Turning, milling, drilling, grinding and so on leaves traces behind on workpieces. Ultrasonic cleaning allows for removing coolant, chips, polishing paste, and other contaminations in a quick, reliable, and economical manner.
When it comes to machining, contamination of the workpieces' surface is inevitable. But soil like residue of cutting fluids, swarf, friction, and polishing paste usually causes problems in upstream processes or may affect function, quality, and lifespan of finished products. That's why machined parts have to be cleaned. In particular, in the production of large lots of chip-formed workpieces parts cleaning can turn out as a brake in the production process. By means of ultrasonic cleaning particulate and film-like soils can be removed not only reliably, but also very quickly and efficiently -- even for parts with difficult to access hollow spaces, for example blind holes, knurling and grooves.
As a rule, good cleaning results can be achieved with an ultrasonic power rating of 8 to 10 W per liter.
Cleaning Effect of Sound Waves
Ultrasonic waves develop their cleaning effectiveness in a liquid bath. It is based on the physical cavitation effect: When a liquid is subjected to ultrasonic sound, the high intensity of the alternating sound pressure during the pulling phase of the oscillation cycle breaks up the liquid -- the cohesive forces are overcome. This causes the formation of millions of microscopically small bubbles. During the subsequent pushing phase, these cavitation bubbles are rendered unstable and collapse (implode), and they generate hydraulic impacts with considerably high energy densities, thus causing micro-currents in the liquid. When these strike a surface, they blast off contamination which has been partially dissolved with a suitable cleaning agent and rinse the "dirt" away.
The Ultrasonic Frequency: A Decisive Criterion
The sound waves are produced by a generator which converts the normal electrical mains frequency of 50 to 60 Hz into high-frequency oscillation. This electromagnetic oscillation is then transformed into mechanical oscillation of the same frequency by so-called sonic transducers.
Ultrasonic cleaning allows for removing particulate and film-like soils not only reliably, but also very quickly and efficiently.
The ultrasonic frequency has a significant influence on cleaning results. Generally speaking, the lower the frequency, the larger the cavitation bubbles and thus the more energy they release. Reference values for the selection of ultrasonic frequencies for cleaning systems used for cleaning machined parts are:
Specifically in the case of components with highly complex shapes, multi-frequency or mixed frequency ultrasonic systems are used to an ever greater extent which generate several frequencies, for example 25 and 40 kHz. As a result of the mixed numbers of larger and smaller cavitation bubbles, ideal cleaning forces are achieved at outside surfaces and boundary surfaces.
Due to the great intensity of the alternating sound pressure during the pulling phase of the oscillation cycle, the liquid is broken up during exposure of the cleaning bath to ultrasonic waves. Cavitation bubbles are formed as a result.
Matched to the Cleaning Task
In addition to the frequency, the number of transducers and their positioning in the bath play a decisive role with regard to the performance of an ultrasonic cleaning system. As a rule, good cleaning results can be achieved with a power rating of 8 to 10 W per liter. This means that for a cleaning bath with 100 liters, 800 to 1000 W of output power is required.
Due to the fact that the sound waves are propagated longitudinally from the sound emitting surface, the arrangement of the transducers has a considerable influence on cleaning results as well. For example, if the transducers are only attached to the floor of the working chamber or cleaning basin, the sound waves are emitted vertically up to the surface of the bath, from where they are reflected back down to the floor. This has consequences for the cleaning of parts with hollow spaces and blind holes: If bubbles form in these areas, the air creates an obstacle for the sound waves and no cleaning takes place. For this reason it must be assured that all hollow spaces are filled with cleaning fluid, which means that the parts should be oscillated or rotated within the bath. As a result of the fact that component shapes are becoming more and more complex, ultrasonic cleaning systems are being equipped with oscillators at multiple locations, i.e. on the floor and the side walls.
