Quality, efficiency, and/or environmental considerations often drive manufacturers, finishers, rebuilders and job shops to consider ultrasonic cleaning. By using ultrasonics in an appropriately designed system, many parts can be cleaned more successfully, often faster, with less operator involvement and without regulated solvents. Industries implementing ultrasonics to improve cleaning and/or rinsing include aerospace, medical, electronics, automotive, jewelry, optics and coating.

A Well-Designed Cleaning Process
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 matches load size, throughput and 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.

Additional mechanical action, supplied here by air spiders, can further enhance ultrasonic cleaning in higher production scenarios.

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.

Selecting an Ultrasonic Tank
As with any type of equipment, ultrasonic cleaning tanks should be matched as closely as possible to the job at hand. Labs and production areas with lower throughputs may choose a single ultrasonic tank followed by a sink rinse. Single tanks typically start at capacities of less than 1 gal. Production areas may consider a multi-station line in which one or more cleaning stations are followed by one or more rinses and an optional dryer. In either case, the ultrasonic tank work area should allow for at least two inches of clearance on all sides of the work load.

Tank construction is typically 300-series stainless steel, insulated to minimize heat loss. Small 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 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 bonded into a stainless steel can 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.

Frequency and Watt Density
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 resonant frequency. This is generally between 25 and 170 kHz but may be as high as 250 kHz, with 25–40 kHz being the most common.

A multiple station automated ultrasonic parts cleaning line for medical parts.

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 16–20 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 removal of smaller particles and where potential damage is a concern—for example, when cleaning polished surfaces, delicate parts, or 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 transducers, which use a single-step process. Also, output from a magnetostrictive transducer is proportionately lower. This is important to note when comparing systems using the two types of transducers. Actual output is similar to that of a piezoelectric transducer with a watt density approximately 60% as much. The 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 chemistry used in an ultrasonic cleaning tank can significantly impact cleaning success. Before the Montreal Protocol in 1988, what we now call “regulated solvents” 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.

Regulation of solvents provided the impetus to shift to water-based ultrasonic cleaning solutions that can deliver better results in many operations. There are three types of aqueous cleaners: acidic, neutral, and alkaline.

Acidic cleaners (pH <6) consist of mineral and organic acids with wetting agents. Generally not used for removal of oil and grease, they are most widely used for the removal of metal oxides. With the addition of ultrasonics this process can be accelerated and the acid used can therefore be less aggressive.

Neutral cleaners (pH 6–8) consist mostly of surfactants. They also contain mild builders and corrosion inhibitors. They are effectively used to remove oil and light grease.

Alkaline cleaners (pH 8–14) are a blend of builders such as potassium and sodium hydroxide, silicates, carbonates, bicarbonates, phosphates, borates, and surfactants. They are best suited for removal of oil, grease, inks and carbonaceous soils.

The efficiency of all 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.

Maximizing Performance
To further increase the chance of success in a production ultrasonic cleaning system, filtration of the cleaning tank is recommended. This is especially important in tanks using bottom-mounted ultrasonics, because 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 Mechanical Action
Ultrasonics coupled with 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.

Heavy oil loading on “before” part is successfully removed on “after” part by ultrasonic cleaning.

The Importance of Good Rinsing
Good rinsing is critical to ultrasonic parts cleaning success. Chemistry residue must be removed., and parts will never be cleaner than the last water they contact. In higher production settings, multi-station rinses are recommended to provide 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 (resistivity) meter 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 required.

Ultrasonics and/or filtration are sometimes recommended in a rinse station if part geometry or the required level of cleanliness 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.

A Design that Delivers
Process design for ultrasonic cleaning systems factors in all of the above considerations. 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 for the right amount of time, following the clean with a sink rinse and evaluating the result.

In higher-production areas, all of the above test-cleaning procedures apply, and loading, fixturing and throughput considerations also become critical to the success of the process design.

Consider, for example, 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 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 along with additional mechanical action in the form of agitation in both ultrasonic cleaning stations to further improve soil release.

Because spot-free rinsing was critical, the system was equipped with multiple DI water counterflow rinses, which also minimized water consumption, followed by air blow-off to expedite drying and a recirculating 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:

• Pretreatment prior to physical or chemical vapor deposition (PVD or CVD)
• Pretreatment for plating
• Removal of heavy soils prior to rebuilding in automotive and aerospace
• Removal of machining oils from stainless steels and aluminum alloys
• Removal of stamping lubricants from stainless steels, copper and mild steel
• Removal of buffing compounds from a variety of metals (automotive and medical parts, jewelry, faucets, hardware, etc.)
• Cleaning of particulates from disc drive hardware
• Removal of wax from glass lenses
• Cleaning of grinding compounds from tool steel hand tools
• Removal of plastic coating and tac from stainless steel sheets
• Cleaning of dirt and oils from high-carbon tool steel tool holders
• Removal of particulates from plastic jewel cases
• Cleaning of grease and dirt from field-tested locomotive shafts prior to performance testing.

Ultrasonics Expedite Passivation
If ultrasonics deliver cleaner parts faster, they can also expedite both nitric and citric passivation processes. Specifications such as ASTM A 967 allow for faster cycle times when ultrasonics are added to the acid tank. The addition of ultrasonics to a subsequent rinse tank can be beneficial in meeting exacting particle counts. Tank size, watt density and ultrasonic frequency guidelines apply.

The best way to see how ultrasonics can improve your 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. PFD