Ultrasonic Cleaning Technical Information & Applications
Explanation of Ultrasonics
A high tech ultrasonic cleaner works as a result of sound waves being introduced into a cleaning liquid by means of a series of transducers mounted onto the cleaning tank. The sound travels throughout the ultrasonic cleaner’s tank creating waves of compression and expansion in the liquid. In the compression wave, the molecules of the cleaning liquid are compressed together tightly. Conversely, in the expansion wave, the molecules are rapidly pulled apart. The expansion is so dramatic that molecules are ripped apart, creating microscopic bubbles. The bubbles are unable to be seen by the naked eye because they are so small and exist for only a split second of time. The bubbles contain a partial vacuum while they exist. As the pressure around the bubbles becomes greater, the fluid around the bubble rushes in, collapsing the bubble very rapidly. Ultrasonic equipment manufacturers take advantage of the associated phenomena to deliver ultra-precise cleaning capabilities. When the bubbles collapse, a jet of liquid is created that travels at an extremely high rate. An associated rise in temperature as high as 5000°C occurs; this is roughly the temperature of the surface of the sun. This extreme temperature, combined with the liquid jet's velocity provides a very intense cleaning action in a very concentrated area that ultrasonic manufacturers can use. Because of the very short duration of the bubble expansion and collapse cycle, the liquid surrounding the bubble quickly absorbs the heat and the area cools rapidly. As a result, the tank and liquid only becomes warm and does not heat excessively due to the introduction of parts in the ultrasonic washing equipment.
Application of Ultrasonic Cleaning
Many articles exist describing "how ultrasonic cleaning works". The goal of this article is to help develop an understanding of the various ultrasonic cleaning applications and components that ensure quality ultrasonic cleaning.
First, establish a cleaning need, along with a determination as to how to measure the level of cleanliness. A few examples of measuring cleanliness include various levels of particle count, microscopic inspection, and a variety of adhesion tests, including the clear tape test that has the ability to remove additional contamination. These are just a few examples of cleanliness measurement.
Seven major concerns related to successful ultrasonic cleaning:
- Proximity to the transducer/part fixture design
- Ultrasonic output frequency
- Watts per gallon
- Loading - the volume (configuration) of the part being cleaned
Typical cleaning times may vary tremendously - how dirty is the part and how clean is clean. As a place to start, a normal trial period is two to ten minutes, since very few parts are sufficiently clean within a few seconds. Ultrasonic cleaning is not just a quick dip and zap, it's clean. Pre cleaning may be required to remove gross contamination or to chemically prepare the parts for a final clean. Some applications require more than one ultrasonic cleaning stage to complete the required cleaning. Ultrasonic agitated rinsing is required in some cases to more thoroughly remove the wash chemicals.
Temperature and chemistry are closely related. Generally, ultrasonic cleaning in an aqueous solution is optimum at 140°F. some high pH solutions will require the temperature to be higher to enhance the synergistic effect of the chemistry. The chemical pH is a good place to start; however, chemistry is not the subject of this article.
The following should be considered the main components of aqueous ultrasonic cleaning chemistry:
- Water - hard, soft, DI or distilled
- Optional ingredients
The chemical formulation must consider all of the above characteristics.
Some chemicals that are designed for spray cleaning, or that include rust inhibitors, are not suitable for ultrasonic cleaning.
PROXIMITY TO THE TRANSDUCER:
The procedure for ultrasonic cleaning is generally as follows: Put parts in basket and place basket through three or four process steps; ultrasonic wash, spray rinse (optional), immersion rinse, dry. Some parts loaded in baskets can mask or shadow from the radiated surface of the ultrasonic transducers. Most ultrasonic cleaning systems are designed for specific applications. Bottom mounted transducers or side mounted transducers are decided upon during the process design stage. Automated systems must specifically address the location of the transducers to insure uniformity of the cleaning. Some parts require individual fixturing to separate the part for cleaning or subsequent processes. Some parts require slow rotating or vertical motion during the cleaning to insure critical cleanliness.
ULTRASONIC OUTPUT FREQUENCY:
Many technical articles claim that high frequencies penetrate more and lower frequencies are more aggressive. The majority of the ultrasonic cleaning that is done in industrial applications today uses 40 kHz as the base frequency. Lower frequencies, such as 20 - 25 kHz, are used for large masses of metal, where ultrasonic erosion is of little consequence. The large mass dampens or absorbs a great amount of the ultrasonic cleaning power.
