Ultrasonic cleaning

Ultrasound

A sound may merely be mechanical vibrations in solid, liquid and gaseous compressible media. As the sound waves propagate, the particles of the medium approach and distance one another. As a result, areas of reduced and increased pressure arise. TA frequency rate is represented by the number of fluctuations in density or pressure per second. The frequency of ultrasonic waves begins at the upper limit of the human hearing range, therefore at 20 kHz.

Use of ultrasound

• Echographs and echocardiographs
• US cleaners and US washing lines
• US stone breakers
• US steel cutters
• Sonars
• US sirens
• US welding, cutting, crushing
• US evaporators
• US coating thickness gauges, materials and irregularities in them
• Etc.

Ultrasonic cleaning

Ultrasonic cleaning is one of the most modern ways of removing dirt today. It also removes dirt in areas that are difficult to access, such as various notches, narrow channels and crevices, tiny holes, overlapping surfaces (scissors), and hollow objects with a narrow mouth such as pipettes, needles, etc. leaving the cleaned area intact. For better efficiency, there is no need either for manual work or high concentrations of detergents. It may be sufficient to use ordinary water, and water-based and nature-friendly cleaning agents.

Turbulence, heat (absorption) and acoustic flows occur in the presence of ultrasound in the liquid. At the same time, the liquid thickens and dilutes. The liquid in the diluted part decomposes due to inelasticity and internal instability caused by impurities and gases. Critically low pressure results in the formation and growth of acoustic cavitation bubbles. The bubble may be empty (incomplete vacuum), may contain gas or steam, or a combination of both. It should be noted that the boiling point is lower at a lower pressure.

In the thickened part of the liquid, so when the pressure is higher, the growth of the bubble stops and begins to decrease. Stable cavitation bubbles only change in size as the pressure changes.

However, unstable bubbles, at a certain size, which depends on the amplitude and frequency of the ultrasound, break up into smaller bubbles very fast. At the same time, they create micro-shock waves. The blows spread to the items present in the cleaning bath and thus remove dirt from them. Bubbles that do not implode at a time of the pressure change, resize or pulse. In moving so, they increase their chemical action (emulsification) and remove dirt.

When they are generated under a layer of dirt, they can break this layer by pulsing. Stable cavitation bubbles may also implode during a next cycle. In addition to the impact of the implosion, there are high local temperatures (5000 ° C), which contribute to a faster chemical reaction or faster removal of dirt.

Factors affecting the intensity of cleaning

The following are some of the factors that affect the intensity of ultrasound cleaning:

• Cleaning agent - The selected cleaner greatly enhances the effect of ultrasonic cleaning. The agent is selected according to the type of base material and the type of dirt. In general, demineralised water is also good for cleaning since it has a lack of ions. The latter are obtained from the dirt during the cleaning process.
• Liquid temperature – Temperature increase causes an increase in the chemical activity of a chemical. The elimination of trapped air in the cleaning agent is also increased. On the other hand, higher temperature also has a negative side by reducing the cleaning effect. The reasons are the decrease in the viscosity of the fluid, the decrease in surface tension and the increase in vapor pressure. Aqueous solutions have maximum efficiency at a temperature of 45 to 55 ° C. In general, the cleaning process finds some compromise with respect to the cavitation resistance of the dirt and the chemical reaction of the detergent to the dirt.
• Surface tension in water - Increasing the surface tension makes it more difficult for bubbles to grow, but the intensity of shrinkage increases. This means that the implosion emits more energy. Such liquid has a lower wetting factor, which may make the cleaning worse. A cleaning agent with a smaller wetting factor covers the object to be cleaned with more difficulty. Lower surface tension facilitates the growth of cavitation bubbles, but they emit less energy when the implosion occurs. Some mean value should be obtained. Surface tension is reduced by adding an agent that increases the wetting factor.
• Liquid density - Liquid density has very little effect on erosion activity. Liquid with a high density does not cavitate. Cavitation requires a lot of ultrasonic energy.
• Liquid viscosity - The viscosity in a liquid does not have a major effect on the cleaning intensity. At very high fluid viscosity, the shock wave pressure caused by the bubble implosion increases. A bad feature of a high viscous liquid is the poorer coating of parts that are cleaned. At the same time, more energy is required to form cavitation, since it is partially absorbed by the liquid with a higher viscosity. A higher viscosity liquid is used in cleaning dirt, which is strongly bonded to the base material and is very resistant to cavitation.
• Fluid cohesiveness - Fluid cohesiveness or an attractive interatomic (inter-molecular) force determines the cavitation point. A liquid with greater cohesiveness requires more energy input to generate cavitation.
• Vapor pressure - Vapor pressure has a significant effect on the intensity of cleaning, as it has an effect on the contraction energy of the bubble. Higher vapor pressure significantly reduces the intensity of the micro-shock wave upon implosion. The lower the vapor pressure, the higher the intensity, but it can pass to prevent implosion with insufficient amount of ultrasonic wave energy (stable cavitation bubble).
• Liquid gas content - Liquid with high dissolved gas content has a lower cleaning efficiency. The mixture of gas and vapor in the cavitation bubble increases the pressure inside it and decreases its impact force upon implosion. The type of gas, or its degradability, also plays an important role. In ultrasonic cleaning, the properties of the gas above the surface of the liquid in which the ultrasonic washing is performed must also be analyzed. This is especially important in closed cleaning systems where increased static pressure is applied.
• Effect of static pressure on erosion activity - Higher static pressure increases the power of the cavitation bubble while also reducing its amount. Increasing static pressure requires more ultrasonic energy.
• Quantity of items in the cleaning tub - The quantity of items in the cleaning tub should not exceed 50% of the volume of the cleaning liquid. In addition to know the occupancy of the cleaning tub, the placement of the items to be cleaned in the bath is also important. It is necessary to know where the ultrasonic transducers are located.
• Ultrasound frequency - The size of the frequency generated by the ultrasonic generator depends on the size of the cavitation bubbles. As frequency increases, more cavitation bubbles are produced, but they are smaller. As a result, they are able to emit less energy. The higher frequency thus enables the cleaning of more sensitive items (semiconductor technology, delicate jewelry ..), or is more in the domain of rinsing an already cleaned object. Also, a higher frequency is more effective in cleaning up dirt particles of the order of a few µm. However, a lower frequency is more useful for dirt with strong cohesion forces and base material cohesion forces. It should be borne in mind that strong cavitation also discharges the base material and that, at a lower frequency, the distribution of cavitation bubbles in the bath is less uniform.
• Signal amplitude (power) or acoustic pressure - Increasing power also increases cavitation power. By increasing the acoustic pressure, the growth time of the cavitation bubble increases, which may exceed the interval of its decrease. As a result, the amplitude of the shock wave generated by the imploded bubble decreases (suffocates).
• Laying of the items to be cleaned – The items have to be fully submerged inside a liquid. It is recommended that they be at least a few inches away from the liquid surface. They should also not be placed directly on the bottom of the bath, as this can prevent them from releasing energy into the water. As a result, high temperatures can be generated on the converters, which can damage them. The items to be cleaned should be at least spaced so that the cleaning agent passes between them. Spacing is particularly important in sensitive areas where scrubbing of the two pieces can result in visible scratches.

Ultrasonic cleaning recommendations

• Before first washing in a fresh liquid, it is recommended that the ultrasound be operated for at least 15 minutes or more to allow the water to degas (degassing).
• It is recommended to wash in the temperature range between 45 - 55ºC, where the maximum cleaning effect is.
• When washing hollow objects and tubes, care must be taken to ensure that the surface is completely covered by water. Care must be taken that no air pockets are present.
• After ultrasound washing, the objects should be rinsed under running water. Demineralized water is recommended. The transfer of items must be carried out as quickly as possible in order not to dry them.
• For heavily soiled items, ultrasonic washing with a strong cleaner and then rinsing with running water is recommended. The next step should be followed by ultrasonic washing with a less aggressive cleaning agent, and rinsing with demineralized water and drying with warm air.
• If there are many tiny ducts on the item to be cleaned that may contain water droplets, it is advisable to purge the item with filtered compressed air after rinsing and drying it with hot air.
• Regular cleaning liquid change is recommended. The interval is determined by validation of the cleaning process.
• It is recommended that the washing be validated and that at least once a year the proper cavitation strength is measured in an ultrasonic cleaner.
• It is not advisable to pour cold water (or vice versa) into the hot bath, as it will shorten the life of the ultrasonic cleaner.
• It is not recommended to place items directly on the bottom of the bath, as this will reduce the effect of ultrasonic washing and may also damage the cleaner.