I. Introduction               
Cleaning techniques are often used in industrial production, cultural relics protection and the treatment of dental diseases. For example: industrial products in electroplating, phosphating, spraying, welding, packaging and assembly line assembly
In order to ensure the quality of the workpiece in the next process, it is necessary to remove grease, dust, rust or residual solvents, adhesives and other contaminants on the surface of the product. Due to environmental pollution and poor protection and other reason.The cultural relics and works of art are corrosion and fouling, to restore the old, dirt and rust on the surface of cultural relics should be cleaned.
Cleaning includes mechanical cleaning, chemical cleaning and ultrasonic cleaning method of traditional mechanical cleaning method, which adopts the scraping and rubbing, brush, sandblasting and other mechanical means to remove surface dirt; wet chemical cleaning method is to use the organic cleaning agent, by spraying and showering, soaking or high frequency vibration ultrasonic cleaning method of surface removal property is to be processed into parts of the cleaning agent, the effect of vibration generated by the ultrasonic removal of dirt. At present, these three cleaning methods are still dominant in our country’s cleaning market, but their applications are greatly limited under the environmental protection and high precision requirements. The mechanical cleaning method can not meet the requirements of high cleanliness of cleaning, cleaning the workpiece is easy to damage the surface area and the chemical cleaning method is easy to cause environmental pollution, the cleanliness is also very limited, especially when the dirt component is complex, must choose a variety of cleaning agent can be cleaned repeatedly meet surface cleanliness requirements. The ultrasonic cleaning method while the cleaning effect is good, but for the cleaning of submicron particles and the size of the cleaning tank incapable of action, limits the scope and complexity of processing parts, and after cleaning the workpiece drying is also a big problem.
In recent years, with the enhancement of people’s awareness of environmental protection, development to the world within the scope of cleaning industry has brought great challenges have emerged, all kinds of cleaning technology for environmental protection, laser cleaning technology is one of them. The laser cleaning technology refers to the use of high energy laser beam irradiation surface of the workpiece, the surface dirt, rust coating or instant evaporation or peeling, high speed and effectively remove attachments or cleaning coating on the surface of the object surface, so as to achieve the clean process. It is a new technology in interaction of laser and material based on, compared with the traditional mechanical cleaning, chemical cleaning, corrosion liquid solid impact cleaning, high frequency ultrasonic cleaning and other traditional cleaning methods have obvious advantages. It is efficient, fast, low cost of substrate thermal load and mechanical load, cleaning for non damage; waste Recyclable, no environmental pollution is safe and reliable, with levels of the cleaning process does not damage the health certificate, the operator can remove all kinds of different thickness and different components is easy to realize automation control, realize remote control cleaning etc..
‖. Principle and method of laser cleaning
Laser is characterized by high directivity, monochromatic, high coherence and high brightness. By focusing the lens and the Q switch, the energy can be concentrated into a very small space and time range. In laserThe cleaning process mainly uses the following characteristics of laser:
1, laser energy can be achieved in time and space on the high concentration, the focus of the laser beam in the vicinity of the focus can produce thousands of degrees or even tens of thousands of degrees of high temperature, so that instant evaporation of dirt, gasification or decomposition.
2, the laser beam has a small divergence angle and a good directivity, and the laser beam can be integrated into different diameter facula through the concentrating system. Under the condition of the same laser energy, controlling the laser spot of different diameter can adjust the energy density of the laser and make the fouling expand. When the expansion force of the dirt is greater than the adsorption of dirt on the substrate, dirt will fall away from the surface of the object.
