Corrugated Cardboard Flexographic Printing Plate Making Process Analysis

Corrugated cartons are made of corrugated cardboard. When printing directly on cartons, in order to ensure the compressive strength of the cartons, strong impacts on the corrugated cardboard should be avoided, but at the same time, good ink transfer should be ensured. Therefore, a flexible version is used. Printing is the best choice.

As we all know, the quality of printing has a close relationship with the quality of platemaking. The quality of our country's flexographic prints is not comparable to that of similar foreign products, which is largely due to the poor quality of platemaking. Carton printing is no exception. In order to obtain high quality prints, we must pay attention to the plate making process.

Flexography has high elasticity, which brings benefits to flexographic printing and also has an adverse effect. The highly elastic material has a large deformation. In the printing, the flexographic plate will have a large deformation, which will cause a large change in the size of the graphic image on the print, and the dot gain will also be more serious. Therefore, in flexo plate making, taking into account the adverse effects of this deformation on the printing, and the corresponding compensation, thus ensuring the quality of printed matter, which is the key to the flexographic plate.

1. Manuscript design requirements

The manuscript is the basis of printing. For a corrugated carton printed with a flexographic plate, the manuscript design has the following requirements.

(1) During the printing process, since the amount of ink and the pressure required for the large area of ​​the field and small texts, fine lines, dots and other details are not the same, so even if they are the same color, do not put on the same plate, Should be printed separately.

(2) In the full-printing of corrugated boxes, in addition to the pressure 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 suitable to superimpose color blocks of the same shape and size in order to avoid overprinting. The texts, patterns or small color blocks should be partially overprinted on the solid color blocks with large areas.

(4) Avoid designing wide and long bars and solids in the axial direction of the printing platen roller. This can cause machine vibration; in addition, avoid the same band pattern as corrugated fiberboard corrugating, otherwise the printing pressure will crush the corrugation. , causing the carton to break.

(5) When designing a layout, a large circular pattern should be avoided, otherwise the printed pattern will become elliptical due to the elongation of the flexible plate in the circumferential direction.

(6) For yin characters, especially small texts, consciously thicken the strokes to avoid thinning or even squeezing the strokes due to ink spreading during printing.

(7) Generally speaking, characters with an area less than 2cm2 cannot be seen at a distance of 5cm. For corrugated box printing, the significance of promotion and promotion of goods is lost. Therefore, the use of too small text should be avoided during design.

(8) Avoid exact overlays of tight seams. Two-color overprinted parts generally need to do trapping 0.1~0.2mm, so as to avoid overprinting is not white.

(9) Minimize the number of overprints, generally below 3 colors. The more the number of registrations, the more severe the compressive strength of the corrugated boxes decreases. Practice has proved that after monochrome printing, the compressive strength of corrugated boxes is reduced by 6% to 12%. After three-color printing, the compressive strength of corrugated boxes is reduced by 17% to 20%.

2. Flexographic deformation and film size compensation

Due to the elasticity of the flexographic plate, after being mounted on the plate cylinder, the plate will undergo bending deformation in the circumferential direction of the cylinder, and hardly deform in the axial direction of the cylinder. This deformation changes the size of the printed graphic. For higher-demand prints, measures must be taken to compensate. The best way is to change the size of the text on the printing film. If you have a circular pattern on the original, you can use the deformable mirror to take a picture with a good compensation factor. The compensation coefficient is the reduction factor of the printing film, that is, the percentage of the printing paper size of the printing film should be reduced in the printing direction. The compensation factor of the graphic size on the printing film can be calculated using the empirical formula of DuPont, USA, see Formula 1.

Compensation coefficient (%) = K/R × 100% 1
In Formula 1, K is the shrinkage 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 plate thickness - support layer thickness) 2
Among them, d is the thickness of the photosensitive resin layer of the flexographic plate.
In Formula 1, R is the printed repeat length and can be calculated according to Equation 3.
R=2π×(r+b+c) 3
Among them, 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. Reproducibility of flexographic version

The size of the outlet determines the reproducibility of the printed matter. Therefore, to ensure the printing quality of the flexible version, it is necessary to control the reproducibility of the flexible version. The dot reproducibility of flexographic printing is different from that of offset printing. As shown in Fig. 1, the dot gain of the flexographic printing is more serious, the tone reproduction range is narrower, about 15% to 85%, and the highlights and midtones ( The percentage of outlets is between 20% and 40%. The increase in the number of outlets is the most serious. This is mainly due to the softness of the flexographic plate. When the plate is made, the small dots are easily lost. When printing, the plate will be deformed, which will increase the density of the dots, and the dots in the dark tone will easily stick.

4. The number of screen lines

At the time of plate making, the choice of the number of screen lines is very important. It determines the reproduction quality of the printed tone levels. The higher the number of screen lines is, the richer the tone level of prints is, but the smaller the network points are, the more serious the network points are when printing, and the more serious the level loss of the dark tone, therefore, the number of screen lines must be correctly selected.

In the flexographic printing, the smallest dot that can be reproduced is determined by the reproducibility of the plate at the time of plate making, and the maximum dot that can be reproduced is determined by the characteristics of dot gain at the time of printing. Because the number of screens is different, the same percentage of printed dots will have different sizes after printing. Therefore, when making a flexographic plate, first select the appropriate number of screens based on the reproducibility of the print. As the flexographic printing dots are more serious, in order to ensure the gradation reproduction range of printed products, the number of screens is generally 133 lines/inch or less. If the resolution of the printing plate is high and the printing conditions are good, the number of screen lines can be achieved. 150 lines/inch, 175 lines/inch. Of course, if the scope of printed media is narrow, the number of screens can be increased.

In addition, for flexographic printing, when selecting the number of screens, the number of anilox rollers to be used in printing should also be considered. The minimum dot size should also ensure that sufficient ink is available. This requires that the aperture area of ​​the anilox roller is smaller The minimum dot area. Usually the number of screen lines should be about 1/3 of the number of anilox rollers.

5. Avoid moire

Since the flexographic printing uses an anilox roller to transfer ink, the inked holes of the anilox roller are engraved at a certain angle, and it is easy to interfere with the screening angle to generate a “rhizard pattern”. Therefore, it is necessary to pay attention to the flexographic plate making. Avoid moire. Since most of the anilox rollers usually engrave ink holes at an angle of 45°, the screening angle should be avoided as far as possible to 45°, typically Y90°, M15°, K30°, C60°, or Y82.5°, M67.5. °, BK37. 5°, C97. 5°. At present, there are many new types of anilox roller ink hole engraving angle has not used 45 degrees, and 60 ° or other angles for carving, which is very beneficial to avoid moiré.

Experiments have shown that if the anilox roll does not experience severe wear, the resulting moire is not severe when a 45° screening angle is used. The main reason for the moiré is that the number of anilox roller lines is not properly matched with the number of screen lines. Therefore, as long as the number of anilox roller lines is correctly matched with the number of screen lines, the moire can be effectively avoided. An important means of avoiding moiré is to use the Nason plot (number of anilox roller lines - number of screen printing screens matching), as shown in Figure 2. The abscissa in the figure indicates the number of anilox roller lines, and the ordinate indicates the number of screen printing screens. The four bold black slashes in the figure indicate the best match between the number of anilox roller lines and the number of screen printing screen lines. There are moiré patterns. The farther away from the thick black line, the more prone to moire. For example, when the number of screen lines is 39 lines/inch, the number of anilox roller lines is 110 lines/inch; if the number of screen lines is 118 lines/inch, the number of screen lines may be 26 lines/inch and 32 lines/inch. Inches, 41 lines/inch. This is a very simple method.