Well-designed ultrasonic cleaning systems have a proven track record of delivering gentle yet thorough parts-cleaning without regulated solvents to industries as diverse as aerospace, electronics, automotive and coating.
A typical ultrasonic parts-cleaning process includes at least one ultrasonic cleaning cycle sized to your particular load requirements, followed by rinsing, and often, but not always, drying. These stages may be accomplished in manual or automated multiple station lines (including strip or reel to reel), as pretreatment steps in plating and finishing lines or even in single tanks alone or in series.
Cleaning success with ultrasonics is directly linked to matching 1) load size, 2) throughput and 3) cleanliness standards with the following:
- The frequency and watt density of ultrasonics in the tank
- The most appropriate cleaning chemistry for the soil(s) and substrate(s)
- The right operating time and temperature for the bath
- The best orientation for the parts as they are processed
- Additional mechanical action (agitation, turbulation, rotation) if required
- Good rinsing
Ultrasonic energy occurs when sound waves are introduced into a solution such as water. The sound waves create microscopic bubbles of solution during periods of positive pressure, which implode and release a burst of energy during periods of negative pressure. This process is called cavitation. It is cavitation that helps to break a soil-bond to expedite parts cleaning. One of the most special aspects of ultrasonics is that the imploding bubbles travel wherever the solution goes, allowing cleaning activity even within complex part geometries.
Controlling the temperature and chemical composition of the solution and the frequency and watt density of the sound waves, determines the level of ultrasonic activity and intensity (the cavitation density) and therefore the cleaning performance in a tank.
When cleaning with ultrasonics, the frequency of the sound waves is matched to the application. For the most part, lower frequencies (20-40 kHz) are safe for most applications and will produce the most intense cavitation energies to remove the most common types of contaminants (oil, grease, metal chips). They are also the most commonly used frequencies.
Higher frequencies (68-250 kHz) will produce smaller cavitation bubbles with less intense energies but more of them. This can be beneficial in the removal of smaller particles and where damage is a concern (polished surfaces, delicate parts, soft substrates).
Making Frequency Work for You
While ultrasonic devices have a natural frequency variation, additional frequency modulation is now available through sweep frequency generators. Frequency-sweep circuitry varies the frequency of the ultrasonic generator to create a more uniform cleaning field by alleviating standing waves and hot spots sometimes characteristic of older equipment. This can be particularly beneficial when cleaning softer substrates where standing waves and hot spots may cause part damage.
Power control circuitry further tailors the output to varying load conditions, thus improving versatility, which is especially useful when different types of parts are being cleaned in the same line.
To meet the challenges of increasingly exacting cleanliness standards for industries such as medical, aerospace, electronics and coatings (including physical vapor deposition and chemical vapor deposition—PVD and CVD), multiple ultrasonic frequencies can further enhance the cleaning process.
The use of multiple frequencies builds on the principle that lower ultrasonic frequencies create larger bubbles which implode with a more aggressive particle separation (cleaning) action, and higher frequencies create smaller bubbles which implode with less aggressiveness but which can travel in solution to geometries where larger bubbles cannot go.
Combining the use of lower and higher frequencies of ultrasonics delivers a wider potential range of cavitation contact without sacrificing cavitation aggressiveness.
In multiple station cleaning lines, the principle of multiple frequencies can be implemented on a tank by tank basis, as one medical manufacturer recently discovered in a new generation cleaning line designed to remove buffing compound from stainless steel parts with microscopic geometries that become compound traps.
One process change on the cleaning line—from 40 kHz of ultrasonics to 25 kHz on the initial cleaning station, cut cleaning time by one-half on the line—thereby meeting particle count requirements while at the same time doubling production.
Adding ultrasonics to a rinse station, and using a different frequency in the rinse station from what the parts were previously exposed to in a cleaning station, can also be useful to remove sub-micron particles to meet specification requirements.
To accomplish multiple frequencies in a single station, the newest ultrasonic technology is the multiple frequency generator which features a range of frequencies (such as 40, 72, 104, and 170 kHz) in one generator. This is a more expensive option that nevertheless is sometimes indicated when cleaning very dissimilar or complex parts in one cleaning tank.
Meeting Your Cleaning Goals with Ultrasonics
The best way to see how ultrasonics will work in your parts-cleaning application is to have potential suppliers test clean parts and recommend a process.
In lower production operations, process development may be as simple as test-cleaning a few parts in a soil- and substrate-appropriate chemistry at the correct temperature and frequency (or frequencies) for the right amount of time, following the clean with a sink rinse and evaluating the result.
When planning and testing to meet higher production goals, all of the above test-cleaning procedures apply and loading, fixturing, and throughput considerations (which may involve multiple clean, rinse, and/or dry stations) also become critical to the success of the process design.