An ultrasonic cleaner produces negative and positive pressure waves that clean even the crevices of parts and small instruments. Ultrasonic cleaning machines agitate water or another cleaning solution by creating high frequency pressure waves. This agitation allows objects that are submerged in the liquid to be thoroughly cleaned in a matter of minutes. To produce these waves, the equipment needs a mechanical vibrating device. Ultrasonic cleaning machine manufacturers make use of diaphragms that are attached to transducers. The transducers produce high frequency. These transducers are attached to a high-frequency electronic generator, and as a result, they vibrate at resonant frequency resulting in amplified vibrations of the ultrasonic cleaner's diaphragm. The amplified vibrations transmitted to the diaphragm are the source of the positive and negative pressure waves that propagate through the created solution in the ultrasonic cleaning tank.
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This process can be compared to the vibrations movements of a loudspeaker, but in the case of sonic cleaners, the frequency is very high. When the waves move through water, they result in the cavitation processes. This unique and highly advanced cleaning process is used in many industries, including automotive, medical, jewelry, sporting goods, and more. Ultrasonic cleaners can be manufactured to be large enough to hold sizable components like car parts or small enough to clean a wedding ring.
For some applications, particularly in industrial settings, a custom ultrasonic cleaner will be necessary. Some companies require customized units that can be made to clean very large components or machines that match a set of exact size specifications due to space limitations in a factory. Some manufacturers work with clients to help create a custom ultrasonic machine that meets their particular needs. Engineers will consult with the client and determine what parameters need to be put on the project in order to make the final product work efficiently with the client's existing systems. They will develop a model first and then move on to building a prototype. If the client needs many of these units, they will optimize the production process to keep costs down.
Ultrasonic System Design
Ultrasonic machining depends on the contaminants on the parts to be cleaned, the geometry of the parts, the material of the parts to be cleaned, the previous layout and design, and the size of the cleaning system required. Part geometry, the production rate, and the cleaning time required go a long way in determining the overall size of the cleaning system. A typical ultrasonic cleaner tank ranges from 20 liters to 400 liters in capacity. Some, however, are larger depending on the applications.
When designing or choosing an ultrasonic cleaner for heavy duty applications, it needs to be industrial and heavy duty. Factors to be considered when purchasing or designing ultrasonic cleaners include the cleaning solution and their temperatures for effective cleaning, the process of rinsing, with or even without ultrasonics, the drying process, automation of the system, and load requirements. When one is purchasing an ultrasonic cleaning system, the manufacturer will assist in most of the decisions to be made, including solutions, tank size, type of transducers, and much more.
- Cleanliness Considerations
- The cleaning stages remove or loosen the contaminants in a conventional ultrasonic cleaning system. Stages as they follow each other result in cleaner parts. The cleaning stage removes contaminants, the rinsing stage removes any loose dirt and detergents on the parts, and the drying stage removes the rinsing water. The whole process is determined practically. Most ultrasonic cleaner manufacturers have a lab where the cleaning system is tried, and errors are eliminated to ensure that the cleaning specifications needed by the end user are met. This means that an ultrasonic cleaner will leave the manufacturer's premises when fully fixed and errors corrected.
- There are a number of ways through which one can test for the cleanliness of parts after the aqueous parts washer has acted on it. The simplest of the methods is the water break test on the cleaned part, which shows whether all the oil has been removed. Other methods are more elaborate and involve the use of stimulated electron emission technology. This method measures contaminants down to Angstrom level.
- Changing Existing Ultrasonic Cleaning Systems
- Where there is an existing system, it is easy to retrofit it to meet the application's needs. An existing system may be a vapor degreaser or a soak tank. Several things need to be considered when retrofitting an existing system. First off, it may be economical to change the solution from an organic solvent to an aqueous cleaner. One needs to know how ultrasonic cleaners work when changing an existing system.
- Transducers can be added to an existing tank by simply perforating the tanking and welding the traducers to the tank. There are also watertight immersible transducers that can be dropped into the tank. Immersible transducers take up room in the tank but take less time and are less labor intensive. Additional work will be done on the tank, mostly removing the degreaser, the cooling coils, and adding fittings for a filtration system.
