TECHNICAL INFORMATION, ARTICLES &
APPLICATIONS |
Jeff Hancock
Blue Wave Ultrasonics
Reprinted from
ASM HANDBOOK®
Volume 5 : Surface Engineering
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ULTRASONIC CLEANING involves the
use of high-frequency sound waves (above the upper range of
human hearing, or about 18 kHz) to remove a variety of
contaminants from parts immersed in aqueous media. The
contaminants can be dirt, oil, grease, buffing/polishing
compounds, and mold release agents, just to name a few.
Materials that can be cleaned include metals, glass,
ceramics, and so on. Ultrasonic agitation can be used with a
variety of cleaning agents: detailed information about these
agents is available in the other articles on surface
cleaning in this Section of the Handbook.
Typical applications found in the
metals industry are removing chips and cutting oils from
cutting and machining operations, removing buffing and
polishing compounds prior to plating operations, and
cleaning greases and sludge from rebuilt components for
automotive and aircraft applications.
Ultrasonic cleaning is powerful
enough to remove tough contaminants, yet gentle enough not
to damage the substrate. It provides excellent penetration
and cleaning in the smallest crevices and between tightly
spaced parts in a cleaning tank.
The use of ultrasonics in cleaning
has become increasingly popular due to the restrictions on
the use of chlorofluorocarbons such as
1,1,1-trichloroethane. Because of these restrictions, many
manufacturers and surface treaters are now using immersion
cleaning technologies rather than solvent-based vapor
degreasing. The use of ultrasonics enables the cleaning of
intricately shaped parts with an effectiveness that
corresponds to that achieved by vapor degreasing. Additional
information about the regulation of surface cleaning
chemicals is contained in the article "Environmental
Regulation of Surface Engineering" in this Volume. The
article "Vapor Degreasing Alternatives" in this Volume
includes descriptions of cleaning systems (some using
ultrasonics) that have been designed to meet regulatory
requirements while at the same time providing effective
surface cleaning.
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In a process termed cavitation,
micron-size bubbles form and grow due to alternating
positive and negative pressure waves in a solution. The
bubbles subjected to these alternating pressure waves
continue to grow until they reach resonant size. Just prior
to the bubble implosion (Fig. 1), there is a tremendous
amount of energy stored inside the bubble itself.
Figure 1
Fig. 1- Imploding
cavity in a liquid irradiated with ultrasound captured in
a high-speed flash photomicrograph. Courtesy of National
Center for Physical Acoustics, University of Mississippi.
Temperature inside a cavitating
bubble can be extremely high, with pressures up to 500 atm.
The implosion event, when it occurs near a hard surface,
changes the bubble into a jet about one-tenth the bubble
size, which travels at speeds up to 400 km/hr toward the
hard surface. With the combination of pressure, temperature,
and velocity, the jet frees contaminants from their bonds
with the substrate. Because of the inherently small size of
the jet and the relatively large energy, ultrasonic cleaning
has the ability to reach into small crevices and remove
entrapped soils very effectively.
An excellent demonstration of this
phenomenon is to take two flat glass microscope slides, put
lipstick on a side of one, place the other slide over top,
and wrap the slides with a rubber band. When the slides are
placed into an ultrasonic bath with nothing more than a mild
detergent and hot water, within a few minutes the process of
cavitation will work the lipstick out from between the slide
assembly. It is the powerful scrubbing action and the
extremely small size of the jet action that enable this to
happen.
Ultrasound Generation.
In order to produce the positive and negative pressure waves
in the aqueous medium, a mechanical vibrating device is
required. Ultrasonic manufacturers make use of a diaphragm
attached to high-frequency transducers. The transducers,
which vibrate at their resonant frequency due to a
high-frequency electronic generator source, induce amplified
vibration of the diaphragm. This amplified vibration is the
source of positive and negative pressure waves that
propagate through the solution in the tank. The operation is
similar to the operation of a loudspeaker except that it
occurs at higher frequencies. When transmitted through
water, these pressure waves create the cavitation processes.
The resonant frequency of the
transducer determines the size and magnitude of the resonant
bubbles. Typically, ultrasonic transducers used in the
cleaning industry range in frequency from 20 to 80 kHz. The
lower frequencies create larger bubbles with more energy, as
can be seen by dipping a piece of heavy-duty aluminum foil
in a tank. The lower-frequency cleaners will tend to form
larger dents, whereas higher-frequency cleaners form much
smaller dents.
