Injection unit (10; 1 10; 210) for injection moulding of plastic material, associated, in a wider moulding system or apparatus, with a hot distribution plate and a back driving plate, comprising: - an injection nozzle (1 1); - a closure pin axially movable, in said injection nozzle, between a first operating position in which the closure pin frees and opens an injection hole so as to allow the flow, through said injection hole, of the plastic material in the molten state towards a mould, and a second operating position in which the closure pin closes the injection hole, so as to interrupt the flow, through said injection hole, of the plastic material towards the mould; and - a guide and cooling assembly (14; 1 14; 214), interposed between said back driving plate and said hot distribution plate; wherein said hot distribution plate is adapted to distribute and feed the molten plastic material to the injection nozzle in order to be injected into the mould, and said driving plate houses the driving means for driving the axial motion of the closure pin between said first and second operating positions; characterized in that said guide and cooling assembly (14; 114; 214) in turn comprises: - a guide and seal element, in particular in the form of a bush, defining a guide hole along which the closure pin extends axially, said guide hole having both a guiding function to guide the same closure pin in its axial motion between said first and second operating positions, and a sealing function against leakage at the outside of the plastic material, in the molten state, distributed and supplied by the hot distribution plate; - a cooling ring; - and pressure means (14c, 14c-1; 14c-2; 14c-3), in particular in the form of an elastic spring or an O-ring type ring, for pressing the cooling ring directly against and in contact with said driving plate; in that said guide and seal element is slidably coupled at an end (14a") with a central hole of the cooling ring and is fixedly attached at the other end to the hot distribution plate; and in that said cooling ring, pressed by said pressure means (14c, 14c-1; 14c-2; 14c-3) against said driving plate, is adapted, in the operation of the injection unit: - to slide laterally with respect to said driving plate because of the different operating temperatures and hence of the different thermal expansions of said hot distribution plate, at a higher temperature, and said driving plate, at a lower temperature, so as to recover this difference in thermal expansion between the two plates (20, 30) and thereby avoid creating dangerous stresses acting on said injection nozzle and in particular on the respective closure pin; and - to efficiently evacuate and transmit the heat from at least one portion (14a") of the guide and seal element to the driving plate, so as to avoid that in the region of the guide and seal element there are reached high temperatures at the origin of leakage, at the outside of the injection nozzle (1 1), of the plastic material in the molten state. - an injection nozzle (1 1); - a closure pin axially movable, in said injection nozzle, between a first operating position in which the closure pin frees and opens an injection hole so as to allow the flow, through said injection hole, of the plastic material in the molten state towards a mould, and a second operating position in which the closure pin closes the injection hole, so as to interrupt the flow, through said injection hole, of the plastic material towards the mould; and - a guide and cooling assembly (14; 1 14; 214), interposed between said back driving plate and said hot distribution plate; - a guide and seal element, in particular in the form of a bush, defining a guide hole along which the closure pin extends axially, said guide hole having both a guiding function to guide the same closure pin in its axial motion between said first and second operating positions, and a sealing function against leakage at the outside of the plastic material, in the molten state, distributed and supplied by the hot distribution plate; - a cooling ring; - and pressure means (14c, 14c-1; 14c-2; 14c-3), in particular in the form of an elastic spring or an O-ring type ring, for pressing the cooling ring directly against and in contact with said driving plate; - to slide laterally with respect to said driving plate because of the different operating temperatures and hence of the different thermal expansions of said hot distribution plate, at a higher temperature, and said driving plate, at a lower temperature, so as to recover this difference in thermal expansion between the two plates (20, 30) and thereby avoid creating dangerous stresses acting on said injection nozzle and in particular on the respective closure pin; and - to efficiently evacuate and transmit the heat from at least one portion (14a") of the guide and seal element to the driving plate, so as to avoid that in the region of the guide and seal element there are reached high temperatures at the origin of leakage, at the outside of the injection nozzle (1 1), of the plastic material in the molten state.