Ultrasonics – Frequency vs. Exposure Time

The blog about maximizing the effect of multiple frequency ultrasonics explained in some detail the mechanics of cleaning using multiple ultrasonic frequencies and the benefits of sequencing through a series of frequencies multiple times to achieve maximum cleaning effect.  This blog will further explore these benefits especially in cases where extended exposure of a part to certain frequencies and power intensities may cause damage to the part.

It is well known that lower ultrasonic frequencies have a higher likelihood of damaging soft substrates including aluminum, brass and others.  This is mainly because lower frequencies produce larger cavitation bubbles which implode releasing more energy than the smaller cavitation bubbles produced at higher frequencies.  It is also well known that if there is a tendency for a particular frequency to damage a substrate, the potential for damage is increased with longer exposure times.  Most substrates survive short exposures to lower frequency ultrasonics without damaging effects.  Yet, the value of using lower frequencies can not be denied.  Using multiple discreet ultrasonic frequencies allows utilization of lower frequencies while minimizing the potential for part damage by limiting the exposure time and/or intensity at lower frequencies.

For example, consider a part that is contaminated with particles of varying sizes which requires the use of multiple ultrasonic frequencies to effectively clean.  Also, let us assume that this part is susceptible to damage due to cavitation erosion if exposed to high power low frequency ultrasonic energy for an extended period of time.  A microprocessor controlled multiple frequency ultrasonic cleaning system offers several options.

1) A frequency sequence that limits the exposure time at lower ultrasonic frequencies.  This provides the benefit of lower ultrasonic frequency to remove larger-sized particles while staying below the time limit threshold for part damage.

2) A frequency sequence limiting the exposure intensity at lower frequencies that may cause part damage.

3) There may also be instances where both time and power as well as the sequence order itself is beneficial.  The following illustrates a sequence utilizing all of the variables.

Realize that the above only illustrates the use of a three frequency system.  Today’s ultrasonic equipment offers as many as 7 frequencies depending on the manufacturer and model.

The next question, of course, is which combination of variables works best for what application.  Unfortunately, this is, as yet, uncharted territory.  There are “indicators” which suggest that particular sequences are more effective than others but the proof, as always, is in the result.  My personal guess is that frequency sequences, time and power will, in time, become as important as today’s time, temperature and chemistry options.  As cleaning requirements become more and more challenging it is good to at least know that the options are available even if not fully explored.  I am sure that diligent researchers will utilize these options to achieve the desired result.

–  FJF  –