First, the choice of power
The ultrasonic cleaning effect is not necessarily proportional to (power × cleaning time). Sometimes, with low power, it takes a long time to remove dirt. However, when the power reaches a certain level, the dirt can be removed quickly. If the power is too high, the cavitation intensity increases significantly, improving the cleaning effect. But for more precise parts, this may cause etching, which is not worth the loss. Additionally, severe cavitation at the bottom of the cleaning tank can lead to increased water spot corrosion. When using organic solvents like trichloroethylene, there are generally no issues, but with water or water-soluble cleaning solutions, water spots are more likely to form. If the surface of the vibrating plate has scratches, cavitation corrosion can become severe under high power. Therefore, the ultrasound power should be chosen based on actual usage conditions.
Second, the choice of frequency
Ultrasonic cleaning frequencies range from 28 kHz to 120 kHz. The physical cleaning force caused by cavitation when using water or water-based cleaning agents is particularly beneficial at lower frequencies, typically around 28-40 kHz. For parts with small gaps, slits, and deep holes, higher frequencies (generally 40 kHz or above, even several hundred kHz) are better. When cleaning watch parts, 400 kHz is commonly used. Using broadband FM cleaning can also enhance the cleaning effect.
Third, the use of the cleaning basket
When cleaning small parts, a basket is often used. However, the mesh of the basket can cause ultrasonic attenuation. It is better to use a mesh of 10 mm or larger when the frequency is 28 kHz.
Fourth, the choice of cleaning fluid temperature
The most suitable temperature for water-based cleaning solutions is 40-60°C. In cold weather, if the solution is too cold, the cavitation effect is poor, leading to reduced cleaning performance. Therefore, some washing machines have heating elements to control the temperature. As the temperature rises, cavitation becomes easier, resulting in better cleaning. However, if the temperature continues to increase, the gas pressure inside the cavities rises, reducing the impact sound pressure. This reflects the balance between these two factors.
Fifth, determining the amount of cleaning fluid and the location of the cleaning parts
Generally, the liquid level of the cleaning fluid should be more than 100 mm above the surface of the vibrating body. Since single-frequency cleaning machines are affected by standing wave fields, the amplitude at the nodes is smaller, while at the antinodes it is larger, causing uneven cleaning. Therefore, the best place for the cleaning items is at the antinode.
Sixth, the ultrasonic cleaning process and selection of cleaning fluid
Before purchasing a cleaning system, it's essential to analyze the parts to be cleaned: define the material composition, structure, and quantity of the parts, and identify the contaminants to be removed. These are crucial for deciding the cleaning method and whether to use an aqueous solution or solvent. A cleaning experiment is also necessary to verify the final process. Only then can a proper cleaning system, well-designed process, and suitable cleaning solution be provided. The physical properties of the cleaning fluid, such as vapor pressure, surface tension, viscosity, and density, significantly affect ultrasonic cleaning. Temperature also influences these factors, thereby affecting cavitation efficiency.
Any cleaning system must use a cleaning fluid. When choosing a cleaning solution, consider three main factors:
1. Cleaning efficiency: Choose the most effective solvent through experimentation. Introducing ultrasound into an existing process usually doesn’t require changing the solvent.
2. Simplicity: The liquid should be safe, non-toxic, easy to operate, and have a long service life.
3. Cost: The cheapest solvent isn't always the most cost-effective. Consider its efficiency, safety, and how much can be cleaned. The solvent must also be compatible with the material being cleaned. Water is common, easy to use, and widely applied, but for certain materials or soils, other solvents may be more appropriate.
Seventh, different cleaning fluids require distinct cleaning systems
An aqueous system typically consists of an open trough, with the workpiece submerged. More complex systems include multiple tanks, circulation filtration, rinsing, and drying units. A solvent system usually involves an ultrasonic vapor phase degreaser, often equipped with waste recovery. The process uses a multi-slot system combining a solvent evaporation tank and an ultrasonic immersion tank. Solvent-soluble soils like oil, grease, and wax are removed by hot solvent vapor and ultrasonic agitation. After cleaning, the workpieces are heated, cleaned, and dried.
Eighth, processing of cleaning parts
Another consideration is the design of the tooling or fixture that holds the cleaning parts. When placed in the ultrasonic tank, the parts and the basket must not touch the tank’s bottom. The total cross-sectional area of the parts should not exceed 70% of the tank’s cross-section. Materials like rubber and non-rigid plastics absorb ultrasonic energy, so they need careful handling. Insulating parts also require special attention. Poorly designed baskets or heavy workpieces can greatly reduce the efficiency of the cleaning system. Hooks, shelves, and beakers can be used to support the parts.
Ninth, cleaning time, type of workpiece, and quantity
The machine's working mode—fully automatic, semi-automatic, or manual—should be considered. Machine size and cost are also important factors.
Tenth, other considerations
Parts heavily soiled are usually pre-cleaned by dipping or spraying before ultrasonic cleaning. This ensures better results. For small or complex-shaped parts, using a net or rotating the object can help achieve even ultrasonic radiation.
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