Q&A on flexographic printing technology (1)
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In recent years, in the printing and packaging industry, everyone has felt a brand-new printing method - flexographic printing, which is coming to us with unprecedented momentum. Flexographic printing is uniquely flexible, economical, and environmentally friendly. It has proven to be the "best and most promising" printing method in developed countries. Since the 1980s, China has introduced dozens of wide-format and narrow-width flexographic printing presses. Due to the lack of understanding of flexographic printing technology and the limitations of some objective conditions, especially related technical equipment, Some imported machines have not been fully utilized, which has affected the company's efficiency and cast a shadow over the introduction, application and localization of flexographic printing technology in China. In response to this situation, in the future when introducing flexographic printing technology and equipment, we must pay attention to the digestion, absorption and application of imported technologies, and find out the working methods suitable for the national conditions.
At present, offset printing is popular in China, and gravure has also occupied a large market in the packaging industry. For the new technology of flexographic printing, it is inevitable to compare it with offset and gravure because of the order of flexographic printing. It will be diverted from the above two printing methods, which requires a sufficient understanding of the characteristics of flexographic printing. After decades of continuous and stable development, the flexographic printing industry in the western developed countries has developed to the point where it can be almost offset with offset printing and gravure printing. People no longer think that flexographic printing can only print low-end files in the field and coarse mesh. Printed. As the voice of protecting the environment grows, the advantages of flexographic printing are becoming more and more obvious. At present, China's flexographic printing industry has just started. It is not realistic to master the experience of foreign countries for decades. Therefore, we must track the development trend of advanced foreign flexographic printing technology, and we must digest, absorb and apply it. The flexographic printing equipment has laid a solid foundation and gradually caught up with the international advanced technology.
The author has been involved in flexographic printing since the beginning of 1993. He used to be the production supervisor of a company's flexographic printing workshop and is now a flexographic printing machine sales engineer. From 1994 to 1995, Zeng Yi went to the United States to inspect the company. After more than two months, he visited experts in various fields such as flexo printing machine, water-based ink, flexographic plate making, anilox roller and die-cutting. He listened to them. The status of printing technology and the opinions and suggestions of flexo printing in China. I am very willing to write these experiences and feelings, and discuss with the seniors and colleagues in the printing and packaging industry. Welcome everyone to criticize and correct.
Flexographic printing is a comprehensive application technology that includes multiple subject areas. The ink design and plate making process are self-contained, and the post-press processing is usually online. Therefore, flexo printing and on-line die cutting and indentation operations are performed. The monitoring methods for the service and the collection of finished products are also unique. For printers, flexographic printing has one of the biggest differences compared to offset and gravure, that is, flexo printing uses water-based inks, while water-based inks have printability and familiar solvent-based inks. Very different. Therefore, we must pay attention to the technical support of the flexographic printing itself, and avoid the experience of offset printing or gravure printing. As for the proper order for flexographic printing, we must fully understand the advantages of flexo printing technology in order to fully utilize the strengths of flexo printing. For example, for plastic film printing, only knowing the process of flexographic printing and comparing it with gravure printing can find a print suitable for flexographic printing. For example, paper printing, only know the cost and life of the die cutting, indentation and other tools, can be compared with offset printing in terms of cost and efficiency.
Regarding the introduction of flexo printing machines, my opinion is that we should first form a certain scale in Beijing, Tianjin, Shanghai, Xi'an, Guangdong and other large and medium-sized cities, which is beneficial to the users of flexo printing machines. Have access to each other to quickly master the modern flexographic printing technology. From the big cities to the surrounding areas, and even drive the development of national flexographic printing.
I have encountered many problems in my work. I believe that companies that have just engaged in flexographic printing will also encounter these problems in the printing process. Therefore, I have summarized these issues and hoped to help readers.
How to equip flexographic printing machine with anilox roller
Most prints have both solid, line, and continuous tone images. In order to meet the needs of various printing products, the user must not adopt a few anilox rolls with several printing units on the flexo printing machine. Take the narrow-width unit type flexo printing machine as an example. At present, most of the domestic introductions are 6+1 type, that is, 6 color groups are designed for multi-color printing, and the last unit can be printed and UV glazed.
