Page 3 - Selected Articles.
CALL FOR CONTRIBUTIONS: I am working on a book for Marcel Dekker on "High-Intensity Ultrasonic Technology and Applications" (in their "Mechanical Engineering Series", edited by Profs. Lynn L. Faulkner and S. Bradford Menkes). This book will focus on the practical application of power (high-intensity) ultrasonics, the use of ultrasonic energy to change materials. Contributions are welcome.
ULTRASONIC CLEANING
{this is a work in process}
I shall define "ULTRASONIC CLEANING" as the application of sound at extremely high intensity and high frequency (normally above human hearing, 20kHz - 20,000 cycles per second - and above) to change surfaces, i.e.: to clean them.
The term "MEGASONICS" is now being used to describe frequencies of 1,000,000Hz (1,000kHz) and above.
Such cleaning is accomplished by accelerating both physical and chemical reactions at the surface.
CLEANING OPERATIONS include:
Surface Cleaning, Preparation, and Treatment - Enhancement of Surfactancy and Detergency - Vapor Degreasing - Turbidity Measurement, etc.
ULTRASONIC CLEANING
Ultrasonic cleaning involves changing the surface of materials by the application of ultrasonics, thereby removing contaminants; it is primarily the removal of contaminant from surfaces of materials through the action of ultrasonically-induced cavitation. Refer to the main ULTRASONICS page, et seq., and the GLOSSARY for more-rigorous descriptions of ultrasonics and cavitation.
The action may occur in plain water but is often enhanced by the addition of surfactants and even detergents. Cavitation can also be induced in solvents, such as hydrocarbons and chlorofluorocarbons (CFCs), but these also have drawbacks of environmental and flammability hazards.
Any bath that is in some way activated by ultrasonics to produce cavitation is thus by definition an ultrasonic cleaner. There is a semantic problem, however, in that the term "cleaner" has two meanings in industry; one, the less technical usage, is a chemical compound used in cleaning, whereas the other, more technical, usage is the bath or tank in which ultrasonic cleaning takes place For the purposes of this text, we shall use the latter; an ultrasonic cleaner shall mean an ultrasonically-activated container or tank (the other usage shall be referred to as a cleaning agent or compound).
Cavitation is the sequential formation and collapse of vapor bubbles and voids in a liquid subjected to acoustic energy at high frequency and intensity. This action is analgous to thermal boiling but without the associated rise in temperature of the mass of liquid, although localized temperatures on the molecular level can be extremely high. The volume within a bath in which active cavitation is generated by a radiating surface is called the cavitation field. Multiple transducers mounted to a radiating surface can generate multiple cavitation fields and the interaction and interference of these fields is a major design problem.
A typical cavitation bubble with the liquid jet, a jet of liquid moving at extreme velocity, resulting from the assymetrical implosion of the bubble in close proximity to the surface to be cleaned, is clearly shown in this dramatic high-speed motion micrograph:
[image from University of Washington, Applied Physics Laboratory (Lawrence Crum, Ph.D.)
- bubble diameter approximately 1mm]
Cavitation initiates most readily at, and proceeds radially outward from, discontinuities (voids, contaminant particles, and such) in the liquid, where bonds between adjacent particles are weakest. Theoretically, a completely pure liquid (an unlikely happenstance) would be virtually inmpossible to cavitate. However, somewhat conversely, once cavitation initiates, any gas bubbles in the bath absorb energy to no avail and must be removed before effective cleaning can proceed; this is normally done by running the tank (degassing it) for a few minutes until free bubbling (see Cavitation) ceases. Probe sonication is at so much higher an energy intensity that this procedure is not normally necessary in that procedure.
Further, a minute amount of surfactant must be present in most cases to assist in wetting the surfaces; unwetted surfaces will NOT be acted upon. Ordinary soaps and detergents are the normal source of surfactancy. As a rule of thumb (this is a very bad pun, as you will see), add only enough surfactant such that the liquid only just begins to feel slippery between the thumb and forefinger. Adding too much surfactant (soap, detergent) will be deleterious to good cleaning.
