Introduction to Laser Cleaning Technology and Its Application in Tire Industry

I. INTRODUCTION In the treatment of industrial production, cultural relics protection and dental diseases, cleaning techniques are often used. For example, when industrial products are used for electroplating, phosphating, spraying, welding, packaging, and assembly of integrated circuits, in order to ensure the quality of the workpieces in the next process, the grease, dust, rust, or residual solvent on the surface of the product must be removed. Condensation and other dirt. 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, dirt and rust on the surface of cultural relics should be cleaned.

Traditional cleaning methods include mechanical cleaning, chemical cleaning, and ultrasonic cleaning. Mechanical cleaning uses scraping, wiping, brushing, and sandblasting to remove surface dirt. Wet chemical cleaning uses organic cleaning agents. Spray, shower, soak or high-frequency vibration and other measures to remove the surface attached to the property Ultrasonic cleaning method is to put the parts to be treated in the cleaning agent, the use of ultrasonic vibration effects to remove dirt. At present, these three cleaning methods still dominate the cleansing market in China, but their application is severely limited under the requirements of environmental protection and high precision. The mechanical cleaning method can not meet the high cleanliness cleaning requirements, and can easily damage the surface area of ​​the cleaned workpieces. The chemical cleaning methods easily lead to environmental pollution and the obtained cleanliness is also very limited. Especially when the dirt components are complex, multiple cleaning agents must be used repeatedly. Cleaning may satisfy the requirements of surface finish. Although the ultrasonic cleaning method has good cleaning effect, it is incapable of cleaning the submicron-sized 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, it has brought tremendous challenges to the development of the cleaning industry worldwide. Various cleaning technologies that benefit the environment have emerged. 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 the dirt, rust or coating instantaneous evaporation or peeling, high-speed and effective removal of the cleaning object surface attachments or surface coating, so as to achieve a clean process . It is a new technology based on the interaction effect of laser and material. Compared with the traditional cleaning methods such as mechanical cleaning, chemical corrosion cleaning, liquid solid strong impact cleaning and high frequency ultrasonic cleaning, it has obvious advantages. It is efficient, fast, low cost, small thermal load and mechanical load on the substrate, cleaning is non-injury; waste can be recovered, no environmental pollutants are safe and reliable, without damaging the operator's health certificate can clear all kinds of different thickness, different The coating sub-cleaning process of the ingredients is easy to realize automatic control, remote cleaning and other remote control.

Second, the principle and method of laser cleaning

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

1. The laser can achieve high concentration of energy in time and space. The focused laser beam can generate high temperatures of several thousand degrees or even tens of thousands of degrees in the vicinity of the focal point, so that the dirt instantly evaporates, vaporizes, or decomposes. 2. The divergence angle of the laser beam is small and its directionality is good. The laser beam can be condensed into light spots of different diameters by the concentrating system. Under the same laser energy conditions, controlling the laser beam spot of different diameters can adjust the energy density of the laser and cause the thermal expansion of the dirt. When the expansion of the dirt is greater than the adsorption of the dirt on the substrate, the dirt will be off the surface of the object. 3, the laser beam can produce ultrasonic waves on the solid surface, generate mechanical resonance, so that the dirt breaks off. Laser cleaning technology is the use of the characteristics of the above laser to achieve the purpose of cleaning. According to the optical properties of the cleaned matrix material and removed dirt, the laser cleaning mechanism can be divided into two categories: one is the use of cleaning substrates (also known as the parent) and surface attachments (dirt) to a certain wavelength of laser light. The absorption coefficient of energy has a big difference. Most of the laser energy radiated to the surface is absorbed by the surface attachments, so that it is heated or evaporated, or instantaneously expanded, and is driven by the formed air flow to separate from the surface of the object and achieve the purpose of cleaning. The substrate does not suffer damage due to its minimal laser absorption energy at this wavelength. For such laser cleaning, choosing the right wavelength and controlling the laser energy size is the key to achieving safe and efficient cleaning. Another type of laser energy absorption coefficient suitable for cleaning the substrate and the surface attachment is not much different, or the substrate is sensitive to the acid vapor formed by the coating heat, or the cleaning method that generates toxic substances after the coating is heated. This type of method usually uses a high-power, high-repetition-rate pulsed laser to strike 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 sound wave generated by the laser, generating a high-energy wave, causing the coating to undergo a small-scale explosion. The coating is pressed into a powder, which is then removed by a vacuum pump and the underlying substrate. It will not hurt.

