THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

480 17. Commercial and Medical Ultrasound ApplicationsFigure 17.1. Conveyor belt for ultrasonic cleaning.principal agent but at higher frequencies the cleaning effect occurs mainly fromagitation. Most cleaning applications are executed in the frequency range of 20–50 kHz, where cavitation effects occur more strongly. Either piezoelectric or magnetostrictivesources are used. The workpiece being cleaned is immersed in atank containing a liquid selected on the basis of its susceptibility to cavitation,its detergent properties, ability to degrease, and so on. Trichlroethylene and cyclohexaneare among the more satisfactory fluids used for ultrasonic cleaning.Standard ultrasonic cleaners range in power from 100 to several thousand wattswith corresponding tanks capacities of 4–160 liters. Multi-kilowatt special systemswith tank capacities of several hundred liters are not uncommon. In recentyears, low-power, inexpensive (below $100) ultrasonic cleaners have been madeavailable, thereby rendering ultrasonic cleaning accessible to small shops andlaboratories.Ultrasound cleaning lends itself to continuous processing in which a series ofworkpieces can be transported on a conveyor belt through a series of processesin separate tanks as shown in Figure 17.1. Ultrasonic cleaning has supersededother older usual methods of cleaning, particularly when these methods are ineffectiveand liable to cause damage. Applications include the removal of lappingpaste from lenses without scratching after grinding, the flushout of greaseand machining particulates from otherwise inaccessible small crevices in enginecomponents, removal of blood and other organic material from surgical instrumentsafter use, and so on. Very delicate parts that can be damaged by cavitationare cleaned by wave agitation at much higher frequencies, from 100 kHz to1 MHz.In general, modern ultrasonic cleaners employ solid-state electronic power suppliesincorporating automatic tuning; thus they do not require operator’s attention.A problem prevalent with all ultrasonic cleaners is the gradual deterioration of thetank due to cavitation erosion. This depends principally on the application, andwell-designed systems can provide years of satisfactory service.