Closure assembly (10), for an injection unit (11) of plastic material (MP), comprising: a closing rod (16); a driving piston (15) adapted to drive an axial movement (f1) of the closing rod (16) between an open position (P1), in which the closing rod (16) opens an injection hole (13′) of a mold (13), and a closed position (P2), in which the closing rod (16) closes the injection hole (13′); an adjustment element or nut (20, 21), provided of a fine-pitch threading (21a), that is adapted to be screwed or unscrewed in one sense or the other (f2) in a corresponding threaded portion (22b) of an internal seat (22), formed in the driving piston (15), in order to regulate axially, in a micrometric way, the closing rod (16) in the closed position (P2); and an elastic washer (30, 31) that is provided for cooperating with a plurality of projections and notches (23a, 23b), formed in such internal seat (22, 22a), in order to fix the adjustment element (21) with respect to the body (15a) of the driving piston (15), once the closing rod (16) has been exactly regulated in the desired closed position (P2). Advantageously the adjustment element (21) is entirely housed in the internal seat (22) formed in the driving piston (15), whereby it does not protrude from the body (15a) of the latter.