We recommend that for 6-15 lines of printing, this 6+1 flexo press should be equipped with 9 anilox rolls. Four 700 lines, ink layer thickness of 2.3BCM (1 billion cubic micrometers / inch), 60 ° anilox roller for layer printing; three 360 ~ 400 line, BCM6.0, 60 ° netting The roller is used for solid printing; two 200-line, BCM15, and 60° anilox rolls are used for gold plating and glazing. If water-based varnish is used, a 360-line anilox roll should be used so that the oil layer is slightly thinner and will not affect the printing speed due to the drying problem of the varnish. Water-based varnishes do not have the special odor of UV varnish. The position of the anilox roll during printing can be determined by testing and comparison. The thickness of the ink layer that the operator can observe during the test mainly depends on the number of lines of the anilox roll and the BCM value.
What problems should be paid attention to during the use of anilox roller
The anilox roller we refer to here refers to the laser-engraved ceramic anilox roller. It is a high-temperature, wear-resistant coating material used in aviation and aerospace. It is used according to a certain density, depth and a certain angle and shape. Laser engraved. The anilox roll is characterized by high cost and wear resistance. If used properly, its life can last for several years; if used improperly, not only will the life be shortened, but also the anilox roll will be scrapped.
During use, the position of an anilox roller on the printing press depends on the specific printing, and the position of the anilox roller is different for different printings. Therefore, the wire roller is often exchanged during printing. At present, the narrow-width machine adopts mostly solid steel anilox rolls, which are very heavy. When installing the anilox rolls, care should be taken to avoid touching the surface cover of the anilox roll to other metal objects. Because the ceramic coating is very thin, it can easily cause permanent damage upon collision. In the process of printing and cleaning the machine, the ink should be prevented from drying on the anilox roller. The special detergent recommended by the water-based ink manufacturer should be used, and the brush should be brushed with stainless steel to ensure clean and thorough cleaning. And develop the habit of observing the anilox roller mesh with a high magnification magnifying glass. Once the ink is found to be deposited at the bottom of the cell and there is a tendency to increase gradually, it should be cleaned in time. If the above method does not work, it can be treated by ultrasonic or sand blasting, but it must be carried out under the guidance of the anilox roller manufacturer.
Under normal service conditions, there is no need to worry about the abrasion of the anilox roller. The wearable parts of the ink transfer system are mainly the doctor blade. Compared with the wear of the anilox roller ceramic coating, it can be said to be negligible. After the anilox roller is slightly worn, the ink layer becomes thin.
What is the reduction?
As we all know, flexographic printing has a phenomenon of unidirectional elongation along the direction of printing paper. The ratio of elongation is related to two factors. One is the diameter of the plate cylinder (including double-sided tape), and the second is printed. The thickness of the plate itself. The larger the diameter of the plate cylinder, the smaller the plate thickness and the smaller the elongation. One concept that needs to be clarified here is that the elongation phenomenon is not caused by pressure in the printing. In fact, the flexographic printing plate has undergone elongation deformation when it is adhered to the plate cylinder by double-sided tape. Therefore, when designing the ink map, it is necessary to reduce the scale, and the ratio is obtained by looking up the table according to the actual thickness of the plate and the actual printing length. If you use computer plate making, you only need to give a one-way reduction instruction before designing the ink separation. If a hand-made film is used, a deforming machine is required, and a scaled-down film can also be obtained.
What is the relationship between the number of printed screen lines and the number of screen lines?
In many articles introducing flexographic printing techniques, the ratio of the number of printed screen lines to the number of anilox roll lines is set to 1:3.5 or 1:4. Based on practical experience and analysis of award-winning products from the American Flexographic Printing Technology Association (FTA) in recent years, the author believes that this value should be higher, about 1:4.5 or 1:5, for individual fine prints. Product, maybe this ratio is even higher. The reason is that the most difficult problem to solve when printing a layered version with a flexographic printing machine is that the dot is enlarged, the anilox roller with a higher number of screen lines is selected, the ink layer is thinner, and the dot expansion deformation is relatively easy to control. If it is found that the ink color is not rich enough during printing, a water-based ink with a higher color density may be selected to ensure the quality of the printed product.
How to design a flexo ink map that is easy to overprint
In the printing process, a key indicator that people often pay attention to is overprinting, because the waste caused by overprinting often accounts for a large proportion of all waste products. Although flexographic press manufacturers have provided a good basis for precision overprinting during design and processing, automatic monitoring systems also provide guarantees for precise overprinting, but the operator's skill level and proficiency also have a great influence on overprinting accuracy. . If you can make some adjustments for convenient overprinting when designing the ink map, it can greatly reduce the difficulty of printing and reduce the scrap rate. The easiest way to do this is to use the reverse white text when designing the original ink draft, or to hollow out between the two colors to form a reversed white outline. You can also cover the edge of the two color overprints with a slightly thicker one. Dark outlines, for the printing of finer prints, can use Trapping technology, the amount of trapping depends on the accuracy of the press, the quality of the paper used for printing and the level of the operator. Trap processing is easy to implement on a computer. If there is no computer, only the ready-made color separation film, you can use a special trapping processor (TrappingModifier), enter the value of the trapping amount, the machine will automatically generate a trapping process. Film. If there is no trapping processor, a transparent spacer can be placed between the color separation sheet and the copy film, and a single point light source copying machine can be used to obtain a similar effect.
