There was a time ten or fifteen years ago when ultrasonic cleaning was considered
something of a novelty. Driving the interest at that time was the Montreal Protocol
and subsequent regulation of chlorinated solvents that meant that parts-cleaning
could no longer be economically or environmentally accomplished using FreonTM
or “1,1,1” or “trichlor.” Today, there’s a generation
of production people out there who don’t even know what “1,1,1”
was or how it was used in their facility.
But today, ultrasonic cleaning has come of age, filling particular kinds of
cleaning needs then as now, and—when properly applied—filling them
better than the previous methods ever did. Over the years, the technology has
steadily improved.
Ultrasonic cleaning can optimize the removal of some types of soils from certain
parts, such as buffing compound from crevices and tiny particles from metalworking
operations. Other excellent applications include precision cleaning of small
objects and electronics assemblies prior to other finishing operations, and
cleaning of valve bodies, transmission parts and sub-assemblies, medical devices
and injection molds.
Sometimes ultrasonics will speed up a cleaning operation that would otherwise
take much longer. For example, carbonization can be removed from injection molds
in minutes instead of hours with the right combination of ultrasonics, heat,
and cleaning solution. In other cases, ultrasonics are used to meet the challenge
of removing small particles from inaccessible areas—such as the sanitization
of medical instruments after manufacture.
Cleaning takes place when high frequency bursts of ultrasonic energy are applied
to a heated liquid cleaning solution that surrounds the parts. This energy produces
a three-dimensional wave pattern of alternating positive and negative pressure
areas within a cleaning tank. The alternating pattern creates bubbles during
periods of negative pressure and implodes them during periods of positive pressure
in a phenomenon known as “cavitation.” The implosion creates a microjet
action that penetrates and cleans areas impossible to reach with brushes, sprays
or dips.
The source of ultrasonic sound waves is a transducer, and there are two types:
magnetostrictive and piezoelectric. Magnetostrictive transducers have a ferrous
core that is oscillated by an electromagnetic field. They are almost always
found in lower frequency applications from 16-20 kHz and are especially suited
to heavy loads and high temperatures. Piezoelectric transducers are typically
ceramic and are highly efficient. Oscillation of piezoelectric transducers is
caused by electrical pulses at the resonate frequency, which is generally between
25 and 170 kHz but may be as high as 250 kHz, with 25-40 kHz being the most
common.
When cleaning with ultrasonics, the frequency of the sound waves is matched
to the application. For the most part, lower frequencies (20-40 kHz) are safe
for most applications and will produce the most intense cavitation energies
to remove the most common types of contaminants (oil, grease, metal chips).
Higher frequencies (68-250 kHz) will produce smaller cavitation bubbles with
less intense energies but more of them. This can be beneficial in the removal
of smaller particles and where damage is a concern (polished surfaces, delicate
parts, soft substrates).
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Cylinders cleaned using ultrasonic cleaning technology. Ultrasonic cleaning is an ideal way for removing small particles from inaccessible areas. It can also speed up cleaning operations that would otherwise take much longer.
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While ultrasonic devices have a natural frequency variation, additional frequency
modulation is now available through sweep frequency generators. Frequency-sweep
circuitry varies the frequency of the ultrasonic generator to create a more
uniform cleaning field by alleviating standing waves and hot spots sometimes
characteristic of older equipment. Power control circuitry tailors the output
to varying load conditions, thus improving versatility, which is especially
useful when different types of parts are being cleaned in the same line. The
newest ultrasonic technology puts more than one frequency in a single generator—a
more expensive option that nevertheless is sometimes required when cleaning
very dissimilar parts in one cleaning line.
Typical tanks range from the small ones used by jewelers or dentists to industrial
strength models holding hundreds or thousands of gallons of solution. Tank size
for a particular application depends on the size and volume of the parts being
cleaned, as well as the substrate and geometry of the parts, and the types of
soils being removed. Immersible ultrasonic transducer canisters can also be
retrofitted into existing tanks. (An added benefit of immersibles in any tank
scenario is that they can be swapped out for repair if required.) The amount
of ultrasonic power in a tank is measured in watts, and the proportion of watts
of ultrasonics to the size of the tank and the mass of the parts is critical.
Undersizing the watt density can mean that production-scale cleaning takes longer
than it should or does not occur properly.
The location of the transducers in the tank can also impact the effectiveness
of the cleaning process. Most commonly, transducers are bottom-mounted. However,
in certain instances where contaminant loading can endanger the transducers
and potentially reduce their effectiveness and life span (buffing and polishing
compounds, paints, inks), transducers can be mounted on the side wall of the
tank. Side mounting can also be indicated when part geometries call for particular
exposure angles to the ultrasonics.
The kind of liquid used is important, as is the temperature. Raising temperature
too high (above about 180F) reduces cavitation pressure and can therefore be
counter-productive.
Before the Montreal Protocol, what we now call “regulated solvents”
were often the cleaning solutions of choice in ultrasonic tanks. Today, they
and a new generation of solvents remain an important option for certain types
of cleaning, as is another class of cleaning solution called “semi-aqueous,”
which mixes solvents and water.
With the regulation of solvents also came the impetus to shift to water-based
cleaning solutions. These have greatly improved in the last ten years, especially
as some new surfactants have been developed for hard surface cleaning. There
are three types of aqueous cleaners: acidic, neutral and alkaline. The efficiency
of all of the cleaners increases in combination with ultrasonics. Also, the
percentage by volume in water and/or the aggressiveness of a cleaner can often
be minimized by augmenting the cleaning action with ultrasonics.
Acidic cleaners (pH less than six) consist of mineral and organic acids with
wetting agents. They are not generally used for the removal of oil and grease,
but are most widely used for the removal of metal oxides. With the addition
of ultrasonics this process can be accelerated and the acid used can therefore
be less aggressive. Neutral cleaners (pH of 6-8) consist mostly of surfactants.
They also contain mild builders and corrosion inhibitors. They are effectively
used to remove oil and light grease. Alkaline cleaners (pH of 8-14) are a blend
of builders such as potassium and sodium hydroxide, silicates, carbonates, bicarbonates,
phosphates, borates and surfactants. They are best suited for the removal of
oil, grease, inks and carbonaceous soils.
Cleaning solution, temperature and the mechanical action of ultrasonics are
a formidable combination against industrial contaminants, but sometimes, additional
types of mechanical action such as rotation, agitation and/or spray under submersion
are required to fully dislodge soils. Filtration of the cleaning tank is usually
recommended to pull particulate out of the bath and extend the solution life,
and surface skimming into a separate overflow weir prevents re-deposition of
soils as the parts exit the cleaning tank. Rinsing is key to overall success,
and in a high production setting multiple stage rinses with high quality water
(counterflowed for conservation if desired) are recommended to assure a spot
free result.
Today, ultrasonic cleaning applications range from removal of machining oils
on stainless steel and aluminum to buffing compound from brass; grinding compounds
from tool steel hand tools; stamping lubricants from stainless steel, copper
and mild steel; particulates from plastic jewel cases; wax from glass and more.
Whether delivered in a single tank or a fully automated multiple-station line,
today’s ultrasonic cleaning technology succeeds with a proven blend of
ultrasonic power, cleaning chemistry, temperature—and a good rinse.