In addition, any surface with a concavity which could trap air or other gases and prevent full wetting of the surface will prevent activity on the that surface. Not only must the surface be wetted, it must be wholly submerged in the liquid, not merely wet. To effect such, the object to be cleaned must be rotated, completely under the surface, if necessary, to discharge any pockets of air or gas such that the gas rises out of the bath.
It is imperative in critical cleaning processes
to assure that there are NO pocketed gases
on the surfaces (internal or external) of the item.
"Blanketing" is a limiting phenomenon in the cavitation field in which the density of the bubble cloud is so great that no further cavitation can take place when additional energy is introduced (analgous to the phenomenon at the temperature of thermal boiling, above which no further change of state occurs ("blanketing" is a term coined by Berliner). The "blanketing threshold" is that intensity of cavitation at which the blanketing phenomenon occurs; for practical purposes, the blanketing threshold may be considered a relative term based on the efficiency of conversion from increased radiated energy to increased cavitation ("blanketing threshold" is also a term coined by Berliner).
The bubble cloud, a cloud of cavitation bubbles which hovers in front of an activated radiating surface, is strongest when the bath has deen degassed first; this entails running a tank for a few minutes until dissolved and suspended air and other gases are driven out by cavitation. Not doing so allows energy to be dissipated in the degassing phenomenon and not to be available for the cleaning process.
Radiation of energy into the bath is done through a diaphragm; this is usually the bottom of the tank, but may also be the side or end wall of the tank or the front surface of an immersible transducer or other radiating acoustic device that transmits ultrasonic energy from the stack or transducer into the liquid (analagous to the diaphragm in an early telephone), in effect thus forming the radiating surface (use of term "diaphragm" in this fashion coined by Berliner).
A tank is considered active if it is fitted with transducers and can be activated to produce cavitation; it is considered a still tank (terms coined by Berliner) if it has not (yet) been activated to produce cavitation or has not been fitted with ultrasonic transducers. A still tank can be activated by insertion of an immersible transducer into the bath.
ACTIVE TANKS
The more common method of inducing cavitation in a cleaning tank is to fasten
transducers to the outer surface of the bottom or sides (or both) of a tank and
thus to energize the inner surfaces of that tank, thereby transmitting ultrasonic
vibration into a liquid bath contained in the tank. Cavitation in the tank
creates shock waves and cleans surfaces of parts and assemblies by
accelerating detergency of cleaning agents in the bath and by mechanically
blasting contaminants off the surfaces. There are generally two styles of
such tanks, small, self-contained models primarily for home and laboratory use,
and larger units consisting of two (or more) modules, a tank and one or more
generators, intended for industrial uses. There is a sizeable overlap in
sizes and applications for the two styles. Off-the-shelf unitized models
range in size and capacity from ½-pint (1 liter) measuring 5" by 5" by 3" deep
(127 x 127 x 76mm) or smaller to 10 U.S. gallons (39 liters) measuring 16" by
14" by 11" deep (406 x 356 x 279mm) and larger. Stock industrial units
with a tank and one or more separate generators are available in sizes and
capacities from 3½-pints (13 liters) measuring 10" by 8" by 10" deep (254 x
334 x 254mm) or smaller to 46 U.S. gallons (167 liters) measuring 24" by 18"
by 24" deep (610 x 457 x 610mm) and larger. Special extra-deep
pipette cleaning models at 13 U.S. gallons (50 liters) measuring 12" by 10" by
25" deep (305 x 254 x 635mm) are also stocked. Larger units are
generally custom built.
Here are some illustrations for such equipment:
{Integrated herein 18 Jan 05, from further down the page.}
[All are by and © 2000 S. Berliner, III - all rights reserved.]
