Ultrasonic cleaning uses high-frequency, high-intensity sound waves in a liquid to facilitate or enhance the removal of foreign contaminants from surfaces submerged in an ultrasonically activated liquid. Ultrasonic technology has more recently been used in a growing number of applications involving chemical processes and surface conditioning, which, although outside the classic definition of cleaning, use basically the same techniques. Demands for increased cleanliness have driven the development of increasingly sophisticated technology in the field, particularly within the past decade. Today it is possible to customize ultrasonic waves to optimize effects in a wide range of applications, as described by Puskas and Piazza (2000). Recent developments in ultrasonic washing of textiles also have occurred.

           Ultrasonic cleaning is a technology unique in its ability to remove contaminants that other technologies cannot remove and in its ability to effectively clean areas that are not accessible using other technologies. Competing technologies include spray washing, turbulation, agitation, and brushing, among others. In general, these technologies are ¡°line of sight¡± in nature; that is, there must be direct access to the contaminated surface for them to be effective. In all of these cases the physical energy is delivered indirectly. In a spray system, for example, the energy for the spray stream is imparted by a pump. Pressurized liquid is delivered to a nozzle, which increases the stream¡¯s velocity and directs it toward the surface to be cleaned. Cleaning is achieved by the impact of the high-velocity stream with the surface being cleaned. Not only is energy lost during each step of the process, but only that energy in the portion of the stream that ultimately impacts the cleaning target provides benefit; the rest is lost. Sprays are not capable of reaching areas hidden from direct access of the spray nozzle to the surface. Cleaning blind holes with spray technology, for example, can only benefit from a flushing action because the spray creates a pressure differential across the hole opening to force liquid in to and out of the hole.

             Another technology, brushing, is similarly limited in that any surface the brush bristle is not able to reach will not be cleaned. In a world where surface geometry of many parts is measured in nanometers, brushes are unable to reach all surfaces of the part to be cleaned.

Ultrasonic cleaning technology, unlike those described above, is able to penetrate and clean any surface that can be reached by a sound-conducting liquid. This means that blind holes, thread roots, parts with complex geometry, minute surface contours, and a number of otherwise impossible cleaning tasks can be easily accomplished using ultrasonic cleaning technology.

             Ultrasonic cleaning is a common procedure for high-quality cleaning, utilizing ultrasonic energy to scrub the parts and a liquid solvent to rinse away the residue and loosened particulate matter. This procedure, rather than using the vapor degreasing technique for precleaning and final rinsing, utilizes manual application of liquid solvents. The process is not limited to any particular solvents and, indeed, organic solvents need not be used. It is widely used with aqueous solutions: surfactants, detergents, and alkaline and acid cleaners. The only real limitations are that the cleaning fluid must not attack the cleaning equipment, fluids must not foam excessively, and the fluids must cavitate adequately for efficient cleaning.2

The process is not as efficient as vapor rinse, solvent wipe, immersion, or spray, but it is suitable for many surface preparation applications and pretreatments. One or a combination of these techniques may be used. A large number of solvents are recommended. Solvent wiping is the most portable and versatile of these methods, but is also the least controllable. There is always a danger of incomplete removal of soil, and spreading of soil in a uniform manner, causing its presence not to be readily visible, and contamination of a surface with unclean wiping materials.

             For general cleaning, wiping materials should be clean, freshly laundered cotton rags, new cheese cloth, or cellulose tissues. For special super-clean applications whereby cleaning must occur in a controlled ¡°clean¡± room, specially processed lint-free polyurethane foam wiping materials are available (from Sills and Associates, Glendale, CA). The solvent should be used only once, and it should be poured onto the wiping material. The wiping material should never be immersed in the solvent. Solvent containers with small openings should be wiped systematically with the solvent-soaked cloth or tissue. The wiping material should be discarded, and the surface should be cleaned again with new solvent and cloth or tissue. This cycle should be repeated until there is no evidence of soil on either the cloth or the cleaned surface.

Although immersion and soaking in a solvent is often sufficient to remove light soil, scrubbing may be required for heavier soils. The most efficient scrubbing method is ultrasonic, as discussed earlier. Other scrubbing techniques include tumbling, solvent agitation, brushing, and wiping. After the parts are soaked and scrubbed they must be rinsed. The quality of cleaning produced by the immersion process depends primarily on the final rinse. The solvent spray cleaning method is efficient because of the scrubbing effect produced by the impingement of high-speed solvent particles on the surface. The solvent impinges on the surface in sufficient quantity to cause flow and drainage, which washes away the loosened soil. Also, because only clean solvent is added to the surface, and scrubbing and rinsing occur, there is no danger of contamination, as there is with the immersion process.