Talking about the Status Quo of Flexographic Printing of Corrugated Cardboard
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First, Introduction
Corrugated paperboard has always been an indispensable material for manufacturing packaging containers because of its unique advantages such as light weight, low cost, high bending strength, high processing and structural adaptability, and good printability. To date, corrugated cardboard has become one of the most widely used packaging materials in modern packaging, and its use is steadily increasing, reaching 125 million tons by 2010. With the fierce competition, commodity packaging puts another functional requirement for corrugated boxes - promotion. Therefore, corrugated boxes must be able to carry rich text, images and other information and have bright colors, such as alcohol and small appliances. Corrugated carton packaging for footwear, hardware, microelectronics, computer software, counter sales, and fast food. Currently, corrugated board printing is commonly used in flexographic printing, gravure printing, and offset printing. Among them, flexographic printing is the most commonly used.
Corrugated boxes use flexographic printing technology in the United States accounting for about 95%, Western Europe accounting for 85%, and Japan accounting for more than 93%. In China, it is gradually accepted by corrugated carton manufacturers. At present, enterprises adopting this technology are estimated to account for more than 50%. Nowadays, more corrugated cardboard multi-color flexographic printing slotting machines are used.
Second, the characteristics of corrugated cardboard flexographic printing
In general, corrugated board flexographic printing has the following characteristics:
1. Not only can copy the real version but also copy the network version; corrugated printing has developed from monochrome, two-color to multi-color network version, meeting the requirements of sales packaging. Now the corrugated paper (box, box) is printed on the screen version, which can be printed on 100 lines/inch on E corrugated and 40 lines/inch on A, B and C corrugated.
2. The flexographic plate can compress and deform. Corrugated paper is the worst thickness error and surface flatness in the printing base. Especially A, C corrugated, thickness error 0.5mm, or even higher. Because the flexible printing plate is made of synthetic rubber as the main body, it can compensate for the thickness error and surface unevenness of corrugated paper by its own compression elastic deformation during the printing process, so as to meet the imprinting requirements of each corrugated paper, the ink color is more uniform and can be guaranteed. Printing quality. The flexographic plate used on the corrugated box is relatively thick, generally 4-7 mm, which is the compression deformation of it. Now in order to reduce costs, 4mm plates and 3mm air cushion rubber lining (not plastic lining), the effect is better.
3. Flexographic printing is a light pressure printing. Corrugated paper is a kind of printing base with uneven pressure. The pressure of corrugated high is high, the pressure of corrugated low is light, and the corrugated low is not exposed to the printing plate. The ink is not printed. Flexographic printing is the pressure in traditional printing. The lightest, commonly known as "0" pressure printing. There is no excessive pressure on the corrugated heights, and the printed ink is soft and flat. Corrugated boxes are printed with screens. Why use fine corrugated? Because the screen printing requirements are higher, the thinner the corrugated, the more uniform the pressure.
4. Flexographic printing can use aqueous inks. Paper is made of fiber interlaced, and its hydrophilicity is better than that of lipophilic. Therefore, as long as it is gently contacted with corrugated paper, almost all of the water-based ink is absorbed by the corrugated paper, so the ink is uniform, flat and thick. Water-based ink is not polluted, low cost, fast drying, and operational use. Flexographic printing uses a short ink path ink transfer system with a metal anilox roll, which can be used with water-based inks and is also a feature of flexo printing.
5. Efficient, high quality, high speed process technology. The corrugated paper is formed by flexographic printing. Since it is printed after corrugated paper molding, the process is simple, and can basically meet the requirements of corrugated paper printing from monochrome to multi-color screen printing, so that high efficiency and high quality can be achieved. In addition, the printing speed is high, the small size is 350 sheets per minute, and the large scale is 170-220 sheets per minute, which is an ideal printing technology.
6. The flexo printing machine has a simple structure and is suitable for making it large. The corrugated box has a large area and a length of 1.6-3.5m. If the lithographic long ink path ink transfer system is used, the equipment is bulky, difficult to manufacture, operate and maintain. The flexographic printing machine has a simple structure, and four rollers can form a color group. The diameter of the roller is suitable to be large, just to meet the requirements of the plate roller, and to improve the mechanical precision and stability. Due to the simple structure of the machine and the large space of the equipment, it can meet the requirements of plate loading and adjustment, and is easy to clean and use.
7. Flexographic printing can be combined with other processes to form a production line, which is also a major feature. Corrugated paper is heavier, and the labor intensity of one sheet is too large to meet the requirements of mass production, and it cannot form a scale. Due to its simple structure, the flexographic printing machine can be combined with other processes to form a production line, which greatly improves work efficiency and reduces labor intensity. Most corrugated flexo presses are now linked to the indentation slot. Good performance indentation, slotting, die-cutting, gluing, box making, counting, etc. The finished product is taken down from the machine and can be directly transported to the vehicle.
Third, the requirements of flexographic printing
1. Plate
The corrugated board has a certain elasticity in the direction perpendicular to the paper surface, and has a cushioning and shock absorbing effect on the package. This is the main reason for the widespread use of corrugated cardboard for the manufacture of crates. But this flexibility brings some difficulty to printing. In order to accommodate a flexible substrate with good ink transfer, the plate must be softer than the plates used in general flexographic printing. In addition to the special fine prints, most corrugated board has a plate hardness of 30 ± 5 Shore.
