Because ultrasonic cleaning can work so well, it is easy to assume you just toss the item to be cleaned into the tank. Not so! For this reason, manufacturers always have questions about ultrasonics. Based on questions posed by clients and students in our training/education programs, here are some steps you can take to make your ultrasonic system run more effectively.
Q: At the beginning of the first shift, ultrasonic cleaning does not work very well. Why is this so?
A: The problem relates to dissolved gases from the tank. In ultrasonics, as high-frequency sound travels through the cleaning chemistry, areas of compression and rarefaction occur. During rarefaction, there is negative pressure. With enough loudness (sufficient amplitude), cavitation, voids, or tears in the cleaning liquid occur. These are sometimes referred to as cavitation or vacuum bubbles. When the bubbles collapse or implode during compression, the energy (pressure and localized temperature) associated with this collapse is responsible for the cleaning power of ultrasonics.
In the morning, it is a near certainty that the cleaning chemistry contains dissolved gases. This means that the ultrasonic bubbles formed during rarefaction contain dissolved gases rather than a near-vacuum. The bubbles are “cushy,” so they do not collapse efficiently. Turning on the ultrasonics for a few minutes prior to processing a product releases the dissolved gases. Some ultrasonic systems are supplied with a degas setting that speeds the process. You should degas cleaning chemistries if the ultrasonics have not been used for awhile, or if fresh cleaning chemistry has been added. If in doubt, then degas.
Q: We have a small, unheated ultrasonic tank. As the day goes on, cleaning efficiency keeps getting better. Is this all due to degassing?
A: Most of the improvement is probably due to temperature effects. In fact, as the day progresses, the ultrasonic transducers generate heat, causing the temperature in the ultrasonic tank to increase, unless a temperature-controlled system is used. As the temperature increases, ultrasonic cleaning performance tends to improve—up to a point. The maximum performance temperature depends on the cleaning chemistry. For water, the maximum temperature is about 55°C. As the temperature increases, the viscosity of the cleaning chemistry decreases, so cavitation becomes increasingly effective. In addition, as the temperature increases, cleaning effectiveness tends to increase, whether or not ultrasonics are used.
Q: Why does ultrasonic performance decline later in the day?
A: As you approach the boiling point of the liquid, performance declines, for reasons related to the degassing issue. The higher the temperature, the higher the vapor pressure of the liquid. So the cavitation bubbles have more vapor; the bubbles become “cushy,” and they do not implode effectively.
Q: The effectiveness of our ultrasonic cleaning line seems to vary throughout the day. Why do we notice the worst problems when our throughput is highest?
A: Your process may have been designed around an average process flow. In actual production, there are peaks and valleys. During peak production, the baskets may be overloaded to maintain flow. In cleaning a large mass of complex parts, such overloading can mean that the cleaning chemistry does not reach the part.
In addition, the ultrasonics may not be reaching the parts. While ultrasonics are omnidirectional, as contrasted to high-pressure spray, there are limits. There can be shadowing effects. You may need to adjust the production schedule so that baskets are not overloaded, so that there is effective cleaning.
Q: We have an ultrasonic cleaning line using 40 kHz. Do we need to switch to a 200-kHz system?
A: Maybe. It depends on your current and anticipated product line and process requirements. Ultrasonic systems have to be optimized or tuned. Historically, the typical ultrasonic frequency (the pitch) was about 18 kHz for heavy industry, with 40 kHz for smaller components.
Gradually, higher frequencies have become increasingly utilized. Higher frequencies mean smaller cavitation bubbles. All other things being equal, this means less product damage. In addition, there are some indications that smaller bubbles may be more effective in penetrating the surface boundary level to remove smaller particles. On the other hand, with high frequencies, there may be insufficient energy to remove large particles. The amplitude or loudness also impacts performance, including the efficiency of soil removal and the potential for product damage. Many other factors, including the specific cleaning chemistry selected, also influence performance.
Cleaning processes involve many variables—and they have to be optimized. Choosing a new system out of a catalog or based on one variable is not likely to lead to profitable production. To maximize your budget, explore the capabilities of your current ultrasonic system and thoroughly evaluate any new systems.
Q: We have heavily soiled parts. We toss parts into the ultrasonic tank, leave them there for hours, and we still have to do touch-up. How long do we need to clean them?
A: Your processing time should be on the order of minutes, not hours. Most particles are removed within the first 30 seconds. If the process takes more than 5 minutes, then something needs to change to take advantage of the power of sound.
Ultrasonic cleaning is a process. The cleaning chemistry might not be optimal. You might need to adjust the temperature. The ultrasonic transducer might be malfunctioning, or it might not be making good contact with the tank. The placement of the transducers or the power density might be incorrect for the task at hand. The samples might not be fixtured properly, so that parts are shadowed.
Q: Can we use our old plastic fixtures for our new ultrasonic cleaning system?
A: In general, no. Plastics tend to be softer and dampen the ultrasonics. Metal fixtures are good; glass or stainless steel beakers can be used as well.
Too often, we see companies test ultrasonic cleaning by putting parts in plastic beakers and then erroneously conclude that ultrasonics do not work for their application. In fact, it is wise to re-evaluate all fixtures when you switch to ultrasonics. Certain wire mesh sizes may dampen the ultrasonics; therefore, it might be better to use a larger mesh or a solid beaker.
Q: We have a small ultrasonic tank, and we have a small-scale process. How long should we ultrasonic clean in acetone?
A: How quickly can you leave the assembly area? DO NOT ultrasonic clean with acetone or other flammable liquids, not even for experimental purposes, not even once in awhile, unless you use a system specially designed for low-flashpoint solvents. Acetone is above its flash point at ambient temperature. Most ultrasonic systems are not designed for use with flammables; the transducers are an ignition source. Playing “Russian roulette” is not advised.
Conclusions
Efficient ultrasonic cleaning is about quality and productivity. In other words, in order to build it right you have to clean it right. However, to make the most of your ultrasonic system, you have to understand what it can and—even more important—cannot do. Take a step back, look at the ultrasonics system, and look at the way your entire cleaning process is set up.
About the Authors
Barbara and Ed Kanegsberg, a.k.a. “The Cleaning Lady and the Rocket Scientist,” are independent consultants at BFK Solutions LLC, Pacific Palisades, Calif. They are experts in critical and industrial cleaning and manufacturing processes. You may reach them at (310) 459-3614 or via e-mail.
