An injection moulding system (10; 110; 210) of plastic material, comprising: a press (11; 111; 211) having a first (11a) and a second (11b) pressure plate, suitable for closing between them a mould (13; 113; 213), by applying on it a closing force (F), and a distribution and injection assembly (12; 112; 212), suitable for distributing and injecting a fluid plastic material (MP) into the mould (13; 113; 213) and in turn comprising an intermediate hot distribution plate (23; 123), interposed between a bottom plate (21) and a nozzle-bearing plate (22), provided for cooperating into contact and in relationship of sliding, along a respective sliding and contact surface (23a), with the end (17a) of one or more injection nozzles (17; 117), fixed on the nozzle-bearing plate (22), in order to distribute the fluid plastic material (MP) to be injected into the mould (13), wherein the distribution and injection assembly (12; 112; 212) is arranged adjacently to and along a side (13c: 113c; 213c), of the mould (13; 113; 213), not associated with the first and the second pressure plate (11a, 11b), so as not to be subject to the closing force (F) applied by the press (11) on the mould, and moreover wherein it (12; 112; 212) is sized (21a; 121a) in such way to ensure a perfect sealing against any outflow or leakage toward the outside of the fluid plastic material (MP) which flows, through such sliding and contact surface (23a), from the intermediate hot plate (23) to the injection nozzles (17) for being injected into the mould, despite the distribution and injection assembly (12; 112; 212) is not subject to the action of the closing force (F), applied by the pressure plates (11a, 11b) of the press (11; 111; 211) on the mould (13; 113; 213) during the step of moulding of a piece (P).