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Ultrasonic Cleaning 

Fundamental Theory 


F. John Fuchs 
Blackstone Ultrasonics 
P.O. Box 220 - 9 N. Main Street 
Jamestown, NY 14702-0220 
716 665-2340 

Abstract - 

A presentation describing the theory of ultrasonics, cavitation and implosion. The importance and 
application of ultrasonics in precision cleaning. Explanations of ultrasonic cleaning equipment options and 
their application. Process parameters for ultrasonic cleaning. Proper operation of ultrasonic cleaning 
equipment to achieve maximum results. 

tn tr Q dp gti Q iH - 

Solvent degreasing using chlorinated and fluorinated hydrocarbon solvents, a process used for 
cleaning by the finishing industry since its very inception, is on its way out, a victim of increased regula- 
tion, the Montreal Protocol and the "Green" movement in general. It is understandable why this method 
achieved its prominent position in metal fininshing - - it is reasonably effective at cleaning and was for- 
merly the most expedient and least expensive cleaning means available. The equipment required is simple 
and inexpensive, and until recently the chemistry was considered environmentally safe. 

Industry is currently in a struggle to replace solvent degreasing with alternative "environmentally 
friendly" means of cleaning. Although substitute water-based, semi-aqueous and petroleum based cherrtis- 
tries are available, they are often somewhat less effective as cleaners than the solvents and may not perform 
adequately in some applications unless a mechanical energy boost is added to assure the required levels of 
cleanliness. Ultrasonic energy is now used extensively in critical cleaning applications to both speed and 
enhance the cleaning effect of the alternative chemistries. This paper is intended to familiarize the reader 
with the basic theory of ultrasonics and how ultrasonic energy can be most effectively applied to enhance a 
variety of cleaning processes. 

What is "Ultrasonics?*' - 

Ultrasonics is the science of sound waves above the limits of human audibility. The frequency of a 
sound wave determines its tone or pitch. Low frequencies produce low or bass tones. High frequencies 
produce high or treble tones. Ultrasound is a sound with a pitch so high that it can not be heard by the 
human ear. Frequencies above 18 Kilohertz are usually considered to be ultrasonic. The frequencies used 
for ultrasonic cleaning range from 20,000 cycles per second or kilohertz (KHz) to over 100,000 KHz. The 
most commonly used frequencies for industrial cleaning are those between 20 KHz and 50KHz. Frequen- 
cies above 50KHz are more commonly used in small tabletop ultrasonic cleaners such as those found in 
jewelry stores and dental offices. 

The Theory of Sound Waves - 

In order to understand the mechanics of ultrasonics, it is necessary to first have a basic understand- 
ing of sound waves, how they are generated and how they travel through a conducting medium. 

Sound Wave Generation - 

A sound wave is produced when a solitary or repeating 
displacement is generated in a sound conducting medium, 
such as by a "shock” event or "oscillatory” movement. The 
displacement of air by the cone of a radio speaker is a good 
example of "oscillatory” sound waves generated by mechani- 
cal movement. As the speaker cone moves back and forth, the 
air in front of the cone is alternately compressed and rarefied 
to produce sound waves, which travel through the air until 
they are finally dissipated. We are probably most familiar 
with sound waves generated by alternating mechanical 
motion. There are also sound waves which are created by a 
single ’’shock” event. An example is thunder which is gener- 
ated as air instantaneously changes volume as a result of an 
electrical discharge (lightning). Another example of a shock 
event might be the sound created as a wooden board falls with 
its face against a cement floor. Shock events are sources of a 
single compression wave which radiates from the source. 

