The History of Ultrasonics
The principle behind ultrasonic cleaning is the phenomenon known as cavitation. Cavitation is best descried as vapor cavities formed between molecules within an aqueous solution under low pressure. These vapor cavities expand and then implode or collapse creating a unique microscopic scrubbing action.
Cavitation was first observed in 1894 when a naval officer noticed tremendous vibrations in the propulsion system of the first British destroyer. These vibrations became more significant as the British Navy built faster ships with more powerful propulsion systems. Unable to solve the problem, the British commissioned a study into the matter. It was eventually determined that the enormous turbulence around the propeller was creating low-pressure areas that fostered the formation of tiny bubbles known as cavitation.
It wasn’t until 1927 that Alfred L. Loomis actually began researching the beneficial effects of cavitation and its relationship to cleaning. Development of ultrasonic cleaning technology was slow and its use modest until the 1970’s when more efficient systems became available.
In the early 80’s ultrasonic equipment was mainly used for specialty clean room applications, the medical field and precision manufacturing..
Research and development of the technology continued, today’s ultrasonic equipment is highly advanced and cost effective. Ultrasonic cleaning or precision cleaning as it is commonly referred to is people safe and environmentally friendly and is has become the preferred choice for most industrial cleaning applications.
Why does it Work?
Ultrasonic cleaning is not just a vibrating process that shakes off dirt but rather a method of generating cavities or cavitation bubbles that enlarge and then implode creating a unique microscopic scrubbing mechanism in a liquid. This is achieved when high frequency sound waves are introduced into the cleaning solution. Sound waves are alternating forces of compression or positive pressure, and expansion, or negative pressure. To create cavitation the ultrasonic generator manufactures high frequency electrical energy that is transmitted to the transducer elements mounted in the cleaning vessel. The transducer’s job is to convert the electrical energy into high frequency sound waves.
The transducers produce alternating waves of compression and expansion by causing the radiating diaphragm to move, or oscillate up and down. During compression the molecules of liquid are compressed tightly together. During expansion the molecules are pulled apart from each other creating a unique mechanical energy. In order to overcome the liquid’s tensile strength (the ability of water molecules to bond together) the sound waves must be of sufficient intensity and frequency. The cavitation bubbles
form and grow rapidly until the pressure around the cavity becomes greater than the pressure inside, at this point the cavitation bubble implodes, releasing the energy it used to create itself.
The illustration shows a microscopic cavitation bubble imploding. As the cavitation bubble expands the pressure outside begins collapsing the bubble walls. At the point of implosion the liquid around the bubble rushes in creating a high-pressure mini-jet stream. The gases inside the cavitation bubble are compressed so rapidly that they reach tremendously high temperatures. This combination of heat and pressure creates a highly efficient process.
A sound cleaning Idea
Ultrasonics is the most thorough process of cleaning known to Science. It is ecologically friendly and user safe. This cleaning process is unique because the cavitation activity created by the ultrasonics cleans the inside as well as the outside. This process is ideal for cleaning objects of complex structure and intricate detail. Blind holes, hidden passageways and cavities can all be addressed during the cleaning process providing the cleaning solution has penetrated all recesses and interior chambers of the item being cleaned.
Ultrasonic cleaning is highly versatile because it is completely adjustable. It can therefore be adapted to virtually any conceivable cleaning application. Ultrasonics can be gentle enough to remove sub micronic particles from silicon chips yet sufficiently aggressive to remove imbedded rust and carbon from metals. The key to successful processing lies in understanding the technology and the requirements of the cleaning application.
Dialing in for Optimum Cleaning Results
The Four Factors
Effective ultrasonic cleaning is achieved by understanding the application and properly combining the following factors: Frequency, Power, Chemistries and Temperature.
