On the current situation of flexographic printing

On the current situation of flexographic printing of corrugated board

I. Introduction

has always been Corrugated board has become an indispensable material for manufacturing packaging containers because of its unique advantages, such as light weight, low price, high bending strength, high processing and structural adaptability, 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, reaching 125 million tons by 2010. With the increasingly fierce competition, commodity packaging puts forward 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 corrugated box packaging for wine, small appliances, shoes, hardware tools, microelectronics products, computer software, counter sales displays, fast food and so on. At present, flexographic printing, gravure printing and offset printing before laminating are commonly used in corrugated board printing. Among them, flexographic printing is the most commonly used

corrugated boxes adopt flexographic printing technology, accounting for about 95% in the United States, 85% in Western Europe and more than 93% in Japan. In China, it is also gradually accepted by corrugated box manufacturers. At present, the enterprises using this technology are estimated to account for more than 50%, and now more corrugated board multi-color flexographic printing slotting machines are used

II. Characteristics of corrugated board flexographic printing

in general, corrugated board flexographic printing has the following characteristics:

1 It can not only copy the field version, but also copy the online version; Corrugated paper printing has developed from monochrome and two-color to multi-color line version, which meets the requirements of sales packaging. Now the line version is printed on corrugated paper (box, box), and it can be printed to 100 lines/inch on e corrugated, and 40 lines/inch on a, B, C corrugated

2. Flexible printing plate can compress and deform. Corrugated paper has the worst thickness error and surface flatness in the printing base material. Especially for a and C corrugated, the thickness error is 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 with its own compression elastic deformation in the printing process, so as to meet the stamping requirements of each corrugated paper, make the ink color more uniform, and ensure the printing quality. The flexo used on the corrugated box is relatively thick, usually mm, which uses its compression deformation. Now in order to reduce the cost, 4mm printing plate and 3mm air cushion rubber lining (plastic lining is not allowed), the effect is better

3. Flexographic printing belongs to light pressure printing. Corrugated paper is a kind of printing base material with uneven pressure. The high pressure of corrugated paper is large, and the low pressure of corrugated paper is light. It will also cause that the low part of corrugated paper cannot contact the printing plate, and the ink cannot be printed. Flexographic printing is the lightest pressure in traditional printing, commonly known as "0" pressure printing. There will be no excessive pressure at the height of the corrugation, and the ink color of the print is soft and flat. Corrugated boxes use line printing, why use thin corrugated? Because of the high requirements of line printing, the thinner the corrugated, the more uniform the pressure

4. Flexographic printing can use water-based ink. Paper is interwoven with fibers, and its hydrophilicity is better than lipophilicity. Therefore, as long as you gently touch the corrugated paper, almost all the water-based ink will be absorbed by the corrugated paper, so the ink color is uniform, flat and thick. Water based ink has no pollution, low cost, quick drying and operation. Flexographic printing uses a short ink path inking system with metal rollers, which can use water-based ink, which is also the feature of flexographic printing

5. High efficiency, high quality and high speed technology. Corrugated paper is printed by flexographic printing after forming. Because it is printed after corrugated paper is formed, the process is simple, and can basically meet the requirements of corrugated paper printing from monochrome to multicolor line version, so it can achieve high efficiency and high quality. Coupled with high printing speed, 350 sheets per minute in small size and 350 sheets per minute in large scale, it is an ideal printing technology

6. Flexo printing machine is simple in structure and suitable for enlargement. The corrugated box has a large area and a length of 1 5m. If the long ink path ink transmission system of lithography is adopted, the equipment is huge and it is difficult to manufacture, operate and maintain. The flexographic printing machine has a simple structure. Four rollers and thousands of such bulletproof plates have been installed in law enforcement agencies and the army, which can form a color group. The diameter of the roller is suitable to be larger, which just meets the requirements of the printing plate roller, and can improve the mechanical accuracy and stability. Due to its simple structure and large equipment space, the machine can meet the requirements of plate installation and adjustment, and it is convenient to clean and use

7. Flexographic printing can form a linkage production line with other processes, which is another major feature of flexographic printing. Corrugated paper is heavy and labor-intensive to move one by one, which can not meet the requirements of mass production and can not form a scale. Due to its simple structure, flexographic printing machine can form a linkage production line with other processes, which greatly improves work efficiency and reduces labor intensity. At present, most corrugated flexographic printing machines are linked with indentation and slotting. For the processes of indentation, slotting, die cutting, gluing, cartoning, counting, etc. with good performance, the finished products can be directly carried on the car and transported away from the machine

III. relevant requirements for flexographic printing

1 The printing plate

corrugated board has a certain elasticity in the direction perpendicular to the paper surface, which has a cushioning effect on the packaging. This is the main reason why corrugated board is widely used to make packaging boxes. However, this elasticity brings certain difficulties to printing. In order to adapt to the flexible substrate and make it have good ink transfer, the printing plate must be softer than the printing plate used in general flexographic printing. Except for particularly fine prints, the plate hardness of most corrugated boards is 30 ± 5 shore

2. Printing machine

corrugated board flexographic printing machine has a variety of specifications, and the small one can print 610mm × 1620mm corrugated board, up to 2030mm large × 4830mm。 Because the printing format is large, the plate cylinder is also large. Generally, the plate cylinder cannot be disassembled. When printing a new job, the flexible plate on the plate cylinder can be replaced

