4 Printing process
Offset printing is the use of oil-water immiscible principle, under a certain printing pressure, the ink on the printing plate is indirectly transferred to the paper through the blanket. In printing, the normal transfer of ink is a condition that determines the quality of prints. In actual operation, there are many factors that affect the quality of ink transfer, such as printing speed, printing pressure, printing color sequence, and ink-water balance.
1) Printing speed
The speed of printing determines the length of time the ink is in contact from the plate to the paper. Printing speed and printing pressure are two factors that influence each other. Under the premise of certain pressure, the faster the speed, the less the contact time, so the lighter the density. In general, the higher the printing speed, the greater the printing pressure should be, so that good ink transfer quality can be achieved. In order to ensure the consistency of the color of printed products, the printing speed must be consistent.
2) Printing pressure
The role of the printing pressure is to eliminate the gap between the various contact surfaces in order to complete the smooth transfer of ink. The ratio of the amount of ink transferred to the surface of the printing material and the amount of ink applied to the printing plate is called the ink transfer rate. In the printing process, it is always desirable to obtain a high ink transfer rate under certain conditions, and the size of the ink transfer rate is related to many factors such as the printing pressure, the printing speed, and the printability of the printing ink. Under certain printing speeds and printing conditions, paper smoothness, ink plastic viscosity and other printability certain conditions, the relationship between the ink transfer rate and the printing pressure is roughly the same: When the printing pressure is too low, the ink and the paper surface can not fully contact, As a result, the imprinting emptiness is incomplete; when the printing pressure is appropriately increased, the ink transfer rate is also increased, almost linearly with the increase in printing pressure, but when it is increased to a certain value, the ink transfer rate no longer follows the printing pressure. The increase will increase. On the contrary, if the pressure is too great, there will be a serious expansion of the network, making the imprints fuzzy and the density decreasing. Moreover, if the pressure is too high, a series of problems such as slipping and friction may occur, resulting in inaccurate overprinting and severely affecting the correct reproduction of colors. Therefore, during the printing process, not only the printing pressure is appropriate, but also the printing pressure must be kept consistent throughout the entire process of printing the same batch of products in order to avoid inconsistencies in color.
3) Printing color sequence
In theory, which color should be printed first and which color should be printed later should be able to obtain the same color rendering effect, which is not the case. First of all, the first printing ink is directly in contact with the paper surface, and a large part of the printing ink is printed on the ink layer. Thus, since the printing material is different, the ink transfer rate is not the same under the same conditions. Second, due to the difference in ink transparency, viscosity, paper absorbency, etc., the surface reflectance of each color ink is not the same after overprinting. Due to the hiding power of ink, the post-color will inevitably obscure part of the pre-colored blotting, resulting in the printed product's color is always focused on the post-color or the mixed color of the post-color, especially the multi-color dot layer is not equal. Part. Due to the inconsistency of the color sequence, the color of the screen is most likely to cause the disadvantages of color cast between print and proof. In order to eliminate color cast between polychrome prints and sample drafts, an important one should unite the color sequence between the two. Therefore, what kind of color sequence is used is a more complicated process problem and must be comprehensively selected according to the nature of the original picture, the characteristics of ink and paper, and the structural characteristics of the printing machine.
4) Ink balance
In offset printing, controlling the ink-water balance is crucial, and improper control of the ink-and-wash balance can also seriously affect the quality of color reproduction. There can be no water in the lithographic printing, but too much or too little water on the printing plate will directly affect the printing quality and smooth production. When the water content is too large, white prints appear in the printed matter, and water traces are generated in the field, causing the prints to appear imaginary, and the shades of ink are different. At the same time, the amount of deformation of the paper increases, and the overprinting is inaccurate; when the moisture is too small, ink is first smeared in the blank part of the printed matter, causing the printing plate to become dirty. As the time goes by, the ink gradually blocks the non-inking. In part, there is a "plain version."
In addition to the above-mentioned several major factors, the increase in the operating time of the printing press causes an increase in the ink temperature. It is unavoidable that an increase in the ink temperature causes a drop in the viscosity of the ink, and the change in the ink viscosity has a significant effect on the split and transfer of the ink. The important influence will inevitably cause changes in the color of prints.
5 Conclusion
Obviously, the above factors do not act in isolation, but are interrelated, interact, and work together. The smoothness of the paper directly affects the ink transfer rate and the adjustment of printing pressure and printing speed. The color of the paper itself directly affects the effect of ink coloration and affects the quality of color reproduction. The paper, ink properties and various factors in the printing process almost all caused a certain degree of dot change. The human visual characteristics also impose certain requirements on the control of dot size. The characteristics of the ink itself also directly affect the various factors in the printing process, and some of the parameters in the printing process can also cause the performance of the ink itself to change. The combination of these parameters together with the pre-press platemaking and post-press processing steps together affect the color reproduction quality of color reproduction. To accurately control and duplicate colors, color separation, Internet access, and printing should be coordinated with each other, and the effects of various factors and their mutual relations should be comprehensively considered in order to achieve the best color reproduction effect. (Author: Du Guihua)