WATTS PER GALLON:
In general, smaller parts, requiring more critical cleaning, require higher watts per gallon to achieve the desired level of cleanliness. Most industrial ultrasonic cleaning systems use watt density from 50 - 100 watts per gallon. However, there is what is known as "the large tank phenomenon". There is an inverse relationship between liquid volume and the power density required to reach cavitation levels in that liquid. Generally, tanks 6-30 gallons might require 60-85 watts per gallon while tanks 50-100 gallons require only about 25-35 watts per gallon. A 2000 gallon tank may require only 10 watts per gallon.
Loading of the part(s) to be cleaned must be considered, with regard to the shape and density. A large dense mass will not allow internal surfaces to be thoroughly cleaned (i.e., metal castings). A rule of thumb for loading is that the load by weight should be less than the weight of half the water volume, i.e., in 5 gallons, approximately 40 lbs. of water, the maximum work load should be less than 20 pounds. In some cases, it is better to ultrasonically clean two smaller loads, rather than one larger load.
The above information is not meant to give all the details to utilize ultrasonic cleaning techniques. This information is to help the process designer gain some insight into the variables of industrial ultrasonic cleaning.
Examples of applications for ultrasonic cleaning
- Aircraft components, brakes, machined parts
- Anything that is honed, lapped, buffed, or polished (ceramic, glass, metals)
- Automotive fuel injector nozzles and components
- Blind cleaning (window blinds)
- Carbide cutting tools
- Cathode ray tube components (TV picture tube components)
- Computer disk drive and head components
- Fire restoration
- Glass substrates
- Glassware cleaning (laboratory glassware)
- Hybrid microelectronic circuits (thin and thick film circuits)
- Hypodermic needle stock (cannulae)
- Jewelry cleaning (new manufacture)
- Lenses, ophthalmic, precision
- Maintenance - cleaning of electronic assemblies
- Maintenance - cleaning mechanical assemblies
- Maintenance - cleaning of food manufacturing filling equipment
- Medical glassware (vials)
- Molds (maintenance cleaning)
- Nuclear decontamination
- Optical components
- Orthopedic implants
- Preparation of metals prior to titanium nitride coating
- Quartz crystals (radio, television, computer, pagers, cellular phones)
- Semiconductor, components, substrates, and sub-assemblies)
- Surgical instruments
- Thick film furnace belt (during operation)
- Weapons (maintenance cleaning)
- Wire dies (drawing dies)
Introduction to ultrasonic cleaning
Ultrasonic energy exists in a liquid as alternate rarefactions and compressions of the liquid. During rarefaction, small vacuum cavities are formed which collapse, or implode, during compression. This continuing rapid process, called cavitation, is responsible for the scrubbing effect which produces ultrasonic cleaning. Three factors affecting the scrubbing action are the degree of liquid degassing, the ultrasonic frequency and the chemical characteristics of the liquid at specific temperatures.
Degassing is the removal of unwanted air from the liquid, typically found in fresh tap water. As the cavities form, they fill with the unwanted air forming bubbles, which resist collapse and tend to remain suspended in the liquid. These bubbles act as "shock absorbers," which materially reduce cleaning efficiency. The amount of air can be reduced by periodically switching off, or modulating, the sound energy to permit adjacent bubbles to coalesce, float to the surface, and escape. The type of modulation is important, for the correct balance between degassing and cleaning efficiency must be selected for each cleaning application.
Frequency affects cleaning efficiency by determining the cavity size. Low frequencies generate large but relatively few cavities with high cleaning power. High frequencies generate a great number of small cavities with good penetrating capability. The selection of the correct frequency is difficult, for it varies with each cleaning application. The frequency also affects degassing, with 40 kHz nearly optimum.
Cleaning efficiency is also affected by the chemical and physical characteristics of the liquid. For best cleaning, the liquid must chemically soften the soil, yet maintain effective cavitation and provide the desired characteristics for rinsing and drying the cleaned parts. Ultrasonic cleaning solutions are broadly characterized as aqueous or non-aqueous. Final selection is dependent upon the overall process considerations for the cleaning application.
