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Pressure waves generated by high-frequency sound waves create micron-size bubbles in the aqueous cleaning solution. This bubble is shown just before implosion, which releases a tremendous amount of energy for cleaning
Ultrasonic cleaning involves the use of high-frequency sound waves (above the upper range of human hearing, or about 18 kHz) to remove a variety of contaminants from parts immersed in aqueous media. The contamination can be dirt, oil, grease, buffing/polishing compounds and mold release agents, just to name a few. Materials that can be cleaned include metals, glass, ceramics and so on. Ultrasonic agitation can be used with a variety of cleaning agents.
Typical applications include removing chips and cutting oils from cutting and machining operations, removing buffing and polishing compounds prior to plating operations and cleaning greases and sludge from rebuilt components for automotive and aircraft applications.
Ultrasonic cleaning is powerful enough to remove tough contaminants yet gentle enough not to damage the substrate. It provides excellent penetration and cleaning in the smallest crevices and between highly spaced parts in a cleaning tank.
The use of ultrasonics in cleaning has become increasingly popular due to the restrictions on the use of chloroflourocarbons such as 1,1,1-trichloroethane. Because of these restrictions, many manufacturers and surface finishers are using immersion cleaning technologies rather than solvent-based vapor degreasing. The use of ultrasonics enables the cleaning of intricately shaped parts with an effectiveness that corresponds to that achieved by vapor degreasing.
How does ultrasonic cleaning work?
Contrary to popular belief, cavitation is the force behind ultrasonic cleaning. It is not the sound waves that scrub the part clean – it is the cavitation process set up by the sound waves that scrub the part clean.
During cavitation, micron-size bubbles form and grow due to the alternating positive and negative pressure waves in a solution. The bubbles subjected to these alternating pressure waves continue to grow until they reach resonant size. Just prior to the bubble implosion, there is a tremendous amount of energy stored inside the bubble.
Temperatures inside a cavitating bubble can reach 9,900F with pressures up to 500 atm. The implosion event, when it occurs near a hard surface, changes the bubble into a jet one-tenth the bubble size, which travels at speeds up to 400 km/hr toward the hard surface. With the combination of pressure, temperature and velocity, the jet frees contaminants from their bonds with the substrate. Because of the inherently small size of the jet and the relatively large energy, ultrasonic cleaning has the ability to reach into small crevices and remove entrapped soils effectively.
An excellent demonstration of this phenomenon is to take two flat glass microscope slides, put lipstick on a side of one, place the other slide over top and wrap the slides with a rubber band. When the slides are placed into an ultrasonic bath with nothing more than a mild detergent and hot water, the process of cavitation will work the lipstick out from between the slide assembly within a few minutes. It is the powerful scrubbing action and the extremely small particle size of the jet action that enable this to happen.
The measurement of cavitation has become increasingly important as the use of ultrasonic cleaning continues to grow. Manufacturers have developed probes and meters of all sorts to determine how much cleaning power, and its uniformity, is in the tank. These devices can be misleading because they typically measure pressure—not cavitation. Cavitation occurs due to pressure waves, but they are two independent processes. Cavitation can actually mask the pressure wave reading.
The most reliable measurement device for cavitation is a simple foil test. To perform the test, a piece of aluminum foil is placed vertically in the cleaning bath down to the bottom of the tank spanning opposite corners. Typical immersion time for the foil is 1 min in the bath. Upon inspection of the foil, a pattern of indentations, and possibly holes (depending on the foil thickness, frequency and power density) should occur. In a properly designed ultrasonic tank, the pattern on the foil should be uniform and cover the entire area. This shows that even cavitation is occurring, and, therefore, cleaning should be even and consistent. If the pattern is inconsistent and there are dead zones, then inconsistent cleaning can occur.
Poor foil tests can occur for a variety of reasons: lack of generator frequency sweep; improperly matched ultrasonic generator to transducer load; piezoelectric transducers that have degraded or disbonded; lack of ultrasonic power for a given tank volume; poor transducer layout; or inability of the cleaning solution to properly degas and cavitate.
In order to produce the positive and negative pressure waves in the aqueous medium, a mechanical vibrating device is required. Ultrasonic manufacturers have made use of a diaphragm attached to high-frequency transducers. The transducers, which vibrate at their resonant frequency due to a high-frequency electronic generator source, induce amplified vibration of the diaphragm. This amplified vibration is the source of positive and negative pressure waves that propagate through the solution in the tank. The operation is similar to the operation of a loudspeaker except that it occurs at higher frequencies. When transmitted through water, these pressure waves create the cavitation process.
