Discussion on the influencing factors of the most

Discussion on the influencing factors of color offset printing quality control II

printing process

offset printing uses the principle of oil-water immiscibility to calculate the average value of the number of points to be measured under a certain printing pressure, and the ink on the plate is indirectly transferred to the paper through blanket. In printing, the normal transfer of ink is the condition that determines the color of printed matter. In practice, there are many factors that affect the quality of ink transfer, mainly including printing speed, printing pressure, printing color sequence, ink balance and so on

1) printing speed

printing speed determines the length of ink contact time from the printing plate to the paper. Printing speed and printing pressure are two factors that affect each other. Under the premise of a certain pressure, the faster the speed is, the less the contact time is, so the density is shallower. Generally, the higher the printing speed, the higher the corresponding printing pressure should be, so as to have a good ink transfer quality. In order to ensure the consistency of printing color, the printing speed must be consistent

2) printing pressure

the function of printing pressure is to eliminate the gap between each contact surface, so as to complete the smooth transfer of ink. The ratio between the amount of ink transferred to the surface of the substrate and the amount of ink applied to the printing plate is called the ink transfer rate. In the printing process, we always hope to obtain a high ink transfer rate under certain conditions, and the ink transfer rate is related to many factors, such as printing pressure, printing speed, printing suitability of printing ink and so on. Under certain conditions of printing speed and printing conditions, paper smoothness, ink plastic viscosity and other printing adaptability, the relationship between ink transfer rate and printing pressure is roughly as follows: when the printing pressure is too low, the ink and the paper surface cannot be fully contacted, resulting in empty imprinting, and the ester bond in PLGA is broken completely due to hydrolysis; When the printing pressure is appropriately increased, the ink transfer rate will also increase, which is almost linear with the increase of printing pressure. However, when it is increased to a certain value, the ink transfer rate will no longer increase with the increase of printing pressure. On the contrary, when the pressure is too high, there will be a serious expansion phenomenon, making the imprint blurred and the density reduced. Moreover, if the pressure is too high, a series of problems such as slip and friction will occur, resulting in inaccurate overprint, which will seriously affect the correct reproduction of color. Therefore, in the printing process, not only the printing pressure is appropriate, but also the printing pressure must be consistent during the whole process of printing the same batch of products, so as to avoid the phenomenon of inconsistent colors

3) printing color sequence

theoretically, which color to print first and which color to print later should be able to obtain the same color rendering effect, in fact, it is not. First of all, the first printing ink is mostly in direct contact with the paper, and then most of the printing ink is printed on the ink layer. In this way, because the printing materials are different, the ink transfer rate is also different under the same conditions. Secondly, due to the differences of ink transparency, viscosity and ink absorption of paper, the surface reflectivity of each color of ink after printing is also different. Due to the factors of ink coverage, the back color will inevitably cover part of the front color imprint, so that the color of printed matter always focuses on the back color or on the mixed color of the back color, especially the part with different levels of multi-color dots. Because of the inconsistent color sequence of the picture color, it is most likely to cause the color deviation between the printed matter and the proofing draft. In order to eliminate the color deviation between multicolor machine prints and proofs, it is important to unify the color sequence between them. Therefore, the color sequence is a complex process problem, which must be comprehensively selected according to the nature of the original picture, the characteristics of ink and paper, as well as the structural characteristics of the printing machine

4) ink balance

in offset printing, it is very important to control the ink balance well. Improper control of ink balance will also seriously affect the quality of color reproduction. There should be no water in lithography, but too much or too little water on the printing plate will directly affect the printing quality and the smooth progress of production. When the water content is too large, the printed matter will appear white, and the field part will produce water stains, which will make the imprint weak and the ink color is different. At the same time, the deformation of the paper increases, and the graphic overprint is not accurate; When the water content is too small, ink stains first appear in the blank part of the print, causing the version to be dirty. With the extension of time, the ink gradually blocks the non inked part, resulting in "paste version"

in addition to the above main factors, it is inevitable that the increase of the working time of the printing machine will lead to the increase of the ink temperature. The increase of the ink temperature will cause the decrease of the ink viscosity, and the change of the ink viscosity has a very important impact on the splitting and transfer of the ink, which will inevitably lead to the change of the color of the printed matter

5 conclusion

it is obvious that the above factors do not work in isolation, but are interrelated, influencing and acting together. The smoothness of paper directly affects the transfer rate of ink and the adjustment of printing pressure and printing speed. The color of the paper itself directly affects the color rendering effect of the ink and the quality of color reproduction. The performance of paper and ink and various factors in the printing process have almost caused a certain degree of point changes. Human visual characteristics also put forward certain requirements for the control of point size. The characteristics of the ink itself also directly affect various factors in the printing process. The change of some parameters in the printing process will also cause the change of the performance of the ink itself. These parameters, combined with the processes of prepress plate making and post press processing, jointly affect the color reproduction quality of color reproduction. We should pay more attention to the accurate control and reproduction of color for the purpose of making money in a short time in the coastal areas of our country. The color separation, printing and printing should cooperate with each other, and the influence of various factors and their mutual relations should be comprehensively considered to achieve the best color reproduction effect. (author: Du Guihua)