The ultrasonic energy is created within a liquid by means of transducers, which convert electrical energy into acoustic energy. These transducers are similar in function to a radio speaker except they function at ultrasonic frequencies (40,000 Hz) and transmit acoustic energy to a liquid rather than to air. The transducers consist of vibrating elements (piezoelectric disc) bolted between thick metal plates. The transducers are bonded to the underside of the tanks containing the cleaning liquid or are encased in stainless steel for immersion within a liquid. For reliability, many transducer modules are uniformly distributed over the tank bottom rather than having a single transducer in the center of the tank working very hard. An electronic generator energizes the transducers. The generator transforms the electrical energy from the wall outlet into a suitable electrical form for efficiently energizing the transducers at the desired frequencies. All ultrasonic cleaning systems consist of the four fundamental components; transducer, generator, container for liquid, and cleaning liquid. The performance and reliability of the system depends upon the design and construction of the transducers and generators. The overall effectiveness of the cleaning is dependent upon the cleaning liquid. The size of the tank is dependent upon the size or quantity of the parts being cleaned. The number of transducers and generators is determined by the tank size. The choice of cleaning liquid depends upon the parts being cleaned and contaminant to be removed.
Versatility of Ultrasonic Power Corporation (UPC) equipment
UPC manufactures engineered equipment that fulfills the broad spectrum of perform¬ance requirements dictated by the various cleaning applications. The varied frequency requirements are met by UPC’s “Simultaneous Multi-frequency®” which provides many frequencies at the same time, thus eliminating the difficulty of choosing a particular frequency for a particular cleaning application.
The exclusive UPC features of “Simultaneous Multi-frequency®” provide the most versatile equipment available on the market. The UPC “Vibra-bar®” transducer module and generator module combine to provide equipment having maximum reliability and simplified field maintenance.
The UPC product line consists of the generator modules and standard “Vibra-bar®” transducer modules. In addition, immersible transducers can be built to be used within existing installations or where application flexibility is required.
Ultrasonic power corporation facilities provide one source for all phases of industrial ultrasonic equipment. All of our products have been designed and developed by UPC engineers and are manufactured by UPC personnel. This experience and ability is available for field assistance on ultrasonic cleaning equipment applications.
The versatile performance of UPC equipment results from the simultaneous presence of many ultrasonic frequencies, 40 through 90 kHz, within the cleaning chamber. The higher frequencies, with their greater penetrating capability, initiate the cleaning by loosening soils in inaccessible areas, such as tapped holes and undercuts. This allows the heavy duty lower frequencies to rapidly and thoroughly complete the cleaning operation. The presence of more than one frequency also reduces the probability of damage to frequency sensitive parts, as it avoids the single strong resonance found in conventional ultrasonic tanks. The cooperating effect of many frequencies insures constantly uniform and thorough cleaning.
“Simultaneous Multi-frequency®” is accomplished by energizing the patented rectangular “Vibra-bar®” transducer module at two critical points with active piezoelectric stacks. This causes complex vibrational modes, which creates more than one frequency in the cleaning bath. This is similar to the creation of various resonant frequencies by vibrating a rectangular, rather than circular, drumhead. The dominant frequency is 40 kHz, but significant energy is produced at other higher frequencies.
Another important UPC advantage is the elimination of undesirable standing wave patterns, which are always present in conventional single frequency ultrasonic cleaners. Because of the coexistence of frequencies with many different wavelengths, UPC equipment provides extremely uniform energy distribution throughout the entire fluid volume. So called "dead" spots simply do not exist with UPC ultrasonic equipment. Degassing is also efficient with UPC equipment because of the presence of the dominant frequency of 40 kHz.
Power intensity control
To further increase the versatility of UPC equipment, an adjustable output power control is on all model 5300 and 5400 series generators. This control, combined with “Simultaneous Multi-frequency®”, extends the equipment cleaning ability to very fragile items such as semiconductor devices, wafers and delicate glass parts. The adjustable output power control provides maximum flexibility over a wide range of cleaning applications. It also maximizes the equipment performance when used for solution degassing.
Modulation sweep control
This generator has the ability to perform in a variety of aqueous and semi-aqueous cleaning chemistries. The generator operates at a base 40 kHz frequency and a 4 kHz sweep (+ 2 kHz and - 2 kHz). Sweep rate is how fast the output sweeps between 38 to 42 kHz. On the front panel of the generator on the right hand side there is a control that controls the sweep rate (300 Hz to 1000 Hz). Turn clockwise to increase sweep rate. For most aqueous solutions the sweep should be set in the full counter clock-wise position (300 Hz). For more viscous solutions and various hydrocarbon-based solutions the sweep typically should be set at full clockwise position 1000 Hz, high enough to reach peak cavitation performance. Certain applications will deviate from these standard sweep rate settings.