The Nature of Sound Waves 

The diagram above uses the coils of a spring similar to a Slinky® toy to represent individual molecules of a 
sound conducting medium. The molecules in the medium are influenced by adjacent molecules in much the same 
way that the coils of the spring influence one another. The source of the sound in the model is at the left. The com- 
pression generated by the sound source as it moves propagates down the length of the spring as each adjacent coil of 
the spring pushes against its neighbor. It is important to note that, although the wave travels from one end of the 
spring to the other, the individual coils remain in their same relative positions, being displaced first one way and 
then the other as the sound wave passes. As a result, each coil is first part of a compression as it is pushed toward 
the next coil and then part of a rarefaction as it recedes from the adjacent coil. In much the same way, any point in a 
sound conducting medium is alternately subjected to compression and then rarefaction. At a point in the area of a 
compression, the pressure in the medium is positive. At a point in the area of a rarefaction, the pressure in the 
medium is negative. 


Cavitation - 

In elastic media such as air and most solids, there is a continuous transition as a sound wave is trans- 
mitted. In non-elastic media such as water and most liquids, there is continuous transition as long as the 
amplitude or ’’loudness" of the sound is relatively low. As amplitude is increased, however, the magnitude 
of the negative pressure in the areas of rarefaction eventually becomes sufficient to cause the liquid to 
fracture because of the negative pressure, causing a phenomenon known as cavitation. 



Pressure Sound Source 

Cavitation "bubbles" are created at 
sites of rarefaction as the liquid 
fractures or tears because of the 
negative pressure of the sound wave in 
the liquid. 

As the wave fronts pass, the cavitation 
"bubbles" oscillate under the influ- 
ence of positive pressure, eventually 
growing to an unstable size. 

Finally, the violent collapse of the 
cavitation "bubbles" results in 
implosions, which cause shock waves 
to be radiated from the sites of the 

The collapse and implosion of myriad cavitation "bubbles" throughout an ultrasonically activated 
liquid result in the effect commonly associated with ultrasonics. It has been calculated that temperatures in 
excess of 10,000°F and pressures in excess of 10,000 PSI are generated at the implosion sites of cavitation 

Benefits of Ultrasonics in the Cleaning and Rinsing Processes - 

Cleaning in most instances requires that a contaminant be dissolved (as in the case of a soluble soil), 
displaced (as in the case of a non-soluble soil) or both dissolved and displaced (as in the case of insoluble 
particles being held by a soluble binder such as oil or grease). The mechanical effect of ultrasonic energy 
can be helpful in both speeding dissolution and displacing particles. Just as it is beneficial in cleaning, 
ultrasonics is also beneficial in the rinsing process. Residual cleaning chemicals are removed quickly and 
completely by ultrasonic rinsing. 


Ultrasonics Speeds Cleaning by Dissolution - 





In removing a contaminant by dissolution, it is necessary 
for the solvent to come into contact with and dissolve the 
contaminant. The cleaning activity takes place only at the 
interface between the solvent and the contaminant. 




As the solvent dissolves the contaminant, a saturated solvent 
layer develops at the interface between the solvent and the 
contaminant. Once this has happened, cleaning action stops as 
the saturated solvent can no longer attack the contaminant. 

Fresh solvent cannot reach the contaminant. 




Ultrasonic cavitation and implosion effectively displace the 
saturated solvent layer to allow fresh solvent to come into 
contact with the contaminant remaining to be removed. This 
is especially beneficial when irregular surfaces or internal 
passageways are to be cleaned. 

Ultrasonic Activity Displaces Particles - 



Some contaminants are comprised of insoluble particles loosly 
attached and held in place by ionic or cohesive forces. These par- 
ticles need only be displaced sufficiently to break the attractive forces 
to be removed. 






Cavitation and implosion as a result of ultrasonic activity 
displace and remove loosely held contaminants such as dust from 
surfaces. For this to be effective, it is necessary that the coupling 
medium be capable of wetting the particles to be removed. 




Complex Contaminants - 

Contaminations can also, of course, be more complex in nature, consisting of combination soils 
made up of both soluble and insoluble components. The effect of ultrasonics is substantially the same in 
these cases, as the mechanical micro-agitation helps speed both the dissolution of soluble contaminants and 
the displacement of insoluble particles. 

