Color printing of corrugated cardboard and factors affecting ink configuration
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For many years, the printing of corrugated board has only applied the functional mark on the packaging box of corrugated cardboard, or the simple line and text monochrome printing, which has backward process, low efficiency and high cost. With the development of the market economy, people's requirements for packaging products continue to increase, and various colorful and well-designed corrugated boxes and shaped corrugated boxes have emerged, which not only increases the added value of products, but also increases the competitiveness of products. Adapted to the development of the international packaging market.
The structure of the corrugated board is shown in Figure 1. The inner layer is a special wave-shaped structure, which cushions the vibration and enhances the impact ability. It is mainly used in the outer packaging of goods. With the appearance of waterproof corrugated cardboard and reinforced corrugated cardboard, corrugated cardboard can also be made into special liquid packaging containers, which expands the application of corrugated cardboard. With the continuous improvement of flexographic printing technology, more and more flexo printing methods are used on corrugated paper. According to relevant statistics, the color printing of corrugated boxes mainly uses flexographic printing technology, 98% in the United States and 85% in Western Europe. The annual printing capacity of corrugated cardboard used for packaging in China exceeds 10 billion m2, facing such a huge market. We need to further improve the lack of printing work for corrugated paper, improve the quality of printing, and meet the needs of market development.
1. Color printing of corrugated cardboard
1.1 Corrugated board uses the advantages of flexo printing
When corrugated cardboard is using flexographic printing technology, due to the elastic deformation of the flexographic plate, it can compensate for the thickness error and surface unevenness in printing, and make the printing ink color uniform; the color printing of foreign corrugated cardboard tends to the white paper E-corrugated cardboard, high-grade The number of printing network lines can reach 120 lines/in, which can print both the physical version and the screen version, and realize four-color printing. The flexographic printing belongs to light-press printing, and the printing pressure is small, which solves the uneven thickness of corrugated paper. The resulting imprinting paste or imprinting is not true; Sangyin uses water-based ink, so it can be transferred by metal anilox roller, and the printing plate only needs to touch the corrugated cardboard gently, the water-based ink is completely absorbed; the flexo printing machine has a simple structure. The four rollers form a color group, which is convenient for loading, adjusting and cleaning. The flexographic printing machine can also be produced in conjunction with other processes, such as indentation, slotting, die cutting, gluing, box making, counting, etc. Linkage production line.
1.2 color flexo printing process of corrugated cardboard
The process of corrugated cardboard color flexo printing is:
Graphic design → image processing → plate making → printing → die cutting → box making
The original design and image processing must fully consider the characteristics of corrugated paper, avoid full-page printing and strip-like graphics in the same direction as the corrugated. The text should not be too small, the ink color should be conspicuous, and the ink with strong tinting strength and high saturation should be selected. The number of screen lines for color painting is generally 60 lines/in, the high-end prints are 100-120 lines/in, the color number is 3-4 colors, and there are also 5-6 colors, and at most 2% of the dots can be printed. When the color separation film is out, comprehensive parameters such as actual ink parameters and dot gain are comprehensively considered, and an excellent balance and gray balance program is prepared, and the tone reproduction curves of C, M, Y, and BK are plotted. In addition, the number of different points of the dot, the width of the flexo press and the printing slot machine, as well as the influence of different materials and pressures should be fully considered. During the plate making process, the flexographic plate is selected from a thicker photosensitive resin plate having a thickness of about 3-7 mm and a hardness of 25-50 Shore Hardness (HS). When the quality of corrugated paper is poor, choose a plate with thicker and lower hardness. If the quality of corrugated paper is good and you need to print small characters and screens, choose a plate with higher hardness. When printing with a thick plate, as shown in Fig. 2, the printing pressure is too small, and the printed substrate is prone to corrugated and "saw"-like patterns, affecting the quality of the printed matter with high number of screens and screen lines; when the printing pressure is too large, printing The substrate is easily deformed and damaged, causing deformation of the resin plate and expansion of the dots. At the same plate thickness and speed, the plate with higher hardness has no obvious enlargement, but the edge imprinted parallel to the corrugated is not solid, the soft version with lower hardness, the imprint is firm, but the edge parallel to the corrugated has expansion. .
American R/bak air-cushion lining technology is currently used in many padding technologies, as shown in Figure 3. The utility model can effectively prevent the deformation of the printing plate and the destruction of the corrugated paper, and make the ink layer more uniform, and the printing tone level and the definition are better. In use, it is only necessary to place the gasket containing microporous bubbles on the back side of the flexographic plate, and the air cushion lining has a buffering pressure during printing, and absorbs between the printing substrate, the printing plate and the plate roller. The resulting pressure is shown in Figure 4. The increase in dot size caused by excessive or too small printing pressure and the deformation and damage of corrugated paper can be reduced or even eliminated, and it has a unique compensation effect on the excessively large plate roller, and is well adapted to various corrugated papers. Sex. Therefore, the air-cushion lining technology gradually replaces the traditional thick printing plate.
