ULTRASONICS
VOLUME
APRIL-JUNE
1
1963
A PRACTICAL INTRODUCTION
ULTRASONIC
by ALAN
TO
CLEANING
E. CRAWFORD*
Cleaning is probably the commonest use to which ultrasound is put, and one which is being
improved continually. This article explains the two most important effects which lie behind
ultrasonic cleaning, cavitation and emulsification, and then shows from a practical point of
view how cavitation, and hence cleaning, is influenced by the type of transducer and the design
of the cleaning tank. Different transducer materials working at different temperatures are
considered, and pulsed systems are compared to continuously vibrating ones. A typical
power supply is described
T
HE application of high power ultrasonic energy in
industry is of comparatively recent origin. While many of
the effects were studied during the immediate pre-war
period, it is only in the last decade that some of the promising results have been applied in practice. There are many
reasons for this delay, but as in many similar research
activities, the transition from the laboratory to the factory
floor is far from a simple scaling-up process. Apart from
the basic problem of power increases, there are others such
as reliability of operation, operator training and practical
economics of running costs and capital outlay. Despite the
progress made in transducer and generator development,
very few of the hundreds of suggested applications have as
yet been adopted in production. Of these applications the
technique of ultrasonic cleaning is predominant.
World
sales of specialized ultrasonic equipment for this purpose
probably already exceed one million pounds sterling each
year, and the process has been accepted for use under the
most stringent conditions.
THEORY
OF OPERATION
The removal of soils or adulterants from a solid surface by
the use of ultrasonic energy is a complex process, and a
number of characteristics play an active part in producing
a cleaning action. Any acoustic wave consists of an
*Sonics Division,
Elliott Bros (London),
Ltd.
alternating pressure front moving at a defined velocity
through a medium. Under normal tank conditions a
standing wave develops owing to reflections, and the wavefront is then stationary, but the alternate pressure and
rarifaction phase still exists. In most liquids there is a
minimum amplitude threshold for a given frequency above
which the phenomenon of cavitation occurs. There is also
a finite wavelength of the sound wave, dependent on the
frequency and the medium in which it is propagated. All
these factors influence the cleaning action. It will be
realized that the alternating pressure will produce high
accelerations in the liquid and this in turn will accelerate
to high velocity any suspended particles.
The ultrasonic intensity in a free acoustic field can be
expressed as
z=$
02
or
P = 2/2pcz
where Z is the intensity level (ergs/cm2/sec), P the peak
sound pressure (dyn/cm2), p the density of the medium
(g/cm3) and c the velocity of sound through it (cm/s). In
practical figures, with an intensity of 5 W/cm2 in water the
peak pressure will be 3.8 x lo6 dyn/cm2 or approximately
3.8 atm. This will always be the minimum value, as in a
standing wave condition it must be multiplied by the
effective standing wave ratio. With the large number of
reflecting surfaces available in a cleaning tank there will
invariably be some standing waves.
ULTRASONICS/
April-June
I963
65
high frequencies are used there is often a considerable hydrodynamic flow of liquid away from the transducer and the
standing wave system is generally disturbed. Suspended
particles are then kept in continuous motion owing to the
overall agitation of the liquid.
Early ultrasonic cleaners operated at frequencies above
250 kc/s and were usually energized as continuous wave
systems. They were relatively inefficient as cleaners since
their action depended entirely on momentum transfer, but
some success was achieved in the cleaning of small mechanisms such as watch assemblies. Reports were also made on
the cleaning of textile waste and woven fabrics, and this was
probably due to the rapid liquid movement through the
threads.l Rapid progress in cleaning did not occur until
low frequency systems were developed and full advantage
was taken of the cavitation effect. High frequency systems
continued to be marketed concurrently under the somewhat
erroneous belief that there was a better cleaning action on
surfaces containing machined cavities of small size, a
relationship between wavelength and physical dimensions of
the cavity being supposed. At a frequency as high as 1 MC/S,
the wavelength in water is still as large as 1a4 mm, and it has
been found that cavities with diameters very much less than
this are effectively cleaned at low frequencies when
cavitation is present.
