What Is Ultrasonic Cleaning?

Ultrasonic cleaning is the use of ultrasonic cavitation, acceleration and direct flow in liquid to directly and indirectly affect the liquid and the dirt, so that the dirt layer is dispersed, emulsified and peeled to achieve the purpose of cleaning. In the ultrasonic cleaners currently used, cavitation and direct flow are more widely used.

Chinese name: Ultrasonic cleaning
Foreign name: ultrasonic cleaning
Principle: Particle oscillation

History development: Megasonic
Constitute: Ultrasound system, etc.
nickname: Ultrasonic decontamination

Ultrasonic cleaning principle

Ultrasound is a sound wave whose frequency exceeds the human hearing range by more than 20 kHz. The propagation of ultrasonic waves depends on the elastic medium. When propagating, the particles in the elastic medium oscillate, and the energy is transmitted through the medium in the direction of the ultrasonic wave. This wave can be divided into longitudinal waves and transverse waves. In solids, both can be transmitted, while in gas and liquid, only longitudinal waves can be transmitted. Ultrasound can cause particle vibration, acceleration of particle vibration and ultrasonic frequency

Ultrasonic cleaning The square of the rate is directly proportional. Therefore, tens of kilohertz of ultrasound will produce a great force. When strong ultrasound propagates in a liquid, it will cause acoustic cavitation due to nonlinear effects. The shock wave emitted when the cavitation bubble suddenly closes can generate thousands of atmospheric pressures around it, which directly and repeatedly impacts the dirt layer, on the one hand, it will destroy the adsorption of dirt and the surface of the cleaning part, and on the other hand, it will also cause the dirt layer. Breaks off the surface of the cleaning element and disperses them into the cleaning solution. Bubble vibration can also scrub solid surfaces. Bubbles can also drill into cracks to make vibrations, which can cause dirt to fall off. For greasy dirt, due to ultrasonic cavitation, the two liquids are rapidly dispersed and emulsified at the interface. When solid particles are wrapped in oil and adhere to the surface of the cleaning part, the oil is emulsified and the solid particles fall off. In the process of cavitation bubbles, the liquid itself will circulate, so-called sound flow. It can make high velocity gradient and viscous stress on the surface of vibrating bubbles, promote the destruction and shedding of dirt on the surface of cleaning parts, and the high-speed microjet generated by ultrasonic cavitation on solid and liquid surfaces can remove or weaken the boundary dirt It corrodes the solid surface, increases the stirring effect, accelerates the dissolution of soluble dirt, and strengthens the cleaning effect of chemical cleaning agents. In addition, ultrasonic vibration causes a large particle velocity and acceleration in the cleaning solution, and also causes frequent and intense impact on the dirt on the surface of the cleaning part. ^[1]

Factors affecting ultrasonic cleaning

Cleaning medium: Ultrasonic cleaning, there are generally two types of cleaning agents, namely chemical solvents and water-based cleaning agents. The chemical action of the cleaning medium can speed up the ultrasonic cleaning effect. Ultrasonic cleaning is a physical effect. The combination of the two effects allows the object to be fully and thoroughly cleaned.

Power density: The higher the power density of the ultrasonic wave, the stronger the cavitation effect, the faster the speed, and the better the cleaning effect. Chemical, corrosion.

Ultrasound frequency: suitable for rough, dirty, initial washing of workpieces, high frequency, strong directivity of ultrasound, suitable for delicate objects cleaning.

High temperature cleaning: Generally speaking, the cavitation effect of ultrasonic waves is the best at 50 ° C ~ 60 ° C. The higher the temperature of the cleaning agent is, the more significant the effect is, and it may fail at high temperature. Usually, when the ultrasonic wave exceeds 85 ° C , The cleaning effect has deteriorated. Therefore, in practical application of ultrasonic cleaning, the working temperature of 50 ° C ~ 70 ° C is used. ^[1]

History of ultrasonic cleaning

Ultrasonic cleaning technology first appeared in the early 1930s. At that time, a technician at a laboratory of American Radio Company in New Jersey, USA tried to clean certain objects with a homemade simple ultrasonic cleaning system, but the test was unsuccessful. On this basis, the ultrasonic cleaning technology has greatly developed in the 1950s, and the ultrasonic working frequency used at that time was between 20 and 40 kHz. Ultrasounds in this range are used in thousands of different working situations, many of which are situations where other cleaning methods do not work well. Ultrasonic waves can apply a very large amount of energy to the workpiece, and are particularly suitable for removing dirt that is firmly attached to the substrate. However, in some cases, the strong energy of ultrasonic waves can also damage the fouled and fragile substrate materials. In the past ten years, there have been some technological innovations in the field of ultrasound, which have improved the safety factor of removing dirt on sensitive substrates. During this period, the ultrasonic technology, especially the medium and high frequency ultrasonic cleaning technology, has made new developments and has become a bright spot in the industry.

