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Many types of parts can be cleaned more successfully, often faster, with less operator involvement and without regulated solvents by using ultrasonics in an appropriately designed parts-cleaning system. Industries that are implementing ultrasonics to improve cleaning and/or rinsing include aerospace, medical, electronics, automotive, jewelry, optics, and coating.
Designing an Ultrasonic Cleaning Process
As with any type of equipment, ultrasonic cleaning systems should be matched as closely as possible to the job requirement. Labs and production areas with lower throughputs may choose a single ultrasonic tank followed by a sink rinse. Single tanks typically start at less than one gallon and up. Higher production areas may consider a manual or automated multiple station line in which one or more ultrasonic cleaning stations are followed by one or more rinses and an optional dryer.
Tank construction is typically 300 series stainless steel, insulated to minimize heat loss. An ultrasonic tank work area should allow for at least two inches of clearance on all sides of the work load.
Small ultrasonic tanks are typically equipped with electric strip heaters, and the ultrasonic transducers are epoxy-bonded or vacuum brazed to a diaphragm usually located in the bottom of the tank.
Larger ultrasonic tanks usually feature heavier duty wall construction and additional wall support. Heat may be supplied in a variety of ways including electric immersion heaters, steam coils, and/or external heat exchangers. Larger tanks may have either diaphragm-bonded transducers, or immersibles which are transducers bonded into a stainless steel can which is then mounted in the cleaning tank. The advantages of immersibles are that a can may be swapped out if required without the whole tank going down, and immersibles can often be retrofitted into existing process tanks to improve cleaning performance.
Ultrasonic Frequency and Watt Density
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 enhances the separation of soils from substrates, often making ultrasonic cleaning faster and more effective than other methods. Imploding bubbles travel wherever the solution goes, allowing cleaning activity even within complex part geometries.
The source of ultrasonic sound waves is a transducer, and there are two types: piezoelectric and magnetostrictive. Piezoelectric transducers typically have a ceramic core that changes structure and emits a sound wave when charged by electrical pulses delivered by a generator at the resonate frequency. This is generally between 25 and 170 kHz but may be as high as 250 kHz, with 25 to 40 kHz being the most common.
Magnetostrictive transducers feature a ferrous core and a two-step sound wave emission process (power to the core + creation of electromagnetic field = introduction of sound wave). They are almost always found in lower frequency applications from 16 to 20 kHz and are especially suited to heavy loads and high temperatures.
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).
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. Power control circuitry 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.
The newest ultrasonic technology is the multiple frequency generator which features a range of frequencies in one generator—a more expensive option that nevertheless is sometimes indicated when cleaning very dissimilar parts in one cleaning line.
Operationally, because the emission of sound waves by magnetostrictive transducers is a two-step process, the energy required to create the sound wave is greater than with piezoelectric, which is a single-step process. Also, the output from a magnetostrictive transducer is proportionately less. This is important to note when comparing quotes on systems using the two types of transducers, as magnetostrictive will appear to have more watts of ultrasonics whereas the actual output is similar to a piezoelectric transducer with a watt density approximately 60 percent as much. The actual watt density required for a particular cleaning application is a function of the size of the tank work area, the mass of the workload, and the type of soil being removed.
Chemistry and Temperature
The kind of chemistry used in an ultrasonic cleaning tank can significantly impact cleaning success. Before the Montreal Protocol in 1988, what we now call “regulated solvents” such as 1.1.1. were often the cleaning solutions of choice in ultrasonic tanks. Today, they and a new generation of solvents remain an important option for certain types of cleaning, as is another class of cleaning solution called “semi-aqueous” which mixes solvents and water. But the biggest change after the Montreal Protocol has been the shift to aqueous cleaners for many types of parts-cleaning applications.
There are three types of aqueous cleaners: acidic, neutral, and alkaline.
The efficiency of all of the water-based cleaners increases in combination with ultrasonics and heat. However, raising the temperature too high (above about 180°F) reduces cavitation pressure and can therefore be counter-productive to ultrasonic cleaning success.
