Flexible Corrugated Printing Process and Failure

At present, the corrugated paperboard in China is printed with a letterpress multi-roll ink printing system and a flexographic printing water-based ink printing system. As the flexographic printing system has good printing quality and high speed, the proportion has rapidly increased and it has become the mainstream of corrugated paper printing.

First, the characteristics of flexible corrugated printing

Flexo printing has the characteristics of quick-drying ink, high-speed printing, and low cost, and is particularly suitable for printing such as corrugated paper which does not require high printing accuracy. Flexo printing corrugated box has the following features: 1 Use of water-based fast-drying ink to replace the past aniline ink, green environmental protection in line with the development of the times; 2 machine structure is simple, the price is relatively cheap, less investment, quick; 3 printing pressure is small, the carton Less strength, less impact on the strength of corrugated board; 4 Flexo printing carton printing accuracy is not high, the conventional screen line number is only 30 ~ 65/in, is a low-precision printing method, the most suitable for printing text lines. 5 The printing format is large, and the maximum width of the wide-format flexographic corrugated box printer can reach 2.0m to 2.8m. 6 flexographic printing plate is resistant to high printing force, and can be reused; ink prices are also low, can be linked to production, such as printing, open bad, indentation, set (sticky) box, bundling and other machine can be completed.

Second, the basic principles of flexible corrugated printing

Flexographic printing is the use of a flexographic printing plate to print by means of an ink transfer roller, which is a type of relief printing process. The printing plates used in the printing plate are all raised in graphic parts. When printing, the anilox roller evenly applies a certain thickness of ink layer to the graphic part of the printing plate, and then under the pressure of the impression cylinder, the graphic part The ink layer is transferred to the surface of the substrate to form a clear graphic. The inking mechanism of a flexographic corrugated paper press is usually a two-roller ink transfer mechanism.

1. Two-roller ink system

As shown in Figure 2-1, the plate cylinder rotates on the side of the impression cylinder and coincides with the linear velocity of the impression cylinder. When stopped briefly, the ink fountain roller and the anilox roller are moved up and out of the plate cylinder, and the ink fountain roller and the anilox roller continue to rotate to facilitate even ink and keep the ink on the anilox roller from drying. The ink fountain roller rotates on one side of the anilox roller, and its tangential speed is lower than the tangential speed of the anilox roller in order to control the ink volume and prevent the ink from splashing. Most printing presses for printing corrugated paper are modular flexographic printing presses. Printing Materials Embossing Rolls Printing Plate Rolling Rollers Anilox Rollers Figure 2-1 Two-Roll Inking System Each unit of this flexographic printing press is independent and horizontally aligned.

2, flexible version of the plate

The flexographic plate is divided into a rubber plate and a photosensitive resin plate: the plate of the rubber plate is selected from natural rubber or synthetic rubber; the types of the photosensitive resin plate are various and can be classified into liquid photosensitive resin plates and Solid photosensitive resin plate. The seamless printing plate is made of a plate-shaped solid photosensitive resin plate. The photosensitive resin flexographic plate is composed of a protective layer, a release layer, a photosensitive layer, an adhesive layer, and a base layer. The function of the protective layer and the release layer is to protect the photosensitive layer; the role of the subbing layer is to support the photosensitive layer; the function of the adhesive layer is to tightly bond the photosensitive layer and the subbing layer together. The photosensitive layer is the main body of the printing plate. After exposure and development, it is a flexible printing plate. The photosensitive layer is mainly composed of a highly elastic unsaturated rubber polymer, a photoinitiator, a cross-linking agent, a sensitizer, a thermal polymerization inhibitor, and the like. It should be noted that due to the rough surface of corrugated paper, a thick flexible plate must be used; if high-grade products are to be printed, a flexographic plate with high resolution and small retractility must be used.

3, texture transfer roller 

Anilox roller is the core of the flexographic printing press. It has many types, such as chrome-plated anilox roller, ceramic anilox roller, etc. No matter which one, its structure, function and principle are the same.

1 Anilox roller structure.

The surface of the anilox transfer roller is provided with a recessed mesh groove line for controlling the ink transfer amount during printing. The surface mesh groove lines of the anilox roller are regularly arranged to form a concave network. The ink transfer performance of the anilox roller is related to the number of lines of the mesh channel, the shape of the cell, and the arrangement of the cells.

