Ultrasonic Cleaning Principles and Precautions

As is well known, the sound heard by humans consists of acoustic signals with a frequency of 20Hz to 20,000Hz. Sound waves above 20,000Hz are called ultrasonic waves. The transmission of sound waves follows a longitudinal sine curve. This means it propagates through alternating layers of high and low pressure.

When a weak acoustic signal acts on a liquid, it creates a certain negative pressure. This leads to the formation of numerous microscopic bubbles within the liquid. Conversely, when a strong acoustic signal acts on the liquid, it creates a positive pressure that crushes these tiny bubbles.

Research has proven that when ultrasonic waves act on a liquid, the rupture of each bubble generates an extremely high-energy shock wave. This is equivalent to an instantaneous high temperature and pressure of up to thousands of atmospheres. This phenomenon is known as the "Cavitation Effect." Ultrasonic cleaning utilizes the shock waves generated by these bursting bubbles to clean and scrub the internal and external surfaces of workpieces.

Classification of Sound Waves

Acoustic waves can be divided into three types: infrasonic, sonic, and ultrasonic.

• Infrasonic waves: Frequencies below 20Hz.

• Sonic waves: Frequencies between 20Hz and 20kHz.

• Ultrasonic waves: Frequencies above 20kHz.

Generally, the human ear cannot hear infrasonic or ultrasonic waves. Due to their high frequency and short wavelength, ultrasonic waves possess excellent directionality and strong penetration capabilities.

The Principle of Ultrasonic Cleaning Machines

The primary principle of an ultrasonic cleaning machine involves a transducer. This device converts the acoustic energy from a power ultrasonic frequency source into mechanical vibrations. These vibrations are radiated into the cleaning solution through the walls of the cleaning tank. Under the influence of these radiated ultrasonic waves, the microbubbles in the liquid maintain their vibration.

When the acoustic pressure or intensity reaches a certain level, the bubbles expand rapidly and then suddenly close. During this process, the moment the bubble closes, it generates a shock wave. This creates a pressure of 10¹² Pa to 10¹³ Pa around the bubble. This massive pressure generated by ultrasonic vaporization can destroy insoluble contaminants and disperse them into the solution.

The Synergy Between Physical Vibration and Chemistry

However, physical vibration alone is often insufficient to resolve complex industrial grease. In the process of ultrasonic cleaning, the formulation of chemical cleaning agents is crucial.

Traditional ultrasonic degreasing formulas often incorporate NP-10 or OP-10 (Alkylphenol Ethoxylates) as core surfactants. These raw materials significantly reduce the surface tension of the cleaning solution. This allows cavitation bubbles to be more easily generated at the oil-dirt interface and "drill" into the gaps of the soil layer. Simultaneously, their strong emulsification capacity quickly encapsulates the oil shaken off by the ultrasound, preventing secondary deposition.

With global environmental regulations such as REACH becoming increasingly stringent, traditional NP/OP-10 are being phased out due to poor biodegradability and toxicity to aquatic life. Currently, high-performance isomerized alcohol ether series (such as the C18 produced by our company) have become the industry mainstream. These alternatives retain the excellent penetration and degreasing power of NP-10 while offering superior biodegradability. They also exhibit more stable cloud point performance under high-temperature ultrasonic conditions (40°C–60°C), significantly enhancing cleaning efficiency and meeting green production requirements.

Cleaning Mechanism Analysis

Ultrasonic waves destroy the adsorption between the dirt and the surface of the cleaning part. On the other hand, they cause fatigue damage to the dirt layer, allowing it to be peeled off. The vibration of gas-type bubbles scrubs the solid surface. Once there is a gap in the soil layer, the bubbles "drill into" it immediately, causing the layer to fall off through vibration.

Due to cavitation, two immiscible liquids are rapidly dispersed and emulsified at the interface. When solid particles are wrapped in oil and adhere to the surface, the oil is emulsified, and the particles fall off on their own. As ultrasound propagates through the liquid, it generates alternating positive and negative acoustic pressures, forming jets that impact the cleaning parts. Concurrently, nonlinear effects produce acoustic streaming and micro-streaming, while ultrasonic cavitation generates high-speed micro-jets at the solid-liquid interface. All these actions destroy contaminants, remove or weaken the boundary soil layer, increase stirring and diffusion, accelerate the dissolution of soluble dirt, and strengthen the cleaning effect of chemical detergents.

Consequently, cleaning occurs wherever the liquid can reach and the acoustic field exists. This technology is particularly suitable for cleaning parts with very complex surface shapes. Most notably, adopting this technology reduces the consumption of chemical solvents, thereby greatly reducing environmental pollution.

Precautions for Using Ultrasonic Cleaning Machines

1. Grounding: The power supply of the ultrasonic cleaning machine and the electric heater must have a reliable grounding device.

2. No Dry Running: It is strictly forbidden to turn on the machine without cleaning liquid. The ultrasonic switch must not be engaged unless the cleaning tank is filled with a sufficient amount of liquid.

3. Heating Safety: Cleaning equipment with heating devices is strictly forbidden from turning on the heating switch when there is no liquid.

4. Transducer Protection: Do not hit the bottom of the cleaning tank with heavy objects (iron parts) to avoid damaging the energy converter chips.

5. Voltage Stability: The ultrasonic generator power supply should use a separate 220V/50Hz line and be equipped with a voltage stabilizer of 2000W or above.

6. Tank Maintenance: The bottom of the cleaning tank must be rinsed regularly. There should not be excessive debris or dirt.

7. New Solution Procedure: Every time the liquid is replaced, wash the parts only after the ultrasound has been started (for degassing).