A blow-molding or stretch-blow molding plant for making containers from thermoplastic polymer blanks, in particular PET, including a blowing bell-nozzle for blowing a pressurized fluid into a blank arranged in a cavity of a mold, the nozzle having one end adapted to be sealingly supported in front against a support plate provided in one surface of the mold capping the neck of the blank; the support plate is ring-shaped having an outline of similar shape as that of the outline of the end of the nozzle, the ring being engaged in a gutter which is hollowed out in the surface of the mold and which surrounds spaced apart the opening through which the neck of the blank passes.