In flexographic printing, the anilox roller uniformly transfers a certain amount of ink to the printing plate, which is both an ink transfer roller and an ink metering roller. The amount of ink is used to achieve precision. The anilox roller is called the heart of the flexo printing machine by the flexographic printing industry. The final printing effect depends to a great extent on the quality and running results of the anilox roller.
Among the quality problems that occur with anilox rollers, most of them are caused by improper cleaning or incomplete cleaning of anilox rollers, except for a few that are caused by their excessive use time and cause serious wear. How to keep the anilox roller in good condition is very important to ensure the printing quality.
The cleaning of the anilox roller can be divided into daily cleaning and periodic cleaning. The most important thing is daily cleaning. After each printing, the anilox roller should be cleaned as soon as possible to remove the residual ink on the anilox roller. Any satisfactory anilox roller cleaning system must completely remove residual ink particles and other residues at the bottom of each mesh, while ensuring that the mesh wall of the mesh is not damaged.
At present, the cleaning process of the anilox roller mainly includes a physical cleaning method and a chemical cleaning method. The following introduces several commonly used cleaning methods and operating essentials.
1. Physical cleaning method
The physical cleaning methods include medium spray washing method (including baking soda spray washing method, dry ice washing method, plastic bead spray washing method, high pressure water washing method), ultrasonic cleaning method, and laser cleaning method.
1. Medium spray cleaning method The spray form of this cleaning method mainly includes positive pressure propulsion type, negative pressure propulsion type and two-stage negative pressure propulsion type.
â‘´ Positive pressure propulsion. The cleaning medium is installed in a sealed hopper, and the pressure of the compressed air (1 ~ 1.2MPa) is used to spray the cleaning medium directly onto the surface of the roller body for cleaning. This method has the advantages of simple structure of the washing machine and large spray surface. However, there are defects such as uneven dispersion of the spray medium, large spray pressure, difficult to control, easy to cause damage to the net hole, and large gas consumption.
figure 1
⑵ Negative pressure propulsion. Using the jet flow principle (see Figure 1), when compressed air travels through the jet, a vacuum is formed in the B cavity, the cleaning medium is drawn into the A tube under the action of atmospheric pressure, and is ejected from the nozzle C under the push of compressed air. The negative pressure propulsion type has the characteristics of low air pressure (about 0.6 ~ 0.8MPa), no damage to the net hole, low gas consumption (about 0.8 cubic meters per minute), and simple structure. The disadvantage is that the uniformity of the ejected media is poor. In addition, the propelling and conveying distance is short, generally effective within 2 to 3 meters.
(3) Two-stage negative pressure propulsion. This is to overcome the shortcomings of the negative pressure propulsion type, enhance the fluidity of the cleaning medium, and achieve the best cleaning effect. The advantage is that the scattering surface is uniform, the distribution of the spray medium is uniform and stable; the air pressure used is lower (about 0.4 ~ 0.6MPa) to ensure that the net hole is not damaged; the transfer distance is large (up to 10 ~ 15 meters). The disadvantage is that it requires certain experience in adjusting the air pressure and air flow of each part, and the structure is relatively complicated.
The jet cleaning method is to use the air flow to blow the small particles of the cleaning medium to the anilox roller to remove the residual ink in the mesh cavity. In general, the nozzle must be carefully positioned above the anilox roller and always move when cleaning. The particles of the cleaning medium must be smaller than the size of the anilox roller mesh holes. The anilox roller of 200 lines / inch has a cell edge length of 0.05 inches (0.127 cm); the anilox roller of 122 lines / inch has a cell edge length of 0.08 inches (0.2032 cm). [next]
In order to ensure the cleaning effect of any kind of medium blowing cleaning method, you must pay attention to the following points:
â‘ The cleaning medium must be softer than the surface coating of the anilox roller, so as not to damage the net hole; moreover, the pressure of the air flow used should be just enough to remove the residual ink in the net hole without damaging the net hole.
â‘¡ The cleaning medium should be tiny particles to ensure that it can hit the bottom of the net hole smoothly.
