An injector for a plastic material injection molding apparatus whose cylindrical pin-shaped valve that is axially displaceable between an advanced fully closed position and a retracted position of maximum opening, is provided in proximity to a distal end thereof with a reduced diameter cylindrical portion which delimits with the outlet end of the tip of the nozzle terminal an annular passage having a constant cross-section. When the pin valve is between fully closed and maximum opening positions, the reduced diameter cylindrical portion can be selectively positioned in intermediate positions so as to vary the axial extension of the annular passage to adjust the pressure drop of the injected plastic material.