corner areas openings for assembly tightening devices 10 which in the assembly serve first for guiding the support plates 1 2 to be stacked on top of each other support plates 1 2 provided with cut diaphragm sections m1 and m2 are placed upon assembly tightener 10 in pairs and with the diaphragms placed on top of each other as shown in fig2 and put together to form a stack of 10 or 20 support plates 1 2 with the aid of the assembly tightener 10 and pressing means not shown the superposed support plates 1 2 are firmly pressed together all non recessed surfaces of support plates 1 2 in this structure rest in plane parallel position on the counter surfaces of the other support plates in this preassembled state a sealing means 15 of a flowable synthetic resin adhesive or thermoplastic material is injected into the perforations 11 the injection may be made into the left lower perforation 11 as shown in fig2 if the package of support plates 1 2 is positioned so that the diagonal of the two opposite perforations 11 is in vertical position the injected sealing means 15 rises in both directions in cutoff channels 11 and grooves 14 in the direction of the highest perforation 11 and displaces the enclosed air the hollow spaces formed by perforations 11 cutoff channels 13 and grooves 14 14 39 since the sealing means 15 consists of plastic material synthetic resin and or adhesive which is flowable but thermally and or agehardenable a stable tight mutual connection of the individual support plates.