< MATERIALS FOR PATTERN CONSTRUCTION
MOULDING SURFACES & UNDERCUTS
Unlike the highly elastic and deformable rubbers used for making lost wax REPRODUCTION MOULDS, sand moulds (once set), are rigid and inflexible. In this respect, moulding in sand has similar technical limitations to the earlier described process of PLASTER PIECE MOULDING, in particular the potential for a pattern LOCK IN. Sand, when mixed with a binder and prepared for moulding, is a plastic material which is easily compressed over the master pattern’s surface and forced into surface detail. If a portion of the surface detail is sufficiently deep or irregular, it can be correctly described as an UNDERCUT FEATURE, this feature can capture the surrounding sand mould and prevent the pattern’s removal later.
The potential for a pattern lock-in can be greatly reduced in part by thoughtful modelling in the artist’s studio, and later, by the use of adaptive moulding techniques in the foundry. Surfaces and textures on master patterns destined for a sand process should be modelled in a way that avoids sharp, acute angled edges and other over hanging ridges – rounded and undulant textures do not usually cause difficulties. Complicated, deep and angular pattern forms should only be used with caution with some attempt made by the artist to anticipate potential moulding difficulties in the foundry. Experienced artists, familiar with patternmaking for sand casting, are often particularly skilled at modifying problematic forms and features – usually by employing a variety of visual devices to give a viewer the impression of greater surface depth and undercut than which actually exists. Visual devices which simplify form are often applied to pattern elements such as clothing creases (lapels/cuffs), such modelling techniques maintain the impression and impact of deep surface relief, without creating exceptionally difficult moulding challenges for the foundry moulder.
Whilst a great deal can be done by the artist to simplify the piece moulding process, inevitably situations arise whereby design necessity overrides technical considerations, and complex forms or undercut features cannot be avoided. Fortunately art foundry craftsmen are especially skilled and inventive when it comes to devising solutions for moulding technically demanding pattern forms.
If an undercut, or any other master pattern feature, cannot be moulded as a single piece without risk of lock-in, the founder adapts their moulding technique to take account of this. The simplest method of accommodating an undercut is to create additional sand pieces around the feature concerned. The specific design of these additional pieces is usually dictated by the peculiarities of the undercut. Whatever method the founder uses, multiple sand pieces are designed to remove from around the master pattern, whereas a single continuous section of sand would lock-in. The pieces are marked or registered in such a way that they can be sequentially replaced back in position, and then held in situ, either by locking into the surrounding PARENT [MOTHER] mould, or by being fixed with a glue. In examples of some very large scale or complicated moulds, the total number of these individual sand pieces may run to several dozen – though the need to construct an intricate piece mould obviously has some significant impact on the time and cost involved in producing a cast.
Another method used by founders to overcome the potential problems posed by severe undercut and intricate surface detail, is to part sand cast and part lost wax cast the sculpture. This combined approach is now common practice, especially for the production of monumental figurative statuary. The use of the lost wax process offers the artist with the freedom to model fine detail on important features such as the figure’s face or hands. These detailed features can then be cast with little difficulty via a rubber mould and lost wax process. Less intricate portions of the pattern can be sand cast separately, then later reunited with the wax cast sections during the chasing process. This combined approach makes best use of the high quality of cast reproduction offered by lost wax and the cost efficiency of a sand process. With the exception of having to divide the master pattern for a combined process and the occasional difficulty of uniting casts formed by contrasting methods (the cast sections having undergone slightly different shrinkage contractions), there are few reasons not to seriously consider this twin process approach for casting appropriate artworks.
SAND PATTERNS FOR COMMERCIAL FOUNDRIES >
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