A bag forming, filling and sealing machine (10) for producing packaging bags (22) with a standing flat or block-shaped bottom has a moulding pipe (12) whose bottom part (27) has two sections (28, 29) that form a transition from a circular supply cross-section (31) and a rectangular discharge cross-section (33), passing by a square intermediate cross-section (32). The intermediate cross-section (32) almost completely compensates differences in length between the surface lines (1a-1b-1c to 8a-8b-8c) of the moulding pipe (12), thus almost completely preventing the packaging material from forming bulges or undulations at the side surfaces of the packaging bags (22).