Flexographic printing technology 14 question and answer selection
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How to apply anilox roller to flexo printing machine
Most prints have both solid, line, and continuous tone images. In order to meet the needs of various printing products, users 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 or 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 and the BCM value of the anilox roll.
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 coating material that is resistant to high temperature and wear 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 is mostly made of 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 the normal use and maintenance 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, the reduction is required, and the ratio is obtained by looking up the sheet thickness 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. 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, and 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 skill level and proficiency of the operator 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, or to hollow out between the two colors to form a reversed white outline. You can also cover the third thicker one at the edge of the two color overprints. Dark outlines, for the printing of finer prints, Trapping technology can be used. 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 the calculator. If there is no calculator, 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 trap. Printed 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 film.
What are the requirements for making flexo types for film?
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 prevents air bubbles from remaining between the film and the flexo plate and affects the quality of the flexographic printing surface. The bead used for the film and the flexographic edge also has an indentation 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 ink layer on the ground, on the other hand, at the time of plate making, Master different exposure times to ensure the quality of the same plate on the Internet and the field. The difference between the image part of the network line and the part of the field 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 excess ink and leave it on the anilox roller. A relatively uniform layer of ink, but its role is far less than the role 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 with each other, the line speeds of the surfaces are different. The line speed of the surface of the rubber inking roller is slightly lower than that of the anilox roller, so the excess ink is "smashed" and the ink acts as a 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 entire 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 a 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 enough ink is evenly applied to the squeegee blade, so that behind the squeegee In order to get a uniform ink layer. 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 easily scrap the squeegee with small defects. You can use the randomizer on other vacant printing units and use it after the defect has been removed. A squeegee can continue to be used even if it is milled a few millimeters over its lifetime.
How to understand the flexographic plate making process as a whole
At present, the flexible plates introduced in China are basically solid photosensitive resin plates, and the thickness of the plates is selected according to the requirements of the flexographic printing machine. The common plate thickness is 1.70 mm and 2.29 mm. The flexographic plate making process transfers the graphic to the printing plate through the negative film, and mainly includes the following processes:
(1) The back side exposure of the plate with ultraviolet light is used to determine the height of the relief on the plate, ie the depth of corrosion, and to cure the substrate.
(2) The printing plate is exposed to the front side under the action of ultraviolet light to form a printed portion on the printing plate and cured.
(3) The plate is brushed in a solvent to remove the unexposed portion of the plate to form a relief.
(4) The plate is dried in a dryer to promote the evaporation of the solvent absorbed in the plate as soon as possible, so that the thickness of the plate is restored to the original standard value.
(5) Post-exposure and de-bonding of the dried plate can further cure the shoulder and the base and improve the printing performance of the flexographic plate. The high-quality flexo plate should have a printing durability of more than 500,000 impressions, or even up to 1 million impressions.
How to calculate the amount of shrinkage of flexographic film
The amount of reduction is only related to the diameter of the plate cylinder (including double-sided tape and plate) and the thickness of the plate itself. Here is a formula for calculating the amount of film shrinkage (percentage) for a flat exposure flexographic plate: The amount of shrinkage (percentage) = K/Rx100%. In the formula, the coefficient K depends on the thickness of the plate used. When the thickness is 1.70 mm, K = 9.89 mm; and when the thickness is 2.29 mm, K = 13.56 mm. R is the printing length of the plate cylinder in millimeters. If the printing length of a certain product is R=235 mm, the thickness of the plate is 1.70 mm, and K=9.89 mm, the amount of reduction is 9.89/235x100%=4.21%. When designing the ink draft, the film should be longitudinally reduced according to the above ratio, and then plated by plane exposure, so that the printed product will meet the size requirements. It should be further explained that the printing length is analyzed and estimated in actual production. The determination of R should consider various factors. For example, when the plate roller gear module is 1/8 inch, the printing length can only be increased or decreased by 3.175 mm steps; other factors also reduce the paper consumption, leaving the sticking plate. The amount of allowance, and the limitations of machining indentation die-cutting tools, etc.
