In recent years, flexographic printing has been widely used in the packaging industry as a new printing method in China. It has the advantages of fast printing speed, high durability, easy operation, inline production of printing and post-processing, short production cycle, etc., and environmental protection, it has attracted more and more attention.
Flexographic printing has a wide range of printing materials and high production efficiency. It uses environmentally friendly ink and UV inks. During the printing process, it can integrate offset printing, gravure printing, and silk screen printing units. Inline production has become the focus of attention today.
The development of flexible version in China is late, the development speed is slow, the application level is low, and there is a big gap compared with European and American countries. However, in recent years, it has become more and more widely used in packaging such as milk packs, self-adhesive trademarks, cigarette labels, wine labels, toothpaste tubes, cartons, etc. Due to the lack of understanding of flexo printing technology and the limitation of objective conditions, it is not yet possible to print products comparable to offset printing.
For the maintenance and maintenance of different flexo printing machines, how to master the flexo printing process, and the adjustment of pressure, let us talk about our views.
1. Maintenance and maintenance of flexo printing machine
The synchronization of the power of the printing unit of the flexo printing machine, the balance of the ink supply, and the balance of the plate roller have certain requirements in the printing process.
The power printed by the flexo printing machine is transmitted from the main motor to the reduction box of each unit, and each color group operates synchronously. In order to make the printing products stable, the gearbox must be well maintained. The reducer is one of the key components of the entire printing press. Pay attention to the oil volume and oil temperature when starting up (the oil is turbid to be replaced), and it is not allowed to start up at high speed without lubrication to avoid damage. When the circumferential register is required, start the circular version motor to make the planetary output carrier produce positive and negative revolutions, which can achieve the superimposition or reduction of the speed of the printing roller at the synchronous speed, thus completing the requirements for register in the circumferential direction.
The balance of the ink volume is completed by the anilox roller and the doctor blade. Anilox roller ink supply is accurate and reliable. There is a ceramic wire roller, which is coated with ceramic on the base roller and laser-engraved. Its precision, surface roughness, shape and position tolerance, hardness, corrosion resistance are all very high and expensive, and it is produced by a professional factory. When using this type of anilox roller, pay special attention to prevent bumps, keep clean and prevent ink from clogging, and clean it immediately after printing. The accuracy of the anilox roller is one of the important factors to ensure the printing quality of the flexographic printing machine.
The anilox roller is a precise ink transfer roller. The amount of ink transfer is determined by the number of mesh lines per inch. The higher the number of lines, the more mesh pits and less ink; the fewer the number of mesh lines, the less mesh pits, the greater the amount of ink. The number of anilox rollers in printing is reasonable, which is determined by the color requirements of the printed product. Screen screening, screen printing should choose high line count, low ink volume; solid printing should choose low line count, large ink volume.
The doctor blade is used to ensure the uniform supply of ink to the printing plate. If you don't need a squeegee, use a rubber roller to transfer ink. The anilox roller is pressed tightly, and the ink transfer is fast and the speed is slow. The anilox roller has a large difference in the amount of ink carried. The amount of ink that rotates fast is more, but the amount of ink that rotates slowly is less. For this purpose, a doctor blade is provided on the flexo printing machine. With the squeegee, the amount of ink transferred from the anilox roller to the printing plate is limited to the amount of ink stored in the anilox roller cell, so that regardless of the speed of the machine, it can ensure the balance of the printing plate ink. Therefore, there are very high requirements for installing scrapers. The scraper should also have certain elasticity, toughness and strength. The blade should be straight and free of burrs and wrinkles. The generally used steel scraper is 0.15mm or 0.2mm.
The installation of the doctor blade can be installed in the forward direction or in the reverse direction. At present, the reverse ink blade is mostly used, and it is combined into the ink chamber. [next]
2. Use anilox roller and scraper
Issues to be aware of
1. The prolonged use of the anilox roller will cause the bearings on both ends to wear out, resulting in the lack of ink in the printed products. At this time, the bearings on both ends should be replaced in time.
Before the printed matter is found, the operator can also check it. A dial indicator can be used to test the circumferential runout of the anilox roller, or it is normal to put it on the scraper frame by hand when running. If there is jitter, replace the bearing in time. In order to ensure the printing precision and the quality of the printed products, the anvil roller, plate roller, and bearing roller three roller runout value should not exceed ≤0.02mm.
2. After the squeegee is worn, it should be replaced in time. Usually pay attention to disassemble the knife holder for cleaning, and clean the ink in the scraper and the knife bar. The ink in the knife holder has not been cleaned for a long time. After the doctor blade is installed, the blade will form a curved shape and not straight. The amount of ink scraped off the anilox roller forms a circumferential ink mark, which is difficult to adjust during printing.
Squeegee materials are divided into stainless steel scrapers and polyester plastic scrapers. Plastic squeegee is generally used for solid layout printing, it is better. Due to the high-speed operation of the flexo printing machine, the knife edge wears faster, which often causes the polyester plastic scrape debris to fall into the ink or be caught between the scraper and the anilox roller, and then passed into the printing plate, affecting the quality of the print. Steel scrapers are generally installed in reverse for shovel scraping due to low wear, with an angle of 30 ° -40 °. The angle and pressure of the scraper depends on the adjustment of the scraper spring (cylinder).