Engineered reliability of UPC equipment
The fundamental reliability of the UPC line is the result of the design and construction of the patented “Vibra-bar®” transducer modules and generator modules. These standard modules are used throughout the entire product line, thereby eliminating "one time untried designs". Required production tooling and facilities have been engineered to insure consistent product quality control. Spreading out from the standard module base are system designs and fabrication that are the result of years of specific field experience in ultrasonic cleaning applications.
“Vibra-bar®” transducer module reliability
The UPC “Vibra-bar®” transducer module consists of two active piezoelectric stacks which energize at two points. Reliability is obtained by the elimination of any adhesive in the active stack. The active stack consists of a piezoelectric (PZT) element bolted between backing plates. The absence of adhesive means the PZT element is "free" to vibrate or distort in any direction. In other designs, the PZT element is clamped in the stack by means of an epoxy adhesive, which prevented the element from movement in more than one direction. Because of the vibrational patterns and differences between the coefficient of expansion of the piezoelectric element and the clamping metals, the adhesive designs cause stresses to be built up in the element, which in time create cracking and failure. The UPC design eliminates adhesive in the stacks by optically polishing all mating surfaces and employing a single center bolt to provide the required clamping. Resiliency and ability to maintain constant compression under all conditions of vibration and temperature is obtained by the compression washers located under the bolt head. To increase reliability, the center bolt is stainless steel. The size of the center bolt is 3/8 of an inch, which is much larger than necessary adding to reliability.
The “Vibra-bar®” transducer module is permanently bonded to the radiating surface by means of a high temperature adhesive. Each stack utilizes only a single PZT element and single center bolt, thus providing increased reliability over designs with several PZT elements and bolts. The PZT element is relatively thin and under compression, which results in a strong structure. The PZT is similar to concrete in that by itself it tends to be brittle, but under compression, it is one of the strongest materials. The PZT in the UPC designs is not the normal commercially available formulation. The UPC formulation has special additives to obtain low dissipation, high density, and low porosity, resulting in improved performance, negligible aging and reliability. The PZT element has a curie temperature above 620°F and during manufacture is heat stabilized and pre-aged at 400°F. The special formulation and pre-aging eliminate any of the previous difficulties with PZT elements, such as changing characteristics after field usage. The field aging of the UPC elements is less than 1 percent.
The acoustical design of the “Vibra-bar®” transducer module is such that the design is a relatively low "Q" (broad band) device compared to other types of single stack designs. This means that the PZT elements do not have to be critically matched.
The reliability of the individual modules is obtained by the simplicity of the design where only a minimum number of components are employed. The UPC design has only about one third the components of similar competitive designs. Also, the design is such that no critical matching or alignment is required. The design employs all solid-state components, further insuring reliable operation.
Built into the design is automatic frequency/load control, which provides uniform and reliable operations under all conditions of loading. This circuitry is simplified because of the low "Q" characteristics of the “Vibra-bar®” transducer module design. The frequency of the generator does not have to be critically matched to the transducer. Therefore, it is relatively easy to detect changes in the load by feedback networks, which can automatically compensate the generator module performance.
Because of the superior reliability, the UPC generators are replacing competitive generators in many installations. UPC engineers can indicate if our generators are suitable for "driving" your particular transducers.
Minimum diaphragm erosion
The greatest factor limiting cavitation amplitude within any ultrasonic cleaning tank is the erosion of the radiating diaphragm. The cavitation causes the surface of the radiating diaphragm to be "etched." This mechanism is affected by the characteristics of the diaphragm as well as by the characteristics of the ultrasonics. Too much etching can render the diaphragm useless within months. With the UPC equipment, high cleaning ability is obtained; yet only a minimum amount of erosion takes place.
Factors affecting erosion are the transducer design, frequency of operation and the type of modulation. The more transducer area bonded to the diaphragm, the lower is the erosion. With the UPC “Vibra-bar®” transducer module design, there is about three times more area of transducer per PZT active element as compared with other designs. This means that the energy is more uniformly distributed, thus minimizing erosion. Experience shows that higher frequencies erode less than lower frequencies. Consequently, the 40 kHz systems have less erosion than 25 or 28 kHz systems. “Simultaneous Multi-frequency®” is superior to all, because not only is the main frequency 40 kHz, but the presence of other high frequencies minimizes the erosion effect. Experience also demonstrates that systems without modulation have greater erosion. The UPC systems have the most advanced type of modulation.
Another factor affecting erosion, independent of the equipment, is the characteristics of the cavitating liquid. Water solutions erode much more than solvent systems. Also, it is true that the higher the temperature of the liquid, the lower the erosion. Since these factors are out of UPC control, it is necessary to have the transducer guarantee be exclusive and not cover the radiating diaphragm (surface).
