The world’s first laser was born in 1960. It was developed by American scientist Professor Theodore Harold Meiman using ruby. Since then, it has opened the door for laser to benefit mankind.
In the following time, lasers used in various fields were born one after another.
The promotion of laser technology has made the rapid development of science and technology in the fields of medical treatment, equipment manufacturing, accurate measurement and remanufacturing engineering, accelerated the pace of social progress, and made important achievements in the application of laser in the field of cleaning.
Compared with traditional cleaning methods, such as mechanical friction, chemical corrosion and high-frequency ultrasound, laser cleaning can realize fully automatic operation.
It has the advantages of high work efficiency, low cost, no pollution to the environment, no damage to the substrate and wide application range of materials.
It fully conforms to the processing concept of green and environmental protection. It is the most reliable and effective cleaning method at present.
Washing is the premise for the detection and processing of waste machine parts.
Using laser cleaning technology can effectively control the surface morphology and surface roughness of the substrate and improve the performance of the substrate after cleaning.
It can also be applied to the manufacturing, surface treatment or remanufacturing of large parts.
Although laser cleaning has not completely replaced the traditional cleaning methods at present, with the enhancement of national awareness of energy conservation and emission reduction in manufacturing industry, laser cleaning will gradually enter people’s life with its unique advantages.
After 2020, when China’s environmental protection laws and regulations are more and more strict and people’s awareness of environmental protection and safety is increasing, the types of chemicals that can be used in industrial production cleaning will become less and less.
How to find a cleaner and non-destructive cleaning method is a problem we have to consider.
Laser cleaning has the characteristics of no grinding, non-contact, no thermal effect and suitable for all kinds of objects. It is considered to be the most reliable and effective solution.
At the same time, laser cleaning can solve problems that cannot be solved by traditional cleaning methods.
Principle of laser cleaning
For example, when submicron pollution particles are adhered to the workpiece surface, these particles often stick very tightly, which can not be removed by conventional cleaning methods, but cleaning the workpiece surface with nano laser radiation is very effective.
In addition, because the laser is non-contact cleaning to the workpiece, it is very safe to clean the precision workpiece or its fine parts, which can ensure its accuracy.
Therefore, laser cleaning has unique advantages in the cleaning industry.
Why can lasers be used for cleaning? Why does it not cause damage to the cleaned object? First, understand the essence of the laser.
In short, the laser is no different from the accompanying light (visible light and invisible light) around us, but the laser uses a resonator to concentrate the light in the same direction, and has better performance than simple wavelength and coordination.
Therefore, theoretically, all wavelengths of light can be used to form a laser.
However, in fact, there are few media that can be excited, so the laser light source that can produce stable and suitable for industrial production is quite limited.
Nd: YAG laser, carbon dioxide laser and excimer laser are widely used.
Because Nd: YAG laser can be transmitted through optical fiber and is more suitable for industrial application, it is also widely used in laser cleaning.
Academically, laser ablation (scientific name of laser cleaning) or light ablation is a process of removing materials from solid (or sometimes liquid) surfaces by irradiating with a laser beam.
At low laser flux, the material is heated and evaporated or sublimated by the absorbed laser energy.
At high laser flux, materials are usually converted to plasma. Generally, laser ablation refers to the removal of materials with pulsed laser, but if the laser intensity is high enough, the materials can be ablated with continuous wave laser beam.
Deep ultraviolet excimer lasers are mainly used for optical ablation. The laser wavelength for optical ablation is about 200 nm.
The depth of absorbing laser energy and the amount of material removed by a single laser pulse depend on the optical properties of the material and the laser wavelength and pulse length.
The total mass of each laser pulse ablated from the target is usually referred to as the ablation rate.
Laser radiation characteristics such as laser beam scanning speed and scanning line coverage will significantly affect the ablation process.
Effect of laser cleaning tire mould
Principle of laser cleaning
In the mid-1980s, scientists such as Beklemyshev and Alrn combined laser technology with cleaning technology and carried out relevant research according to the actual work needs. Since then, the technical concept of laser cleaning was born.
