A resin mold, typically made from flexible rubbers, allows both natural and synthetic resins to be cast into pre-determined shapes. Resin molds can be used to create plastic-like parts for use in manufacturing or for lightweight jewelry components. A silicone resin mold provides the most versatility, as it will not require a separating agent to prevent the resin from sticking. Other resin mold materials include latex, plaster, metal, and fiberglass. Softer mold materials, such as silicon and soft rubbers, make ejecting the finished resin piece simpler, but rigid mold materials, such as plaster, fiberglass, metal, and wood, are less likely to warp with repeated use.
When creating a resin mold, a three-dimensional (3D) object is used to give shape to the mold. This object is called a model. Complex resin molds are made by placing a model in a container and pouring clear rubber around the object. After the rubber dries, it can be cut into two pieces, each piece resulting in a mold for one half of a resin piece.
Sometimes pliable resin molds can lose their shape after several uses. Backup molds are more rigid molds used to support flexible rubber molds during the resin-casting process. A backup mold prevents the primary rubber molds from distorting when casting resin is added. These molds can be made from plaster, fiberglass, or sometimes wood.
Once a resin mold has been created and a separating agent applied, casting resin is poured into the mold to cure. Most resin molds can be used multiple times to create nearly identical resin pieces, especially if used with a backup mold to prevent distortion. Coloring agents can be added to clear resin to change the color of the finished pieces. Small objects or additives can be suspended and sealed within clear resin if added during the casting process.
Natural resin comes from coniferous trees, such as pine, folded birch, and many varieties of palm trees. The sap of these trees, made up of volatile fluid terpenes, is extracted in a viscous state and hardens over time. Natural resin often contains impurities and can be quite unpredictable when exposed to heat.
Synthetic resins are produced through polymerization of molecules to produce a highly stable viscous substance similar to that found in trees. Such synthetic resins have gained popularity due to their purity and more predictable behavior. Low viscosity clear resin, found in craft supply stores, must be combined with a hardening catalyst before it will cure.