Tool cleaning in the injection molding facility was critical and time consuming.Personnel used solvents,rags, abrasive pads, and dental instruments to break the soil bonds of plastic residue, gassing, flashing, and mold release from the mirror finishes of the cores and cavities used to mold personal health-care packaging. The work was slow, tedious, and risky—a tool valued at $50,000 or more could be scarred in the slip of a hand.

To improve the process, an ultrasonic cleaning system was installed consisting of a tank with appropriate chemistry and filtration, followed by a rinse and corrosion inhibitor. Using a manual crane to suspend heavy frames or baskets of tool parts, an operator dipped the load into the cleaning station and resumed other activities while the system did the cleaning work. Ultrasonic cleaning minimized the risk to the tools by eliminating harsh abrasives, freed personnel for assembly and repair work to keep tools in production, and removed the soils that used to take an hour or more in under fifteen minutes.

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, but not all, parts can be cleaned more successfully with less operator involvement and without regulated solvents.

Steps Involved in Ultrasonic Cleaning
A typical ultrasonic parts-cleaning process includes at least one ultrasonic cleaning cycle sized to 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.

An immersible ultrasonic transducer and generator

Cleaning success with ultrasonics is directly linked to matching load size, throughput and cleanliness standards with the following:

• the frequency and power 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 and
• good rinsing.

What Makes Ultrasonics So Special?
Ultrasonic energy occurs when sound waves are introduced into a liquid 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 and 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 power density of the sound waves, determines the level of ultrasonic activity and intensity (the cavitation density) and therefore the cleaning performance in a tank.

Ultrasonic Cleaning Tank

What to Look for in an Ultrasonic Cleaning 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 less than one gallon. Higher production areas may consider a multiple 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.

Tanks are typically constructed of 300 series stainless steel, insulated to minimize heat loss. Small tanks are often 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 transducers bonded into a stainless steel can that 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 often can be retrofitted into existing process tanks to improve cleaning performance.

Selecting Ultrasonic Frequency and Power
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 to 40 kHz being the most common.

Magnetostrictive transducers have 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. 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.

Parts processed without ultrasonics still show soil

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 (the all-important watt density) whereas the actual output is similar to a piezoelectric transducer with a watt density approximately 60% 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” 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 solution called “semi-aqueous” which mixes solvents and water.

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

Acidic cleaners (pH less than 6.0) consist of mineral and organic acids with wetting agents. They are generally not used for the removal of oil and grease, but 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.

Parts after ultrasonics are added to the process show significantly less soil.

Neutral cleaners (pH of 6.0–8.0) consist mostly of surfactants. They also contain mild builders and corrosion inhibitors. They are used to remove oil and light grease.

Alkaline cleaners (pH of 8.0–14.0) are a blend of builders such as potassium and sodium hydroxide, silicates, carbonates, bicarbonates, phosphates, borates, and surfactants. They are best suited for the removal of oil, grease, inks, and carbonaceous soils.

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.

Maximizing 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 buildup 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 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 submersion) 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.

Why Rinsing Is So Important
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, counterflowing (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 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).

Bringing It All Together
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 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. However, production-scale testing also suggested that two additional steps would be required to break the soil bond while meeting production goals: 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 counterflow (cascading) rinses, which also minimized water consumption, followed by air blow off (to expedite drying to meet production goals) 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, and a system that meets production and personnel goals.

Ultrasonics have proven similarly successful in numerous other cleaning applications, including:

• pretreatment prior to Physical Vapor Deposition (PVD) and/or Chemical Vapor Deposition (CVD)
• pretreatment prior to plating
• removal of heavy soils prior to rebuilding in automotive and aerospace
• machining oils from stainless steel and aluminum
• stamping lubricants from stainless steel, copper and mild steel
• buffing compounds from a variety of metals (motorcycle and automobile parts, medical parts, jewelry, faucets, hardware)
• particulates from disc drive hardware
• wax from glass lenses
• grinding compounds from tool steel hand tools
• plastic coating from stainless steel sheets
• dirt and oils from high-carbon tool steel tool holders
• particulates from plastic jewel cases,
• grease and dirt from field-tested locomotive shafts prior to performance testing.

The best way to see how ultrasonics would work in your parts-cleaning application is to approach potential suppliers about test cleaning parts. Review test results, process recommendations and proposals, and, before ordering equipment, be sure that provisions are made in equipment, chemistry, fixturing and cycle times to meet your throughput
goals. PFD