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Art foundries that employ the lost wax method to create bronze castings have used a variety of flexible mold materials over the years. Since the lost wax procedure involves casting wax, polyurethanes, polysulfides or silicones can be used since each withstands repeated casting with wax. The best choice among these options is influenced by several factors, which will vary depending on the object being molded. Some primary factors to be considered are: 1) type of model material, 2) size of the model, 3) storage requirements for the completed mold, 4) available equipment, and 5) price of the mold material.
Model materials interact differently with various liquid mold rubbers. This is important to realize since most foundries accept pieces from a number of sculptors using different clays, etc. Water clays need to be sealed and released properly before polyurethanes are used, since moisture can adversely affect the surface of the rubber. Polysulfides and silicones are less likely to react with water clays, although it is generally advisable to seal such masters. Sulfur clays may virtually preclude the use of silicone mold rubbers since sulfur inhibits the cure of many silicone mold surfaces. Inhibition results in a thin, sticky, uncured layer of silicone which never fully cures. Tin-catalyzed (condensation-cured) silicones are less susceptible to inhibition than platinum-catalyzed (addition-cured) systems, but caution is still recommended. A small test cure should always be done before starting an actual project. For more information on model preparation, please consult Polytek®'s Manual & Catalog or individual technical bulletins.
The size of the model will also play a role in mold rubber choice. Small to midsize models can be poured or brushed. All of the mold rubbers listed above can be poured or brushed. For brushing, Cab-O-Sil® or, in certain cases, liquid thickeners are added to the mixed rubber to make them thixotropic so liquid doesn't run off vertical surfaces. Self-thickening Polygel® rubbers can be used directly without the need for additional thickening. When mixed 1:1, Polygel® becomes thixotropic, making it the easiest brushable system to use. As models get larger, spraying rubber becomes a viable option. Polygel® Spray 50 has been specifically designed for such applications. Spraying is an economical alternative to brushing for larger models since it greatly reduces the time it takes to apply the rubber, as well as helps to reduce the amount of wasted product. Silicones can also be sprayed, although the equipment needed is much more expensive than that used with Polygel®. Polygel® can be sprayed with equipment costing as little as $3,500, whereas silicone sprayers cost upwards of $15,000. A silicone sprayer is not only a costly initial investment, but is also a commitment to use the most expensive type of mold rubber.
If pouring is the technique of choice, additional equipment may be necessary when using certain silicone rubbers. Silicones often have a much higher viscosity than polyurethanes or polysulfides, so it is advisable to use a vacuum chamber to degas the mixed rubber. Vacuuming insures the speedy removal of trapped mixing air and better, bubble-free mold surfaces when using silicone. Vacuuming may be used when silicones are brushed as well, but the brushing action itself helps to remove trapped air. There are low-viscosity silicones available (such as PlatSil® 71-10) which may not require degassing.
Most foundry customers want their molds stored for lengthy periods of time in the event additional castings are required in the future. Polysulfides, polyurethanes (such as Polytek®'s 74 Series rubbers), and platinum silicones (the PlatSil® 71 and 73 Series systems) are the best mold materials for long-term storage. Tin silicones have a cured shelf life of one to several years, making them much less attractive in this regard. Polysulfides may be susceptible to cold-flow, meaning, if not properly supported in a shell, they can deform with age. Platinum silicones and most polyurethanes (Poly 74 Series rubbers) are permanent rubbers when cured, making them the best choices for the long haul. If storage time is important, these considerations can prevent the costly process of having to make a second mold in the future.
Last but not least, mold material costs are important
to understand. Silicones are the most expensive rubbers, followed by polysulfides,
then by polyurethanes. The relative low cost of polyurethanes, coupled
with the fact they can be poured, brushed, or sprayed with easy 1:1 mix
ratios, makes them the most popular mold rubber now used by foundries.
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