Cavitation action in an ultrasonic cleaner tank results from the violent implosion of millions of microscopic bubbles every second. The implosion creates a temperature of 5000˚C (>9000˚F) and a jet of plasma impacting the objects being cleaned. It is this action that quickly strips away dirt and other contaminants.
Cavitation bubbles are created by ultrasonic transducers that in turn are driven by an ultrasonic generator operating at a set frequency, examples being 40 kHz at a lower level and 130 kHz at an upper level. (Several models are available with selectable dual frequencies.) Lower frequencies create more vigorous cleaning action than higher frequencies. Because of this, they are generally used for removing coarse contaminants. Higher frequencies are used for cleaning products that are more delicate or have highly polished finishes.
While properly operated and controlled ultrasonic cleaning is generally safe for a wide range of products, the key criterion being “properly operated.” This is a combination of the cleaning solution composition, bath temperature and the ultrasonic frequency employed. The guiding factor is the items being cleaned and what is being removed.
This suggests that ultrasonic cleaning does have the potential to damage products, and why expert advice should be sought when operating an ultrasonic cleaning system, especially if the parts being cleaned are of an unusual dimension or composition. Nevertheless, some basic rules generally apply.
Ultrasonic cleaning solutions, for example, are formulated for specific cleaning operations. Solution selection is based on what is being cleaned, the contaminants removed, the product configuration, its composition and post-cleaning requirements.
The pH of a solution – acidic, neutral or alkaline – is also an important consideration, the selection of which is likewise governed by what is being cleaned and what is being removed.
As noted above, delicate or highly polished surfaces on materials such as glass and aluminum should be cleaned at high frequencies such as 130 kHz to avoid being damaged by the more aggressive cavitation action at 35 or 40 kHz. Even then damage is possible because under normal operating conditions the ultrasonic waves create fields of relatively high and low energy in the bath. These are called standing waves. It means that surfaces receive uneven intensities of cavitation, which manifests itself in streaks of microscopic erosion. To counteract this, a Sweep mode is engaged to distribute the cleaning action more evenly in the bath. This is accomplished automatically by continuously shifting the frequency over a narrow range.
The Sweep mode proves useful across the broad spectrum of ultrasonic cleaning operations in terms of increasing efficiency. But it also helps protect very thin or delicate parts that might begin to vibrate at the same resonance frequency of the transducers and shatter or otherwise be damaged. Before initiating a full-scale cleaning operation for such parts, trial runs should be made to determine the optimum ultrasonic power and frequency.
As another but not final example of avoiding damage, metal parts being cleaned in the same bath should be of the same composition, and ideally should not be in contact with each other. Contact us for advice if this proves impractical.
We at Tovatech hope these points prove helpful in avoiding damage that can result from improperly operated ultrasonic cleaning baths. You can rely on our professionally trained ultrasonic cleaning specialists to recommend equipment and procedures tailored to your specific needs.
——————-
What experiences have you had with damage caused by ultrasonic cleaning? How did you overcome the problem?
Tags: ultrasonic, ultrasonic cleaner