CAVITATION
Fig. 1. Focusing
effect of a tube segment
transducer
The maximum acceleration of any suspended particle
will be the peak particle acceleration in the beam. This can
be expressed as
where w is the angular frequency. For 5 W/cm2 at a
frequency of 20 kc/s the maximum acceleration is 3.34 x
IO6 cm/s2 or 3,400 g. The displacement amplitude is then
given by
X=w
A
and at this frequency and intensity the amplitude will be
approximately 2.12 X low4cm.
With operation at the high-frequency end of the ultrasonic spectrum, these figures change appreciably, particularly the amplitude and acceleration. For 10 W/cm2 in
water at a frequency of 1 MC/S, the peak particle acceleration becomes 2.35 x 10s cm/s2 or 240,000 g. Correspondingly, the amplitude decreases to 6 x low6cm.
The direct effect of high intensity ultrasonic waves on a
contaminated surface in water is to vibrate small particles
of the soiling medium by a transfer of momentum from the
moving liquid to the particles. The movement will depend
on the size of the particles and the frequency; a wavelength
appreciably greater than the particle size will excite the
particles into oscillation, and larger sizes will move at
decreasing amplitudes. The accelerating force is opposed
by the particle inertia and thus has less influence on large
bodies. Loosely adhering particles on soiled surfaces are
therefore removed and remain in suspension adjacent to
the surface if a standing wave is present. When relatively
66
uLmsoNrcs/April-June
1963
It is now considered that the primary cleaning effect is
caused by cavitation. Under normal circumstances this
phenomenon occurs when the peak alternating pressure in
the ultrasonic wave exceeds the external atmospheric
pressure. The rapid variation in pressure produces gas- or
vapour-filled voids in the liquid. The eventual collapse of
these voids generates intense shock waves with pressure
amplitudes several orders larger than the initiating pressure.
When a bubble collapse occurs adjacent to a soiled surface,
particles of adulterant are disrupted from the surface and
dispersed. Increasing cavitation will eventually reach a
point where the surface itself is abraded and it is therefore
necessary to impose a limit on the acoustic intensity when
delicate objects are being cleaned. For some years a
qualitative test of cleaning efficiency has been to immerse
a piece of aluminium foil in the ultrasonic bath and to note
the time for disruption of the foil.
There are three phases in the cavitation process : an initial
liberation of dissolved gas resulting in large numbers of
visible bubbles, the stimulation of resonance in bubbles of
a finite size, and the eventual collapse of the bubble, producing a large amplitude shock wave. The first phase of
gas liberation occurs at quite low sonic intensities and is
seen over a wide frequency range. Fig. 1 shows the focusing
effect of a tube segment transducer in which the streams of
gas bubbles liberated by cavitation trace out the beam path.
At the focus there is a high concentration of acoustic
energy, and the cavities formed at this centre are no longer
filled with gas. The pressure amplitude exceeds the vapour
pressure of the liquid and the cavities are filled with vapour.
The collapse of vapour cavities produces shock waves of
even greater amplitude than those produced by gas-filled
bubbles.
Cavitation stimulated by acoustic energy is basically due
to resonance effects in the bubbles. The bubble usually
forms round a nucleus such as a microscopically small
suspended particle or the minute particles of gas clinging to
a boundary layer in the liquid. Owing to the alteration in
wavelength, bubbles of different sizes will resonate as the
frequency is changed. At any given intensity and frequency,
only cavities within a particular range of sizes will be
stimulated into cavitation, the upper size limit being
approximately that at which the bubble will resonate. For
example, the resonant radii of air bubbles in water at 1 atm
are about 0.6 mm at 10 kc/s, 0.05 mm at 100 kc/s and
0.004 mm at 1 MC/S. Cavitation commences with a growth
stage in which bubbles too small to cavitate are set into
oscillation by the sound field and grow by a rectified
diffusion process until they reach the resonant size. The
oscillations then rapidly increase in amplitude and result in
an eventual collapse. This violent collapse produces large
shock waves that can exceed 1,000 atm.2 The diameter of
the void must be smaller than the resonant size, and so an
increase in frequency will result in fewer bubbles being
available for the occurrence of rectified diffusion and in an
increase in the minimum power required to produce
cavitation. The ratio of the initial radius to the collapse
radius decreases with frequency and the generated shock
wave will have a smaller amplitude with increasing
frequency.3
With a frequency of 10 kc/s the acoustic intensity for
cavitation in aerated water at room temperature and
atmospheric pressure is about O-3 W/cm2. This increases to
Fig. 2. Cleaning a wafer switch by cavitation
l-0 W/cm2 at 100 kc/s and 500 W/cm2 at 1 MC/S. The
surface tension, and thus the vapour pressure, influences
these onset figures, and many organic liquids will cavitate
at somewhat lower ones. Similarly, a reduction in atmospheric pressure also reduces the thresholds.