2. Printing machine
Corrugated board flexographic printing presses are available in a variety of sizes, from small to 610mm x 1620mm corrugated cardboard, up to 2030mm x 4830mm. Because the printing format is large, the plate cylinder is also very large. Generally, the plate cylinder can not be disassembled, and the flexographic plate on the plate cylinder can be replaced when the new job is printed.
3. Ink
Most corrugated cardboard is printed with water-based inks. This is because, first of all, the text or simple pattern printed on corrugated board does not need to be very glossy (only for the quality requirements of some cartons), water-based ink can meet the quality requirements of the print. Secondly, corrugated board is a loose fiber fabric with strong water absorption. After applying water-based ink, the drying method is mainly based on osmosis drying, and the drying speed is fast, and no drying device is needed. Again, water-based inks are less polluting and low cost
4. Reticulated roller
The reticulated roller is the core of the flexographic printing machine. It has many types, such as chrome-plated anilox roller and ceramic anilox roller. The structure, function and principle are the same.
The surface of the anilox roller is provided with a concave mesh groove line for controlling the amount of ink conveyed during printing. The surface mesh grooves of the anilox roller are regularly arranged to form a recessed cell. The ink transfer performance of the reticulated transfer roller is related to the number of mesh lines, the shape of the cells, and the arrangement of the cells.
The function of conveying the ink is as follows: the metal anilox roller (2) after the ink on the surface of the roller is squeezed by the scraping rubber roller (7), when it is in contact with the flexographic plate (3) pasted on the plate roller (6) Most of the ink stored in the cell is evenly transferred to the surface of the flexographic plate (3), and printed onto the corrugated cardboard plate (5) by the plate roller (6). The inking printing system on the flexographic printing press is composed of a metal mesh transfer roller, a scraping rubber roller and a plate roller. Each time the metal mesh transfer roller runs, it realizes a complete functional process of metering, ink storage, ink distribution and ink transfer.
Since the corrugated box flexo printing started late in China, the printing technology level needs to be improved. So some places still need improvement
Fourth, corrugated carton flexographic printing should pay attention to the problem
1. Printing pressure
The flexo printing pressure is generally 1-3 kg/cm2, and the corrugated box printing pressure is generally 1-2 kg/cm2. Because of the pressure of the printing plate and the reaction force of the impression cylinder, the phenomenon of light color and color cast often occurs during the printing operation. When the amount of ink is “not enough”, the printing pressure of the plate cylinder is often increased, in an attempt to achieve the ink color. The result is a serious edge effect, such as: dot enlargement, text blurring, and squeezing a limited amount of ink to an unwanted location. When printing on corrugated paper, the corrugated portion is partially raised due to the heavy pressure, so it is better to have a resilient pad to make up for it. In the layer printing, the dot value increase is serious, which will cause the whole page dot to develop to a high-profile, and the dark tone part becomes a serious color cast in the field, so that the entire picture is unclear, the stereo is not strong, and the edge effect of the field, lines and text appears. It doesn't look full, it's not thick, and the font is imprinted, so using the right embossing is especially critical. The quality of corrugated carton printing is currently poor, largely due to improper printing pressure. In actual operation, in addition to the elastic cushioning force, it is necessary to pay attention to the adjustment and balance of the press pressure. Generally, the pressure should be adjusted until the dot is enlarged and the edge effect is not obvious.
2. Corrugated paper quality
The corrugated paperboard paper should be stable in weight, uniform in thickness, reasonable and uniform in moisture content, and ensure good absorption and rapid drying of the ink. High quality paperboard is a prerequisite for high print quality.
Finally, it is equally important to ensure that the printed colors are consistent between batches of carton products, and that the same printing parameters (printer speed, pressure, ink viscosity, ambient temperature, etc.) are recorded and maintained.
3. Keep the position accurate
First of all, when making the plate, the pattern and the relative position of the characters are accurate, and the overprint is accurate when the plates are overlapped. The plate should be centered on the basis of the plate, and the pad and plate should be installed flat. If necessary, use the positioning. system. For most carton manufacturers, in the production of plates, whether it is rubber or resin, in order to save production costs, the imposition method is adopted, which puts higher requirements on plate-making, so the position of the plate must be guaranteed. accurate. The production, assembly, and correction of the plates must be serious and careful, and the position of each version must be accurate.
4. Quality of water-based ink
The printing color is formed by the color of the ink. Therefore, the quality of the water-based ink is related to whether the color of the carton printing is pure or not, and it is the key to the bright color, which directly affects the consistency of the printing color. Therefore, it is necessary to pay special attention to the selection. Whether it is to directly purchase the finished ink or to prepare the ink by itself, the technical specifications of the ink should be strictly controlled. There is no obvious difference between high-quality ink and color standard, and the hand feel is delicate, viscous and suitable in concentration; no precipitation or agglomeration is diluted by adding water or alcohol solvent; no foam is generated by stirring with wooden stick; mixing with other inks has good affinity, all good The phenomenon of compatibility, the color of the ink is pure, and there is no abnormal reaction.
5. Flexo 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, 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
Where 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.
V. Future prospects
With the continuous development of society and the improvement of standard consumption level, the printing of corrugated boxes must gradually develop to high-grade, high-quality, multi-color, and visual effects. This requires us to continuously optimize printing technology, corrugated carton packaging and printing industry. We should speed up the transformation of technical equipment and open up the future, so that China's corrugated box industry is based on packaging countries.