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The basic components of an
ultrasonic cleaning system include a bank of ultrasonic
transducers mounted to a radiating diaphragm, an electrical
generator, and a tank filled with aqueous solution. A key
component is the transducer that generates the
high-frequency mechanical energy. There are two types of
ultrasonic transducers used in the industry, piezoelectric
and magnetostrictive. Both have the same functional
objective, but the two types have dramatically different
performance characteristics.
Piezoelectric transducers
are made up of several components. The ceramic (usually lead
zirconate) crystal is sandwiched between two strips of tin.
When voltage is applied across the strips it creates a
displacement in the crystal, known as the piezoelectric
effect. When these transducers are mounted to a diaphragm
(wall or bottom of the tank), the displacement in the
crystal causes a movement of the diaphragm, which in turn
causes a pressure wave to be transmitted through the aqueous
solution in the tank. Because the mass of the crystal is not
well matched to the mass of the stainless steel diaphragm,
an intermediate aluminum block is used to improve impedance
matching for more efficient transmission of vibratory energy
to the diaphragm. The assembly is inexpensive to manufacture
due to low material and labor costs. This low cost makes
piezoelectric technology desirable for ultrasonic cleaning.
For industrial cleaning, however, piezoelectric transducers
have several shortcomings.
The most common problem is that the
performance of a piezoelectric unit deteriorates over time.
This can occur for several reasons. The crystal tends to
depolarize itself over time and with use, which causes a
substantial reduction in the strain characteristics of the
crystal. As the crystal itself expands less, it cannot
displace the diaphragm as much. Less vibratory energy is
produced, and a decrease in cavitation is noticed in the
tank. Additionally, piezoelectric transducers are often
mounted to the tank with an epoxy adhesive, which is subject
to fatigue at the high frequencies and high heat generated
by the transducer and solution. The epoxy bond eventually
loosens, rendering the transducer useless. The capacitance
of the crystal also changes over time and with use,
affecting the resonant frequency and causing the generator
to be out of tune with the crystal resonant circuit.
Energy transfer of a piezoelectric
transducer is another factor. Because the energy is absorbed
by the parts that are immersed in an ultrasonic bath, there
must be a substantial amount of energy in the tank to
support cavitation. If this is not the case, the tank will
be "load-sensitive" and cavitation will be limited,
degrading cleaning performance. Although the piezoelectric
transducers utilize an aluminum insert to improve impedance
matching (and therefore energy transfer into the radiating
diaphragm), they still have relatively low mass. This low
mass limits the amount of energy transfer into the tank (as
can be seen from the basic equation for kinetic energy, mv2
). Due to the low mass of the piezoelectric transducers,
manufacturers must use thin diaphragms in their tanks. A
thick plate simply will not flex (and therefore cause a
pressure wave) given the relatively low energy output of the
piezoelectric transducer. However, there are several
problems with using a thin diaphragm. A thin diaphragm
driven at a certain frequency tends to oscillate at the
upper harmonic frequencies as well, which creates smaller
implosions. Another problem is that cavitation erosion, a
common occurrence in ultrasonic cleaners, can wear through a
thin-wall diaphragm. Once the diaphragm is penetrated, the
solution will damage the transducers and wiring, leaving the
unit useless and requiring major repair expense.
Magnetostrictive Transducers
are known for their ruggedness and durability in industrial
applications. Zero-space magnetostrictive transducers
consist of nickel laminations attached tightly together with
an electrical coil placed over the nickel stack. When
current flows through the coil it creates a magnetic field.
This is analogous to deformation of a piezoelectric crystal
when it is subjected to voltage. When an alternating current
is sent through the magnetostrictive coil, the stack
vibrates at the frequency of the current.
The nickel stack of the
magnetostrictive transducer is silver brazed directly to the
resonating diaphragm. This has several advantages over an
epoxy bond. The silver braze creates a solid metallic joint
between the transducer and the diaphragm that will never
loosen. The silver braze also efficiently couples the
transducer and the diaphragm together, eliminating the
damping effect that an epoxy bond creates. The use of nickel
in the transducers means there will be no degradation of the
transducers over time; nickel maintains its magnetostrictive
properties on a constant level throughout the lifetime of
the unit. Magnetostrictive transducers also provide more
mass, which is a major factor in the transmission of energy
into the solution in the ultrasonic tank. Zero-space
magnetostrictive transducers have more mass than
piezoelectric transducers, so they drive more power into the
tank, and this makes them less load-sensitive than
piezoelectric systems.
