Abstract: An inflatable core for use in forming a thermal break in a metal frame for a door and/or window by moulding and curing thermal barrier material, such as resin, in two parallel, laterally spaced, elongate gaps defined between respective pairs of confronting elongate surfaces of the inner and outer frame elements which when assembled form a box-section member. The gaps are converted into respective mould cavities for the resin by the pressure of shoes against the elements adjacent the respective gaps to close one side thereof, the shoes being removed only after the resin has cured to leave the finished thermal break. The shoes are carried on opposite walls of a laterally expansible tube which is inserted into the box-section and is expanded to press "non-stick" surfaces of the shoes against the respective gaps ready for the moulding and curing of resin therein, while maintaining the gap dimensions.