Some components, such as those manufactured of highly-polished aluminum, can quickly be damaged by ultrasonic cleaning action due to cavitational erosion of the parts, a condition where the ultrasonic scrubbing action actually erodes the surfaces of the parts.  When these parts are cleaned in a standard ultrasonic cleaner, the surface of the parts appears mottled, and covered with a pattern of small comet-like spots.  These "comets" are created when the ultrasonic activity essentially drills holes in the surface of the objects being cleaned.  The head of the comet is the location of most intense ultrasonic activity, while the tail of the comet represents the direction that the fluid was blasted away during the cleaning action. The distance between neighboring "comets" and the degree of damage produced will be dependent upon the ultrasonic frequency in use.

When ultrasonic cleaning action strikes the surface of a part, neighboring liquid is blasted away from the part in a direction which is dependent upon the angle between the ultrasonic cavitation, and the surface of the part.  If the cavitation produced is perfectly perpendicular to the surface of the part, the neighboring liquid is displaced evenly around the central strike location, thereby producing a circular spot of erosion.  However, this is rarely the case.  In most cases, cavitational action is not perfectly perpendicular to the surface of the parts.  As such, neighboring fluid tends to be displaced in a specific direction, which yields the comet-like appearance found.  Damage can be produced after only a few minutes of ultrasonic cleaning, depending upon the sensitivity of the parts being cleaned, cleaning agents in use, and operational ultrasonic frequency.

When cleaning sensitive materials, Zenith typically installed higher ultrasonic frequencies in the system to produce a more evenly distributed cleaning action which is less prone to the "hotspots" that are produced in lower frequency ultrasonic systems.  In addition, parts are typically oscillated, either continuously or intermittently, to scan the object past areas of intense ultrasonic activity rather than allowing the parts to remain stationary.  This is very similar to heating food in a microwave oven.  By moving the object, a more evenly distributed cleaning effect is produced which can completely prevent damage to sensitive devices in most cases.

Other objects, such as very thin glass, certain semiconductors, and other similar objects may also be damaged by the ultrasonic activity produced.  The use of higher operational frequencies combined with oscillation can prevent damage to these components as well, except possibly the cleaning of thin glass.  These materials may fracture by sonic resonance rather than by cavitational erosion. 

Painted objects may also be damaged by ultrasonic cleaning action.  Since the ultrasonic cleaning system can not differentiate between contamination and paint, the ultrasonic activity will evenly attack both paint and contamination.  It is quite possible that paint having a weak bond to the surface, or paint on corners or in crevices of the object, will be removed during the ultrasonic cleaning process.  Since this damage is caused by cavitational erosion, the same remedy applies; higher ultrasonic frequencies combined with oscillation.

Parts can also be damaged by the misapplication of cleaning fluids for a particular application.  Aluminum and brass require the use of cleaning fluids which are specifically formulated for these metals to prevent darkening or oxidation of the part surface.

When customers approach Zenith to clean a potentially sensitive components, Zenith requests that parts are submitted for ultrasonic testing to ensure that damage potential can be eliminated prior to the purchase of an ultrasonic cleaning system.

Parts which May be Damaged by Ultrasonic Cleaning:

• Highly Polished Softer Metals such as Aluminum and Magnesium
• Thin Glass, Quartz, or Silicon Wafers.
• Painted Objects
• Damage can be Prevented by Oscillating Parts during Ultrasonic Cleaning to Scan items past Standing Waves (hotspots) in the Fluid.

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Zenith Mfg. & Chemical Corp.
85 Oak St.
Norwood, NJ  07648-0412
800-432-SONIC (7664)
201-768-6999
sales@zenith-ultrasonics.com