Ultrasonic activity has also been demonstrated to speed or enhance the effect of many chemical re- 
actions. This is probably caused mostly by the high energy levels created as high pressures and tempera- 
tures are created at the implosion sites. It is likely that the superior results achieved in many ultrasonic 
cleaning operations may be at least partially attributed to the sonochemistry effect. 

A Superior Process - 

Ultrasonic energy has been proven to be more effective at enhancing cleaning than other alterna- 
tives, including spray washing, brushing, turbulation, air agitation, and electro-cleaning in many applica- 
tions. The ability of ultrasonic activity to penetrate and assist the cleaning of interior surfaces of complex 
parts is especially noteworthy. 

Uitrasomc E q ui p ment - 

To introduce ultrasonic energy into a cleaning system requires an ultrasonic transducer and an ultra- 
sonic power supply or "generator." The generator supplies electrical energy at the desired ultrasonic fre- 
quency. The ultrasonic transducer converts the electrical energy from the ultrasonic generator into me- 
chanical vibrations. 

Ultrasonic Generator - 

The ultrasonic generator converts electrical energy from the line which is typically alternating 
current at 50 or 60Hz to electrical energy at the ultrasonic frequency. This is accomplished in a number of 
ways by various equipment manufacturers. Current ultrasonic generators nearly all use solid state technol- 

60 Cycles 

20,000+ Cycles 

There have been several relatively recent innovations in ultrasonic generator technology which may 
enhance the effectiveness of ultrasonic cleaning equipment. These include square wave outputs, slowly or 
rapidly pulsing the ultrasonic energy on and off and modulating or "sweeping" the frequency of the 
generator output around the central operating frequency. The most advanced ultrasonic generators have 
provisions for adjusting a variety of output parameters to customize the ultrasonic energy output for the 

Square Wave Output - 

Applying a square wave signal to an ultrasonic transducer results in an acoustic output rich in har- 
monics. The result is a multi-frequency cleaning system which vibrates simultaneously at several frequen- 
cies which are harmonics of the fundamental frequency. Multi-frequency operation offers the benefits of 
all frequencies combined in a single ultrasonic cleaning tank. 


Pulse - 

In pulse operation, the ultrasonic energy is turned on and off at a rate which may vary from once 
every several seconds to several hundred times per second. 

Pulsed Ultrasonic Output 


The percentage of time that the ultrasonic energy is on may also be changed to produce varied results. At 
slower pulse rates, more rapid degassing of liquids occurs as coalescing bubbles of air are given an oppor- 
tunity to rise to the surface of the liquid during the time the ultrasonic energy is off. At more rapid pulse 
rates the cleaning process may be enhanced as repeated high energy ’’bursts” of ultrasonic energy occur 
each time the energy source is turned on. 

Sweep - 

In sweep operation, the frequency of the output of the ultrasonic generator is modulated around a 
central frequency which may itself be adjustable. 

Sweep Ultrasonic Output 




A A A 

-Sweep Period 


A A A 


A A 


A A 



Minimum Maximum 
Frequency Frequency 

Various effects are produced by changing the speed and magnitude of the frequency modulation. The fre- 
quency may be modulated from once every several seconds to several hundred times per second with the 
magnitude of variation ranging from several hertz to several kilohertz. Sweep may be used to prevent 
damage to extremely delicate parts or to reduce the effects of standing waves in cleaning tanks. Sweep 
operation may also be found especially useful in facilitating the cavitation of terpenes and petroleum based 
chemistries. Pulse and sweep operation may be especially useful in facilitating the cavitation of terpenes 
and petroleum based chemistries. 

Ultrasonic Transducers - 

There are two general types of ultrasonic transducers in use today: Magnetostrictive and piezoelec- 
tric. Both accomplish the same task of converting alternating electrical energy to vibratory mechanical 
energy but do it through the use of different means. 


