| Casting with Plastic Resins |
Casting is the process of producing a part in a mold. Resins can be cast as a pourable liquid, or they can be "laid up"; laminated by hand with reinforcing cloth of glass or carbon fiber. Here we shall deal mostly with poured casting technique, since lay-up is rarely used by artists. Resin properly filled can cast solid figures 36 inches tall- most sculpture is within those parameters. Most resins can be cast in properly-separating flexible molds made of polysulfide and urethane, but the best results are obtained from silicone molds, which require no separator coat. They can also be cast in rigid molds that have draft (without undercuts, and slightly bigger at the bottom than the top) or composed of multiple drafted sections, if the molds are properly assembled. Rigid molds can be of plaster, fiberglass, metal, and even wood. There are a few types of resin available for casting: acrylics, polyesters, urethanes, and epoxies are the materials most used by artists. To work with acrylics requires some equipment. Casting acrylic 'water-clear' means subjecting the polymer/monomer "dough" to high pressure and high temperature in an autoclave, using special molds. I know a company that has spent 4 years and untold thousands of dollars just trying to develop a reliable acrylic casting technique. Because of this, and the fact that uncured acrylic is highly toxic, most artists should avoid using it and it will not be covered here. Instead, we will concentrate on those resins easily cast in standard molds; the pourable resins that set at room temperature. Polyester resin is the most common castable resin. It is a catalytic system wherein you add a small amount of MEK peroxide to accelerate the polymerization of the liquid resin (which would eventually cure on its own, particularly if left in the sun.) You can control the curing time by varying the amount of catalyst you add. Polyester resin is the resin component of most fiberglass layups and is normally glass clear. Understand that glass is not all that clear-- it's actually green, and so is polyester resin. It can be filled for thicker castings and to affect its appearance. It can be tinted colors both transparent and opaque using special pigments, and, for outdoor use, can have a UV inhibitor mixed in. Polyester resin comes in laminating [hot-promoted] and casting [slow-promoted] formulas. Do not attempt to cast with laminating resin, as it is meant to produce enough heat to cure in thin section. Also, its surface will not fully cure while exposed to air. In laminating fiberglass, this property, called "tack" ensures the next coat will bond well. So when laying up, the final coat needs wax mixed in it, to achieve a good, glossy cure. |
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