Curved surfaces of a preform of thermoplastic adhesive provide improved regulation of heating and exclusion of gas as surfaces to be bonded are heated and pressed against the thermoplastic adhesive preform. Particulate or filamentary materials can be added to the thermoplastic adhesive for adjustment of coefficient of thermal expansion or further increase of heat transfer through the adhesive or both. The preform is preferably fabricated by molding, preferably in combination with die-cutting of a preform of desired volume from a web of approximately the same thickness as the completed bond.