In recent years, the printing level of corrugated board has increased a lot. With the increase of equipment, plates, and inks, the application of printing dots with up to 100 lines on corrugated board can often be seen. Due to the rapid development of this industry, and the corresponding theoretical knowledge has not kept pace with, encountered some of the more common problems or refer to some foreign materials or to the relevant suppliers, but the effect is not very good, because foreign ink, materials Equipment and customer requirements are different from those in the country, and suppliers may conceal some reasons and issues in order to safeguard their own interests. The author sums up some experiences from the production of corrugated boxes and communicates with many colleagues here. I hope there are some deficiencies in the exhibition.
The main components of the printing unit The printing unit of the corrugated board is the same regardless of the domestic or imported principle. The difference is the materials used, the preparation and the transfer method. The printing unit is mainly composed of anilox roller, printing roller, rubber roller (squeegee), embossing roller and ink system.
Their role is as follows:
Anilox: The heart of the press. At present, two main materials are used: metals and ceramics. There are many holes on the surface of the anilox roller to carry the transferred ink, so how many cells and the shape of the cells in the unit length determine the amount of ink transferred by the printing machine and the degree of uniformity. At present, the number of network points per inch is generally used. To represent. The higher the mesh line is, the smaller the amount of ink transferred is, and it is proved that the hexagonal ink of the mesh shape 600 is more uniform. The difference between the metal roller and the ceramic roller lies in the wear resistance. The wear resistance of the ceramic roller is far better than that of the metal roller. According to the usage of the company, the actual service life of the ceramic anilox roller is more than 5 times higher than that of the metal anilox roller. The main cause of the abrasion of the anilox roller is the damage of particles and other external forces in the ink path, and the hard object is prevented from touching the anilox roller during installation and use. The anilox roller's cell must be clean. If there is dry ink in the cell, it will also affect the amount of ink transferred. Mesh roller must be cleaned periodically. There are many ways to clean it. Carton manufacturers generally use special cleaning liquid to circulate in the ink circuit or apply special cleaning liquid to the surface of anilox roller with a special brush (wire brush for ceramic roller; metal roller With copper wire brush). In addition, you can also ask a professional company to clean.
Rubber Roller (Scraper): Used to remove excess ink outside the anilox roller's mesh pocket. The rubber roller has a certain middle height to keep the entire roller surface in uniform contact with the anilox roller under a certain pressure. The middle height of the rubber roller is a key factor in maintaining the balance of lateral ink volume. In addition, the rubber roller and the anilox roller contact with a certain pressure, will have a certain cleaning effect on the network, so many manufacturers have found that the use of scraper anilox roller often need cleaning, but the use of rubber roller in high-speed production of ink will Gradually increase, this is due to the large amount of ink resulting in the proliferation of dots or the dark color of the main reason for printing, in order to avoid this situation, scraper approach came into being. Most of the scrapers currently used are chambered spatulas, which use a certain pressure to press a group of plastic or steel sheets against an anilox roll, using the sheets to deposit the ink outside the anilox roller net. The advantage of the scraper is that the amount of ink does not change with the change of the vehicle speed. The disadvantage is that it is easy to wear. There is a certain amount of running-in time for the new knife to be replaced, and the anilox roller net hole is easy to block.
Printing roller: used to install the printing plate. Plate installation must be tight, flattened and fastened. If you do not tighten or flatten, overprinting may occur, resulting in excess printing. Corrugated board printing uses a flexographic printing plate. Under certain pressure, the printing plate is deformed to facilitate the full contact between the printing plate and the corrugated cardboard surface. In order to reduce the deformation of the printing plate (the corrugated printing is obviously reflected on the printing product), one type is usually used at present. R-bak pad, generally as a standard configuration, the author's use of the effect is more obvious. Embossing roller: used to control the pressure of cardboard and printing plate, corrugated board printing is light pressure printing, but due to the uneven surface of corrugated cardboard, it needs a certain pressure to make the printing plate completely contact with the corrugated board to transfer the ink to the surface of the corrugated cardboard. . Directly related to the printing effect is the printing plate, ink, and precision of the equipment.
Printing plate: At present, corrugated paperboard is used in a large number of photosensitive resin plates. The surface smoothness, ink transfer capacity, hardness, and durability of different photosensitive resin plates are different. To control the cost, a large number of photosensitive resin plates with lower prices are used. The current trend, but in the actual production need to consider the requirements of printing products, can not blindly save the printing plate costs, otherwise it is worth the candle. A good plate, as long as it is used correctly, can be used with 1 million impressions. A printing plate of the author's company has been used for nearly 3 years with more than 1.2 million impressions printed. It is a four-color dot printing and is still in use. In terms of the overall design of the printing plate, large-color printing in the field requires a plate with a low hardness, while fine text and dots need a plate with a higher hardness. Do not use a cloth with too high a hardness when cleaning, and do not use great efforts. After use, wipe it clean and store it in a vertical suspension. Usually a qualified plate, the adjustment time is short, the production efficiency is high, but there are many uncertainties in the production of the printing plate: plate design, production.
