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 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.

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).

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.