Methods of forming dental appliances. The methods may include partially curing a prepolymer mixture in a first mold to form a partially cured polymer mixture. The first mold may have an internal cavity that is shaped to provide the partially cured polymer mixture with a first shape having a channel in accordance with a dental arch. The first shape may have a greater thickness at locations corresponding to where a gingival line of the dental appliance will be formed compared to a thickness at occlusal surfaces of the dental appliance. The methods may include modifying the first shape of the partially cured polymer mixture to form a shell having cavities shaped to receive teeth. The methods may also include fully curing the shell by exposing the partially cured polymer mixture to energy until the partially cured polymer mixture thermosets.