Ultrasonic cleaning

It is a deep surface cleaning process that uses ultrasound to agitate a fluid with a cleaning cavitating effect. Ultrasonic tanks are available in different sizes, from small portable or benchtop units with an internal volume of less than 0.5 litres, to large industrial units with volumes approaching 1,000 litres, which can be integrated with automated multi-stage washing systems.

The principle of these ultrasonic devices is to convert the sound energy of the ultrasonic frequency source into mechanical vibrations through a transducer. Transducers are usually piezoelectric (e.g. made from lead zirconate titanate, barium titanate, etc.), but are sometimes magnetostrictive. The vibration generated by the ultrasonic wave is transmitted to the detergent liquid through the wall of the washing tank so that the microbubbles in the contained liquid can continue to vibrate under the action of the sound wave, destroying and separating the dirt adsorbed on the surface of the object. The bubbles collapse with enormous energy, reaching high temperatures and pressures on the surfaces involved.

The lower frequencies (20-25 kHz) are best for cleaning large artefacts, for example automotive components and have enough strength to remove grease and stubborn dirt, present for example in engine blocks and moulds. They are clearly not suitable for delicate surfaces or with particular finishes.

For most applications, 40 kHz is the best choice because it represents the best balance between power and cavitation bubble size. This is why it is used in over 90% of all industrial ultrasonic cleaning systems. Cavitation bubbles at 40 kHz are about one micron in size, small enough to fit into small cracks and blind holes. It's also powerful enough to remove stubborn contaminants yet gentle enough for all but the most fragile materials.

The higher frequencies (68 kHz to 170 kHz) should be used on items that require particularly gentle, sub-micron cleaning. These higher frequencies produce submicron-sized cavitation bubbles that can navigate into the smallest cracks. Higher frequencies are often used to clean pharmaceutical equipment, medical implants, titanium components, delicate electronics and precision optics.

Depending on the morphology of the object and the type of dirt to be removed, the process can be very rapid, providing optimal cleaning in just a few minutes; in other cases, longer times or repetitions of treatment are necessary. The ultrasonic process is quick and effective, on the other hand it has a considerable energy cost and a greater initial investment in the plant than a normal washing tunnel.

In summary, ultrasonic treatment is successfully used to clean a multitude of manufactured goods, including industrial parts, jewelery and trinkets, mechanical components, tools, small parts, firearms, fuel injectors, etc. Most hard, non-absorbent materials (metals, plastics, etc.) and not chemically attacked by the cleaning liquid are suitable for ultrasonic cleaning.

The process is preferably suitable for the removal of dust, dirt, oil, pigment, rust, grease, algae, fungi, bacteria, limescale, polishing compounds, fluxing agents, fingerprints, wax, soot, release agents, biological soil such as blood, and so on.

Objects should not rest on the bottom of the device during the cleaning process, as this would prevent cavitation on the part of the object not in contact with the fluid.

Cleaning solutions

In some circumstances, ultrasonic cleaners can be used with only normal water, but in most cases a cleaning solution is used. This solution must be designed to maximize the effectiveness of ultrasonic cleaning, guaranteeing speed of dissolution, penetration and lowering of surface tension to aid cavitation.

When cleaning metals, are usually used alkaline cleaning solutions to allow the removal of contaminants typical of this sector, i.e. stubborn oils and greases. The solutions are generally heated, even around 50–65 °C. The surfactants used for this application must have a good degree of ethoxylation as well as stability suitable for medium-high temperatures, thus avoiding the floating of the surfactant particles. Foam formation is generally a secondary problem since the ultrasonic waves will automatically continuously disturb the solution, breaking the bubbles and keeping any kind of foam under control (up to a certain percentage of use).

Due to the high level of evaporation and vapors developed during the process, another characteristic must be considered when choosing the surfactant, namely its eco-compatibility and non-toxicity towards the operators.

In addition to detergent solutions and surfactants, mixtures of water and soluble hydrocarbons with a low degree of volatility or 100% hydrocarbon solutions can be used (historically, toxic solvents such as carbon tetrachloride and trichloroethane were used industrially, but have been gradually eliminated). The use of mixtures based only on hydrocarbons (such as high molecular weight glycols, ethylene or propylene carbonates) limits evaporation and reduces the possibility of oxidation of the pieces to a minimum; on the other hand, after the treatment it will be necessary to use more energy for drying the pieces.

In the opposite way, acid solutions can be used to pickle metals, if traditional treatments are not sufficient, for example due to excessively extensive oxidation or particular deposits inside pipes and complex artefacts. The acid will greatly amplify the pickling and oxidizing action, therefore a minimum and suitable percentage will be needed so as not to re-oxidize the piece immediately after extraction from the bath; corrosion inhibitors, buffered acids or mixtures of organic acids are almost mandatory.

Even for paint stripping, the ultrasound system is advantageous as with a minimum consumption of chemical product it is possible to have a rapid removal of the paint film, even on tough multilayers. Thanks to the power of the waves, slightly acidic or alkaline solutions can be used, containing small amounts of co-solvents and surfactants.

Safety

Ultrasonic cleaners emit irritating high-frequency noises and even in case of temporary exposure hearing protection is required.

Avoid using flammable cleaning solutions, the temperature increases during the process even if no heat is administered (some industrial units are specifically certified as explosion-proof).

When the unit is operating, contact with the cleaning solution may cause thermal or chemical injury; The action of ultrasound is relatively benign to living tissue but can cause discomfort and skin irritation.

The devices are electrically powered, the operator must therefore be careful when working with liquids (especially during the extraction or handling of the pieces) to avoid spills and create unwanted short circuits.