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in Liquid Rubber & Plastic for Moldmaking & Casting

No. 23 • Spring 1998


	Sculpture & Foundries

Art Foundry Molding
Flexible Mold Material Options: Urethane, Polysulfide, or Silicone?

     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.


Tips from the Mold Room

Have you ever wished you had a third hand when cutting a seam in a rubber mold? Here's a trick from the Polytek mold shop. Use a hammer to pound an ordinary table fork as flat as possible. Place the flattened fork in a vise and bend the tines at a 90-degree angle to the rest of the fork. Your fork should now look like a small rake. Attach the bent fork securely to a work bench, tines pointed down, so it overhangs about 2 inches. Attach the fork at two points so it can't pivot. The fork can now be used as a stationary "claw" to grip one side of a partially cut seam. While one hand pulls the seam open, the other hand is free to cut.




	Mold-holding claw for easier cutting.

A table fork is ideal, since the tines grip the rubber securely but won't cut into the rubber. This method is equally effective for the zigzag cut frequently used in total-encapsulation block molds, as well as a straight cut following a cut with a mold key knife.

The Best Mold Release for Silicones
     Do not use a silicone mold release with silicone rubbers. Although it is often done and often works just fine, more dependable clean release is obtained when the surface to be covered with silicone rubber is coated with a   petroleum-based release agent. Our tests have shown that occasional sticking results when silicone releases, including Pol-Ease^® 2300, are used beneath both tin and platinum RTV silicone mold compounds.
     The most widely used release is petroleum jelly dispersed in mineral spirits at a 5% to 10% concentration. This solution is sprayed onto the non-porous surface with a simple plant-type spray bottle, and the solvent is then allowed to dry. Light brushing with a soft, dry brush may spread the thin film evenly over the surface; take care not to brush away the coating.
     The petroleum jelly solution can be made by warming it to a liquid state, then stirring or shaking in the mineral spirits. Either fast-drying spirits like VM&P naphtha or regular paint thinner (not lacquer thinner) can be used, but caution should be exercised with respect to flammability of the faster-drying naphthas.
     Of course, the model surface and inside of the mold shell must be nonporous prior to applying most release coatings. Porous surfaces can be sealed with lacquers, paints, paste wax, petroleum jelly, and other suitable sealers. In some cases, we have found the mineral spirits petroleum jelly solution will act as both a sealer and release agent. Remember, a small test cure against any questionable surface is strongly recommended, to be sure proper release and cure is obtained.

Bubble-Free Plaster Castings
If you pour plaster into rubber molds, you cannot do without Pol-Ease^® Mold Rinse.
Pouring plaster into a dry rubber mold results in air bubbles trapped against the surface, yielding a pitted casting. Perfect casts result if your rubber mold is wet with diluted Pol-Ease^® Mold Rinse. The difference is like magic! The cost is practically nothing.
Pol-Ease^® Mold Rinse is made specially for casting plaster into rubber molds. A 5-gallon pail of Mold Rinse costs $27.00; it is diluted with 10 parts water to yield 55 gallons. The rubber mold should be wet with the diluted rinse by any suitable method (like dipping, spraying, or brushing) just prior to pouring your plaster. The rinse must not dry out; the mold surface must be wet with rinse when the plaster is poured. The results are so dramatic, it is hard to believe!

The Trouble with Soap
Potter's soap, green soap, Murphy's Oil Soap, and other aqueous soap solutions are fine for sealing bare, damp plaster and are traditionally used by the plaster trades; they work with plaster because the soap reacts with the plaster surface to form a nonporous waxy film. However, trouble often results when soap is used, especially on nonplaster surfaces, as it remains reactive and can interfere with the proper cure of all types of liquid rubbers.
Even traces of unreacted soap on properly soaped damp plaster will poison the cure of some platinum-cured silicones. We have even seen soap picked up by a urethane rubber mold inhibit platinum silicones poured into that urethane mold. Thus, to be safe, soaps should not be used as a release agent or sealer under most liquid rubbers or resins.


*New!*
SPRAY PLASTIC Coat Carved Foam Make Mold Shells New, tough Poly Plastics and the Polygel^® Sprayer allow sculptured foam and other surfaces to be coated with a thin, tough, plastic coating. *Call for more information on your specific requirements.*



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