A radiating diaphragm that uses
zero-space magnetostrictive transducers is usually 5 mm
(3/16 in.) or greater in thickness, eliminating any chance
for cavitation erosion wearthrough. Heavy nickel stacks can
drive a plate of this thickness and still get excellent
pressure wave transmission into the aqueous solution.
In summary, the advantages of
zero-space magnetostrictive transducers are:
- They are silver brazed for
permanent bonding with no damping effect
- They provide consistent
performance throughout the life of the unit with no
degradation of transducers
- Their high mass results in high
energy in the tank and less load sensitivity
- Their thick diaphragm prevents
erosion wear-through
The magnetostrictive transducer is
not as efficient as a piezoelectric transducer. That is, for
a given voltage or current displacement, the piezoelectric
transducer will exhibit more deflection than the
magnetostrictive transducer. This is a valid observation;
however, it has offsetting disadvantages. The efficiency of
concern should be that of the entire transducing system,
including not only the transducer but also the elements that
make up the transducer, as well as the diaphragm. It is the
inferior mounting and impedance matching of a
piezoelectric-driven diaphragm that reduces its overall
transducing efficiency relative to that of a
magnetostrictive transducer.
The ultrasonic generator
converts a standard electrical frequency of 60 Hz into the
high frequencies required in ultrasonic transmission,
generally in the range of 20 to 80 kHz. Many of the better
generators today use advanced technologies such as sweep
frequency and autofollow circuitry. Frequency sweep
circuitry drives the transducers between a bandwidth
slightly greater and slightly less than the center
frequency. For example, a transducer designed to run at 30
kHz will be driven by a generator that sweeps between 29 and
31 kHz. This technology eliminates the standing waves and
hot spots in the tank that are characteristic of older,
fixed-frequency generators. Autofollow circuitry is designed
to maintain the center frequency when the ultrasonic tank is
subjected to varying load conditions. When parts are placed
in the tank or when the water level changes, the load on the
generator changes. With autofollow circuitry, the generator
matches electrically with the mechanical load, providing
optimum output at all times to the ultrasonic tank.
Ultrasonic tanks
are generally rectangular and can be manufactured in just
about any size. Transducers are usually placed in the bottom
or on the sides, or sometimes both when watt density (watts
per gallon) is a concern. The transducers can be welded
directly into the tank, or watertight immersible units can
be placed directly into the aqueous solution. In some
instances the immersibles may be mounted at the top of the
tank, facing down. For applications such as strip cleaning,
one immersible is placed on top and one on the bottom, with
minimal distance between them. The strip is then run through
the very high energy field. A tank should be sturdy in
construction, ranging from 11 to 14 gauge in thickness.
Larger, heavy-duty industrial tanks should be 11 to 12 gauge
and should contain the proper stiffeners for support due to
the weight of the solution.
Return to Top
The solution used in ultrasonic
cleaning is a very
important consideration. Solvents such as
1,1,1-trichloroethane and freon have been used effectively
for many years, with and without ultrasonics. However, with
the advent of the Montreal protocol, which calls for
elimination of key ozone-depleting substances by 1996,
companies are searching for more environmentally friendly
methods to clean their parts. Chemical formulators are
developing products that meet the demands of cleaning
operations, yet are compatible with the health and
well-being of society.
Whenever possible, it is best to
use a water-based detergent in the ultrasonic cleaning
process. Water is an excellent solvent, nontoxic,
nonflammable, and environmentally friendly. However, it can
be difficult and expensive to dispose of soiled water.
Rinsing and drying can also be difficult without detergents.
High surface tension exists in solutions without detergents,
thus making rinsing difficult in hard-to-reach areas.
Detergents can therefore be added to lower the surface
tension and provide the necessary wetting action to loosen
the bond of a contaminant to a substrate. As an added bonus,
the cavitation energy in a water-based solution is more
intense than in an organic solvent.
Table 1 is a guide for selection of
appropriate cleaning agents for use with ultrasonic
cleaning. Additional information about many of these agents
is available in the other articles in this Section of the
Handbook.
Solution temperature
has a profound effect on ultrasonic cleaning effectiveness.
In general, higher temperatures will result in higher
cavitation intensity and better cleaning. However, if the
temperature too closely approaches the boiling point of the
solution, the liquid will boil in the negative pressure
areas of the sound waves, reducing or eliminating cavitation.
Water cavitates most effectively at about 70ºC (160ºF); a
caustic/water solution, on the other hand, cleans most
effectively at about 82ºC (180ºF) because of the increased
effectiveness of the chemicals at the higher temperature.