In recent years, people have found that using megasonic waves (depending on the frequency of ultrasonic waves, 40 kHz and below are called conventional or low-frequency ultrasonic waves, and 1000 kHz and above are called high-frequency ultrasonic waves, also known as mega-frequency ultrasonic waves, referred to as mega-sonic waves. The cleaning can remove the ultrafine dirt particles on the surface of the semiconductor material, and it will not damage the surface of the base material. This technology has quickly gained popularity. ^[1]

Ultrasonic cleaning process

Generally speaking, the cleaning process depends on the ease and quantity of the objects being cleaned. The main cleaning process is as follows:

1) Hot dip or spray wash: The purpose is to soften, separate and dissolve the pollutants on the workpiece, and reduce the load of the next cleaning process.

2) Ultrasonic cleaning: Use the strong cavitation and vibration generated by ultrasonic waves to peel off the dirt on the surface of the workpiece, and at the same time decompose and emulsify the greasy dirt.

3) Cold rinsing: Use the flowing clean water to wash off the dirt that has fallen off but still floating on the surface of the workpiece.

4) Ultrasonic rinsing: The solvent is clean water. After the workpiece is immersed, the ultrasonic waves will be used to clean the dirt floating on the sides, corners and pores of the workpiece.

5) Rinse with hot and cold water: further remove the dirt particles hanging on the surface of the workpiece.

6) Hot air drying: Use a certain temperature and wind speed to quickly dry the surface of parts. ^[2]

Composition of ultrasonic cleaning machine

The ultrasonic cleaning machine consists of the following parts:

1) Ultrasound system: including transducer and ultrasonic generator.

(1) Transducer: The transducer is fixed with a special high-temperature, vibration-resistant, high-viscosity resin glue supplemented by a special method, never falling off, and can withstand high temperatures of 100 ° C to 150 ° C.

(2) Ultrasonic generator: The high-power ultrasonic generator can be composed of super audio IGBT power electronic devices as the main components. This type of ultrasonic generator has an advanced circuit, a complete structure, and a sensitive and reliable integrated control system. Various ultrasonic generators can work independently or in multiple groups in parallel to complete large-scale cleaning projects.

2) Heating and temperature control system. The heater is usually made of stainless steel pipe, which can resist acid and alkali. The purpose of heating is to heat the cleaning agent to increase the washing effect of the washing machine. Automatic temperature control, can be adjusted at will within the appropriate range.

3) Cleaning tank: The cleaning tank is generally made of stainless steel and welded by argon arc welding. The tank body is provided with a slag discharge inspection port and a heat insulation sound insulation layer. The water level should be at least 200 mm higher than the transducer box.

4) Tank liquid circulation filtration system. A filter is provided in the system to dynamically filter the tank liquid to maintain the cleanliness of the tank liquid. When the workpiece comes out of the tank, the filtered liquid flows through the spraying part of the upper part of the tank to rinse the workpiece once, in order to flush out the oil stains adhered to the surface when the workpiece exits the tank, so as to avoid contamination of the next tank liquid.

5) Conveying system: Determine the conveying method and control method of the ultrasonic cleaning machine according to the shape, volume and batch of the workpiece to be cleaned.

6) Spray rinsing system: According to the surface condition of the workpiece being cleaned, some washing machines are equipped with a spray rinsing process, which organically combines ultrasonic cleaning and spray cleaning.

7) Drying system: According to the condition of the workpiece being cleaned, some washing machines are equipped with a drying system. The drying system is mainly controlled by a heater, a fan, and a blowing nozzle.

Ultrasonic cleaning cleaning agent

Technical characteristics of ultrasonic cleaning agent

The cleaning effect is good, the cleanliness is high, and the cleanliness of all the workpieces is consistent.

Fast cleaning speed, improve production efficiency, no manual contact with cleaning liquid, safe and reliable.

Deep holes, fine seams and hidden parts of the workpiece can also be cleaned.

No damage to the surface of the workpiece, saving solvent, heat energy, work place and labor.

