Metal Finishing Guide Book

2013

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Mega is not scientifically defined. A commonly accepted limit is frequencies exceeding 250 kHz. Megasonic transducers don't produce cavitation bubbles, and aren't commonly used in metal finishing operations, except to remove surface rust and other dense debris in remanufacturing operations. THERE IS MORE THAN ONE FREQUENCY The reason waves (fluctuations) of pressure are valued is Figure 3. Piezoelectric vs. magnetostrictive operation. that they produce cavitation bubbles. Collapse of those bubbles releases high levels of energy which can interrupt local collections or networks of debris (soil). That's metal cleaning! Larger bubbles, which will ultimately release more energy per bubble when collapsed, are formed when there is more time for them to do so. This means when the frequency is low. Said another way, a lower frequency generates wave fronts with a longer time interval between them, thereby allowing more time for bubble growth. Smaller bubbles are produced when the frequency produced by the transducer is higher. Calculated bubble size vs. frequency is shown in Figure 2. But there is another factor affecting energy release: That's the number of bubbles produced. More bubbles are produced at higher frequencies because there are more opportunities to do so — more cycles of compression and rarefaction (expansion). Essentially, the energy released to do cleaning work on surfaces is the product of the volume of each bubble times the number of bubbles. In other words, for the same power input from the transducer to the liquid tank: • A low frequency will produce fewer cavitation bubble implosions each with higher release of energy, and — • A higher frequency will produce more cavitation bubble implosions each with lower release of energy. The two different types of operation with the same power level are illustrated in Figure 3. Which would you prefer? CHOOSING THE RIGHT FREQUENCY What's significant is that the cleaning capabilities will be quite different in these two examples even though the power level is the same. The value of that difference will depend upon the nature of the cleaning work to be done. The right ultrasonic frequency is that which best matches the cleaning capability to the needed cleaning performance. Some have referred to collapse of cavitation bubbles as being "pecked to death by ducks." This is because other mechanical actions such as blast cleaning with 72

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