By John Biagiarelli
Lewis Cleaning Systems

Having reviewed numerous articles about ultrasonic cleaning, the majority of authors tend to concentrate on theoretical parameters while providing little or no information about practical applications. This does not help most manufacturing engineers solve their primary problems: Can ultrasonic cleaning help me? How does it get my parts clean? Is ultrasonics right for my application? How does it compare to other types of cleaning?

How Ultrasonic Cleaning Works
Ultrasonic cleaning is the introduction of high-frequency sound waves, usually between 20-80 kHz, into a liquid. The resulting action is called “cavitation.” Cavitation is created by high and low pressure areas produced in the solution as the sound waves pass through it. In low-pressure areas, microscopic “vapor bubbles” form. The pressure rises rapidly as the next sound wave passes through the area, violently imploding the minuscule bubbles and radiating outward the energy that does the cleaning. At 20 kHz, this is happening 20,000 times per second.

The Transducer. The ultrasonic transducer converts high-frequency electric energy into mechanical motion. The device physically oscillates at its resonant frequency. There are two basic types of transducers: magnetostrictive and piezoelectric. Magnetostrictive transducers are metallic, usually made of laminated nickel and typically silver-brazed to the diaphragm or radiating plate. Piezoelectric types are composed of man-made crystals and mounted in various ways, the most common of which is epoxy bonding.

The Generator. The generator converts low frequency line power at 50-60 Hz to high frequency power at 20-80 kHz or higher, which matches the resonant frequency of the transducer. Generators designed to drive piezoelectric transducers often have automatic tuning or frequency controls to compensate for fluctuations in the resonant frequency of the crystal transducer. Typically, magnetostrictive generators do not have automatic tuning because the resonant frequency of the magnetostrictive transducer is more stable.

Ultrasonic Cleaning Applications
In general, ultrasonics is used in precision cleaning and heavy-duty cleaning applications of complex manufactured metallic parts. It has the ability to clean in narrow crevices and small holes that would not be easily accessible by a spray washer or other methods of cleaning. Difficult soils, such as buffing compounds and baked-on carbon, are also good candidates for ultrasonics. In other words, the more complex the part configuration and more stubborn the contaminant, the more likely ultrasonics will be of benefit to the cleaning process.

There are also many others factors that contribute to the cleaning process equation. Among these are the types of soil, chemical limitations, temperature, cycle time, piece part volume, etc.

Though there are three energy forces key to any cleaning application, scientifically speaking, there are actually four. The three typical energies available in almost every cleaning application are: thermal energy; chemical energy; and mechanical energy. The success of a cleaning application depends on the balance and relationship of these three types of energy. The fourth parameter, however, is time. Time will not only increase the effectiveness of all three energies, but it will also increase or decrease production rates in the particular step of the manufacturing process.

Ultrasonics is only one type of mechanical energy and is only one part of the equation for a successful cleaning process. Other forms of mechanical energy used in cleaning include simple immersion, spray, turbulation, agitation and rotation. To a great extent, the type of mechanical energy selected depends on the relationship of the heat applied to the process and the type of chemistry used. Many times companies only consider the chemical aspects when converting a vapor degreaser, trading the solvent for another chemical. A more successful approach considers all of these parameters.

Ultrasonics is not a magic force nor is it right for every application. It is just another form of mechanical energy that can enhance a liquid chemical process.

Using a combination of these energies and finding the optimum balance for your application can be a challenging project. The relationship of the chemistry, bath temperature and degree of mechanical motion the part can withstand are all critical factors in choosing the right cleaning method.

Real Life Examples
A manufacturer of aviation hardware was using a vapor degreaser to remove oil, followed by a 50% nitric acid bath at 170F to deoxidize aluminum parts prior to brazing. By using ultrasonics, it was possible to combine both processes in one cleaning line that integrated the following steps:

1. Ultrasonic degrease in an aqueous-based solution—pH neutral at 140F
2. Hot water rinse
3. Ultrasonic cleaning in 5% citric acid bath at 140F
4. Deionized rinse at 160F
5. Deionized rinse at 160F
6. Hot air dry

The capital equipment cost to the manufacturer was $160,000. However, because of savings in operating costs by eliminating the degreaser and eliminating disposal of the nitric acid waste, the return on investment was less than 18 months.

