cleaning, pretreatment & surface preparation
PRIMER ON ULTRASONIC TRANSDUCERS
BY JOHN DURKEE, INDEPENDENT CONSULTANT SPECIALIZING IN METAL AND
CRITICAL CLEANING, HUNT, TEXAS
This article is about technology common to many metal cleaning shops — ultrasoniccleaning systems.Ultrasonic Transducers—How They Work. These
equipment components are
used in both aqueous and solvent cleaning applications. Chiefly employed forremoving solid particulate matter, they are agents of agitation that can dislodgesoil components that can’t be removed solely by chemical action.In common use for decades, they are becoming (or have become) commodityequipment products despite the best efforts of suppliers to providedifferentiation.Ultrasonic transducers produce waves of fluid pressure that bombard partsurfaces (and all surfaces under immersion). The waves are produced by diaphragmsthat vibrate under immersion in fluids. The device producing the
vibration is called a “transducer.”Frequency of vibration is high—from tens of thousands to hundreds of thousandsof oscillations (cycles) per second (cps or Hertz). Consequently, the effectof each cycle of vibration is negligible—but their cumulative and continuouseffect can be either positively or negatively dominant.There are two methods by which transducer diaphragms are caused to vibrate.
PIEZOELETRIC TRANSDUCERS
A piezoelectric material has two unusual and interrelated characteristics. Theyare basically the reverse of one another:
• When a force is applied to a piezoelectric material, a tiny electric currentis produced.
• When an electric current is passed through piezoelectric materials theydeform, i.e., change in size (volume) by a few percent.
It is the latter characteristic that produces a vibrating diaphragm. A rigidconnector (arm) causes the diaphragm to move slightly when the piezoelectricmaterial changes shape upon application of an electric current (see Figure 1).Repeated application of the electric current, followed by its relaxation, enablesa diaphragm to move forward and backward in one direction.Most piezoelectric materials are ceramics, many of which contain silicon, lead,aluminum, or titanium oxides
.
MAGNETOSTRICTTIVE TRANSDUCERS
There is a magnetic analog to the piezoelectric effect. A ferromagnetic material(magnetic Iron) will respond mechanically to magnetic fields. This effect is called“magnetostriction.”Magnetostrictive materials transduce or convert magnetic energy to mechanicalenergy. As with the piezoelectric effect, the reverse is also true.When a magnetostrictive material is magnetized, it elongates—that is, itUltrasonic Frequency, kHzBubble Diameter, micronschanges dimension in onedirection. As shown inFigure 1, that dimensionalchange can be used to causea diaphragm to move thoughdriven by a different factor.Most magnetostrictivematerials are metal alloys of
nickel or contain significantquantities of nickel compounds.Magnetostrictivetransducers are not used atfrequencies above 30 kHz.The main reason is that thedifficulty and cost of controllingthe motion of the relatively
large mass of material(dense nickel) associated withmagnetostrictive transducerelements becomes too severeat frequencies above that level.
MAKING A CHOICE
Some may inform managersthat the choice is between thehigher purchase price and longermaintenance life of magnetostrictivetransducers vs. the opposite for piezoelectric transducers, or toachieve a lower level of operating noise. That’s a false choice. The choice shouldbe totally based on the character of the parts.
• No one, for example, would consider using magnetostrictive transducersfor cleaning of disk drive components where piezoelectric transducersare commonly used. The components would “dance” in the water bathand be destroyed.
• Nor would anyone consider using piezoelectric transducers for removalof scale prior to painting of small engine blocks for lawn mowers.Nothing would be removed.Ultrasonic TransducersÑ How to Choose the Frequency. Ultrasonic cleaningsystems use two familiar types of ultrasonic transducers: piezoelectric and magnetostrictive.Both generate cavitation bubbles. This section will cover how tochoose the right kind of cavitation bubbles, and the right type of transducer togenerate them. A
TWO TYPES OF FREQUENCY
The two prefixes normally attached to the word sonic are ultra and mega. Ultra refersto frequencies above those identified by the human ear — i.e., those above ~18 kHz.Ultrasonic transducers are the type most commonly used, with frequencies above18 kHz, and below ~ 250 kHz. A manager purchasing a ultrasonic system withoutspecifying the frequency would probably receive one operating at 40 kHz.Figure 1. Diaphragm changes relative to application ofcurrent.Figure 2. Bubble diameter relative to ultrasonic frequency.Mega is not scientificallydefined. A commonly acceptedlimit is frequencies exceeding250 kHz. Megasonic transducers
don’t produce cavitationbubbles, and aren’t commonlyused in metal finishing operations,except to remove surfacerust and other dense debris inremanufacturing operations.
THERE IS MORE THAN
ONE FREQUENCY
The reason waves (fluctuations)of pressure are valued isthat they produce cavitationbubbles. Collapse of thosebubbles releases high levels of energy which can interrupt local collections ornetworks of debris (soil). That’s metal cleaning!Larger bubbles, which will ultimately release more energy per bubble whencollapsed, are formed when there is more time for them to do so. This meanswhen the frequency is low.
