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Some Common Questions regarding ultrasonic cleaning are:-
Q.
What is Ultrasonic Cleaning
A. 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, there is a tremendous amount of energy stored inside the
bubble itself.
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 suction jet about one-tenth
the bubble size, which travels at speeds up to 400 km/hr away from
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.
For a Wikipedia guide to ultrasonic cleaning
click here.
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A. "Cavitation" is the rapid
formation and collapse of millions of tiny bubbles (or cavities) in
a liquid. Cavitation is produced by the alternating high and low
pressure waves generated by high frequency (ultrasonic) sound.
During the low pressure phase, these bubbles grow from microscopic
size until, during the high pressure phase where they are compressed
and implode.
Examples of cavitation bubbles in
action.
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A. "Degassing" is the initial removal
of gases present in the solution. Non removal or "trapped Gas" in
the cleaning solution leads to gaseous cavitation where bubbles
cannot effictivly form, useful cavitation occurs after gasses have
been removed from the cleaning solution, leaving a vacuum in the
formed bubble. When the high pressure wave hits the bubble wall, the
bubble collapses; it is the energy released by this collapse that
will assist a detergent in breaking the bonds between parts and
their soils.
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Q. How do I
"Degas" my ultrasonic Cleaner?
Once filled with water the
ultrasonics should be left operating for a few minutes so that
excess air is driven from the liquid. Tiny bubbles will be seen to
rise to the surface but this is gaseous cavitation and not suitable
for cleaning.
At the end of the de-gasing
period, a hissing sound and a churning of the liquid surface (cold
boiling) should occur indicating vaporous cavitation. At this point
switch off the ultrasonics add chemical as required and repeat this
cycle.
Once cold boiling returns the
liquid is now in the correct mode for ultrasonic cleaning.
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A. There are many considerations
important to ultrasonic cleaning. Optimizing these variables will
produce the best cleaning. The most important decisions to be made
are choosing the proper cleaning solution, cleaning at the right
temperature for the correct amount of time, and choosing the right
size and type of ultrasonic cleaner.
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A. Soils adhere to the parts... if
they didn't, the soil would just fall off the parts! The purpose of
the solution is to break the bonds between parts and their soils.
Water alone has no cleaning properties. The primary purpose of the
ultrasonic activity (cavitation) is to assist the solution in doing
its job. An ultrasonic cleaning solution contains various
ingredients designed to optimize the ultrasonic cleaning process.
For example, increased cavitation levels result from reduced fluid
surface tension. An ultrasonic solution will contain a good wetting
agent or surfactant.
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A. Modern ultrasonic cleaning
solutions are compounded from a variety of detergents, wetting
agents and other reactive components. A large variety of excellent
formulations are available, designed for specific applications.
Proper selection is crucial for acceptable cleaning activity and to
preclude undesirable reactivity with the part being cleaned. A
crucial part of Hilsonic's service is that we work with the client
offering advise on cleaning solutions as necessary.
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A. Flammables or solutions with low
flash points should only be used on a suitable Hilsonic's ATEX EX
rated system. The energy released by cavitation is converted to heat
and kinetic energy, generating high temperature gradients in the
solution, and can create hazardous conditions with flammable
liquids. Acids, bleach and bleach by-products should generally be
avoided, but may be used with indirect cleaning in a proper indirect
cleaning container, such as a glass beaker, and appropriate care.
Acid and bleach will damage stainless steel tanks, and/or create
hazardous conditions.
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A. With certain cautions, ultrasonic
cleaning is considered safe for most parts. While the effects of
thousands of implosions per second is very powerful, the cleaning
process is safe since the energy is localized at the microscopic
level. The most important cautionary consideration is the choice of
cleaning solution. Potentially adverse effects of the detergent on
the material being cleaned will be enhanced by the ultrasonics.
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A. Cleaning solutions should be
replenished when a noticeable decrease in cleaning action occurs, or
when the solution is visibly dirty or spent. A fresh batch of
solution at each cleaning session is usually not required.
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A. The solution level should always
be maintained. Maintaining the proper solution level provides
optimum circulation of solution around parts, and protects heaters
and transducers from overheating or stress.
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A. Cleaning time will vary, depending
on such things as soil, solution, temperature and the degree of
cleanliness desired. Highly visible removal of soils should start
almost immediately after the ultrasonic cleaning action begins.
Cleaning time adjustment is the easiest (and most often misapplied)
factor used to compensate for process variables. Although new
application cycle duration can be approximated by an experienced
operator, it usually must be validated by actual use with the chosen
solution and the actual soiled parts.
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A. The primary purpose of the unit
heater is to maintain a solution temperature between cleaning
cycles. The tremendous energy released by cavitation will generate
the heat for cleaning.
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A. Most poor cleaning usually results
from improper control of one or more process variable(s); such as
choosing the wrong detergent solution, insufficient heat, or not
allowing enough time for the particular soil to be removed. If you
suspect that your ultrasonic cleaner is not cavitating properly,
there are two simple tests you can perform: the "glass slide" test
and the "foil" test.
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A. Wet the frosted portion of a glass
slide with tap water and draw an "X" with a No. 2 pencil from corner
to corner of the frosted area. Making sure that the tank is filled
to the fill line, immerse the frosted end of the slide into fresh
cleaning solution. Turn on the ultrasonics. The lead "X" will begin
to be removed almost immediately, and all lead should be removed
within ten seconds.
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A. Cut three small pieces of aluminum
foil about 4" x 8" each. Fold each piece over a rod that you will
use to suspend the foil in the tank. A clothes hanger works well.
Your cleaner should be filled with an ultrasonic cleaning solution,
degassed, and brought up to normal operating temperature. Suspend
the first "square" in the center of the tank and the other two a
couple of inches from each end of the tank. Make sure that the tank
is filled to the fill line, and turn on the ultrasonics for about
one minute. Remove the foil and inspect: All three pieces of
aluminum foil should be perforated and wrinkled to about the same
degree.
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A. Items being cleaned should never
be placed directly on the tank bottom. Transducers (which produce
the ultrasound) are sometimes bonded to the bottom of the tank.
Items resting directly on the tank bottom can damage the transducers
and/or reduce cavitation. Additionally, a tray or beaker will
position the item within the optimal cleaning zone of the tank. The
tray or beaker will also hold the load together and allow for easy,
no-touch removal, draining and transport of the items to the next
step in the cleaning process.
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A. Heat usually enhances and speeds
up the cleaning process, and most detergent solutions are designed
to work best at an elevated temperature. The best way to find the
optimum temperature, which will give you the fastest, cleanest and
safest results, is to run tests. Usually, the best results are
within the 50°C to 75°C range.
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A. Rinsing is recommended to remove
any chemical residue, which could be harmful to the part. Parts can
be rinsed right in your ultrasonic cleaner, using a clean water
bath, or in a separate tub containing tap, distilled or deionized
water.
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A. Low solution levels can seriously
damage your cleaner. Running your unit continuously runs the strong
risk of lowered levels as the solution evaporates, especially when
heated. Getting into the habit of shutting off the ultrasonics when
not in use, and monitoring the solution level when in use, will
yield many years of trouble free service from your ultrasonic
cleaner.
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If you require any further information on any of the above points
please feel free to
contact us and we will be happy to answer your queries.
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