This article presents detailed information about ultrasonic cleaning.
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Ultrasonic cleaning is a type of cleaning process which uses cavitation induced by alternating compression and rarefaction cycles at ultrasonic frequencies. Ultrasonic frequencies are sound waves vibrating at 20 kHz or higher. In this process, the part is immersed in a tank filled with a formulation of cleaning solution. Cleaning solution concentration, tank temperature, and immersion time is carefully controlled to produce the necessary cleaning effect. The cavitation causes small but powerful agitations between the debris and the surface of the part. This method effectively cleans all surfaces that can be reached by the cleaning fluid.
Cavitation is the phenomenon where tiny bubbles or voids are formed in a liquid due to the rapid decrease in pressure. These voids are instantaneously imploded by the rapid increase in pressure creating a shockwave. Collapsing these voids impart cyclic stresses that erode surfaces due to repeated implosions. Instantaneous cavitations are called inertial or transient cavitation. This type can erode surfaces and is one of the main problems in operating pumps since it rapidly decreases the life of the equipment.
Using lower energy acoustic oscillations create voids that do not implode but pulsate about an equilibrium radius. These are known as non-inertial or stable cavitation. Sometimes, lower energy cavitation is enough to overcome particle-to-substrate adhesion forces.
Acoustic oscillations are simply alternating and repeating high- and low-pressure waves. High-pressure results in compression, while low pressure results in rarefaction. The low-pressure period creates voids from the sudden vaporization of the liquid. In the second half of the cycle, the high-pressure period occurs which compresses or contracts the void. These voids are microscopic in scale which cannot be seen during operation, but these are high energy localized regions that can have temperatures of up to 5,000 K and pressures of 500 atm. Imploding voids can have microscopic jet velocities of around 300 m/s.
It is important to note that the amplitude of the sound waves alone does not determine which type of cavitation can occur. There is no exact mathematical equation to describe its generation. Parameters such as medium composition, solute concentration, and temperature can affect the process. A combination of inertial and non-inertial cavitation can exist throughout the cleaning process.
An ultrasonic cleaner can be divided into two main parts. First are the components that generate the acoustic waves; and second, the components that hold the liquid and the parts to be cleaned. This applies to all ultrasonic cleaners, regardless of type, form, function, and application. Below are the major components of an ultrasonic cleaning machine.
An ultrasonic transducer converts a form of energy, usually electrical or mechanical, into an ultrasonic vibration. The two main types of ultrasonic transducers used for cleaning are piezoelectric and magnetostrictive. These transducers use special materials that create minuscule changes in their geometry, usually in the order of 10-6 m/m, upon application of electricity or magnetic field.
The main advantage of using piezoelectric transducers is their energy efficiency. This is due to the direct conversion of electrical energy into mechanical energy. Energy losses from this conversion only result in internal friction and heat which is typically 5%. Thus, 95% of the power from the generator is delivered to the tank utilized for cleaning. The overall efficiency of ultrasonic cleaning machines using piezoelectric transducers is around 70%.
On the other hand, there are downsides in using this type of transducers since they are negatively affected by aging and are less reliable. The performance of piezoelectric materials decreases over time. This is due to the depolarization of the charge carriers in the crystal which causes a significant reduction in its strain characteristics. However, this effect can be predicted and countered by pre-aging the material since degradation tends to slow down over time.
Reliability is lower since mounting of the transducer is only through adhesives. The epoxy bond is fatigued by cyclic loading over time which eventually loosens. Nevertheless, developments in the design of epoxy mountings of piezoelectric transducers are made more reliable that is guaranteed to last for 10 years. These workarounds on the mentioned disadvantages make piezoelectric transducers more popular than magnetostrictive.
Upsides of using magnetostrictive transducers over piezoelectric are its reliability and resistance to degradation over time. Magnetostrictive transducers can be mounted by braze-bonding which cannot easily loosen, in contrast to epoxy bonds used in piezoelectric transducers. Epoxy bonds also create a damping effect which decreases the amplitude of the applied acoustic wave. Regarding its stability, ferromagnetism is a material‘s inherent property that does not decay over time.
As discussed earlier, this type of transducers has lesser efficiency. One reason is that there are two energy transformation steps involved: electrical to magnetic, then magnetic to mechanical. In magnetic systems, 50% of the energy is lost due to the heating of the coils and to the effects of hysteresis. Magnetostrictive transducers have an overall efficiency of about 30 to 40%.
The ultrasonic generator is the main component of an ultrasonic cleaner. This part receives power and converts it into a suitable form for energizing the transducer at the desired frequency. The standard electrical frequency of power utility systems is 50 and 60 Hz. Since ultrasonic frequencies range from 20 kHz and above, the power supply frequency must be changed to the appropriate range which depends on the type of contaminant to be removed and the mechanical strength of the part. Lower ultrasonic frequencies tend to form larger cavitation bubbles that produce more powerful oscillations and implosions that are suited for stronger, more durable parts. In cleaning small and delicate parts such as semiconductors and jewelry, higher frequencies are desired.