More important, the ultrasonic cleaning process was much safer and gave the user a dramatic increase in quality. The application of ultrasonics allowed the manufacturer to switch from nitric acid to much less hazardous citric acid by adding a high degree of mechanical energy to the process. In addition, ultrasonics enhanced a neutral pH bath for thorough degreasing, eliminating the need for 1,1,1-trichloroethane.

Another example of the successful application of ultrasonics is in plating—especially in reel-to-reel plating of continuous strip material. Most plating lines use strong alkalines at 170F and electrocleaning for soil removal at the beginning of the line. In many cases, the strip or parts have gone through a vapor degreaser. The problems with this type of cleaning are that it limits line speed and high-pH chemistries are usually not free rinsing. This causes contamination of the plating baths through dragout and high rinse water use. The parts also must be degreased prior to plating because a high pH tends to emulsify oil and to deplete the chemistry.

By using high-intensity ultrasonics, most of these problems are eliminated. Ultrasonics is being used in reel-to-reel plating with a pH-neutral detergent at 140F in place of electrocleaning. Because of the neutral detergent, any oil will float and can be separated with a coalescer. This often leads to the elimination of the vapor degreaser prior to plating. It is free-rinsing, and, in most cases, the line speed can be doubled. The advantages are many. The plating baths do not get contaminated, water use is cut in half, product quality is more consistent, chemical and waste treatment costs are lower and output of the line is doubled.

For fluorescent penetrant inspection (FPI), the cleaning requirements are for both before and after. Before penetrants are applied, the parts and potential cracks and crack defects on them should be degreased and any particulate contamination should be removed. Static soaks are slow and cannot penetrate deep fine cracks. Ultrasonic rinsing has been found to enhance crack indications with brighter more defined fluorescence.

After FPI, it generally is essential to remove FPI materials before further processing of the parts such as weld repair, plating, coating or other finishing processes.
Generally, ultrasonic cleaning for FPI can provide a higher quality of flaw detection, a faster process and ultimately reduce costs.

These examples are only three of hundreds like them that illustrate the benefits of applying the proper type of mechanical energy to a process.

Candidates for Ultrasonic Cleaning
The following list is typical of parts cleaned in ultrasonic processes:

• Diesel fuel nozzles
• Battery cans
• Turbine engine blade and vanes
• Diecastings
• Bearings, races and rings
• Textile spinnerettes
• Aircraft fuel nozzles
• Ratchet handles
• Flatware
• Valve plates, refrigeration
• Stamped parts (pieces or strips)
• Continuous strips, up to 200 fpm
• Pistons
• Golf club heads
• Piston rings
• Computer disc drive hubs
• Valves, lifters
• ABS—valve bodies
• Polished brass hardware
• Continuous wire and cable

When trying to determine if ultrasonic cleaning can help you, keep these things in mind:

1. Is the part a complex shape?
2 Does the part have small crevices, blind holes or deep recesses?
3. Is the part delicate? Will strong agitation damage it?
4. Will strong chemistry damage the part?
5. Will high temperature affect the part?
6. Is the cleaning cycle time limited because of part volume?

If the answer to most of these questions is “yes,” then your parts are good candidates for ultrasonic cleaning. Future trends are dictating cleaner parts, better quality and a safer environment. Ultrasonics can be a tool to help you.

Selecting Ultrasonic Equipment
Once you have determined that ultrasonic cleaning is right for your application, the next question to be answered is what equipment best suits your application.

