Chances are when you hear the word “weld,” you don’t think about plastics. Instead, you are probably picturing the traditional process of welding metal. However, in the plastics industry, welding two plastics is a common practice, though it differs from traditional welding. Rather than adding material and melting everything together, ultrasonic welding uses equipment to vibrate two materials against each other at high frequency. The friction between the two surfaces causes them to melt and bind together on a molecular level. Compared to other connection methods (like using hardware), the connection achieved through ultrasonic welding is more permanent, water-resistant, and impact resistant.
How it works
Ultrasonic systems require a power supply, converter, booster, and horn. The power supply converts standard AC voltage into a desired voltage frequency. The frequency then travels to a converter, which translates the electrical signal into mechanical vibrations. Next, the vibrations are transferred through the booster and horn (designed for specific applications). Since ultrasonic welding requires the booster and horn assembly to be applied to the part with a specific force, it’s common to have the booster and horn assembly mounted to a pneumatic or servo-driven actuator.
Joint Design
The connection point of the two materials is critical to the performance of the ultrasonically welded joint. At the joint, there needs to be a feature called an energy director, which is a small pointed ridge molded into one of the surfaces. The ridge is typically a triangular shape with the point facing toward the opposing surface. During the welding process, the energy director melts into the other surface to secure a solid bond.
From a design perspective, the pointed ridge should be between a sixty and ninety-degree angle and have a height ranging from .008 inches to .025 inches. Designs will, of course, vary based on the application requirements and materials. The images below show some examples of joint designs.
Butt Joint
Step Joint
Tongue and Groove Joint
Shear Joint
Cost
The cost of ultrasonic welding varies by application. However, like many value-added applications, the cost factors include time, materials, and equipment. Ultrasonic welding is a relatively fast process and could be completed by an operator in tandem with the injection molding process. Upfront costs could include welding equipment like horns (designed for the specific application) and fixturing. Because of the custom nature of injection molding applications, it’s best to get a molder involved early. The molder can help you find the best ways to meet design requirements and get the best pricing for both short- and long-term applications.
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