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Introduction to laser cleaning technology and its application in the tire industry
I. Introduction
Cleaning techniques are often used in industrial production, heritage conservation, and the treatment of dental diseases. For example, in the assembly of electroplating, phosphating, spraying, welding, packaging and integrated circuits, in order to ensure the quality of the workpiece in the next process, it is necessary to remove the grease, dust, rust or residual solvent on the surface of the product, and stick it. Contaminants such as conjunctants. Due to environmental pollution and poor protection, many cultural relics and artworks are gradually being rusted and defaced. In order to restore the old appearance, it is necessary to clean the dirt and rust on the surface of the cultural relics.
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 increasing awareness of environmental protection, the development of the cleaning industry around the world has brought enormous challenges. Various cleaning technologies that are conducive to environmental protection have emerged. Laser cleaning technology is one of them. The so-called laser cleaning technology refers to the use of a high-energy laser beam to illuminate the surface of the workpiece, so that the surface of the dirt, rust or coating is instantaneously evaporated or peeled off, and the surface attachment or surface coating of the cleaning object is removed at high speed, thereby achieving a clean process. . It is a new technology based on the interaction between laser and matter, and has obvious advantages compared with traditional mechanical cleaning methods, chemical corrosion cleaning, liquid solid impact cleaning, high frequency ultrasonic cleaning and other traditional cleaning methods. It is efficient, fast, and low in cost. It has low heat load and mechanical load on the substrate, and the cleaning is non-injury. The waste can be recycled, and the environmental pollutants are safe and reliable. The health certificate can be removed without damage to various thicknesses and differentities. The sub-cleaning process of the coating is easy to achieve automatic control, enabling remote remote cleaning.
Second, the principle and method of laser cleaning
The laser is characterized by high directivity, monochromaticity, high coherence and high brightness. Through the focus and Q-switching of the lens, energy can be concentrated into a small spatial range and time range. In the laser cleaning process, the following characteristics of the laser are mainly utilized:
1. The laser can achieve high concentration of energy in time and space. The focused laser beam can generate several thousand degrees or even tens of thousands of degrees of high temperature near the focus, so that the dirt instantaneously evaporates, vaporizes or decomposes. 2. The divergence angle of the laser beam is small and the directivity is good. The laser beam can be integrated into the spot 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. 3. The laser beam can generate mechanical resonance by generating ultrasonic waves on the solid surface to break and fall off the dirt. 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 light. 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 heated or vaporized to evaporate, or instantaneously expands, 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, and converts 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 the coating to explode in a small range, 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: 1. Laser dry cleaning method, that is, direct radiation decontamination using pulsed laser; 2. Laser + liquid film method, that is, first depositing a liquid film on the surface of the substrate, and then irradiating with laser Decontamination; 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. 3, laser + inert gas method, that is, while the laser radiation, with inert gas blowing to the surface of the substrate, when the dirt is peeled off the surface will be immediately blown off the surface by the gas to avoid surface re-contamination and oxidation; After the laser has loosened the dirt, it is cleaned by non-corrosive chemical methods. Currently, the first three methods are commonly used. The fourth method is only used for the cleaning of stone artifacts, and a complete laser cleaning system includes: laser or YAG laser), sample processing station, light guiding system, console, auxiliary system (such as calorimeter, beam) Profiler and its accessories). For the laser + liquid film cleaning method, there is also a liquid film deposition system. When using a laser + inert gas cleaning method, an inert gas entry and removal system should also be included.
Third, the application status of laser cleaning
Stone carvings and stone carvings, such as the high-grade stone art, have become the earliest applications of laser cleaning technology due to their extremely fine and fragile surface structure. It has been found that the use of a laser to remove dirt from the surface of a stone artifact has its unique advantages in that it can precisely control the movement of the beam over a complex surface, removing dirt without damaging the artifact stone. For example, in September 1992, the UNESCO-listed World Cultural Heritage Protection Organization carried out the maintenance of the famous British Yaming Cathedral to commemorate the twentieth anniversary of the organization. The exquisite marble carvings on the door of the Virgin on the west side of Yaming Cathedral are the key to the project. In the one-year maintenance project of the Notre Dame, the maintenance personnel used the laser to remove the black scale layer covering the marble engraving pattern by a few millimeters thick. The original color of the marble surface was reflected, which made the exquisite carving reappear. Glorious. Another example: the stone carvings of Insbrundier, one of the most important stone carving collections in the UK, have been laser-cleaned and have the same effect. The surface of the stone after laser cleaning was observed with an electron microscope. It was found that the structure of the stone after laser cleaning did not change, and the surface to be cleaned was smooth and flat without damage. This is completely different from the surface cleaned by the microparticle spray method (blasting method). Damage to the surface structure of the marble after microparticle spray cleaning is inevitable, especially for marble surfaces with existing sulfate scales. Observation by electron microscopy also found that after laser irradiation, the properties of the rock material under the surface layer were neither degraded nor changed. At present, the use of laser to clean the surface dirt of high-grade stone materials such as limestone and marble has become a new and promising business project. In addition to the cleaning of stone materials, laser cleaning has a good effect in the cleaning of glass, metal, mold, magnetic disk and various microelectronic products. In the production of automobile tires, the bottom and peripheral patterns of the tire mold are every It needs to be cleaned once every two to three weeks, and the entire mold should be thoroughly cleaned every few months. The traditional method is to use chemical immersion or sand blast cleaning, which is expensive, noisy, and pollute, and also affects the surface quality of the mold. In addition, this type of cleaning method can not achieve on-line cleaning. It takes a long time to disassemble the mold and clean the mounting mold before cleaning, which affects the normal operation of the production line. Online cleaning can be achieved by cleaning the mold with laser technology. Depending on the amount of organic deposits on the mold surface and the size of the compound, it takes only 45-90 minutes to clean a set of molds (including loading and unloading and cleaning parts on both sides of the mold). Since the surface of the mold can reflect the laser beam, the surface of the mold is not damaged by the laser. Laser cleaning is highly flexible and users can use this technology to replace other methods in use. At present, the YAG laser has been used in the tire production line to realize the online cleaning of the tire mold, and has been applied to the industry. When the mold needs to be cleaned, the vulcanizer is turned on at the end of the vulcanization, and the laser mold cleaning system is moved to the vulcanizer to clean the mold without waiting for the mold to cool. Also note that the time to clean the mold is independent of the temperature of the mold. The operator can start cleaning by pressing the head of the laser cleaning system against the mold. During the cleaning process, the laser beam scans the surface of the mold and illuminates the surface of the mold from four different angles, thus ensuring that all types of molds are cleaned. After half of the cleaning, the head of the washing machine is moved to the other half of the mold for cleaning. After the mold is completely cleaned, move the cleaning unit to another vulcanizer mold to be cleaned. The cleaned vulcanizer can be put into production immediately. In addition to cleaning the mold, industrial applications include marking the tires, removing the coating of components such as brake wires or stripping of wires on flat conductors.