- In some systems, there is an inventory of baskets that is used to hold parts in readiness for cleaning. These baskets are needed to save on the replacement costs. The configuration of the mesh hole sizes inhibits the cavitation process. The best mesh size is one that is greater than 200 or less than 10.
- Ultrasonic Part Handling
- It is important to understand the geometry of the parts to know how they will be placed in the tank. This is where one needs to know how ultrasonic cleaners work. When cleaning, large parts can be suspended on a hoist and small parts held on a basket. It is important to ensure that there is no air trapped anywhere in between the parts. When an air pocket is formed, the cleaning effect is not able to reach that part and cleaning is not effective. To prevent ineffective cleaning in the tank, parts should be rotated during cleaning. Rotating can be done manually or by a rotating arm on an automated operator. Small parts should be physically separated when placed in the basket. Knowing how an ultrasonic cleaning works will help the end user take care of the system for long product life.
Types of Transducers
Transducers are one of the components of an ultrasonic cleaner that determines its power. These transducers come in different frequencies ranging from 20kHz to 80kHz, for industrial cleaning. It is the resonant frequency of a sonic cleaner's transducer that determines the size of the resonant bubbles and their magnitude. Lower frequency transducers produce large bubbles with high energy. Lower frequency transducers create huge dents on the item they are cleaning, while the high-frequency transducers create small dents.
Transducers are attached to the radiating diaphragm, a tank filled with an aqueous solution, and an electrical generator. An ultrasonic cleaner uses transducers to create high-frequency mechanical energy. In industry cleaning, two types of transducers are used; both the transducers have the same function, which is to create mechanical energy at high frequency, but their performance is different. These transducers are magnetostrictive and piezoelectric.
- Piezoelectric Transducers
- Components of Piezoelectric Transducers
- There are many components that make Piezoelectric transducers. The basic components are two strips of tin sandwiching a ceramic crystal. The generator applies voltage to the tins, which creates a displacement on the ceramic crystal. This displacement is referred to as the piezoelectric effect. When transducers are mounted on the diaphragm at the bottom of the walls of the tank, the displacement in the crystal results in a movement in the diaphragm making a pressure wave that is transported through the tank's solution. The mass of the ceramic crystal does not match that of the stainless steel diaphragm and as such, an aluminum block is used as an intermediate to enhance impedance matching, ensuring there is efficient transmission of the vibratory energy from the crystal to the diaphragm.
- Piezoelectric transducers are easy to assemble and are more cost-effective. However, they have a number of disadvantages experienced when used for industrial cleaning. One of the problems that may be experienced with this type of transducer is their performance deteriorates with time. The ceramic crystal used tends to depolarize itself with time and as a result, a substantial reduction in strain characteristics of the crystal is witnessed. When the crystal cannot expand maximally, it cannot produce enough displacement to move the diaphragm to full effect. The reduced vibratory energy is witnessed in the reduced cavitation in the tank.
- Piezoelectric transducers are attached to the solution tank with epoxy adhesive. The problem with epoxy adhesive is that it fatigues at high heat and high frequencies, which are characteristic of the solution tank and the transducer. When the epoxy bond loosens, the transducer becomes useless.
- Again, the ceramic crystal loses its capacitance with time as it is used. With time, the crystal's resonant circuit falls out of tune with the generator. Even with an aluminum insert to improve impedance matching, piezoelectric transducers still have relatively low mass, and energy transmission is still low. Energy is transmitted to parts that are immersed in an aqueous solution and as such, a substantial amount of energy is needed if cavitation has to occur efficiently. The low mass of the crystal affects energy transmission in the tank. To this end, ultrasonic cleaning machine manufacturers use very thin diaphragms in piezoelectric tanks.
