Precision Ultrasonic Cleaning Systems


3678 Bassett Street Santa Clara, CA  95054 Phn: (408) 727 8388 Fax: (408 ) 727 8997

By using an electrical generator that puts out a high frequency signal [20 to 250 kHz] the transducer rapidly induces compression and rarefaction waves in the liquid. During the rarefaction cycle the liquid is torn apart. This creates a vacuum cavity within the liquid. These cavities will grow larger and smaller as the compression waves are continued. When the cavity reaches a certain size [based on the frequency and the wattage of the signal] the cavity can no longer retain its shape. The cavity collapses violently and creates a temperature of 5,000 degrees centigrade and a jet of plasma that impacts against whatever object is in the tank. There are millions of these bubbles created and collapsing every second in an ultrasonic tank.  

Compression & Reareification Waves in an ultrasonic Tank   
It is these collapses that clean the part. The jet will explode the dirt or any other material off the surface of the part. By adding soap or other chemical to the water in an ultrasonic tank, you can increase the effectiveness of the cleaning operation. Heat also improves Ultrasonic Cleaning by eliminating entrapped air in the water and making the detergent more effective. The best temperature to clean with is 80% of the boiling temperature of the solution. You should always use a basket to hold the parts you are cleaning. Never put parts directly on the bottom of an ultrasonic cleaner. Top

Transducers
A transducer is a pizeo ceramic material that when excited by an electrical pulse will physically change shape. The reverse is also true. When you physically induce force on a transducer it will produce an electrical current in proportion to the force induced. There are also some other types of transducers such as ferrite and certain combinations of metal and minerals such as quartz that will produce a similar effect. All ultrasonic cleaning system use the pizeo ceramic type with a few exceptions. A physical mass and shape of the transducer determine it's resonant point. [ The frequency at which it will change shape] Most transducers have more than one natural resonance point. A 40 kHz transducer can be designed to produce a secondary resonance point of 170 or 68 kHz. There is a slight power loss at this secondary frequency but it is minor as  far as the cleaning effect. This requires a generator capable of producing a signal at both frequencies. The generator is in effect 2 separate generators linked by a switch or Programmable logic controller. The resonant mass of the transducer is also determined by how the final construction of the transducer is accomplished.

Inertial Mass and acceleration of the transducer assembly and the transducers ability to resonate in harmony with the output frequency of the generator will determine the power of the ultrasonic cleaning system. Newton’s second law [ F=ma ] states that force is equal to inertial mass times acceleration. A transducer emits ultrasonic vibrations by rapidly expanding and contracting in resonance with the frequency of the generator output when it equals the primary operating frequency of the transducer According to this law a heavier faster accelerating transducer will produce more cleaning force than a lighter, slower accelerating transducer. Top

This is the primary reason why virtually all-40 kHz ultrasonic transducer manufactures incorporate a resonant mass in their transducer assembly. A resonant mass is a precisely machined steel or stainless metal block, which has been perfectly sized to resonate with the transducer output. Its sole purpose is to add mass to the transducer assembly, since a 40 kHz is small and light in weight. Supposedly, the cleaning power of the piezoelectric 25 kHz ultrasonic cleaning systems stem from the frequency of the ultrasonics, yet the resonant mass of the 25 kHz assembly is roughly 6 times larger than that of the 40 kHz assembly. Why not simply place very large, heavy resonant mass on any transducer assembly.

The answer lies in the term resonant. A metal object will vibrate only at the primary frequency depending on mass and size. If the resonant mass chosen is not designed to resonate in harmony with the transducer both at the primary and secondary frequencies, the mass will oppose the force of the transducer and cause a decrease in efficiency. The waves will act against each other. This is another reason why altering the output frequency of the generator has not been shown to work well except when the generator operates at the secondary harmonic of the transducer. If the transducer changes its frequency, it no longer resonates in harmony with the resonant mass, which gives the ultrasonic system its power. Changing or sweeping the frequency feeding the transducer beyond a narrow range is self-defeating. Except when operating at the higher secondary frequency harmonic of the transducer. The design of the resonant mass must take into account the entire range of frequencies the transducer will generate.

The acceleration of the transducer makes up the other half of the second law, the faster the transducer accelerates the greater amount of force it will exert. Since the 170Hz systems must produce approximately four times as many pulses per second as a 40 kHz transducer, it accelerates faster leading to higher power output. The second law applies to all ultrasonic cleaning systems regardless of frequency. Top

Typical Transducer Configurations  
The difference between a 170 kHz system and lower frequency systems is immediately apparent when seen side-by-side. The unit is amazingly quiet. Since 170kHz is four times as far from the hearing range of the human ear, it does not produce as many random sub-harmonics, which leads to noticeably less irritating noise. The surface of the cleaning fluid also appears entirely different to that of ordinary ultrasonics. The 170 kHz systems have small, evenly distributed waves rippling across the entire surface, while lower frequency units have virtually no ripples.

The power of the 170 kHz system is immediately felt when the hands are submerged into the system. When positioned in some orientations, the sensation borders on painful. Yet even with the amazing cleaning power, the 170kHz system is gentle. Testing with sensitive metals also showed no signs of metal etching [Cavitation Erosion] common with some ultrasonic systems.

To transmit the vibrations a transducer must be firmly and acoustically linked to the diaphragm of the tank [bottom of the tank in most cases.] This is accomplished by epoxy bonding or by a metal brazing process.

The placement of the transducers on the bottom of the tank will have an effect on the even distribution of energy within the tank.  The ideal diaphragm is round [much like an acoustic speaker.] Since we make square ultrasonic tanks care must be taken in their design so that the distribution is as even as possible. Top

Multiple simultaneous frequencies can be generated from a stack or multi component transducer assembly by bonding 2 or more stacks of transducers to a bar and energizing the entire bar. The bar itself will generate additional frequencies. The bar with the transducer assemblies is bonded to the bottom of the tank. Several of these bars will make up an ultrasonic system dependent on power required.

