A Relative Comparison of Flexible Mold Materials and Their Attributes Whenever a mold making project is begun, it is always a good idea to review some basic requirements before starting. First, what casting medium is going to be cast into the mold? Second, how many reproductions are necessary? Third, how long is the mold to be kept? Fourth, what form of mold is to be made? (Is the mold going to be an open face block or slab mold, is it going to be shelled, etc.) Once these questions have been answered, it is then much simpler to choose the proper mold material for any given project. POLYMER USES ADVANTAGES DISADVANTAGES POLYESTER A highly versatile resin used for producing everything from boats to statuary. Low cost, easy to use, very forgiving as far a accepting additives. Available in water clear version for clear casting Somewhat noxious odor, relatively high shrink rate in unfilled castings, potential for high exotherm during cure. Polyester resin is probably the most versastile resin available. It can be used for casting, laminating or coating. A wide variety of fillers is available to enhance the resin to simulate everything from marble to bronze. It can be thickened or reinforced by the addition of various silica thickeners and fibrous reinforcing agents such as glass, polypropylene, graphite or kevlar. EPOXY Used mainly as a component in fiber reinforced laminates for boats and aircraft. Commonly used in adhesives. Widely varied resins and hardeners available to "fine tune" for any specific application. Moderate to high cost depending on resin type and hardener used. Can cause skin sensitization. Innumerable variations available for any one of a number of applications. Primarily one or two basic resins are used for these systems with different hardeners. Can also be used for "cold casting", but is more expensive than polyester and not all fillers are compatible with all systems. POLYURETHANE Probably the largest family of polymers in use today. These materials are used for everything from auto bumpers to refrigerator magnets. Because of their current popularity, there are a lot of choices out there. Some set in as little as three minutes; others can be used to simulate thermoplastics such as ABS. Polyurethane foam can be used to simulate wood. High cost, moisture sensitivity, varying degrees of tenacity. Polyurethanes are somewhat toxic and can cause systemic sensitization. Highly aggressive when casting into molds; a good release agent is always necessary. This family of products contains some of the most user friendly materials as well as the most high tech products available today. Not easily modified because of moisture sensitivity, polyurethanes are usually used as supplied by the manufacturer, but everything from low density foams to heavily filled resins are available. Once cured these materials are very stable and resistant to attack by most solvents.