This article presents detailed information about ultrasonic cleaning.
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Ultrasonic cleaning involves a specialized technique where sound waves at ultrasonic frequencies—typically starting from 20 kHz—create cavitation through alternating compression and rarefaction. In this method, the item is placed in a tank filled with a specific cleaning solution. The concentration of this solution, along with the temperature of the tank and the duration of immersion, are meticulously regulated to achieve optimal cleaning results. The cavitation effect generates intense agitation that dislodges debris from the item's surface, ensuring a thorough clean of all accessible areas.
Cavitation occurs when small bubbles or voids form in a liquid due to a sudden drop in pressure. These bubbles quickly collapse as pressure rises again. The repeated implosion of these voids generates cyclic stresses that can cause surface erosion. The type of cavitation that happens instantaneously is known as inertial or transient cavitation. This process can lead to significant surface damage and is a major concern in pump operations, as it drastically shortens the equipment's lifespan.
Acoustic oscillations with lower energy levels can generate voids that oscillate around a stable size rather than collapsing. This is referred to as non-inertial or stable cavitation. In some cases, this lower-energy cavitation is sufficient to break the adhesion forces between particles and surfaces.
Acoustic oscillations consist of alternating waves of high and low pressure. The high-pressure phase causes compression, while the low-pressure phase leads to rarefaction, creating small voids through the rapid vaporization of the liquid. During the subsequent high-pressure phase, these voids are compressed. Although these voids are microscopic and not visible during operation, they generate high-energy localized areas with temperatures reaching up to 5,000 K and pressures of around 500 atm. The implosion of these voids can produce microscopic jets with velocities up to 300 m/s.
It's crucial to understand that the amplitude of sound waves alone does not dictate the type of cavitation that will occur. There isn't a precise mathematical formula to describe cavitation generation. Factors such as the composition of the medium, solute concentration, and temperature can influence the process. During cleaning, both inertial and non-inertial cavitation may be present simultaneously.
An ultrasonic cleaner consists of two primary components: the parts responsible for generating the acoustic waves and those that contain the liquid and the items being cleaned. This structure is common to all types of ultrasonic cleaners, regardless of their specific design, purpose, or application. Here are the key components typically found in an ultrasonic cleaning device.
An ultrasonic transducer transforms energy—typically electrical or mechanical—into ultrasonic vibrations. The two primary types of transducers used in cleaning applications are piezoelectric and magnetostrictive. These devices utilize specific materials that undergo tiny geometric alterations, generally around 10-6 meters per meter, when exposed to electrical currents or magnetic fields.
Piezoelectric Ultrasonic Transducers: This type of transducer converts alternating electrical current (AC) directly to mechanical energy from the phenomenon known as the inverse-piezoelectric effect. Piezoelectricity happens when materials release electrical energy when stressed. The opposite effect, inverse piezoelectricity, is used for ultrasonic transducers where the application of an electric field to a piezoelectric material causes changes to the electric charge carriers in the material‘s crystal structure. The realignment of these charge carriers results in the elongation or contraction of the crystal. Popular piezoelectric materials used are lead zirconate titanate (PZT) and barium titanate.
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, typically of around 5%, only result from internal friction and heat. Thus, 95% of the power from the generator is delivered to the tank and utilized for cleaning. The overall efficiency of ultrasonic cleaning machines using piezoelectric transducers is around 70%.
On the other hand, there are downsides to using this type of transducer 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, pre-aging the material can predict and counter this effect since degradation tends to slow down over time.
In addition, the reliability of these transducers is lower because the transducers can only be mounted through adhesives. The epoxy bond can become fatigued by cyclic loading over time and eventually loosen. Nevertheless, developments in the design of epoxy mountings of piezoelectric transducers are more reliable and guaranteed to last for ten years. These workarounds on the mentioned disadvantages make piezoelectric transducers more popular than magnetostrictive.
Magnetostrictive Ultrasonic Transducers: Magnetostrictive transducers operate on the principle of magnetostriction. Magnetostriction is the phenomenon in which a ferromagnetic material changes its dimension when a magnetic field is applied. When an external magnetic field is introduced to the material, its magnetic domains change their orientation and realign to the applied magnetic field. This effect allows the direct conversion of electromagnetic energy into mechanical energy. Nickel is a widely used material for ultrasonic cleaning.
The 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 transducer has lesser efficiency. One reason is that two energy transformation steps are 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 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 suited for stronger, more durable parts. Higher frequencies are desired for cleaning small, delicate parts such as semiconductors and jewelry.
