Article Contents

1. Introduction
2. Sweeping Frequency (1)
3. Sweeping Frequency (2)
4. Sweeping Frequency (3)
5. Power Control
6. Center Frequency Control (1)
7. Center Frequency Control (2)
8. Center Frequency Control (3)
9. Center Frequency Control (4)
10. Conclusion

Ideal Ultrasonic Parameters for Delicate Parts Cleaning
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Introduction

The various ultrasonic parameters, or degrees of freedom, available to the process engineer define what the ultimate limits are for the cleaning process. The traditional degrees of freedom available in an ultrasonic cleaning system have included modulation of a single center frequency (sweep), variable duty cycle, and amplitude control at a single frequency. All of these variables allow control of gross, or macroscopic, variables, such as raw power, into the fluid. The latest class of aqueous cleaning technology allows all of the aforementioned parameters, but at multiple center frequencies in a single process tank. Multiple center frequencies allow precise microscopic tuning of the energy in the individual cavitation event. Understanding and optimizing the parameters yields maximal cleaning efficiency with minimal substrate damage. Unlike many popular ultrasonic papers available, the descriptions here are fully referenced and arrived at through sound physical reasoning. This paper intends to undertake the ambitious task of illuminating the various physical effects of these parameters in such a way as to shed as much light and as little heat as possible on this often less than intuitive subject. It is the authors' desire that this paper is not a final description, but the beginning of a dialog.

The primary physical phenomenon behind the technology of ultrasound is an event known as cavitation. Indeed, any proper treatment of ultrasonic cleaning must begin with a discussion of cavitation. Cavitation is the creation and subsequent collapse of microscopic bubbles within a liquid. These bubbles are formed when a large pressure gradient is introduced into a fluid. During the low-pressure part of a sound wave the fluid is put into tension. When the amplitude of this sound wave exceeds the local tensile strength of the fluid, a void, or cavity, is created in the medium. This cavity grows for the rest of the half cycle of sound. As this bubble grows, both dissolved gasses and fluid vapor diffuse through the walls of the cavity and into the bubble via a process known as rectified diffusion. As the pressure associated with the sound wave begins to go positive, one of two things can happen. The bubble, which grew to a certain size (R₀), during the half cycle, can partially collapse. In this case the entrained gasses act as a shock absorber and the bubble may undergo further stable oscillations. The second possibility is that the bubble can suffer complete implosion, an event labeled transient cavitation. Both of these processes re-radiate absorbed energy from the incident acoustic field. It is this re-radiated energy that impinges upon a substrate and does the bulk of the cleaning. Cavitation is one of nature's most efficient and dramatic amplifiers of energy density currently known. Each bubble collapse is accompanied by the local generation of temperatures on the order of thousands of degrees centigrade and pressures exceeding hundreds of atmospheres. Though recognized for almost a century¹, physicists have yet to construct a complete description of this final collapse. Although much of the final implosion event is shrouded in mystery, the bubble dynamics prior to this are quite well understood.^2,3,4

Armed with this phenomenological explanation of cavitation we can begin to examine the effects of the various ultrasonic parameters. The relevant questions to be asked are how does the generator impart energy to the transducers, how do the transducers impart that energy to the cavitating cleaning solution and then how is the sonic energy exciting the part being cleaned? It is very important to realize that the cleaning system is composed of both the cleaning solution and the part or parts being cleaned.

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