An electronic device may have electronic components enclosed within a plastic housing. The plastic housing may include housing members that are joined using heat activated thermoset polymer film. The heat activated thermoset polymer film may be heated using a metal structure such as a strip of metal that is placed along a joint between the housing members. The temperature of the metal strip and associated layers of the thermoset polymer film may be raised by applying current to the metal strip using an external tool or the electronic components within the housing. Heat activated thermoset polymer film may be heated using a reactive multilayer metal foil. Heat sink layers of metal may be interposed between the reactive multilayer metal foil and the thermoset polymer film. A heated metal ring or a heated edge portion of a plastic housing member may also be used to heat the thermoset polymer film.