The resonant frequency of the transducer determines the size and magnitude of the resonant bubbles. Typically, ultrasonic transducers used in the cleaning industry range in frequency from 20-80 kHz. The lower frequencies create larger bubbles with more energy. The lower frequency cleaners will show a more aggressive dimple pattern during a foil test while higher frequency cleaners will show less, if any at all.
|Table I - Comparison of Options|
|Material of Construction||Types of Parts||Contaminants||Suitable Cleaning Agent|
|Iron, steel, stainless steel||Castings, stampings, machined parts, drawn wire, diesel fuel injectors||Chips, lubricants, light oxides||Neutral to highly alkaline|
|Oil-quenched, used automotive parts, fine-mesh and sintered filters||Carbonized oil and grease, carbon smut heavy grime deposits||Highly alkaline|
|Bearing rings, pump parts, knife blades, drill taps, valves||Chips, grinding, lapping and honing compounds, oils, waxes, abrasives||Moderately alkaline|
|Castings, machined parts||Buffing and polishing compounds, miscellaneous machining, shop and other soils||Neutral to highly alkaline|
|Aluminum, zinc||Castings, machinings, used automotive carburetor parts, valves, switch components drawn wire||Chips, lubricants, general grime, buffing and polishing compounds||Neutral to moderately alkaline, specially inhibited to prevent etching of metal|
|Copper, brass (also silver, gold, tin, lead, solder)||Printed circuit boards, wave guides, switch components, instrument connector pins, machinings||Chips, shop dirt, lubricants, light oxides, flux residues, buffing and lapping compounds||Neutral to moderately alkaline, silicated or neutral synthetic (possible with ammonium hydroxide for copper oxide removal)|
|Various Metals||Heat-treated materials, brazed parts||Oxide coatings||Moderately to strong inhibited proprietary acid mixtures specific for the oxide and base metal of the part to be cleaned.|
The basic components of an ultrasonic cleaning system include a bank of ultrasonic transducers mounted to a radiating diaphragm, an electrical generator and a tank filled with an aqueous solution. A key component is the transducer that generates the high-frequency mechanical energy. There are two types of ultrasonic transducers used in the industry—piezoelectric and magnetostrictive. Both have the same functional objective, but the two types have dramatically different performance characteristics.
Piezoelectric transducers are made of several components. A piezoceramic (usually lead titanate) disc is sandwiched between two strips of tin. When voltage is applied across the strips, it creates a displacement in the material known as the piezoelectric effect. When these transducers are mounted to a diaphragm (wall or bottom of the tank), the displacement in the piezoceramic causes a movement of the diaphragm, which, in turn, causes a pressure wave to be transmitted through the aqueous solution in the tank. Because the mass of the ceramic is not well matched to the mass of the stainless steel diaphragm, an intermediate aluminum block is used to improve impedance matching for more efficient transmission of vibratory energy to the diaphragm. The assembly is inexpensive to manufacture due to low material and labor costs. For industrial cleaning, however, piezoelectric systems have several shortcomings.
The most common problem is that the performance of a piezoelectric unit deteriorates over time. This can occur for several reasons. The piezoceramic tends to depolarize over time and with use, causing a substantial reduction in its strain characteristics. As the material expands less, it cannot displace the diaphragm as much. Less vibratory energy is produced, and a decrease in cavitation is noticed in the tank. Additionally, piezoelectric transducers are often mounted to the tank with an epoxy adhesive, which is subject to fatigue at the high frequencies and high heat generated by the transducer and solution. The epoxy bond eventually loosens, rendering the transducer useless. The capacitance of the piezoceramic also changes over time and with use, affecting the resonant frequency and causing the generator to be out of tune with the mechanical resonant circuit.
Energy transfer of a piezoelectric transducer is another factor. Because the energy is absorbed by the parts that are immersed in an ultrasonic bath, there must be a substantial amount of energy in the tank to support cavitation. If this is not the case, the tank will be “load sensitive” and cavitation will be limited, degrading cleaning performance. Although the piezoelectric transducers use an aluminum insert to improve impedance matching (and therefore energy transfer into the radiating diaphragm), they sill have relatively low mass. This low mass limits the amount of energy transfer into the tank (as can be seen from the basic equation for kinetic energy, 1/2mv2).
Due to the low-driving mass of the piezoelectric transducers, manufacturers must use thin diaphragms in their tanks (typically 14-16 gauge). A thick plate simply will not flex (and therefore cause a pressure wave) given the relatively low energy output of the piezoelectric transducer. Over time, cavitation tends to erode at the diaphragm and can pit through this thin wall, allowing the cleaning solution to leak through the tank. It is generally less expensive to replace the tank and transducer system (or immersible pack) at this point rather than repair it. With high purity water systems or aggressive cleaning solutions, cavitation erosion is accelerated. Manufacturers of piezoelectric systems offer chrome plating of the diaphragm or titanium nitride coating to reduce cavitation erosion.
