System for the injection moulding of plastic material, in particular of pieces (PZ, PZ-10) exhibiting the form of a solid of revolution with external and/or internal cylindrical surfaces or the like, comprising: - an injection device for the injection of a plastic material, in the molten state, into a mould (11, 11b); and - a circular diaphragm for distributing circularly and evenly into the cavity of the mould, through an opening or circular injection gate having a circular form, the plastic material that is injected by the injection device; characterized by control means (21, 22, 23, 24), including a heating inductor, adapted to be driven in frequency, and a temperature sensor, both arranged in the zone of said circular injection gate, in that said control means are adapted to constantly control, during a moulding cycle of a piece and as a function of the temperature detected by said temperature sensor, the temperature (T, T+, T-) of the zone of said circular injection gate, by suitably driving said heating inductor, and to control in real time the phases of said moulding cycle, including the phases of injection of the plastic material into the mould and of the opening and closing of the mould, and in that said control means (23, 24) are further adapted to control in real time, through said heating inductor, the temperature (T, T+, T -) of the zone of said circular diaphragm, during the injection phase of the plastic material into the mould, also by taking into account data (D-1, D-2), contained in a data base, the data concerning the characteristics (D-1, T1, T2, T3, T4, D-2), in particular physical and rheological (D-2, G', G ", tan(d)), of the plastic material that is used to mould the piece (PZ, PZ-10). - an injection device for the injection of a plastic material, in the molten state, into a mould (11, 11b); and - a circular diaphragm for distributing circularly and evenly into the cavity of the mould, through an opening or circular injection gate having a circular form, the plastic material that is injected by the injection device; characterized by control means (21, 22, 23, 24), including a heating inductor, adapted to be driven in frequency, and a temperature sensor, both arranged in the zone of said circular injection gate, in that said control means are adapted to constantly control, during a moulding cycle of a piece and as a function of the temperature detected by said temperature sensor, the temperature (T, T+, T-) of the zone of said circular injection gate, by suitably driving said heating inductor, and to control in real time the phases of said moulding cycle, including the phases of injection of the plastic material into the mould and of the opening and closing of the mould, and in that said control means (23, 24) are further adapted to control in real time, through said heating inductor, the temperature (T, T+, T -) of the zone of said circular diaphragm, during the injection phase of the plastic material into the mould, also by taking into account data (D-1, D-2), contained in a data base, the data concerning the characteristics (D-1, T1, T2, T3, T4, D-2), in particular physical and rheological (D-2, G', G ", tan(d)), of the plastic material that is used to mould the piece (PZ, PZ-10).