Said another way, a lower frequency generates wave fronts with a longer timeinterval between them, thereby allowing more time for bubble growth.Smaller bubbles are produced when the frequency produced by the transduceris higher. Calculated bubble size vs. frequency is shown in Figure 2.But there is another factor affecting energy release: That’s the number ofbubbles produced. More bubbles are produced at higher frequencies becausethere are more opportunities to do so — more cycles of compression and rarefaction(expansion).Essentially, the energy released to do cleaning work on surfaces is the productof the volume of each bubble times the number of bubbles.In other words, for the same power input from the transducer to the liquidtank:
• A low frequency will produce fewer cavitation bubble implosions eachwith higher release of energy, and —
• A higher frequency will produce more cavitation bubble implosions eachwith lower release of energy.The two different types of operation with the same power level are illustratedin Figure 3. Which would you prefer?
CHOOSING THE RIGHT FREQUENCY
What’s significant is that the cleaning capabilities will be quite different inthese two examples even though the power level is the same. The value of thatdifference will depend upon the nature of the cleaning work to be done.The right ultrasonic frequency is that which best matches the cleaningcapability to the needed cleaning performance.Some have referred to collapse of cavitation bubbles as being “pecked to death
by ducks.” This is because other mechanical actions such as blast cleaning withFigure 3. Piezoelectric vs. magnetostrictive operation.solid media or impact from a pressurized fluid jet apply such different stress to
soil elements and the surface on which they lay. To complete this analogy, blastand pressurized jet cleaning technologies might be thought of as being “eatenby a T-Rex dinosaur.” (See Table 1).
Consider Table 1, in which this analogy is presented in a generalized visualform. Also remember that it’s an analogy!The point of this presentation is that the transducer frequency should bechosen to match the nature of the cleaning task. Each choice of frequency willbe more useful when applied to a specific type of soil material, and will havedifferent effects on the underlying surface. Said another way, use the right tool
(frequency) for each job (cleaning situation).And how is the right tool to be identified? Managers should organize and witnesscleaning demonstrations using actual soiled parts with facilities provided by
suppliers. These parts should be cleaned using several transducer configurationsand the performance evaluated by the normally used cleaning test. Let the detailsof the application reveal the right choice of frequency.
A NEW FREQUENCY SWEEPS CLEAN
Selection of a transducer which radiates pressure waves into fluid and onto partsurfaces at a selected, constant, and fixed frequency may solve cleaning problems(as above), but also create concern about part integrity.
Any single wave frequency can— and is— likely to resonate within the liquidvolume as it reflects off the walls which contain the liquid, and the parts.Resonance is the term for coordination of the pressure amplitudes whichoccur at the constant wave frequency. Pressure values (amplitudes) can combineif the wave frequency doesn’t change.This isn’t bad, if there isn’t some threshold pressure which can harm the parts.
But delicate parts will fracture when excited into resonance. This outcome wascatastrophic for those removing particles from fragile parts such as those usedin disk drives.
The solution developed was to force the transducer frequency to vary overa small range by changing the frequency of the alternating current suppliedto the piezoelectric crystal. This prevented wave resonance, and application ofunwanted high pressure forces to fragile parts.Deliberate variation of frequency around a central value is known as “sweep.”The amount is usually 1 or 2 or 3 kHz for a transducer designed to produce pressurefluctuations at 40 kHz. This capability is now a standard feature of nearlyall commercial ultrasonic transducer systems — whether to be used with fragiledisk drive components or used with sturdy drive gears.
Table 1. Visualization of Various Ultrasonic Transducer Applications
Magnetostrictive Piezoelectric
20 kHz 40 kHz 68 kHz 104 kHz 170 khz
T-Rex eating man Pelican eating
fish
Hornbill eating
beetle
Cockatoo eating
nuts in shell
Finch eating
seeds
74
0
0%
20%
40%
60%
80%
100%
500 1,000
Power Level, watts
Cleaning performance1,500 2,000Ultrasonic Transducers—How toChoose the Power Level. It is ahuman characteristic to believe“more is better.” This characteristicis reflected in the financialadvice: “bears make money,
bulls make money, and hogs getslaughtered.” Another exampleof this characteristic is thechoice by many users of everlargerpower ratings for sonicpoweredtransducer systems.There are at least three factors to be considered by a manager when choosingthe power level for the ultrasonic transducers in a cleaning system: parts, cycletime, and tank size.
POWER TO THE PARTS
A generalized relationship between cleaning effectiveness for a properly designedsystem is illustrated in Figure 4. Note that the relationship is “S-shaped (asymptotic):”
• Modest application of ultrasonic power has only minor effects. This isbecause an adequate number of cavitation bubbles of sufficient sizehasn’t been produced.
• At some level of applied power, the ultrasonic cleaning system performswell, as designed.
• When a high level of cleaning performance has been achieved, there islittle gain by applying additional ultrasonic power.
• In the latter situation, if removal of the small levels of remaining soilis necessary, a secondary cleaning process should be employed ratherthan force this cleaning process to perform beyond its capability.
Without regard to the character of the parts, there is a suitable rangeof power levels. There is no point to paying for more power, or trying toeconomize by paying for less.