The main feature of the flexographic printing machine is that the ink path is short, and the anilox roller is a key component for ink transfer and ink leveling. The structure and number of lines of the anilox roller determine the ink transfer performance and range of use of the anilox roll. In corrugated board flexo printing, the number of anilox rolls is usually 180-300 lines/in, and the ratio of the number of line points to the number of lines of the anilox roll is 1:4-1:5. In addition, the roller arrangement of most corrugated board flexographic printing presses is that the impression cylinder is on the upper side and the plate cylinder is on the bottom, and the bottom surface of the paperboard is printed so that the dust and shredded paper on the printing surface can be naturally dropped.
After the corrugated cardboard is printed, according to the design requirements of various packaging boxes and boxes, the corresponding die-cutting method and the box-making equipment are used for die-cutting, and finally the desired products are produced.
2. factors affecting ink configuration
2. 1 printing ink of corrugated cardboard
The biggest feature of corrugated paper is that the paper is rough, the surface is not flat enough, the ink penetrates quickly, and the surface is mostly dark brown. Therefore, the basic color of the water-based ink is required to be bright, the coloring power is strong, and the hiding power is high; and the penetration and drying performance are fast; the re-solubility must be good in the printing process to ensure smooth ink-forming process; The requirements of the packaging, the imprint should also have good abrasion resistance and water resistance.
Corrugated printing inks are mainly made of 5 different materials, including coloring (10-40%), resin or adhesive components (20-28%), solvent components (33-50%), alkaline The composition of the substance (4-6%) and the additive became (3-4%). Among them, changes in binders and additives play the most important role. As the quality of corrugated packaging increases, the amount of moisture absorbed by the substrate decreases, relying more on evaporation similar to the drying principle. After the ink is printed on the substrate, the moisture evaporates on the one hand to the surrounding environment and on the other hand is absorbed by the substrate. The rate of evaporation of water depends on the water content, the dewatering characteristics of the binder, and the volatility of the water.
2.2 Factors affecting ink configuration
(1) Colorant
The colorant is the most expensive component in the ink configuration. The colorant used in the corrugated printing ink is mainly a basic polymer synthetic pigment, which is characterized by being insoluble in the binder resin paint, low toxicity and abrasion resistance, and improving the pigment. Wettability. In some cases, the adhesive can also be colored using very clear pigment chips. The chrome yellow lead and other heavy metal pigments that were originally used were gradually replaced.
(2) Adhesive
Alkali-soluble resins and disperse-phase resins are two types of adhesives suitable for use in corrugated printing inks. The alkali-soluble resin is cationic, and its polymer skeleton contains a functional carboxylate, and the carboxylate and the basic base form a water-soluble salt. which is
Acid + alkaline base = salt + water
This reaction is reversible, and the resin will return to an insoluble state when the steady pH is lowered. As shown in Figure 5. The second resin is a dispersed phase polymer whose resin particles are controlled by the suspension force generated by the hydrophilic tension. When the steady pH value is lowered, the tension is dissipated, causing the resin to coalesce. This reaction is non-reverse, once dried, The ink layer is difficult to redissolve. It can be seen that the door is suitable for the configuration of corrugated ink, which has higher solid content and less water, and naturally requires less water to be removed, thereby accelerating the drying speed.
In order to improve the printing quality of corrugated paper, it is necessary to obtain a good remelting system, that is, a system capable of self-reweting when driven by anilox and a three-dimensional ink roller. This system is obtainable by a low molecular weight alkali soluble polymer having excellent remelting properties. The remelting properties of corrugated printing inks are related to the volatility of the alkali materials used. Sodium hydroxide is the best material for this purpose, it does not volatilize and imparts a permanent water solubility to the ink. In addition, monoethanolamine or triethanolamine is used instead of ammonia in the original ink for ink configuration. The use of an alkali-soluble polymer allows the pigment particles to be wetted. Under the same color strength, the dehydration is less, the wet coating amount can be applied, the drying speed is faster, and a higher coloring effect can be obtained. The tone level is clearer.
(3) Additives and other ingredients
Additives account for a small proportion of the configuration, but they can change the properties of the ink, such as the addition of additive polyethylene wax, can improve the wear resistance of printed matter. In the printing process, the corrugated ink is easy to generate foam in the ink fountain, the foam climbs over the ink fountain, blocks the pipe, flows to the three-dimensional ink roller and the substrate, and causes printing errors, so it is necessary to add antifoaming agent and foaming in the corrugated printing ink. Agent, but must pay attention to the anti-foaming agent and anti-foaming agent can not be excessive, otherwise it will cause printing defects.
The surface tension of the coated paper substrate is 36-42 dynes/cm, and the surface tension of the water is 72 dynes/cm. The surface tension of the water is much larger than the tension of the substrate. In order to obtain a planar coating effect, the ink is applied to the ink. A surface wetting agent is added to reduce the surface tension of the resin or water mixture, but a suitable wetting agent is selected which otherwise causes or aids in the generation of foam. The surface tension of alcohol is very low. High-quality corrugated printing inks often contain a small amount of alcohol to improve the low foaming and surface wetting properties, while helping to improve the drying of the ink and benefit the ink transfer process.
The preparation of corrugated ink is a complicated balancing process of various materials, and the proper arrangement of ink components can greatly improve the quality of corrugated board printing.