Laser cleaning is an effective method to remove the solid surface of different materials and sizes of dirty particles and film layer. Through the high brightness and good directional continuous or pulsed laser, through optical focusing and spot shaping to form a specific spot shape and energy distribution of the laser beam, irradiated to the surface of the contaminated material to be cleaned, the attached contaminant materials absorb laser energy, will produce a series of complex physical and chemical processes such as vibration, melting, combustion, and even gasification, and finally make the contaminant from the surface of the material Even if the laser action on the cleaned surface, the vast majority are reflected off, the substrate will not cause damage, so as to achieve the cleaning effect. The following picture: thread surface rust removal and cleaning.
Laser cleaning can be classified in accordance with different classification standards. Such as in accordance with the laser cleaning process on the substrate surface is covered with liquid film is divided into dry laser cleaning and wet laser cleaning. The former is the direct irradiation of laser contaminant surface, the latter needs to be applied to the laser cleaning surface moisture or liquid film. Wet laser cleaning of high efficiency, but the laser wet cleaning requires manual coating of liquid film, which requires the liquid film composition can not change the nature of the substrate material itself changes. Therefore, relative to the dry laser cleaning technology, wet laser cleaning has some limitations on the scope of application. Dry laser cleaning is currently the most widely used laser cleaning method, which uses the laser beam to irradiate the surface of the workpiece directly to remove particles and thin films.
Laser Dry Cleaning
The basic principle of laser dry cleaning is the particle and the material substrate by laser irradiation, the instantaneous conversion of absorbed light energy into heat, causing the particle or substrate or both instantaneous thermal expansion, between the particle and the substrate instantly generated an acceleration, the force generated by the acceleration to overcome the adsorption between the particle and the substrate, so that the particle from the substrate surface.
According to the different absorption methods of laser dry cleaning, laser dry cleaning can be divided into the following two main forms:
1. For the melting point is greater than the parent material (or laser absorption rate differences) of the dust particles: particles absorb laser irradiation is stronger than the absorption of the substrate (a) or vice versa (b), then the particles absorb laser light energy converted to thermal energy, causing thermal expansion of the particles, although the amount of thermal expansion is very small, but the thermal expansion are in a very short period of time, so there will be a huge instantaneous acceleration on the substrate, while the substrate counter-action on the particles, the force to overcome the mutual adsorption force, so that the particles from the substrate, the principle of schematic diagram as shown in Figure 1.
2. For the lower boiling point of the dirt: surface dirt directly absorb laser energy, instant high temperature boiling evaporation, direct vaporization to remove the dirt, the principle as shown in Figure 2.
Laser Wet Cleaning Principle
Laser wet cleaning is also known as laser steam cleaning, as opposed to dry, wet cleaning is in the presence of a thin layer of a few microns thick liquid film or media film on the surface of the cleaning parts, the liquid film by laser irradiation liquid film temperature rises instantly and produces a large number of bubbles to gasification reaction, gasification explosion generated by the impact of particles and substrate to overcome the adsorption force between. According to the particles, liquid film and the substrate on the laser wavelength absorption coefficient is different, laser wet cleaning can be divided into three types.
1. Strong absorption of laser energy by the substrate
Laser irradiation onto the substrate and liquid film, the absorption of the laser by the substrate is much greater than that of the liquid film, so explosive vaporization occurs at the interface between the substrate and the liquid film, as shown in the figure below. Theoretically, the narrower the pulse duration, the easier it is to generate superheat at the junction, which results in a greater explosive impact.
2. Strong absorption of laser energy by the liquid membrane
The principle of this cleaning is that the liquid film absorbs most of the laser energy, and explosive vaporization occurs on the surface of the liquid film, as shown in the figure below. At this time, the efficiency of laser cleaning is not as good as when the substrate absorption, because at this time the explosion impact on the surface of the liquid film. While the substrate absorption, bubbles and explosions occur at the intersection of the substrate and liquid film, the explosive impact is easier to push the particles away from the substrate surface, therefore, the substrate absorption cleaning effect is better.
