The butt joint with energy director is the most common joint design used in ultrasonic welding, and the easiest to mold into a part. The main feature of this joint is a small 90° or 60° triangular shaped ridge molded into one of the mating surfaces. This energy director limits initial contact to a very small area, and focuses the ultrasonic energy at the apex of the triangle. During the welding cycle, the concentrated ultrasonic energy causes the ridge to melt and the plastic to flow throughout the joint area, bonding the parts together.

For easy-to-weld resins (amorphous polymers such as ABS, SAN, acrylic and polystyrene) the size of the energy director is dependent on the area to be joined. Practical considerations suggest a minimum height between .008" and .025" (.2 and .6 mm).

Crystalline polymers, such as nylon, thermoplastic polyesters, acetyl, polyethylene, polypropylene, and polyphenylene sulfide, as well as high melt temperature amorphous resins, such as poly-carbonate and polysulfones are more difficult to weld. For these resins, energy directors with a minimum height between .015" and .020" (.4 and .5 mm) with a 60° included angle are generally recommended.

The 90° included angle energy director height should be at least 10% of the joint width, and the width of the energy director should be at least 20% of the joint width. Image 1 (below) shows a butt joint with a 90° included angle energy director. With thick-walled joints, two or more energy directors should be used, and the sum of their heights should equal 10% of the joint width.

To achieve hermetic seals when welding poly-carbonate components, it is recommended that a 60° included angle energy director should be designed into the part. The energy director width should be 25% to 30% of the wall thickness. Image 2 (below) shows a butt joint with a 60° included angle energy director. Image 3 (below) shows how the ports should be dimensioned to allow for the flow of molten material from the energy director throughout the joint area.