Author: Beijing Institute of Graphic Huang Beiqing
Corrugated cardboard box is made of corrugated cardboard. When printing directly on the cardboard box, in order to ensure the compressive strength of the cardboard box, strong impact on the corrugated cardboard should be avoided, but at the same time, it is necessary to ensure that the ink transfer is good. Printing is the best choice.
As we all know, the quality of printing has a close relationship with the quality of plate-making. The quality of our flexographic printing products is not as good as that of similar foreign products. To a large extent, it is caused by the poor quality of plate-making. Carton printing is no exception, if you want to get high-quality prints, you must pay attention to the plate making process.
The flexographic plate has high elasticity, which brings benefits to the flexographic printing, but also brings adverse effects. The highly elastic material deforms greatly, and the flexographic printing plate will deform greatly during printing, which will cause a large change in the size of the graphic on the printed matter, and the dot increase will be more serious. Therefore, when making flexographic plates, Taihu Lake took into account the adverse effects of this deformation on printing and made corresponding compensations to ensure the quality of the printed matter. This is the key to flexographic plate making.
1. Design requirements for manuscripts
The manuscript is the basis of printing. For corrugated cardboard boxes that use flexographic printing, the design of the manuscript has the following requirements.
(1) During the printing process, the ink volume and pressure required for large areas and small text, thin lines, dots and other details are not the same, so even if they are the same color, do not put them on the same printing plate. Should be separately plate printing.
(2) When performing full-page printing on a corrugated cardboard box, in addition to the pressing effect of the embossing roller on the corrugated cardboard, the ink also has a wetting effect on the paper surface, which reduces the compressive strength of the corrugated cardboard box, so full-page printing should be avoided .
(3) It is not advisable to overprint two color blocks of the same shape and size, so as to avoid overprinting, and local text, patterns or small color blocks should be overprinted on the solid color block with a larger area.
(4) Avoid designing wide and long bars and solids along the axial direction of the printing plate cylinder, which will cause machine vibration; in addition, avoid the band pattern in the same direction as the corrugated cardboard corrugation, otherwise the printing pressure will crush the corrugation , Causing the carton to break.
(5) When designing the layout, large circular patterns should be avoided. Otherwise, the printed patterns will become elliptical due to the flexographic elongation in the circumferential direction.
(6) For the negative characters, especially the small ones, the strokes should be thickened consciously to avoid the strokes becoming thinner or even muddled due to ink spreading during printing.
(7) Generally speaking, texts with an area of less than 2cm2 cannot be seen beyond a distance of 5cm. For corrugated carton printing, the meaning of publicity and promotion of goods is lost. Therefore, the design should avoid using too small text.
(8) Avoid precise overprinting of tight stitches. The two-color overprinted parts generally need to be trapped with 0.1 ~ 0.2mm to avoid whiteness when the overprint is poor.
(9) Minimize the number of overprints, generally less than 3 colors. The greater the number of overprints, the more severe the compressive strength of the corrugated box decreases. Practice has proved that after single-color printing, the compressive strength of corrugated boxes decreases by 6% ~ 12%, and after three-color printing, the compressive strength of corrugated boxes decreases by 17% ~ 20%.
2. Flexible plate deformation and film size compensation
Because the flexible plate has elasticity, after being installed on the plate cylinder, the plate will bend and deform in the circumferential direction of the cylinder, but will hardly deform in the axial direction of the cylinder. This deformation changes the size of the printed text. For the more demanding prints, measures must be taken to compensate, and the best way is to change the size of the graphic on the printing film. If there is a circular pattern on the original, you can use the deformable mirror to shoot according to the calculated compensation factor. The compensation factor is the reduction factor of the printing film, that is, the percentage of the graphic size of the printing film in the printing direction. The compensation factor for the size of the graphic on the printed film can be calculated using the empirical formula of DuPont, see formula ①.
Compensation coefficient (%) = K / R × 100% ①
In formula ①, K is the reduction of the printing plate, which is a constant related to the thickness of the photosensitive resin layer, and can also be calculated according to formula ②.
