A very important aspect of thermoset resins is their cure cycle. Unsaturated polyester and vinylester, along with epoxy, require time and temperature in order to achieve what we call “Crosslinking.” This is the the “set” part of thermoset, and is the permanent and irreversible chemical bonds in the resin. The amount of time and temperature is dependent upon the formulation of the resin, the ratio of resin-to-hardener, and the presence of additional chemicals used to modify the properties.
Outside of the chemistry, the control of the time and temperature is important to the curing of the resin. If the actual temperature is outside the range of the intended formulated temperature, it will affect the curing reaction. If the part is demolded too early, the resin will continue to cure, but the final shape of the part may not match the mold. The manufacturer of the resin is the very best source for information on the recommended cure time and temperature.
As these resins change from liquid to solid states, there is a certain amount of shrinkage involved. A part made on a female mold will shrink towards the center, and a part made on a male mold will tighten around that mold. This shrinkage factor depends upon the resin chemistry and its additives, but is generally less than 3% by volume. This is why male molds more difficult to demold, and the design of the mold needs to account for part shrinkage and part removal.