An ultrasonic device for a polymer extruder comprising: a chamber provided with an inlet bore for melted pressurized polymer, an outlet bore for melted pressurized polymer, and a sonotrode housing bore, said chamber being connectable or integrable in a polymer extruder apparatus for the feeding of melted pressurized polymer to the inlet bore; an ultrasonic head including at least one sonotrode, the ultrasonic head being provided with a distal portion and with a proximal portion separated by a first nodal plane, wherein the distal portion is completely constituted by said sonotrode and it is inserted in the chamber through the sonotrode housing bore projecting in cantilever into the chamber, the distal portion being intended to remain in contact with the melted pressurized polymer contained in the chamber, and wherein the proximal portion, that is at least partially constituted by said sonotrode, remains outside the 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; a ring seal in contact with the ultrasonic head in coincidence with the first nodal plane and the ring seal being configured to seal the sonotrode housing bore of the 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, parallel to the first surface, an anchoring device attached to, or resting on, said second surface, a pressure device, configured to apply pressure to said anchoring device in a direction perpendicular to the first surface, and to transmit said pressure to the ring seal through the compression of the proximal portion of the ultrasonic head producing a tight sealing of the chamber. a chamber provided with an inlet bore for melted pressurized polymer, an outlet bore for melted pressurized polymer, and a sonotrode housing bore, said chamber being connectable or integrable in a polymer extruder apparatus for the feeding of melted pressurized polymer to the inlet bore; an ultrasonic head including at least one sonotrode, the ultrasonic head being provided with a distal portion and with a proximal portion separated by a first nodal plane, wherein the distal portion is completely constituted by said sonotrode and it is inserted in the chamber through the sonotrode housing bore projecting in cantilever into the chamber, the distal portion being intended to remain in contact with the melted pressurized polymer contained in the chamber, and wherein the proximal portion, that is at least partially constituted by said sonotrode, remains outside the 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; a ring seal in contact with the ultrasonic head in coincidence with the first nodal plane and the ring seal being configured to seal the sonotrode housing bore of the 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, parallel to the first surface, an anchoring device attached to, or resting on, said second surface, a pressure device, configured to apply pressure to said anchoring device in a direction perpendicular to the first surface, and to transmit said pressure to the ring seal through the compression of the proximal portion of the ultrasonic head producing a tight sealing of the chamber.