Flexographic printing technology is the main printing method of corrugated board.
For a long time, corrugated board has become an essential material for manufacturing packaging containers because of its unique advantages such as light weight, low price, high bending strength, strong adaptability to processing and structuring, and good printability. So far, corrugated board has become one of the most widely used packaging materials in modern packaging, and its consumption is still rising steadily. It is predicted that before 20 10, the world demand for cartons and paperboard will increase by 2.8% every year, and it will reach 654.38+25 million tons by 20 10.
With the intensification of competition, commodity packaging puts forward another functional requirement for corrugated boxes-promotion. Therefore, corrugated boxes should be able to carry rich text, images and other information, and have bright colors, which are used to package alcohol, small household appliances, shoes, hardware tools, microelectronics products, computer software, counter sales exhibits, fast food and so on. At present, flexographic printing, gravure printing, offset printing and laminating are commonly used for printing corrugated board, among which flexographic printing is the most commonly used.
As we know, corrugated board can be divided into "pre-printing (brushing)" and "post-printing (brushing)" according to whether there are images and words on the paper. In short, "pre-printing (brushing)" refers to printing tissue paper (roll by roll) before corrugated board production, and then sending the printed tissue paper to a corrugated board machine for lamination to produce paperboard. And "post-printing (brushing)" refers to printing on the corrugated board that has been made. However, whether it is "pre-printing" or "post-printing", the most important printing method is flexographic printing, in which satellite web printing machine (up to 4 or 6 colors) is used for pre-printing, and unit sheet-fed printing machine (generally less than 4 colors) is used for post-printing.
According to relevant statistics, the flexographic printing technology of corrugated boxes accounts for about 95% in the United States, 85% in Western Europe, more than 93% in Japan, and it is estimated that it accounts for more than 50% in China. The wide application of flexographic printing technology in corrugated paper industry is inseparable from its many advantages.
Advantages of Flexographic Printing Applied to Corrugated Board
The advantages of adopting flexographic printing technology for corrugated board mainly include the following points:
1. Flexographic printing has good flexibility, so that the printing process can achieve good printing effect under less printing pressure. When printing on the surface of corrugated board, it can reduce the influence of uneven pressure on corrugated board, make up for the defect of uneven thickness of corrugated board substrate and reduce the phenomenon of "washing board"
2. After photochemical treatment, the quality of flexographic printing plate is better than the previous rubber plate, and sometimes the color separation is more obvious.
3. The water-based ink used in flexographic printing is pollution-free and harmless to human body, which meets the requirements of environmental protection. Flexo printing machine uses metal roller to transfer ink, and the printing plate only needs to touch corrugated board lightly, and the water-based ink is completely transferred.
4. The flexo printing machine has a simple structure, and four rollers form a color group, which is convenient for plate mounting, adjustment and cleaning. Flexographic printing machine can also be combined with other processes, such as indentation, slotting, die cutting, gluing, box making, counting and other processes to form a joint production line, which greatly improves the production efficiency.
5. Flexo printing machine operation training is also relatively easy. Because the anilox roller is used for quantitative inking, the requirements of ink coloring can be met when the roller rotates 1 ~ 2 revolutions during printing, and the color difference of the whole batch of printed matter is extremely small, which reduces the paper consumption.
6. Large format flexographic printing machine can meet the requirements of large carton production, which just adapts to the development trend of corrugated board to large format.
Several stages of corrugated board printing
1. Monochrome printing of words or lines. This kind of printing machine has low printing accuracy, the ink must be beaten evenly by several cots, and the drying time after printing is long, which is not suitable for high-speed printing. Printed matter plays a simple identification role, not a decorative role.
2. Color printing of text or line color blocks. This is mainly overprinted by a three-color printing machine with water-based ink, with high precision. The printing ink is water-based ink with fast drying speed, full color and good printing effect. However, due to the limitations of printing machines and other factors, its performance content has not been widely used.
3. Print halftone patterns directly. At this time, the printing machine is already a three-color or four-color flexographic printing machine with high accuracy. The number of lines of the anilox roller is generally 250 lines/inch, which has a good reproduction effect on some halftone patterns. However, due to the limitation of printing machine accuracy and printing color number, the performance of layering is not very strong.
4. Color preprint. Color preprinting is the direct printing of web, and exquisite patterns can be printed by multi-color group (4-8 colors), high-performance and high-precision satellite printing machine. The ink used is alcohol ink, which has fast drying speed, bright color and strong expressive force. In order to improve the surface gloss, you can also cover it with a layer of varnish to make the printed pattern bright and not easy to wear. The printing plate is a thin resin plate, and through fine color separation, the level of the pattern can be completely reproduced. The contents of printed patterns are also more diversified and colorful, becoming real sales packaging, beautifying goods and promoting sales.