Ultrasonic cleaning encompasses a wide range the frequencies, the most commonly used range for industrial applications is 20 to 80 kHz (1 Kilohertz, is equal to 1,000 cycles per second, the prefix kilo means 1000; the word Hertz refers to a change in polarity or direction of current flow). There are also megahertz cleaners with frequencies starting at 1 MHz or 1 million cycles per second. Megahertz cleaners are used for specialty cleaning applications such as removing sub-micron particles from semi-conductors/wafers.
Selecting the appropriate frequency has a direct impact on the cleaning results. An 80 kHz frequency for example will produce a high density of small cavitation bubbles in the cleaning vessel, although there is a high concentration of cavitation per cubic inch, these bubbles release only small amounts of energy during implosion. Conversely, a lower frequency of 25 kHz will produce larger cavitation bubbles, although these bubbles are much more powerful they are less abundant so in effect a lower frequency of 25 kHz will produce a lesser amount cavitation per square inch but provides more aggressive cleaning action.
For optimum cleaning results correctly matching the frequency to the cleaning application is important. The cleaning of intricate surgical instruments for example would be better served by higher frequencies of 60 to 80 KHz while heavy duty degreasing applications such as automotive parts cleaning would require the intense energy of lower frequencies such as 25 to 40 kHz.
Ultrasonic cleaning performance can be further enhanced by incorporating a feature known as “Wave Sweep”. This unique feature enables the ultrasonic components to vary their set operating or mean frequency by allowing the ultrasonic generator to cycle through a pre determined range of frequencies.
Wave Sweep can be beneficial because it effectively reduces standing waves or dead zones inside the cleaning bath. Dead zones occur when ultrasonic sound waves of the same frequency but opposite phase (wave of expansion versus wave of compression) collide with each other canceling each other out.
The power of an ultrasonic cleaner can be measured by watts per gallon. This is determined by calculating the cleaning tanks fluid volume (number of gallons at operating level) then dividing that number into the total watts of power generated by the ultrasonic components. Example: If operating with at 30 gallons fluid volume with 1800 total watts of component power (1800 watts divided by 30 gallons = a power ratio of 60 watts per gallon)
Strong cavitation intensity is directly related to ultrasonic power and weak activity in the cleaning tank will yield slow and inconsistent results. Although effective cleaning requires ample power, as power is increased substantially above the cavitation threshold cavitation intensity levels off. Ultrasonic power alone does not clean, what really counts is the ability of the ultrasonic components to convert that power into efficient mechanical energy.
Optimum cleaning efficiency is always the goal and the complexity of the cleaning application generally determines the frequency and power ratio required. Today’s versatile multi-tank systems are designed so that one set of ultrasonic components can be transferred from one tank to another. These (all in one systems) consist of different shaped tanks that allows the operator to address various part sizes and unusual item configurations. In order to achieve uniform cleaning all tanks comprising the system should have similar fluid capacity or watts per gallon ratio at operating levels.
Not all cleaning agents work effectively in ultrasonics and many chemical characteristics must be considered in the formulation of an ultrasonic cleaning agent. A properly formulated cleaning agent will facilitate an abundance of cavitation activity and create an optimum cleaning environment. Ineffective cleaning results can often be attributed to the use of incompatible cleaning agents.
Factors that must be addressed when formulating an ultrasonic cleaning agent:
1) Wetting Qualities Wetting agents have a direct bearing on how readily and intensely cavitation will take place. The correct formulation can neither be too high nor to low.
2)Surfactant Qualities The chemicals ability to hold or suspend contaminants must be facilitated otherwise contaminate may settle back onto the items being cleaned.
3)Deflocculating Qualities The solution must be tailored specifically for the application and be capable breaking the attracting forces that adheres the contamination to the item.
4) Safety All chemical formulations must be user safe and environmentally compatible.
Under certain conditions several types of contaminates can be removed with a compatible all-purpose ultrasonic cleaning agent. However for heavy duty and specialty cleaning applications, chemicals specifically formulated for the precise application yields best results. This is especially true for difficult residues such carbon, rust, paint and varnishes.
