packaging containers of single - use disposable type which are manufactured from flexible paper / plastic laminate normally display sealing fins which extend over one or more of the walls of the packaging container . in parallelepipedic packaging containers , triangular corner flaps moreover occur over which extend the sealing fins . the corner flaps are folded down and secured on the final formation of the packaging container against its side wall . this however entails that the material with many times thickness must be double - folded , which entails deformations and stretching which damages the packaging material so that this no longer remains completely liquid - tight and gas - tight . according to the invention , the above - outlined drawbacks may be obviated in that the fibre layer included in the material is heated to a temperature of between 80 and 250 ° c . in the fold region , whereby the material softens so that the downward folding of the corner flaps together with subjacent parts of the sealing fin may take place without the material being damaged . the liquid - tightness and gas - tightness of the finished packaging container will hereby be considerably improved , which is crucially important in the packaging of , for example , liquid foods .