3, laser beams can produce ultrasonic waves on the solid surface, resulting in mechanical resonances, causing the dirt to break and fall off. Laser cleaning technology is to use the characteristics of the laser to achieve the purpose of cleaning. According to the optical properties of the substrate material and the removal of dirt can be washed, laser cleaning mechanism is divided into two categories: one is used to clean the substrate (also called matrix) and surface attachment (dirt) absorption coefficient of a wavelength of laser energy has great difference. Most of the laser energy radiated to the surface is absorbed by the surface attachment, which is heated or vaporized, evaporated or expanded at once, and is driven by an air stream that is separated from the surface of the object to achieve the cleaning purpose. The substrate has minimal energy absorbed by the laser wavelength and will not suffer damage. To select the proper wavelength and control the laser energy is the key to the safe and efficient cleaning of the laser cleaning. Another type of laser energy for surface cleaning and matrix attachment absorption difference coefficient, or substrate is sensitive to acid vapor coating formed by heating, cleaning or coating method after heating will produce toxic substances such as the. The method usually uses a high power, high repetition rate laser pulse to clean the surface and convert some of the beam into sound waves. The sound waves hit the hard surface after the incident wave generating part and the return of the laser interference generated high waves, so that the coating small range of explosion, the coating was pressed into powder, and then removed by the vacuum pump, and the substrate is not damaged under.
From the method of analysis, there are 4 kinds of laser cleaning methods:
1, laser dry cleaning method, that is, the use of pulsed laser radiation decontamination;
2, the first film + laser method, depositing a layer of liquid film on the surface of the matrix, and then use the laser radiation decontamination; when the laser irradiated on the liquid film on the liquid film, sharp heat, explosive vaporization, explosive shock wave to loose surface dirt. With the shock wave flying away from the surface of the processed object, the purpose of decontamination is achieved.
3, laser + inert gas, which is in the laser radiation at the same time, blown to the surface of the substrate with an inert gas, when the dirt from the surface after stripping will be gas blowing away from the surface, to avoid secondary pollution and surface oxidation;
4, the use of laser dirt loose, and then use non corrosive chemical cleaning. At present, the first 3 methods are often used. Fourth kinds of cleaning method applies only to the stone relics, and a complete system of laser cleaning include: laser or YAG laser), sample processing, light guide system, console and auxiliary system (such as the calorimeter and beam profile instrument and its accessories).
For laser and liquid film cleaning, there is also liquid film deposition system. When laser + inert gas cleaning methods are used, inert gas entry and removal systems shall also be included.
III. Application status of laser cleaning
Stone carvings and stone carvings, such as the ages of high-grade stone art, because of its extremely fine and vulnerable surface structure, laser cleaning technology has become the earliest application of the field. It has been found that the removal of dirt on stone artifacts by laser has unique advantages. It can precisely control the light beam moving on the complex surface and remove the dirt without damaging the stone. For example, in September 1992, UNESCO organized the world cultural heritage protection organization to commemorate the establishment of the organization, 20th anniversary, the implementation of the famous British Church of St. Charles’s maintenance project. The exquisite marble sculpture of the Notre Dame gate on the west side of Ya Ming cathedral is the key to the project. In the door of the one-year maintenance, maintenance personnel with the aid of a laser, a laser beam to remove the cover on the marble carved on a few millimeters thick black scale layer, the original color of the marble surface reflected, the exquisite carving splendor. Another example: one of Britain’s most important stone collection at the stone collection by laser Bulundier yingsi after cleaning, also obtained the same effect. People observed the stone surface after laser cleaning with electron microscope, and found that the structure of the stone has not changed after laser cleaning, and the cleaned surface is smooth and flat, without any damage. This is completely different from the surface after cleaning with a particle ejection method (sandblasting). It is difficult to avoid the damage of marble surface structure after particle injection cleaning, especially for the marble surface with sulfate scale. The observation of the electron microscope also shows that the properties of the rock materials under the surface layer are neither degraded nor changed after laser irradiation.