Applicable with All Cleaning Agents
Ultrasonic cleaning is part of the wet chemical cleaning techniques and is applicable with aqueous cleaning agents as well as solvents.
Cleaning baskets fabricated of round wire enable all-around and good access to the parts for the ultrasonic waves and cleaning media.
For the selection of an adequate cleaning agent the maxim "like dissolves like" (similia similibus solvuntur) applies. This means, if the chipping process is carried out with a non-polar cutting fluid, e.g. mineral oil or grease, a solvent is usually the right choice. Chips and particles, which are polar contaminations, forfeit their adhesion to the workpiece surface through the removal of the oil or grease and will be carried away by the ultrasonic cleaning effect.
Polar substances such as aqueous coolants, polishing pastes, chips, friction, and salt, dissolve in aqueous cleaning agents (polar media). These media are available as pH-neutral, alkaline and acidic cleaning agents. Usually the compatibility of the material to be cleaned and the achievement of required cleaning results are adjusted by cleaning tests conducted in the test facilities of the system or cleaning agent manufacturer. In order to provide for stable cleaning results, aqueous cleaning processes require frequent process monitoring. It is recommended to check process parameters, e.g., concentration of detergent, temperature, ultrasonic power, rinsing water quality and filter runtime.
Polar solvents and modified alcohols combine the advantages of aqueous cleaning agents and solvents.
Ultrasonic cleaning processes using solvents as a cleaning agent are carried out in fully closed, single-chamber cleaning systems with integrated distillation units for reconditioning the solvent.
By using ultrasound in combination with a cleaning agent which is matched to the respective type of contamination cleaning time can be reduced by as much as 90%. At the same time, cleaning agent consumption can also be reduced. The wrong influence would affect the process stability and cleaning result in a negative way. For example, if very oily parts are cleaned by an aqueous cleaning process, bath lifespan can be reduced significantly, the required cleanliness will often not be achieved and at the same time process costs will increase.
System Concepts for Ultrasonic Cleaning Processes
Ultrasonic cleaning is mostly used in single-chamber and multi-tank dipping systems. Single-chamber cleaning systems are equipped with a closed working chamber, and one or more tanks for fluids. The parts to be cleaned are fed into the chamber in baskets as bulk goods or in workpiece-carriers. The working chamber is then successively flooded with the cleaning and rinsing fluid and, where necessary, with preservation or passivating agents. Ultrasonic can be used as needed during all steps. In the optimum case the medium will be filtered in a bypass line during filling and draining and throughout the cleaning cycle itself. Drying is likewise carried out in the chamber. In case solvents are used as cleaning agent, the ultrasonic cleaning process takes places in fully closed, single-chamber cleaning systems with integrated distillation units for reconditioning of the solvent. These allow for re-circulation of the utilized solvents.
Ultrasonic multi-tank dipping systems operated with aqueous media are frequently used for fine cleaning tasks and applications, where high throughput is required.
Multi-tank dipping systems are used only in combination with aqueous cleaning agents. These open systems allow for any number of treatment modules to be arranged in series. Since that offers a high diluting effect, fine cleaning is one typical field of application. Another one is the high throughput capacity, given that several batches can be treated in the different baths simultaneously. The parts to be cleaned are delivered to the various immersion baths using trolleys or vertical step conveyors.
Using the Optimal Cleaning Tank
The product carrier also has a significant influence on the cleaning result and duration. Whether the parts are cleaned in bulk or staged, cleaning baskets made of stainless steel round wire are recommended. They allow access to the parts for the ultrasonic waves and cleaning media.
Overly dense loading of the bath, for example parts stacked closely on top of each other, or large volumes of bulk goods, prevent the ultrasonic waves from reaching all of the workpiece surfaces to be cleaned. As a rule of thumb one can say that the surfaces to be exposed to ultrasonic waves should not be bigger than the sound emitting surface. At the same time, the mass of the part(s) should not exceed 50% of the volume of the bath.
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