Comes of Age
by Brian Sutton, Miraclean, reprinted
with permission from Products Finishing Magazine (Gardner Publications),
November 2003, pp. 40-43.
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—when properly applied—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).
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.
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
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.