When powered by electrical energy, which is in resonance with operation frequency of the transducer, the transducer vibrates in harmony with the output of the generator powering it. If the output of the generator does not closely match the operating frequency of the transducer, efficiency and power sharply drop. When transducers are shipped the operating frequency range of the transducers is given as a range. (i.e.: 170 kHz + 1-1) Top

However this only indicates an operating range, not the most efficient operating frequency. The transducer will function at the outer limits of its range, but marginally and with a general lack of efficiency. This is the reason for the general ineffectiveness of sweep frequency circuits at lower ranges. Sweep Frequency Circuits are an attempt a curing the level sensitivity problem associated with lower frequency ultrasonics. It is theorized that by sweeping the output frequency of the ultrasonic generator, a multi frequency effect is produced. Sweep Frequency circuits are more effective at the high end range of frequencies, i.e., 60 kHz up   Top

Multi Frequency Ultrasonic Cleaning Systems
Multi Frequency Ultrasonic Cleaning systems consist of 3 basic types;
Type 1 is a system that has 2 or more independent sets of transducers bonded to the tank. Each set of transducers is controlled by its own generator and the frequency is determined by the generator The frequency is determined by which set of transducers is operational at a given time. A tank that has both 40 and 68 kHz transducers mounted will run at either 40 or 68 kHz independently or both 40& 68 kHz simultaneously. This can be controlled either by a manual switch or by a Programmable Logic Control for automatic operation. The limitations of such a system are the physical constraints that are imposed by the tank size and the bonding area for the transducers that is available. A possible disadvantage to running both frequencies at the same time is that too much power can be in the tank at any one time. This could cause cavitation erosion of the part. Power intensity control is a must for this type of system  Top

Type 2 is a system that has 1 set of transducers and a generator that will by means of a Programmable Logic Control switch the frequencies from one to another. In most cases only one frequency at a time can be present in the ultrasonic tank. This type of system eliminates the problem of bonding enough transducers to the tank but limits the system from creating more than one frequency at a time. 

There is a 3rd type of system that combines both of the above, and that is a system that has the ability to run banks of transducers [ ie, 2, 4, or 6, out of 12 ]at a particular frequency for a specified time. This system can be run in either mode and has the added advantage of limiting the total power avoiding part damage.

Ultrasonic Generators 
An Ultrasonic generator energizes the transducers. The generator transforms the electrical energy from the power source into a suitable form for efficiently energizing the transducers at the desired frequencies. The generator produces a electronic signal of high voltage and sends it to the transducers. When the transducers receive the signal they will respond by changing shape as long as the signal is applied. The response range of the transducer determines the frequency of the generator. Since the response range of the transducer is narrow the signal from the generator must be close to the response range of the transducer.

The generator is designed to power a specific number of transducers. Each transducer requires a minimum amount of voltage to activate, usually about ¾ of the maximum voltage of the transducer.

A typical generator usually has controls for varying the amount of power to the transducers,[ Power Intensity] and a built in frequency sweep that will vary the frequency sweep rate  [ Sweep Frequency rate control] over the operation range of the transducer. A control that automatically turns the signal on an off very rapidly is sometimes provided to help degas the cleaning solution.

Most generators are designed in modules that will operate a specific amount of transducers. The most common are 250, 500, 750, and 1000-Watt sizes, simply adding additional generator modules to the system can operate Transducer stacks of any size. Top

All ultrasonic cleaning systems consist of the four fundamental components of Transducer, Generator, Tank and the cleaning solution. 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 of the parts benign cleaned. The number of transducers and generators is determined by the tank size. The choice of the cleaning solution depends on the parts being cleaned and the contaminates to be removed. Top

Sweep Frequency Control
Sweep Frequency is a circuit designed into an ultrasonic generator that will cause the signal that is sent to the transducer to vary slightly in frequency over a set period of time. The result of this will move the standing wave up and down within the tank. This distributes the energy more evenly throughout the tank. It also prevents delicate parts from coming into resonance with the frequency. Delicate parts may be severely damaged if this happens. 

 The amount the base frequency is shifted and the rate at witch it is shifted is determined by the circuitry. In some cases the generator has a control that allows the operator to vary the shift rate. Most generators will not allow the operator to choose the range. This is because the transducer has a very narrow band of operation. If the frequency is adjusted to far off this band, the transducer will fail to activate. The power will be lost and damage may result to the transducer.

A good design for sweep frequency will automatically vary the sweep rate on a constant basis, so that no resonant power pulse from the generator will move the part into resonance.  

Power Intensity Control
Power intensity control is designed to allow the operator of the tank to reduce the overall voltage going to the transducers bonded on the bottom of the tank.  It essentially moves the power pulse off the peak of the waveform. This reduces the power sent to the liquid reducing the size of the cavitations bubbles implosion force. Since a transducer requires a minimum voltage to operate, the power intensity control can only reduces the power of a 500-watt system by 25 to 30%. If you reduce the power to the transducers any further they will stop operating. Power intensity is standard on all our systems; this allows you to match the cleaning power of the system for maximum cleaning and minimum damage to the part. It is required when you are cleaning delicate parts.  

Full Wave / Half Wave and Pulse Controls Top
Some generators have a switch that will cut out the pulse to the transducers every other half of the wave. This will allow the gas to escape more quickly from the solution. In tanks with higher frequencies the higher frequency aids in the degas process as well. With or without a degas control a tank will degas in about 20 min. on its own.

