An apparatus for forming an object (2) from a dose (3) of plastics comprises die means (54) provided with a cavity (53) for receiving said dose (3) and with at least two parts (35) that are reciprocally movable for shaping an undercut portion of said object (2), fluid means for applying a clamping force so as to keep said at least two parts (35) in contact; said fluid means is such as to increasediscontinuously said clamping force from an initial force that is applicable when said dose (3) is substantially contained in said cavity (53) to a final force applicable when said at least two parts(35) shape said undercut portion.