A method for belling pipes made of thermoplastic material comprising the following steps: - preparing a forming pad designed to deform into the shape of a bell an end portion of a pipe made of thermoplastic material, the pad having an axially symmetric shape about a respective central axis, and having a tapered front part, to facilitate insertion of the pad inside the pipe, and a substantially cylindrical rear part, - preparing an annular gasket on the pad in a predetermined position, the annular gasket being designed to be stably positioned in the bell to be formed, - preparing an annular element fitted to and sliding on the rear cylindrical part of said pad and designed to move relative to said pad in the direction of its axis, to adopt different reciprocal positions, said annular element having a first cylindrical portion fitted on said rear part of the pad, a flange supporting the cylindrical portion, and a plurality of actuating means, designed to slide the above-mentioned cylindrical portion relative to the pad, - heating the end portion to a predetermined temperature designed to allow the deformation, - moving relative to each other the end portion of the pipe and the pad, towards each other in the direction of a longitudinal central axis of the pipe, for fitting the end portion on the assembly of the pad and the annular gasket, - retaining the annular gasket in its predetermined position during the relative movement of the end portion and pad, - generating a negative pressure at a space between the pad and the end portion of the pipe at the annular gasket, - moving the annular element relative to the pad so that the annular element reaches a contact position in which its cylindrical portion is placed frontally in contact with an end face of the end portion of the pipe, - forcing an end edge of the portion against the gasket, characterised in that the step of forcing is actuated by activating the actuating means, with the annular element in the contact position, so that the actuating means exert an axial pushing action on an end face of said portion towards a front face of the gasket to optimize the adhesion of the wall of the pipe on this surface of the gasket, said axial pushing action on the pipe, being exerted by said actuating means through the annular element, constituting a combined bending and compressive stress. - preparing a forming pad designed to deform into the shape of a bell an end portion of a pipe made of thermoplastic material, the pad having an axially symmetric shape about a respective central axis, and having a tapered front part, to facilitate insertion of the pad inside the pipe, and a substantially cylindrical rear part, - preparing an annular gasket on the pad in a predetermined position, the annular gasket being designed to be stably positioned in the bell to be formed, - preparing an annular element fitted to and sliding on the rear cylindrical part of said pad and designed to move relative to said pad in the direction of its axis, to adopt different reciprocal positions, said annular element having a first cylindrical portion fitted on said rear part of the pad, a flange supporting the cylindrical portion, and a plurality of actuating means, designed to slide the above-mentioned cylindrical portion relative to the pad, - heating the end portion to a predetermined temperature designed to allow the deformation, - moving relative to each other the end portion of the pipe and the pad, towards each other in the direction of a longitudinal central axis of the pipe, for fitting the end portion on the assembly of the pad and the annular gasket, - retaining the annular gasket in its predetermined position during the relative movement of the end portion and pad, - generating a negative pressure at a space between the pad and the end portion of the pipe at the annular gasket, - moving the annular element relative to the pad so that the annular element reaches a contact position in which its cylindrical portion is placed frontally in contact with an end face of the end portion of the pipe, - forcing an end edge of the portion against the gasket, characterised in that the step of forcing is actuated by activating the actuating means, with the annular element in the contact position, so that the actuating means exert an axial pushing action on an end face of said portion towards a front face of the gasket to optimize the adhesion of the wall of the pipe on this surface of the gasket, said axial pushing action on the pipe, being exerted by said actuating means through the annular element, constituting a combined bending and compressive stress.