Number of reticulate lines: The number of network sites per cm in the shortest direction of the network spacing is the number of reticulate lines, which represents the distribution density of the network sites. The number of reticulate lines of the anilox roller influences the amount of ink transferred: the greater the number of reticulate lines, the greater the density of the cell sites, the smaller the volume of the cell sites, and the smaller the amount of ink transferred. Corrugated printing generally uses a low number of anilox rollers (typically 65 lines/in or less).

The shape of the net: One is the three-dimensional shape of the net. There are many types of cell sites, which are basically pyramidal and prismatic.

2 characteristics of roller roller

The meshes spread on the same size of the cells and distributed evenly, which can effectively transfer and control the ink volume, making the ink film uniform in thickness and uniform in ink volume. In addition, there is less occurrence of a phenomenon in which ink is not easily applied due to the difference in the viscosity of the ink. The high precision of the anilox roller ensures that the ink can be transferred under a small pressure and the ink-moistening phenomenon on the edges of the graphic is reduced. The ink is evenly distributed, and edges are not easily hardened. The use of anilox roller to transmit ink, there is little ghosting or levers and other ink transmission failure, high-speed printing less ink phenomenon. The ink volume of the ink inking device using the anilox roller is easy to adjust, the thickness of the transferred ink layer is thin and very uniform, so that the dot color printing becomes possible, and the ink film density is consistent from the minimum dot to the spot. The anilox roller has high durability and corrosion resistance, especially the application of laser engraved ceramic anilox roller greatly improves the service life of the anilox roller and the stability of ink transfer.

3, flexible version of corrugated printing process.

4, loading

Flexographic printing is letterpress printing. When the printing is in direct contact with the corrugated medium, the printing pressure is lighter. Therefore, the flatness of the flexo plate is higher than that of the metal plate. The factors affecting the flatness of the printing plate, besides the flatness of the printed version, should also pay attention to the printing plate base and the plate cylinder cleanness and flatness. The procedure for loading the plates is: 1 Cleaning the plate of the printing plate: There must be no greasy and photopolymer residue on the plate. When cleaning the substrate, the solvent must not come into contact with the surface of the photosensitive layer. 2 Clean the plate cylinder: Before affixing the plate, first use a fine gauze to scrub the surface of the plate with the printing plate. If oil stains or dirt can be used, clean it with gasoline. 3 Adhesive tape: Take a protective film with double-sided adhesive tape on one side, and attach the adhesive tape to the plate cylinder at the position where the plate is installed. When attaching, the hand and neck should be large and firmly adhered. When hand pressing, remove the protective film from the center of the tape and then remove the protective film on the other side. 4 Paste the plates: Arrange the flexographic plates on the platen tape by positioning them. When aligning, you should align the vertical and horizontal coordinate lines on the platen roller. Pay attention to the bite and drag of the plate. The position must be correct and not skewed. At the time of sticking, a wood hammer is tapped to attach any part to the surface of the plate cylinder and the entire plate is glued. 5 Seal the edge of the printing plate: After the printing plate is pasted, the sealant should be added around the edges to prevent the ink and solvent from eroding the double-sided adhesive tape when printing and cleaning the printing plate, to avoid the occurrence of shelling between the printing plate and the roller. phenomenon.

5, printing

1 printing pressure Flexo printing pressure should be light, in order to clear graphics can print. The pressure is too heavy, the flexographic plate is squeezed and deformed, the lines and dots spread outwards, and the lines become thicker. If the pressure is too light, the picture is not clear. The adjustment of the printing pressure should be based on the printing quality standard. The graphic and ink of the printing should be consistent and clear. To adjust the flexographic printing pressure, the following three aspects should be noted: the flatness of the printing plate; the pressure between the anilox roller and the plate roller should be appropriate, so that the two rollers are tangent and the optimum printing pressure is selected. Because this is related to the control of the ink volume and the consistency of the ink color; the pressure between the embossing roller and the plate roller should be appropriate, the surfaces of the two rollers should be tangent, and the two ends of the roller should be consistent with the intermediate pressure.