Cavitation  Erosion
Cavitation erosion is the effect of the part being left for too long a period in the tank. When the cavitation bubble collapses it generates a temperature of 5,000 degrees C and a shock wave that travels over 500 miles per hour. This will cause the cavitation to erode the part being cleaned. In lower frequencies such as 20kHz Cavitation erosion will eventually over a long [years ] period of time will eat though the bottom of the tank. If your part has a smooth soft surface it can also be eroded by this effect. These effects are most prominent with lower frequencies. Higher frequencies will also cause cavitation erosion, but it will take a relatively long cleaning time to see the effects. By adjusting the power and frequency of an ultrasonic system to the level where it cleans the part, without eroding the part you can avoid damage to the part. The higher the frequency, the more evenly spread out the power.  This produces more even cleaning on the part. Higher frequencies also produce smaller cavitation bubbles and can clean smaller particles. Top

Damage can also occur when the part is extremely fragile and it is placed in a position in the tank that suspends it so that part of the object is in an area of compression and part is in an area of reareification. This is more evident in the lower frequencies [ 20-40 kHz.]  For this reason most delicate parts are cleaned in a high frequency ultrasonic tank. [ 70 to 200 kHz.] By distributing the total energy of the tank over a greater number of energy peaks the overall effect is to create a very homogeneous power distribution and subject the part to an even level of energy.

Differences in Ultrasonic Cleaners
Ultrasonic cleaners come in three categories, First; the small toy like systems that are sold by companies for cleaning contact lenses, etc. These are very light duty cleaners with small transducers and very simple generators that are not very efficient. They should never be used for critical cleaning jobs.    

The second type are the dental cleaners and small lab size tabletop cleaners, these have a heaver transducer and a better generator and are efficient in cleaning small parts and lab glass. They can be used for longer periods of time. Most of these cleaners have the generator incorporated into the same case as the tank. There are some drawbacks to this especially if the system is heated. If the system is heated the extra heat generated by the heater can affect the electronics if they are in the same cabinet, causing premature failure of the generator.  They are not rated for 24 hr 7 day operation and should not be used for production cleaning. tm associates table top systems use a separate enclosure for the generator so that our table top systems are rated for heavy duty use. 

The third type is the industrial heavy-duty type with very heavy compound transducers and rugged generators that produce high wattage per transducer. The Generators are separate from the tanks and can be remote mounted up to 10ft from the tank. These clean more efficiently and can be used for production cleaning. Tanks are heavy-duty welded 316L stainless steel with overflow and drain ports.  Top

Ultrasonic Tank Construction  
There are several types of tank used in ultrasonic cleaners. The most common tank used in tabletop systems is a deep drawn tank that is one piece. These tanks come in a range of sizes from about 5” x 5” x 4” up to 20” x 11” x 8” They are food service tanks [like the ones you see in a buffet line] and are extremely well made and ideal for table top systems or small consoles. The tanks can be electro polished for particle shedding and a number of drains and overflows can be added. Being one-piece tanks they almost never develop leaks. The only disadvantage is the number of sizes available and the depth is limited to no more than 8”.

The second type of tank most commonly used is a heavy gauge stainless steel of either 304 or 316L grade welded construction. [The tanks are made of panels welded together]. These tanks can be custom made to any size and shape requirement. Overflows and spray bars can be included in this type of tank. This type of tank is mostly used for industrial type cleaning. Top

The third type of tank is a coved welded tank designed for Ultra Clean applications. These tanks are always made of 316 L stainless steel and have the inside edges curved and ground smooth so that particle retention is greatly reduced. The tanks are always acid treated to harden the interior surface and electro polished. Most have overflows added for use within a console. Needless to say these are the most expensive type of tank.

 The choice  of metal, and the manner in which it is welded also have an effect on the performance of the final ultrasonic tank. When you design an ultrasonic tank you are in effect designing an audio system and considerations such as load, shape and diaphragm thickness are important.

 It was for exactly those reasons that tm associates developed the L-2001 ultrasonic probe. With this instrument we can compare effects of the ultrasonics in different tank designs and also tune the tanks so that the effects are relatively the same.

 Your specific requirements will dictate the type of tank to choose.  Top

Immersible Transducers
An Immersible transducer is nothing more than a tank that is turned inside out. It is simply a watertight box usually about 3-4 inches thick and what ever length and width is required with transducers bonded to the upward facing surface. A watertight cable connects the transducer to the generator with a coaxial cable. The transducer can be placed in any tank and will turn it into an ultrasonic tank. The advantage of using an Immersible is that any existing size or shape tank can be turned into an ultrasonic tank very quickly.

Sono Chemisitry
Sono-Chemistry is the use of ultrasonic energy to promote chemical and physical reactions. By sonicating a liquid, powder or compound chemical and physical reactions can be speeded up or can be made to form new compounds. The immense temperatures and pressures and the quick heating and cooling cycles generated on a microscopic scale by the ultrasonics, enable high energy chemistry.

The basics of Sono-Chemistry rely on the creation of the cavity vacuole in the liquid by the ultrasonic frequency. The collapse of the vacuole creates a temperature of 5,000 deg C and a pressure of about 1,000 atmospheres at a time scale of a micro-second, and heating and cooling rates of above 10 billion degrees C per second. The combination of these reactions enable extreme chemical reactions on a microscopic scale.

Sono-Chemistry is used in degassing of liquids, production of homogeneous metals, sonolysis of molecules, rupture of polymers, cell disruption, mass transfer and many other applications. There are numerous applications for sono-chemical reactors. Top

Cleaning Power

Cleaning Power of an Ultrasonic Tank
There is no industry recognized term for “ cleaning Power” .Usually each manufacturer making a claim in this manner has his own standard or interpretation. Therefore what may be strong in his eyes may be weak when compared with another cleaner. The cleaning ability of a cleaner depends on many factors, such as; actual electrical power input, cleaning solution, transducer matching physical construction, and other factors. tm associates uses average electrical power. 

