Create Config
Edit Name Description Configuration Type Type

Choosing the Right Joint Design for Ultrasonic Plastic Bonding: A Primer


David Cermak

Dukane Choosing the Right Joint Design for Ultrasonic Plastic Bonding Banner

When it comes to plastic bonding, selecting the appropriate joint design is a critical factor that will significantly impact the success of your project. In this guide, we will introduce different joint designs and help you determine which one suits your specific needs. 

Energy Director for Simple Bonding 

An energy director is the most common joint design used in ultrasonic welding. It involves molding a small, raised feature onto one of the components that helps to initiate the melt in the intended locations. These features are typically triangular, but also can be round if you are using a Dukane Infinity Series Ultrasonic Welder to joint your parts. A simple energy director is usually between 0.25-0.75mm tall and provides a strength-only bond. If your assembly requires a sealed joint or for the weld joint to help with part-to-part orientation, additional features are required.  

Shear Joint for Sealed Parts 

When a sealed assembly is imperative, a shear joint stands out as the most effective choice. A shear joint is an intentionally molded interference fit between your two plastic parts. It requires tight molding tolerances and dimensionally stable parts, but with those controls provides the best path to a leak-free assembly. It is ideal for smaller parts, especially when component geometry does not allow for the presence of an energy director. Shear joints are particularly effective at welding semi-crystalline resins that struggle to bond with energy directors. The key drawback to this joint design is sensitivity to changing molding conditions, so using a shear joint requires a consistent molding process to see the best success. When properly done, however, shear joints provide the strongest possible ultrasonic welds. 

Step-Style Joint for Part-to-Part Location 

If your project requires precise part-to-part location, a step-style joint might be the solution. This design not only locates the parts together but also controls external flash. It’s a great choice for strength-only bonds where part alignment is not provided by other features. Because a step-style joint does not control internal flash, seal quality will vary, especially when welding more challenging resins. Step-style joints can be adapted to include either energy directors or shear interference.  

Tongue-and-Groove Energy Director for Sealed Parts 

For applications where sealed parts are necessary, a tongue-and-groove energy director emerges as the easiest and most robust option. While it requires thicker part walls, it can reliably seal parts of various sizes. This design is particularly effective with amorphous resins. If your assembly has the room to include this geometry, it is the first and best choice to achieving a great weld.  

Consult Dukane’s Application Engineers 

Navigating the nuances of joint design can be challenging. Multiple variants of all the joint designs listed in this primer exist and are used. When uncertainty arises, don't hesitate to seek guidance from Dukane’s Application Engineers. Their expertise can help you make informed decisions, ensuring that the chosen joint design aligns seamlessly with your project requirements.