Apparatus for injection-moulding of plastic materials comprising a hot runner (1), at least one injector (2) including a nozzle mobile within which is a valve pin (3) driven by a rotary electric motor (4) and an associated transmission including a screw-and-nut assembly (10) for converting the rotation of the shaft (5) of the electric motor (4) into a translation of the valve pin (3). At least the rotary electric motor (4), and the screw-and-nut assembly (10) are set parallel alongside one another.