Sonotrode, system and method of reshaping a connecting element

A method of reshaping a connecting element to form a closing head, the method comprising:

• providing a connecting element in the form of a rivet, the connecting element defining a longitudinal axis and being at least partially formed by either a tubular wall or a solid post with an outer circumferential surface and an end face;
• providing a sonotrode aligned with the longitudinal axis, the sonotrode having a cavity, the cavity at least partially having the negative shape of the closing head to be formed, wherein the cavity provides a contact surface designed to come in contact with the connecting element during reshaping;
• initially bringing the contact surface of the sonotrode in contact with the connecting element at the end face or in a transition section between the outer circumferential surface and the end face;
• applying an ultrasonic vibration via said sonotrode to the connecting element thereby initially softening or melting the material of the connecting element at the end face or in the transition section between the outer circumferential surface and the end face;
• after bringing the contact surface of the sonotrode in contact with the connecting element, moving said sonotrode and the connecting element relative towards each other over a stroke (s) in a direction along the longitudinal axis, thereby direct softened material or melt to flow along and in permanent contact to the outer circumferential surface of the connecting element over at least 15% of said stroke (s).