Application of Laser Cleaning Technology

Cleaning technology is often used in industrial production, heritage conservation and dental treatment. For example, in the assembly of electroplating, phosphating, spraying, welding, packaging and integrated circuit of industrial products, in order to ensure the quality of the workpiece in the next process, it is necessary to remove grease, dust, rust or residual solvents, binders and other dirt on the product surface. Due to environmental pollution and poor protection, many cultural relics and works of art are gradually being corroded and dirty. In order to restore the old appearance, it is necessary to clean the dirt and rust on the surface of cultural relics.

Traditional cleaning industrial equipment has a variety of cleaning methods, mostly using chemical and mechanical methods for cleaning. Today, as environmental protection regulations become more stringent and people are becoming more aware of environmental protection and safety, the types of chemicals that can be used in industrial production cleaning will become less and less. Laser cleaning has the characteristics of no grinding, non-contact, low thermal effect and cleaning of objects suitable for various materials. In the surface contamination of the workpiece, the bond between the workpiece surface attachment and the surface is mainly due to the following various forces. Among them, capillary force, adsorption force and electrostatic force are the most difficult to destroy, so laser cleaning is the need to overcome these forces. The smaller the size of the particles, the greater the acceleration required to remove them. This is why it is difficult to remove the surface attachments of small diameters by conventional cleaning techniques.

The traditional cleaning methods include mechanical cleaning method, chemical cleaning method and ultrasonic cleaning method. Mechanical cleaning method uses mechanical means such as scraping, rubbing, brushing, sand blasting to remove surface dirt; wet chemical cleaning method uses organic cleaning agent to pass Spraying, leaching, soaking or high-frequency vibration to remove surface adhesion property Ultrasonic cleaning method is to put the treated parts into the cleaning agent, and use the vibration effect generated by the ultrasonic waves to remove the dirt. At present, these three cleaning methods still occupy a dominant position in China's cleaning market, but their application is greatly limited under the requirements of environmental protection and high precision. The mechanical cleaning method cannot meet the requirements of high cleanliness cleaning, and it is easy to damage the surface area of ​​the workpiece to be cleaned. The chemical cleaning method is easy to cause environmental pollution, and the cleanliness obtained is also limited. Especially when the dirt composition is complicated, it is necessary to use a plurality of cleaning agents repeatedly. Cleaning can meet the requirements of surface cleanliness. Although the ultrasonic cleaning method has a good cleaning effect, it is incapable of cleaning submicron-sized dirt particles. The size of the cleaning tank limits the range and complexity of the processed parts, and the drying of the workpiece after cleaning is also a major problem.

In recent years, with the enhancement of people's awareness of environmental protection, the development of cleaning industry in the world has brought great challenges. Various cleaning technologies which are beneficial to environmental protection have emerged at the historic moment. Laser cleaning technology is one of them. The so-called laser cleaning technology refers to the use of high-energy laser beam irradiation on the surface of the workpiece, so that the surface of dirt, rust or coating instantaneous evaporation or peeling, high-speed and effective removal of the surface of clean objects or surface coating, so as to achieve a clean process. It is a new technology based on the interaction effect between laser and material. Compared with traditional cleaning methods, such as mechanical cleaning, chemical corrosion cleaning, strong impact cleaning of liquid and solid, high frequency ultrasonic cleaning, it has obvious advantages. It is efficient, fast, low cost, low heat load and mechanical load on the substrate, cleaning is non-damaging; waste can be recycled, no environmental pollution is safe and reliable, no damage to the operator health certificate can remove various thicknesses, different components of the coating sub-cleaning process easy to achieve automation control, remote remote control cleaning and so on.

The principle and method of laser cleaning are characterized by high directivity, monochrome, high coherence and high brightness. By focusing the lens and Q-switch, the energy can be concentrated in a very small space and time range. In the laser cleaning process, the following characteristics of laser are mainly used:

Laser can achieve a high concentration of energy in time and space. The focused laser beam can produce thousands or even tens of thousands of degrees of high temperature near the focus, which can instantly evaporate, gasify or decompose dirt.

The divergence angle of the laser beam is small and the directivity is good. The laser beam can be focused into spots of different diameters by the concentrating system. Under the same conditions of laser energy, controlling the laser beam spot of different diameters can adjust the energy density of the laser to cause the dirt to expand. When the expansion force of the dirt is greater than the adsorption force of the dirt on the substrate, the dirt will leave the surface of the object.

The laser beam can generate mechanical resonance by generating ultrasonic waves on the solid surface, causing the dirt to break and fall off. The laser cleaning technology utilizes the characteristics of the above laser to achieve the purpose of cleaning. According to the optical properties of the substrate to be cleaned and the dirt to be removed, the laser cleaning mechanism can be divided into two categories: one is to use a cleaning substrate (also called a mother) and a surface attachment (dirt) to a certain wavelength of laser. The absorption coefficient of energy has a large difference. Most of the laser energy radiated to the surface is absorbed by the surface deposits, which is evaporated by heat or vaporization, or instantaneously expanded, and is driven by the formed airflow to separate from the surface of the object for cleaning purposes. The substrate does not suffer damage due to the minimal energy absorption of the laser light at this wavelength. For such laser cleaning, choosing the right wavelength and controlling the laser energy is the key to safe and efficient cleaning. Another type of cleaning method is that the laser energy absorption coefficient of the cleaning substrate and the surface attachment is not much different, or the substrate is sensitive to the acidic vapor formed by the heat of the coating, or the toxic substance is generated after the coating is heated. This type of method usually uses a high power, high repetition rate pulsed laser to impact the surface being cleaned, converting part of the beam into sound waves. After the sound wave hits the lower hard surface, the returned part interferes with the incident acoustic wave generated by the laser, generating a high-energy wave, causing a small-scale explosion of the coating, the coating is pressed into a powder, and then removed by the vacuum pump, and the underlying substrate It will not be damaged.

From the method analysis, there are four kinds of laser cleaning methods: laser dry cleaning method, that is, direct pulse radiation decontamination using pulse laser; laser + liquid film method, that is, first depositing a liquid film on the surface of the substrate, and then decontaminating with laser radiation; When the laser is irradiated onto the liquid film, the liquid film is heated abruptly, causing explosive vaporization, and the explosive shock wave loosens the dirt on the surface of the substrate. And with the shock wave flying away from the surface of the processed object, to achieve the purpose of decontamination. The method of laser + inert gas, that is, while irradiating the laser, the surface of the substrate is blown with an inert gas. When the dirt is peeled off from the surface, it is immediately blown off the surface by the gas to avoid re-contamination and oxidation of the surface; the laser is used to loosen the dirt. After that, it is cleaned by non-corrosive chemical methods. Currently, the first three methods are commonly used.