EFFECT OF CYCLE TIME
Cycle time (contact time with ultrasonic agitation) should be viewed similarly.Cleaning quality will have the same general (“S-shaped”) relationship vs timeas seen in Figure 4.
• Parts just “dipped” into the ultrasonic tank will not be well cleaned.
• Parts “cooked” as some like their steak to be well done will not becleaned to a premium level.
Doubling the cycle time will not double the cleaning quality. For a properlydesigned cleaning system, if the production rate is raised and the associated cycletime shortened, cleaning quality will suffer only to a modest degree.As benchmarks, a cycle time of 2 minutes contact would be quite short, butperhaps satisfactory. A cycle time of 5 to10 minutes would be quite long, butperhaps necessary.Figure 4. Generalized relationship between cleaningeffectiveness and power.
Tank Volume, gal
0 20
20
0
40
60
80
100
120
140
160
180
200
40 60 80 100
Power Density, watts/gal
EFFECT OF TANK SIZE
Size (fluid volume) of the tank in which the cleaning work is being done matters— substantially. Less power is used in tanks with smaller volumes — substantially.Ultrasonic power level is normally specified as a density — power per volume.Specifications for standard tank systems produced by four major U.S. suppliershave been collected. The suppliers are identified only as “A,” “B,” “C,”
and “D.”The power density provided in standard systems is graphed in Figure 5. Recallthat supplier “A” is not necessarily providing superior cleaning systems becausetheir systems have a higher power density, nor is supplier “C” supplying inferiorsystems.
EFFECT OF PART SIZE
Ultimately, all mechanical energy added to a cleaning or rinsing tank by ultrasonictransducers is converted to heat.¥ The mechanical energy is consumed in doing frictional work Ñ eitheragainst the mass load of parts, against the walls of the tank, withinthe water, or as heat and additional frictional forces produced by thecollapse of cavitation bubbles.Consequently, if the parts are a large dense mass of metal (castings, forgings,
etc.), more ultrasonic power will be required to compensate for that absorbed bythe metal. If parts are left too long within an ultrasonic-powered cleaning tank,they — along with the fluid within the tank and the tank walls — will becomewarm. Further, if the parts occupy a large amount of the volume within a tank,it is likely that internal surfaces may not be effectively cleaned.Some suppliers recommend that the weight of parts in a ultrasonic cleaningtank be no more than about one-third to one-half of the weight of water in thetank. (This author’s experience favors the lower value.) Such a recommendation
doesn’t mean that more large systems be purchased; it may only mean thatmultiple loads be processed in a smaller and less expensive machine.
TEST, TEST, TEST!
A manager’s objective, in every demonstration with a supplier’s ultrasonic (ormegasonic) facilities, should be to identify the power level and the cycle timethat should be used to design a commercial system.
Figure 5. Power density provided in “standard” ultrasonic cleaning systems.
• Excess power has negligible value. A good manager should not pay forthat. It will only serve to overheat the cleaning tank!
• Excess cycle time is a waste of productivity. A manager should not standfor that.
Remember: Generalized relationships and specific recommendations don’tnecessarily relate directly to actual performance data.
MORE THAN JUST EQUIPMENT
Performance of sonic-powered cleaning system, for a given set of parts, is related tomuch more than the choice of frequency and sweep rate, tank size, and power level.
• Chemicals, and their concentration, affect performance. But there areother factors that can be significant, or not, which are not so obvious.Some observed by this author are:
• Tank configuration — depth vs open area (more shallow tanks generallyuse power more efficiently),
• Tank configuration — presence of unusual shapes where waves aren’treflected back onto parts,
• Positioning (racking) of parts within the open volume of a tank,
• Location of transducers within a tank (bottom, sides, etc.)
• Operating temperature (an optimum is around 160°C ,
• Residual gas (air) content (there should not be any),
• Water quality (reduced mineral content is better),
• Smoothness of the part surface,
• Excess fluid circulation can reduce effectiveness,
• Waveform of the ultrasonic-produced pressure pulses (take caution overclaims where non-sinusoidal waveforms are preferred),
• Anything present on the part surface that would prevent it from beingwetted (and submerged).
• Accumulation of debris within the tank (clean tank; clean parts).
It isn’t that ultrasonic cleaning in static tanks isn’t reproducible. It very oftencan be and is so. Ultrasonic cleaning is reliable very often. However, specificresults (claims by single vendors of superior performance in unique applications)can often be difficult to reproduce in ultrasonic systems provided byother vendors.In other words, if a supplier can back up a claim with repeatable performance
data with your parts, a manager should give great priority to that supplier inthe selection process.Ultrasonic technology should be integral to — and designed for — a cleaningmachine. This article has shown how a manager can be certain that is done.
ABOUT THE AUTHOR
John Durkee is the author of the book Management of Industrial Cleaning Technology andProcesses, published by Elsevier (ISBN 0-0804-48887). In 2013, Elsevier will publish inprint his two landmark books Science and Technology of Cleaning with Solvents [ISBN9781455731312], and Handbook of Cleaning Solvents, (ISBN-13:978145573144) as wellas a 4-part e-Book Design of Solvent Cleaning Equipment.