Some generators are only able to provide a fixed frequency, while others can provide sweeping frequencies. Since multiple transducers deliver ultrasound to the tank, an array of fixed frequency transducers creates hot spots and dead zones in the tank which produce variations in the cavitation produced. Hot spots tend to have higher cavitation activity which can erode surfaces of delicate parts. Dead zones, on the other hand, are areas where no cavitation is created. To solve these problems, sweep frequency generators are used. In sweep frequency generators, the delivered frequency on the transducer array fluctuates from a center frequency which is the average frequency used. The variation from the center frequency is called the sweep bandwidth. By varying the frequency, hot spots and dead zones are constantly moved. Thus, there is no particular location for hot spots and dead zones. This, somehow, eliminates the problem.
Feedback systems are used to maintain the center frequency of the wave. When cleaning parts with different weights and geometries, it produces different interactions with the acoustic waves. With a sweeping system, the feedback system detects changes with the load on the generator. The generated frequency is changed accordingly enabling the output to be optimum at all times.
The tank contains the cleaning solution and the part to be cleaned. This is also where the transducers are mounted, usually at the sides or bottom of the tank. The tank must be durable enough to resist erosion from ultrasonic cavitation and must be corrosion-resistant to withstand chemical attack from the cleaning solution. That being said, ultrasonic tanks are usually manufactured entirely from stainless steel. Common surface finishings applied to tanks are electropolishing to reduce surface roughness and titanium nitride (TiN) coating deposited by physical vapor deposition (PVD) to prevent erosion.
Most ultrasonic machines are designed to operate with the part placed at or near the center of the tank. Since the parts are usually denser than the fluid, they sink at the bottom of the tank. Contact with the tank walls affects the delivered waves making the frequency lower and the cleaning less effective. Moreover, high vibrations can cause damage, particularly for delicate parts with tiny assemblies. Baskets are usually made of stainless-steel mesh.
Heat is supplied by heating elements integrated into the tank assembly. Heat must be high enough to promote an increased amount of cavitation, cavitation intensity, and chemical solution cleaning ability, but low enough to prevent degrading any special compounds added to the cleaner.
Different factors can affect the cleaning quality of the machine. These can be factors that affect the ultrasonic cavitation produced or the dissolution, emulsion, and reaction of the contaminants with the liquid. Below are the major factors to consider.
The cleaning solution not only acts as the medium where the ultrasonic wave propagates but also influences the amount and size of the cavitation bubbles produced. The properties of the cleaning solution have significant effects on the cavitation induced. There are a few parameters involved which are:
Temperature affects the amount and intensity of cavitation by modifying the properties of the cleaning solution. The temperature has a direct effect on the properties of the liquid. By increasing the temperature, vapor pressure increases while surface tension, viscosity, and density decrease. High vapor pressure and low surface tension, viscosity, and density tend to create more cavitation activity.
Temperature not only increases the effect of cavitation but the cleaning solution's effectiveness as well. Usually, higher temperature results in more chemical activity. Higher temperatures also promote better mass transport. This means debris, oils, and other contaminants removed from the part are easily dispersed and dissolved.
Chemicals are added to improve cleaning efficiency by modifying the cleaning liquid‘s properties to facilitate better cavitation. Aside from using ultrasonic cavitation, chemicals are also used to help in dissolving and separating contaminants removed from the part. These may be present in the ultrasonic bath or the succeeding rinsing stages. Water is used as the general solvent for ultrasonic cleaning applications since it is cheap, readily available, and has the right range of properties under ambient temperatures. Chemicals such as alkaline detergents, acidic solutions, enzymes, and other special chemicals are added to modify its properties and include additional functions.
Dissolved gases decrease cavitation intensity by acting as a cushion during the bubble implosion. During the negative pressure phase, voids are formed from the vaporization of the liquid. When dissolved gases are present in the liquid, they migrate and diffuse into the bubble. As the positive pressure phase occurs, the vaporized liquid and the dissolved gases that migrated into the bubble are compressed. The collection of these dissolved gases into the bubble prevents the void from collapsing. The resulting cavitation is only oscillating bubbles that have lesser intensity.
It is important to degas the liquid first. This is done by operating the cleaner without load. Dissolved gases that are collected make the bubble larger, becoming more buoyant, and eventually rises to the surface of the liquid. Once no more rising bubbles are observed, the liquid is ready for operation.
Certain frequency range performs better in specific applications than other frequencies. There is no specific frequency that fits all applications. A general rule would be lower frequencies tend to produce greater cavitation, while higher frequencies produce less intense but finer cavitation. Low frequencies are not effective at removing microscopic particles. Surfaces have tiny troughs where particles can lodge into. Below are some of the frequency ranges used and their application.
The power delivered into the tank must be sufficient to create cavitation. Typical ultrasonic generator power density is 100 W per gallon. Liquid volume and power density have an inverse relationship with each other. As the volume is increased, the required power density decreases that usually bottoms at a specific value depending on the design of the system.
This chapter discussed different types of cleaning machines according to form and construction. These machines can operate at different frequency ranges and can use different cleaning solutions. Below are the three main types.
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