ANSWER: A couple of things. First, energy is transmitting through the stud to the base. Try pre-triggering the system so that the horn starts vibrating before it contacts the stud. This will help maintain an out-of-phase relationship between the stud and horn which, in turn, will prevent the coupling that's occurring. You should also try altering the velocity, or descent of the horn. Consider a servo welder with patented melt-match technology which will allow you to match the velocity with the flow of molten plastic. Another problem might be at the base of the stud, where you probably have a 90° angle. That's acting as a stress riser, and energy tends to migrate there. That's bad--radius the base of the stud.
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For more information or help with operating your machines, think about attending one of our ultrasonic training sessions! The next session is October 16th-17th at Dukane. There are seats available but they tend to fill up quickly as the time gets closer, so be sure to sign up ASAP to guarantee your seat.