Analysis of Several Factors Affecting Quality Control of Color Offset Printing

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 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.

The quality of the prepress plate has a direct impact on the quality of the printed matter. In the process of plate making, the cause of the color difference is quite a lot. Here, only the use of offset printing as a color reproduction method, the introduction of several major factors affecting the color quality .

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.

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 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. 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 effect on the reproduction of the color.

In color printing, the quality of prints is often measured by the ink's on-site density, dot gain, print contrast, and overprint rate. These parameters are closely related to the ink film thickness on printed sheets. 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 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. 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. 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 ink's own defects 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. The smoothness, ink absorption, surface strength, whiteness, sizing, and stretchability of the paper are all closely related to the quality of the 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 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 print products with even color even if the thickness of the ink layer is slightly changed. Offset paper is poor and the transfer rate is 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 yellow, blue, or reddish color of the paper can cause color shifts in the print, affecting the hue, lightness, and saturation of the print. The whiteness of the paper also affects the contrast in the picture layer. The whiteness of the paper is good, and the 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.

printing art

Offset is the use of oil-water immiscible principle, certain printing pressure, the ink on the printing plate 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, the factors affecting the quality of ink transfer include 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 a certain pressure, the faster the speed, the less the contact time and 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

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. When the pressure increases, when the pressure is too high, there will be a serious expansion of the network, making the imprints fuzzy and the density decreasing. 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.

3) Printing color sequence

The first printing ink is directly in contact with the paper surface. A large part of the ink is printed on the ink layer. 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 inconsistent color sequence, the color of the screen is most likely to cause the defects of color cast between the printed matter and the proof. In order to eliminate the color shift between the multi-color machine prints and the proofs, the color sequence should be unified.

4) Water and 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.