In the products produced by offset printing, such as the covers of books and periodicals, packaging bags, packaging boxes, etc., color blocks with large area solid or flat screen are often designed. For the color block, the most basic and most important print quality requirement is that the color should be uniform and uniform. However, large-scale color patches often show cloud-like or speckled color irregularities and irregularities in production, and are often referred to as imprinted flowers. This article discusses various reasons that may affect the emptiness of offset printing, for reference for colleagues in the printing industry.
1 Printing pressure problem
Printing pressure is the basis for the transfer of ink from the printing plate to the surface of the substrate through the blanket. Since the blanket surface and the paper surface are not completely smooth, only when the contact pressure reaches a certain level can the blanket and paper surface be deformed sufficiently to achieve full contact and achieve ink transfer. For paper with poor smoothness, the printing pressure must be increased during printing to make the paper and ink fully contact, and sufficient ink transfer, otherwise it will cause uneven ground color; the thicker the paper, the more loose the structure (low tightness), pressure The greater the amount of plastic deformation in the vertical direction of the paper during printing, the greater the amount of compression between the rubber cylinder and the impression cylinder. For a printing machine, the printing pressure is generally greater than the printing plate pressure, and printing thick cardboard is greater than printing relatively thin, smooth and tight coated paper, and the printing pressure is greater.
Usually when the blot is found, the first thing that comes to mind is the lack of pressure, but if the pressure is normal when printing on coated paper, the ink color is uniform. When changing to white paperboard, it is desirable to increase the printing pressure a little, but if the pressure is increased In the future, the unevenness of the ink color has not been significantly improved, indicating that the main cause of the blotting is not this. It should be considered from other aspects, and the pressure cannot be increased repeatedly to avoid causing other ills.
2 Blanket problem
1) Elasticity and surface wetting properties of the blanket
The surface of the blanket must be smooth so that the printed dots can have a clear outline, but a blanket with a too smooth surface has poor ink adhesion performance and difficult ink transfer, and when the blanket surface is too smooth, it is very easy to adsorb Impurities such as paper wool. Therefore, the surface of the offset printing blanket has been properly roughened so that its surface is evenly distributed with countless fine capillary structures. If the printed product is mainly based on the printed mesh tone image, a blanket with a fine capillary structure should be used to help the dots be clear and complete; if the printed product is mainly printed with a large area of ​​background color, the surface should be relatively thick and uniform The rubber blanket is conducive to uniform and thick background. The hardness of the blanket is roughly between 50 and 80 Shore. The dots printed by the rubber blanket with high hardness are smooth and clear, but when printing the background color on relatively rough paper, it is better to use a rubber blanket with a lower hardness, which is conducive to the transfer of ink.
During the printing process, the blanket will gradually aging after long-term use, making its surface smooth, the wettability of the ink decreases, the absorption and transfer ability of the ink becomes poor, and the rubber hardens and loses its elasticity. It is not conducive to the ink on the surface of the blanket to keep sufficient contact with the surface of the paper, resulting in poor ink transfer, blotting and other ills. Therefore, the severely aged rubber blanket should be replaced even if there is no other hard damage. When printing on coated paper with a smooth surface, even if the elasticity is slightly poor, the effect on the printing effect may not be obvious, but when changing the cardboard with a poor surface smoothness, if the surface elasticity of the blanket is not good enough, the blotting may be more obvious . If you try to increase the printing pressure again and again to make up, it will make the plastic deformation of the blanket more severe and the elasticity worse, so it cannot solve the problem of imprinting.
2) The blanket is stretched too tight
Due to the incompressible nature of volume, rubber becomes thinner after being stretched, the thickness of the lining is reduced, and the elasticity is reduced. For air cushion blankets, even the micro-foam sponge layer will fail. The tighter the stretch, the more the elasticity will decrease, which will affect the ink transfer ability of the blanket and may cause the blot to bloom. In order to save energy, some people are accustomed to putting a sleeve on the handle of the T-wrench when tightening the blanket, so it is easy to make the blanket stretch too tight. It is best to use a torque wrench with a controllable torque to tighten the blanket .
3 Paper problems
When printing coated paper, if the thickness of the coating of the paper is uneven or the distribution of adhesives and other substances in the coating is uneven, when the ink is transferred to the surface of the paper, the uneven absorption of the ink on the surface of the paper will also cause blotting. Flower, this kind of printing hair failure may occur on the monochrome machine or any one-color printing of the multi-color machine.
