Ultrasonics, The Ultimate Clean
By Dick Herman, PRO TRACK
MODEL RACING INTERNATIONAL VOLUME 2 #2 SUMMER 1990
Our world of hobby, whether it be R/ C, slot pacing, trains or whatever, is continually expanding and refining itself through the application of new technology. With ideas and techniques we generate, plus some we borrow from other industry, we keep pushing ourselves and our equipment to the cutting edge. We can find good examples of this right on our work bench. Take solid state electronics for example, our world has really opened up with the miniaturization it offers us. R/C receivers are now about as big as a single vacuum diode was back in the 60's. Electronic speed controls is another good example. These were not marketed for R/C much before 1986, and today they predominate. R/C and slot tire compounds are being specially formulated with as much consideration as the rubber being raced at Indy and Daytona. Plastic materials account for a very large portion of things built for R/ C. slot. boats, trains and planes. Plastics with names like Lexan', "Kydex", "ABS", "Polystyrene", "Nylon", just to name a few. Some of these plastics had to wait for a technological break-thru before they could become a reality and today we adapt these break-thru materials to our everyday endeavors without a second thought.
A technology that is rapidly bringing the hobbyist to a higher plane can be found in an ultimate cleaning system that uses ultrasonics. What used to pass for "clean" in the past, today, just isn't enough. We used to think that if soap and water was good for dishes and taking showers, nothing but a harsh solvent would really do for our beloved motors, bearings and gears! Yes, gasoline, lighter fluid, trichlor, lethal carbon tet!, these were the kinds of solvents that we thought of when asked "what's best?" We weren't swayed by those technical terms found on the label of household dish washing soap, it just wasn't enough. But tell me. today, do you eat off of those dishes? Do you suppose they're clean? Of course they are. Why, with Moms' elbow grease, detergents and a solvent (water), the greasiest dish in the world will come clean. What was the technology used here? In brief, (1) Agitation, (2) Detergent. (3) Water. What! no trichlor?
Now, how does this end up in a discussion of high technology? Well, I'd like to regress back to the part about Moms' elbow grease and expand on it. Suppose your mother (or wife as the case may be) could bombard that dish with scouring pads, a few million say, and do it at 55,000 times per second! Of course this must be done below the surface of the water so that the particles of grease etcetera can be emulsified and surrounded with a soap film where they will not be able to re- adhere to the dish. This is something similar to what happens in a ultrasonic cleaner although ultrasonics is somewhat more complex and will do a more efficient job. I'll explain, but first, I'd like to define some terms used in describing ultrasonics.....
CAVITATION. In our case it is: "The formation of a gas (vapor) filled cavity within a liquid created by mechanical forces"... ULTRASONIC. Pertaining to frequencies just above the range of human hearing, hence above 20,0000 hertz... TRANSDUCER. Any device which converts one form of energy to another form, i.e. alternating electrical impulses converted to mechanical oscillations... DETERGENT. A synthetic cleansing agent resembling soap in its ability to emulsify oil and hold dirt and which contain surfactants... Now, ultrasonic cleaning.......
Theory of Ultrasonics
Ultrasonic cleaning depends upon cavitation. the rapid formation and violent collapse of minute bubbles or cavities in a cleaning liquid. This agitation by countless small and intense imploding bubbles creates a highly effective scrubbing of both exposed and hidden surfaces of parts immersed in the cleaning solution. Cavitation is produced by introducing high frequency (ultrasonic), high intensity sound waves into a liquid. Consequently, the three essential components of any ultrasonic cleaning system are: a tank to contain the cleaning liquid, a transducer which is attached to the tank and converts electrical energy into mechanical energy and an ultrasonic generator.
The heart of any ultrasonic cleaning system is the transducer. There are basically two types. One is electrostrictive which means that the transducer changes physical dimension when placed in a varying voltage electric field, this is known as the "piezoelectric effect' The other type of transducer is made form magnetostrictive matter which changes dimension in a varying magnetic field. Varying the voltage of the electrostrictive transducer, for example, generates mechanical pulses which are transmitted through the tank wall to the cleaning solution contained therein.
Regardless of the type of transducer used, the common factor is the intensity of cavitation produced. Ultrasonic energy, like any sound wove. is a series of pressure points or rather a series of compressions and rarefactions (opposite of compression). In operation. the liquid will actually be pulled apart at the rarefaction stage and small bubbles or cavities will be formed. (Fig 1) With the immediately following compression state, the bubbles will collapse or implode throughout the liquid, creating an extremely effective force which is uniquely suited to cleaning. This process is known as cavitation.