- If a thick plate is used in piezoelectric tanks, it will not flex, given the low energy that these transducers produce. However, when thin diaphragms are used, they oscillate at an upper harmonic frequency when driven at a certain frequency and thereby create smaller implosions. There is also the problem of cavitation erosion where the diaphragm can wear, causing the solution to damage transducers and the wiring. This may require major repairs.
- Magnetostrictive Transducers
- Created for durability when used for industrial applications. Zero-space magnetostrictive transducers are created with nickel laminations tightly attached together with an electric coil attached on top of the nickel stack. When current from the electrical generator is passed through the coil, a magnetic field is created, which is analogous to the piezoelectric effect when the crystal expands. When alternating current is passed through the coil, the nickel stack vibrates at the current's frequency.
- The nickel stack of the transducer is silver brazed to the resonating diaphragm directly. This presents many benefits over the epoxy bond used in piezoelectric transducers. The silver braze ensures there is a metallic joint that does not loosen with time and there will be no damping effect that affects the transducer. Again, the use of nickel in the transducer reduces degradation; nickel will maintain its magnetostrictive properties throughout the life of the transducer.
- Magnetostrictive transducers have more mass, which is important in the transmission of vibratory energy in the aqueous parts washer tank. By driving more power, the tanks are less load-sensitive compared to piezoelectric systems.
- However, the magnetostrictive transducer is not as efficient compared to a piezoelectric transducer. This means that the piezoelectric transducer will produce more deflection for a given voltage displacement than magnetostrictive transducers. However, overall, a magnetostrictive transducer is more efficient in producing a vibratory effect; the overall efficiency is determined by the various parts of the transducer and the tank of the ultrasonic cleaner.
Ultrasonic Cleaning Solvents
How do ultrasonic cleaners work? High frequency sound waves generated by transducers clean parts, guns, and other items in industries. One of the most important components of sonic cleaners is the aqueous solution in the tank. To create this solution, ultrasonic cleaning machine manufacturers have used Freon and 1,1,1-Trichloroethane as solvents for many years, but due to the need to eliminate ozone depleting substances, companies have started looking for new ways to keep the environment safe and still clean parts.
- Water Based Solutions
- Today, most ultrasonic cleaning tank solutions are water. To this end, a water soluble detergent is recommended. Water has many advantages over other solvents used in the tank. It is non-flammable, non-toxic, and environmentally friendly. It is, however, expensive to dispose of water once it is soiled. Again, rinsing and drying without the use of detergents is challenging. Water based detergents are used to break the surface tension of water and provide the wetting required to break the bond between a substrate and a contaminant.
- As an advantage, the cavitation process in a water-based solution is more intense than in an organic based solution. Different parts will require different solutions. Solutions may be high caustic, alkaline, chlorinated solvent degreaser, or silicate.
- Temperature of the Solution
- The temperature of the solution determines the effectiveness of ultrasonic cleaning. High temperatures are preferred in ultrasonic machining as this causes high cavitation energy and, consequently better cleaning. Nevertheless, the solution's temperature should not move too close to the boiling point of the solution. As the liquid boils, it moves to the waves' negative pressure areas, thereby canceling cavitation. Water is best as a solvent when used at 70 degrees Celsius and caustic/water solution is best used as a solvent at 82 degrees Celsius, thanks to the upped effectiveness of the chemicals when used at high temperatures. For effectiveness, solutions used in the cavitation tank should be used at 6 degrees Celsius lower than their boiling points.
Producing Ultrasonic Cleaning Solutions
Most manufacturers will not produce a custom ultrasonic cleaning solution for a client unless the client orders multiple units at the same time. The cost of engineering and producing just one unit is often much too high to make it reasonable. Exceptions are sometimes made when the machine can be assembled using modified parts and components from products that the manufacturer already offers. Large ultrasonic parts washers can be made with multiple chambers, so in one machine, there can be separate areas for cleaning various components. Due to industry regulations, some applications may require the machine to have more safety and control features, so a manufacturer can add these features to ensure that industry standards are met. These regulatory demands are most common in the medical and pharmaceutical industries, where strict procedures for cleaning objects and operating any machinery are absolutely necessary to maintain protocol.