The ultrasonic cleaning method exceeds the conventional cleaning methods in general, especially the surface of the workpiece is more complicated, such as mechanical parts with uneven surfaces and blind holes, and some particularly small products with high requirements on cleanliness, such as clocks and watches. Precision machinery parts, electronic components, circuit board components, etc., can achieve very good results using ultrasonic cleaning. The principle of ultrasonic cleaning is that the high-frequency oscillation signal sent by the ultrasonic generator is converted into high-frequency mechanical oscillation by the transducer and propagated to the medium-cleaning solvent. The ultrasonic wave is densely and equally radiated forward in the cleaning liquid, making the liquid Flowing to produce tens of thousands of tiny bubbles.

These bubbles form and grow in the negative pressure area where the ultrasonic waves propagate longitudinally, and close rapidly in the positive pressure area. In this process known as the "cavitation" effect, the bubble closure can form an instantaneous high pressure in excess of 1,000 atmospheres, and the instantaneous high pressure is continuously generated like a series of small "explosions" that continuously impact the surface of the object, making the surface The dirt in the gap is quickly peeled off, so as to achieve the purpose of purifying the surface of the object.

The mechanism of ultrasonic cleaning mainly has the following aspects: Because a strong shock wave is generated when the cavitation bubble bursts, a part of the dirt layer is peeled off, dispersed, emulsified, and peeled off under the action of the shock wave. The air bubbles generated by cavitation penetrate the gaps and voids between the dirt layer and the surface layer formed by the impact. Due to the small bubbles and sound pressure expanding and contracting simultaneously, the physical force like peeling repeatedly acts on the dirt layer, dirt The layers are peeled off layer by layer, and the air bubbles continue to penetrate inward until the dirt layer is completely peeled off. This is the secondary effect of cavitation. Impact of cleaning solution ultrasonic vibration on dirt in ultrasonic cleaning. Ultrasonic accelerated chemical cleaning agent (RT-808 ultrasonic cleaning agent) dissolves dirt, combining chemical and physical forces to accelerate the cleaning process.

The main parameters of ultrasonic cleaning

Frequency: 20 ~ 90KHz

Cleaning medium: Ultrasonic cleaning, generally two types of cleaning agents: chemical solvents, water-based cleaning agents (RT-808 ultrasonic cleaning agent), etc. The chemical action of the cleaning medium can accelerate the ultrasonic cleaning effect. Ultrasonic cleaning is a physical action. The two functions are combined to fully and thoroughly clean the object.

Power density: power density = transmission power (W) / emission area (cm2) is usually 0.3W / cm2. The higher the power density of the ultrasound, the stronger the cavitation effect, the faster the speed, and the better the cleaning effect. However, for precision, high surface finish objects, long-term high power density cleaning can cause "cavitation" corrosion on the surface of the object.

Ultrasonic frequency: The lower the ultrasonic frequency, the easier the cavitation generated in the liquid, the greater the intensity and the stronger the effect. It is suitable for the initial washing of workpieces (rough and dirty). If the frequency is high, the directivity of the ultrasonic wave is strong, which is suitable for fine object cleaning.

Cleaning temperature: Generally speaking, the cavitation effect is best when ultrasonic is at 30 -40 . The higher the temperature of the cleaning agent, the more significant the effect. Generally, when ultrasonic waves are actually used, a working temperature of 50 ° C-70 ° C is used.

Application field of ultrasonic cleaning

Ultrasonic cleaning is widely used in surface spray treatment industry, machinery industry, electronics industry, medical industry, semiconductor industry, watch and jewelry industry, optical industry, textile printing and dyeing industry. Other industries, etc., and provided by Yijing ultrasonic cleaning, as follows:

1. Surface spraying treatment industry: (cleaning attachments: oil, mechanical chips, abrasives, dust, polishing wax) remove carbon deposits before plating, remove oxide scale, remove polishing paste, remove oil and rust, clean before ion plating, Phosphating treatment, metal workpiece surface activation treatment, etc. Stainless steel polishing products, stainless steel knives, tableware, knives, locks, lighting, hand decorations before spray coating, cleaning before plating.

2. Machinery industry: (Cleaning attachments: cutting oil, abrasive particles, iron filings, dust, fingerprints)

Removal of rust-proof grease; cleaning of measuring tools; degreasing and derusting of mechanical parts; engines, engine parts, gearboxes, shock absorbers, bearings, grease nipples, cylinder bodies, valve bodies, carburetor and automotive parts and chassis paint Cleaning before degreasing, derusting, and phosphating; dredging and cleaning of filters, piston parts, and filters. Precision machinery parts, compressor parts, camera parts, bearings, hardware parts, molds, especially in the railway industry, are very suitable for degreasing and decontamination of train cabin air conditioners, and for rust prevention, derusting, and degreasing of various train head components.