For theoretical considerations, it has been estimated that a pressure of more than 10,000 psi and a temperature greater than 20,000 degrees F can exist within the collapsing bubble, and shock waves radiate in all directions at the instant of collapse. The energy released from a single cavitation bubble is extremely small, but many millions of bubbles collapse every second. Cumulatively, the effect is very intense and produces the intense scrubbing action characteristic of ultrasonics not only on the surface, but also in holes and crevices.
Cavitation only occurs when the local pressure on the liquid is reduced to a value less than its vapor pressure. The amplitude of the ultrasonic waves generated by the transducer must be great enough to satisfy this condition. The minimum amount of power necessary to initiate cavitation is referred to as the 'threshold of cavitation" Different liquids will have different thresholds and the thresholds must be exceeded to achieve ultrasonic cleaning. It is only that ultrasonic energy above the threshold that is contributing to the formation of cavitalion bubbles and to ultrasonic cleaning.
An increase in ultrasonic energy above the threshold level will result in increases in cleaning up to a certain point. There is a level beyond which the liquid will be incapable of accepting increases in power and. at this point, the cavitation near the transducer radiating face will be of such violence as to cause the liquid carrier to become elastic, thus either reducing or eliminating further transmission of energy into the liquid. This effect is known as 'surface cavitation'. At the other end of the scale, there is a certain threshold below which cavitation will not occur.
Let us consider an ultrasonic cleaning tank with a fixed power level and certain liquid level requirement. This tank when severely underfilled with liquid will be subject to "surface cavitation" That same tank when filled with the required amount of liquid but also over loaded with parts to be cleaned may not be as efficient as desired. In this case, the "threshold of cavitation" was probably only moderately attained. Manufacturers recommendations on liquid level and part loading level should be followed for best results.
I think that it is obvious by now that the ultrasonic devices which we're talking about are above and beyond the electro-vibrating type of cleaners that classify themselves as "sonic", and operate at a vibratory rate of 60 cycles/second. For true cavitation to occur, the oscillation must be above 20.000 hertz.
An important factor in ultrasonic cleaning concerns the loading of ports and the design of the basket or container for holding those parts. Loading of parts in the ultrasonic cleaning tank should be such that neither the objects nor the basket are on the tank bottom. The sum of the objects cross-sectional area should nol exceed 70% of the tank's cross- sectional area. Parts may have to be specifically oriented and multiple immersions be performed to obtain optimum cleaning if they cannot be completely immersed. Rubber and soft plastics will absorb ultrasonic energy and result in inconsistent cleaning. Rigid plastics, however, will respond to ultrasonic energy.
THE CLEANING PROCESS
In considering the cleaning process the cleaning solution is of ultimate importance Fortunately, water is one of the best liquids for this process While all of a liquids physical properties will have an effect on ultrasonic cleaning. the effects of vapor pressure, surface tension, viscosity, and density are the most pronounced Since temperature influences these properties, it will have an influence on the effectiveness of cavitation.
Considering the effects of these four key phystcal properties on cavitation. studies have shown that high density. low viscosity, and middle range surface tension and vapor pressure are the ideal conditions for most intense cavitation Due to temperature effects on these four physical properties, the most intense cavitation will be considerably below the liquid's boiling point, but not so low as to get into the adverse regions of too low vapor pressure or too high surface tension Different liquids will have different temperatures at which cavitation intensity will be the greatest because of the difference in physical properties Thus the cavitation intensity will be less either below or above this ideal temperature
Any cleaning system should be designed for use with the proper cleaning solution In selecting the cleaning solution, three factors should be considered independently, interactions between them can sometimes dictate an opposite choice is wiser based on safety, productivity or environmental considerations And naturally, the cleaning solution must be compatible with the materials to be cleaned as well as effective in removing the soils, oils and other contaminants on the part An additional consideration is the material of construction in the cleaner itself. Cleaners designed for aqueous systems should not be used with other solvents.
WHY AN ULTRASONIC CLEANING SYSTEM ?
Because ultrasonic energy penetrates into crevices and cavities, any type of part or assembly con be cleaned In many coses ultrasonic cleaning is the only way to meet specifications, as in the cleaning of precision space hardware assemblies and optical, and medical systems as well
Ultrasonic cleaning is faster than any conventional cleaning method in the removal of soil and contamination from parts Entire assemblies can be cleaned without disassembly Often, its labor saving advantages make ultrasonics the most economical way of cleaning
Ultrasonics offers unmatched cleaning consistency, whether pieces to be cleaned are large or small, simple or complex, handled singly, or grouped together The net result will be the ultimate clean Ultrasonics.. the cutting edge.
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