At present, the most important part of color offset quality control is color control. The quality of color reproduction directly affects the quality of prints. The print copy process of a color manuscript can usually be expressed as a combination of two processes of color decomposition and color reproduction of image information. In actual operation, due to the unsatisfactory conditions, the existence of color difference is inevitable, and the color difference mainly comes from three aspects. One is the color shift caused by the original itself due to the shooting process and materials, and the color shift caused by discoloration (color attenuation) of the color medium; the second is the color difference during the color decomposition process, mainly the light source, lens, color filter, and photomultiplier. Errors such as tubes and photosensitive materials; third, color differences in the color reduction process, mainly errors in paper, ink, and printing processes. In addition, the quality of the prepress plate has a direct impact on the quality of the printed matter. Moreover, the reasons for the color difference during the plate making process are also quite numerous. No analysis is performed here, but only when using the offset printing as a color reproduction method. Several major factors of color quality are described in detail.

1. Network changes

Color prints are used to reproduce the color and tone of originals using overprinting of yellow, blue and green plates with different dot angles. A black version is also added to make up for the lack of dark tone. In offset printing, due to various reasons, it is inevitable that outlets will be expanded.

Changes in dot size can cause changes in the color of prints, and it is worth noting that the depth of printed matter is not always linear with the percentage of dots. At the end where the optical density is small, the effect of the increase in the dot area on the hue density is much smaller than the end where the optical density is large. In other words, to make a difference between two brighter tones, it is necessary to make a large change in coverage area of ​​the dot area; on the contrary, to make two darker tones distinguishable, dot area coverage is only It can be achieved with minor changes. It also shows that in the bright tuning area, small changes in the percentage of dots have less influence on the hue and the eyes are less noticeable; in the darkening tone, slight changes in the percentage of the dots will cause significant changes in the hue. Therefore, the effect of the dot size change on the image copy color is complicated. In actual operation, the control of the dot size of the dark tone portion is particularly important.

2. Ink

The structure and properties of the ink itself have a direct effect on the reproduction of the color.
In color printing, the quality of printed products is often measured by the ink's solid density, dot gain, print contrast and overprint rate, and these parameters are all closely related to the ink film thickness on the print sheet. The ink layer is too thin, the ink cannot completely cover the printing area of ​​the substrate, the color is necessarily light and the gloss is not enough. Usually, the thickness of the ink film is about 1.0 μm, and the thickness of the ink film increases, and the density does not increase. On the contrary, because the ink layer is too thick, the dot network will increase, the level will be parallel, the tone and color will undergo major changes, resulting in color cast. Therefore, controlling the ink film thickness is an important means of color reproduction. The thickness of the ink film is not only restricted by the nature of ink and paper, but also affected by printing conditions and environmental factors. To sum up, there are mainly the following points: printing pressure, printing speed, ink supply, paper performance, blanket quality, and temperature and humidity. Different inks have different ink film thicknesses due to the difference in their performance, such as different coloring power, in order to achieve a certain required density value.

Therefore, controlling the ink film thickness is the key, and the ink's own defects should also be understood. It is well known that the ideal three-primary inks are mixed in equal proportions to produce neutral ash, and the actually used three primary colors inks are not mixed in equal proportions, and they cannot produce neutral colors and only produce dark colors. The correct reproduction of the balance is the basis of color printing, and the ink itself is not ideal, which obviously brings certain difficulties to the correct reproduction of colors.

At the same time, the color range (color gamut) that can be reproduced by the ink overprint is the smallest among all color gamuts. The inks produced by different manufacturers and the different types of inks produced by the same manufacturer are different in the range of colors that they can reproduce. When printing, it is necessary to select inks with a large color gamut as much as possible. The larger the color gamut, the smaller the loss of color information, and the higher the fidelity of color reproduction; conversely, the smaller the color gamut, the greater the loss of color information. Correct reproduction of colors.

3. Paper

Different printing materials have different printing aptitudes. The smoothness, ink absorption, surface strength, whiteness, sizing, and stretchability of the paper are all closely related to the quality of the printed matter. This paper mainly discusses the effect of paper's smoothness and whiteness on color reproduction. The smoothness of the paper determines the degree of contact between the paper and the plate. Paper smoothness and ink transfer rate are proportional to each other within a certain range. When the smoothness is high, even if the ink layer of the printing plate is very thin, the ink connecting material will quickly form a fixed ink layer on the paper surface, which is favorable for overprinting of the next printing color, but when the ink layer is increased to a certain thickness The transfer rate no longer rises. Coated paper has a higher degree of smoothness than offset paper, and can still obtain prints with even color even when the thickness of the ink layer is slightly changed. On the other hand, offset paper is inferior and the transfer rate is low, which is not conducive to the reduction of color.

The whiteness of paper, the degree of whiteness of paper, is the ability of paper to reflect the visible light at each wavelength. Printing paper, except for a few non-ferrous papers, is mostly white, and the paper is different. For printing, of course, the higher the whiteness of the paper, the better. Only the paper that is nearly pure white can reflect all the visible light, and the color characteristics of the ink can be fully utilized. The yellow, blue or reddish color of the paper will cause the printing product to have a color cast that will affect the hue, lightness, and saturation of the printed product. The paper's degree will also affect the contrast in the image layer, and the whiteness of the paper will be good. The sense of layering and sharpness are also correspondingly improved. On the contrary, the colors of printed products are dark and contrast is poor.

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) Print speed The print speed 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) The pressure of the printing pressure The role of printing pressure is to eliminate the gap between the contact surfaces so as to complete the smooth transfer of the 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. The printing process always expects high ink transfer rates under certain conditions. 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 not complete; when the printing pressure is appropriately increased, the ink transfer rate is also increased, and it is almost linear with the increase in printing pressure, but when it is increased to a certain value, the ink transfer rate no longer depends on the printing pressure. The increase will increase. On the contrary, when the pressure is too high, the network will be severely punished and expanded, making the imprints fuzzy and the density decreasing. Moreover, the pressure is too high. A series of problems such as slipping and friction will also occur, resulting in inaccurate overprinting and seriously 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 true. 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. In this way, due to the different printing materials, the ink transfer rate under the same conditions is not the same. Secondly, due to the differences in ink transparency, viscosity, and ink absorbency, 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 a part of the pre-colored blot, resulting in printed matter. The color is always focused on the back or on the mixed colors of the back. In particular, that part of the overlapping multi-color dot levels. 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 will severely punish 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 several major factors. Only the increase of the working time of the printing press causes the increase of the ink temperature. It is unavoidable that the increase of the ink temperature will cause the decrease of the viscosity of the ink. The change of the viscosity of the ink has a very important influence on the division and transfer of the ink, and it will inevitably cause the color of the printed matter. The change.

5 Conclusion

Obviously, the above factors do not act in isolation. But they are related to each other, influence each other, 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. Certain parameters and changes in the printing process can also cause changes in the performance of the ink itself. 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.

Source: Chongqing Printing and Printing