A method for manufacturing an injection nozzle unit for the moulding of plastics materials, comprising the steps of: - providing a nozzle (11, 12) with a tubular metal core forming a central longitudinal injection duct and a substantially cylindrical, outer, lateral surface, - manufacturing a heater/diffuser device including a metal heat diffuser element of tubular cylindrical shape with an open annular cross-section, and - mounting said heater/diffuser device around the cylindrical outer surface of the nozzle core, with a substantially cylindrical internal surface of said heat diffuser element fitting the cylindrical outer surface of the nozzle core, and with two facing longitudinal free edges (30e, 30f) of the same heat diffuser element being spaced apart circumferentially (d), being free of one another (d), and defining between them a longitudinal space extending along a generator line of the cylindrical surface of the heat diffuser element mounted around the cylindrical outer surface of the metal nozzle core, wherein the step of manufacturing said heater/diffuser device and the respective metal heat diffuser element of tubular cylindrical shape with said open annular cross-section comprises the following steps: - producing, by starting with a flat metal sheet, a substantially rectangular flat plate having a height (h) and a width (w) corresponding respectively to the length of the nozzle core and to slightly less than the circumference of its cylindrical outer surface; - processing an internal face of said plate, or alternatively of the starting metal sheet, corresponding to the internal surface of said heat diffuser element, so as to form a channel-like seat having a substantially C-shaped or U-shaped cross-section suitable for accommodating a wire resistor exactly, or with a predetermined minimum clearance, or with slight interference; - fitting the wire resistor in said channel-like seat; - placing said plate, with the resistor fitted therein, in a plurality of tools (P1, P2, P3, P4) adapted to bend the plate, so as to bend the two opposed longitudinal edges (30e, 30f) of the plate in the direction in which the respective internal face faces, and to impart a C-shape or a U-shape to the plate; and - finally bending said plate to give said heat diffuser element its final open tubular cylindrical shape, wherein the final bending step of the plate for giving the heat diffuser element its final open tubular cylindrical shape is carried out by using and placing, into contact with the internal face of the plate, a cylindrical tool, having a diameter slightly smaller than that of the cylindrical outer surface of the nozzle core on which the heat diffuser element has to be fitted, whereby the heat diffuser element has an internal diameter, in a free or undeformed condition, slightly smaller than that of the cylindrical outer surface of the nozzle core and consequently it is mounted on said cylindrical outer surface with radial interference so as to grip the nozzle core resiliently, and wherein the distance (d) between the longitudinal free edges (30e, 30f) of the heat diffuser element mounted around the cylindrical outer surface of the metal nozzle core is such that, when the nozzle unit is heated by the supply of electrical current through the wire resistor, the heat diffuser element is substantially free to expand thermally by extending in the circumferential direction around the nozzle core without moving radially away from the outer surface of the nozzle core. - providing a nozzle (11, 12) with a tubular metal core forming a central longitudinal injection duct and a substantially cylindrical, outer, lateral surface, - manufacturing a heater/diffuser device including a metal heat diffuser element of tubular cylindrical shape with an open annular cross-section, and - mounting said heater/diffuser device around the cylindrical outer surface of the nozzle core, with a substantially cylindrical internal surface of said heat diffuser element fitting the cylindrical outer surface of the nozzle core, and with two facing longitudinal free edges (30e, 30f) of the same heat diffuser element being spaced apart circumferentially (d), being free of one another (d), and defining between them a longitudinal space extending along a generator line of the cylindrical surface of the heat diffuser element mounted around the cylindrical outer surface of the metal nozzle core, - producing, by starting with a flat metal sheet, a substantially rectangular flat plate having a height (h) and a width (w) corresponding respectively to the length of the nozzle core and to slightly less than the circumference of its cylindrical outer surface; - processing an internal face of said plate, or alternatively of the starting metal sheet, corresponding to the internal surface of said heat diffuser element, so as to form a channel-like seat having a substantially C-shaped or U-shaped cross-section suitable for accommodating a wire resistor exactly, or with a predetermined minimum clearance, or with slight interference; - fitting the wire resistor in said channel-like seat; - placing said plate, with the resistor fitted therein, in a plurality of tools (P1, P2, P3, P4) adapted to bend the plate, so as to bend the two opposed longitudinal edges (30e, 30f) of the plate in the direction in which the respective internal face faces, and to impart a C-shape or a U-shape to the plate; and - finally bending said plate to give said heat diffuser element its final open tubular cylindrical shape,