Some ultrasonic generators are designed to operate at a fixed frequency, while others offer sweeping frequencies. When multiple transducers are used to deliver ultrasound to the tank, fixed-frequency transducers can create areas of uneven cavitation, including hot spots and dead zones. Hot spots, where cavitation activity is intense, can potentially damage delicate surfaces, whereas dead zones are regions with little to no cavitation. To address these issues, sweep frequency generators are employed. These generators vary the frequency delivered by the transducer array around a central average frequency, known as the sweep bandwidth. By continuously altering the frequency, sweep generators help to move hot spots and dead zones around, preventing any specific area from becoming problematic.
Feedback systems are employed to ensure the ultrasonic wave remains at its optimal center frequency. Different weights and shapes of cleaning parts interact differently with the acoustic waves. In a sweeping system, the feedback mechanism monitors changes in the load and adjusts the generated frequency as needed. This adjustment helps maintain optimal performance of the output 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 attacks from the cleaning solution. That being said, ultrasonic tanks are usually manufactured entirely from stainless steel. Common surface finishings applied to tanks are to prevent erosion include electropolishing to reduce surface roughness and titanium nitride (TiN) coating deposited by physical vapor deposition (PVD).
Ultrasonic machines are typically designed for optimal performance with parts positioned at or near the center of the tank. However, since these parts are often denser than the cleaning fluid, they tend to sink to the bottom. This can interfere with the acoustic waves, reducing the frequency and effectiveness of the cleaning. Additionally, intense vibrations can potentially damage delicate components or intricate assemblies. To address these issues, stainless steel mesh baskets are commonly used to hold and suspend parts within the tank.
Heat is supplied by heating elements integrated into the tank assembly. Heat must be high enough to promote an increased cavitation, cavitation intensity, and chemical solution cleaning ability but low enough to prevent degrading any special compounds added to the cleaner.
Several factors can influence the effectiveness of the cleaning process in an ultrasonic machine. These factors can impact the ultrasonic cavitation as well as the dissolution, emulsion, and reaction of contaminants with the cleaning liquid. Here are the key considerations to keep in mind.
The cleaning solution serves as both the medium for ultrasonic wave propagation and a key factor in determining the quantity and size of cavitation bubbles generated. Consequently, the properties of the cleaning solution play a crucial role in the effectiveness of cavitation. Several parameters influence this process, including:
Vapor Pressure: In fluid machinery, particularly pumps, the vapor pressure of the fluid is important as it directly affects the susceptibility of cavitation within the pump. Cavitation is formed when the pressure of the liquid falls below its vapor pressure. Thus, liquids with higher vapor pressure can easily develop cavitation since less effort is needed to go below the point of vaporization point. This means less power is required. However, since less power is required to form the bubble, less energy is absorbed and less energy will be released. Therefore, liquids with moderate vapor pressure are desired.
Surface Tension: Like vapor pressure, surface tension affects the formation of cavitation bubbles. High surface tension means more force is required to break the cohesive forces between the liquid molecules; thus, more energy is required to produce cavitation. Still, high surface tension is necessary to store large amounts of energy in the bubble. Moreover, surface tension affects the "wetness" of the solution. Making the solution "wetter" means better coverage of small areas on the surface of the part.
Temperature influences cavitation by altering the properties of the cleaning solution. As the temperature rises, the vapor pressure of the liquid increases, while its surface tension, viscosity, and density decrease. Higher vapor pressure combined with lower surface tension, viscosity, and density generally leads to greater cavitation activity.
Temperature not only enhances cavitation but also improves the effectiveness of the cleaning solution. Typically, higher temperatures boost chemical activity and facilitate better mass transport. This leads to more efficient dispersion and dissolution of debris, oils, and other contaminants removed from the parts.
Chemicals are introduced to enhance cleaning efficiency by altering the properties of the cleaning solution to promote better cavitation. In addition to aiding ultrasonic cavitation, these chemicals help dissolve and remove contaminants from the parts. They can be present in the ultrasonic bath itself or used during subsequent rinsing stages. Water is commonly used as the base solvent in ultrasonic cleaning due to its cost-effectiveness, availability, and suitable properties at ambient temperatures. To tailor the solution for specific tasks, chemicals such as alkaline detergents, acidic solutions, enzymes, and other specialized agents are added to adjust the solution's properties and provide additional cleaning functions.
Dissolved gases can reduce cavitation intensity by cushioning the implosion of bubbles. During the negative pressure phase, voids are created as the liquid vaporizes. When gases are dissolved in the liquid, they move into the bubbles. As the positive pressure phase begins, both the vaporized liquid and the dissolved gases are compressed. The presence of these gases within the bubbles prevents complete collapse, resulting in oscillating bubbles with lower cavitation intensity.
Degassing the liquid is a crucial step, which involves running the cleaner without any load. This process allows dissolved gases to accumulate and form larger bubbles, which become buoyant and rise to the surface. Once no more bubbles are seen rising, the liquid is considered ready for use.