Ultrasonic cleaning baths usually incorporate an arrangement of transducers positioned at the base of the tank. The
liquid snrface and the sides of the tank constitute reflecting
faces, and articles placed in the tank also provide surfaces
from which a standing wave formation will be created. This
means that gaseous bubbles will collect at nodal points in
the standing wave and provide further reflecting surfaces.
In early equipments using continuous wave power supplies
it was necessary to move the articles in the bath to avoid the
masking effect of the reflecting layers. It was realized that
by pulsing the electrical input, time would be allowed
between pulses for the bubbles to disperse and promote
continuous cavitation over the entire volume of liquid
within the sound field. A pulsed system also somewhat
simplifies the electrical circuit, and most of the present-day
equipments incorporate pulsed supplies, usually at twice
mains frequency. Some designs use a swept-frequency
generator to achieve the same purpose, and this also allows
unselected transducers to be used. With transducers of
high Q-factor a multiple element array would normally
require close frequency matching to ensure that full power
is delivered by each element. When a sweep frequency of a
bandwidth covering the frequency spread of the transducers
is used, there will always be one or more transducers
delivering full power at any given time. However, acoustic
power within the bath must be uneven owing to the random
excitation of the individual elements.
The cleaning effect due to intense cavitation is strikingly
shown in Fig. 2 where dirt from a wafer switch is being
rapidly dispersed.
SURFACE ADULTERATION
The removal of dirt from contaminated surfaces is a
problem experienced in all industries. The contaminant can
be of many forms and be held to the surface by a number of
forces. In the simplest case a clean surface will collect dust
by atmospheric deposition. Electrostatic forces will sometimes prevent removal but generally there is no cleaning
problem. Perhaps the most common soiling is found where
solid particles are bonded to the surface by an oily film.
This film can be either organic or inorganic, and if partial
carbonization has occurred because of heat, its removal will
present greater difficulties. Finally, there is contamination
due to a chemical bond such as an oxide film. In practice,
combinations of all three types of soiling are generally
experienced.
One of the widely known effects of ultrasonic cavitation
is that of emulsification. Oil can be finely dispersed through
water to form an emulsion and this process forms the basis
for simple ultrasonic cleaning. A soiled oily surface
immersed in water and subjected to. intense ultrasonic
energy will have the soiling removed by two separate
processes. The oil film initially emulsifies leaving solid
particles adhering to the surface. These are in turn dispersed
by the vibration amplitude, leaving a clean surface. When
oxide film or scale is present, it is generally necessary to
soften or re-form the surface by chemical means before a
cleaning action can take place.
It is obvious that, while water can be used, there are
distinct advantages in choosing a solution that will dissolve
uLTRAsoNICs/April-June
1963
67
or alter the bonding medium. In the early days of ultrasonic
cleaning it was common to employ organic solvents such as
trichlorethylene.
Developments in surface chemistry have
now resulted in a range of water-soluble detergents that
show many advantages over organic solvents, and these
have largely replaced them in practical cleaning systems.
The correct choice of a solvent or detergent is important
and is made after a study of the type of soiling present.
TRANSDUCER
SYSTEMS
Two methods of converting electrical energy to high-power
ultrasonic energy are at present available. Magnetostrictive
transducers use the change in dimensions produced in
Fig. 3. Construction
of a Multipower transducer
A. Radiating
face; B. Low density material;
C. High density material; D. Cone spring washers;
E. Piezoelectric ceramic discs(4 in all); F. Compression bolt
certain materials when they are placed in a magnetic field.
