the main mechanism of the ultrasonic cleaner is ultrasonic cavitation. The intensity of ultrasonic cavitation is related to acoustic parameters, the physicochemical properties of the cleaning solution and environmental conditions. How can the cleaning effect be made more obvious?
1. Selection of ultrasonic sound intensity or sound pressure
In the cleaning liquid, negative pressure only occurs when the amplitude of the alternating sound pressure exceeds the static pressure of the liquid. In the ultrasonic cleaning tank, the sound intensity must be higher than the cavitation threshold to generate ultrasonic cavitation. For general liquids, the cavitation threshold is approximately 1/3 watt per square centimeter (the sound pressure is proportional to the sound intensity). When the sound intensity increases, the ratio of the maximum radius to the initial radius of the cavitation bubble increases, and the cavitation intensity also increases. That is, the higher the sound intensity, the more intense the cavitation, which is conducive to the cleaning effect. But it's not the case that the greater the sound power, the better; the sound intensity is too high. A large number of useless bubbles will be generated, increasing scattering attenuation and forming a sound barrier. At the same time, the increase in sound intensity will also increase nonlinear attenuation. All these will weaken the cleaning effect in areas far from the sound source. For some stubborn dirt that is difficult to clean thoroughly, such as oxides on metal surfaces and dirt in the holes of chemical fiber spinnerets, a higher sound intensity is required. At this time, the surface to be cleaned should be close to the sound source. In this case, trough cleaners are mostly not used. The rod-shaped focused transducer is directly inserted into the cleaning solution close to the surface of the cleaning object for cleaning. 2. Frequency Selection
The ultrasonic cavitation threshold is closely related to the frequency of ultrasonic waves. The higher the frequency, the higher the cavitation threshold. In other words, the higher the frequency, the greater the sound intensity or sound power required to generate cavitation in a liquid. At low frequencies, cavitation is prone to occur. Meanwhile, at low frequencies, the compression and thinning effects on liquids have longer time intervals. This enables the bubbles to grow to a larger size before they collapse, increasing the intensity of aeration and facilitating the cleaning effect. At present, the working frequencies of ultrasonic cleaners are roughly divided into three frequency bands based on the objects to be cleaned. Low-frequency ultrasonic cleaning (20-50KHz), high-frequency ultrasonic cleaning (50-200 KHZ), and megahertz ultrasonic cleaning (700KHz - above 1MHz). Low-frequency ultrasonic cleaning is suitable for the surfaces of large components or situations where the bonding strength between dirt and the surface of the cleaning object is high. At the lower end of the frequency, the cavitation intensity is high, which is prone to corrode the surface of the cleaning parts and is not suitable for cleaning components with high surface finish. Moreover, the cavitation noise is large. At a frequency of around 40KHz, under the same sound intensity, the number of cavitation bubbles generated is greater than that at 20KHz, with stronger penetrating power. It is suitable for cleaning workpieces with complex surface shapes or blind holes, and the cavitation noise is relatively small. However, its cavitation intensity is relatively low, making it suitable for cleaning situations where the adhesion between dirt and the surface of the object to be cleaned is weak. High-frequency ultrasonic cleaning is applicable to the fine cleaning of computers and microelectronic components, such as disks, drives, read-write heads, liquid crystal glass and flat panel displays, micro-components and polished metal parts, etc. These cleaning objects are required not to be subject to cavitation corrosion during the cleaning process. It should be capable of washing away dirt at the micrometer level. Megahertz ultrasonic cleaning is suitable for cleaning integrated circuit chips, silicon wafers and thin films, etc. It can remove micron and sub-micron scale contaminants without causing any damage to the cleaned parts because no cavitation occurs at this time. Its cleaning mechanism mainly involves the effects of sound pressure gradient, particle velocity and sound flow. Its characteristic is strong directionality in cleaning, and the parts to be cleaned are generally placed in a direction parallel to the sound beam. 3. The influence of the physical and chemical properties of the cleaning solution on the cleaning effect: The selection of cleaning agents should be considered from two aspects: on the one hand, cleaning agents with good chemical action effects should be chosen based on the nature of the dirt; On the other hand, it is necessary to select a cleaning agent with appropriate surface tension, vapor pressure and viscosity, as these properties are related to the strength of ultrasonic cavitation. When the surface tension of a liquid is high, cavitation is less