Choosing the correct power level for an ultrasonic tank
The average Watts per gallon of ultrasonics should be between 50 to 100 Watts. This is the average rating and can be adjusted dependent on the cleaning application. To calculate the power requirements use the following formula;

 L x(in) W(in) x. (H –2”) /231*100=Avg. Watts power. It is important to remember that ultrasonic companies can rate the watts of ultrasonic energy in two ways; Peak & Average. Peak watts are the start up requirement and Average Watts are the continuous operating wattage. Base all calculations on Average Watts.

Most companies will offer a power intensity control as an option. This control will lower the Wattage of the ultrasonics to any desired level on the top of the power curve. Below 50% there is not enough energy to activate the transducers. (ie: if you have a 100 watt avg. ultrasonic tank  you will be able to adjust the power from 50 watts to 100 watts below 50 watts the transducers will not operate.) Top

The power intensity control is a good option where you will be cleaning delicate parts that may be subject to cavitation erosion or in a situation where the ultrasonics is being used in a plating operation or in other chemical processes.  

Effects of different frequencies and multiple frequencies
For a given cleaner the lower frequencies are much stronger because they concentrate the available power in fewer bands of cleaning. The higher the frequencies the more the available power is evenly distributed throughout the tank area. Most companies that claim to have multiple frequencies vary the power to the transducers to push them off of their natural frequency. This only works to a limited degree as any transducer has a natural frequency that at which it will resonate best. If they are pushed to far off the natural frequency they will dissipate the power in the form of heat, and will not generate cavitation bubbles. Any ultrasonic unit will generate more than one frequency; most units will generate sufficient high frequencies in addition to the fundamental low frequency to provide a practical balance for general cleaning applications. Higher frequencies such as 65 – 70 or 170 kHz generate much smaller cavitation bubbles and will remove smaller particles more evenly than lower frequencies. There is some loss of efficiency when you run a 40 kHz transducer at 170 kHz resonant point. The loss is only about 5% of the total power of the system and for all practicable purposes is irrelevant. Top

It is important to remember  power in an ultrasonic cleaner is usually rated in total average Watts. or Peak Watts. Watts average is what the cleaner will draw during continuous operation. Peak Watts is what the cleaner draws on start up. Watts average is the better determination of cleaning power.

There are specific cases where tanks are constructed with two or three types of transducers, thus enabling true multi frequency operation. These require different generators to run each set of transducers. Some transducers can be operated at more than one frequency. These transducers have a natural harmonic at a higher frequency such as 40 kHz and 170 kHz;  by including 2 different frequency driver boards in a single generator the tank can be switched from one frequency to the other.

There is development work in progress to manufacture a transducer material that will have the ability to run at any frequency imposed by the generator. This will also require generators that can change output to delivery the different frequency signals to the transducers. these will be very expensive systems as the construction of the generator becomes extremely complex.

Tank Size & Loading
The total surface area of the work to be cleaned, measured in square inches, should not be greater  than the tank volume, measured in cubic inches., or about 230 square inches of cleaning area per gallon of tank capacity. The size of the tank should be such, that when the work is loaded into the basket, there is at least 1.5 inches on each side and top and at least 2 inches of liquid on the bottom are free of parts. Top

Parts arrangement in an ultrasonic cleaner
Never put the parts on the bottom of an ultrasonic tank. This is like putting your thumb on a speaker diaphragm in a radio. You will prevent the correct movement of the diaphragm [bottom or side of the tank] and interfere with the creation of ultrasonic energy.

Parts should be racked in a basket or work holder designed to handle your specific part. This is very important in high end cleaning systems where you want the cleanest part possible. You should always use a stainless steel basket, as softer materials will absorb the ultrasonic energy. Never use plastic or other soft materials. If your part is easily damaged or scratched, stainless steel racks with Nylobond or Teflon coatings are available.

Parts should be arranged in a single layer, this gives the cleaning fluid an opportunity to circulate and remove particulate from the immediate area of the part. When removing the parts from the cleaning solution a single layer prevents the upper parts from shedding particles on the lower parts.  

 Heat in the Ultrasonic Cleaning Process 
As you heat water, the entrapped air in the water,is forced out. As you reach 80% of the boiling rate of a solution, it holds the least amount of dissolved gas.

Most detergents are designed to work at an elevated temperature. If you are using DI water it is much more effective at a higher temperature than cold. Particulate, oils and other contaminate dissolve more readily in hot solutions. Degassing also takes place at a quicker rate. Top

Flammable Solvents in an Ultrasonic Cleaner
Flammable solvents can be used in special types of ultrasonic tanks that are custom built for that purpose. You should make every effort to find a nonflammable solvent or process before resorting to using flammable solvents in an ultrasonic system. The tank must be constructed so that a freeboard equal to the depth of the liquid in the tank is included in the design. Other considerations are the inclusion of a condenser coil in the freeboard area that will re condense the vapor from the flammable solvent so that it does not escape the tank.  The tank must also have a sealed transducer compartment that is purged with an inert gas. This prevents any sparks or other electrical malfunction from igniting any vapor that may have found its way to that area.  

Since most flammable solvents have a low density they form a fuel air mixture [ the Military uses this principal as a very effective bomb] above the liquid when ultrasonically activated. For this reason tanks should be mounted in a stainless steel fume hood and the generators located outside any flammable area. Infra red fire suppression systems are required by most local rules.

