Ultrasonic cleaning uses ultrasound energy to remove contaminate particles from the surface of an object that requires cleaning. It is proving an effective replacement for traditional cleaning methods that employ the use of toxic solvents, acid baths, high-pressure water jets and hand scrubbing.
There are five major advantages of ultrasonic cleaning that is today widely recognised:
- Consistent cleaning results that are not dependent on operator skill or temperament.
- Work, health and safety benefits for operators with hands-off cleaning and reduced contact with harmful or harsh chemicals.
- Increased productivity through multiple parts being cleaned simultaneously and the reduced need to disassemble components before cleaning.
- Variable and non-contact scrubbing action to remove only the contamination through frequency and power selection.
- The ability to clean hard to reach areas such as small aperture holes, or tubes such as those found in radiators that are traditionally difficult and time consuming to clean.
What is Ultrasonics?
Ultrasonics can be defined as the creation of sound waves at frequencies above 20kHz. Electrical energy can be converted to ultrasonic sound waves (above the range of human hearing of 16–20 kHz) through transducers or sonotrobes, coupled with generators, management and control electronics.
Sound acts as a source of energy, and sound waves are composed of alternating zones of high and low pressure which alternately compress and stretch the liquid structure. Cavities form in the liquid in the stretched zone, creating empty volumes into which molecules of liquid migrate as a vapour, thus creating bubbles of vapour.
These bubbles are then subjected to those same vibrational stresses within the liquid, increasing in internal pressure and temperature as they accumulate more vapour during each vibration cycle.
A point is reached where the bubbles are unable to retain any more vapour and they eventually collapse. The internal pressure in the bubbles at the point of collapse is approximately 2000 atmospheres (30,000psi) and the temperature is about 50,000°C.
At the point of collapse, the unstable bubbles release energy causing shock waves which transfer focused mechanical, heat, and vibrational energy to the surrounding liquid. Typically, the high-shear energy wave that results from the collapse of each bubble in the liquid medium and at the surface of solid boundaries can travel at some 570 km/h.
PHOTOGRAPH OF A ‘CAVITATION BUBBLE’ RECORDED BY PROFESSOR LAWRENCE CRUM, DIRECTOR, CENTER FOR INDUSTRIAL AND MEDICAL ULTRASOUND, UNIVERSITY OF WASHINGTON, USA. THE AUTHORS ARE GRATEFUL TO PROFESSOR CRUM FOR PROVIDING THE IMAGE FOR PUBLICATION.
The role of ultrasonic chemicals
Detergents function two ways in the ultrasonic cleaning process:
They are surface wetting agents and as such improve the wetting of both the oscillating surfaces of the cleaning bath and the work piece. This ensures that there is intimate contact between the transmitter of ultrasonic energy – the oscillating surfaces of the bath – and the liquid which transmits this energy in the form of cavitation bubbles to the work piece. The intimate wetting of the work piece ensures that cavitation bubbles can be created at the surfaces to be cleaned
The detergent also assists in the removal of surface contaminants by wetting the contaminant particles and “dissolving” oily substances which may be present.
Because of their molecular structure they are best used in very small concentrations. In significant concentrations they can reduce the surface tension of the water too much, thus reducing the power of the cavitation bubbles by causing them to collapse prematurely.
There are many different types of detergents – foaming and non-foaming, anionic and non-ionic – and different contaminants require a detergent type and concentration specific to that contaminant. Formulation of cleaning solutions containing detergents is therefore carried out by our technical team and should always be used as directed.
Importance of degassing before use
Commonly, public water supplies contain microscopic air bubbles. These bubbles absorb ultrasonic energy which could otherwise have been used to create cavitation bubbles.
Water used in ultrasonic baths are therefore degassed with ultrasound before use. Ultrasound causes these microscopic bubbles to join together to form larger bubbles that then float to the surface where they collapse.