A nozzle comprises a hollow element comprising a recess and an injection hole (6) to inject fluid plastic into a mould, a heating tip which penetrates inside the recess and forms, together with the recess itself, a holding cavity (5) into which the channels (14) run. The holding cavity (5) comprises narrow section (52) which separates the holding cavity (5) into a rear portion (51) and a front portion (50), in order to reduce or completely prevent exchanges of fluid plastic between the two areas.