The overlay accuracy of a typical corrugated box machine is one millimeter. When the gray scale exceeds 92%, the screen cannot be clearly reproduced. The high-light part of the network cannot be guaranteed. However, the customer's design draft will not take this into consideration. This requires communication with the customer and slightly changes the design. : Decrease the accuracy of overprinting, put proper bleeding lines (adding overlapping parts), and reduce gradients. Also consider the accuracy of the equipment, the number of anilox rollers, etc.
I do not know whether your colleagues have encountered this situation: In different network lines, prone to the gray point of the connection is different, in this case to sum up the empirical data, in the design to avoid the corresponding grayscale. It is also necessary to pay attention to summarizing the experience in setting the shape of the dots. Circular and diamond dots are better for printing, and square dots are more likely to cause dots to stick. Sometimes in order to express the design requirements, you can also consider using both FM and AM. At the time of design, the position of the layout will need to be properly set to the '+' line to prepare for the typesetting. In the actual production, it is usually found that the '+' overprint is accurate. However, the actual printing error is about 1 millimeter. According to the author's observation, it is found that the direction of the paper is mainly related to the number of printing plates in the printing direction. The more plates are distributed in the paper feeding direction, the greater the elongation after printing. One meter is about 1.1 mm in length. In order to avoid the occurrence of this situation, there are two methods to solve: the plate is set at both ends of the plate (must be in contact with the corrugated cardboard); before setting the '+', the plate with less feeding direction is stretched.
The printing plate production mainly has the following aspects: film quality, back exposure, main exposure, plate washing, drying and post-processing.
Film (negative film) must meet the needs of plate making, graphic and clear, accurate size and size specifications; use of frosted film; black density of 5.5 or more.
The duration of the exposure determines the thickness of the plate substrate. The longer the exposure time, the thicker the substrate.
The main exposure: the photosensitive resin plate is supported with the film facing down and the protective film facing up. If the exposure time is too short, the graphic gradient will be too straight, the lines will be curved, and the small words and small dots will be washed away. On the other hand, if the exposure time is too long, the edition will be applied and the writing will be blurred. If there are large and small characters on the same plate, thick and thin lines, the black film may be used to cover the individual exposures as the case may be. The fine parts will not be lost due to flushing to ensure the plate quality. During exposure, it is necessary to take precautions to prevent the film from being heated and foaming, resulting in the plate being scrapped. You can use the exposure for a period of time and pause for a period of time. After cooling, continue exposure.
Wash plate: wash and dissolve the unexposed part, leaving the photopolymerized relief. The length of the washing process is determined by the thickness of the printing plate and the depth of the print. The washing time is too short. The unsensitized resin on the plate will affect the plate-making depth. If the plate washing time is too long, the plate will swell and the fine part will be deformed. Or shedding.
Drying: Remove the solvent from the plate wash to restore the original plate thickness. The baking temperature is generally between 50-60 degrees. The baking time is determined by the thickness of the plate and the length of the plate-washing time, generally two hours thick and one hour thin. Too long baking time and too high baking temperature will make the printing plate brittle, which will affect the printing life. If the baking temperature is too low, the drying time will be prolonged. If the baking time is too short, the printing plate will appear rotten.
After treatment: that is, in addition to sticking and after exposure. The photosensitive resin is hardened completely to achieve the required hardness index, and eliminate the tackiness of the printing plate to facilitate ink transfer. The post-processing time was obtained from the test with the aim of not cracking or sticking.
Ink: Ink is the key part of the printing quality, and it is the most difficult point in the carton industry. The author's relevant experience is not much, just know that the ink is quite different, and a good ink has fewer problems in the production process. , And poor ink not only prints poor results, it may also damage the anilox roller and rubber roller. Many of the problems in production are related to the quality of the ink, such as: dirty printed surfaces or ink dots, printed colors that are too light, ink foam, holes in the blot, or fish-eye holes, ink penetration , India / overprint quality poor sets, ink coverage is uneven, ink color density and brightness is not standard, poor wear resistance, fade, etc. are all related to the ink. The performance of the ink is mainly determined by the colorant and the resin. The auxiliaries only have some improvement effects. If problems occur, they cannot only resort to additives, and ink suppliers should also re-adjust ink formulations to solve them.