How to do back exposure test
We can first determine the back exposure time for different corrosion depths by performing a back exposure test on the platesetter. For comparison purposes, we used a method of multiple exposures of the same plate with different exposure times. Take a piece of material that is about 320 mm long and 100 mm wide. When back exposure is performed on the platesetter, we use a red cover strip to partially cover the area, and the plate-making machine exposes multiple times, resulting in five different back exposure times. The printing plate of the area. In addition, a blank of about 20 mm is left at one end of the plate for the plate washer to hold. Suppose we use the Dutch-made AV-500 combination plate-making machine and GRACE's 1.70 mm thick flexible plate. After partial covering and multiple exposures, we get 0 time exposure on the same plate. Five parts of 15, 20, 25, 30 seconds, after exposure, put on the drum of the brush plate for washing, about 7 minutes. It is then dried in a drying oven at a temperature of 50. c, the time is 1.5 hours. Finally, the plate is removed, and four different plate thickness values on the plate can be measured with a caliper, while the unexposed portion of the plate leaves only a hard polyester base.
Through the above tests, the plate making machine can be quickly familiarized, and the test results are also instructive for adjusting the corrosion depth of the printing plate. It can be seen from the above experiments that the length of the back exposure time affects the thickness of the plate base. Therefore, for different printed images, different back exposure times should be used for plate making. Still taking the 1.70 mm thick flexible plate as an example, the reverse white and large solid parts on the plate have a deeper corrosion depth (ie, the plate base is thinner), up to about 1.0 mm, and the back exposure time is the shortest; The ordinary image part has a moderate depth of corrosion, about 0.85 mm, and the back exposure time is moderate. As for the small lines and the dot part, the corrosion depth should be the shallowest, about 0.63 mm, so the back exposure time needs to be the longest.
In addition, there are two points worth noting: when the printer switches to a flexographic version of another thickness or a flexographic version of another brand, the back exposure test must be repeated. As the UV lamp is used for a longer period of time, its light intensity will be reduced. At this time, the back exposure time should be adjusted accordingly.
How to do a positive exposure test
The front exposure, also called the main exposure, forms the printed portion of the flexographic image. Appropriate positive exposure and back exposure work together to achieve a high-quality printing version that meets the requirements of various graphic types. In general, underexposed exposure will cause the bottom of the word shoulder to be brushed off, which will affect the service life of the printing plate; excessive positive exposure will cause insufficient corrosion, which may cause paste, which will result in waste during printing.
Before performing the front exposure test, we recommend that the operator prepare a test "sample" for the flexographic version. The "sample" should contain small anti-white, ordinary anti-white and solid, ordinary graphic, small Lines and characters, dots and other basic printed graphic types, and a certain number of network lines (such as 150lpi) from 10% high to 100% of the field are reflected in the "sample".
The positive exposure test can be performed by using a "swatch" on the platesetter by partial occlusion and multiple exposures. In the early stage, we can start with a less difficult business, from easy to difficult, accumulate experience, and fully master the technology of the flexible version. In theory, the finer the image, the longer the positive exposure time is required, but remember that it works in conjunction with the back exposure. For example, anti-white characters require less back exposure and positive exposure, while small dots require longer shallow exposure depths, so their back exposure and positive exposure time are longer.
After the general exploration and practice, the plate-making operator should understand the limits of his own plate making, such as the thinnest lines, the smallest characters, the most dense dots and 3% of the outlets can stand up, 50% of the outlets "Opening up" and so on.
What should be paid attention to when washing and drying?
As the number of times of solvent use increases, the time for brushing the plate should be appropriately extended; adjust the position between the fine brush and the plate drum according to different plates, thereby adjusting the pressure on the surface of the fine brush and the plate; if the system is particularly fine The mesh point should be based on the standard thickness position to further reduce the pressure of the fine brush and prevent the dots from being washed away.
The drying temperature is about 50. c, the time is about 1.5~2 hours; should be taken out several times during the drying process, check the surface of the layout for solvent spots or other bristles, use a clean cloth without fibers, dip some fresh solvent, gently wipe The surface of the plate is then placed back in the drying oven to continue drying.
Post-exposure and de-bonding of the plate can greatly improve printability. The prepared flexographic plate is preferably placed for 12 hours and then tack-printed, in order to completely evaporate the residual solvent absorbed into the printing plate during the plate making process.