Piezoelectric transducers are based on the dimensional
change occurring in some crystal structures when an
electrostatic field is present. Both systems are used in
ultrasonic cleaning baths, but magnetostrictive transducers
are mainly restricted to applications where localized highintensity fields are required.* The first high frequency
generators used quartz plates cut from single crystals, but
the high impedance of the transducers necessitated very high
energizing voltages and accurate frequency adjustment
because of the high Q-factor of the system. The development of barium titanate piezoelectric ceramic provided a
more adaptable .material, but manufacturers were still
biased towards the high frequency end of the ultrasonic
spectrum and barium titanate plates were used to generate
frequencies between 100 and 400 kc/s.5 The mechanical
Q-factor of barium titanate ceramic is very much lower than
that of quartz and so is the electrical impedance at
resonance. Power supplies became simpler, requiring less
precise frequency control and lower energizing voltages.
During the early 195Os, studies of the mechanism of
cavitation conclusively showed the advantage of reducing
68
ULTRASONICS/A~~~I-J~~~
1963
the frequency and as a result it steadily decreased, eventually
reaching 40 kc/s. Being a mechanically resonant structure,
a transducer is generally operated as a half-wavelength
element. In barium titanate this corresponds to a thickness
of about 2 in and is the largest dimension for a practical
single element transducer. Before a polycrystalline piezoelectric can be energized, it is necessary to prepolarize it by
means of a large electrostatic field, and there are many
difficulties in polarizing a thickness of more than 2 in.
Similarly, the energizing voltage must also be increased
with increasing thickness and this produces problems in
generator design.
Barium titanate has a Curie point at about 100°C;
that is, at this temperature the initial polarization of the
material is permanently destroyed. Dielectric and mechanical losses cause a normal temperature rise when the
material is driven at cavitation power densities, and without
the provision of elaborate cooling systems the operation of
a barium titanate cleaning bath is restricted to about 85°C.
More recent developments in cleaning fluids have produced
solutions which should be used at temperatures of over
lOO”C,and the limitations of barium titanate have restricted
their general employment. The discovery of the piezoelectric effect in polarized lead zirconate-titanate ceramic
gave a further impetus to ultrasonic cleaning by providing
a material capable of operating safely up to at least 250”C6>’
The greatly increased efficiency gave promise of higher
power densities, but once again thickness limitations
governed the lower frequency limit. With lead zirconatetitanate there is even difficulty in reaching 40 kc/s, and
because of this limitation, the material is rarely used as a
half-wavelength resonator for cleaners.
To solve this problem the principle of mass loading was
employed. In the early days of submarine detection
Langevin showed that a sandwich composed of a thin slice
of piezoelectric material and two metal masses could be
excited into resonance at a frequency governed by the
dimensions of the metal blocks.8 The quartz crystal used
by Langevin has been replaced by barium titanate or lead
zirconate-titanate and it is now possible to design transducers operating down to at least 10 kc/s.
Sandwich transducers are generally assembled using
epoxide resins as the jointing medium. During the extension
phase of the cyclic motion of the transducer, considerable
stresses are imparted across the cement joint and this limits
the maximum amplitude. A new form of sandwich transducer based on a pre-stressed structure overcomes this
problem. Known as the Multipower transducer it combines a bolted structure with asymmetrical operation giving
a single-face power gain. s~l” Dissimilar metals are used for
the two loading blocks, usually aluminium and steel.
To follow the law of conservation of momentum, the
amplitude at the face of the aluminium block must be
higher than that at the steel face. By using the lighter metal
as the radiator, considerably less energy is dissipated at the
unused back face and the overall power gain into the liquid
is made at least three times a normal symmetrical,system.
The construction of this transducer is shown in Fig. 3.
These are essentially systems with a relatively low mechanical
Q-factor, and with a bandwidth of 4 kc/s around a designed
resonant frequency of 20 kc/s, and thus obviate the
necessity of tuning controls or feedback circuits. The bolted
structure does not generally require cementing between the
ceramic and metal masses since the mechanical bias
suppresses the extension phase.
