cushion - shaped packing containers made of a liquid - tight tube which is sealed together at both ends by transverse sealing fins , cannot be used for pressurized contents , because the flexible plastic material of which the containers are made cannot be joined with sufficient strength in the sealing fins . a strengthening element for the packing containers , and in particular for their sealing fins , is disclosed . the sealing fins are strengthened by an external strengthening band which joins the sealing fins . at the same time the fins are held folded down against the outside of the container so that the internal pressure in the container no longer acts upon the material layers sealed together in the sealing fin in a direction away from each other , but is transmitted instead to the seal between the respective fin and the strengthening band which seal can readily be given the required strength . the invention also relates to a method for the manufacture of the packing containers described .