ULTRASONIC CLEANING SELECTION

The principle for ultrasonic cleaning machines is that an oscillating signal at a frequency higher than 20 kHz is firstly generated and amplified by an ultrasonic generator, and after being converted into high-frequency mechanical vibration energy by the inverse piezoelectric effect of the ultrasonic transducer (shock), it further forms an acoustic radiation during its spreading in the medium--cleaning solution to make the cleaning liquid molecules vibrate and produce numerous tiny bubbles. The bubble forms and grows in the negative pressure zone along the ultrasonic propagation direction, rapidly closes in the positive pressure zone to generate an instantaneous high pressure of thousands of atmospheres, and blasts to form numerous microscopic high-pressure shock waves acting on the surface of the workpiece to be cleaned. This is the "cavitation effect" in ultrasonic cleaning. Ultrasonic cleaning machine is based on the basic principle of this “cavitation effect”. Therefore, ultrasonic cleaning is excellent for workpieces with complex internal and external structures, microscopic uneven surfaces, slits, small holes, corners, dead angles, and dense components, with an effect that is unmatched by other cleaning methods. As the ultrasonic frequency increases, the number of bubbles also increases and the blasting impact is weakened. Therefore, the high-frequency ultrasound is particularly suitable for the cleaning of small particles without damaging the surface of the workpiece, including electronic FPCB printed circuit board cleaning agent, environmentally friendly industrial metal cleaning agent, electrolytic ultrasonic cleaning solution, etc.

EXPANSION AND BURSTING OF CAVITATION BUBBLES (IMPLOSION)

Bubbles are generated by applying high frequency (ultrasonic frequency), high intensity sound waves to a liquid. Therefore, any ultrasonic cleaning system must have three basic components: a tank for holding the cleaning solution, a transducer for converting electrical energy into mechanical energy, and an ultrasonic generator for generating high-frequency electrical signals.

TRANSDUCER AND GENERATOR

The most important part of the ultrasonic cleaning system is the transducer. There are two types of transducers, one being the magnetic transducer made of nickel or a nickel alloy, and the other, the piezoelectric transducer made of lead zirconate titanate or other ceramics. When a piezoelectric material is placed in an electric field of varying voltage, it deforms, which is called the 'piezoelectric effect'. Relatively speaking, magnetic transducers are made of materials that deform in a changing magnetic field. Regardless of what kind of transducer is used, the most basic factor is the strength of the cavitation effect it produces. Ultrasound, like other sound waves, is a series of pressure points, a wave of alternating compression and expansion. If the sound energy is strong enough, the liquid is pushed away during the expansion phase of the wave, thereby generating bubbles; while during the compression phase of the wave, the bubbles burst or impinge instantaneously in the liquid, producing a very effective impact force, which is especially suitable for cleaning. This process is called cavitation.

SOUND WAVE COMPRESSION AND EXPANSION

Theoretically, a burst of cavitation bubbles produces a pressure of more than 10,000 psi and a high temperature of 20,000 °F (11,000 °C), and the shock wave radiates outward rapidly at the moment of its burst. The energy released by a single cavitation bubble is small, but there are millions of cavitation bubbles bursting at the same time every second. Therefore, the cumulative effect is very strong, and the powerful impact it produced will peel off the dirt on the surface of the workpiece. This is the hallmark of all ultrasonic cleaning. If the ultrasonic energy is large enough, cavitation will occur everywhere in the cleaning solution, therefore the ultrasonic waves can effectively clean tiny cracks and holes. Cavitation also promotes chemical reactions and accelerates the dissolution of the surface film. However, cavitation occurs only in the region where the liquid pressure is lower than the gas pressure in the bubble, so that the ultrasonic wave generated by the transducer must be sufficiently large to satisfy this condition. The minimum power required to generate cavitation is referred to as the cavitation critical point. Different liquids have different cavitation critical points, so the ultrasonic energy must exceed the critical point to achieve the cleaning effect. That is to say, only when the energy exceeds the critical point, cavitation bubbles can be generated for ultrasonic cleaning.

IMPORTANCE OF FREQUENCY

Noise is generated when the operating frequency is low (within the human hearing range). When the frequency is below 20 kHz, the working noise not only becomes very large, but may exceed the limits of safety noise as stipulated by occupational safety and health laws or other regulations. In applications where high power is required to remove dirt without regard to surface damage, a lower cleaning frequency from 20 kHz to 30 kHz is often chosen. Cleaning frequencies in this frequency range are often used to clean large, heavy parts or high density material workpiece. While high frequencies are often used to clean smaller, more delicate parts or to remove tiny particles. Besides, high frequencies are also used in applications where damage is not allowed on the surface of the workpiece. The use of high frequencies improves cleaning performance in several ways. As the frequency increases, the number of cavitation bubbles increases linearly, producing more dense shock waves that can enter into smaller gaps. If the power remains constant, the cavitation bubbles become smaller and the energy released is correspondingly reduced, thus effectively reducing damage to the surface of the workpiece. Another advantage of high frequencies is the reduced viscous boundary layer (Ponuri effect), allowing the ultrasound to 'discover' extremely fine particles.