Different frequency ranges are better suited to specific applications, and no single frequency is ideal for all uses. Generally, lower frequencies generate more intense cavitation, while higher frequencies produce less intense but finer cavitation. Lower frequencies are less effective at removing very small particles, as surfaces often have tiny crevices where particles can become trapped. Below are some common frequency ranges and their typical applications.
To effectively generate cavitation, the power supplied to the tank must be adequate. The standard power density for ultrasonic generators is around 100 watts per gallon. There is an inverse relationship between liquid volume and power density: as the volume increases, the necessary power density decreases, typically reaching a minimum value based on the system's design.
This chapter covered various types of cleaning machines based on their form and construction. These machines operate at different frequency ranges and utilize various cleaning solutions. Here are the three main types discussed.
Single-tank Ultrasonic Cleaners: Single-tank ultrasonic cleaners are standalone machines suitable for cleaning small to medium-sized parts. More advanced designs use single tanks that have multiple functions by combining cleaning, rinsing, and drying steps. Small scale applications such as jewelry, laboratory equipment, and surgical equipment cleaning only need a cleaning tank. Rinsing may be done through a separate, ordinary water bath, while drying is done by ambient air.
Multiple-tank Ultrasonic Cleaners: This type has separate tanks for the different steps of the cleaning process. The most common is having a three-tank system in which each tank is a station that performs either cleaning, rinsing, or drying. For production lines with higher throughput, multiple cleaning tanks are employed. Multiple cleaning tank systems can have pre-wash stages to remove loose debris, while other tanks perform ultrasonic cleaning. Fully automatic systems also use gantry robots to pick and carry the baskets containing the parts. The gantry lowers the basket into a tank for a specific amount of time, then transfers the basket onto the next station.
Immersible Ultrasonic Cleaners: Immersible (submersible) ultrasonic cleaners are detached ultrasonic transducers and generator systems that are used for new cleaning systems to add an ultrasonic cleaning function, or for retrofitting existing ultrasonic cleaning systems to improve cleaning performance. Immersible transducers can be submerged at the sides or bottom of the tank. The drop-in location depends on the load, geometry of the tank, and the volume of liquid solution. This type of ultrasonic cleaner is highly versatile since more transducers can be added which can be added at different locations. Also, the transducers can be transferred from one tank to another.
Ultrasonic Rod Transducers: Ultrasonic rod transducers have a single piezo element that creates ultrasonic vibrations in a cylindrical tube, a design that allows the ultrasonic waves to radiate in all directions from the source. When an ultrasonic rod transducer is placed in a tank, the surface area of every item is exposed to the cleaning process without any dead spots.
The power output of an ultrasonic rod transducer is up to 2 kW, with different lengths to fit the needs of any type of industrial cleaning. The single-point attachment of an ultrasonic rod transducer makes it possible to use them in closed systems, chamber systems, or open cleaning tanks with the capability of being adapted to vacuum cleaning processes and positive pressure processes at temperatures of up to 203 °F (95 °C) with short cleaning cycles. To meet the demands and requirements of modern cleaning operations, ultrasonic rod transducers are made of various materials, including stainless steel, titanium aluminum, and pure titanium.
Although an ultrasonic rod transducer's multidirectional attribute is ideal, the design has advantages beyond that single feature. They can be used to clean cylindrical surfaces and are well-suited for applications where debris breakdown is required.
Unlike dual-head transducer systems, single piezo crystals in ultrasonic rod transducers are easily replaced, which means less maintenance and longer use of the transducer.
The versatility of ultrasonic cleaners is enhanced by the variety of detergents used in the cleaning process, ranging from acidic to highly alkaline solutions. Understanding the different types of detergents helps prevent cleaning errors and avoids the removal of crucial components like waxes, lacquers, coatings, and anti-corrosion layers. Choosing the right detergent is a critical aspect of ultrasonic cleaning that requires careful attention.
Alkaline Solutions: Alkaline solutions can be used with a wide range of temperatures to remove salts, oxides, organic soils, metal chips, and grease. They have a pH of 10 or higher and contain caustic soda according to the required cleaning strength. Moderate alkaline solutions are used for cleaning metals, ceramics, glass, and most plastics.
The use of alkaline solutions is due to their effectiveness in removing organic contaminants such as oil, grease, and waxes. Oils do not easily dissolve in water due to surface tension. A component of alkaline detergents, a wetting agent, reduces the water's surface tension, enabling oils to be dissolved. Stronger alkaline solutions convert oils into soap to make them soluble in water.
When selecting an ultrasonic cleaning solution, the initial step is to consider the industry in which the detergent will be used and the specific components that need cleaning. While many manufacturers offer general-purpose detergents suitable for various ultrasonic cleaners, it’s crucial to ensure that the chosen detergent is compatible with the composition and structure of the items being cleaned. As with any cleaning solution, it is essential to review the guidelines and chemical composition of the detergent before making a final choice.
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