likely to occur. However, when the sound intensity exceeds the cavitation threshold, the energy released by the collapse of cavitation bubbles is also large, which is conducive to cleaning. Liquids with high vapor pressure will reduce the cavitation intensity, and liquids with high viscosity are also less likely to generate cavitation. Therefore, detergents with high vapor pressure and high viscosity are not conducive to ultrasonic cleaning. In addition, both the temperature and static pressure of the cleaning solution have an impact on the cleaning effect. When the temperature of the cleaning solution rises, the number of cavitation nuclei increases, which is beneficial for the generation of cavitation. However, if the temperature is too high, the vapor pressure in the bubbles increases, and the cavitation intensity will decrease. Therefore, the selection of temperature should take into account the influence on the cavitation intensity simultaneously. It is also necessary to consider the chemical cleaning effect of the cleaning solution. Every liquid has a temperature at which cavitation is most active. The most suitable temperature for water is 60-80℃, during which cavitation is most active. When the static pressure of the cleaning solution is high, cavitation is less likely to occur, so the effect of ultrasonic cleaning or treatment in a closed pressurized container is relatively poor. 4. Other factors Affecting the Ultrasonic cleaning Effect
The flow rate of the cleaning solution also has a significant impact on the ultrasonic cleaning effect. It is best for the liquid to remain stationary during the cleaning process, at which point the growth and closure movements of bubbles can be fully completed. If the flow rate of the cleaning solution is too fast, some cavitation nuclei will be carried away by the flowing liquid, while others will leave the sound field before reaching the entire process of growth and closure movement, thus reducing the overall cavitation intensity. In the actual cleaning process, sometimes to prevent dirt from re-adhering to the cleaned items. The cleaning solution needs to be constantly updated. At this time, it should be noted that the flow speed of the cleaning solution should not be too fast to avoid reducing the cleaning effect. The acoustic characteristics of the items to be cleaned and their arrangement in the cleaning tank also have a significant impact on the cleaning effect. Cleaning objects with high sound absorption, such as rubber and fabric, have poor cleaning effects, while cleaning items with strong sound reflection, such as metal parts and glass products, have good cleaning effects. The smaller side of the cleaning parts should face the sound source and be arranged at a certain distance. The items to be cleaned must not be placed directly at the bottom of the cleaning tank. Especially for heavier cleaning parts, it is necessary to avoid the vibration of the bottom plate of the groove and also to prevent the cleaning parts from scratching the bottom plate and accelerating cavitation corrosion. It is best to hang the cleaned items in a tank or hold them in a metal basket and hang them. But it should be noted that it should be made of metal wire. And as much as possible, use fine wires to make baskets with larger gaps to reduce sound absorption and shielding. The content of gas in the cleaning solution also affects the ultrasonic cleaning effect. If there is residual gas (non-cavitation nucleus) in the cleaning solution, it will increase the loss of sound propagation. In addition, the gas that diffuses into the cavitation bubble during its movement will reduce the intensity of the shock wave when the cavitation bubble collapses, thereby weakening the cleaning effect. Therefore, some ultrasonic cleaning equipment has the function of degassing. When starting up, it first performs vibration at a power level lower than the cavitation threshold, and degassing is carried out through pulse or intermittent vibration. Then increase the power to the normal cleaning power level for ultrasonic cleaning.
some ultrasonic cleaning equipment is designed with a vacuum pumping device {also known as vacuum degassing or negative pressure cleaning), whose purpose is to reduce the residual gas in the cleaning solution. 5. The influence of standing waves
The cleaning tank is a confined space. When ultrasonic waves propagate from the sound source to the liquid surface. At the interface between a liquid and a gas, the sound pressure is reflected back to form a standing wave. The characteristic of a standing wave is that the sound pressure is the lowest in some areas of the liquid space and the highest in others. This will cause uneven cleaning. To reduce the influence of standing waves, sometimes the cleaning tank is deliberately made into an irregular shape to avoid the formation of standing waves. Nowadays, in terms of ultrasonic power supplies, a sweeping frequency method is adopted, which makes the point with the minimum sound pressure not fixed in one place but constantly moving. To achieve a more uniform cleaning. < / p > < p > ultrasonic cleaning machine: < a href = "http://www.kejeme.com" > www.gzkjm.com < / a > < / p >