Problems existing in offset printing of miniature corrugated board
Commonly used miniature corrugated boards are F-type (0.75mm), G-type (0.5mm), N-type (0.46mm) and O-type (0.3mm), all of which are composed of three layers: face paper, matte paper and base paper. At the same time, miniature corrugated board also has the following characteristics: ① high strength, which can enhance the protection function of products, and the strength is 40% higher than that of thick board; ② Light weight, 40% lighter than thick cardboard and 20% lighter than coated corrugated cardboard; ③ Smooth surface, exquisite pattern, bright color and strong visual effect.
Offset printing is the most widely used and mature printing method. At present, it accounts for more than 50% of all printing shares in the country, mainly paper printing. Micro corrugated board direct offset printing is a brand-new printing technology, which has reached a high level in printing effect and printing quality, basically comparable to the printing quality of paper, and has become the first choice for high-grade corrugated boxes and carton packaging, which can bring huge economic benefits to packaging and printing enterprises and is one of the main directions of offset printing development in the future. But we also see that there are still many problems in offset corrugated board:
1. It affects the strength of corrugated board. Greater offset pressure will reduce the compressive strength of corrugated board; Using fountain solution will reduce the surface strength and compressive strength of paperboard due to water absorption.
2. There is a "washboard" phenomenon. The washboard phenomenon is one of the most common quality problems in corrugated board printing. If the pressure and ink quantity are not well controlled during printing, the washboard phenomenon will occur.
3. Ink and wash are unbalanced. In offset printing, the balance between ink and ink has the greatest influence on the quality, especially the excessive moisture has a greater influence on micro corrugated board.
4. pH of fountain solution. Excessive acidity will slow down the drying of ink and corrode the printing plate; If the acidity is too weak, an effective hydrophilic protective layer cannot be formed in the blank part of the printing plate.
5. The performance of the blanket. The performance of blanket includes surface performance and compression deformation performance. Surface performance is the guarantee of ink absorption and transfer, while compressive deformation performance is the basis of obtaining high-quality images on corrugated board.
Matters needing attention in flexographic printing of water-based ink
When using water-based ink in flexographic printing, we should pay attention to the following items while understanding the characteristics of water-based ink:
1. As water-based ink is a pure water-based system, it is forbidden to mix water-based ink with alcohol-based ink or solvent-based ink, and it is also forbidden to add organic solvent to water-based ink, so as not to cause printing failure and affect printing quality.
2. Fully stir before use to keep the ink composition uniform; Pay attention to measuring viscosity when using. Water-based ink can be stirred evenly in a barrel with a stirring rod, and then the viscosity is measured and poured into an ink tank. If the viscosity is too high, an appropriate amount of diluent can be added; If the viscosity is too low, it should be adjusted with thickener.
3. Pay attention to the change of ink viscosity and pH value when printing. Because in the printing process, because of the evaporation of water, the viscosity of water-based ink will increase and the pH value will decrease, and the pH value should be controlled between 8.5 and 9.5. If it exceeds this range, it can be adjusted with pH stabilizer to keep the viscosity and pH value stable. In addition, if it is found that the viscosity of the ink used is slightly higher, an appropriate amount of water can be added to adjust it.
4. After use, the ink should be sealed immediately to prevent dust from falling and the shell from drying. After printing, the remaining water-based ink should be collected and put back in the original bucket for next use. If the remaining ink contains impurities, it should be filtered and used with new ink.
5. Waterborne ink should be stored in a cool and ventilated place at room temperature of 5℃ ~ 50℃ to avoid exposure to high temperature or scorching sun. Generally speaking, water-based inks can be stored for 1 year without deterioration under normal conditions.
6. After printing, the residual ink on the anilox roller and Mo Dou can be rinsed with clean water when it is not completely dried. If you can't wash it with clean water, you can wash it with cleaning agent.
7. Reasonable use of ink additives.
Development trend of corrugated board flexographic printing
Flexographic printing of corrugated board presents the following six development trends:
1. From monochrome to multicolor;
2. Develop from low network cable to high network cable;
3. From sheet-fed printing to web printing;
4. Developing from low-speed printing to high-speed printing;
5. Develop from small format printing to large format printing;
6. There are more and more sales packages of micro corrugated board.
Flexographic printing has always been the main part of corrugated board printing, but with the expansion of the market demand for micro corrugated board, direct offset printing will be widely used in micro corrugated board, and many printing machinery manufacturers are also doing research in this field, so colleagues engaged in flexographic printing should pay close attention to it.
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