Ultrasonic cleaning is a process that uses non-audible sound waves (usually from 20 to 400 kHz) and a suitable cleaning solvent (occasionally ordinary tap water) to rapidly clean objects. The objects are placed in a tank with liquid flooded with high-frequency ultrasound energy that gently removes any contaminants from the surface of the item.
Simply put, ultrasonic cleaners work similarly to automatic dishwashers, although obviously the process is slightly more refined. While automatic dishwashers are used for pans and bowls, ultrasonic cleaners are utilized for items difficult or impossible to clean using other methods.
Cavitation or how does ultrasonic cleaning work?
The secret of ultrasonic cleaning is a process called cavitation. Cavitation is a result of ultrasonic energy which is produced by converting high-frequency electrical energy with the help of a transducer placed at the bottom of the tank.
The high-frequency sound waves (or ultrasonic energy) enter the fluid in the tank, and that causes the rapid formation and collapse of small bubbles – a process known as cavitation. These small bubbles move at high velocity inside the tank which makes them implode (not explode) on the surface of any item immersed, releasing a reasonable amount of energy.
As the bubbles continue imploding and cavitation occurs, the cleaning solvent moves into the gap left behind by bubbles, mildly removing any dirt, tarnishing, and contaminants from both the surface and the most intricately shaped parts of the object.
There are more than enough variables to take into account when cleaning items using ultrasonic cleaners: heat, frequency, power, cleaning solvent type, and time – all of these affect the process, but we are safe to assume that the core of the device is the transducer.
The ultrasonic equipment’s ability to clean even the most persistent contaminants from objects is determined by the transducer. Therefore, the cleaning power of the ultrasonic cleaner derives from the transducer’s performance, making this component a key part of the entire process.
There are two main types of transducers: the piezoelectric (or electrostrictive) transducer and the magnetostrictive one.
Both types are sensitive to the current provided by the unit’s generator and start vibrating at ultrasonic frequencies which then cause the bottom and the sides of the tank to vibrate. This process makes the tank serve as a membrane, and the vibration forms the bubbles that implode on contact with items immersed in the ultrasonic cleaner.
Another important part of the ultrasonic cleaning process is the ultrasonic frequency as this is what determines the size of the bubbles.
Lower frequencies like 25 kHz (or 25,000 cycles per second) produce significantly larger bubbles that implode more vigorously than the bubbles created at higher frequencies such as 80 or 130 kHz that are at the root of a more gentle cleaning movement.
To remove contaminants from items made of cast iron or metal, one should use lower frequency cleaners whereas softer metals or objects made of plastic and items with polished surfaces should be cleaned at higher frequencies. That’s because smaller bubbles manage to penetrate difficult areas such as crevices and small holes and protect polished surfaces.
Where do the contaminants go?
To continue with our automatic dishwasher analogy – in the case of the dishwasher, the dirt goes down the drain. However, in the case of the ultrasonic cleaner, the contaminants remain in the cleaning solution – they either sink to the bottom or float to the surface or simply remain in suspension in the liquid, depending on what kind of cleaning solution is used.
Because of this, the efficiency of the ultrasonic equipment will eventually decline, and when that happens, the solvent must be drained and disposed of along with the contaminants, according to the manufacturer’s instructions and local regulations.
Ultrasonic cleaning applications
Ultrasonic cleaning is extensively used across many industries, from jewelry and watchmakers workshops to electronic repair shops. They can also be used in hospitals, factories, and other similar businesses.
Ultrasonic cleaners are generally customized to suit a certain type of items. For example, tabletop and benchtop cleaners are excellent for cleaning small or delicate objects like laboratory glass and plasticware, surgical instruments, optical parts (like telescope mirrors), or carburetor components.
On the other hand, precision cleaning is mostly used for fragile items that have been damaged by other cleaning methods (like coins, gramophone records or jewelry).