As we all know, the binding force between pollutants and matrix is divided into covalent bond, double dipole, capillary action and van der Waals force.
If this force can be overcome or destroyed, the decontamination effect will be achieved.
Laser cleaning is to use the characteristics of laser beam with large energy density, controllable direction and strong convergence ability to destroy the bonding force between pollutants and matrix or directly vaporize pollutants, so as to reduce the bonding strength between pollutants and matrix, so as to clean the workpiece surface.
The schematic diagram of laser cleaning is shown in Fig. 1.
When the pollutant on the workpiece surface absorbs the energy of laser, it overcomes the force between the pollutant and the substrate surface after rapid gasification or instantaneous thermal expansion.
Due to the increase of heating energy, the pollutant particles vibrate and fall off from the substrate surface.
Fig. 1 Schematic diagram of laser cleaning
The whole laser cleaning process is roughly divided into four stages:
- Laser gasification and decomposition
- laser stripping
- Thermal expansion of pollutant particles
- Substrate surface vibration and pollutant separation
Of course, when applying laser cleaning technology, we should also pay attention to the laser cleaning threshold of the cleaned object and select the appropriate laser wavelength to achieve the best cleaning effect.
Laser cleaning can not only change the grain structure and orientation of the substrate surface without damaging the substrate surface, but also control the substrate surface roughness, so as to enhance the comprehensive properties of the substrate surface.
The cleaning effect is mainly affected by beam characteristics, physical parameters of substrate and dirt materials, and the absorption capacity of dirt to beam energy.
At present, laser cleaning technology includes dry laser cleaning technology, wet laser cleaning technology and laser plasma shock wave technology.
Dry laser cleaning
That is, the pulse laser directly irradiates and cleans the workpiece, which increases the energy absorption temperature of substrate or surface pollutants, produces thermal expansion or substrate thermal vibration, and then separates them.
This method can be roughly divided into two cases:
One is that surface pollutants absorb laser expansion;
The other is that the substrate absorbs laser to produce thermal vibration.
Wet laser cleaning
Wet laser cleaning is to pre coat the liquid film on the surface before the pulse laser irradiates the workpiece to be washed. Under the action of the laser, the temperature of the liquid film rises rapidly and gasifies.
At the moment of gasification, a shock wave acts on the pollutant particles to make them fall off from the substrate.
This method requires that the matrix can not react with the liquid film, so it limits the range of application materials.
Laser plasma shock wave
Laser plasma shock wave is a spherical plasma shock wave produced by breaking through air medium during laser irradiation.
The shock wave acts on the surface of the substrate to be washed and releases energy to remove pollutants;
The laser does not act on the substrate, so it does not damage the substrate.
Laser plasma shock wave cleaning technology can clean particle pollutants with tens of nanometers, and there is no limit on the laser wavelength.
In actual production, different test methods and related parameters should be selected according to the needs to obtain high-quality cleaning workpieces.
In the process of laser cleaning, the evaluation of surface cleaning efficiency and quality is an important standard to determine the quality of laser cleaning technology.
Compared with traditional cleaning methods such as mechanical friction cleaning, chemical corrosion cleaning, liquid solid strong impact cleaning and high-frequency ultrasonic cleaning, laser cleaning has obvious advantages.
2.1. Laser cleaning is a “green” cleaning method, which does not need to use any chemical agent and cleaning solution.
The cleaned waste is basically solid powder, small in volume, easy to store and recyclable, which can easily solve the environmental pollution caused by chemical cleaning;
2.2. The traditional cleaning method is often contact cleaning, which has mechanical force on the surface of the cleaned object, damages the surface of the object or the cleaning medium is attached to the surface of the cleaned object, which cannot be removed and produces secondary pollution.
The non grinding and non-contact of laser cleaning can solve these problems;
2.3. Laser can be transmitted through optical fiber and cooperate with manipulator and robot to facilitate long-distance operation, and can clean parts that are difficult to reach by traditional methods, which can ensure personnel safety in some dangerous places;
2.4. Laser cleaning can remove various types of pollutants on the surface of various materials and achieve the cleanliness that cannot be achieved by conventional cleaning.