From the methodological analysis, there are four kinds of laser cleaning methods: 1. Laser dry-cleaning method, which uses pulsed laser direct radiation decontamination; 2. Laser+liquid film method, that is, firstly deposits a layer of liquid film on the surface of the substrate, and then uses laser radiation. Decontamination; When the laser light is irradiated on the liquid film, the liquid film is heated rapidly, resulting in explosive vaporization. The explosive shock wave loosens the surface of the substrate. And with the shock wave flying away from the surface of the object to achieve the purpose of decontamination. 3, laser + inert gas method, that is, in the laser radiation at the same time, with inert gas blown to the surface of the substrate, when the dirt peeled from the surface will be immediately blown away from the surface of the gas to avoid re-contamination and oxidation of the surface; After the laser loosens the dirt, it is cleaned with a non-corrosive chemical method. At present, the first three methods are commonly used. The fourth method is only applied to the cleaning of stone artifacts, and a complete laser cleaning system includes: laser or YAG laser), sample processing table, light guide system, console, auxiliary system (such as calorimeter, beam) Profiler and its accessories). For the laser + liquid film cleaning method, there is a liquid film deposition system. When laser + inert gas cleaning methods are used, inert gas inlet and discharge systems should also be included.
Third, the application of laser cleaning

The age-old, high-grade stone works such as stone carvings and stone carvings have become the earliest areas of application of laser cleaning technology due to their extremely delicate and delicate surface structure. It has been found that the use of laser to remove the dirt on the surface of stone artifacts has its unique advantages. It can very accurately control the movement of the light beam on a complex surface, remove the dirt without damaging the artifact stone. For example, in September 1992, UNESCO's World Cultural Heritage Conservation Organization commemorated the organization's 20th anniversary and implemented maintenance work on the famous British Armenian Cathedral. The exquisite marble carvings on the west side of the Cathedral of the Imam are key to the project. During the one-year maintenance of the Virgin Mary, the maintenance staff used a laser beam to remove a few millimeters thick layer of black dirt covered by the marble carving pattern. The original color of the marble surface is reflected and the beautiful carving is reproduced. Brilliance. Another example: After Insbrentier's collection of stone sculptures, one of the most important stone collections in the UK, has been cleaned by lasers, the same results have been obtained. People observed the laser-cleaned stone carving surface with an electron microscope and found that the structure of the stone after the laser cleaning did not change, and the cleaned surface was smooth and flat with no damage. This is completely different from the surface cleaned by the micro-particle spray method (blasting method). The damage of the marble surface structure after particle jet cleaning is unavoidable, especially for marble surfaces with existing sulfate scale. Electron microscope observations also revealed 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 of limestone, marble and other high-grade stone materials 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, molds, disks, and various microelectronic products. In the production of automobile tires, the bottom of the tire mold and its surrounding patterns are Two to three weeks need to be cleaned once every few months to thoroughly clean the entire mold. The traditional method is to wash with chemical spray or sandblasting, which is not only expensive, noisy, and serious pollution, but also affects the surface quality of the mold. In addition, this type of cleaning method cannot realize on-line cleaning. The disassembly mold before cleaning and the installation mold after cleaning take a long time, which affects the normal operation of the production line. Using laser technology to clean the mold can achieve online cleaning operations. According to the number of organic attachments on the surface of the mold and the difference in the structure of the rubber, it takes only 45-90 minutes to clean a set of molds (including loading and unloading and cleaning the parts on both sides of the mold). Since the mold surface can reflect the laser beam, the mold surface is not damaged by the laser. Laser cleaning is highly flexible, and users can use this technology to replace other methods being used. At present, YAG lasers have been used on the tire production line to achieve online cleaning of tire molds, and have been applied to the industry. When the mold needs to be cleaned, the vulcanizer is turned on at the end of 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 starts the cleaning by bringing the head of the laser cleaning system against the mold. During the cleaning process, the laser beam scans the surface of the mold and irradiates the surface of the mold from four different angles. This ensures that all types of mold can be cleaned. After cleaning half, move the head of the washing machine to the other half of the mold for cleaning. After the mold is completely cleaned, the cleaning device is moved to another vulcanizer mold to be cleaned. The washed vulcanizer can be put into production immediately. In addition to cleaning the molds, industrial applications include marking the tires, removing the coating of components such as brake wires or stripping of wires on flat wires.

