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voltage, high-frequency signal. It also contains the programming necessary to operate the actuator and stack in a controlled manner to achieve a desired weld result. The actuator, either pneumatically or electric servo-operated, and available as a stand-alone benchtop unit or integrated into an automated system, moves the ultrasonic tooling toward the parts to be joined. It applies the needed force to the materials to help create the welding conditions. The ultrasonic stack completes the system. It transfers vibratory energy, through direct contact with the parts, to the sealing/joining surface. The stack typically consists of three items: the transducer or converter (described above), which contains the piezoelectric ceramic crystals that oscillate at the frequency of the applied power-supply signal. As these crystals oscillate, they physi- cally expand and contract, creating measurable mechan- ical motion (referred to as peak-to-peak amplitude) in the output side of the transducer. The second section, the booster, with an attached ring in its mid-section, serves two functions: It acts as a mounting point for the stack into the actuator, and also serves to amplify or reduce the output motion created in the transducer. The third and final component of the stack is the horn (sonotrode) that will contact the parts to be joined. The horn will be designed to match the profile of rigid parts to be joined or can have a sealing profile added to its contact face in a film/textile application. For each application, the horn is designed to combine with the other stack components to reach the optimum level of amplitude output to allow ultrasonic welding to occur as efficiently as possible. TYPICAL SETBACKS Issues usually occur in one of four areas: 1. Equipment: The ultrasonic welding equipment or various welding components are not suited to the application. 2. Process parameters: The parameters used are not suited to the parts being joined. 3. Materials: Changes are made in the type, composition, or phys- ical/mechanical characteristics of the materials used in the parts. 4. Part design: Certain details of the part's geometry are not suited to repeatable or successful welding. It should also be noted that sometimes a problem identified in one area may expose a weakness or deficiency in another area. Let's start with equipment. It is easy and usually logical to think the equipment and approaches that produce successful welds in one application will do so in another. But that is not universally true. Worldwide, 20-kHz ultrasonic welders are by far the most widely used; due to their versatility, these welders can deliver high-power (up to 6000 W) and high-amplitude outputs, and they can accommodate a wide range of available tooling sizes. For a contract manufacturer that produces ultrasonically welded parts, 20-kHz equipment can be a great investment since it offers the promise of future use in many applications. However, there are some instances—especially with small and delicate parts—where the high-power, high-amplitude capabilities of 20-kHz equipment may prove too "aggressive" for certain assemblies, potentially resulting in damage. One possible solution is to reduce the input amplitude, but this won't work if the amplitude applied is below the recommended level for the polymer being welded. QUESTIONS ABOUT ULTRASONIC WELDING? Visit the Welding, Bonding and Assembly Zone. Heeding "a squeaky wheel" sooner rather than later may well permit identification and resolution of a problem before production is adversely affected. 20 kHz Power Supply 50/60 kHz FIG 2 Ultrasonic Welder Components Basic components of an ultrasonic welder. Actuator/ Stand Power Supply Ultrasonic Stack Converter Booster Horn Converter Booster Horn FIG 1 The power supply takes a standard electrical line voltage and converts it to an operating frequency. @plastechmag 57 Plastics Technology U LT R A S O N I C W E L D I N G