Optimizing Solution Performance
To further increase ultrasonic cleaning success in a production driven ultrasonic cleaning system, filtration of the cleaning tank is recommended. This is especially important in cleaning tanks in which the ultrasonics are bottom-mounted, as particulate build up decreases ultrasonic efficiency. Cartridge and bag filters are available for various types and levels of particulate and are selected based on soil loading considerations.
To further optimize the ultrasonic cleaning process, surface skimming (sparging) of the cleaning solution into a separate weir means that clean parts won’t be pulled through floating contaminants on exit from the cleaning station.
An added bonus of filtering and sparging is longer cleaning tank solution life, which can also be further extended with coalescing and/or ultrafiltration units.
Fixturing and Additional Mechanical Action
Ultrasonics plus the right chemistry and temperature make a powerful parts cleaning package. This is especially true when parts can be racked or fixtured to present their toughest soils toward the ultrasonics, when the parts do not nest or mask each other, and when the parts are not allowed to trap air pockets which prevent solution movement and cavitation.
In higher production scenarios, additional mechanical action is sometimes required to compensate for load requirements and part geometries, and to realistically translate lab-testing results up to production levels.
Agitating platforms, rotation, spray, and turbulation (spray under immersion) are some of the additional mechanical actions that can further enhance cleaning performance and improve process times. Test cleaning is the best way to determine if additional mechanical action is recommended, and if so, what type is most desirable.
Good Rinsing is Key
Good rinsing is critical to ultrasonic parts cleaning success. First, chemistry residue needs to be removed. Second, parts will never be cleaner than the last water they contact. In higher production settings, multiple station rinses are recommended with each one accomplishing successive dilutions of the cleaning chemistry.
To optimize water use and conservation, counter-flowing (cascading from the last rinse back to the first rinse) is suggested, as it reuses water in more than one station. A conductivity or resistivity meter on the line can also facilitate water conservation by calling for fresh water only when quality drops below a preset standard. If parts need to be spot free, RO or DI water will be the required feed.
Ultrasonics and/or filtration are sometimes recommended in a rinse station if the geometry of the parts or the level of cleanliness required suggests that they would be of particular value (to meet a clean room or military specification, for example). Varying the frequency of rinse tank ultrasonics can also enhance results.
Ultrasonic Cleaning Success Stories
A well-designed ultrasonic parts-cleaning system can make the difference between success and failure in a cleaning operation. One such case was a medical manufacturer who was removing buffing compound from complex parts. The parts were put in baskets and manually immersed in chemistry and rinses, but were not always clean at the end of the process. The operator then re-cleaned each part individually by hand. As throughput requirements grew, the approach was not reliable or cost-effective, and the manufacturer sought an ultrasonic alternative.
The complex geometry of the parts and the buffing compound soil made the parts good candidates for ultrasonic cleaning. A chemistry especially formulated to remove buffing compound was tested and approved during the ultrasonic test cleaning process.
Production-scale testing also suggested that two additional steps would be required to deliver the desired results and efficiency: a second ultrasonic cleaning station would be added and additional mechanical action in the form of agitation in both ultrasonic cleaning stations would further improve soil release.
Because spot-free rinsing was critical, the system was equipped with multiple DI water counter-flow (cascading) rinses, which also minimized water consumption, followed by air blow off (to expedite drying to meet production goals) and a re-circulating hot air dry. Automation controlled process steps and times, assuring standardized performance.
Assessment of and attention to part substrates and geometries (critical clean areas), buffing compound (soil) characteristics, rinsing and drying requirements, and throughput resulted in an automated system that effectively cleans, rinses, and dries the parts without operator intervention during or after the process. The system meets production and personnel goals as well as increasingly exacting FDA standards.
Ultrasonics have proven similarly successful in numerous other cleaning applications, including:
The best way to see how ultrasonics can improve a parts-cleaning application is to approach potential suppliers about test cleaning parts. Review test results, process recommendations, and proposals, and, before ordering any equipment, be sure that provisions have been made in equipment, chemistry, fixturing, and cycle times to meet or exceed your throughput goals.