An ultrasonic cleaner's prime mover.
During a negative-going pressure event, with contamination present in the aqueous medium, vapour is formed which, during subsequent positive going pressure events, is transformed into micron-sized bubbles. Once formed, these are alternatively expanded and compressed. Since more energy is taken into a bubble during a negative-going pressure event than is pushed out during a positive-going pressure event, the bubble continues to expand until its resonant size is attained, whereupon the bubble implodes. Collectively such bubbles become the prime (re)movers of contaminant and oxides from a |dirty' surface. The imploding phenomenon occurs whether the ultrasonic pressure wave is generated by piezoelectric (PZT) or magnetostrictive (MST) transducers.
When these micron-sized bubbles reach the climax of implosion, it is possible to create the temperature existing on the sun's surface, the pressure existing in the deepest ocean trench and the rate of cooling of molten metal splattered onto a liquid helium cooled surface.
In an implosion event lifetime of only a few millionths of a second, the temperature and pressure within an imploding bubble can reach 5500 [degrees] C and 4.92 x [10.sup.6] kg/sq meter respectively. The tongue-like micro-sized jet appearing at the centre of an imploding bubble manifests itself only when the bubble is close to a surface and exists only for the duration of the implosion event. This jet is propelled towards the surface at speeds of 400 km/hr. It's little wonder then that no manmade aqueous pressure application device can mimic the microscopic cleaning capability released by propagating an alternating ultrasonic pressure wave through an aqueous medium.
The temperature and pressure generated during bubble implosion is independent of bubble size. While a larger bubble contains more energy per implosion event the generated temperature and pressure will be no greater than that existing in a smaller bubble.
Bubble size is inversely proportional to the applied ultrasonic frequency. The higher the frequency, the smaller the bubble. A 20 kHz bubble at resonant size is 150 microns in diameter. Imploding bubble events are otherwise known as cavitation. This exists in two states, stable and transient. In the former the bubble simply oscillates in size in sympathy with the applied frequency. The latter connotes the imploding event. Stable cavitation usually but not necessarily precedes transient cavitation.
On the basis of energy content, a 20 kHz implosion event lasts longer than an 80 kHz implosion event, but we're only talking a few microseconds difference. Practically speaking, over the frequency range 20 to 80 kHz, all ultrasonic cleaners, if properly designed, are inherently capable of the same degree of intensity and therefore cleaning effect.
At an applied ultrasonic frequency of 20 kHz, cavitation events can be induced in tap-water with ultrasonic intensities lower than 1 watt/ [cm.sup.2]. A 1 watt/[cm.sup.2] intensity corresponds to an alternating pressure of [+ or -] 1.8 atmosphere. If the applied ultrasonic frequency is increased above 20 kHz, correspondingly higher levels of electrical input power are required by the transducer to generate the same level of alternating pressure in an aqueous medium.
Transient cavitation is a mixed blessing. While it is solely responsible for the effectiveness of this type of ultrasonic cleaning, it is also responsible for the presence of cleaning tank wall or bottom etching, particularly in the vicinity of the transducers. It is intuitively obvious that the microscopic jet bombardment of a surface is primarily responsible for surface cavitation erosion wear-through. Some manufacturers will not warrant their ultrasonic cleaning system against this occurrence. If, however, the tank wall or bottom to which the transducers are attached is thick enough, then cavitation erosion is self-arresting and in time ceases. Manufacturers able to employ thick wall tanks are also able to warrant their ultrasonic cleaners against cavitation erosion wear-through.
Microscopic jet formation cannot get established over an area unless that area is flat to an extent several times larger than the size of the resonant bubble. The act of etching and eroding caused by the presence of these microscopic jets leads, in time, to the destruction and reduction of flat tank surface areas, leading in turn eventually to the termination of cavitation erosion at all eroded surface locations.
Thus we know that over the frequency range of 20 kHz to 80 kHz cleaning action should be virtually independent of frequency. Nevertheless, to obtain the same level of cleaning action, a lower less electrical input power than a high frequency cleaner. We know also that in the vicinity of the transducers the tank wall or bottom must be thick enough to resist the combined effects of cavitation erosion and transducer tank bottom or wall flexural stressing. The only practical way of knowing whether a tank is thick enough is to require its manufacturer to warranty against cavitation erosion wear-through.
This brings us to why today two different ultrasonic transducer activation technologies still exist. Some 20 years ago it was reported by a prestigious university that the ceramic-like material comprising a PZT transducer was inherently more efficient at transducing electrical energy into displacement amplitude than was a nickel-metal MST transducer. Incidentally, too, the PZT transducer and its associated generator would cost less to manufacture.
As a result, most of the manufacturers in the ultrasonic cleaning industry at that date switched to PZT technology. Today, PZT technology dominates the ultrasonic cleaning marketplace worldwide.
In selecting one transducer technology or the other, the end-user should set aside the superficial 20-year-old material transduction efficiency factor and instead consider all factors relevant to a desirable ultrasonic cleaning device; viz,
* A very low level of induced pressure intensity is required to stimulate transient cavitation in an aqueous medium. This intensity is easily obtainable from either transducer technology.
* In ultrasonic transient cavitation cleaning, only the imploding bubble is responsible for the resulting cleaning effect. A manufacturer can create the conditions necessary for transient cavitation to occur in an aqueous medium but cannot control the resulting cleaning event once implosion is initiated, except for making the imploding events uniform throughout a tank.
* Today, as it has always done, transduction efficiency (electrical input power to pressure intensity W/[cm.sup.2] measured in the aqueous medium) depends more on transducer mounting methods and acoustic matching techniques than it does on the inherent transducing efficiency of the material comprising the transducer.
* Today, modern MST transducing efficiency, as defined above, is at least equivalent to that available from PZT and incidentally the level of associated acoustically radiated noise is just as low.
* Relative to PZT, MST transducers are: impervious to mechanical shock, impervious to very high temperature exposure; MST transduction efficiency remains constant throughout the transducer's lifetime; MST transducers exhibit MTBF's (lifetimes) well in excess of two trillion hours; and MST transducers operate at low electrical voltages.
Particularly for highly reliable industrial ultrasonic cleaning applications and end use is advised to examine the attributes of both PZT and MST ultrasonic cleaners since while initial cleaning performance may be identical between PZT and MST, consistent cleaning action, over the long term, resides with MST.
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|Title Annotation:||Pre-treatment, Deburring and Blasting|
|Author:||Vago, Robert E.|
|Date:||Mar 1, 1992|
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