An injection unit for injection moulding of plastic material, associated, in a moulding system, with a hot distribution plate and a back driving plate, comprising:
- an injection nozzle;
- a closure pin axially movable, in the injection nozzle, between a first operating position in which the closure pin frees and opens an injection hole so as to allow a flow through the 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 the injection hole of the plastic material towards the mould;
- and
- a guide and cooling assembly, interposed between the back driving plate and the hot distribution plate;
- wherein the 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 the back driving plate houses the driving means for driving an axial motion of the closure pin between the first and second operating positions;
- wherein the guide and cooling assembly comprises:
- a guide and seal element in the form of a bush defining a guide hole along which the closure pin extends axially, the guide hole having both a guiding function to guide the axial motion of the closure pin between the first and second operating positions, and a sealing function against leakage at an outside of the plastic material in the molten state and distributed and supplied by the hot distribution plate;
- a cooling ring;
- and
- a pressure means in the form of an elastic spring or an 0-ring type ring configured to press the cooling ring directly against and in contact with the back driving plate;
- in that the guide and seal element is slidably coupled at an end with a central hole of the cooling ring and is fixedly attached at the other end to the hot distribution plate;
- and
- in that the cooling ring, pressed by the pressure means against the back driving plate, is adapted, in operation of the injection unit:
- to slide laterally with respect to the back driving plate because of the different operating temperatures and hence of the different thermal expansions of the hot distribution plate, at a higher temperature, and the back driving plate, at a lower temperature, so as to recover this difference in thermal expansion between the back driving plate and the hot distribution plate and thereby avoid creating dangerous stresses acting on the injection nozzle and on the closure pin;
- and
- to efficiently evacuate and transmit heat from at least one portion of the guide and seal element to the back driving plate, so as to avoid that high temperatures are reached in a region of the guide and seal element at an origin of leakage, at an outside of the injection nozzle, of the plastic material in the molten state.