Although there are many variables that effect cavitation intensity temperature is the single most important parameter.
There are many properties related to liquid that have a direct affect on cavitation intensity. Changes in temperature can increase or decrease fluid viscosity. The viscosity of a cleaning solution should be minimized in order maximize the effects of cavitation. Heavy liquids are generally lethargic and cannot react quickly enough to form cavitation bubbles that aggressively implode. Since the viscosity of most liquids is reduced when the temperature is increased the most effective ultrasonic cleaning occurs when temperatures of 160 F or greater are present.
The solubility of gases in the liquid and their rate diffusion as well as vapor pressure are affected by temperature. Liquids at high temperatures give up their dissolved gases more readily than solutions at lower temperatures. For optimum cleaning results the cleaning solution must contain as little dissolved gas as possible. The gasses in the liquid are released during the (De-Gassing Stage) or bubble formation phase when the ultrasonic activity is first engaged. These gasses prevent the bubbles form aggressively imploding which is required for the desired ultrasonic cleaning effect. The amount of dissolved gas in a liquid is increasingly reduced as the liquid temperature rises. As the liquid is brought closed to its vapor pressure, cavitation bubbles are filled with the vapor of the cavitating liquid and then implode violently creating an extremely efficient cleaning action. As temperatures approaches boiling levels, cavitation intensity is reduced as the liquid begins to boil at the cavitation sites.
Ultrasonics and the Restoration Industry
The first ultrasonic cleaners were introduced to the restoration industry in the early 90’s. Although cleaning results were encouraging they were very often inconsistent. This was mainly due to a lack of available processing education. Contents restoration is a complex undertaking and involves dealing with various item categories, material classifications, residue types and residue levels. All of this information must be actively utilized during packout and contents processing. Since there is no single cleaning procedure that can effectively address all of these content variations.
The following contents information should be known prior to processing:
1) Item categories are:
a) General Contents (includes all common household items)
b) Delicates (includes collectables antiques and scheduled items etc.)
c) Electronics and Electrical Items
d) Metals (oxide removal/rust and heat scale)
2) Level of residue/contamination, heavy, medium, light residue, hot or cold smoke.
3) Material make up: plastic, glass, metal, wood, rubber, fabric, clay, porcelain and painted surfaces.
4) Residue type: natural, protein, synthetic, chemical, petroleum and mold.
When mold spores are present all hard content items are candidates for ultrasonic cleaning, most items can be successfully restored. When dealing with mold contamination content items should always be handled with extreme care and is accordance with strict industry standards and protocols.
Efficient contents processing begins at the scene of the fire where content items are properly organized and cataloged. Content boxes should clearly labled so they can be distributed to the proper work stations upon arrival at the warehouse.
Restoration procedures are determined by the category of contents being restored as well as the level and type of resides present. Cleaning protocols are adjusted to accommodate specific categories of contents as required.
The objective of ultrasonics cleaning is to create a continuous flow of content items moving through the cleaning line without interruption. This is achieved through the use of assembly line techniques similar to those used by the manufacturing industry. Proper implementation of these time tested cleaning techniques deliver optimum cleaning results and high flow production.
Ultrasonic cleaning technology has evolved from a highly specialized precision process to a multi purpose cleaning method routinely used for a variety of industrial applications? From the most delicate of cleaning tasks to heavy duty industrial challenges successful results are achieved by matching the cleaning application with the correct equipment specifications and cleaning protocols. Due to the versatile nature of this technology new cleaning application are continuously being discovered and there seems to be no end in sight. It appears that ultrasonic cleaning has truly gone from evolution to revolution.
by David Mazur
Biographical Information about the Author:
David Mazur is the Owner and Founder of Ultrasonics International Corp. (UIC). The Company began in 1988 and was incorporated in California in 1992. Mr. Mazur is in charge of Applications Research and Product Development.
Abridged version of an article the author sent us for use in our Blindcleaners.biz Newsletter.