Ultrasonic Cleaning,Ultrasonic Cleaning,Ultrasonic Cleaning,Ultrasonic Cleaning
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.

Temperatures inside a caviting 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
process.
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.

Equipment
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, 1
2
mν2). 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, and nickel has a unique property of
expanding or contracting when it is exposed to the 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 stainless steel 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.

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