The relationship between ink and dot gain
Many factors in the ink such as pigments, tackiness, viscosity, and resin are all related to dot gain. The strength of the ink - its color strength - plays an important role in the dot enlargement. The ink is pressed by the printing plate and the blanket, and then squeezed between the blanket and the paper. The ink layer formed after two extrusions is usually very thin. The color intensity of the ink depends in part on the pigment in the ink. Pigments are often the most costly part of the ink. However, reducing pigments also reduces the tinting strength of inks. When the ink layer is thick at the time of printing, dot network expansion will occur. The coloring power of the ink is high, and the printed ink layer is thin, so that the desired ideal color can be obtained.
In the control of network expansion, ink adhesion is also very important. The higher the viscosity of the ink, the smaller the dot gain. However, if the viscosity of the ink is too high and its degree exceeds the strength of the paper, the paper will â€œfluffâ€ and even severely tear the paper.
Viscosity, also known as the flow properties of inks, depends primarily on VanWater and pigments. Of course, temperature and water also affect viscosity. At low temperatures, the viscosity of the ink is high, and the dot gain is low; at high temperatures, the ink viscosity is low, and dot gain is high. As we all know, in the printing process, the ink viscosity changes with the temperature.
In plain printing, the feed rate of water has an effect on the viscosity of the ink and the dot gain. When the ink absorbs water during the printing process, the viscosity decreases and the viscosity decreases, which affects the dot gain. Too much water will reduce the viscosity of the ink and increase the dot gain. As a result, overprinting will also occur. Conversely, if there is a lack of water, the viscosity of the ink will increase and paper fluff will occur.
According to another study, soy-based inks have less effect on dot gain than petroleum-based inks. As the degree of expansion of outlets has decreased, the contrast of printed products has also increased.
The relationship between paper and outlet expansion
Before printing, proper paper must be selected for the printed products. In particular, considering the factors of dot gain, the choice of paper should be more careful.
The printability of the paper depends on its surface characteristics, such as the fast acceptance of the transfer ink, and the greater the dot enlargement of the print. Absorption is a property that determines the speed and amount of ink penetration into paper. The better the paper's absorption performance, the higher the percentage of dot gain. When the ink is fixed on the absorbent paper, penetration and diffusion occur. The rate of ink absorption by the paper plays a key role in the extent of dot gain. If printing 300 lines/inch prints, it is unwise to use highly absorbent paper.
The flatness of the paper is the degree of deviation between the actual surface of the paper and the ideal plane. The relationship with outlet expansion is that the higher the paper flatness is, the smaller the outlet expansion is and the lower the flatness is, the larger the outlet expansion is.
There are many types of coated paper on the market, such as glossy coated paper and matt coated paper. Coated paper helps to improve the fidelity of the dot-printing and the sharpness of the color because it is less absorbent than non-coated paper. As we all know, low-absorbency papers have small dot gains.
Non-coated paper has high absorbency, and highly absorbent non-coated paper prints high-line images, resulting in a high percentage of dot gain.
Recycled paper, theoretically, has the same dot enlargement as the base paper, but it is actually larger than the original paper.
The relationship between the printing press and outlets
There are many factors that cause dot gain in printers, such as printing machine speed, wetting fluid, blankets, ink rollers, and so on. Printers generally believe that web printing is more extensive than web printing.
The blanket is wrapped around the blanket cylinder and the image is received from the plate and transferred to the paper. There are two kinds of rubber blankets, traditional blankets and compressible blankets, and the latter produces clearer dots. Under pressure, the imprinting line of the blanket can be raised on one side or both sides. The imprinting line is the line of contact between the blanket cylinder and the impression cylinder. Traditional blankets do not have compressible properties. Under pressure, it is necessary to make every effort to restore the original shape and produce friction on the printing plate. This frictional effect not only shortens the service life of the printing plate, but it also causes such dot enlargement phenomenon called ghosting. Like a traditional leather-like cloth, the compressible blanket has a synthetic rubber surface layer and several layers of fiber backing. In addition to several fiber layers, the backing also includes one or more layers such as foam, cork, or non-woven fibers, which will produce compression properties on the imprinting line. This design accommodates greater squeezing forces and improves ink transfer performance.
Due to the large lining range of the compressible blanket, the lining allows greater tolerances, which is important because improper pressure leads to excessive dot gain. Choosing the thinnest liner thickness required will control dot gain. Although the lining latitude of the compressible blanket is large, we suggest that it should not exceed the range of 0.004-0.006 inches.
Water, ink volume and the expansion of outlets
On the offset printing press, the inking device not only applies ink to the printed image, but also performs other functions such as ink distribution to improve the performance of the ink and to receive the wetting liquid to emulsify the ink. The above factors will affect the expansion of outlets. The thicker the ink layer provided by the ink roller system, the more non-directional dots are expanded.
The wetting system applies a water-based wetting fluid to the printing plate, which functions to quickly and completely separate the image and non-image portions of the plate. The operator must maintain a balance between water supply and ink supply. Too much water may cause the viscosity of the ink to decrease, thereby increasing the dot gain.
Although electronic prepress technology, laser graphic output technology, and optical density measurement technology provide some means for the control of dot gains, the complexity of the lithographic printing process and the ever-changing technological changes will still bring printing trouble to the printers. Understanding how dot gains occur, where they occur, and how they are controlled will certainly help printers get high-quality prints.
Reprinted from: Printing Industry
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