A typical low-intensity laboratory-type ultrasonic cleaning tank will usually have
a deep-drawn tank and may have one or more transducers bonded to the
bottom or side-wall of the tank to energize the wall or bottom as a diaphragm,
passing vibrational energy through virtually unimpeded and cavitating the
water or other liquid inside. Diagramatically, it looks like this:
Basic Ultrasonic Cleaner
Impressing too much voltage onto a crystal bonded on in this manner can
cause it to shatter. For more energy, especially in industrial usage, a
STACK similar to that used in a high-intensity ultrasonic probe-type
processor is used. The working parts inside the CONVERTOR of a
high-intensity ultrasonic probe, the STACK, are shown on
Ultrasonics Continuation Page 3; here, for comparison, is a typical (and
similar) stack used in a heavy-duty industrial cleaning tank or immersible
transducer. Note how the front driver here is a negative-gain device;
its function is to pass as much energy as possible into the wall/base of the tank
at moderate amplitude (too high an amplitude and the tank will hole through
from excessive cavitation and parts may be eroded):
Industrial Heavy-Duty Cleaner Stack
Now see how a cleaner stack is used with a tank:
Industrial Heavy-Duty Cleaner
Industrial tanks are usually made of bent and welded sheet metal.
In addition, magnetostrictive transducers may be fitted; see
MAGNETOSTRICTIVE TRANSDUCERS on Ultrasonic Cleaning continuation page 1.
IMMERSIBLE TRANSDUCERS
An immersible transducer is a radiating device sealed in a housing (usually stainless steel), the forward or front surface of which is the radiating surface, and which can be submerged under the surface of a liquid bath to energize the liquid to produce cavitation. An immersible transducer placed in a still tank turns that tank into an ultrasonic cleaner. The immersible transducer is, in effect, a standard tank everted (turned inside out) with the radiating surface on the outside and the transducers on the inside, as can be seen from the diagrammatic sketch, below:
[Image by and © 2000 S. Berliner, III - all rights reserved.]
Immersible Transducer
Immersibles can have one transducer stack (radiator), or two or more stacks in a row or an array, depending on the size tank to be energized and the power to be transmitted. The sketch does NOTshow the method by which electrical energy is transmitted to the electrodes; it must go through or over the tank wall or up through the tank bottom and then through the housing (box), with all interfaces completely water/liquid tight. A common way to simplify this is to use a piece of stainless steel conduit hanging over the rim of the tank; in that way only the entry into the housing (box) need be liquid-tight. The cable can also be in a flexible conduit, usually at extra charge.
# - the following paragraphs are only "teasers" - advisory
items for which more work needs to be done and illustrations need to
be added.
# Ultrasonic Vapor Degreasing
A vapor degreaser is a cleaning tank in which solvent is evaporated and cleaning and drying are done sequentially by immersing the part(s) to be cleaned in the vapors above a heated solvent tank (and perhpas in the hot solvent itself) and the withdrawing it (them) into the hot atmosphere above the vapor phase to dry. An ultrasonic vapor degreaser adds ultrasonics to the heated tank. Because of the stress on environmentally-safe solvents, non-CFC agents have been developed to keep this useful technique from being consigned to the scrap heap of technological history.
# Printed Circuit Boards and Chips
A whole separate discipline has arisen for cleaning critical Printed Circuit Boards and Chips; techniques such as Surface Wave Technology are used.
# High-intensity Cleaning of Porous Media
This started with Pall Corporation's HIPS© machine of the 1960s (the HyperIntense Proximal Scanning Ultrasonic Filter Element Cleaner, on which your author cut his cavitational teeth), which cleaned reusable fluid filter elements for aircraft and submarine and other similar critical uses by exposure to cavitation in a cleaning solvent directly under a ½" x 2½" (12.7mm x 63.5mm) horn. The cleaning solvent was pumped under pressure backward through the porous filter medium, usually sintered pleated wire mesh cylinders, to flush away contaminant and the unit provided means for performing bubble testing for maximum pore size determination and differential pressure testing for cleanliness evaluation. Since then, various other devices have been developed for like use.
# High-intensity Cleaning of Surfaces for NDE
Your author developed this technique (a form of turbidity measurement), in which an active ultrasonic probe tip is held in close proximity to a flooded surface, blasting contaminant away which is then measured by a particle analyzer, for an Australian steel firm ca. 1987.