It takes a certain amount of energy to achieve the threshold of cavitation, which is the level of energy required to achieve cavitation. The problem is that when you put a load in the tank, you attenuate or absorb energy. If you do not have a good watt density (power level), you can fall below the threshold and cavitation ceases. In other words, the heavier the load, the more power or watt density needed. Some ultrasonic manufacturers calculate in terms of watt density per area of radiating face, such as 5-7 w/in2. Other manufacturers recommend power levels in terms of volume—typically 40-75 w/gal. Because the displacement of the ultrasonic transducer is a mass relationship, a volumetric determination is more realistic. It is desirable to have the transducer/diaphragm moving mass exceed the mass of the cleaning load. This usually assures sufficient driving energy to compensate for load attenuation.

In most general degreasing applications, 30-50 w/gal is adequate. However, in more rigorous applications, such as removing burned-out carbon in turbines or removing a diamond-lapping compound, watt densities of 100-150 w/gal are common.

The number of transducer elements is often used as a yardstick to measure how much power is in a tank. This is not an accurate method and should not be used to compare manufacturers. Most types of crystal transducer elements range from 20-50 watts per transducer. Magnetostrictive transducer elements can range from 140-400 watts per transducer. You would need twice as many crystal transducers to get the same energy in a given volume as with typical magnetostrictive transducers. This is why the w/gal approach is a more reliable method of determining if the power available will be adequate.

Piezoelectric and magnetostrictive devices vary in their construction, power output per transducer and methods of attachment (for example, epoxy-bonded or silver-brazed). The efficiencies in general, considering all parameters, are similar. Typically, piezoelectric transducers are found in lower power applications and magnetostrictive transducers in higher power ranges. It is safe to say that the more power a unit has, the more expensive it will be. Therefore, if you are cleaning lightly soiled parts or printed circuit boards, a low-watt density is adequate. But, if you are decarbonizing jet engines or cleaning large textile spinnerettes, a high-watt density tank is a much better choice.

Another consideration is liquid displacement. As a general rule, part volume should not displace more than 25% of the total tank volume. The reason for this is to maintain enough liquid level to cover the part. You do not want to displace solution out of the overflow weir every time you put a basket of parts in the tank or you will have to add water constantly to maintain the proper level.

There are two things that are usually overlooked that will greatly enhance any equipment selection. First is to make sure that the ultrasonic tank has adequate filtration and a sparge/overflow system with oil separation when oil is being removed from the parts. This serves two purposes. It keeps the floating oil from redepositing on the parts and reduces the particulate level in the tank. It has the additional benefit of enhancing the cavitation because high amounts of suspended solids also attenuate, or reduce, the ultrasonic activity. The money spent on filtration will pay back in reduced chemical consumption, cleaner parts, more consistent quality and reduced cycle times. Removing as much oil as possible will give similar benefits. The preferred type of oil separation is usually a coalescing system. Oil wheels work but do not remove oil fast enough in a production situation.

The second criteria to consider in selecting ultrasonic equipment is an adequate rinse system. As the old saying goes, “You cannot get clean dishes out of dirty dish water.” The same applies to clean parts. Parts will only be as clean as the final rinse. At a minimum, a two-tank cascading system should be considered.

Ultrasonic Rinsing
Ultrasonic rinsing typically is not considered because of cost considerations. It adds 30-40% in cost to most cleaning lines. However, if the holes in your part retain water due to capillary action, then an ultrasonic rinse may be justified. This is because a heated static rinse will not remove trapped chemicals and debris from tiny capillary holes. Small blind holes are rinsed by diffusion rather than by flushing. Ultrasonics will add the impingement energy necessary to flush out these difficult areas. This is especially true for FPI cleaning. Users have found that ultrasonic rinsing provides a cleaner, brighter, more defined defect indication.

Typical parts that require an ultrasonic rinse are turbine engine components, textile spinnerettes, ultra-high pressure diesel fuel nozzles, hypodermic needles and parts that nest or stick together. In a cascading system, the ultrasonic rinse should be the final rinse. Again, the best way to make your final process determination is by doing careful lab analysis.

Be sure to work with an equipment sup-plier who can help you develop a process, pinpoint the right chemistry and select the proper equipment. Most major ultrasonic and cleaning equipment suppliers have laboratory facilities and will do sample parts cleaning for you at no charge. Take advantage of the resources available to you.