NASA Technical Reports Server (NTRS) 19950025362: Ultrasonic cleaning: Fundamental theory and application

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N95-31783

Ultrasonic Cleaning

Fundamental Theory
and

Application

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.

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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.

370

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.

Rarefaction

^Compression

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
collapse.

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
bubbles.

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.

371

Ultrasonics Speeds Cleaning by Dissolution -

Part

Being

Cleaned

Contaminant

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.

Contaminant

Saturated

Solvent

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.

Contaminant

Saturated

Solvent

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 -

Insoluble

Contaminant

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.

Insoluble

Contaminant

Part

Being

Cleaned

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.

Part

Being

Cleaned

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-
ogy*

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.

373

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

Period

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 A AAA

V

v

A A A

-Sweep Period

h

A A A

\A

A A

AAA

A A

\J

\J

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.

374

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.

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min

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Laminated Nickel Strips
Attached to Output
Diaphragm by Silver Brazing

Electrical Coil Wrapped
Around Nickel Strips

Oscillating Magnetic
^ Reid

375

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-
tency.

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.

376

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

377

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.

378