Ultrasonic Cleaner

Ultrasonic cleaning uses cavitation bubbles induced by high frequency pressure (sound) waves to agitate a liquid. The agitation produces high forces on contaminants adhering to substrates like metals, plastics, glass, rubber, and ceramics. This action also penetrates blind holes, cracks, and recesses. The intention is to thoroughly remove all traces of contamination tightly adhering or embedded onto solid surfaces. Water or solvents can be used, depending on the type of contamination and the workpiece. Contaminants can include dust, dirt, oil, pigments, rust, grease, algae, fungus, bacteria, lime scale, polishing compounds, flux agents, fingerprints, soot wax and mold release agents, biological soil like blood, and so on. Ultrasonic cleaning can be used for a wide range of workpiece shapes, sizes and materials, and may not require the part to be disassembled prior to cleaning.

Objects must not be allowed to rest on the bottom of the device during the cleaning process, because that will prevent cavitation from taking place on the part of the object not in contact with solvent.

Ultrasonic cleaning is a process that uses ultrasound (usually from 20–400 kHz) and an appropriate cleaning solvent (sometimes ordinary tap water) to clean items. The ultrasound can be used with just water, but use of a solvent appropriate for the item to be cleaned and the type of soiling present enhances the effect. Cleaning normally lasts between three and six minutes, but can also exceed 20 minutes, depending on the object to be cleaned.

Ultrasonic cleaners are used to clean many different types of objects, including jewelry, lenses and other optical parts, watches, dental and surgical instruments, tools, coins, fountain pens, golf clubs, fishing reels, window blinds, firearms, car fuel injectors, musical instruments, gramophone records, industrial parts and electronic equipment. They are used in many jewelry workshops, watchmakers' establishments, and electronic repair workshops.

Ultrasonic Principle

In an ultrasonic cleaner, the object to be cleaned is placed in a chamber containing a suitable solution (in an aqueous or organic solvent, depending on the application). In aqueous cleaners, surfactants (e.g., laundry detergent) are often added to permit dissolution of nonpolar compounds such as oils and greases. An ultrasound generating transducer built into the chamber, or lowered into the fluid, produces ultrasonic waves in the fluid by changing size in concert with an electrical signal oscillating at ultrasonic frequency. This creates compression waves in the liquid of the tank which ‘tear’ the liquid apart, leaving behind many millions of microscopic ‘voids’ or ‘partial vacuum bubbles’ (cavitation). These bubbles collapse with enormous energy; temperatures and pressures on the order of 5,000 K and 135 MPa are achieved; however, they are so small that they do no more than clean and remove surface dirt and contaminants. The higher the frequency, the smaller the nodes between the cavitation points, which allows for cleaning of more intricate detail.

Ultrasonic transducers showing ~20 kHz and ~40 kHz stacks. The active elements (near the top) are two rings of lead zirconate titanate, which are bolted to an aluminium coupling horn.

Transducers are usually piezoelectric (e.g. made with lead zirconate titanate (PZT), barium titanate, etc.), but are sometimes magnetostrictive. The often harsh chemicals used as cleaners in many industries are not needed, or used in much lower concentrations, with ultrasonic agitation. Ultrasonics are used for industrial cleaning, and also used in many medical and dental techniques and industrial processes.

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