A method for manufacturing a hollow body, typically a tank, from a plastic material. The method includes molding a preform into shells in a molding tool; joining an insert to an inner surface of a shell of said shells, the insert defining a sub volume within the hollow body to be made; joining together the shells to form the hollow body by closing the molding tool and applying a main pressure to a main volume enclosed by the shells. It is proposed to apply a sub pressure to the sub volume while applying the main pressure to the main volume, such that a pressure difference between the main volume and the sub volume is bigger than 0.20 bar (P1′−P2′>0.20 bar) and smaller than 3.00 bar (P1′−P2′