An ultrasonic device for a polymer injector apparatus including: a fusion chamber provided with a loading opening for polymer in pellet, powder or tablet form, an actuated plunger displaceable within the fusion chamber adapted to modify the volume of said fusion chamber, an outlet bore for melted polymer connected to a mould, and a sonotrode bore; an ultrasonic head that includes at least one sonotrode, the ultrasonic head being provided with a distal portion and with a proximal portion separated by a first nodal plane of the sonotrode, wherein the distal portion is completely constituted by said sonotrode and is inserted in the fusion chamber through the sonotrode bore projecting in cantilever into the fusion chamber, the distal portion being intended to remain in contact with the polymer contained in the fusion chamber, and wherein the proximal portion, that is at least partially constituted by said sonotrode, remains outside the fusion chamber and in connection with an ultrasonic transducer and includes at least one second nodal plane away from and parallel to the first nodal plane, and wherein, a ring seal in contact with the ultrasonic head in coincidence with the first nodal plane and the ring seal is configured to seal the sonotrode bore of the fusion chamber that contains the melted pressurized polymer; characterised in that the first nodal plane is coplanar with a first annular surface of the ultrasonic head on which an annular surface of the ring seal rests parallel to the first surface; the second nodal plane is coplanar or adjacent to a second surface of the ultrasonic head on which an anchoring device, in cooperation with a pressure device, applies pressure that is transmitted to the ring seal through the proximal portion of the ultrasonic head producing a tight sealing of the fusion chamber. a fusion chamber provided with a loading opening for polymer in pellet, powder or tablet form, an actuated plunger displaceable within the fusion chamber adapted to modify the volume of said fusion chamber, an outlet bore for melted polymer connected to a mould, and a sonotrode bore; an ultrasonic head that includes at least one sonotrode, the ultrasonic head being provided with a distal portion and with a proximal portion separated by a first nodal plane of the sonotrode, wherein the distal portion is completely constituted by said sonotrode and is inserted in the fusion chamber through the sonotrode bore projecting in cantilever into the fusion chamber, the distal portion being intended to remain in contact with the polymer contained in the fusion chamber, and wherein the proximal portion, that is at least partially constituted by said sonotrode, remains outside the fusion chamber and in connection with an ultrasonic transducer and includes at least one second nodal plane away from and parallel to the first nodal plane, and wherein, a ring seal in contact with the ultrasonic head in coincidence with the first nodal plane and the ring seal is configured to seal the sonotrode bore of the fusion chamber that contains the melted pressurized polymer; characterised in that the first nodal plane is coplanar with a first annular surface of the ultrasonic head on which an annular surface of the ring seal rests parallel to the first surface; the second nodal plane is coplanar or adjacent to a second surface of the ultrasonic head on which an anchoring device, in cooperation with a pressure device, applies pressure that is transmitted to the ring seal through the proximal portion of the ultrasonic head producing a tight sealing of the fusion chamber.