# High-intensity Cleaning of Deep Holes
Misonix (formerly Heat Systems) pioneered this technique, in which very thin probes and even activated wires are run into holes to blast contaminant out and, if the probe is hollow and pressurized, flush out the detritus.
For reference, Cup Horns (see the processing section of the main ultrasonics page) can also be used for precision cleaning of small items.
For terminology, see the Ultrasonics Glossary page.
# Generators (syn. Power Supplies) for ultrasonic cleaners are usually rather different than those for processing and welding, having either loose frequency control or even deliberate frequency sweeping and multiple frequencies, this to avoid localized hot spots in the bath from standing wave formation.
[The author has always suspected that certain cleaning units advertising a wide range of frequencies are simply covering up really poor frequency control! There ARE some units that truly do offer multi-frequency and sweep-frequency operation but - - - caveat emptor!]
# Magnetostrictive cleaners are used especially for very high power applications and those requiring extremes of temperature.
# Piezoelectric cleaners are by far the most common ones in use today.
Megahertz (MHz) cleaners (operating at or above 1,000,000 cycles per second) have been developed, especially for microelectronics use, even up to 20-50MHz.
It should be noted here (as it is elsewhere on these pages) that higher frequencies
yield smaller cavitation bubbles and thus give better penetration into fine holes and
crevices and do less damage to components, but have less shock front energy
(intensity). The trade-off is that cleaning time may have to be increased for full
cleaning of items at higher frequencies. Similarly, lower frequencies give better
cleaning of heavy, bulky objects, but may cause greater cavitational erosion of the
workpiece surface.
MAGNETOSTRICTIVE TRANSDUCERS - moved
to Ultrasonic Cleaning continuation page 1 on 13 Feb 2005.
You may wish to look at the website of Precision Cleaning Magazine.
WHAT'S NEW?
What's new is that I had put some illustrations, for equipment that somehow
never got into the preceding text, down here, but now have integrated them
where they belong, above. (18 Jan 05)
Cleaning Berliner Gramophone Disk Recordings, Edison Cylinder Recordings, and the like - one should use the highest frequency commercially available (80KHz or higher, preferably much higher) and at very low energy (variable output would be desirable), to keep the cavitation implosions from eroding off the peaks of the tracks. Do NOT ask me which manufacturer makes such machines; I do not keep track (deliberate pun) of such.
MUCH MORE TO FOLLOW
You may wish to visit the main Ultrasonics page, et seq., as well as the Ultrasonics Glossary page {also in process}.
THUMBS UP!
THUMBS UP! - Support your local police, fire, and emergency personnel!
S. Berliner, III
To contact S. Berliner, III, please click here.
To tour the Ultrasonics pages in sequence, the arrows take you from the main Ultrasonics Page (Ultrasonics index, Applications List, Keywords/Applications Index, and Brainstorming) to Page A ("Condensed Guide to Ultrasonic Processing" and "A Popularized Guide to Ultrasonic Processing"), Page 1 (with "A Popularized Guide to Ultrasonic Cavitation" and Tubular Horns), Page 1A ("Amplitude Measurement", Free Bubbling, Bubble Entrapment, Foaming and Aerosoling, and Extenders), Page 2 (More on Cavitation and "Ultrasonics and Fine Particles"), Page 3 ("Ultrasonic Sterilization and Disinfection","Ultrasonics, Hearing, and Health", Ultrasonics and Living Organisms, and What's New?), Glossary Page, Cleaning Page (Immersible Transducers and What's New?), Bibliography Page 1 (Reference Books on Acoustics, Vibration, and Sound), Bibliography Page 2 (Sonochemistry), and Bibliography Page 3 (Selected Articles).
© Copyright S. Berliner, III -
1999, 2001, 2002, 2004,
2005
- All rights reserved.
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times since the counter was installed.
[The information on Cleaning
Solutions, Detergents, Precautions, and Suggestions, applies equally to any home or
hobby uses, as well as to many light industrial applications.]