The invention concerns a method and an assembly for over-moulding an at least partially tubular element by plastic material compression. Said tubular element has an inner surface (22a) and extends along a elongation direction (8). The invention is characterized in that the method comprises the following steps: a) using a mechanical device (6) extending between a proximal end and a free end, and having a rigidity such that it extends in its radially restricted state along a substantially rectilinear axial direction (8) and said shape is self-maintained. b) placing the mechanical device in its radially restricted state, c) maintaining the mechanical device by its proximal end and inserting the mechanical device inside the tubular element by translation along said elongation direction, d) bringing the mechanical device in its radially expanded state, e) closing said mould for compressing the plastic material.