High quality corrugated box flexo printing plate making process
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The corrugated carton is made of corrugated cardboard. When printing directly on the carton, in order to ensure the compressive strength of the carton, it should avoid strong impact on the corrugated board, but at the same time ensure the ink transfer is good. Therefore, the flexo plate is adopted. 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 flexographic prints in China is not comparable to that of similar foreign products, which is largely due to the poor quality of plate making. Carton printing is no exception. To get high-quality prints, you must pay attention to the plate making process.
The flexo plate has a high elasticity, which brings benefits to flexographic printing and also has an adverse effect. The highly elastic material has a large deformation, and the flexographic plate produces a large deformation during printing, which causes a large change in the size of the printed matter on the printed matter, and the dot size is also increased. Therefore, in the flexographic plate making, Taihu Lake considers the adverse effects of such deformation on printing and compensates accordingly to ensure the quality of the printed matter, which is the key to the flexographic plate making.
1. Design requirements for the original
The manuscript is the basis of printing. For corrugated boxes that use flexographic printing, the design of the manuscript has the following requirements.
(1) In the printing process, since the amount of ink and pressure required for large areas of solid and small characters, thin lines, dots, etc. are not the same, even if they are the same color, do not put them on the same plate. Should be separately plated.
(2) When full-page printing is carried out on corrugated boxes, in addition to the pressing effect of the embossing roller on the corrugated board, the ink also has a wetting effect on the paper surface, which reduces the compressive strength of the corrugated box, so full-printing should be avoided. .
(3) It is not advisable to overprint two color blocks of the same shape and size to avoid inaccurate overprinting. The text, pattern or small color block should be partially overprinted on the solid color block with 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 same strip pattern in the direction of corrugated cardboard corrugation, otherwise the printing pressure will crush the corrugated , causing the carton to break.
(5) A large circular pattern should be avoided when designing the layout, otherwise the printed pattern becomes elliptical due to the elongation of the flexographic plate in the circumferential direction.
(6) For negative characters, especially small characters, it is necessary to consciously bold the strokes to avoid thinning or even dying of the strokes due to ink spreading during printing.
(7) Generally speaking, texts with an area of less than 2 cm2 are invisible outside the distance of 5 cm, and the meaning of propaganda and promotional items is lost for corrugated box printing. Therefore, too small text should be avoided in design.
(8) Avoid precise overprinting of tight seams. The two-color overprinting part generally needs to be trapped by 0.1~0.2mm to avoid whitening when the overprinting is not good.
(9) Minimize the number of overprints, generally should be less than 3 colors. The more the number of overprinting, the more severe the compressive strength of the corrugated box is lowered. Practice has proved that after monochrome printing, the compressive strength of corrugated boxes is reduced by 6% to 12%, and after three-color printing, the compressive strength of corrugated boxes is reduced by 17% to 20%.
2. Flexographic deformation and film size compensation
Since the flexographic plate is elastic, after being mounted on the plate cylinder, the plate will be bent and deformed in the circumferential direction of the drum, and hardly deformed in the axial direction of the drum. This deformation causes the size of the printed graphic to change. For more demanding prints, measures must be taken to compensate, the best way is to change the size of the graphic on the film. If there is a circular pattern on the original, you can use the deformed mirror to take a good compensation factor. The compensation coefficient is the reduction factor of the printing film, that is, the percentage of the graphic size of the printing film should be reduced in the printing direction. The compensation coefficient of the graphic size on the printing film can be calculated by the empirical formula of DuPont of the United States, see Equation 1.
Compensation coefficient (%) = K / R × 100% 1
In Formula 1, K is the shrinkage amount of the printing plate, which is a constant relating to the thickness of the photosensitive resin layer, and can also be calculated according to Formula 2.
K2πd=2π×(total plate thickness-support layer thickness) 2
Wherein d is the thickness of the flexographic photosensitive resin layer.
In Equation 1, R is the printing repeat length and can be calculated according to Equation 3.
R=2π×(r+b+c) 3
Where r is the plate cylinder radius, b is the thickness of the adhesive liner, and c is the total thickness of the printing plate.
3. Modular reproducibility of flexographic plates
The size of the dot determines the tone reproducibility of the printed matter. Therefore, to ensure the printing quality of the flexographic plate, the dot reproducibility of the flexographic plate should be controlled. The dot reproducibility of flexographic printing is different from that of offset printing. As shown in Fig. 1, the dot density of flexographic printing is more serious, the tone reproduction range is narrower, about 15%~85%, and the highlight and middletone ( The dot gain of the dot percentage between 20% and 40% is the most serious. This is mainly due to the softness of the flexographic plate. The small mesh points are easy to be lost during plate making, and the printing plate will be deformed during printing, which makes the dot increase serious, and the dot of the dark tone is easy to paste.
4. Selection of screening lines
When making plates, the choice of the number of screen lines is very important, which determines the reproduction quality of the print tone level. The higher the number of screen lines, the richer the tone level of the printed matter, but the smaller the dot size, the more serious the dot gain during printing, and the more serious the layer loss of the dark tone part. Therefore, the number of screen lines must be correctly selected.
At the time of flexographic printing, the minimum dot which can be reproduced is determined by the reproducibility of the plate at the time of plate making, and the maximum dot which can be reproduced is determined by the characteristics of the dot gain at the time of printing. Since the number of screen lines is different, the size of the same percentage of dots is different after printing. Therefore, in flexographic plate making, the appropriate number of screen lines should be selected according to the tone reproducibility of the printed matter. Due to the serious increase of the dot size of the flexographic printing, in order to ensure the reproduction range of the print, 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 be appropriately increased.
In addition, for flexographic printing, when selecting the number of screen lines, the number of anilox lines used in printing should also be considered. It should be ensured that the minimum dots can also obtain sufficient ink, which requires that the opening area of the anilox hole of the anilox roller is smaller than The area of the smallest dot. Usually, the number of screen lines should be about 1/3 of the number of screen rolls.
5. Avoid moiré
Since the flexographic printing uses an anilox roller to transfer ink, and the inking hole of the anilox roller is engraved at a certain angle, it is easy to interfere with the screening angle to produce a "moire pattern". Therefore, attention should be paid to the flexographic plate making process. Avoid moiré. Since most anilox rolls usually engrave ink holes at a 45° angle, the screen angle should be as close as possible to 45°, typically Y90°, M15°, K30°, C60°, or Y82.5°, M67.5. °, BK37. 5 °, C97. 5 °. At present, there are many new anilox rollers that do not use 45° engraving angles, but use 60° or other angles for engraving, which is very beneficial to avoid moiré.
Experiments have shown that if the anilox roller does not suffer from severe wear, the resulting moiré is not severe when using a 45° screen angle. The main cause of the moiré is that the number of anilox lines is not properly matched with the number of screen lines. Therefore, as long as the number of anilox lines and the number of screen lines are correctly matched, the moiré can be effectively avoided. An important means of avoiding moiré is to use a Nassen diagram (the number of anilox rolls - the number of screens added to the screen), as shown in Figure 2. The abscissa in the figure indicates the number of anilox lines, and the ordinate indicates the number of screen lines in the plate. The four thick black lines in the figure indicate the best match between the number of anilox lines and the number of screen lines added. Moiré is generated, and the farther away from the thick black line, the more likely it is to produce moiré. For example, when the number of screen lines is 39 lines/inch, the number of anilox lines is 110 lines/inch; if the number of anilox lines is 118 lines/inch, the number of screen lines can be 26 lines/inch, 32 lines/ Inches, 41 lines/inch. This is a very easy way.