Technical Report RT-ATCP-02
Ultrasonic cleaning: overview and state of the art
ATCP Physical Engineering ww.atcp-ndt.com / [email protected] São Carlos - Brazil
Author: Antônio Henrique Alves Pereira (Pereira A.H.A.)
[Reviewed and published online on April 5 of 2010]
The necessity of cleaning systems for the removal of contaminants is present in various sectors, from the industrial, passing through services up to the hospital sector, mainly for maintenance of equipment and devices, for products surface preparation for reuse or for cleaning processes during the manufacture. The main types of cleaning systems can be divided into 8 groups :
- By solvents
- By emulsions
- By bath of molten salts
- By pickling
The selection of the cleaning system is usually determined by the following variables:
- Nature of the contaminant
- Nature of the object to be cleaned
- Degree of cleanliness required
- Geometry of the objects to be cleaned
- Quantity and frequency
- Necessity of automated processes
- Environmental restrictions and standards
- Costs and budget available
A detailed discussion of all aspects of industrial cleaning processes can be found in ASM Handbook, Volume 5 - Surface Engineering. Of the mentioned systems, the cleaning by bath of molten salts and by solvents have suffered a highlighted decline in recent decades in developed countries due to increasing restrictions on the use of toxic and aggressive substances to the environment. The main alternative to the use of these processes has been the acid, alkaline or emulsion cleaning, together with the mechanical and ultrasonic cleaning. Besides being less harmful to the environment and health, these processes facilitate the deployment of automated systems.
In Brazil, until the 90s, the most sophisticated cleaning systems (automated and ultrasonic) were imported and used, mostly, by multinational companies who bought them in their countries of origin. The simplest systems were developed and used by small and medium enterprises, are often similar to the first imported analog versions. With the acceleration of globalization in the 90s and the ecological constraints, there arose a great demand among small and medium enterprises for more efficient cleaning systems that collaborate with the increasing competitiveness of its products and to be environmentally friendly, such as ultrasonic and automated systems for medical and hospital applications. This new market niche has already been successfully explored in Brazil for several Brazilian and foreign companies.
The technology of ultrasonic cleaning uses the cavitation and momentum transfer, induced phenomena’s by the propagation of acoustic waves of high intensity, with frequencies above the human audible limit (approximately 18 kHz) in liquid media. It is the most efficient non-abrasive method for cleaning and do not uses chemical dissolution of the substrate. Associated with other methods such as alkaline, acid or emulsion's cleaning, the ultrasonic cleaning is capable of removing more complex contaminants without compromising the integrity or damage the surface that is being cleaned, is particularly effective in cleaning objects with cavities, holes and recesses. Today is extensively used in the metalworking, automotive, aerospace and optical industries, for the removal of metallic and fatty residues of machining process and handling.
In Figure 1 we can see a typical industrial system for ultrasonic cleaning. There are three tanks in series: The first performs a coarse cleanup (with ultrasound of 25 kHz), the second removes microscopic particles that resist the action of the first (with ultrasound of 40 kHz), and the third one rinse, avoiding drying of any remnants of previous solutions of the tanks (which would limit the efficiency of the process). In this system, the ultrasonic sources are coupled to the bottom of the tanks and excited by a generator. The automation of this configuration is not complex, just a robot with two degrees of freedom to perform the movement of baskets of parts over the three tanks.
In Table I we can see a comparison between the main cleaning processes used in the metalworking industry . Undoubtedly, the ultrasonic cleaning is the state of the art in efficiency and repeatability, and leaves no doubts about its convenience and cost of operation, but is the most expensive technology.
Despite high costs, the demand for ultrasonic systems is growing rapidly, driven mainly by the previously mentioned environmental restrictions of the other technologies. The world's largest manufacturers are multinationals like Crest, Branson and Amsonic. In Brazil, stand out Unique and CTA of Brazil, and several manufacturers of small equipment for laboratories and clinics (equipment with just one tank of up to 5 liters).
MACROSCOOPICAL ASPECTS OF CLEANLINESS
Cleanliness, in general, consist in remove permanently contaminants from a substrate, which may be the surface of any object. To carry out cleaning, you must perform work to remove the contaminants, breaking chemical bonds and overcoming the force of electrical attraction and Van der Waals, and ensure that this removal is permanent, preventing the electrical attractive force re-deposit contaminants . Therefore, the cleaning of a substrate is not a simple task, especially if: The required degree of cleanliness is high, the contaminants are chemically inert, object to be cleaned has recesses and cavities or cannot undergo chemical or mechanical abrasion.
In cleaning systems by ultrasound, who performs the job of removing contaminants and keep them away from the substrate (usually with chemical help) are two phenomena of the propagation of high-intensity sound: Cavitation and momentum transfer. The macroscopic behavior of these phenomena in the cleaning process are as follows :
Increasing the dispersion and dissolution of solid films and liquids Erosion
Fatigue and breakdown of contaminant layers
Removal of air bubbles of small pits and grooves
Cavitation is the main effect on systems that operate at frequencies up to 100 kHz, and momentum transfer in systems operating at frequencies near 1 MHz (known as megasonics systems).
Dispersion and increase of the dissolution of solid films and liquids
Figure 2 shows the initial and evolved condition of a substrate immersed in a static chemical bath. Since the action of the bath only occurs at the interface, via dissolution, as the bath reacts with the contaminant forms a saturated layer, leading to reduced dissolution rate or even the stagnation of the process, making essential the mechanical action. When the object to be cleaned has a complex geometry, with cavities and crevices, often the mechanical agitation generated by air bubbles, propellers or agitators, is not enough, requiring the use of ultrasound.
With the presence of an ultrasonic field of high intensity in the liquid medium, the phenomenon of cavitation takes place, which we can briefly describe as the appearance of vapor bubbles that collapse, generating large differential point of pressure and temperature. In Figure 3, a pictorial representation of the action of cavitation in the dispersion of saturated bath layer and in the mechanical removal of the contaminant.
Among the myriad of contaminants, we have the chemically inserts, which are the most difficult's to remove due to the requirement of using vigorous mechanical action. For these applications, the ultrasonic cleaning systems are particularly suitable because of the erosion generated by cavitation, avoiding direct contact of the object to be cleaned with brushes or other external physical agents. See Fig.4.
In addition to cavitation, another phenomenon of propagation of intense ultrasonic fields has been explored, the momentum transfer, which becomes important at frequencies above 1 MHz. In those systems these contaminants are removed by the shear force generated by a noise jet (See Fig.5). It is also this phenomenon the principle of ultrasonic nebulizers.
Removal of layered contaminants and air bubbles
In his propagation, the ultrasonic wave generates the expansion and contraction of air bubbles that may be trapped in holes and cavities, often preventing a thorough cleaning of the object by hindering the access of the chemical bath. These cycles leads to fatigue of layered contaminants and facilitates the removal of bubbles trapped by surface tension (during the expansion, volume increases, and consequently, the buoyant force that removes the bubble), see Figure 6. The phenomena of cavitation and the momentum transfer will be detailed and explained later.
 COTELL,C.M.;SPRAGUE,J.A.;SMIDT,F.A.;ASMHandbook,Volume5/Surface Engineering: The Materials Information Society.