Why Use Attachments for Ultrasonic Horns?
by Gary Clodfelter
Plastics Assembly Technologies, Inc. (PATSONICS)
Question: What attachments can be added to ultrasonic horns? In what cases would these attachments be necessary?
Answer: There are situations when it is necessary to add other components to an ultrasonic horn. One of these situations occurs when the mechanical vibrations created from ultrasonic oscillation causes damage to the part at locations outside of the weld area. Thin top covers might diaphragm and cause a hole or mark to appear on the part. A boss or appendage without adequate radii might detach when the part is ultrasonically vibrated. In situations like these, it is not uncommon to add a nodal plunger to the horn. A nodal plunger can be added by attaching a mount to the nodal point of an ultrasonic horn. The nodal point of the horn is, in theory, the place on a horn where there is no vibration. This nodal position exists in all horns and provides an excellent point of attachment for secondary devices. By the way, the two faces of the horn are called the anti-node.
By adding a nodal attachment, features can be added to the horn that will contact the part and isolate the vibratory energy at the problem area. This damping effect has been very effective in isolating the energy at a local position, while still allowing the horn to vibrate and create a weld at the joint. By isolating or damping the energy, the damage is resolved.
Another reason for adding a nodal attachment is to serve as a clamp. This can be particularly effective with a spring-loaded nodal plunger. The nodal plunger comes in contact with the part to be held in place during the ultrasonic process and because the hold down is spring-loaded, the horn continues to travel to the work piece, allowing the assembly operation to occur.
One other type of attachment that is found on horns is the addition of a vacuum feature. The vacuum line usually is attached to the horn at the nodal point and provides a feature used to hold a part in position before the horn is delivered to the part piece to be welded. While this feature can be used as an alignment device, it is preferable to have the part designed so the alignment occurs within the part and nothing is attached to the horn before it descends to weld the part.
Another addition to an ultrasonic horn is the various types of coatings added to the face of the horn to reduce part marking and increase wear resistance. The types of coatings that have been used are broad, but the more typical coatings seen are chrome, anodizing and carbide coating. The chrome and anodized coatings are used with aluminum horns to prevent oxidation and reduce part marking. The carbide coating is used with titanium to improve wear resistance when welding abrasive materials.