A mould carrier unit having a stretching rod (10) borne by a slide (13) mobile between an inactive position and an active position where its free end reaches the bottom of the mould (2); a nozzle (15) including a fixed body (16) extended by a bell (20), which is mobile from an inactive position where its free edge (25) is held substantially at the same level as the free end (11) of the stretching rod (10) to an active position where it is sealingly applied against the inlet surface (4) of the mould (2), to close the circuit of the blowing fluid. The bell (20) includes a bar (21) provided with a stop (23), which stop (23) cooperates with a support (24) formed on the slide (13) to permit the bell (20) to pass from its active position to the inactive position with the same law of motion as the stretching rod (10).