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 color decomposition and color reproduction of image information. In actual operation, the color difference is inevitable due to unsatisfactory conditions. There are three main sources of color difference. One is the color shift caused by the original itself due to the shooting process and materials, and the color shift caused by the color medium color (color attenuation); the second is the color difference in the color decomposition process, mainly the light source, the lens, the color filter, and the photomultiplier. Errors such as tubes and photosensitive materials; third, color differences in the color reduction process, mainly errors in paper, ink, and printing processes.
The quality of prepress plate has a direct impact on the quality of the printed matter. In the process of plate making, there are quite a few reasons for the color difference. Here is a brief introduction to several major factors affecting the color quality when offset printing is used as a color reproduction method. .
Network changes
Color prints use the yellow, blue and green plates of different dot angles to overprint to reproduce the color and tone of the manuscript. A black plate 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.
The change of the dot size will cause the color of the printed material to change. The depth of the printed color is not always linear with the dot percentage. At the end where the optical density value is small, the effect of the increase in dot area on the hue density is much smaller than the end where the optical density value is large. To make a difference between two brighter shades, it is necessary to make a large change in the area coverage of the dots; instead, to make the two darker shades distinguishable, the coverage of the dot area needs to be changed only slightly. Can be achieved. In the highlight area, small changes in the percentage of dots have less influence on the hue, and the eyes are less noticeable; in the shadow tone, slight changes in the percentage of dots will cause the hue to change significantly. Therefore, the effect of the dot size change on the color of the image copying is complicated. In actual operation, it is particularly important to control the dark spot portion.
Ink
The structure and properties of the ink itself have a direct influence on the reproduction of the color.
In color printing, the quality of printed matter is often measured by the density of ink on the spot, dot gain, print contrast, and overprint rate, and these parameters are closely related to the thickness of the ink film on the printed 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 1.0um, and then the thickness of the ink film is increased, and the density will not increase. On the contrary, because the ink layer is too thick, the dots will increase, the gradation will be parallel, the tone and color will all undergo major changes, resulting in color cast. Therefore, controlling the ink film thickness is an important means of color reproduction. In addition to ink and paper properties, ink film thickness is also affected by printing conditions and environmental factors. Due to the difference in performance of different inks, such as different tinting strengths, the thickness of the ink film is different in order to achieve a certain required density value.
Controlling the ink film thickness is the key, and the defects of the ink itself should also be understood. The ideal three-primary inks are mixed in equal proportions to produce neutral ash, and the actually used three-primary inks are mixed in equal proportions, producing only dark colors. The correct reproduction of the gray 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.
The range of colors (color gamut) that ink overprint can reproduce is the smallest of all color gamuts. The inks produced by different manufacturers, as well as different types of inks produced by the same manufacturer, have different ranges of colors that they can reproduce. When printing, it is necessary to select ink with a large color gamut. The larger the color gamut, the smaller the loss of color information and the higher the fidelity of color reproduction.
Paper
Different printing materials, printability is not the same. Paper's smoothness, ink absorption, surface strength, whiteness, sizing, and stretchability are all closely related to the quality of printed matter.
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 a certain range. When the smoothness is high, even if the ink layer of the printing plate is 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, Changes can still obtain prints with uniform colors; offset papers are poor and transfer rates are low, which is not conducive to color reproduction.
The whiteness of paper, that is, the degree of whiteness of paper, is the ability of paper to reflect the visible light at various wavelengths. Printing paper, except for a few colored papers, is mostly white, and the whiteness of different papers is not the same. For printing, 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 partial color of the paper is common, bluish or reddish, which will cause the product to have a color cast, affecting the hue, lightness, and saturation of the printed product. The whiteness of the paper also affects the contrast in the picture layer. The whiteness of the paper is good, and the sense of layering and sharpness of the picture are correspondingly improved. On the contrary, the color of the printed product is dark and the contrast is poor.