Aluminum laminae are heat-sealed onto the inside of flat-bottomed plastic bags so that, by bending the lamina double after the bag has been cut open to remove part of its contents, the bag can be reclosed. The apparatus for sealing the laminae on can be incorporated in an existing bag-filling installation. An aluminum strip is supplied to a punching device from a supply roll. A punched-out aluminum lamina is placed upon the upper part of a conveyor lever. The lever is rotated through 180.degree. so that the aluminum lamina to be sealed on comes to be positioned between two layers of a web of plastic material fed by rollers. The ends of the lever are preheated by heating elements in order to accelerate the sealing-on operation. The aluminum lamina is heat-sealed onto the inside of the folded web in such a way that the longer edges of the lamina form an angle of about 45.degree. with the flat bottom of the bag. The rate of production of the existing filling installation can be maintained despite incorporation of the sealing-on apparatus.