOULDS is that they do not deform when pulled away from either the moulded master pattern, or later, the cast wax pattern. This means that a plaster mould will lock-in to any pattern undercut. There are however, moulding techniques which enable reasonable amounts of undercut and overhang to be accommodated by a rigid mould. These adaptive techniques usually require the moulder to make an independent ‘piece part’ specifically designed to remove cleanly from the undercut feature. The piece part (or parts) are constructed first, then the main part of the mould is formed around the rest of the pattern and piece parts, thus seating the loose pieces in position. To disassemble the mould, the outer case of the mould (sometimes called the ‘mother’ or ‘parent’ mould) is removed first, this reveals the piece parts which can be pulled away from the pattern at a tangent to the undercut.
Other disadvantages of plaster piece moulds include plaster’s tendency to adhere to pattern surfaces. Later on in the casting process, wax may stick to the plaster mould whilst slush casting wax copies. In both cases the use of a release agent is essential if adhesion is to be avoided. Larger plaster piece moulds can warp and distort if not properly reinforced with internal scrim bandaging and an external support frame. Piece moulds can also be heavy, complicated, and time consuming to construct. Despite these drawbacks, a well made piece mould can be something of an artwork in it’s own right, and are generally perfectly serviceable for foundry use if cared for properly.
However well made, plaster piece moulds have only a very limited role in the modern foundry. RTV rubbers, especially polyurethane, are considered relatively inexpensive even for moulding larger designs, and many of the master patterns forms deemed suitable for piece moulding (typically large scale with simple forms and smooth surfaces), are often best sand cast anyway. On rare occasions, a plaster piece mould can be used as a viable alternative to rubber moulding – and this option can prove to be a very quick and cost effective method of creating a reproduction mould. Many foundries still use plaster moulds for casting regular shaped production items such as wax sheets, POURING CUPS and RUNNER/RISER bars.
Glass reinforced resins can also be used to create reproduction piece moulds. Resin has the advantage of being lightweight and [effectively] non-porous. Large resin moulds can be prone to flexing, so if this material is used it is important to stiffen any sizable mould case with a laid in reinforcement – steel bar is usual. Although resin surfaces are not as porous as those of untreated plaster, it is nonetheless essential to treat the moulding surface with a suitable release agent before applying wax. Resin moulds are also well suited for casting in concrete and other abrasive materials.
REPRODUCTION MOULD CONSTRUCTION >
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TIP: If a pattern is being piece moulded with plaster, its surfaces should be sealed and treated with a release agent like clay slip to prevent the plaster case adhering. Similarly, the case itself should be either soaked in water or sealed and treated (petroleum jelly is a common release), before wax is applied. |
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