Perhaps the most critical facet of ultrasonic welding is joint design. It should be considered when the parts to be welded are still in the design stage, and 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.

NOTE: Joints with energy directors are not recommended for use with crystalline materials when high strength or hermetic seals are required. When these criteria must be incorporated in crystalline assemblies, the shear joint should be considered. 

Butt Joint with Energy Director

The butt joint with energy director is the most common joint design used in ultrasonic welding, and the easiest to mold into a part. 

Step Joint with Energy Director

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.

Tongue and Groove Joint with Energy Director

This joint is used primarily for scan welding, self-location of parts, and prevention of flash both internally and externally. 

Shear Joint

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

Scarf Joint

The scarf joint, illustrated in Figure 11, is generally recommended for high-strength hermetic seals on parts with circular or oval designs, especially crystalline resins.