A method for manufacturing a packaging container consisting of a bottom and from this bottom up-folded side walls, which are fixable relative to each other in an erected state by liquid-tight side joints, wherein a fiber based packaging material preferably having at least one side coated with a heat sealable layer is formed into said packaging container in a mold having two relative to each other moveable halves constituting an outer part and an inner part for injection molding in which a plastic melt of preferably thermoplastic, liquid-tight, joins the side walls of the packaging material with their edge parts by said side joints and creates a circumferential opening edge of thermoplastic formed integrally with said side joints, said plastic melt during injection molding only fills first cavities on the inside of the packing material and second cavities designed to accommodate the edge parts of the side walls, where the plastic melt during injection fills these cavities (22,23) without contact with the outside of the packaging material and thus connects the sidewalls of the packaging material with each other, while the plastic melt also fills the cavity of the circumferential opening edge for the creation of a liquid tight cup shaped packaging container, whereby in case of injection molding of the packaging container, the side walls of the packaging material are pressed against the outer part of the mold by the incoming plastic melt, which is controlled so that the plastic melt from the injection point until it reaches the cavity is distributed into two main flow channels (28,29) separated from each other, and further up into the side joints or in the so-called leg-formed cavities and there branches into a plurality channels, that the molding outer part of the mold is designed with flat surfaces for receiving the packaging material which is pressed directly against the same without overlapping, and that the inner part of the mold having edge provided longitudinal recesses, the edges (25,26) of which are intended to press the packaging material between itself and the outer part of the forming mold and are extending just outside and on each side along the side joins, said edges (25,26) during their contact against the packaging material will be pressed against said packaging material during the injection molding and forming first cavities between themselves within or inside second cavities, which are formed and intended to receive the side joints of the packaging material for enabling filling of the plastic mass both in between the edge parts of the side walls and in the first cavities, forming the recesses located between the edges (25,26), characterized in that a protruding part, working as a stacking device for stacking multiple cups into each other, towers the pressure of the plastic melt when reaching this point, whereby the protruding part, located just adjacent to the packing material side ends, creates a controlled deceleration of rushing plastic flow, by a relatively large volume of the protruding part, which when filled with plastic melt lowers the pressure and thus creates a delay in the filling of the cavity, which facilitates for the plastic flow to press the packaging material against the outer part of the mold, while first thereafter also fills the strip-formed legs and side joints upwards.