Photobook printing is a commodity and artwork that depends on visual arbitration. Its quality depends on many factors, and paper characteristics are undoubtedly one of the important factors. The paper with excellent performance can complete the ink transfer completely, so that the graphics and text can be reproduced on the paper clearly and full, so as to obtain a satisfactory reproduction effect. However, due to the limitations of materials such as paper and process technology, most color continuous tone images cannot be faithfully copied and reproduced, but can only make the printed image as close to the original as possible.
1. The influence of paper properties on the gloss of printed matter
In the printing process, the ink film is transferred to the surface of the paper and filled with the unevenness of the paper surface, making the surface of the printed product quite smooth. The specular reflection of light on the surface of the ink film of the printed matter determines the size of the gloss of the printed matter, and the surface of the ink film is closely related to the characteristics of the paper.
1. Ink film thickness
The thickness of the ink film is the main factor affecting the gloss of the printed matter. After the paper absorbs the ink binder to the maximum, the remaining binder remains in the ink film, which can effectively improve the gloss of the printed matter. The thicker the ink film, the more remaining binder, the more beneficial to improve the gloss of the printed matter. The study found that the tendency of gloss to increase with the thickness of the ink film varies with different papers. Figure 1 shows the relationship between the gloss of the printed product and the thickness of the ink film printed on the four different papers with the same ink. It can be seen from the figure that although the ink is the same, the tendency of the gloss of the printed products formed by different papers varies with the thickness of the ink film. When the ink film of the high-gloss coated paper is thin, the gloss of the printed matter decreases with the increase of the thickness of the ink film. This is because the ink film masks the original high gloss of the paper itself, and the gloss formed by the ink film itself is due to the paper. Absorb and decrease. As the thickness of the ink film increases, the higher the original gloss of the paper itself, the more difficult it is to reflect through the ink film. When the thickness of the ink film reaches 1.5 μm, the paper absorbs the binder substantially saturated. Afterwards, as the thickness of the ink film continues to increase, the more retention material remains, the gloss will continue to increase. When the ink film is thin, the gloss of the coated cardboard printed matter increases rapidly with the increase of the ink film thickness. After the ink film thickness increases to 3.8 μm, the gloss no longer increases with the increase of the ink film thickness. But no matter what kind of paper, the amount of gloss increase with the increase of ink film thickness should have a certain limit.
2. The effect of paper properties on the gloss of printed matter
The paper forms a capillary network structure due to the interweaving of fibers, which has a large number of pores and becomes the basis for ink absorption. The balance between the absorption time of the ink capillary material and the fixation time of the capillary material in the paper capillary network determines the degree of penetration of the captive material when the gloss of the printed matter is formed. Generally, the gloss of printed matter decreases with the increase of paper absorption capacity.
The smoothness of the paper and its own gloss affect the gloss of the printed matter. High smoothness is conducive to the formation of uniform and smooth ink film, thereby improving the gloss of printed products. The good gloss of the paper itself can improve the reflectivity of the ink film, especially for transparent ink. The results of two inks printed on 4 coated papers with different gloss. It can be seen from the figure that the printed gloss of both inks increases with the increase of the gloss of the paper itself. The correlation between the gloss of the printed matter and the gloss of the paper itself is stronger than the correlation between the gloss of the printed matter and the absorption capacity of the paper.
As can be seen from the above discussion, there is also a correlation between paper smoothness, gloss and absorption capacity. It is easy to explain this from the papermaking process. For example, calendering can improve the smoothness and gloss of the paper, while also reducing the amount of porosity in the paper, thereby reducing the absorption capacity of the paper. In addition, the pH value on the surface of the paper is also a factor that can not be ignored in the formation of the gloss of the printed matter. The high pH value is conducive to the drying of the ink, which is conducive to improving the gloss of the printed matter.
Second, the effect of paper characteristics on the reproduction of the tone of the printed image
Ideal tone reproduction is the most important process for obtaining high-quality printed images. Most of the bad prints are caused by problems in tone reproduction.
The primary requirement for image copying is that the copied image visually matches the original. But in fact it is impossible in most cases. Table 1 lists the density range of the copied images of different types of paper, and the density range of the original document is 0 ~ 2.7 (3.0). Obviously, in most cases, the density range of the original is larger than the density range of the printed image, and as the quality of the paper decreases, the smaller the density range that can be copied, the same is true for saturation and color.
The solution is to compress the tone range of the original to match the reproducible density range of the paper.
3. The influence of paper characteristics on the increase of dots
1. Increased outlets
The increase of the dots has a greater effect on the reproduction hue change than any other variable. The amount of ink printed on the paper will affect the dot gain, and the dot gain will affect the printing contrast. When printing black-and-white or color images, the increase in dots will change the contrast of the picture and cause the loss of image detail and clarity. In multi-color printing, the increase of the dots will cause the loss of contrast, the image is dark and dark, the dots are dead and cause a sudden color change.
2. The effect of paper characteristics on the increase of dots
The smooth surface of the ink produces the best solid density on higher-quality paper. When the paper quality decreases, the solid ink density decreases, resulting in a certain dot expansion, which affects the quality of the printed matter.
The higher the number of screen lines, the smaller the dot diameter, and the tiny pits on the paper can also distort or omit fine printing dots. Therefore, fine prints with higher screen counts require higher paper smoothness. This is a principle of choosing paper. But in actual work, it is also possible to properly adjust some properties of the ink or change the process parameters to adapt to this characteristic of the paper.
Fourth, the effect of paper characteristics on the accuracy of printing overprint
For fine prints, registration requirements are higher, and the general tolerance is within 0.05mm. In multi-color printing, there are many reasons for the inaccuracy of overprinting, but the deformation of the paper due to the change in water content during the printing process is an important factor.
As we all know, the change of paper moisture content not only causes the paper to expand or contract, but also changes the local size of the paper due to the change of water content, thus causing other forms of deformation, such as curling and wrinkling. This kind of deformation seriously affects the printing workers who are concerned about the effect of relative humidity on the moisture content of the paper.
â‘ The room temperature is constant. The rate of change of water content caused by the change of relative humidity at high humidity is much larger than the rate of change of water content caused by the change of relative humidity at medium humidity; the same is true at low humidity. From this perspective, printing is most beneficial under moderate humidity conditions.
â‘¡ The two form a closed hysteresis loop. This phenomenon is called the hysteresis hysteresis of paper. The performance is as follows: if the paper reaches equilibrium water volume under a certain relative humidity, if it is hygroscopic (dehumidified) and then returns to the original relatively humid ambient air, its water content will increase (decrease) than the original. .
In order to keep the moisture content of the paper uniform on the entire paper surface and adapt to the temperature and humidity of the printing workshop; at the same time, in order to reduce the sensitivity of the paper to the environmental humidity and improve the stability of the paper size, generally before printing , To adjust the humidity (hanging) treatment.
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