3. Ink

the vast majority of corrugated cardboard is printed with water-based ink. This is because: first of all, the text or simple patterns printed on corrugated board do not need to be very shiny (only for the quality requirements of some cartons), and water-based ink can meet the quality requirements of printing. Secondly, corrugated board is a loose fabric with strong water absorption. After coating with water-based ink, its drying method is mainly osmotic drying, with fast drying speed, and there is no need to set up a drying device. Thirdly, water-based inks have less pollution and low cost

4 Ink transfer roller

ink transfer roller is the core of flexographic printing machine. It has many types, such as chrome plated roller, ceramic roller, etc. no matter which one, its structure, function and principle are the same

the surface of the ink transfer roller is provided with a concave groove line for controlling the ink transfer volume during printing. The surface grooves of the ink transfer roller are regularly arranged to form a concave hole. The ink transfer performance of the ink transfer roller is related to the number of grooves, the shape of holes, and the arrangement of holes

the function of transferring ink is shown in the figure: when the metal ink transfer roller (2) after squeezing the ink on the surface of the roller by the ink scraping rubber roller (7) contacts the flexographic plate (3) pasted on the plate roller (6), most of the ink stored in the hole is evenly transferred to the surface of the flexographic plate (3), and printed on the corrugated box board (5) through the plate roller (6). The ink transfer printing system on flexographic printing machine is composed of metal ink transfer roller, ink scraping rubber roller and plate roller. Every time the metal ink transfer roller runs, it realizes a complete functional process of metering, ink storage, ink homogenization and ink transfer

due to the late start of corrugated box flexographic printing in China, the printing technology level needs to be improved. Therefore, some places still need to be improved

IV. problems needing attention in corrugated box flexographic printing

1 Printing pressure

flexo printing pressure is generally kg/cm2, while the printing pressure of corrugated boxes is generally kg/cm2. Because of the pressure of the printing plate and the reaction force of the embossing cylinder, the phenomenon of light color and color deviation often occurs in the process of printing operation. When it is considered that the ink volume is "insufficient", it often increases the printing pressure of the printing plate cylinder in an attempt to meet the ink color requirements, resulting in serious edge effects, such as dot expansion and blurred text, squeezing the limited ink volume to unwanted parts. When printing on corrugated paper, the higher part of the corrugated paper appears like a washing board slot due to heavy pressure. Fortunately, there is an elastic pad to supplement it, which can be slightly avoided. When printing in layers, the point increase value is serious, which will lead to the development of the whole page point to a high profile, and the dark part will become the field, resulting in serious color deviation, so that the whole picture level is unclear, the three-dimensional is not strong, and the field, lines and words have edge effects, which looks not full, not thick, and the font is pasted. Therefore, it is particularly critical to use the appropriate stamping force. At present, the poor printing quality of corrugated boxes is largely due to improper printing pressure. In practice, in addition to using the buffer force of the elastic pad, it is also necessary to pay attention to the adjustment and balance of the press pressure. Generally speaking, the pressure should be adjusted until the point is enlarged and the edge effect is not obvious

2. The quality of corrugated paper should ensure that the gram weight of corrugated paper is stable, the thickness is uniform, the moisture content is reasonable and uniform, and the ink is well absorbed and quickly dried. High quality paperboard is the premise of high printing quality

finally, it is equally important to ensure that the printing colors of carton product batches are consistent, and to record and maintain the same printing parameters (printing machine speed, pressure, ink viscosity, ambient temperature, etc.)

3. Keep the position accurate

first of all, during plate making, ensure that the relative size and position of the pattern and text of the position are accurate, and the overprint is accurate when the printing plate overlaps. The printing plate should be installed based on the center line, the pad and the printing plate should be installed smoothly, and the positioning system should be used when necessary. For most carton manufacturers, when making a printing plate, whether it is a rubber plate, a resin plate or carrying items to accept the resonance of transportation, in order to save production costs, they adopt the method of making up plates, which puts forward higher requirements for plate making and pasting, so it is necessary to ensure the accurate position of the printing plate. The production, assembly and correction of printing plates should be serious and careful, and the position of each plate should be accurate

4. The quality of water-based ink

the printing color is formed by the color of water-based ink. Therefore, the quality of water-based ink is related to the purity of carton printing color, which is the key to bright color, and directly affects the consistency of printing color. Therefore, special attention should be paid to the selection. Whether you buy finished ink directly or prepare ink yourself, you should strictly control the technical indicators of ink. There is no obvious difference between high-quality ink and color code, and the hand feel is fine, viscous and the concentration is appropriate; Dilute with water or alcohol solvent without precipitation and caking; Stirring foam free phase change materials with wooden sticks has the advantages of high thermal efficiency, high storage density, and almost constant temperature when storing or releasing energy; When mixed with other inks, the color matching affinity is good, there is no immiscibility, and the prepared ink is pure without abnormal reaction

5. Deformation of flexo and film size compensation

due to the flexibility of flexo, after it is installed on the plate cylinder, the plate will bend and deform along the circumference of the cylinder, while it will hardly deform along the axial direction of the cylinder. This deformation changes the size of the printed text. For printing materials with higher requirements, measures must be taken to compensate, and the best way is to change