A method for manufacturing an exchanger includes the steps of folding sheet material in a series of pleats, placing the folded sheet material in a casing having an opening defined by walls with distal edges, covering the distal edges with end portions of the sheet material, and affixing a cover to the casing by simultaneously melting a portion of the cover and the sheet covered distal edges of the walls, and contacting the cover with the sheet material covering the distal edges of the walls. The method may also include melting a portion of the cover and/or the casing and placing the melted cover on the casing so that the melted portion contacts pleats of the sheet material, and permitting the melted material to cool and solidify and form a bond with the sheet material. The method may further include melting the side edges of the folded sheet material as well as the four edges of the casing and cover and the inner surfaces of the side walls, and then bonding the side walls to the melted folded material and the melted casing cover edges.