At present, YAG lasers have been used on the tire production line to achieve online cleaning of tire molds, and have been applied to the industry. If the system is running at full capacity, the original tire factory can clean 1500 molds a year, which can save production time 6000h; tire production can increase about 30,000 sets. Not only environmental protection, but also considerable economic benefits.

The application of laser cleaning technology has been popularized and applied in Europe for several years now, and there are products. However, our country has only just started, mainly because the investment in laser equipment is expensive, and there is still a certain difficulty in universal application. Laser cleaning technology is compared with traditional cleaning technology. There are unparalleled advantages. At present, high-efficiency and low-cost cleaning methods for tire molds are in urgent need of promotion. According to incomplete statistics, there are thousands of tire and product manufacturers in China, and there are no less than 2,000 companies that produce more than one million tires per year. There are tens of thousands of molds owned by manufacturers of one million tires per year, and one set of molds needs to be cleaned once in about three 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 the tire mold cleaning or other cleaning industry, and has a very good prospect for development.
Fourth, laser cleaning benefit analysis

The following is an example of laser cleaning of tire molds to illustrate the economic benefits of laser cleaning compared to off-line sandblast cleaning and dry ice cleaning equipment. Laser cleaning technology has obvious advantages: fast cleaning, low labor intensity, no wear and no risk to operators

However, the initial investment of the equipment is relatively high, reaching 30-60 million US dollars. Therefore, the factory needs to establish a reasonable investment recovery plan. A typical JET laser system factory can obtain investment recovery within 18 months. Short downtime for sulfuration, low labor costs, low mold wear, and lower production costs are potential benefits. For example, a 20,000-daily tire equipment is required to be cleaned once per day on 8 vulcanizers (16 modules). It is assumed that 3 vulcanizing machines are used for each shift or 9 vulcanizing machines are cleaned every day (some factories clean twice) Remove the two molds from the vulcanizing machine to perform off-line cleaning. It takes about 15 hours of operation and 10 hours of downtime. If the laser is used to clean the molds, 0.3 hours and 3 hours of downtime are needed to complete the cleaning. A vulcanizing machine can save 14 hours of work and 7 hours of downtime. Assume also that there are only 10 cleanings (5 vulcanizers) and 5 offline cleanings in the mold shop. Laser cleaning will generate huge returns: 70 hours of daily work and 35 small downtimes. Based on a working day of 320 days a year, 22400 hours of work and 11200 hours of boarding time can be added each year.

Maintenance and maintenance costs for laser cleaning equipment should also be taken into account. To clean the laser mirror and remove any residue deposited on the filter, the device must be maintained for 30 minutes every week, and the main components must be inspected every 4 weeks for 60 minutes. . Every 6 months, the device is routinely serviced and the laser system is run in accordance with the manufacturer's requirements. Most mechanical parts can have a lifetime of more than 10 years like a laser rack, and some laser parts require replacement after about 300 hours of use. These parts can be replaced on site during routine preventive maintenance. Laser device plus one year warranty. Including the replacement of wearing parts and providing typical spare parts, the total operating cost is about 4-8 dollars for the first hour. All devices have modems (modulators) installed so that manufacturers can provide remote services.

Based on the above factors, overall, the economic benefits of laser cleaning technology are very significant.

V. Summary and Prospect Laser cleaning technology has great advantages over traditional cleaning technologies. It can bring great benefits to the society in terms of economic benefits and “green” projects. The market prospect is quite broad. However, China's current research in this area is not enough. It is entirely possible to use our existing laser technology to develop complementary laser cleaning equipment, and to put it into production and realize industrialization. With the decrease of the cost of lasers, laser cleaning technology will gradually be popularized, which is also important for promoting the development of high-tech industries.