â‘¢ The ejected medium should be kept in a uniform state.
â‘£ The spray angle should be able to be adjusted and controlled. The nozzle must be perpendicular to the surface of the anilox roller, so as to ensure that the cleaning medium is directly blown into the anilox, rather than the wall of the anilox, to avoid damage to the anilox roller.
⑤ The distance between the nozzle and the surface of the anilox roller should be able to be adjusted to ensure that the sprayed medium can form a scattering surface and be evenly sprayed on the surface of the roller body. Of course, the distance between the adjusted nozzle and the surface of the anilox roller should always be consistent to achieve a uniform cleaning effect.
â‘¥ The pressure of the propulsion air should not be concentrated at one point. In addition, the principle of removing the residues of the net cavity without damaging the net cavity is the principle.
⑦The moving speed of the spray head should mainly match the rotation speed of the anilox roller, and should generally be controlled at about 100 mm / min.
⑧ Can repeat the medium spray cleaning of the anilox roller for a long time to avoid damage to the anilox roller mesh wall. For the same anilox roller, the medium jet cleaning method is best not to be used as a routine cleaning method every day, but only as an auxiliary periodic maintenance method, or only to restore the severely clogged mesh cavity to the original ink capacity .
According to the different media used in jet cleaning, there are currently the following cleaning methods:
â‘´Baking soda powder washing method. This is a powdered cleaning medium made of baking soda powder (sodium bicarbonate) and sodium dicarbonate. It uses the spraying force to remove the residual ink, resin, coating materials, etc. that have dried up in the net hole to achieve the purpose of cleaning. Its advantages are: low pressure spray washing, generally using a safe pressure of 0.4MPa. In addition, the texture of baking soda powder is much softer than the surface coating of the anilox roller, the particles are smaller, they can enter the mesh hole, the cleaning efficiency is higher, and the surface safety of the anilox roller and the mesh wall can be ensured. Baking soda beads are an easily soluble, safe, non-toxic and harmless substance, which is beneficial to environmental protection. Baking soda powder cleaning is not suitable for anilox rolls with medium and high lines above 600 lines, because it easily causes scratches on the wall of the net.
⑵ Dry ice spray washing method. This is the use of liquid CO2 to make solid CO2 dry ice. The prepared dry ice particles or dry ice cubes are loaded into the cleaning equipment. Under the drive of compressed air, high-density dry ice particles or fine powder are washed with compressed air to be washed The surface of the object, thus playing a cleaning role.
Before dry ice spray washing, the dry ice should be crushed to make particles with a particle size of about 1 micron before use. The unique feature of dry ice cleaning is that the dry ice particles vaporize at the moment of impact, and the solid CO2 sublimates quickly into a gas, which expands nearly 800 times in volume in a few thousandths of a second, which causes a micro explosion at the impact point. As CO2 evaporates, all that remains is dirt that needs to be removed. Due to the temperature difference between the dry ice particles and the cleaned surface, thermal shock occurs. As the temperature of the material decreases and the brittleness increases, the dry ice particles can crush the dirt layer.
The main advantages of dry ice cleaning are: â‘ cost savings. Dry ice online cleaning saves a lot of downtime, no need to increase labor and production equipment, reduce costs. â‘¡Extend the service life of the equipment. â‘¢ No damage to the anilox roller, on-machine control elements, switches, etc. â‘£Safety and environmental protection. Dry ice cleaning does not pollute the environment and does not produce waste residues. However, dry ice cleaning is easy to cause the ceramic surface to burst, and it cannot be used to clean high-line anilox rollers. Because dry ice cannot reach the bottom of the net hole, of course, it cannot remove impurities at the bottom of the net hole. Therefore, the dry ice cleaning method is generally only suitable for cleaning anilox rolls with a low number of anilox lines.
(3) Plastic fine-bead spraying method. The plastic beads used in this method are specially made of polyethylene. When the anilox roller is cleaned, it is ejected from the nozzle at a pressure of about 0.5MPa. The abrasive is not used, and the polyethylene plastic medium is used. The particles are very soft. Polyethylene has sharp edges. When it comes into contact with the anilox roller, it deforms first and then rebounds, so that the residual ink residue, resin, paint, etc. in the mesh can be taken away to achieve the purpose of full cleaning. There is no damage to the cell surface of the ceramic anilox roller.