Solvents should be used at temperatures at least 6ºC (10ºF)
below their boiling points (Ref 2).
Table 1
Solutions Used With Ultrasonic Cleaning Of Various Parts
(Source: Ref. 1)
|
Material of construction |
Types
of parts |
Contaminants |
Suitable cleaning agent |
| Iron, Steel, Stainless
steel |
Castings, stamping,
machined parts, drawn wire, diesel fuel injectors |
Chips, lubricants, light
oxides |
High caustic with
chelating agents |
| Iron, Steel, Stainless
steel |
Oil-quenched, used
automotive parts; fine mesh and sintered filters |
Carbonized oil grease,
carbon smut, heavy grime deposits |
High caustic, silicated |
| Iron, Steel, Stainless
steel |
Bearing rings, pump
parts, knife blades, drill taps, valves |
Chips; grinding, lapping
and honing compounds; oils; waxes and abrasives |
Moderately alkaline |
| Iron, Steel, Stainless
steel |
Roller bearings,
electronic components that are affected by water or pose
dryer problems, knife blades, sintered filters |
Buffing and polishing
compounds; miscellaneous machining, shop and other soils |
Chlorinated-solvent
degreaser (inhibited trichloroethylene, for example) |
| Aluminum and zinc |
Castings, open-mesh air
filters, used automotive carburetor parts, valves, switch
components, drawn wire |
Chips, lubricants and
general grime |
Moderately alkaline,
specially inhibited to prevent etching of metal, or
neutral synthetic (usually in liquid form) |
| Copper and brass (also
silver, gold, tin, lead, and solder) |
Printed circuit boards,
waveguides, switch components, instrument connector pins,
jewelry (before and after plating), ring bearings |
Chips, shop dirt,
lubricants, light oxides, fingerprints, flux residues,
buffing and lapping compounds |
Moderately alkaline,
silicated, or neutral synthetic (possibly with ammonium
hydroxide for copper oxide removal) |
| Magnesium |
Castings, machined parts |
Chips, lubricants, shop
dirt |
High caustic with
chelating agents |
| Various metals |
Heat treated tools, used
automotive parts, copper-clad printed circuit boards, used
fine-mesh filters |
Oxide coatings |
Moderately to strong
inhibited proprietary acid mixtures specific for the oxide
and base metal of the part to be cleaned (except
magnesium) |
| Glass and ceramics |
Television tubes,
electronic tubes, laboratory apparatus, coated and
uncoated photographic and optical lenses |
Chips, fingerprints,
lint, shop dirt |
Moderately alkaline or
neutral synthetic |
| Plastics |
Lenses, tubing, plates,
switch components |
Chips, fingerprints,
lint, shop dirt |
Moderately alkaline or
neutral synthetic |
| Various metals, plastics
(nylon, Teflon, epoxy, etc.), and organic coatings when
water solutions cannot be tolerated |
Precision gears,
bearings, switches, painted housings, printed circuit
boards, miniature servomotors, computer components |
Lint, other particulate
matter, and other light oils |
Trichlorotrifluoroethane
(fluorocarbon solvent), sonic-vapor degreaser |
Return to Top
Considerations in the design of any
cleaning system include the contaminants on the part(s), the
required cleanliness level, the geometry and material of the
part(s), the quantity to be processed, and the previous
system design and layout (if applicable). The part geometry,
production rate, and cleaning time required will determine
the size of the cleaning system, once the overall process
has been decided. Typical tanks range from 20 to 400 L (5 to
1000 gal), and some are even larger.
Industrial, heavy-duty applications
require industrial, heavy-duty ultrasonic equipment. Other
factors that need to be considered are cleaning solutions
and temperatures, rinsing (with or without ultrasonics),
drying, automation, and load requirements. Most
manufacturers of ultrasonic cleaning systems will assist in
these decisions and will offer laboratory services and
technical expertise. A typical system is shown in Fig. 2.
Figure 2
Fig. 2- Automated
ultrasonic cleaning system. This system is designed to
clean intricate metal hearing-aid components using a
neutral-pH solution at 60ºC (140ºF) and three rinse stages
at 70ºC (160ºF). Basket rotation (1 to 3 rpm) is used
during each stage to ensure adequate cleaning and rinsing.
The system computer controls all functions, including the
hoist, and allows for storage of different process
parameters for different types of parts. Courtesy of Blue
Wave Ultrasonics.
Cleanliness Considerations.