3. Both the substrate and the liquid membrane jointly absorb laser energy
At this time, the cleaning efficiency is very low, after the laser irradiation to the liquid film, part of the laser energy is absorbed, the energy is dispersed throughout the liquid film inside, the liquid film boiling to produce bubbles, the remaining laser energy through the liquid film is absorbed by the substrate, as shown in the figure. This method requires more laser energy to produce boiling bubbles before the explosion occurs. Therefore the efficiency of this method is very low.
Wet laser cleaning using the substrate absorption, as most of the laser energy is absorbed by the substrate, will create a liquid film and substrate junction overheating, bubbles at the interface, compared with dry cleaning, wet is the use of the junction bubble explosion generated by the impact of laser cleaning, while you can choose to add a certain amount of chemical substances in the liquid film and pollutant particles to chemical reaction to reduce the particles and substrate The adsorption force between the material, in order to reduce the threshold of laser cleaning. Therefore, wet cleaning can improve the efficiency of cleaning to a certain extent, but at the same time there are certain difficulties, the introduction of liquid film may lead to new contamination, and the thickness of the liquid film is difficult to control.
Factors Affecting the Quality of Laser Cleaning
Effect of Laser Wavelength
The premise of laser cleaning is laser absorption, therefore, in the choice of laser source, the first thing to do is to combine the light absorption characteristics of the cleaning workpiece, choose a suitable wavelength laser as the laser light source. In addition, foreign scientists experimental research shows that cleaning the same characteristics of the pollutant particles, the shorter the wavelength, the stronger the cleaning capacity of the laser, the lower the threshold of cleaning. It can be seen that, in order to meet the material light absorption characteristics of the premise, in order to improve the effectiveness and efficiency of cleaning, should choose a shorter wavelength of laser as a cleaning light source.
Effect of Power Density
In the laser cleaning, the laser power density there is an upper damage threshold and the lower cleaning threshold. In this range, the greater the laser power density of laser cleaning, the greater the cleaning capacity, the more obvious the cleaning effect. Therefore should not damage the substrate material in the case, should be as high as possible to increase the power density of the laser.
Effect of Pulse Width
The laser source of laser cleaning can be continuous light or pulsed light, pulsed laser can provide a very high peak power, so it can easily meet the threshold requirements. And it was found that in the cleaning process on the substrate caused by the thermal effects, pulsed laser impact is smaller, continuous laser caused by the thermal impact of the region is larger.
The Effect of Scanning Speed and Number of Times
Obviously in the process of laser cleaning, the faster the laser scanning speed the fewer times, the higher the efficiency of cleaning, but this may cause a decline in the cleaning effect. Therefore, in the actual cleaning application process, should be based on the material characteristics of the cleaning workpiece and the pollution situation to choose the appropriate scanning speed and the number of scans. Scanning the overlap rate and so on will also affect the cleaning effect.
Effect of the Amount of Defocusing
Laser cleaning before the laser mostly through a certain combination of focusing lens for convergence, and the actual process of laser cleaning, generally in the case of defocusing, the larger the amount of defocusing, shining on the material the larger the spot, the larger the scanning area, the higher the efficiency. And in the total power is certain, the smaller the amount of defocusing, the greater the power density of the laser, the stronger the cleaning capacity.
Summary
Since laser cleaning does not use any chemical solvents or other consumables, it is environmentally friendly, safe to operate and has very many advantages:
1. green and environmentally friendly, without the use of any chemicals and cleaning solutions,
2. cleaning waste is mainly solid powder, small size, easy to collect and recycle,
3. Cleaning waste smoke is easy to absorb and handle, low noise, no harm to personal health,
4. Non-contact cleaning, no media residue, no secondary pollution,
5. Selective cleaning can be achieved, no damage to substrates,
6. No working medium consumption, only consume electricity, low cost of use and maintenance,
7. Easy to achieve automation, reduce labor intensity,
8. Suitable for hard-to-reach areas or surfaces, for hazardous or dangerous environments.
Maven Laser Automation Co., Ltd. is a professional manufacturer of laser welding machine, laser cleaning machine, laser marking machine for 14 years. Since 2008, Maven Laser focused on the development and production of various types of laser engraving/welding/marking/cleaning machine with the advanced management, strong research strength and steady globalization strategy, Maven Laser establish more perfect product sales and service system in China and around the world, make the world's brand in laser industry.
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Post time: May-05-2023