2 printing ink flexo printing Anilox roller used to transmit ink, small network, the texture of printing ink should be fine, the color should be bright, the viscosity of the ink should match the printing speed. Corrugated paper printing generally use water-soluble quick-drying ink, water-based ink for short, its linking material is the oil and water miscible system formed by resin and water under the action of surfactant. The stability of the system is poor. The stability of water-based inks depends on the pH value: When the pH drops to about 7.8 or lower, the ink begins to cloud and thicken, tends to conjunctiva, and hinders the normal ink transfer. Alkali is too strong and has an effect on the anilox roll. Generally, the pH value is controlled at 8 to 9, and the water-based ink is alkaline. Similar to a soap solution, it is easy to foam. Compared with solvent inks, water-based inks have lower viscosity, save solvent, have better safety, less environmental pollution, and no toxicity; however, they have an impact on the stretchability of paper and errors in overprinting. More suitable for printing corrugated paper and lower quality prints. In the flexo printing process, solvent transfer to the ink, shades of ink, balance of colors, and resistance to printing plates are of great concern. In the printing process of flexible corrugated board, the main use of water as a solvent, the ink on the cardboard quickly dry.
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Third, the flexible version of corrugated paper printing failure analysis

1, the outline of the edge of the ink is too heavy, the middle of the lighter colors, fine text is not clear

Cause: The printing plate is not even, there is wear and tear, and the fine brushwork words are blocked, hanged, brushed or embossed.
Solution: 1 Adjust the pressure between the metal anilox roller, plate, and impression roller to minimize the contact pressure. 2 Adjust the layout height (pad), including layout grinding and back-tapping tape. For example, when field and fine strokes are combined together, the field part should be slightly higher. 3 Adjust ink according to graphic conditions. For example, when there are many fields, you should use harder ink and increase printing pressure.

2, ink film is too thick, excessive ink overflow graphics, line rough ratio.

Cause: The viscosity of the ink is too high, the printing pressure is insufficient; the ink is too thin to give too much ink, and the anilox roller cable is too thick.
Solution: Reduce the viscosity of the ink, increase the printing pressure, thicken the ink, and reduce the amount of ink; use an anilox roller with a finer mesh.

3, ink film is too thin

Causes: The viscosity of the ink is too low, the anilox roller is too thin, the anilox roller and the ink roller are worn out, the ink is stirred unevenly before use, and the printing pressure is too small.
Solution: Add the original ink to increase the viscosity, use a coarse mesh anilox roller, replace the anilox roller, ink roller, stir the ink evenly, and increase the printing pressure.

4, paste version, blocking version

Cause: The printing plate is too shallow, the printing pressure is too large, and the ink supply is excessive; the ink viscosity is too high, the ink pigment particles are too coarse; the anilox roller mesh hole is blocked.
Solution: Replace or deepen the plate; adjust the printing pressure; reduce the ink supply; reduce the viscosity of the ink, replace the ink; clean the anilox roller.

5, print products are not clear, some halo and feather-like, edge burr.

Cause: The ink is drying too fast; the pressure between the anilox roller and the printing plate is too large or uneven; the ink is too viscous to cause fluffing on the plate and the printing plate wears; the printing speed is too slow, and the pigment component in the ink is too much or not well dispersed.
Solution: Slow drying agent to reduce the printing ink drying speed; adjust the printing pressure; reduce the viscosity of the ink and clean the printing plate, check the wear of the printing plate; improve the printing speed, add the solvent in the ink and stir evenly.

6, plate wear

Cause: The printing pressure is too large, the contact is too strong; the quality of the printing plate is bad; the ink is improperly selected, and the printing plate is corroded; the ink particles are too thick to guarantee the printing quality.
Solution: Reduce printing pressure, make reasonable contact, fully pad or replace new plates, change ink, and replace fine ink.

7, prints show many holes like a hole

Causes: Improper adjustment of press pressure, slight corrosion of printing plate, uneven surface of substrate, granular bulge, dry ink on anilox roller, abrasion of anilox roller, poor ink transfer, ink thinning, low viscosity, printing ink The layer is too thin; mechanical impurities adhere to the plate surface, the ink wettability is poor, the ink viscosity is too low or the drying is too slow, the ambient temperature is too low, and the humidity is too high.
Solution: increase the printing pressure; improve the quality of the printing plate, change the anilox roller, thicken the ink, increase the viscosity, adjust the thickness of the ink layer; remove the impurities on the layout, add additives to improve the ink wettability, increase the ink viscosity and dryness, Increase room temperature and reduce humidity.

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