TANK DESIGN
Since no acoustic system can be studied without consideration of the complete structure, it is necessary to design
cleaning tanks based on correct acoustic principles if
maximum cleaning efficiency is to be obtained. There
would be little point in mounting one transducer with a face
area of 4 in2 in the centre of a tank bottom with an area of
100 in2 unless localized cleaning was required. The thin wall
of a typical cleaning tank does not act as part of the transducer face but is merely a transmission diaphragm.
Secondary vibrations of the tank structure play a minor
part in the distribution of cavitation zones and a study of
tank parameters has been the subject of a recent paper.ll It
is general to cover the tank floor with transducers to ensure
a minimum number of “dead spots” where little cleaning
Fig. 4. Tank assembly with 12 transducers
and a total input of 500 W
Fig. 5. Immersible transducer and lead
action is found. A typical tank assembly is shown in Fig. 4
where 12 transducers are energized by an average total input
power of 500 W. Each transducer is driven at an intensity
of 1.6 W/cm2 of radiating area. If internal losses in the
transducers are assumed to be negligible, the total output of
the units averaged over a tank bottom area of 813 cm2
results in a tank power density of 0.62 W/cm2. In all
Multipower cleaning tanks there are approximately 25 W/l
of liquid at average power, and twice this at peak power.
The power supply used with this tank consists of a fully
transistorized push-pull self-excited oscillator using multiple
parallel-connected germanium transistors. Operating frequency is centred at 20 kc/s, which is determined by the
inductance of a ferrite-cored output transformer and a
suitable capacitance. Full-wave silicon rectifiers are used in
the d.c. power supply, but the filter capacitor is of small
value, leaving the generator output waveform heavily
modulated at twice the mains frequency.
Frequency modulation of sufficient magnitude to eliminate manual tuning of the low Q-factor transducers is
developed by allowing the transformer ferrite core to
saturate periodically. Peak transistor current due to the
unsmoothed supply voltage is sufficient to alter the transformer inductance, and thus frequency modulates the selfexcited oscillator.
The compact layout possible with this design enables
generators to be stacked as modules. Immersible transducers of the type shown in Fig. 5 allow very large cleaning
installations to be built. Each generator energizes two or
more transducers, and thus provides an adaptable cleaning
system for complex operations. The packing density of the
power supplies is very high, 25 kW being easily contained in
a standard 6 ft rack.
CONCLUSIONS
The demand for reliability in industry has stimulated the
search for cleanliness. Clean rooms and mechanized
assembly procedures ensure that soiling does not occur after
piece parts are completed, but manufacturing operations
invariably produce contaminants.
The rapid growth of
ultrasonic cleaning has been due to the many man-hours
entailed in normal cleaning methods. Ultrasonic cleaners
save time and often produce results unequalled by any other
method, ensuring a progressive future for this technique.
REFERENCES
1. “High frequency ultrasonic generator,” Mullard Ltd., 1949.
2. CRAWFORD,ALAN E., “Ultrasonic engineering,” Butterworths
(1955).
3. NOLTINGK,B. E. and NEPPIRAS,E. A., Proc. Phys. Sot. London,
Series B 63, 674 (1950); 64, 1032 (1951).
4. MYERS, J. A. and GOODMAN, J. E., Pharm. J., 585, December
(1960).
5: AT~ERTON,L., Brit. Comm. and Electronics, 138, March (1957).
6. JAFFE,B., ROTH. R. S. and MARZULLO,S., J. appl. Phys. 25.
809, (1954)
7. CRAWFORD,A. E., Brit. J. appl. Phys. 12, 529 (1961).
8. LANGEVIN,P., Brit. Pat. No. 145,691: 1921.
9. RWH, S., Brit. Pat. No. 686,784: 1957.
10. CRAWFORD,A. E. Paper No. 23 at 4th I.C.A., Copenhagen,
August, 1962.
11. ROD, R. L., Paper presented at Brno, September, 1962.
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69