Magnetostrictive - 

Magnetostrictive transducers utilize the principle of magnetostriction in which certain materials 
expand and contract when placed in an alternating magnetic field. Alternating electrical energy from the 
ultrasonic generator is first converted into an alternating magnetic field through the use of a coil of wire. 
The alternating magnetic field is then used to induce mechanical vibrations at the ultrasonic frequency in 
strips of nickel or other magnetostrictive material which are attached to the surface to be vibrated. Because 
magnetostrictive materials behave identically to a magnetic field of either polarity, the frequency of the 
electrical energy applied to the transducer is 1/2 of the desired output frequency. Magnetostrictive 
transducers were first to supply a robust source of ultrasonic vibrations for high power applications such as 
ultrasonic cleaning. 

Because of inherent mechanical con- 
straints on the physical size of the hardware as 
well as electrical and magnetic complications, 
high power magnetostrictive transducers 
seldom operate at frequencies much above 
20 kilohertz. Piezoelectric transducers, on the 
other hand, can easily operate well into the 
megahertz range. 

Magnetostrictive transducers are gener- 
ally less efficient than their piezoelectric 
counterparts. This is due primarily to the fact 
that the magnetostrictive transducer requires a 
dual energy conversion from electrical to 
magnetic and then from magnetic to mechani- 
cal. Some efficiency is lost in each conver- 
sion. Hysteresis effects also detract from the 
efficiency of the magnetostrictive transducer. 

Piezoelectric - 

Piezoelectric transducers convert alternating electrical energy directly to mechanical energy through 
use of the piezoelectric effect in which certain materials change dimension when an electrical charge is 
applied to them. 

Electrical energy at the 
ultrasonic frequency is supplied to 
the transducer by the ultrasonic 
generator. This electrical energy is 
applied to piezoelectric element(s) 
in the transducer which vibrate. 
These vibrations are amplified by 
the resonant masses of the 
transducer and directed into the 
liquid through the radiating plate. 

Early piezoelectric transduc- 
ers utilized such piezoelectric 
materials as naturally occuring 
quartz crystals and barium titanate 
which were fragile and unstable. 
Early piezoelectric transducers 
were, therefore, unreliable. 

2 mi mi 



2 ■iiininmiii 

2 ■iiiiniiifiiiii 

Laminated Nickel Strips 
Attached to Output 
Diaphragm by Silver Brazing 

Electrical Coil Wrapped 
Around Nickel Strips 

Oscillating Magnetic 
^ Reid 


Today's incorporate stronger, more efficient and highly stable ceramic piezoelectric materials 
which were a result of the efforts of the US Navy and its research to develop advanced sonar transponders 
in the 1940's. The vast majority of transducers used today for ultrasonic cleaning utilize the piezoelectric 
effect, and its research to develop advanced sonar transponders in the 1940's. The vast majority of 
transducers used today for ultrasonic cleaning utilize the piezoelectric effect. 

Ultrasonic Cleaning Equipment - 

Ultrasonic cleaning equipment ranges from the small tabletop units often found in dental offices or 
jewelry stores to huge systems with capacities of several thousand gallons used in a variety of industrial 
applications. Selection or design of the proper equipment is paramount in the success of any ultrasonic 
cleaning application. 

The simplest application may require only a simple heated tank cleaner with rinsing to be done in a 
sink or in a separate container. More sophisticated cleaning systems include one or more rinses, added 
process tanks and hot air dryers. Automation is often added to reduce labor and guarantee process consis- 

The largest installations utilize immersible ultrasonic transducers which can be mounted on the sides 
or bottom of cleaning tanks of nearly any size. Immersible ultrasonic transducers offer maximum flexibil- 
ity and ease of installation and service. 