The flexo printing machine operates for a long time and wears the doctor blade faster, so the correct installation of the doctor blade is also particularly important for printing. When the squeegee blade is worn out, it is necessary to clean the ink in the blade holder and the pressure blade. If there is dry ink in the knife holder and the knife bar, it will cause the blade edge to be curved and not straight after being mounted on the blade, which will also cause circumferential ink marks on the anilox roller. It is difficult to adjust during printing and affect the quality of printing.
Clean the squeegee every time after printing, and wipe the squeegee with alcohol to evaporate the water on the squeegee to avoid rusting of the squeegee and affect the uneven ink scraping during printing. [next]
Third, the printing pressure of the flexographic plate
Flexographic printing is a direct printing method with less pressure, which is called "light kiss" printing. Because the pressure of flexographic printing is very small, the size of the pressure will obviously change the quality of the printing, so the control of the pressure is very important.
Different models have different adjustment methods. For example, the adjustment of the "Black Bull" flexo printing machine is basically the same as that of the Yajiafa flexo printing machine.
Since pressure is the most sensitive issue in flexographic printing presses (unlike gravure printing, the printing quality is not affected by pressure), flexographic printing press operators should pay attention to the adjustment of pressure during machine operation. When the general flexible printing plate is completed, the thickness error of the entire printing plate should be controlled within 0.015mm ~ 0.02mm. When the pressure is greater during printing, the printing plate transfers more ink to the substrate. Therefore, in order to ensure that the entire pattern part can be uniformly inked without causing hair, paste, etc., the printing pressure, that is, the compression of the printing plate should be strictly limited to 0.1mm (screen version 0.02mm). If the pressure is too large, the text and image will be crushed, the dots will be deformed and connected to each other, and it is prone to problems such as unrealism or the edges of the pattern not printed. The flexo plate is compressed by 0.01mm, and the embossing force is doubled (1 ～ 3kg / cm2). The dot increase of the screen version is serious, which will cause the dark tone part of the full version dot to become solid, and the dots appear hollow, dots Not smooth. Due to the change of the dot level, the color will be severely cast, so that the entire screen looks unclear, unclear, and the subtle levels cannot come out, and the three-dimensional sense is not strong; too many middle levels will make the screen dark, and the color restoration is not good. Brightness and other issues seriously affect product quality. Edge effects and hard mouths appear on the ground, lines, and text, the imprint looks uneven, the ink color is not thick, and the text is pasted, so that it loses the characteristics of strong color and clear peak of the letterpress printing.
The size of the dot determines the color reproduction of the printed matter.
According to the principle of geometry, the increase of the dot is that the edge part spreads out evenly. With different percentages of dots, the edge increase of each proportion is consistent. Under the conditions of a certain pressure on the dots of the same area and different shapes, the dots with a larger circumference will have a larger dot expansion rate.
Therefore, the quality of flexographic printing is closely related to the pressure of operation adjustment. The size of the printing embossing force should be adjusted to the point where the dots do not expand or the expansion rate is the smallest, the edge effect and the hard mouth are not visible or not obvious, which is crucial. When adjusting the pressure, the pressure should be zero, and the pressure on both sides should be consistent (the balance line and the test strip should be made on both sides of the plate during plate making to ensure that the pressure on both sides of the printing plate is consistent). The actual printing depends on the specific conditions of the layout, for example, the pressure when printing a fine screen version is relatively small, and the pressure when printing a solid version is relatively large. In addition, the adjustment of the pressure also includes the adjustment of the contact pressure between the surface of the anilox roller and the surface of the printing plate. When the pressure between the two is too large, the ink will be squeezed from the surface of the plate to the periphery of the entire graphic part and accumulate A paste plate is formed on the shoulder. At this time, even if the printing pressure is appropriate, the dot overlap phenomenon will inevitably appear on the printed product; otherwise, if the pressure between the two is too small, it will affect the transfer of ink. Before using a set of plate rollers each time, be sure to use the "plate gauge" (feel gauge) to carefully adjust. Under the condition of ensuring the printing pressure, the pressure between the printing plate and the anilox roller should be the thickness of the printing plate + the thickness of the double-sided adhesive-0.01mm, and the pressure between the printing plate and the substrate should be the thickness of the printing plate + the double-sided adhesive Thickness + substrate thickness-about 0.01mm to determine the above two pressures. When printing on the ground, it is necessary to avoid excessive printing pressure. Generally, the ghost image on the edge cannot be recognized by the naked eye.
Without measuring the gauge (feel gauge), the paper of the substrate can be sandwiched between the printing plate and the impression cylinder, and then gently pulled, after a little force can be pulled out, basically Stress is more appropriate.
The printing pressure is also related to the printing speed. The printing speed is fast, and the contact time between the printing plate and the substrate is short. The printing pressure should be appropriately increased, otherwise the printing pressure should be reduced.
The pressure of the anilox roller and the plate roller is constant (0.02mm ~ 0.03mm). The pressure of the plate roller and the printing roller is adjustable according to the performance of the substrate.
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