Parts Washing and Cavitation: The Ultrasonic Answer

Cavitation is produced in the process of Ultrasonic Cleaning that, in conjunction with a proper detergent, breaks down contaminants such as dirt and grime. As sound waves at specifically selected frequencies are generated throughout the liquid bath, they produce a scrubbing action upon the surface of the object to dislodge particles. Ultrasonic parts washing has been proven to be among the one of the most efficient cleaning methods, particularly when the parts are of intricate design or are unable to tolerate some of the stronger chemicals used for more durable parts.

The dynamic components of ultrasonic cavitation are millions of microscopic bubbles that are created when high frequencies are generated in the liquid bath. The selected frequencies to be used are relative to the attributes of the parts to be cleaned, For instance, higher frequencies generate streams of small, moderately active bubbles that are effective for cleaning sensitive substrates, whereas, lower frequencies generate fewer ultrasonic events but produce ultrasonic bubbles that are larger and release more energy at the substrate thereby work much better for heavier, larger and more durable industrial parts.

The action of the bubbles themselves is a complex series of transformations that begins with the bubbles looking as one would expect: round bubbles. As they come into contact with any surfaces, they implode, morphing roughly into a c-shaped bubble. The implosions release energy that enhance the chemical action as well, stripping off contaminants which are then washed away from the parts.

RAMCO’s Preventative Approach

There are a number of factors to be taken into consideration in evaluating the effectiveness of an ultrasonic cleaning system.

Entrapment

One such factor is air entrapment. This can happen when parts are multi-planar, have fine undercutting, or have blind holes. Such characteristics can cause air to become “trapped” in areas and become an obstacle, preventing those areas from being cleaned. If the all the part surfaces are not fully immersed in the liquid detergent the ultrasonic process will not work in those areas that are not in full contact with the liquid detergent.

To overcome the difficulties associated with air entrapment, a number of things need to be taken into account, especially since it’s virtually impossible to change the shape of a given part:

• The placement/positioning of the parts to be washed. With careful positioning, it’s possible to ensure all surfaces are properly flooded.
• Upon submerging the parts a careful inspection of their positions can be made to determine if any trapped air can be manually released.
• Gentle agitation with or without solution turbulation can help bring solution in contact with part irregularities.
• Rotation of parts is very effective but such a procedure is not always feasible since it can cause damage to certain parts. However, when it can be done without risk the parts’ integrity this process can irrigate problem areas and release trapped air.

Cavitation Erosion

Additional care must be taken if any given parts might be subject to damage from the ultrasonic forces at work. Some materials are sensitive to vibrations or to being subjected to force, resulting in erosion of the part’s surface. Higher frequencies, being less forceful, are less likely to cause damage than the lower frequencies.

The parts to be cleaned are not the only components at risk for surface erosion. The washing machinery itself is subject to this condition as well. Possibly the most crucial area is cavitation erosion of the radiating surface of the ultrasonic tank or transducers – a topic that, more often than not, is rarely mentioned among manufacturers of ultrasonic cleaning systems. At RAMCO we face these concerns head-on, understanding that this type of erosion will eventually occur over time; we only use immersible transducers which can be easily replaced when the time comes.

Above: Cavitation erosion on the surface of a transducer.

During the ultrasonic parts washing process, the powerful ultrasonic forces unleashed on the radiating surface of the transducer can actually remove metal from that surface, causing pitting and compromising the transducer’s integrity. The factors at play here are a) the operating time – the more time and the longer the machinery is used, the greater the chances of erosion; b) the degree of corrosive properties of the chemical agents being used; and c) the specific temperature being employed to do the cleaning – in general, the higher the temperature, the greater the risk.

Taking such factors into account can help extend transducer life. In addition, hard chrome plating of the transducer’s surface as well as utilizing heavy gauge metal for the transducer face have both been shown to be effective. However, it is virtually impossible to completely stop cavitation erosion from occurring.

In this regard, RAMCO’s recommendations as to what type of transducer is to be used will go a long way toward extending transducer life.

Matching the Machine to the Task

Mitigating these potential issues is of major priority when pairing parts with the right ultrasonic parts washing machines and solutions. The size and shape of ultrasonic tanks can have significant impact upon fluid circulation and agitation. Furthermore, parts that are submerged into the tank can actually absorb, to varying degrees, the ultrasonic energy that is being generated. For example, any parts located directly on or too close to the ultrasonic radiating surfaces can adversely impact performance.

Enhanced Cleaning with the Multiple Rollover Process

Occasions may arise when it’s necessary to augment the ultrasonic parts washing process in order to ensure the best results. To that end, RAMCO pioneered the technique of Multiple Rollover. There are situations where the ultrasonic cleaning process is not effective in and of itself. An example is in the removal of heavily soiled parts for mass transit and overhaul operations. In these cases, the surface layers of contamination are caked up with dirt, grease and grime which requires a thorough prewash using turbo/agitation to remove the heavy soil prior to the ultrasonic process. The RAMCO Multiple Rollover process combines both methods. Superior washing results are achieved by alternating between ultrasonic “bursts” and turbo/agitation hydraulic flushing.

Specifically, during solution turbulation, as the cleaning agents move into and around the parts, gross contaminants can be flushed from recessed passages, blinds holes and other regions of part irregularities. Using Multiple Rollovers combined with proper cleaning agents, correct ultrasonic frequencies and regulated exposure times are critical for parts which may be vulnerable to excessive ultrasonic exposure, such as aluminum components common to the aerospace industry. Using this process, they can be thoroughly cleaned without risking damage. Multiple Rollover, in addition to it cleaning benefits, can markedly reduce washing times.

Standard RAMCO platform oscillation moves the parts through the solution and can be used intermittently as well as in combination with ultrasonics in situations where stationary cleaning outcomes might otherwise fall short. Not only is ultrasonic parts washing enhanced by the agitation, but cleansing action is further improved as the cleaning agent flows across the parts surfaces with every oscillation. In addition, contaminants caught in any of the entrapment regions are systematically flushed away and drained.

For a free consult with one of our experts, contact the RAMCO factory at 800-553-3650.