Small amounts of flammable solvents such as alcohol may be used in an ultrasonic cleaner only if the solvent is in a beaker that is immersed in the water of the tank. The beaker should be capped and the entire unit should have adequate ventilation. Care must be exercised when using this method to prevent fire.

The cost of a system of this type is 5-10 times the cost of a tank that uses water as the base cleaning liquid. It is imperative to follow all the requirements when using flammable solvents in any ultrasonic system. You should check with your plant facilities personnel and with the ultrasonic manufacturer for complete information. Failure to do so could result in explosion and fire.

Degassing and how it affects  cleaning  
Any liquid will retain dissolved oxygen, for an ultrasonic cleaner to work properly the dissolved oxygen must be driven out of solution. The cleaning power of an ultrasonic tank comes from the violent collapse of the vacuum cavity or bubble. If there is any dissolved gas in the cleaning fluid it will migrate to the area of lowest pressure [ the vacuum cavity] preventing the violent collapse of the cavity and reducing the cleaning power of the tank.

Frequency Choice Top
Today you can get ultrasonic systems with frequencies ranging from 20 to 250 kHz. Which one to choose depends on: what your cleaning job is, what type of soil is to be removed and how clean your part needs to be. In reality, most systems to day incorporate more than one frequency of ultrasonics. They may use 40 / 70 / 170 for a graduated cleaning process. A general list of frequencies that are most common follows:

20-40 kHz Heavy duty cleaning for things like engine blocks, heavy metal, heavy greasy soils.
40-70 kHz General cleaning of machine parts, optics etc. very good at removing small particles.
70-200 kHz ultra fine gentle cleaning of optics, semiconductor wafers, disk drives, etc.  

Measuring Ultrasonic Power Levels in an Ultrasonic Tank
Power levels in an ultrasonic tank are usually expressed in Watts. E.G. a 500 watt system has 500 watts of average power produced by the generator. This does not mean that you have 500 watts of cleaning power. There are losses in any system. In an ultrasonic system there is a small power loss in the transducer[ they will warm up considerable from the friction of moving rapidly] There is also a loss in the energy required to penetrate and move the tank bottom. [ diaphragm] There is also the power lost to the parts and absorbed by the work holders themselves, finally there is a loss to the cleaning liquid inertia.[ this is expressed in the fact that ultrasonic energy will heat up the liquid in a tank. All of these losses of power might approach 10-20 watts dependent on the system construction. Power loss is also affected by the temperature , chemical make up and density of the cleaning fluid.

Some companies are now including a power output meter in the generator itself. This type of indicator will show the output of the generator is does not show the power existent in the ultrasonic tank. There is only one way to measure the power output of an ultrasonic tank with any reliability. That is by use of a power intensity meter that will measure the energy given off by the collapse of the cavities. No two ultrasonic tanks generate the same amount of power there is always a slight difference in the construction of the generator, transducers, and tank.Top

A good power meter should show you the frequency and sub frequencies generated in the tank and an oscilloscope view of the waveform. It should also be able to display the True RMS power and the peak to peak power of the energy levels. The system should be capable of recording any of these parameters and playing them back at a later time for comparison purposes. The probe should be small enough to fit into and between you parts when in the tank so that power readings can be taken around the parts and work holder. If you do not need all the sophistication of a full fledged metering system there are systems that measure RMS only. This will give you an indication of the power in any area of the tank. Top

Cleaning Chemistry

Fluid Surface Activity
Surface activity does not necessarily indicate strength. A highly active fluid surface can be caused by the fluid level, especially in those cleaners, which have a high sensitivity to fluid level-, those that have a high standing wave ratio- those having dual generator units, which are not properly frequency matched. Cleaners having strong surface activity sometimes completely die out whenever the least change occurs in the fluid level or when a workload is placed in the tank. After a few minutes the tank should return to normal operation.

De Ionized Water in the Cleaning ProcessTop
DI or deionized water is simply water that has had most of the mineral content and dissolved ion content removed. It is pure water. The cleanliness of the water is rated as its resistance value. [Pure water will not conduct electricity] The rating is in Meg Ohms.]  

Standard tap water is brought to a temperature of about 72 degrees f and run though a . 5-micron filter to remove particulates, a carbon filter then removes any organics within the water. A reverse osmosis filter removes further contaminants and a mixed bed resin filter removes the final dissolved minerals. This is a very basic process for production of DI water on a small scale. Larger systems are very complicated and use pumps and re- circulation loops to provide larger volumes of DI water. 

DI water is important in high end cleaning for the following reasons;

1.        Since it has all or most of its mineral content removed it is very hungry to acquire minerals from your parts [mostly from the dirt and contaminates on the surface of your parts.]

2.        Because it is such an active cleaner it is Ideal for use in ultrasonic cleaning systems with or without detergents.

3.        DI water leaves no residue on your parts, so you have no water spotting when they are dried.

4.        In combination with a detergent it makes an extremely good cleaning solution.

5.        DI water will remove any remaining detergent or soaps from your parts in the rinsing phase and since it has no mineral content it leaves no residue on the parts.

6.        In test performed with an ultrasonic cleaner DI water with detergents outperformed chlorinated solvents, solvents will only remove the particular contaminates it is designed for DI water will remove a large range of contaminants.

7.        Detergents and other cleaning agents perform better is solution with DI water. The cleaning action is enhanced and the detergent is not wasted in converting the mineral content of the water, all of its cleaning action can be directed to the part.
Whenever possible use DI water in your ultrasonic tank for both cleaning and rinsing. Top

The importance of Cleaning Chemicals
Solutions are the most single influential variable in ultrasonic cleaning. Properties of the specific fluid interact greatly, that is, some fluids operate quite well at ambient temperatures while others operate better at 140 to 160 degrees F. Some fluids require wetting agents [surfactants, detergents] to effectively transfer the ultrasonic energy into the solution. Water always requires a wetting agent and operates better at the higher temperatures. The choice of a detergent is dependent on the type of soils to be removed. This is one of the more important choices to make in any ultrasonic cleaning process.

