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Ultrasonics, The Power of Sound Part 2 - Variables
Barbara Kanegsberg, BFK Solutions LLC
“Oh, we can’t change any of the settings on the ultrasonic system; it was set up that way five years ago.”
“Does it clean the parts?”
“Nope!”
Effective ultrasonic cleaning depends on understanding and controlling the variables. We’ll provide an overview of
Frequency
Amplitude
Cleaning Chemistry
Sweep
Temperature
Pressure
Time
Review:
In Part one of this series on ultrasonic cleaning (Ref. October, 2011 Clean Source), compared ultrasonics with other cleaning techniques. Because ultrasonics are omnidirectional, the technique is often the best option for cleaning components or products with complex configuration. We also discussed extreme views about ultrasonics. Some manufacturers are afraid of using the technique; others simply flip the switch and assume that the system will magically remove all the soil without any damage to components. Both of these extreme views are fanciful and both can result in process problems. An effective critical cleaning process depends on refining the variables. We’ll discuss a few common ultrasonic variables.
Frequency
All other things being equal, the higher the frequency, the smaller and more “gentle” are the cavitation bubbles. Smaller bubbles can get into tighter spaces; and, some (but not all) studies indicate that they are better able to penetrate the boundary layer of liquid near the soil. One convenient way to think about frequency is to use the analogy between a delicate soprano and a bass.
Amplitude
Loudness also matters. Think of a tenor crooning softly versus soprano “singing” in an ear-piercing, high-pitched scream. An analogous situation holds with ultrasonics. The aggressive cleaning power of, say a 40HKz system, may be able to be somewhat tempered by adjusting the amplitude.
Cleaning chemistry
Physical properties of the cleaning agent, like density, surface tension, and viscosity, influence the effectiveness of cavitation. For example, it is more difficult to get bubbles to form and collapse in water alone than in water to which a surfactant has been added, because the surfactant lowers the surface tension. It may be difficult to get very viscous, soy-based cleaning agents to cavitate without adjusting the amplitude and/or frequency.
Sweep
Sweep ultrasonic systems employ a slight, continuous variation in the frequency; such systems have been found to promote effective cavitation. “Sweep sonics” are also said to eliminate (or at least decrease) the number of standing waves. Some commercial systems, like those designed for cleaning jewelry, do not have sweep. Choosing a system with sweep is a good idea, both to achieve more consistent properties throughout the tank and to provide greater flexibility in the choice of cleaning agent.
Temperature
At higher temperature, cavitation is enhanced – up to a point. As you increase the temperature, you decrease the viscosity, so the cavitation bubbles can more readily form and implode. However, after a certain point - and that point varies with the cleaning agent - the vapor in the bubbles increases to the point where the bubbles become “cushy.” Such bubbles do not implode well.
Pressure
Why should you have to worry about pressure? Sometimes pressure is an issue because pressure, like temperature, influences the physical properties of the cleaning agent and therefore influences the effectiveness of cavitation. Some enclosed, contained cleaning systems allow you to operate at other than atmospheric pressure. For example, “airless” cleaning systems, used where the cleaning solvent must be contained because of safety and environmental considerations, are operated at reduced pressure. If you decrease the pressure, you decrease the boiling point; this impacts cavitation effectiveness, particularly at higher temperatures since vapor pressures are higher (“cushier”) than at normal atmospheric pressure.
Time
Unless the ultrasonic process is totally ineffective, the longer you clean, the more you erode – you either erode the soil or, potentially, the surface of the part. If ultrasonic cleaning is going to work, soil removal should happen within a few minutes, at most. Some people turn on the transducers, toss in the part, and walk away for a few hours. They claim the part is getting clean. What we sometimes observe is that the transducers heat the tank to the point where cavitation is no longer effective. In such cases, the ultrasonic tank has been turned into a very expensive heated immersion tank.
Playing with the variables intelligently
You now have some basics of ultrasonics variables. There are still more factors to consider; we will continue the discussion in future articles. Adjusting the variables gives you lots of flexibility to improve the ultrasonic-based cleaning process. The secret is to change the variables systematically.
In future articles we’ll discuss more about optimizing the variables; and we’ll cover topics like fixturing and the impact of the parts being cleaned.
