Consumer packages for, for example, liquid or pumpable foods are often manufactured from web or sheet-shaped material.  The material may be a plastic film or a packaging laminate which includes layers of different material types.  At least one surface layer of the packaging material often consists of thermoplastic which, on the one hand, ensures the liquid-tightness of the material and, on the other hand, makes it possible to thermoseal the material to itself in liquid-tight fashion.  A packaging material may also be given satisfactory tightness against both gas and liquid penetration with the aid of thin layers of so-called barrier plastics, for example ethyl vinyl alcohol (EVAL).  Different methods may be employed in order to ensure that the material obtains self-supporting properties, i.e. displays sufficient rigidity to be usable for the manufacture of configurationally stable, e.g. parallelepipedic, packaging containers.  The material may, for example, include a layer of flexurally rigid material, for example paperboard or metal, or be given the desired rigidity in that the layers which are included in the material and which per se are of flexible plastic material may, with the aid of, for example gas (normally air), be kept at such mutual spaced apart relationship that so-called bulkhead effect is achieved. 
A number of variations of the above-mentioned packaging material types are known in the art, and one packaging laminate whose flexural rigidity is achieved with the aid of gas-filled cells acting as spacers is disclosed and described, e.g. in European Patent Application EP 94105450.4.  The packaging laminate disclosed in this publication comprises a chamber defined between the laminate layers, the chamber being divided by means of a number of linear, substantially parallel seals, into a large number of inflatable cells.  The cells, which extend substantially horizontally over the walls of the finished packaging container, keep the material layers included in the laminate at such spaced apart relationship from one another that the previously mentioned bulkhead effect is achieved, which, despite the absence of conventional rigidifying material layers, results in a configurationally stable packaging container possessing superior rigidity.  The packaging container is substantially paralelepipedic with a number of continuous wall panels which, along a vertical edge of the packaging container, are united in a liquid-tight, longitudinal sealing joint.  When the cells of the packaging laminate are filled with gas, a contraction of the material takes place, with the result that the height of the finished packaging container is less than the height (length) of corresponding packaging material in the deflated state.  The height loss which takes place when the individual cells are filled with gas is, however, counteracted at the vertical corner of the packaging container, where the sealing joint is located, with the result that the packaging container has gently curved configuration.  Attempts to make the sealing joint narrower in order thereby to reduce its obstructive effect on the shrinkage of the material in connection with its inflation have only been partially successful and there is thus a need in the art to provide an arrangement which greatly reduces or wholly obviates the above mentioned drawback and results in the packaging container having, in the inflated "finished" state, uniformly high vertical edges and thereby a straight, symmetric (e.g. parallelepipedic) appearance.