Album printed matter is a kind of merchandise and artwork that rely on visual arbitration. Its quality depends on many factors. Paper properties are undoubtedly an important factor. The paper with excellent performance can completely complete the transfer of ink and make the picture clearly and plumply reproduced on the paper so as to obtain a satisfactory copying effect. However, due to limitations of paper and other materials and process technologies, most color continuous tone images cannot be faithfully copied and reproduced, but only the printed image can be as close to the original as possible.
First, the effect of paper properties on print gloss
In the printing process, the ink film is transferred to the surface of the paper and fills the uneven surface of the paper, making the surface of the print very smooth. The specular reflection of light on the surface of the printed ink film determines the gloss of the print, and the surface of the ink film is closely related to the characteristics of the paper.
Ink film thickness
Ink film thickness is the main factor affecting the gloss of prints. After the paper absorbs the ink binder to the maximum extent, the remaining binder remains in the ink film, which can effectively improve the gloss of the print. The thicker the ink film, the more the remaining linking material, the more conducive to improving the gloss of the print. Studies have found that the tendency of gloss to increase with ink film thickness varies from paper to paper. Figure 1 is a graph of the relationship between gloss and ink film thickness of prints printed on the same ink on four different papers. As can be seen from the figure, although the inks are the same, the tendency of the gloss of prints formed by different papers varies with the ink film thickness is different. When the ink film is thin, the glossiness of the printed matter of the high-gloss coating paper decreases as the thickness of the ink film increases. This is because the ink film masks the original higher glossiness of the paper itself, and the ink film itself forms a glossiness due to the paper. Absorption decreases. As the ink film thickness increases, the original higher gloss of the paper itself is more difficult to reflect through the ink film. When the ink film thickness reaches 1.5 Î¼m, the absorption of the web by the paper is substantially saturated. Afterwards, as the thickness of the ink film continues to increase, the more link material remains, the more the gloss increases. When the ink film is thin, the gloss of the paperboard board is rapidly increased with the increase of the thickness of the ink film. After the thickness of the ink film is increased to 3.8Î¼m, the gloss does not increase with the increase of the ink film thickness. However, regardless of the type 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 prints
Paper forms a capillary network structure due to the interweaving of fibers and has a large amount of pores, which serves as a basis for ink absorption. The balance between the absorption time of the ink binder and the bonding time of the binder by the paper capillary network determines the degree of penetration of the binder during gloss formation. The gloss of general prints decreases with the increase of paper absorption capacity.
Paper smoothness and its own gloss affect print gloss. High smoothness is conducive to the formation of a uniform and smooth ink film, thereby improving the gloss of prints. The good gloss of the paper itself can increase the reflectance of the ink film, especially for transparent inks. The results of printing two inks on four different luster coated papers. It can be seen from the figure that the gloss of the two inks increases with the improvement of the gloss of the paper itself. The correlation between the gloss of the print and the gloss of the paper itself is stronger than the correlation between the gloss of the print and the absorbency of the paper.
From the above discussion, it can be found that there is also a correlation between paper smoothness, gloss, and absorptive capacity. This is easily explained from the papermaking process. For example, calendering can improve paper smoothness and gloss while also reducing the amount of paper porosity, thereby reducing the paper's absorbency. In addition, the paper surface pH is also a non-negligible factor in the formation of glossy prints, high ph value, is conducive to ink drying, which is conducive to improve the gloss of prints.
Second, the paper characteristics of the print image tone reproduction
The ideal gradation reproduction is the most important process link for obtaining high-quality printed images. Most of the bad prints are caused by problems with gradation reproduction.
The primary requirement for image copying is that the copied image visually matches the original. However, in most cases it is impossible to achieve. Table 1 lists the density ranges of the reproduced images of different types of paper, and the density of the original document ranges from 0 to 2.7 (3.0). Obviously, in most cases, the density of the original document is greater than the density range of the printed image, and as the quality of the paper decreases, the range of the reproducible density is smaller, as is the case with saturation and color.
The solution is to compress the gradation range of the original so that it matches the reproducible density range such as paper.
Third, the paper characteristics of the network increase
1. Dot increase
Dot gain has a greater effect on copy hue change than any other variable. The amount of ink printed on the paper will affect the dot gain, while the dot gain will affect the print contrast. When printing a black-and-white or color tone image, the dot gain will change the picture contrast and cause loss of image detail and sharpness. In multi-color printing, dot gain can result in loss of contrast, dark images, dead spots, and sudden color changes.
2. The effect of paper properties on dot gain
The smooth surface of the ink produces the best on-site density on higher quality paper. When the quality of the paper is reduced, the density of the ink in the field is reduced, resulting in a certain dot gain, affecting 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 that can distort or miss the small plate dots. Therefore, the finer prints with the highest number of screens require higher paper smoothness. This is a principle of using paper. However, in practice, some properties of the ink may be properly adjusted or the process parameters may be changed to adapt to the characteristics of the paper.
Fourth, the impact of paper properties on the printing overlay accuracy
For fine prints, the registration requirements are relatively high, generally within 0.05mm. In multi-color printing, there are many reasons for overprinting, but the paper deformation caused by changes in the water content during the printing process is an important factor.
It is well-known that changes in the water content of the paper not only lead to the expansion or contraction of the paper, but also changes in the local size of the paper due to changes in the water content, and thus generate other forms of deformation, such as curling, wrinkling, and the like. The printer who is seriously affected by this deformation is concerned with the effect of relative humidity on the moisture content of the paper.
1The room temperature is certain, and the change rate of the water content caused by the change of relative humidity when the humidity is high is much greater than the change rate of the water content caused by the change of the relative humidity when the humidity is high; the same is true when the humidity is low. From this point of view, printing is best performed in moderate humidity conditions.
2 Both form a closed hysteresis loop. This phenomenon is called hysteresis hysteresis of the paper. The performance is: if the paper reaches equilibrium water in a certain relative humidity condition, if it is reabsorbed (dehumidified) and then returned to the original relatively humid ambient air, its water content is increased (reduced) compared with the original. .
In order to keep the moisture content of the paper uniform on the entire paper surface, and adapt it to the temperature and humidity of the printing shop; 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 conduct humidity control (hanging) processing.
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