A device and method for the fluid-tight sealing of two partially overlapping packaging components, one being a tubular base member formed from a blank of composite cardboard/plastics material, and an additional packaging component forming the upper packaging portion or packaging base, having at least one mandrel and one die, the mandrel having at least two expansion elements which are moved from the operating position thereof towards each other into a rest position, the expansion elements having sealing jaws which, in the operating position thereof, form a closed circumferential outer contour, which corresponds to an inner contour formed by an opening in the die, there remaining between the inner contour and outer contour a narrow annular gap in order to apply a pressing force from the inner side to the overlap region of the packaging components.