Ultrasonic Cleaning Comes of Age

by Miraclean, reprinted with permission from Products Finishing Magazine (Gardner Publications), November 2003, pp. 40-43.

There was a time ten or fifteen years ago when ultrasonic cleaning was considered something of a novelty. Driving the interest at that time was the Montreal Protocol and subsequent regulation of chlorinated solvents that meant that parts-cleaning could no longer be economically or environmentally accomplished using FreonTM or “1,1,1” or “trichlor.” Today, there’s a generation of production people out there who don’t even know what “1,1,1” was or how it was used in their facility.

But today, ultrasonic cleaning has come of age, filling particular kinds of cleaning needs—when properly applied—better than the previous methods ever did. Over the years, the technology has steadily improved.

Ultrasonic cleaning can optimize the removal of some types of soils from certain parts, such as buffing compound from crevices and tiny particles from metalworking operations. Other excellent applications include precision cleaning of small objects and electronics assemblies prior to other finishing operations, and cleaning of valve bodies, transmission parts and sub-assemblies, medical devices and injection molds.

Sometimes ultrasonics will speed up a cleaning operation that would otherwise take much longer. For example, carbonization can be removed from injection molds in minutes instead of hours with the right combination of ultrasonics, heat, and cleaning solution. In other cases, ultrasonics are used to meet the challenge of removing small particles from inaccessible areas—such as the sanitization of medical instruments after manufacture.

Cleaning takes place when high frequency bursts of ultrasonic energy are applied to a heated liquid cleaning solution that surrounds the parts. This energy produces a three-dimensional wave pattern of alternating positive and negative pressure areas within a cleaning tank. The alternating pattern creates bubbles during periods of negative pressure and implodes them during periods of positive pressure in a phenomenon known as “cavitation.” The implosion creates a microjet action that penetrates and cleans areas impossible to reach with brushes, sprays or dips.

The source of ultrasonic sound waves is a transducer, and there are two types: magnetostrictive and piezoelectric. Magnetostrictive transducers have a ferrous core that is oscillated by an electromagnetic field. They are almost always found in lower frequency applications from 16-20 kHz and are especially suited to heavy loads and high temperatures. Piezoelectric transducers are typically ceramic and are highly efficient. Oscillation of piezoelectric transducers is caused by electrical pulses at the resonate frequency, which is generally between 25 and 170 kHz but may be as high as 250 kHz, with 25-40 kHz being the most common.

When cleaning with ultrasonics, the frequency of the sound waves is matched to the application. For the most part, lower frequencies (20-40 kHz) are safe for most applications and will produce the most intense cavitation energies to remove the most common types of contaminants (oil, grease, metal chips). Higher frequencies (68-250 kHz) will produce smaller cavitation bubbles with less intense energies but more of them. This can be beneficial in the removal of smaller particles and where damage is a concern (polished surfaces, delicate parts, soft substrates).

Cylinders cleaned using ultrasonic cleaning technology. Ultrasonic cleaning is an ideal way for removing small particles from inaccessible areas. It can also speed up cleaning operations that would otherwise take much longer.

While ultrasonic devices have a natural frequency variation, additional frequency modulation is now available through sweep frequency generators. Frequency-sweep circuitry varies the frequency of the ultrasonic generator to create a more uniform cleaning field by alleviating standing waves and hot spots sometimes characteristic of older equipment. Power control circuitry tailors the output to varying load conditions, thus improving versatility, which is especially useful when different types of parts are being cleaned in the same line. The newest ultrasonic technology puts more than one frequency in a single generator—a more expensive option that nevertheless is sometimes required when cleaning very dissimilar parts in one cleaning line.

Typical tanks range from the small ones used by jewelers or dentists to industrial strength models holding hundreds or thousands of gallons of solution. Tank size for a particular application depends on the size and volume of the parts being cleaned, as well as the substrate and geometry of the parts, and the types of soils being removed. Immersible ultrasonic transducer canisters can also be retrofitted into existing tanks. (An added benefit of immersibles in any tank scenario is that they can be swapped out for repair if required.) The amount of ultrasonic power in a tank is measured in watts, and the proportion of watts of ultrasonics to the size of the tank and the mass of the parts is critical. Undersizing the watt density can mean that production-scale cleaning takes longer than it should or does not occur properly.

The location of the transducers in the tank can also impact the effectiveness of the cleaning process. Most commonly, transducers are bottom-mounted. However, in certain instances where contaminant loading can endanger the transducers and potentially reduce their effectiveness and life span (buffing and polishing compounds, paints, inks), transducers can be mounted on the side wall of the tank. Side mounting can also be indicated when part geometries call for particular exposure angles to the ultrasonics.

The kind of liquid used is important, as is the temperature. Raising temperature too high (above about 180F) reduces cavitation pressure and can therefore be counter-productive.

Before the Montreal Protocol, what we now call “regulated solvents” were often the cleaning solutions of choice in ultrasonic tanks. Today, they and a new generation of solvents remain an important option for certain types of cleaning, as is another class of cleaning solution called “semi-aqueous,” which mixes solvents and water.

With the regulation of solvents also came the impetus to shift to water-based cleaning solutions. These have greatly improved in the last ten years, especially as some new surfactants have been developed for hard surface cleaning. There are three types of aqueous cleaners: acidic, neutral and alkaline. The efficiency of all of the cleaners increases in combination with ultrasonics. Also, the percentage by volume in water and/or the aggressiveness of a cleaner can often be minimized by augmenting the cleaning action with ultrasonics.

Acidic cleaners (pH less than six) consist of mineral and organic acids with wetting agents. They are not generally used for the removal of oil and grease, but are most widely used for the removal of metal oxides. With the addition of ultrasonics this process can be accelerated and the acid used can therefore be less aggressive. Neutral cleaners (pH of 6-8) consist mostly of surfactants. They also contain mild builders and corrosion inhibitors. They are effectively used to remove oil and light grease. Alkaline cleaners (pH of 8-14) are a blend of builders such as potassium and sodium hydroxide, silicates, carbonates, bicarbonates, phosphates, borates and surfactants. They are best suited for the removal of oil, grease, inks and carbonaceous soils.

Cleaning solution, temperature and the mechanical action of ultrasonics are a formidable combination against industrial contaminants, but sometimes, additional types of mechanical action such as rotation, agitation and/or spray under submersion are required to fully dislodge soils. Filtration of the cleaning tank is usually recommended to pull particulate out of the bath and extend the solution life, and surface skimming into a separate overflow weir prevents re-deposition of soils as the parts exit the cleaning tank. Rinsing is key to overall success, and in a high production setting multiple stage rinses with high quality water (counterflowed for conservation if desired) are recommended to assure a spot free result.

Today, ultrasonic cleaning applications range from removal of machining oils on stainless steel and aluminum to buffing compound from brass; grinding compounds from tool steel hand tools; stamping lubricants from stainless steel, copper and mild steel; particulates from plastic jewel cases; wax from glass and more. Whether delivered in a single tank or a fully automated multiple-station line, today’s ultrasonic cleaning technology succeeds with a proven blend of ultrasonic power, cleaning chemistry, temperature—and a good rinse.