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Complaints from customers about poor ink absorption on paper have always been a headache for papermakers, and the causes of poor ink absorption vary widely. It is difficult to find out whether it is a paper problem or a printing process. Specifically, the poor ink absorption should be divided into two cases of poor ink absorption capacity and uneven ink absorption. The main form of poor ink absorption is speckle pattern, which is generally found in cyan and magenta ink printing. It is mainly printed in solid color, while black ink has higher color intensity and lower yellow color intensity, even if uneven ink absorption occurs. It is also not easy to detect.
Speckle type
The offset process produces the following three types of markings.
Back peeling pattern <br> This is caused by the deterioration of the ink transfer ability from the blanket to the paper. The root cause is that the ink is unstable or the solvent/grease is separated from the pigment too quickly. Any change in ink transfer rate will form a streak.
Water Interference Patterns <br> When the paper does not absorb the carrier of the ink, it reduces the amount of ink transfer and forms water interference spots.
Wet-sticking markings <br> When the order of viscosity and pH between inks is not suitable (printing color sequence is generally arranged from high to low depending on the viscosity of the ink), after the second printing is printed It will form markings. However, these inks do not appear to be streaked when printed separately.
In addition, streaking may occur due to plate mounting errors, changes in the elastic force of the blanket, changes in drive gear marks, and ink balance.
Causes of speckles <br> Paper imperfections produce streaks, which are unintended, varying, uneven light reflections in a uniformly printed area, in the form of fuzzy clouds, granules, blocks Shape or other regular pattern. We know that this is because uneven light reflection causes us to see the markings, so what causes the light to be unevenly reflected? Although the coated paper is highly smooth, if you look at it with a high magnification magnifier, you will still see the surface of the paper being pitted. When printing, the ink first fills the potholes before it covers the surface of the paper, which results in uneven thickness of the ink layer on the surface of the print. When light hits the surface of the printed matter, part of the light passes through the surface of the paper, part of the light is absorbed by the surface of the paper, part of the light is absorbed by the ink layer, and most of the light is diffused by the surface of the paper, and the light reflected from the thick layer of the ink will Less (higher density), where the ink layer is thinner, more light is reflected (lower density), and when the difference reaches a level that can be recognized by the human eye, it appears as a streak.
All factors affecting the ink receptivity, ink absorption and thickness of the ink layer may cause poor ink absorption and streaking, which are mainly caused by three major aspects: paper, ink and printing process.
Although paper, ink, printing processes, and other printing problems can cause poor ink absorption and streaking, in most cases, many factors work together. Paper uniformity and surface characteristics form the most basic form of the marking, its density, surface roughness, fiber content, filler content, sizing degree, surface pH changes, etc. will affect the ability of the ink to transfer to the printing surface and its absorption, Permeation and drying characteristics. Because the uniformity of the paper also affects the content of impurities and additives, the change in fiber internal distribution (meanity) is usually the source of the above changes. At the same time, poor uniformity will have a negative impact on the diffuse reflection coefficient and opacity. .
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