Maximizing the Ultrasonic Cl eaning Process - 

Process Parameters ■ 

Effective application of the ultrasonic cleaning process requires consideration of a number of para- 
meters. While time, temperature and chemical remain important in ultrasonic cleaning as they are in other 
cleaning technologies, there are other factors which must be considered to maximize the effectiveness of 
the process. Especially important are those variables which affect the intensity of ultrasonic cavitation in 
the liquid. 

Maximizing Cavitation 

Maximizing cavitation of the cleaning liquid is obviously very important to the success of the ultra- 
sonic cleaning process. Several variables affect cavitation intensity. 

Temperature is the most important single parameter to be considered in maximizing cavitation in- 
tensity. This is because so many liquid properties affecting cavitation intensity are related to temperature. 
Changes in temperature result in changes in viscosity, the solubility of gas in the liquid, the diffusion rate 
of dissolved gasses in the liquid, and vapor pressure, all of which affect cavitation intensity. In pure water, 
the cavitation effect is maximized at approximately 160°F. 

The viscosity of a liquid must be minimized for maximum cavitation effect. Viscous liquids are 
sluggish and cannot respond quickly enough to form cavitation bubbles. The viscosity of most liquids is 
reduced as temperature is increased. 

For most effective cavitation, the cleaning liquid must contain as little dissolved gas as possible. 

Gas dissolved in the liquid is released during the bubble growth phase of cavitation and prevents its violent 
implosion which is required for the desired ultrasonic effect. The amount pf dissolved gas in a liquid is 
reduced as the liquid temperature is increased. 

The diffusion rate of dissolved gasses in a liquid is increased at higher temperatures. This means 
that liquids at higher temperatures give up dissolved gasses more readily than those at lower temperatures, 
which aids in minimizing the amount of dissolved gas in the liquid. 


A moderate increase in the temperature of a liquid brings it closer to its vapor pressure, meaning that 
vaporous cavitation is more easily achieved. Vaporous cavitation, in which the cavitation bubbles are filled 
with the vapor of the cavitating liquid, is the most effective form of cavitation. As the boiling temperature 
is approached, however, the cavitation intensity is reduced as the liquid starts to boil at the cavitation sites. 

Cavitation intensity is directly related to Ultrasonic Power at the power levels generally used in ul- 
trasonic cleaning systems. As power is increased substantially above the cavitation threshold, cavitation in- 
tensity levels off and can only be further increased through the use of focusing techniques. 

Cavitation intensity is inversely related to Ultrasonic Frequency . As the ultrasonic frequency is in- 
creased, cavitation intensity is reduced because of the smaller size of the cavitation bubbles and their resul- 
tant less violent implosion. The reduction in cavitation effect at higher freqencies may be overcome by in- 
creasing the ultrasonic power. 

Maximizing Overall Cleaning Effect 

Cleaning Chemical selection is extremely important to the overall success of the ultrasonic cleaning 
process. The selected chemical must be compatible with the base metal being cleaned and have the ca- 
pability to remove the soils which are present. It must also cavitate well. Most cleaning chemicals can be 
used satisfactorily with ultrasonics. Some are formulated especially for use with ultrasonics. However, 
avoid the non-foaming formulations normally used in spray washing applications. Highly wetted formula- 
tions are preferred. Many of the new petroleum cleaners, as well as petroleum and terpene based semi- 
aqueous cleaners, are compatible with ultrasonics. Use of these formulations may require some special 
equipment considerations, including increased ultrasonic power, to be effective. 

Temperature was mentioned earlier as being important to achieving maximum cavitation. The ef- 
fectiveness of the cleaning chemical is also related to temperature. Although the cavitation effect is maxi- 
mized in pure water at a temperature of approximately 160°F, optimum cleaning is often seen at higher or 