At present, the YAG laser has been used in the tire production line to realize the online cleaning of the tire mold, and has been applied to the industry. If the system is running at full load, the original tire production plant will clean 1500 sets of molds a year, which can save 6000 hours of production time; the tire production can increase by about 30,000 sets. Not only environmental protection, but also economic benefits.
The application of laser cleaning technology has been promoted and applied in Europe for several years, and there are already existing products. However, China has only just started, mainly because of the high investment in laser equipment, and the general application has certain difficulty. Compared with traditional cleaning technology, laser cleaning technology is compared. There are incomparable advantages. At present, high-efficiency and low-cost cleaning methods for tire molds are urgently needed to be promoted. According to incomplete statistics, there are thousands of tire and its products manufacturers in China, and there are no fewer than 2,000 enterprises with an annual output of more than one million sets of tires. Manufacturers with an annual output of one million sets of tires have tens of thousands of molds, and a set of molds need to be cleaned after about 3 weeks. It can be seen that the promotion and application of laser technology cleaning is of great significance. It is believed that on the basis of continuous improvement of this technology, with the mass production of equipment, laser cleaning technology will play an important role in tire mold cleaning or other cleaning industry, and has a good development prospect.
Fourth, laser cleaning benefit analysis
The following is an example of laser cleaning of a tire mold to illustrate the economic benefits of laser cleaning compared to offline blast cleaning and dry ice cleaning equipment. Laser cleaning technology has obvious advantages: fast cleaning, low labor intensity, no wear, no danger to operators
However, the initial investment in equipment is relatively high, reaching $300,000-$600,000. Therefore, the factory needs to establish a reasonable investment recovery plan. A typical JET laser system plant can achieve investment recovery within 18 months. Shorter vulcanization downtime, low labor costs, less mold wear and lower production costs are potential benefits. For example, a machine with a daily output of 20,000 tires is required to be cleaned once a day by 8 vulcanizers (16 modules). It is assumed that 3 vulcanizers are cleaned per shift or 9 vulcanizers are cleaned every day (some factories clean twice) It takes about 15h of operation and 10h of downtime to remove the two molds from the vulcanizer. If the two halves of the mold are cleaned by laser, 0.3h and 3h of downtime are required. A vulcanizer can save 14 hours of operation and 7 hours of downtime. It is also assumed that only 10 cleanings (5 vulcanizing machines) and 5 offline cleanings in the mold shop will result in huge returns: 70 hours of operation per day and 35 small downtimes, then According to the working day of 320 days a year, 22400 hours of operation and 11,200 hours of machine time can be added each year.
The cost of repair and maintenance of the laser cleaning equipment should also be taken into account. To clean the laser mirror and remove the residue deposited on the filter, the device should be maintained for 30 minutes per week, and the main components should be repaired for 60 minutes every 4 weeks. . The unit is routinely maintained and the laser system is operated every 6 months according to the manufacturer's requirements. Most mechanical components have the same life span of 10 years as laser frames, and some laser components require replacement after approximately 300 hours of use. These parts can be replaced in the field during routine preventive maintenance. The laser unit plus its one-year warranty. Including the replacement of consumables and the provision of typical spare parts, the total operating cost is approximately $4-8 for the first hour. All devices are equipped with a Modem (Modulation Regulator) so that the manufacturer can provide remote service.
Based on the above factors, the overall economic benefits of laser cleaning technology are very significant.
V. Summary and outlook
Laser cleaning technology has great advantages over traditional cleaning technology. It can bring great benefits to the society in terms of economic benefits and "green" engineering, and the market prospect is quite broad. However, China's current research in this area is still not enough. It is entirely possible to use our existing laser technology to develop supporting laser cleaning equipment and put it into production and industrialization. As the cost of lasers declines, laser cleaning technology will gradually become popular, which is also of great significance to promote the development of high-tech industries.