A high tech ultrasonic cleaner works as a result of sound waves being introduced into a cleaning liquid by means of a series of transducers mounted onto the cleaning tank. The sound travels throughout the ultrasonic cleaner’s tank creating waves of compression and expansion in the liquid. In the compression wave, the molecules of the cleaning liquid are compressed together tightly. Conversely, in the expansion wave, the molecules are rapidly pulled apart. The expansion is so dramatic that molecules are ripped apart, creating microscopic bubbles. The bubbles are unable to be seen by the naked eye because they are so small and exist for only a split second of time. The bubbles contain a partial vacuum while they exist. As the pressure around the bubbles becomes greater, the fluid around the bubble rushes in, collapsing the bubble very rapidly. Ultrasonic equipment manufacturers take advantage of the associated phenomena to deliver ultra-precise cleaning capabilities. When the bubbles collapse, a jet of liquid is created that travels at an extremely high rate. An associated rise in temperature as high as 5000°C occurs; this is roughly the temperature of the surface of the sun. This extreme temperature, combined with the liquid jet's velocity provides a very intense cleaning action in a very concentrated area that ultrasonic manufacturers can use. Because of the very short duration of the bubble expansion and collapse cycle, the liquid surrounding the bubble quickly absorbs the heat and the area cools rapidly. As a result, the tank and liquid only becomes warm and does not heat excessively due to the introduction of parts in the ultrasonic washing equipment.
APPLICATION OF ULTRASONIC CLEANING
Many articles exist describing "how ultrasonic cleaning works". The goal of this article is to help develop an understanding of the various components that ensure good ultrasonic cleaning.
First, establish a cleaning need, along with a determination as to how to measure the level of cleanliness. A few examples of measuring cleanliness include various levels of particle count, microscopic inspection, and a variety of adhesion tests, including the clear tape test that has the ability to remove additional contamination. These are just a few examples of cleanliness measurement.
Seven major concerns related to successful ultrasonic cleaning:
Proximity to the transducer/part fixture design
Ultrasonic output frequency
Watts per gallon
Loading - the volume (configuration) of the part being cleaned
Typical cleaning times may vary tremendously - how dirty is the part and how clean is clean. As a place to start, a normal trial period is two to ten minutes, since very few parts are sufficiently clean within a few seconds. Ultrasonic cleaning is not just a quick dip and zap, it's clean. Pre cleaning may be required to remove gross contamination or to chemically prepare the parts for a final clean. Some applications require more than one ultrasonic cleaning stage to complete the required cleaning. Ultrasonic agitated rinsing is required in some cases to more thoroughly remove the wash chemicals.
Temperature and chemistry are closely related. Generally, ultrasonic cleaning in an aqueous solution is optimum at 140°F. some high pH solutions will require the temperature to be higher to enhance the synergistic effect of the chemistry. The chemical pH is a good place to start; however, chemistry is not the subject of this article.
The following should be considered the main components of aqueous ultrasonic cleaning chemistry:
Water - hard, soft, DI or distilled
The chemical formulation must consider all of the above characteristics.
Some chemicals that are designed for spray cleaning, or that include rust inhibitors, are not suitable for ultrasonic cleaning.
PROXIMITY TO THE TRANSDUCER:
The procedure for ultrasonic cleaning is generally as follows: Put parts in basket and place basket through three or four process steps; ultrasonic wash, spray rinse (optional), immersion rinse, dry. Some parts loaded in baskets can mask or shadow from the radiated surface of the ultrasonic transducers. Most ultrasonic cleaning systems are designed for specific applications. Bottom mounted transducers or side mounted transducers are decided upon during the process design stage. Automated systems must specifically address the location of the transducers to insure uniformity of the cleaning. Some parts require individual fixturing to separate the part for cleaning or subsequent processes. Some parts require slow rotating or vertical motion during the cleaning to insure critical cleanliness.