What are the requirements for making flexo types?
When the flexible plate is placed on the front side of the plate making machine, in order to ensure the quality of the film and the flexographic printing surface, the plate making machine is equipped with a vacuum pump to vacuum, and the side of the film that requires the film to be close to the flexo must be rough (matte). This can prevent air bubbles from remaining between the film and the flexo plate and affect the quality of the flexographic printing surface. The bead used for the pressure film and the flexographic edge should also be indented and can be used for lateral air conduction. In addition, there are specific requirements for the blackness and transparency of the film.
How to make high quality cable and field versions on one plate at the same time
Generally speaking, in the case where the flexo printing unit is sufficient, the same color of the screen and the solid version should be divided into two, because the screen printing requires a thinner ink layer, while the field and line versions need to be thicker. The ink layer. However, in some special cases, it is necessary to make the same color of the network cable and the field image to the same version. In this case, on the one hand, in the printing, the choice of anilox roller should be compromised, usually it is preferred to meet the printing requirements of the screen version, at the expense of the thickness of the solid ink layer, on the other hand, at the time of plate making, Master different exposure times to ensure the quality of the same plate and the field. The difference between the image portion of the network cable and the solid portion is mainly due to the different depth of corrosion. In order to "stay" in the process of brushing the printing plate, the part of the wire needs to have a strong foundation, so the corrosion depth is shallower. Different corrosion depths can be obtained with different back exposure times by masking the local method. The front exposure time should also be done in a similar way, but it is said that the current flexographic plate manufacturer has introduced a new type of flexible plate, regardless of whether the printed part is a cable or a field, this plate only needs a uniform front exposure. Time, avoiding the trouble of multiple exposures, making flexo plate making more convenient.
Why is the rubber ink roller very easy to wear out?
The rubber squeezing roller is driven by the gear of the anilox roller. There are two functions, one is to refer the ink to the ink transfer system from the ink tray; the other is to “smash” the smudged ink to leave on the anilox roller. The next layer is a relatively uniform ink layer, but its effect is far less than that of the reverse doctor blade. Regardless of whether the machine is printed or not, once the ink is filled into the ink tray, the rubber ink roller and the anilox roller should start to rotate. However, once there is no ink or water between the two rolls, that is, in the case of complete drying, it should be absolutely avoided that the two rolls are rotated under the condition that the surfaces are in contact. Because although the surfaces of the two rollers are in contact, the surface speeds of the surfaces are different, and the line speed of the surface of the rubber inking roller is slightly lower than that of the anilox roller, so that the ink of the crucible is "snapped" and the ink acts as The lubricant between the two rolls. Under the condition that there is no water or ink between the two rollers, if the two rollers contact and rotate, the hard surface of the rubber roller will be worn out in a very short time, and the damage will be intensified, so that the whole rubber ink roller is scrapped. Therefore, it should be remembered that the rubber roller and the ceramic anilox roller cannot be "dry-grinded".
What should I pay attention to when using a doctor blade?
Whether the squeegee is used in the ink transfer system is optional. For rough printing or UV glazing, it is not necessary to use the squeegee, just adjust the pressure between the rubber squeegee and the anilox roller. A substantially uniform ink layer can be obtained. For the printing of general printed products, the squeegee should be used. At this time, the pressure between the rubber roller and the anilox roller should be smaller to ensure that there is enough ink to evenly hit the squeegee blade. A uniform ink layer can be obtained later. In addition, the screws on the squeegee holder should be carefully adjusted. The screws can change the contact pressure between the squeegee and the surface of the anilox roller. If the above method does not work, that is, if the smooth and uniform ink layer is still not obtained, the squeegee blade can be inspected to see if it is physically damaged or raw, or the screw of the fixed squeegee holder is loose, causing the blade and the net. The roller axes are not parallel. Do not scrap the squeegee with small defects. It can be worn on other empty printing units, and then used after the defects are worn away. A squeegee can continue to be used even if it is milled a few millimeters over its lifetime.