At present, the use of laser cleaning limestone, marble and other high-grade stone material surface fouling work has become a new and promising business projects. In addition to clean stone materials, has a good effect in cleaning glass, metal, mold, disk and various microelectronic products in laser cleaning, the automobile tire production, the bottom and the surrounding pattern every two to three weeks of tire mold needs to be cleaned once every few months to the whole die a thorough cleaning. The traditional method is to use chemical solution soaking or sand blasting cleaning, not only expensive, noisy, serious pollution, but also affect the surface quality of the mold. In addition, such cleaning methods can not be achieved online cleaning, cleaning before the removal of the mold and cleaning after the installation of the mold takes a long time, affecting the normal operation of the production line. The cleaning operation can be realized by cleaning the mold with laser technology. Depending on the amount of organic attachment on the mold surface and the structure of the adhesive material, it takes only 45-90 minutes to clean a mold (including loading and cleaning parts on both sides of the mold). As the mold surface can reflect the laser beam, the mold surface will not be damaged by laser. Laser cleaning has high flexibility, and users can use this technology instead of other methods that are being used. At present, foreign YAG laser has been used to realize the on-line cleaning of tire mold in tire production line, and has been applied to industry. When the mold needs cleaning, the vulcanizing machine is opened at the end of vulcanization, and the laser mold cleaning system is moved to the vulcanizing machine before cleaning the mold without waiting for die cooling. Also note that the cleaning time has nothing to do with the mold temperature. The operator can clean the head of the laser cleaning system and start cleaning. In the cleaning process, the laser beam scans the mold surface and irradiate the mold surface from four different angles, so that all types of molds can be cleaned. After cleaning half, the head of the cleaning machine is moved to the other half of the mold for cleaning. After the mold is completely cleaned, the cleaning device is moved to another vulcanizing machine mold to be cleaned. The cleaned vulcanizing machine can be put into production immediately. In addition to cleaning dies, industrial applications include marking of tires, stripping of components such as brake lines, or stripping of wires on flat wires.
IV. Analysis of laser cleaning efficiency
The laser cleaning of the tire mold is taken as an example to illustrate the economic benefits of laser cleaning compared with off-line sand blasting cleaning and dry ice cleaning equipment. Laser cleaning technology has obvious advantages: fast cleaning, low labor intensity, no wear and tear, no danger to the operator, however, the initial investment of equipment is higher, reaching $30-60. Therefore, the factory needs to establish a reasonable investment recovery plan. A typical JET laser system factory will be able to make investment recovery within 18 months. Low lead time, low labor cost, less die wear, and lower production costs are potential benefits. For example, a Nissan 20000 tire curing machine equipment, according to 8 (16 modes) an average requirement of cleaning every day, assuming each class cleaning 3 curing machine or daily cleaning 9 curing machine (some factory cleaning two times) from the vulcanizing machine, remove the two halves of the mold for offline cleaning, about 15h operation and 10h down time, such as the use of laser cleaning of the two halves of the mold will need 0.3H, and 3H for downtime, so you can save 14h after cleaning a vulcanizing machine operation and 7h downtime. Suppose, just 10 times cleaning (5 vulcanizing machine), 5 times for offline cleaning in the mould workshop, the laser cleaning will produce huge returns: can save 70 hours a day and 35 hours of operation downtime, so by working day 320 days a year, can increase the homework for 22400 hours and 11200 hours of time each year.
Maintenance and maintenance of laser cleaning equipment should also be counted as clean and residue removal of laser deposition on the filter, a week maintenance for 30min equipment maintenance, every 4 weeks for the main components of 60min. Every 6 months, according to the manufacturer’s requirements, routine maintenance of the equipment, and run a laser system. Most mechanical components, like laser frames, have a life span of more than 10 years, and some laser components require replacement after use of about 300h. These parts can be replaced on-site during routine preventive maintenance. Laser device plus its one year warranty service. Including replacement of wearing parts and providing typical spare parts, the operation cost is about 4-8 dollars at the first hour. All devices are equipped with Modem (modulation regulators) so that manufacturers can provide remote services.
Considering the above factors, the economic benefit of laser cleaning technology is remarkable.
V summary and Outlook
Compared with the traditional cleaning technology, laser cleaning technology has great advantages. It can bring great benefits to society in terms of economic benefits and "green" project, and the market prospect is quite broad. But in our country, the research in this area is still not enough, if we can use the existing laser technology, laser cleaning equipment supporting the development of the investment and the production, industrialization, it is entirely possible. With the decrease of laser cost, laser cleaning technology will gradually be popularized, which is important for promoting the development of high and new technology industry.