This cleaning method requires an alkaline recovery device, which is specially used to remove the steel scraps from the scraper. Residual ink, impurities, etc. are collected in a collection bag, and the polyethylene plastic media particles are returned to the air blowing unit and reused.
â‘· High pressure water washing method. It uses high-pressure water to directly spray the anilox roller to wash off residual ink, and can be used with alkaline cleaning media. The disadvantage of this method is that the residual ink in the mesh cavity will often be washed into the bottom of the mesh cavity by high-pressure water, thereby affecting the cleaning effect. The sewage discharged after washing by this method directly affects the environment. [next]
2. Ultrasonic cleaning method
Ultrasonic cleaning is an efficient cleaning method, but it should be noted that the ultrasonic device should be adjusted to a frequency suitable for cleaning. Most printing plants that use ultrasonic cleaning should also use strong corrosive solutions to improve efficiency and reduce cleaning time. Because flexographic printing mostly uses water-based inks, there is an immersion process before cleaning by ultrasonic cleaning method, which is used to soften residual impurities such as residual ink in the anilox of the ceramic anilox roller. This procedure depends on the size of the roller and the clogging of the ink, and generally takes 0.5 to 24 hours. Ultrasonic cleaning method uses ultrasonic waves to generate ultrasonic cavitation phenomenon-the formation of subtle bubbles in mechanical form, these bubbles will inwardly explode when they touch the surface of the anilox roller under pressure, and the energy released by the inward explosion will form a stirring action Or it can be used to remove impurities such as residual ink from the anilox roller.
The ultrasonic cavitation intensity is affected by the following factors:
â‘´ Ultrasonic intensity. It is best to refer to the ultrasonic intensity standards recommended by the cleaning equipment manufacturer.
⑵Ultrasonic frequency. The ultrasonic frequency is inversely proportional to the intensity of the cavitation phenomenon. As the frequency increases, the bubbles become smaller and the cavitation intensity also becomes smaller. In general, it is sufficient to select the bubble generated by the minimum frequency of 40,000 Hz.
⑶ Soaking temperature. The temperature of the soaking solution is generally maintained at 60 ° C to 70 ° C.
â‘· Cleaning time. The cleaning time is the most important factor. The longer the anilox roller is placed in the ultrasonic solution, the more easily its mesh holes are damaged. Many ultrasonic cleaning methods use a pre-soak procedure, followed by ultrasonic cleaning for 5 minutes, or a shorter time to protect the anilox roller from damage.
3. Laser cleaning method
This is a new cleaning method developed in recent years. It uses a laser device with special optics to clean the anilox roller. In the cleaning system, the laser is placed in an optical device to protect the surface of the ceramic anilox roller, but it can evaporate impurities such as residual ink on the anilox roller. It is said that this cleaning method can clean the laser engraved ceramic anilox roller with a maximum screen number of 1200 lines / inch, and remove dry ink, fusion agent, wax, silicon, polytetrachloroethylene, plate particles and even plate particles from the mesh Scraper particles, etc., without damaging the anilox roller mesh holes.
In terms of ink supply and cleaning, it is used to quantitatively supply ink to printing parts and automatically clean the anilox roller, cavity scraping system, ink pump and ink tube. The cleaning sequence is controlled by the computer, which improves efficiency and quality. [next]
Second, the chemical cleaning method
The chemical cleaning method is also called alkaline solvent cleaning method. It uses corrosive alkaline (pH value 8 to 14) cleaning agent to soften, dissolve the residue in the net hole by painting, drip, spraying or soaking. Then wash with high-pressure water. According to the strength of the cleaning agent, it can be divided into strong alkaline cleaning method: the pH value of the cleaning agent is 12-14, which is an efficient cleaning agent. However, it has the disadvantages of strong corrosion to metal materials and certain harm to human contact, so it is not suitable for anilox rollers with low corrosion resistance. Neutral alkaline cleaning method: The pH value of the cleaning agent is 10 to 12, which is a very widely used mild cleaning agent, which can remove the colorant, but may leave residues, which affects the cleaning effect. Weak alkali cleaning method: the pH value of the cleaning agent is 8 to 10, which is an effective cleaning agent. But it is easy to corrode the net hole and cause damage to the net hole.
