3 The effect of absorption Absorption of paper refers to the paper's ability to absorb water or other solvents. For printed paper, absorbency refers to the degree of absorption of the binder, solvent, and water in the ink. Absorption of the paper has a great influence on the color printing quality, especially the saturation and vividness of the color, and also has a great influence on the surface glossiness of the printed matter. It is also a necessary condition for the smooth progress of printing, especially offset printing. Many print failures are caused by the inability of the paper to absorb the ink and the printing conditions used (including printing pressure, ink type, temperature and humidity, etc.). Most of the binders used in offset printing inks consist of dry vegetable oils, synthetic resins, and a certain amount of high-boiling point kerosene. When the ink is transferred to paper, the paper absorbs high-boiling point kerosene and a small amount of dry vegetable oil. The material loses its solubility, fixes on the surface of the paper, and then, depending on the oxygen in the air, the binder oxidizes, polymerizes the conjunctiva, gradually dries, and wraps the pigment particles in it. If the absorption capacity of the paper is too large, most of the ink will be absorbed by the paper, the proportion of the binder in the ink film will decrease, the concentration of the pigment will be over-saturated, and the leveling of the ink will be affected, and the pigment particles will not be sufficient. The result of the protection is that the ink is not strong, the powder is dripping, the print is printed, and the print is dry. The color of the print is dull and dull. In order to solve the above drawbacks, if one-sided use of the method to increase the amount of ink, the result is that although the ink has increased, but will cause the network to expand and deform, resulting in a large number of print levels, especially dark tone levels. If the absorbency of the paper is too small, fixing the dried conjunctiva at the time of printing is very slow, and it easily causes the back of the print to become dirty. Therefore, mastering the absorptivity of the paper is an important aspect of rationally designing the printing process, studying the printing mechanism, and improving the quality of the printed matter. 4 Effect of whiteness The whiteness of paper is essentially the degree of reflection of visible light on the surface of the paper and can be expressed as a percentage. The whiteness of paper is the basis for the vividness of prints. Although completely pure white paper does not exist, if you can approach pure white paper that can reflect almost all visible light, the color characteristics of the ink can be fully utilized; if the paper is color-tinged, it is often bluish due to the raw materials of the paper. It may be yellowish or grayish. Such paper cannot reflect light of various wavelength bands in equal proportions, or it cannot reflect all colors. Therefore, the use of color-shifted, partially-gray paper for printing can affect the hue, lightness, and saturation of the print. The degree of whiteness of the paper directly affects the contrast of the printed matter and the reproducibility of the image. In general, a high degree of whiteness will result in a darker ink, a higher level of gradation and sharpness in the picture, a clearer color in printed images, and a stronger contrast in tone levels. On the contrary, the whiteness of the paper is poor, and the tone and color gradation of printed images are also gray. 5 Effect of paper efficiency The paper surface efficiency of paper is essentially the combined effect of paper glossiness and absorbency. The surface state and ink absorption of the paper will simultaneously affect and change the hue and gradation of the ink of the three primary colors, which in turn will cause changes in the color tone of the color mixing ink and affect color reproduction. However, different papers, both of which vary in performance, are printed using the same ink, and their color reproduction is sometimes very different, sometimes it looks essentially the same. 6 Summary The difference in ink transfer rate and gloss caused by the surface characteristics of the paper ultimately affects the color change of the printed matter, especially the effect on saturation. The paper with good surface characteristics can obtain good printing effect during printing, and has strong ability of expressing color saturation, and at the same time, the range of the color gamut being expressed is also relatively large, so it is possible to obtain printed materials with vivid colors and rich gradation. In the printing process, there is interaction and connection between paper, ink and printing press. Therefore, for the printing worker to print high-quality products, a detailed analysis of the surface characteristics of the paper used must be carried out. The surface properties are quantitatively or qualitatively tested and the experimental data is analyzed. The printing process is adjusted according to the analysis results. An evaluation system for the printability of the paper is established, and the most suitable paper is selected so that a satisfactory print can be obtained. (Author: Luoguang Lin Li Ying)