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Casting resins fall into three classes, polyester, epoxy and polyurethane resins. Polyester resin is most commonly used in the construction of molded reinforced fiber and composite products, so its mechanical properties are reinforced with the fiberglass itself. The polyester resin used in the molding application is a viscous liquid (like honey) requiring the addition of catalysts and accelerators to complete the curing process. Polyester resins are contact products which require no pressure to cure and can be cured from a fluid or solid state. There are a number of safety issues in using polyester resin as it can be dangerous with its very noxious odor. The user must wear a respirator and work with a hood or in a well ventilated area. In addition, it is not UV light resistant which will cause it to yellow with time. Because it does cure very hard, polyester resin projects may break if dropped on a hard surface.
The advantage of polyester resin is that it is easy to use and is the lowest cost resin among resin products. However, there are a number of critical disadvantages in that the resin without the addition of fiberglass has poor mechanical properties, it’s emission during use in open molds is considered carcinogenic and it has a high and unpredictable shrinkage rate.
Most of us are familiar with polyester resin as it is the binder in fiberglass products – it is the polyester resin that produces the distinct and often off putting odor of fiberglass. Because of its health hazards we do not offer this product.
Though a respirator is not needed while using epoxy resins, epoxy resins get more expensive the clearer you want your finished casting to be. Unlike faster setting polyurethane resins, epoxy resins can take from several hours to several days to completely cure.
Then there are polyurethane casting resins. They are fast setting and our product offerings do not need a respirator as they are very low in odor emission. These polyurethane casting resin products are two-part, 1-to-1 mix ratio formulas that set from 5 to 20-minutes. The two varieties offered here are opaque and a water clear resin. The opaque resins set to an ivory color in approximately 5-8 minutes. The clear casting resins require more setting time up to about 25-minutes. The advanced formula clear resins set with fewer air bubbles than most available formulas. We even offer a unique polyesher resin formula that will expand (point up) your castings up to 160%
However, no water clear polyurethane casting resin is completely bubble free. Therefore, the use of a pressure pot is recommended to perfect the casting without bubbles. The advantage of polyurethane resins over polyester resins is that these formulas are very low in odor compared to carcinogenic producing smells of polyester resins.
Our polyurethane resins are also the perfect choice for cold casting when combined with our metal, stone and wood powders to create beautiful cold castings that exhibit the realistic look of rich metals, stone and wood..
As a caution, polyurethane is very moisture sensitive. It will not cure well in humid climates or in molds that contain water. You can color polyurethane before casting using our Circius Polyurethane Dyes.