Due to the strong penetration of the cleaning agent in the chemical cleaning method, the cells of the anilox roller of more than 1000 lines can also penetrate to the bottom, so it has a good cleaning effect and is widely used.
The chemical cleaning method can be generally divided into a chemical solution immersion method and a chemical solvent brushing method.
1. Chemical solvent immersion method This method requires special equipment, first use chemical cleaning agent to dissolve the ink and other substances remaining in the net hole, and then use high-pressure water flow to wash away the dissolved residue. During operation, first place the anilox roller in the washing machine, and add cleaning agent, soak the anilox roller in whole or in part in the tank. The liquid level of the cleaning agent is so that 1/2 to 1/3 of the anilox roller is The immersion prevails, the cleaning agent is heated to 40 ℃ ~ 60 ℃ degree, the anilox roller is cleaned for about 2 hours under the condition of non-stop rotation, let the solvent dissolve and soften the dried ink, and then spray the rotating anilox roller with high pressure water, Remove residues such as cleaning fluid and dissolved ink, and finally dry with high-pressure gas. However, this method has a problem with the treatment of cleaning wastewater. A separation device should be equipped to separate the water from the residual ink and the chemical cleaning agent.
2. Chemical solvent scrubbing method Chemical solvent scrubbing method is to spray chemical solvent on the surface of the anilox roller, and select a suitable fine brush according to the material of the anilox roller (stainless steel wire brush for ceramic anilox roller, copper wire brush for metal anilox roller) ), Scrub repeatedly. This method is only suitable for cleaning the anilox roller of coarse wire. The advantages are simple equipment, convenient operation, low cost, easy cleaning and environmental protection. The disadvantage is that it can't be deeply brushed into the net hole, it is difficult to clean the dirt and accumulated ink at the bottom of the ink hole, so the cleaning is not thorough and the volume of the net hole cannot be restored. [next]
3. Anilox roller coating / cleaning
Concurrent cleaning
Before using the new anilox roller on the machine, pretreat the surface of the mesh, such as coating a thin layer of water-repellent polymer coating on the surface of the ceramic anilox roller to prevent ink and mesh The surface of the cavity is in direct contact, so that the printing ink does not accumulate on the bottom of the cell. This method of applying a coating on the anilox roll surface for easy cleaning in the future is coating / cleaning combined cleaning.
The coating / cleaning method has the following characteristics: (1) The water-repellent polymer is used as the coating of the ceramic roller, which not only can seal the porous structure of the cell surface, but also avoid direct contact with ink and cleaning agent, The coating will also play a good role in protecting the anilox roller. ⑵ The surface tension of the inner coating of the cell is lower than the surface strength of the ceramic, which can improve the thin ink performance of the cell and the transfer rate of the ink. ⑶Because of the protective effect of the coating, it can be cleaned with non-corrosive cleaning agents, thereby improving the safety and environmental protection of the cleaning operation. ⑷ Can avoid corrosion and damage to the anilox, so that the anilox roller can maintain a good working state for a long time, thereby obtaining significant economic benefits.
The coating / cleaning process mainly has two methods: on-machine cleaning and off-line cleaning.
The coating / cleaning system is suitable for new or fully cleaned anilox rolls, whether it is a wide-width or narrow-width flexo printing machine. However, the effect of cleaning the anilox roller with residual ink in the mesh hole is not good. In addition, pay attention to the dilution and application of the paint, and pay attention to the coating liquid in the net hole, otherwise it will not protect the net hole because the coating is too thin, which directly affects the cleaning effect. The coating / cleaning process is currently an ideal anilox roller cleaning system. With the development of high-performance coating materials and the further improvement of process technology, it will become the main method of anilox roller cleaning in the future.
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