Joint Design Guidelines Used in Ultrasonic Welding

The most critical factor of part design for ultrasonic welding is the joint design, or the configuration, of the two mating surfaces. It should be considered while the parts to be welded are still in the design stage and then be incorporated into the molded parts.

There are a variety of joint designs, each with specific features and advantages. Their selection is determined by such factors as type of plastic, part geometry, weld requirements, machining and molding capabilities, and cosmetic appearance.

The four main joint designs are Butt, Step, Shear, and Tongue and Groove. The illustrations below show an example of each one of these joints. Notice the placement of the energy director, or the triangularly shaped ridge molded into one of the mating plastic parts. This ridge limits initial contact to a very small area. The ridge melts during ultrasonics causing plastic to flow through the joint and bond the parts together.

There are many variations of each of these four joint designs. Toman can assist you in determining the proper joint design for your welding application.

Contact Toman ThermoSonics for any questions regarding ultrasonic welding joint design guidelines.


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, amorphous polymer resins 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 inch (.2 and .6 mm).

Crystalline polymers such as nylon, thermoplastic polyesters, acetal, polyethylene, polypropylene, and polyphenylene resins such as polycarbonate and polysulfones, are more difficult to weld. For these resins, energy directors with a minimum height between .015 and .020 inch (.4 and .5 mm) with a 60-degree, included-angle are generally recommended. The 90-degree, 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 (see figure above.)

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 polycarbonate components, it is recommended that a 60 degree included-angle energy director be designed into the part. The energy director width should be 25% to 30% of the wall thickness (see figure above.)

With assemblies whose components are made of identical thermoplastics, the energy director can be designed into either half of the assembly. However, beware when designing energy directors into assemblies consisting of one part made of co-polymers or terpolymers such as ABS and another part made of a homopolymer such as acrylic. For maximum strength, the energy director should always be incorporated into the homopolymer half of the assembly.

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The step joint with energy director is illustrated at the left. This joint molds readily and provides a strong, well-aligned joint with a minimum of effort. This joint is usually stronger than a butt joint due to the fact that material flows into the vertical clearance.

The step joint provides good strength in shear as well as tension and is often recommended where good cosmetic appearance is required. When working with crystalline materials, a 60-degree, included-angle energy director should be used instead of the 90-degree, included angle energy director.

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The shear joint, or interference joint, shown at the right is generally recommended for high-strength hermetic seals on parts with square corners or rectangular designs, especially with crystalline resins.

Initial contact is limited to a small area which is usually a recess or step in either of the parts. The contacting surfaces melt first, then, as the parts telescope together, they continue to melt along the vertical walls. The smearing action of the two melt surfaces eliminates leaks and voids making this the best joint for strong hermetic seals.

The shear joint requires weld times in the range of 3-4 times that of other joint designs because larger amounts of resin are being melted. In addition, a certain amount of flash will be visible on the surface after welding.

Several important aspects of the shear joint should be considered:

  • The top part should be as shallow as possible
  • The outer walls should be well supported by a holding fixture
  • The design should allow for a clearance fit
  • A lead-in should be incorporated (see Figure A at right)
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The tongue and groove joint with energy director is illustrated in the figure at the right. This joint is used primarily for scan welding, self-location of parts, and prevention of flash both internally and externally. It provides the greatest bond strength of all the joints.

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Contact Toman ThermoSonics today for an evaluation of your ultrasonic welding joint designs.