Magnetostrictive transducers are known for their ruggedness and durability in industrial applications. They operate on the inherent property of nickel that when it is subject to a magnetic field, it contracts. This property does not degrade over time or with higher temperatures. There are two types of magnetostrictive transducers, and it is important to differentiate between the two.
Spaced laminated designs use corrugated laminates so that the transducer stack resembles a honeycomb from the top. This reduces the amount of nickel required to create the transducer, and reduces the cost. It is epoxy bonded to the radiating diaphragm, similar to most piezoelectric systems. Being that it is magnetostrictive, performance does not deteriorate over time. However, due to its low mass, it is again attached to a thin 14-gauge diaphragm and is subject to cavitation erosion. This type of transducer also has the potential to detach from the diaphragm due to the epoxy bond.
Zero-spaced laminate magnetostrictive transducers consist of nickel laminations attached tightly together with an electrical coil placed over the nickel stack. When an alternating current is sent through the magnetostrictive coil, the stack vibrates at the frequency of the current.
The nickel stack of the magnetostrictive transducer is silver brazed directly to the resonating stainless steel diaphragm. This has several advantages over an epoxy bond. The silver braze creates a solid metallic joint between the transducer and the diaphragm that will never loosen. The silver braze also efficiently couples the transducer and the diaphragm together, eliminating the damping effect that an epoxy bond creates. The use of nickel in the transducers means there will be no degradation of the transducers over time. Nickel maintains its magnetostrictive properties on a constant level throughout the lifetime of the unit. Magnetostrictive transducers also provide more mass, which is a major factor in the transmission of energy into the solution in the ultrasonic tank. Zero-space magnetostrictive transducers have more mass than piezoelectric transducers, so they can drive more power into the tank, making them less load-sensitive than piezoelectric systems.
A radiating diaphragm that uses zero-space magnetostrictive transducers is usually 3/16 inch or greater in thickness, eliminating any chance for cavitation erosion. Heavy nickel stacks can drive a plate of this thickness and still get excellent pressure wave transmission into the aqueous solution.
The magnetostrictive material is not as efficient as the piezoelectric material. That is, for a given voltage or current displacement, the piezoelectric material will exhibit more deflection than the magnetostrictive material. This is a valid observation; however, the efficiency of concern should be that of the entire transducing system, including the transducer, the elements that make up the transducer as well as the diaphragm and the effectiveness of the bond to the diaphragm. It is the inferior mounting and impedance matching of a piezoelectric-driven diaphragm that reduces its overall transducing efficiency relative to that of a magnetostrictive transducer.
The ultrasonic generator converts a standard electrical frequency of 60 Hz into the high frequencies required in ultrasonic transmission.
The ultrasonic frequency generator plays a key role in the overall performance of the system. Earlier generator designs developed a fixed frequency signal designed to operate at the resonant frequency of the transducer system. For example, if the design frequency of the transducer was 20 kHz, the generator developed exactly a 20 kHz signal. The problem with this design is that it tends to create hot spots and standing waves in the tank bath, providing uneven cleaning in different tank areas.
The development of sweep frequency generators eliminated these problems. In most any ultrasonic cleaning system, there are multiple transducers that make up the transducer system. Due to manufacturing tolerances, it is highly unlikely that each transducer will have exactly the same resonant frequency. With a fixed frequency generator, the transducers that are closest in resonant frequency will operate the most efficiently in their respective areas, creating an uneven cavitation pattern. By sweeping the frequency just slightly above and below the center frequency, all transducers see their resonant frequency at the rate of sweep and maximum efficiency occurs, eliminating hot spots. Sweep frequency also eliminates standing waves from occurring by repeatedly overlapping the wavelength.
Many ultrasonic generators also have “autofollow” circuitry. Autofollow circuitry is designed to maintain the center frequency when the ultrasonic tank is subject to varying load conditions. When parts are placed in the tank or when the water level changes, the load on the generator changes. With autofollow circuitry, the generator matches electrically with the mechanical load, providing optimum output at all times to the ultrasonic tank.
Ultrasonic tanks are generally rectangular and can be manufactured in just about any size. Transducers are usually placed in the bottom or on the sides, or sometimes both when watt density (watts/gal) is a concern. The transducers can be welded directly into the tank, or watertight immersible units can be placed directly into the aqueous solution. In some instances, the immersibles may be mounted at the top of the tank facing down. For applications such as strip cleaning, one immersible is placed on the top and one on the bottom with minimal distance between them. The strip is then run through the very hard energy field. A tank should be sturdy in construction, ranging from 11-14 gauge in thickness. Larger, heavy-duty industrial tanks should be 11-12 gauge and should contain the proper stiffeners for support due to the weight of the solution.
