Soprano or Bass—What Ultrasonic Frequency to Use? Part 2: Being TACTful

As a professional in product cleaning, you probably have encountered the acronym TACT. The letters represent four key facets for effective cleaning, Temperature, Action, Chemistry, and Time. Action is the facet usually associated with ultrasonics. In Part 1, we focused on cleaning action and the relationship to frequency. However, ultrasonic cleaning involves all four facets. Read on and I’ll explain how the other facets play a strong, interactive role determining what frequency to use and in maximizing productivity.

Chemistry
The chemical characteristics of the soil to be removed and of the cleaning agents is important in ultrasonic cleaning. Solvency is a measure of how effective a cleaning agent is to chemically unbind soil from the surface. In addition to solvency, a cleaning agent may reduce surface tension, an important factor for cavitation. In some cases, the extremely high local temperature (20000 deg C), and high pressure (10000 psi) associated with cavitation collapse can enable chemical reactions, a phenomenon known as sonochemistry. We have observed marked differences in ultrasonic processes among aqueous cleaning agents and this effectivity difference is also frequency dependent. Some agents work better at 132 kHz than at 45kHz; with others the reverse is shown. Therefore, the combination of ultrasonic frequency and cleaning agent must be tested together and not be assumed from behavior at another frequency.

Temperature
Temperature is the measure of the internal energy of an object. When energy flows into a liquid bath, kinetic molecular motion increases and is measured as the temperature of the bath. Ultrasonic transducers or heaters in the tank add energy causing the bath temperature to increase. As it heats, the viscosity of the fluid decreases and the vapor pressure of the liquid increases. These phenomena affect properties of cavitation bubbles. They may form more easily due to lower viscosity but create a less violent shock wave due to the cushioning effect of vapor in the bubble. There is an optimum temperature for ultrasonic cleaning. This temperature is a function of the constituents of the cleaning bath, including cleaning agents and the ultrasonic frequency.

Time
Usually with cleaning processes, the longer the process, the more soil gets removed. One of the major advantages with ultrasonic cleaning is that it is fast; cleaning times are shorter than soaking in a still or even a stirred bath and this leads to greater product throughput. However, both in non-ultrasonic and ultrasonic processes, too much time can be a detriment. The product surface may be altered, frequently in a negative way. This is especially important when considering the operating frequency for ultrasonics. The key question is “how long is needed to remove the soil of interest and will the product surface be damaged during that time?”

Another aspect of ultrasonic frequency is the effect on particle removal. Generally, the smaller the particle, the higher the frequency needs to be for effective removal. Therefore if cleaning is needed to remove small particles from a delicate surface, a higher frequency ultrasonic system would be the likely choice. For large particles or heavy grease on a rugged surface, a low frequency device is likely to be adequate.

There is a lot more to ultrasonic cleaning than just choosing the frequency. There are many variables and options including sweep frequency, fixturing, and monitoring. Come to the Product Quality Cleaning Workshop in May 2020. Learn more about ultrasonics and experience both 45kHz and 132kHz systems during the lab exercises.