K2πd = 2π × (total thickness of printing plate-thickness of supporting layer) ②
Where d is the thickness of the photosensitive resin layer of the flexographic plate.
In formula ①, R is the printing repeat length, which can be calculated according to formula ③.
R = 2π × (r + b + c) ③
Where r is the radius of the printing plate cylinder, b is the thickness of the adhesive liner, and c is the total thickness of the printing plate.
3. Flexibility tone reproduction
The size of the dots determines the tone reproducibility of the printed matter. Therefore, to ensure the printing quality of the flexible plate, the dot reproducibility of the flexible plate should be controlled. The dot reproducibility of flexographic printing is different from that of offset printing. As shown in Figure 1, the dot increase of flexographic printing is more serious, the reproduction range of the tone is narrower, about 15% to 85%, and the highlight and middle tone ( The percentage of dots between 20% and 40%) is the most serious. This is mainly due to the flexibility of the flexographic plate. Small dots are easy to lose during plate making, and the printing plate will be deformed during printing, which makes the dots increase seriously, and the dots in the dark tone part are easy to die.
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4. Selection of the number of screens
When making a plate, the selection of the number of screen lines is very important, it determines the reproduction quality of the tone level of the printed matter. The higher the number of screen lines, the richer the tone level of the printed matter, but the smaller the dots, the more serious the dot increase during printing, and the more serious the level loss of the dark tone part. Therefore, the number of screen lines must be selected correctly.
In flexographic printing, the smallest dot that can be reproduced is determined by the reproducibility of the printing plate during platemaking, and the largest dot that can be reproduced is determined by the characteristic of dot gain during printing. Because the number of screen lines is different, the size of the same percentage of dots after printing is different. Therefore, when making a flexographic plate, the appropriate number of screen lines should be selected according to the tone reproduction of the printed matter. Due to the serious increase in the dots of flexographic printing, in order to ensure the tone reproduction range of the printed matter, the number of screen lines is generally below 133 lines / inch. If the resolution of the printing plate is high and the printing conditions are good, the number of screen lines can also be reached. 150 lines / inch, 175 lines / inch. Of course, if the tone range of the printed matter is narrow, the number of screen lines can also be increased appropriately.
In addition, for flexographic printing, the number of anilox rollers used for printing should also be considered when selecting the number of screen lines to ensure that the smallest dots can also get enough ink, which requires that the opening area of the anilox roller ink hole is less than The area of the smallest outlet. Generally, the number of screen lines should be about 1/3 of the line number of the anilox roller.
5. Avoid moire
Because flexographic printing uses anilox rollers to transfer ink, and the anilox roller's ink holes are engraved at a certain angle, it is easy to interfere with the screen angle and produce "craze". Therefore, pay attention to the flexographic plate making Avoid moire. Since most anilox rollers usually engrave ink holes at a 45 ° angle, the screen angle should try to avoid 45 °, generally Y90 °, M15 °, K30 °, C60 °, or Y82.5 °, M67.5 °, BK37. 5 °, C97. 5 °. At present, there are many new anilox roller ink hole engraving angles are no longer 45 °, but 60 ° or other angles are used for engraving, which is very beneficial to avoid moire.
The experiment proves that if the anilox roller does not cause serious wear, the moiré produced by the 45 ° screen angle is not serious. The main cause of moire is that the number of lines of the anilox roller and the number of screen lines are mismatched. Therefore, as long as the number of lines of the anilox roller matches the number of screen lines correctly, moire can be effectively avoided. An important method to avoid moire is to use Nassen diagram (the number of lines of the anilox roller-the number of plates plus screen lines to match), as shown in Figure 2. The abscissa in the figure represents the line number of the anilox roller, and the ordinate represents the number of screen line of the printing plate. The 4 thick black diagonal lines in the figure indicate the best match between the line number of the anilox roller and the plate screen line. Moire occurs, the further away from the thick black line, the easier it is to produce moire. For example, when the number of screen lines is 39 lines / inch, the number of screen lines is 110 lines / inch; if the number of screen lines is 118 lines / inch, the number of screen lines can be 26 lines / inch, 32 lines / inch Inches, 41 lines / inch. This is a very convenient method.
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