THE SUPERIORITY OF ULTRASONIC CLEANING

HIGH PRECISION: Since the energy of the ultrasonic waves can penetrate tiny gaps and small holes, it can be applied to any part or assembly cleaning. When the parts to be cleaned are precision parts or assemblies, ultrasonic cleaning often becomes the only choice meeting their special technical requirements;FAST: Ultrasonic cleaning is much faster than conventional cleaning methods in dust removal and descaling of workpieces. The assembly can be cleaned without disassembly. The advantage of labor-saving makes ultrasonic cleaning the most economical way to clean;CONSISTENT: Whether the parts being cleaned are large or small, simple or complex, single or batch or on an automatic assembly line, ultrasonic cleaning can achieve unmatched uniform cleanliness superior than hand cleaning.

ULTRASONIC CLEANING PROCESS AND SELECTION OF CLEANING SOLUTION

Before purchasing the cleaning system, the following application analysis should be carried out concerning the parts to be cleaned: clarify the material composition, structure and quantity of the parts to be washed, and analyze and clarify the dirt to be removed, which are all decisive prerequisites for determining what kind of cleaning method should be used and whether to apply an aqueous cleaning solution or a solvent. The final cleaning process still requires a cleaning experiment to verify. Only in this way can a proper cleaning system, a well-designed cleaning process and a cleaning solution be provided.Considering the influence of the physical properties of the cleaning solution on ultrasonic cleaning, the vapor pressure, surface tension, viscosity and density are the most significant factors among all. And since temperature can affect these factors, it also affects the efficiency of cavitation.Any cleaning system must use a cleaning solution. When it comes to the selection of it, there are three factors to consider:1. Cleaning efficiency: Experiment must be done in the selection of the most effective cleaning solvent. If ultrasound is introduced into an existing cleaning process, the solvent used usually does not have to be changed;2. Simple operation: The liquid used should be safe and non-toxic, easy to operate and long in service life;3. Cost: The cost of using the cheapest cleaning solvent is not necessarily the lowest. Lots of factors must be taken into consideration during the usage, like the cleaning efficiency and safety of the solvent, how many workpieces can be cleaned with a certain amount of solvent so as to achieve the maximum utilization, etc. Of course, the cleaning solvent chosen must achieve expected cleaning effect and should be compatible with the material being cleaned. As water is the most common cleaning solution, systems using water-based solution are easy to operate, low in cost, and widely used. However, for certain materials, as well as dirt and the like, which are not suitable for aqueous solutions, there are still many solvents to choose from.

TWO CLEANING SYSTEMS DISTINGUISHED BY DIFFERENT CLEANING SOLUTIONS

Aqueous system: usually consists of an open tank, with the workpiece submerged in it. A complex system consists of multiple tanks with a circulation filtration system, a shower tank, a drying tank, and other accessories.Solvent system: mostly ultrasonic vapor phase degreasing cleaning machine, often equipped with a waste liquid continuous recovery device. The ultrasonic vapor phase degreasing cleaning process is accomplished by an integrated multi-tank system consisting of a solvent evaporation tank and an ultrasonic immersion tank. Oil, grease, wax, and other solvent-soluble dirt can be removed by the action of hot solvent vapor and ultrasonic agitation. After a series of cleaning processes, the unloaded workpieces are heated, cleaned, and dried.

CLEANING PARTS PROCESSING

Another consideration for ultrasonic cleaning is the design of the upper and lower materials of the cleaning parts, or to say, the tooling for placing the cleaning parts. When the cleaning parts are in the ultrasonic cleaning tank, neither the cleaning parts nor the cleaning basket should touch the bottom of the tank. The total cross-sectional area of the cleaning parts should not exceed 70% of the cross-sectional area of the ultrasonic tank. Rubber and non-rigid materials absorb ultrasonic energy, so care should be taken when using such materials for tooling. Insulating cleaning parts should also be given special attention. If the tool basket is not designed properly, or the workpiece is too heavy, even the efficiency of the best ultrasonic cleaning system is greatly reduced. Hooks, shelves and beakers can be used to support the cleaning parts.