Importance of Minimizing Dissolved Gas 

Negative Pressure Atmospheric Pressure 

Cavitation Bubble Growing Bubble Starts to Collapse 

During the negative pressure portion of the sound wave, the liquid is tom 
apart and cavitation bubbles start to form. As a negative pressure develops 
within the bubble, gasses dissolved in the cavitating liquid start to diffuse 
across the boundary into the bubble. As negative pressure is reduced due to 
the passing of the rarefaction portion of the sound wave and atmospheric 
pressure is reached, the cavitation bubble starts to collapse due to its own 
surface tension. During the compression portion of the sound wave, any gas 
which diffused into the bubble is compressed and finally starts to diffuse 
across the boundary again to re-enter the liquid. This process, however, is 
never complete as long as the bubble contains gas since the diffusion out of the 
bubble does not start until the bubble is compressed. And once the bubble is com- 
pressed, the boundary surface available for diffusion is reduced. As a result, cavitation bubbles formed in 
liquids containing gas do not collapse all the way to implosion but rather result in a small pocket of 
compressed gas in the liquid. This phenomenon can be useful in degassing liquids. The small gas bubbles 
group together until they finally become sufficiently buoyant to come to the surface of the liquid. 

Maximum Pressure 


lower temperatures because of the effect that temperature has on the cleaning chemical. As a general rule, 
each chemical will perform best at its recommended process temperature regardless of the temperature 

effect on the ultrasonics. For example, al- 
though the maximum ultrasonic effect is 
achieved at 160°F, most highly caustic cleaners 
are used at a temperatures of 1 80°F to 190°F 8 

because the chemical effect is greatly en- o 
hanced by the added temperature. Other | 

cleaners may be found to break down and lose 
their effectiveness if used at temperatures in 
excess of as low as 140°F. The best practice is 
to use a chemical at its maximum recom- 
mended temperature not exceeding 190°F. 

100 120 140 160 180 200 220 

Temperature °F 

Degassing of cleaning solutions is 

extremely important in achieving satisfactory cleaning results. Fresh solutions or solutions which have 
cooled must be degassed before proceeding with cleaning. Degassing is done after the chemical is added 
and is accomplished by operating the ultrasonic energy and raising the solution temperature. The time 
required for degassing varies considerably, based on tank capacity and solution temperature, and may 
range from several minutes for a small tank to an hour or more for a large tank. An unheated tank may 
require several hours to degas. Degassing is complete when small bubbles of gas cannot be seen rising to 
the surface of the liquid and a pattern of ripples can be seen. 

The Ultrasonic Power delivered to the cleaning tank must be adequate to cavitate the entire volume 
of liquid with the workload in place. Watts per gallon is a unit of measure often used to measure the level 
of ultrasonic power in a cleaning tank. As 

tank volume is increased, the number of watts 
per gallon required to achieve the required 
performance is reduced. Cleaning parts that 
are very massive or that have a high ratio of 
surface to mass may require additional ultra- 1 
sonic power. Excessive power may cause | 
cavitation erosion or "burning” on soft metal * 
parts. If a wide variety of parts is to be 
cleaned in a single cleaning system, an 
ultrasonic power control is recommended to 
allow the power to be adjusted as required for 

various cleaning needs. 

Part Exposure to both the cleaning chemical and ultrasonic energy is important for effective clean- 
ing. Care must be taken to ensure that all areas of the parts being cleaned are flooded with the cleaning 
liquid. Parts baskets and fixtures must be designed to allow penetration of ultrasonic energy and to posi- 
tion the parts to assure that they are exposed to the ultrasonic energy. It is often necessary to individually 
rack parts in a specific orientation or rotate them during the cleaning process to thoroughly clean internal 
passages and blind holes. 

Conclusion ■ 

Properly utilized, ultrasonic energy can contribute significantly to the speed and effectiveness of 
many immersion cleaning and rinsing processes. It is especially beneficial in increasing the effectiveness 
of today’s preferred aqueous cleaning chemistries and, in fact, is necessary in many applications to achieve 
the desired level of cleanliness. With ultrasonics, aqueous chemistries can often give results surpassing 
those previously achieved using solvents. Ultrasonics is not a technology of the future — it is very much a 
technology of today.