It can also selectively clean the pollutants on the material surface without damaging the material surface;
2.5. Laser cleaning has high efficiency and saves time;
2.6. Although the one-time investment in the purchase of laser cleaning system is high in the early stage, the cleaning system can be used stably for a long time, with low operation cost and only electricity charge per hour.
The process of pulsed Nd: YAG laser cleaning depends on the characteristics of the optical pulse generated by the laser and is based on the photophysical reaction caused by the interaction between high-intensity beam, short pulse laser and pollution layer.
Its physical principle can be summarized as follows:
a) The beam emitted by the laser is absorbed by the contaminated layer on the surface to be treated.
b) The absorption of large energy forms a rapidly expanding plasma (highly ionized unstable gas) and produces shock waves.
c) The shock wave turns the pollutants into fragments and is removed.
d) The light pulse width must be short enough to avoid heat accumulation that destroys the treated surface.
e) Experiments show that when there are oxides on the metal surface, the plasma occurs on the metal surface.
The plasma is generated only when the energy density is higher than the threshold, which depends on the contaminated layer or oxide layer removed.
If the energy density exceeds this threshold, the base material will be destroyed.
In order to effectively clean the substrate material on the premise of ensuring the safety of the substrate material, the laser parameters must be adjusted according to the situation, so that the energy density of the optical pulse is strictly between the two thresholds.
Each laser pulse removes a certain thickness of the pollution layer. If the contamination layer is thick, multiple pulses are required for cleaning.
The number of pulses required to clean the surface depends on the degree of surface contamination. An important result of the two thresholds is the self-control of cleaning.
The number of pulses required to clean the surface depends on the degree of surface contamination.
An important result of the two thresholds is the self-control of cleaning.
Light pulses with an energy density higher than the first threshold will remove contaminants until the base material is reached.
However, because its energy density is lower than the failure threshold of the substrate material, the substrate will not be damaged.
Laser cleaning can be used not only to clean organic pollutants but also to clean inorganic substances, including metal corrosion, metal particles, dust, etc.
Some practical applications are introduced below. These technologies have been very mature and widely used.
4.1. Mold cleaning:
Every year, tire manufacturers all over the world manufacture hundreds of millions of tires. In the production process, the cleaning of tire molds must be rapid and reliable to save downtime.
Traditional cleaning methods include sandblasting, ultrasonic or carbon dioxide cleaning, but these methods usually must be moved to the cleaning equipment for cleaning after the high heat mold is cooled for several hours.
The cleaning takes a long time and is easy to damage the accuracy of the mold. Chemical solvents and noise will also produce problems such as safety and environmental protection.
Using the laser cleaning method, because the laser can be transmitted by optical fiber, it has deep elasticity in use;
Because the laser cleaning method can be connected with optical fiber to guide the light to the dead corner of the mold or the part that is not easy to remove for cleaning, it is convenient to use;
Since the rubber is not gasified, it will not produce toxic gas and affect the safety of the working environment.
The technology of laser cleaning tire mold has been widely used in European and American tire industry.
Effect of laser cleaning tire
Although the initial investment cost is high, the benefits obtained in saving standby time, avoiding mold damage, working safety and saving raw materials can be quickly recovered.
According to the cleaning test of the laser cleaning equipment on the production line of the tire company, it can clean a set of large truck tire molds online in only 2 hours.
Compared with conventional cleaning methods, the economic benefits are obvious.
The anti stick elastic film on the mold of the food industry needs to be replaced regularly to ensure hygiene.
Laser cleaning without chemical reagent is also particularly suitable for this application.
Mold cleaning effect
4.2. Cleaning of weapons and equipment:
Laser cleaning technology is widely used in weapon maintenance.
The laser cleaning system can remove rust and pollutants efficiently and quickly, and select the removal parts to realize the automation of cleaning.
Using laser cleaning, not only the cleanliness is higher than that of the chemical cleaning process, but also there is almost no damage to the object surface.
By setting different parameters, a dense oxide protective film or metal melting layer can also be formed on the surface of metal objects to improve surface strength and corrosion resistance.
