Abstract: A core usable as a rotor in humidity exchangers and heat exchangers is disclosed.The core is comprised of alternate layers of flat and corrugated sheet material so arranged that the corrugations form with the flat layers a plurality of fine channels running substantially parallel to each other and axially to the center of the core. The corrugated layers in the axial direction extend at least at one end face of the core beyond the flat layers and the extending corrugated edges are reinforced. A method and apparatus for forming the core is also disclosed.