I have written before that there are four major variables we consider in cleaning – – Time, Temperature, Chemistry and Agitation. Based on some recent feedback from the field, however, I guess I need to provide a little more emphasis on the benefits and definition of agitation as it relates specifically to ultrasonic cleaning processes.
“Agitation” is a pretty loose definition that can be applied to a wide variety of mechanical processes. For example, ultrasonic cavitation and implosion provide agitation on a micro-scale. Turbulation, part rotation, physical part displacement and even spraying a part with liquid are forms of agitation on a larger scale. It is difficult to think of a cleaning situation where agitation of one form or another is not beneficial. Selection of the agitation alternative and how it is applied is a critical part of the process development process. In some cases two (or more) forms of agitation applied in a specific way may maximize process results.
Enough for generalities – – let’s look at an example or two that illustrate the benefits of agitation in cleaning. Two earlier blogs explain the benefits of ultrasonic cavitation and implosion in removing soluble contaminants and particles.
Ultrasonic Cleaning – Soluble Contaminants
Ultrasonic Cleaning – Removing Particles
In both cases there are two possible scenarios to consider that may limit the effectiveness of ultrasonic agitation (cavitation and implosion) alone. The first is an excess amount of contamination. Ultrasonics, as we know, is great for removing thin layers of contamination but falls short when it comes to removing a thick buildup of contamination. The application of more aggressive agitation (spraying, for example) may remove a significant portion of the contamination allowing ultrasonics to be effective in removing the final layers of contaminant. In this case, agitation acts as a “pre-clean.” In another case, ultrasonic cavitation and implosion might be effective in removing a contaminant but, once removed, the released contaminant may not disburse and, as a result, limit the effectiveness of ultrasonic cleaning as the solution surrounding the cleaning site becomes saturated with contaminant. Note – This can occur with either soluble or insoluble contaminants. In this case, larger scale agitation may be used to disburse the released contaminants to allow continued efficiency of the ultrasonic cleaning process. In both cases, larger scale agitation benefits the effectiveness of ultrasonic cleaning.
Although beneficial to the overall process, agitation can, when improperly applied, be detrimental to ultrasonic cleaning. One notable example is that of turbulation as discussed in the blog Ultrasonics and Turbulation Don’t Mix! Although turbulation (high velocity jets of liquid provided by submerged nozzles, pump returns etc.) can be beneficial in some cleaning processes, any significant turbulation in an ultrasonic cleaning tank has been demonstrated to severely diminish the effectiveness of ultrasonic cleaning.
Probably the most effective means of adding agitation to an ultrasonic cleaning tank is to physically move the parts relative to the cleaning liquid. Although this is commonly done using an “agitation platform,” similar benefits can be derived by rotation or any other means that moves the parts being cleaned relative to the liquid cleaning media. One common mistake in the design of agitation systems is that agitation is too slow to provide a beneficial effect. Vertical displacement of a part 2 or 3 inches at a rate of 5 or 6 oscillations per minute seldom provides a significant cleaning benefit. Twenty to 30 oscillations per minute (less, of course as the displacement magnitude is increased) provides much better results.
Over the years, I have seen such substantial benefits using agitation in the form of part displacement through a liquid during ultrasonic cleaning that I would recommend its use whenever possible within budgetary and physical constraints. It is a tool that is, in my opinion, underutilized.