In a typical aqueous ultrasonic cleaning system, it is the
cleaning stage(s) that will remove or loosen the
contaminants. The following rinse stage(s) remove any
remaining loosened soils and residual detergent, and a dryer
removes any remaining rinse water. The overall process of
the system is usually determined experimentally. Most
reputable industrial cleaning equipment manufacturers have
an applications lab where, through a process of experience,
trial, and error, a properly designed cleaning process can
be determined to meet the cleanliness levels specified.
There are a variety of ways to
check for cleanliness. Some are as simple as a water break
test on the part to see if most oil has been removed. Others
are as elaborate as surface quality monitoring that uses
optically stimulated electron emission technology to measure
thin films of contaminants down to the Angstrom level.
Changing Existing Systems.
If a current system exists, such as a vapor degreaser or
soak tank, several things need to be considered. It may be
practical, and possibly most economical, to retrofit the
existing unit from one that uses solvent an organic solvent
to one that uses an aqueous cleaner. Ultrasonic transducers
can be added to an existing tank by cutting a hole in the
tank and welding the transducer(s) in, or by simply dropping
a watertight immersible unit into the tank. The latter
method will take up some room in the tank, but it requires
less labor. Additional work may have to be done to the tank,
such as removing the cooling coils from the vapor degreaser,
adding additional fittings for a filtration system, and so
on.
In some existing systems, there is
a large inventory of stainless steel baskets for handling
the parts throughout the cleaning system. If possible, it is
best to use these baskets due to the relatively high cost of
replacement. In ultrasonic cleaning, the mesh size or hole
configuration of the basket is very important. Some mesh
sizes will inhibit the cavitation process inside the basket,
thereby affecting the overall cleaning capability. Mesh
sizes greater than 200 mesh or less than 10 mesh work best.
An interesting note is that ultrasonic activity will pass
through a variety of media. For example, solution A placed
in a Pyrex beaker will cavitate if placed in solution B,
which is cavitating in an ultrasonic tank.
Additional information on adapting
vapor degreasing systems for ultrasonic immersion cleaning
is provided in the article "Vapor Degreasing Alternatives"
in this volume.
Part Handling.
The geometry of the parts must be carefully analyzed to
determine how they will be placed in the cleaning tank.
Large parts, such as engine blocks, can be suspended
directly from a hoist, whereas smaller parts will usually be
placed in a basket. The most important factor in parts
placement is to be sure that air is not trapped anywhere
inside the part. If an air pocket is allowed to form, such
as in a blind hole that would be facing downward toward the
bottom of the tank, the cleaning solution and effects of
cavitation will not be able to reach this particular area.
The part will have to be rotated somehow in the tank during
the cleaning process to allow the cleaning solution to reach
the area where air was previously trapped. This can be
accomplished either manually, by the attending operator, or
by a rotating arm on an automated lift mechanism.
It is best if small parts can be
physically separated when placed in a basket. An example
would be to place machined valve bodies in a basket with
some type of divider or locator for each one. Many times,
however, in high output lines it is not possible to separate
parts physically, such as in the manufacture of electrical
connector pins where thousands of parts may need to be
cleaned at one time because of the high production output
and the small size. Ultrasonic agitation will be able to
reach between these parts and allow the solution's scrubbing
power to remove the contaminants, even if the parts are
stacked on top of one another. On the other hand, rinse
water may not remove all of the residual detergent , and a
dryer has a very hard time removing moisture from embedded
parts. The problem is easily solved by having an automated
hoist with a constant rotating fixture on the arm that
allows the basket to tumble at 1 to 3 rpm. This rotation
allows the parts to tumble slowly and exposes the embedded
pieces for proper rinsing and drying.
1.
Ultrasonic Cleaning, Tool and Manufacturing Engineers
Handbook, Vol. 3, Materials, Finishing, and Coating,
C. Wick and R.F. Veilleux, Ed., Society of Manufacturing
Engineers, 1985, p 18-20 to 18-24
2.
F.J. Fuchs, Ultrasonic Cleaning, Metal Finishing
Guidebook and Directory, Elsevier Science, 1992, p
134-139
Reprinted with permission from
"Ultrasonic Cleaning", published in the ASM Handbook,
Vol. 5, Surface Engineering, p 44-47, copyright 1994,
ASM International, Materials Park, OH 44073-0002. Although
this article is being used with permission, ASM did not
prepare the version for Web display. For information about
purchasing ASM Handbooks in print or on CD-ROM, visit ASM
International on the Web at
http://www.asm-intl.org
or call 1-800-336-5152 ext. 5900.
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