 The intensity with which cavitation takes place in a liquid medium varies greatly with the Colligative properties of that medium which include vapor pressure, surface tension, viscosity, and density as well as any other property that is related to the number of atoms, ions or molecules in the medium. In ultrasonic cleaning applications, the surface tension and the vapor pressure characteristics of the cleaning fluid play the most significant part in determining cavitation intensity and cleaning effectiveness. The energy required to form a cavitation bubble in a liquid is proportional to both surface tension and vapor pressure. The higher the surface tension of a liquid, the greater will be the energy that is required to produce a cavitation bubble and the greater will be the shock wave energy that is produced when the bubble collapses, In pure water whose surface tension is about 72 dynes/cm sq. , cavitation is produced only with great difficulty at ambient temperatures. It is easily produced when a surface-active agent is added to the liquid, reducing the surface tension to about 30dys/cm sq. When the vapor pressure of a liquid is low, as is the case with cold water, cavitation is difficult to produce but becomes less and less so as the temperature is increased. Every liquid has a characteristic temperature relationship in which cavitation exhibits maximum activity within a fairly narrow temperature range.

 The flow characteristics or reological properties of the cleaning applications static fluid conditions are highly conducive to the formation of the standing wave pattern that characterize intense ultrasonic fields, and hence it would seem likely that cavitation intensity would be maximized under such conditions. Optimum performance is seldom achieved in static fields since continuous purification of the cleaning fluid either by overflow or by recycle filtration [filtering up to 50% of the tank volume per minute] is a prerequisite to effective cleaning.  When the filtered liquid is properly introduced into the bath little or no cavitation is lost.  In fact, improvement in overall surface impingement and homogeneity of cleaning can be attained with this method. Top

Aqueous Cleaning Solutions  
Aqueous cleaners are designed to reduce the surface tension of the water and also to provide a chemical reaction with the type of soils it is designed to remove.

The chemicals in an aqueous cleaner may vary from soaps to surfactants to acids or chelating agents, builders, saponofiers, alkaline or combinations of the above. The cleaning solutions may be Ionic or non-Ionic dependent on the application.

De ionized water itself can be an effective cleaning agent in some circumstances. It is always preferable to use DI water as the major portion of the cleaning fluid as it is pure water and does not have minerals or other contaminants in it. It provides an excellent vehicle for the detergent and there no chance of depositing minerals on the substrate. This will aid in the rinsing of the detergent and will provide spot free drying. Top

Aqueous Cleaners/Soaps and Detergents  
Aqueous cleaning works by the detergent actually bonding to the dirt (oil, grease, or particulate) and the mechanical action of the ultrasonics flushing this new compound into solution. This reaction of alkaline detergent with fatty acids is the saponification of dirt emulsifying into solution. 

A typical detergent is made up of several agents which work in combination to accomplish cleaning. Surfactants  (wetting agents) reduce the surface tension of the dirt, allowing the cleaning agents to penetrate. Saponifiers combine with the fatty acids and the flocculent to disburse the dirt into tiny particles. The mechanical action of the cleaning system flushes away the contaminants, one microscopic layer at a time.                                                                                                                                                                       

Acidic type cleaners are typically used for removal of scale, rust and calcium deposits. Heavy acid cleaners are used for pickling parts as they are cleaned. Since in a good ultrasonic system, parts are striped to the bare metal and all natural oxides are removed, the part will oxidize rapidly after extraction unless protected either by pickling or by addition of rust inhibitors to the cleaning solution 

Solvent Cleaners  
Solvent cleaners generally have a lower surface tension than water and are much denser. Solvents work on the basis of dissolving the contaminant. The extremely low surface tension of a solvent permits it to penetrate fine cracks or blind holes and dissolve organic oils and other contaminants. The solvents penetrating action as well as its chemical action remove Inorganic. Solvents can be blends, azeotropes or mixtures of both solvent and water. Solvents require special handling and design in selecting the correct ultrasonic system. In most cases since solvents are denser than water additional ultrasonic power is required to induce cavitation. Most solvents are specific in type of soil removed and care must be used in selecting the correct solvent. 

HFC solvents contain carbon, hydrogen and fluorine and are not ozone depleting. As a class hydrofluorocarbons remain the only option, which could be non-flammable, because they contain fluorine. Flammable solvents such as isopropyl, turpines and ethers are also usable in an ultrasonic system, however specific modifications must be made to any system that uses flammable solvents. Top

Process  
Ultrasonic Cleaning Consoles
An ultrasonic console is a series of ultrasonic tanks, spray rinse tanks, dump rinse tanks, ultrasonic tanks and dry tanks that are contained within a single machine. They are either controlled individually, passing the work manually from tank to tank, or may be completely automated and the work moved from station to station by an automatic work handling system. A console is effective when you have a large volume of parts to clean or are looking for extreme cleanliness of your parts.

Consoles vs. single tanks
If you are cleaning in a laboratory, or just a few small parts, you can safely use a small industrial type tank to clean your parts. The parts can be cleaned in beakers with both rinse and clean solutions in different beakers This gives the same cleaning process as a console but is much more labor intensive. The tank should have heat, power intensity and sweep frequency controls and a beaker holder insert that will allow you to suspend beakers at the correct positions in the tank  

Ultrasonic Detergent Tanks
The ultrasonic detergent tanks are the primary means of removing the particulate from your parts. In a high end cleaning system they are designed to minimize the retention of particulate. A series of ultrasonic detergent tanks are used for high end cleaning. The lowest frequency tank is first, [it will remove the largest particles first.] then immersion in a spray rinse tank to flush off any remaining particles, and soap. The next step is  cleaning in a higher frequency tank to remove smaller particles. Followed by another spray rinse, The finial step is the highest frequency tank to remove the smallest particles, again followed by a spray rinse.   