4. Pharmaceutical industry: (Cleaning attachments: blood, gelatin, dust, fingerprints, blood stains, proteins; chemical synthesis vibrational dissolution in pharmaceutical research and development, etc.)

Disinfection, sterilization, cleaning of syringes, surgical instruments, droppers, research and experimental appliances, glass containers, dental appliances, esophagus mirrors, bronchoscopes, rectal mirrors, microscopes, etc.

5. Semiconductor industry: (Cleaning attachments: blood, gelatin, dust, fingerprints, blood stains, proteins) High-cleanness cleaning of semiconductor wafers.

6. Watch and jewelry industry: (Cleaning attachments: paint, water, grease, dyes, plastic residues, dust, fingerprints)

Remove dust, oxide layer, polishing paste, precious metals, decorations, gauges, watch bands, watch cases, hands, digital dials, sludge, etc.

7. Optical industry: (cleaning attachments: paint, water, oil, dye, rust, plastic residue, dust, fingerprints)

Glass lenses, resin lenses, microscopes, telescopes, camera lenses, coated glass, prisms, lenses, and other optical products are cleaned before and after coating and before assembly. In the optoelectronic industry, they are mainly used for ITO conductive glass, LCD substrate cleaning, and liquid crystal chip packaging Residual LCD cleaning.

8. Textile printing and dyeing industry: (cleaning attachments: fingerprints, dust, ink, dyes, plastic residues, rubber residues) ,,

Cleaning, spinneret, drawing board, textile spindle, fiber wire (stainless steel wire, nickel wire, copper wire, etc.) for degreasing and decontamination

9, other industries: (cleaning attachments: hand tar, dust, fingerprints, sweat stains) seals, number plates, coins, high-grade pottery, silver products, gold products, bank magnetic cards, etc. ^[1]

New development of ultrasonic cleaning

With the expansion of the scope of application, new developments have also been made in ultrasonic cleaning technology. Traditional ultrasonic cleaning equipment is difficult to ensure the uniformity of parts cleaning due to the low degree of automation. In recent years, automatic ultrasonic cleaning equipment with high automation and strong flexibility has gradually appeared. Not only realize the automatic control and batch operation of ultrasonic cleaning, but also stabilize the cleaning process and improve the cleaning quality. This type of ultrasonic cleaning equipment combines ultrasonic cleaning with chemical cleaning, rinsing, dehydration, drying and other processes, so it has very high cleaning efficiency. In the aspects of transmission, drying and cleaning, PLC control is usually used to achieve full automation of the cleaning process.

With the implementation of the "Overall Elimination Plan for China's ODS in the Cleaning Industry", the ODS organic solvents previously used in the cleaning industry will be gradually prohibited. Hydrocarbon solvents are popular because of their many advantages and their economic and environmental protection. However, the flash point of hydrocarbon solvents has limited its application in ultrasonic cleaning. The introduction of ultrasonic hydrocarbon vacuum cleaning in the field of ultrasonic cleaning not only overcomes the disadvantages of hydrocarbon solvents, but also achieves the purpose of environmental protection. Moreover, ultrasonic hydrocarbon cleaning under vacuum can also strengthen the cleaning effect, improve the cleaning effect and achieve a high degree of automation. Ultrasonic hydrocarbon vacuum cleaning has become the development direction of environmentally friendly ultrasonic cleaning in the future.

In recent years, there has also been a special application of ultrasonic cleaning-ultrasonic descaling. Ultrasonic descaling mainly uses the ultrasonic strong sound field to treat the fluid, which causes a series of changes in the physical form and chemical properties of the scale-forming substances in the fluid under the action of the ultrasonic field, making it dispersed, crushed, loose, loose and difficult to adhere to the tube, Dirt builds up on the wall. As a special application of ultrasonic cleaning, ultrasonic antiscaling is widely used in the field of antiscaling and descaling of boilers, heat exchange devices and pipes.

In addition, Ren Jinlian and others also proposed a new method of ultrasonic cleaning, ultrasonic vibration cleaning method, which is an ultrasonic vibration cleaning method that does not require a cleaning liquid and is different from the conventional ultrasonic cleaning mechanism. This method makes use of the characteristics that ultrasonic waves can cause great acceleration and force on the particles of the medium when they propagate in the solid medium.The ultrasonic waves are transmitted to the workpiece to be cleaned through the horn and the vibrating head, so that the workpiece's medium particles are at a high speed in the equilibrium position. Vibration causes the dirt to be loosened and detach from the workpiece, thereby achieving the purpose of cleaning. At present, the ultrasonic vibration cleaning device has been used in a kinescope automatic production line. ^[3]