Frequency and cleaning
Until the last 5 years, ultrasonic cleaning was typically limited to the lower end of the ultrasonic spectrum—20 and 30 kHz for magnetostrictive systems and 25 and 40 kHz for piezoelectric systems. Recently, manufacturers have developed systems that will operate at frequencies well above 100 kHz. These higher frequencies are less aggressive on sensitive substrates and can clean contamination levels down to the submicron range. They can be ideal for applications such as cleaning a fragile disk drive or semiconductor components but lack the power required for typical precision industrial applications. The primary frequency range for the typical industrial process continues to be 20-40 kHz.
So, is there an ideal frequency for a particular application? Provided everything else is equal, the lower the frequency, the more powerful the cavitation process and the larger the imploding bubbles. The higher the frequency, the less aggressive cavitation is and the smaller the implosions. It should follow that for large parts and areas with heavy contamination levels, lower frequency is best. And, for small parts with tiny blind holes and intricacies that contain small particles, the higher the frequency the better. However, this is not always the case. Removing a contaminant from a substrate requires a certain amount of power, and a higher frequency system may not be able to create this level of energy. Cavitation and its power levels are affected by several variables: Ultrasonic frequency; time; chemistry concentration; load size; contamination level; power density; detergent or solvent type; part geometry; basket/rack configuration; contamination geometry; cavitation uniformity; bath temperature; part material; contamination type; and bath filtration.
The pressure waves tend to be absorbed by the mass, geometry and material composition of the load and racking system. If the pressure waves are not powerful enough to overcome this absorption, then cavitation will be affected, no matter what the frequency. Cleaning solutions, whether aqueous or solvent based, will cavitate differently depending on their surface tension, viscosity and vapor pressure. Temperature of the cleaning bath may also affect cavitation as well as the ability of the solution to clean the contaminant.
So to say that a particular frequency is best for a certain part type and/or contaminant is virtually impossible. It is one variable in the equation and is independent of many other variables.
A major manufacturer of metal cutting tools used trichloroethylene to remove hydrocarbon-based machining oils from the tools prior to final coating. A high cleanliness level of these tools was critical since the manufacturer also offered PVD coating on these tools, which was done in-house.
A corporate initiative to eliminate solvents required the manufacturer to search for an alternative aqueous cleaning system. While aqueous cleaning for the application may have seemed straightforward, the manufacturing process created several hurdles: 1) The throughput requirement was substantial as was the variety of taps and end mills; and 2) The parts were required to be cleaned in their traveler bins, which are constructed of plastic. The plastic material protects the delicate threads on the tools while vertical plastic dividers in the bins protect the tools from one another.
While the throughput requirements were high, an automated system of proper proportions could certainly accomplish the task. The real difficulty was cleaning the heavy, hydrocarbon-based cutting fluid between each tool’s threads, which were hidden in the partitions of the bin. Parts and bins were sent to manufacturers of various types of aqueous cleaning equipment, including spray, vertical agitation and ultrasonic systems. Ultrasonic cleaning showed the most promise.
The spray washer’s jets were deflected by the compartment areas of the bins and couldn’t reach between the parts and the vertical partitions. The vertically agitated washer’s shear cleaning force was impeded by the plastic mesh on the bottom of the bin and not able to adequately penetrate into the tools’ threads.
While ultrasonic cleaning showed potential, it alone was not the ultimate solution. The plastic mesh bin absorbed a significant amount of the ultrasonic energy; however, combined with the proper alkaline cleaning solution enough energy was available to adequately break the bond of the cutting oil from the tool, even deep into the threads of the larger end mills. Additional agitation, in the form of spray-under-immersion, provided the extra boost for complete removal of all oil from the parts and bins.
The ultimate process consists of an automated, five-stage aqueous cleaning system:
- Heated alkaline cleaning stage with ultrasonic agitation, spray-under-immersion agitation (intermittent with ultrasonics) and surface skimming for oil removal;
- Heated, deionized immersion rinse with spray-under-immersion agitation;
- Heated, deionized immersion rinse;
- Heated, rust-inhibitor stage;
- Recirculating hot air dryer.
This system and process was implemented more than four years ago and successfully cleans all of the production in the plant.
Ultrasonic cleaning will continue to grow and expand driven by new and demanding manufacturing processes, stringent quality requirements and an environmentally conscientious society. Its use in many cleaning applications can be the critical element that determines the successful process.
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