Spray Rinse Tanks
A spray rinse tank is usually made of virgin polypropylene  [a plastic material that is low shedding] and is slightly larger then the ultrasonic tanks in the system. It has 2 or more spray bars with multiple nozzles in them. These can be adjusted to focus the spray of DI water on a load of parts.  All wetted parts are designed to be compatible with DI water. A point of use DI water heater is used with a spray rinse tank to provide more effective rinsing. 

Whenever a load of parts is removed from an ultrasonic cleaning tank the parts should be rinsed. Any loose particulate is removed and the majority of the detergent film is removed. The load of parts is now ready for the next stage of cleaning.  Top

Dump Rinse Tank  
A dump rinse tank, is used much in the same way as a spray rinse tank with additional steps built in. A dump rinse has a 4 sided overflow and a large [2-3” dia ] solenoid controlled drain plug in the bottom. It also has spray bars in the top of the tank to spray DI water on the parts.  A Programmable Logic Control or PLC controls the tank sequence. The sequence of events is as follows;
A.       Parts are placed in tank and the cycle start button is activated
B.       The tank sprays water on the parts [with drain closed] until the tank overflows
C.       At a preprogrammed time the drain opens and all the water is dumped from the tank immediately
D.       The drain closes and the cycle is repeated as many times as programmed.

The Dump Rinse is used in conjunction with the spray rinse and ultrasonic tanks in a high end cleaning system. 

Ultrasonic Rinse Tanks
An Ultrasonic Rinse Tank is a standard ultrasonic tank that has a continuous DI water inflow from a regulated source. The amount of DI water can be controlled from a valve either manually or PLC controlled. The DI water overflows a dam and carries any particulate out of the tank. The ultrasonics help keep the particulates from re depositing on the parts. The frequency of the ultrasonic rinse tank is usually the highest in the cleaning system.  Top

Point of use DI Water Heater
A Point Of Use DI water heater uses ceramic heating elements to raise the temperature of DI water instantly. When the water is flowing the heater is activated by a flow switch and heats the water on demand. How hot it gets depends on the flow rate and the size of the DI point of use heater.

 DI water is extremely active. It will pull ions from any surrounding metal, including stainless steel. To heat DI water in a stainless steel vessel is possible; though if it remains standing in the vessel, it will eventually pick up ions and deteriorate. A better method is to heat the DI water only when it is needed.

Ultra pure DI water is rated at 18 Meg Ohms this is extremely pure water. Lesser grades of DI water are also used for cleaning. [10 meg ohm etc.] It is so clean that if you drank it would pull the ions from your body chemistry and would cause serious health problems. 

Filtration in Ultrasonic Cleaning Systems
The action of the ultrasonics combined with the detergent, will loosing the particles on the surface of the part and suspend these particles within the cleaning solution. If you are cleaning a large amount of parts or require ultra high end cleaning; it is recommended that you use filtration and additional ultrasonic cleaning tanks, in series with a spray rinse, in between each ultrasonic clean station. Cleaning Solvents and detergent solutions are also recommended.

The importance of using clean solvents cannot be overemphasized. Exposure of substrates to dirty solvents could easily result in the deposition of soils that are more difficult to remove than the original contaminants.  Top

The need for filtration of cleaning solutions is even greater when the ultrasonic process is operated on a continuous basis to clean consecutive batches of substrates. Under such circumstances, lack of filtration would lead to rapid contamination of the tanks and to the deterioration of cleaning ability. The above chart summarizes the cleaning efficiencies that were obtained in filtered and unfiltered three tank cleaning systems. After only seven batches of substrates, the cleaning in the first unfiltered tank fell to 40%ppm while that in the third tank fell to 80% By contrast, cleaning in the first tank of the filtered system remained at almost 90% efficiency, while in the third tank, essentially complete cleaning was achieved. It is evident that in order for an ultrasonic cleaning system to function effectively on a continuous basis, it must have two essential elements: 1: a filtration system to remove soils as they are displaced from substrates: 2: at least two consecutive cleaning tanks in series. Top

Rinsing 
In most cases rinsing will improve the final result. When you use a detergent of any type you will leave a residue of that detergent on a part. The best way to remove this residue is to rinse in DI water spray and or another Ultrasonic tank filled with a continuous over flow of Hot DI water. This will remove any traces of the cleaning chemical.  

Spray Rinse Tanks are designed to be a little larger than the ultrasonic tank that is used. The spray is delivered from 2 or more spray bars on the topsides of the tank. The drain is usually twice the size of the inlet. In most cases a point of use DI water heater is used to deliver a hot spray rinse. The tanks can be made of Polypropylene or stainless steel. 

Ideal process for Ultrasonic CleaningTop
Any ultrasonic tank will clean a part. The determining factor is how clean do you want the part. If you are looking for a level of cleanliness such as laser parts or semiconductor wafers you need to take additional steps. The most common ultrasonic cleaning process consists of 5 steps.

1.        Ultrasonic clean in a hot detergent solution  
The action of the ultrasonics combined with the detergent will loosing the particles on the surface of the part and suspend these particles within the cleaning solution. If you are cleaning a large amount of parts or require ultra high end cleaning, it is recommended that you use filtration and additional ultrasonic cleaning tanks, in series with a spray rinse in between each ultrasonic clean station. Cleaning Solvents and Detergent Solutions.