Due to the uneven coating, the absorption performance of fountain solution on the surface of the paper will also be different. When printing on a multi-color machine, the blank fountain solution on the printing plate that will be printed first will be transferred to the paper. The absorption of the plate solution is not uniform, and some parts are not fully absorbed. The transfer effect of the post-printing ink to this part is not good, resulting in uneven imprinting of the color block. This kind of blotting hair caused by wet repulsion usually appears in the subsequent color printing of the multi-color machine other than the first color, and it appears in the blank area of ​​the first color printing. If it is printed on the solid color of the first color printing, it is not This will happen. Under the premise of not getting dirty, try to reduce the supply of fountain solution, and at the same time clean the paper wool and paper powder accumulation on the blanket in time to reduce the amount of fountain solution transferred to the surface of the paper, which can improve the phenomenon of blotting.
As we all know, the first color mark of wet overprint will leave a mark on the blanket of the latter color, and then transferred to the substrate. Therefore, when printing with a multi-color machine, in addition to the last color, the other colors will not be overprinted with the subsequent colors, but will be overprinted multiple times. The uneven coating of the paper surface causes the ink to penetrate and dry at different parts of the paper surface. When the ink film of this color and the blanket of the next unit contact and separate again, the separated and broken parts of the ink film are different, resulting in imprinting Uneven. This kind of printed hair appears on the color block of the first color except the last color when printed by the multi-color machine. The difference in the absorption and penetration of ink on the surface of the paper due to the uneven coating of the paper surface. This difference increases with the increase of the printing pressure and increases with the extension of the penetration time. Therefore, reduce the printing pressure appropriately and speed up The printing speed can reduce the difference between the ink film on the surface of the paper and the blanket of the next unit before contact, thereby improving the uneven imprint.
To determine whether the flower is caused by the paper, you can illuminate the back of the paper with light or the front with UV light to see whether the structure of the paper matches the pattern of the imprint. In addition, the absorption characteristics of the ink on the surface of the paper can also be confirmed by doctoring the test ink.
4 Poor overprint
If the color block is obtained by two or more colors overprinting, if the overprinting effect of the later printing color on the first printing color is not good, the obtained overprinting color will appear unevenly. For the dry overprinting of the monochrome machine, the overprinting and printing will occur mainly because too much drier is added when printing the first printing color, which makes the first printed blot dry and solidify, the surface is bright and smooth, and the color of the ink layer after printing The ink is difficult to attach, causing the so-called "ink crystallization" phenomenon. For wet overprinting of multi-color machines, it is due to improper color sequence arrangement or poor adjustment of ink viscosity that leads to poor overprinting, so-called "reverse overprinting", which can be solved by changing the color sequence or adjusting the ink viscosity.
5 Uneven coating
More and more packaging prints use glazing to enhance the aesthetic performance of the prints, improve the performance of the prints, and serve the purpose of enhancing added value. Because the ink layer is thick when printing the color block, in order to dry in time to avoid smudging the back, often need to add a drier. When the drying agent added during printing is excessive or the ink is crystallized for too long, it will cause poor adhesion on the imprint when the glazing coating is applied. The color patches before glazing appear to be uniform, but due to uneven coating after glaze coating, the color patches after glazing appear uneven. Therefore, when the printed matter to be glazed in the subsequent process is printed, you can not add too much drier; if this phenomenon has already occurred, you can add 5% lactic acid to the glazing paint to enhance the adhesion of the glazing paint.
6 Ink is too thick
The viscosity and adhesion of the ink are too large, it is not easy to distribute the ink, and the ink color of the printed product cannot be evenly served; the rougher the paper, the lower the viscosity of the ink, which is beneficial to the ink leveling and achieve uniform ink color.
Uneven color block printing is more common when using white cardboard as a substrate to print packaging carton products. It is not easy to feel when printing yellow or other light colors, and it is more obvious when printing dark blue and other colors with high intensity serious. When someone encounters this kind of problem, they tend to increase the printing pressure and increase the thickness of the ink layer to reduce uneven printing. Increasing the thickness of the ink layer can improve the uneven imprinting phenomenon, but it is also easy to cause problems such as back rubbing and excessive powder spray drier. Therefore, accurately analyze the various causes of the ink color of the printed matter, so as to adopt corresponding technological measures to avoid blindness in the process operation, and can print the color blocks evenly under the appropriate printing pressure and ink layer thickness. It is very important to improve production quality and efficiency.
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