Plaster itself may be cast, as can other chemical setting materials such as concrete or plastic resin - either using single use waste molds as noted above or multiple use piece molds, or molds made of small ridged pieces or of flexible material such as latex rubber (which is in turn supported by an exterior mold). When casting plaster or concrete the finished product is, unlike marble, relatively unattractive, lacking in transparency, and so is usually painted, often in ways that give the appearance of metal or stone. Alternatively, the first layers cast may contain colored sand so as to give an appearance of stone. By casting concrete, rather than plaster, it is possible to create sculptures, fountains, or seating for outdoor use. A simulation of high quality marble may be made using certain chemically set plastic resins (for example epoxy or polyester) with powdered stone added for coloration, often with multiple colors worked in. The later is a common means of making attractive washstands, washstand tops and shower stalls, with the skilled working of multiple colors resulting in simulated staining patterns as is often found in natural marble or travertine.
Shell molding
Shell molding is also similar to sand molding except that a mixture of sand and 3-6% resin holds the grains together. Set-up and production of shell mold patterns takes weeks, after which an output of 5-50 pieces/hr-mold is attainable. Aluminium and magnesium products average about 13.5 kg as a normal limit, but it is possible to cast items in the 45-90 kg range. Shell mold walling varies from 3-10 mm thick, depending on the forming time of the resin.
There are a dozen different stages in shell mold processing that include:
Initially preparing a metal-matched plate
Mixing resin and sand
Heating pattern, usually to between 505-550 K
Inverting the pattern (the sand is at one end of a box and the pattern at the other, and the box is inverted for a time determined by the desired thickness of the mill)
Curing shell and baking it
Removing investment
Inserting cores
Repeating for other half
Assembling mold
Pouring mold
Removing casting
Cleaning and trimming.
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