The importance of using clean solvents cannot be overemphasized. Exposure of substrates to dirty solvents could easily result in the deposition of soils that are more difficult to remove than the original contaminants.             

2.        Spray Rinse or Dump Rinse with hot DI or filtered water  
As you remove the part from the ultrasonic cleaning tank some particles will tend to re-deposit on the part. To remove these parts a spray rinse is required. Hot DI spray rinse is the most effective way to remove particles loosened by ultrasonic cleaning. A Dump Rinse type tank may be substituted for a spray rinse tank. Spray rinsing is required each time a parts basket is removed from the ultrasonic cleaning tank or from an ultrasonic rinse tank.  

                  3.        Ultrasonic Rinse in Hot DI water
          Ultrasonic rinsing in hot DI water is accomplished by placing the parts in an ultrasonic tank that is equipped with an overflow, [either 1,2, ( or )4 sided.]
        DI water is constantly entering the tank at a specific rate and is overflowing and carrying the particulates out of the tank. The ultrasonic action helps shake
          loose any remaining particulates. The exact design of the tank depends on your requirements.  

    4.        Spray rinse or Dump Rinse in DI or filtered water  
The last step your part should see is a spray or dump rinse in hot DI water. This removes any remaining particulate making the part ready for drying.  Top

                 5.        Filtered Air Dry  
         Dry the parts in hot filtered air [HEPA type filter] this prevents re-deposition of particulate on the part. The dryer should be set for about a 150 to
         170 degrees F., depending on the configuration and load. The dryer should be designed so that the air passes through the dryer one time only and then is
         exhausted. This drastically decreases drying time and prevents any contamination of the part. Drying tanks should be electro-polished to prevent particle
         retention. Parts should then be stored in a clean atmosphere.  How may of the above steps you need depends on how clean you want your part. For high levels
         of  cleanliness several ultrasonic clean at progressively higher frequencies are required. Several spray rinses and ultrasonic rinses are also required. It is important
         to remember that a good rinsing step will improve the results of any cleaning process. Top

The power intensity control is a good option where you will be cleaning delicate parts that may be subject to cavitation erosion, or in a situation where ultrasonics are  used in a plating operation or in other chemical processes.

Other Drying Methods
There are several other methods of drying parts, such as vacuum drying, where extremely dry parts are required. The parts are placed in a vacuum  oven and the pressure is reduced. The oven is back filled with hot N2 gas that heats the part. When the pressure is reduced again, the moisture is sublimated off the parts and pumped out after the next backfill cycle. This method takes time but produces an extremely dry part.

Infrared drying uses a series of lamps to heat the part and drive off the moisture. Usually used in a conveyer type oven. Hot air drying tunnels with air knives are also used. The exact method of producing a dry part will depend on your final process requirements.  

Measuring Cleanliness
There are several ways to check parts for the correct level of cleanliness

Visual Inspection: The simplest of all tests, the visual inspection test is examining the part under a 10 to 20 x magnification and subjectively determining the parts condition. 

Water Break Test: This is the least expensive and quickest method. The water break test is simply dropping a small drop of water on the surface of the part. If the water beads up, the part is not clean. If the water sheets evenly across the surface of the part then all oils and particulates are removed. This test works well with metals and smooth surface parts. 

Black Light Inspection: In a darkened room or area the parts are inspected under a black light [ UV ]. Organics and certain types of particulate become easily visible under this type of light.

Resistivity / Conductivity Test: This works well with smaller parts that are hard to inspect or configured so that the water break test is not easily accomplished. Select a small number of parts and place them in a holder or basket that is clean. Place the basket in a beaker filled with a known amount of DI water at a specific Meg Ohm rating. [ Make sure the interior of the beaker is clean and always measure the Meg ohm rating of the test water before placing your parts in the beaker. Immerse the beaker in an ultrasonic tank and clean for 1 min. Measure the resistivity of the water again. The new resistivity level will indicate how clean the parts are. [ You will never reach the original level of resistivity because the DI water will pull ions from the beaker and from your parts. ] A visual or other test will establish a basis for comparison. This type of system can be built into a rinse tank in a console and be set to control the rinse time. Top

Photon Emission: An expensive system that emits a beam of electrons on the part and counts the excited state of organics and other chemistry via a detector. This type of system is very expensive but also very accurate.

Particulate Laser Counting: A laser is shone thru a sample of the rinse water and particulate levels are counted and recorded. It works by detecting the amount of laser light scattered [ lost to the detector] by the particulate in the liquid. Top

 Problems

Basic  problems that occur in Ultrasonic Cleaning
      1.        Overloaded tanks; too many parts in a tank will reduce cleaning effectiveness.

2.        Using an underpowered tank to try and clean heavy industrial parts. Some lab type cleaners are not suited for heavy duty cleaning of parts; they are made to clean one or two parts at a time  because of their low power level.

3.        Choosing the wrong detergent or not using the proper amount will reduce cleaning effectiveness. Remember more is not always better when it comes to detergents. Use the manufactures recommendation.

4.        No ultrasonic cleaning action. Check to see if the tank is a. turned on b. timed out, c. plugged in . If you have a separate generator make sure that the generator is connected to the tank. If all this is OK and the tank still doesn’t work, call the manufacturer's and get them to repair it. Ultrasonic tanks use high voltage and are extremely dangerous to repair if you do not understand how they work. Don’t try this yourself.

5.        Low power or loss of cleaning action. Let the tank degas for at least 15 min. if no increase in power is noted, dump the old solution, refill with fresh water and detergent and repeat the degas process. If you're still having problems, call the manufacturer. If you smell any burning of electrical components, shut the tank down immediately and call the manufacturer. It's possible that a transducer shorted from a water leak from the tank, or some other reason. It is not safe to repair this yourself.