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Understand and address the likely origins of welding problems to keep production on track. How to Solve Common Ultrasonic Welding Problems Ultrasonic welding is a widely recognized and accepted process for joining thermoplastic materials. It offers many advantages, including process reliability and repeat- ability, lower energy usage than other joining techniques, material savings (because there is no need for consumables, such as glue or mechanical fasteners), and labor savings. But as with any process, there are situations where apparent problems with this technique may interrupt the production process. The key to resolving and avoiding these problems is to understand their likely origins. Processors that are successful in using ultra- sonic welding typically share two principal traits: they have a well-docu- mented, validated welding process; and that process is supported and maintained by a resident well-trained "champion." If one or both of these important factors are not present, you'll likely very soon call for help. Even with both present, it is possible that you'll need some help or technical assistance at least once in a while. HOW THE PROCESS WORKS Before examining common causes of ultrasonic welding problems, let's take a moment to understand the welding cycle itself. In ultra- sonic welding, high-frequency vibrations are applied the surfaces of two parts by a vibrating tool, commonly called a "horn" or "sonotrode." Welding occurs as the result of frictional heat generated at the interface between the parts. The ultrasonic vibrations are cre- ated by a series of components—the power supply, converter, booster, and horn—that deliver mechanical vibration to the parts. As shown in Fig. 1, the power supply takes a standard elec- trical line voltage and converts it to an operating frequency. In the following example, we will utilize a common ultrasonic welding frequency of 20 kHz, though welding can take place over a range of 15 to 60 kHz to meet specialized needs. In operation, the power supply sends electrical energy at the specified frequency through an RF cable to the converter. The converter utilizes piezoelectric ceramics to convert the electrical energy to mechanical vibrations at the operating frequency of the power supply. This mechanical vibration is either increased or decreased based on the configura- tion of the booster and horn. The proper mechanical vibration amplitude is determined by an applications engineer and is based on the thermoplastic materials used in the parts. The parts to be welded are put under a mechanical load, generally with a pneumatic actuator that holds the booster and horn. Under this load, the mechanical vibrations are transmitted to the interface between the material surfaces, which focuses the vibrations to create intermolecular and surface friction. This friction creates heat and a subsequent melt, which solidifies into a welded bond. The basic components of an ultrasonic system are a power supply, an actuator, and a stack (see Fig. 2). The power supply takes line voltage at a nominal 120-240V and transforms it into a high- By David Dahlstrand Branson Ultrasonics Higher-frequency welders are considered "more gentle" in the application of ultra- sonic energy to parts. High-frequency vibrations are applied to two parts' surfaces by a vibrating tool, commonly called a "horn" or "sonotrode." 56 JUNE 2017 Plastics Technology PTonline.com Troubleshooting