The waste removed by laser basically does not pollute the environment, and can also be operated remotely, effectively reducing the health damage to operators.
4.3. Removal of old aircraft paint:
In Europe, laser cleaning systems have long been used in the aviation industry.
The surface of the aircraft needs to be repainted after a certain period of time, but the original old paint needs to be completely removed before painting.
The traditional mechanical paint removal method is easy to damage the metal surface of the aircraft and bring hidden dangers to safe flight.
If multiple laser cleaning systems are used, the paint layer on the surface of an A320 Airbus can be completely removed within two days without damaging the metal surface.
4.4. Cleaning of building exterior wall:
With the rapid development of China’s economy, more and more skyscrapers have been built, and the cleaning problem of building exterior walls is becoming more and more prominent. Laserlaste laser cleaning system provides a good solution for cleaning building exterior walls through optical fibers up to 70 meters long.
It can effectively clean all kinds of pollutants on all kinds of stones, metals and glass, The cleaning efficiency is many times higher than that of conventional cleaning.
It can also remove black spots and color spots on various stone materials of buildings.
The experiment of laser cleaning system on buildings and stone tablets in Songshan Shaolin Temple shows that laser cleaning has a very good effect on protecting ancient buildings and restoring their appearance.
4.5. Cleaning in the electronic industry
The electronic industry uses laser to remove oxides: the electronic industry needs high-precision decontamination, which is especially suitable for the laser to remove oxides.
Before welding the circuit board, the component pins must be completely deoxidized to ensure the best electrical contact, and the pins must not be damaged during the decontamination process.
Laser cleaning can meet the use requirements and has high efficiency. One pin only needs to irradiate the laser once.
4.6. Precision degreasing cleaning in precision machinery industry:
In the precision machinery industry, it is often necessary to remove esters and mineral oils used for lubrication and corrosion resistance on parts, usually by chemical methods, and chemical cleaning often still has residues.
Laser deasterification can completely remove esters and mineral oil without damaging the surface of parts. The pollutant removal is completed by the shock wave.
The explosive gasification of the thin oxide layer on the surface of parts forms shock wave, which leads to pollutant removal rather than mechanical interaction.
The material is thoroughly de esterified and used for cleaning mechanical parts in the aerospace industry.
Laser cleaning can also be used to remove oil esters in machining of mechanical parts.
4.7. Pipeline cleaning in the reactor of nuclear power plant:
The laser cleaning system is also used to clean the pipes in the reactor of the nuclear power plant.
It uses optical fiber to introduce high-power laser beam into the reactor to directly remove radioactive dust, and the cleaned materials are easy to clean.
Moreover, due to the remote operation, the safety of staff can be ensured.
To sum up, laser cleaning plays an important role in many fields, and can play an important role in automobile manufacturing, semiconductor wafer cleaning, precision parts processing and manufacturing, military equipment cleaning, building exterior wall cleaning, cultural relics protection, circuit board cleaning, precision parts processing and manufacturing, LCD cleaning, chewing gum residue removal and other fields.
Main cleaning methods
There are four main laser cleaning methods:
① Laser dry cleaning method, i.e. direct radiation decontamination by pulsed laser;
② Laser + liquid film method, that is, first deposit a layer of liquid film on the substrate surface, and then decontaminate with laser radiation;
③ The method of laser plus inert gas, that is, while laser radiation, inert gas is blown to the substrate surface.
When the dirt is stripped from the surface, it will be blown off the surface immediately by gas to avoid re pollution and oxidation of the surface;
④ After the dirt is loosened by laser, it is cleaned by non corrosive chemical method.
The first three methods are most commonly used. The fourth method is only used in the cleaning of stone cultural relics.
Internationally, laser cleaning technology has been applied to stone materials for more than ten years.
In China, laser cleaning of stone materials started late. Because the investment of laser equipment is still relatively expensive, it is still difficult to popularize it.
However, laser cleaning technology has incomparable advantages over traditional cleaning methods.
With the continuous improvement of technology and mass production of equipment, laser cleaning technology will play an important role in the cleaning industry of stone materials.