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
(p. 7)

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Center Frequency Control (continued)

From a cleaning perspective, there is much research on particle removal efficiencies (PRE) at different frequencies. It is observed that low frequency ultrasound has superior PRE for large particles, and that high frequency ultrasound is best suited for submicron particle removal¹⁰. Thus, in an optimized single process, one would employ low frequency ultrasonics (few, high energy events) to remove large particles and / or gross contamination, and high frequency ultrasonics (many small low energy events) to remove submicron particles. This constitutes the ideal cleaning process, in which a part can be exposed to relatively low frequency ultrasound, i.e. 40 kHz or 72 kHz, for short amounts of time and then to high frequency ultrasound, i.e. 104 kHz or 170 kHz for long times. Such a process would avoid the damage often associated with low frequency ultrasonics but run the gamut, from large to submicron sized particles, with excellent particle removal efficiency. The most recent technological advances in ultrasonic systems allow such a processing scheme to be realized. There is a new class of liquid cleaning and processing equipment in which there is one transducer array and one generator that produces ultrasound at the primary resonance, or one of a number of overtones, of that transducer array for some given period of time. After this programmed time, the frequency then discontinuously jumps, as specified by the process engineer, to a different overtone of the transducer array for another specified time before discontinuously jumping to a third overtone, and so on.

The improved part cleanliness is best demonstrated by graphs of percent particle removal versus particle size. It has been well established that higher frequencies remove a higher percentage of small particles than do low frequencies.¹⁰ There is some minimum size that a frequency removes efficiently, by the same token there is a maximum size particle that any frequency can remove efficiently. If this curve is assumed to be Gaussian in nature, then the graph shown in figure 7 results for a selected center frequency.

The dotted line in figure 7 represents 100%. The reciprocal of particle size was used on the x-axis to spread out the small particle size numbers.

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