Ease of maintenance of equipment
The UPC designs have been engineered for reliability, plus the ability for rapid and simplified field ultrasonic cleaner maintenance.
The construction of the “Vibra-bar®” transducer module, which incorporates only a single center bolt and employs no adhesives, permits field replacement of all elements. The only required tool is a torque wrench. The circuit design is such that component replacement is simplified since there is no requirement for critical matching.
The construction of the generator chassis permits easy access to the electronic components. Access is obtained by the removing the four cover screws. To insure continuous production, a whole chassis module may be rapidly interchanged without tools. Only UPC technicians should repair the generator circuit. Dangerous high voltages are present within the generator.
“Vibra-bar®” transducer module description
The “Vibra-bar®” transducer module consists of a radiating bar approximately 2" x 5 1/4", which is permanently attached to the stainless steel radiating surface by high temperature bonding. Located on the radiating bar are two active piezoelectric stacks. Each stack consists of PZT elements bolted between the radiating bar and “backing plate”. One surface of the PZT element is electrically insulated from the backing plate by means of an insulator. The PZT element is specially formulated to obtain low dissipation, high density and low porosity, which results in, improved performance and negligible aging. The element is further stabilized and pre aged during manufacture at 400°F to insure less than 1 percent field aging. The curie temperature of the element is above 620°F. Electrical voltage is applied across the PZT element by means of stainless steel electrodes specifically designed to provide reliable means of attaching wires. All parts in the stack have a center hole to permit clamping by means of the single stainless bolt. All mating surfaces permit clamping by means of the single stainless bolt. All mating surfaces are finely polished which eliminates the need for any adhesives in the stack construction. The absence of stack adhesive permits the PZT disc to be "free" to vibrate and distort in many different planes. Uniform compression on the PZT element is maintained under all operating conditions by means of compression washers under the bolt head. This design results in a rugged construction with long life and efficient operation.
“Simultaneous Multi-frequency®” is accomplished by driving the radiating bar at the two areas of the active stacks. Each stack is energized by the 40 kHz oscillation from the generator modulated by either the full wave or half wave mode of operation. The absence of any stack adhesive permits the stack to vibrate and resonate not only in the thickness mode, but also in other modes such as circular and transverse. The two point driving of the “Vibra-bar®” transducer module causes it to resonate and distort in complex modes, which creates more than one frequency in the cleaning tank. This process is similar to the creation of various resonant frequencies by vibrating a rectangular rather than a circular drumhead. If a circular drumhead is vibrated, it will resonate in only a single frequency. The radiating bar being rectangular instead of circular and larger than the driving stacks will resonate in more than one fundamental frequency. It will, in effect, have several fundamental frequencies plus the harmonics of all the fundamental frequencies. A circular transducer will have only a single fundamental frequency. The result in the tank is a dominant frequency of 40 kHz, but additional significant energy is produced at other and higher frequencies in the range of 50-90 kHz, by the distortion of the radiating bar. These other frequencies are in addition to the normal harmonics of the fundamental frequencies.
Generator module description
A generator module consists of a narrow profile cabinet. The cabinet has four rubber feet, a carrying handle and a 6 foot long power cord. The power cord from the cabinet has a conventional three pronged plug. The generator needs to be grounded through the third prong of the power cord or by the use of a conventional grounding lug receptacle plug. Be sure the generator is plugged in the correct AC line voltage.
The model 5300 (500 watt module) is designed to energize six “Vibra-bar®” transducer modules (12 piezoelectric stacks) the model 5300 (250 watt module) is designed to energize three “Vibra-bar®” (6 piezoelectric stack) transducers. The chassis module contains the fan, (FET) transistors; control circuit, power circuit, and RFI (radio frequency interference) filter circuit.
The circuit is designed to provide a constant output power even as the temperature of the cleaning solution increases. The generator circuit compensates for a wide variety of load conditions. Because of the circuit design, no harm will result if the output of the generator is either "short circuited" or "open circuited" for a short period of time. These conditions can arise in the field if the transducer cable is not connected to the chassis module, or if a short accidentally develops in the transducer cable.
The adjustable power output control is a circuit that determines the power level applied to the transducers. The built in wattmeter (optional display) circuit monitors the actual power and makes adjustments to keep the power constant regardless of liquid temperature and load in the cleaning bath. The power control circuitry provides the user with a very precise control from zero watts to maximum power. Therefore, the power setting can be tailored to all cleaning applications.