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The relationship between ink and dot expansion
Many factors in the ink, such as pigment, viscosity, viscosity, and resin, are related to dot expansion. The strength of the ink-its coloring power-plays an important role in expanding the dots. The ink is squeezed by the printing plate and the blanket, and then by the squeeze between the blanket and the paper. The ink layer formed after two squeezes is usually very thin. The color intensity of the ink is partly determined by the pigment in the ink. Pigments are often the largest part of the cost of chemical conversion in inks. However, reducing pigments also reduces ink tinting power. When printing the ink layer is thick, the problem of dot expansion will occur. The ink has high coloring power, and the printed ink layer is thin, so that the desired ideal color can be obtained.
In the expansion of control dots, the adhesion of ink is also very important. The higher the viscosity of the ink, the smaller the dot expansion. However, if the viscosity of the ink is too high, and the degree exceeds the strength of the paper, the paper will "fluff", and even severely tear the paper.
Viscosity, also known as the flow properties of ink, mainly depends on the water and pigment. Of course, temperature and water also affect viscosity. When the temperature is low, the viscosity of the ink is high, and the degree of dot expansion is low; when the temperature is high, the viscosity of the ink is low, and the degree of dot expansion is high. Everyone knows that during the printing process, the viscosity of the ink will change with the temperature.
In lithography, the feed rate of water has an effect on ink viscosity and dot expansion. After the ink absorbs moisture in the printing process, the viscosity will decrease and the viscosity will decrease, thus affecting the expansion of the dot. Too much water will reduce the viscosity of the ink and increase the degree of dot expansion. Therefore, the problem of overprinting will also occur. Conversely, if there is no water and the viscosity of the ink increases, the paper fluffing problem will occur.
According to another study, soybean-based inks have a smaller effect on dot expansion than petroleum-based inks. As the expansion of the outlets is reduced, the contrast of the printed products is also increased.
The relationship between paper and network expansion
Before printing, you must choose the appropriate paper for the printed product, especially considering the factor of the expansion of the dot, and the choice of paper should be more cautious.
The printing suitability of paper depends on its surface characteristics, such as the rapid acceptance of transfer ink, the greater the degree of expansion of the printed matter. Absorbency is a characteristic that determines the speed and amount of ink penetration into the paper. The better the absorption performance of the paper, the higher the percentage of dot expansion. When the ink is fixed on the absorbent paper, it will penetrate and diffuse. The speed of paper absorption of ink plays a key role in the expansion of dots. If printing 300 lines per inch, 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 dot expansion is that the higher the paper flatness, the smaller the dot expansion, and the lower the flatness, the greater the dot expansion.
There are many types of coated paper on the market, such as glossy coated paper and matte coated paper. Coated paper helps to improve the fidelity of dot printing and the clarity of colors, because its absorption is lower than that of uncoated paper. As we all know, the low absorption paper, the network expansion is also small.
Non-coated paper has high absorption performance, and high-absorbency non-coated paper prints high screen image, and as a result, the dot expansion percentage is high.
Recycled paper, in theory, the degree of network expansion is the same as the original paper, but in fact it is larger than the original paper.
The relationship between printing presses and network expansion
There are many factors that cause the dot expansion of the printing press, such as printing machine speed, dampening fluid, blanket, ink roller and so on. Printers generally believe that the dot expansion of web printing is greater than that of sheet printing.
The blanket is wrapped around the blanket cylinder, receives the image from the printing plate, and then transfers it to the paper. There are two types of blankets, traditional blankets and compressible blankets, the latter produces clear dots. Under pressure, the impression line of the blanket will bulge on one or both sides. The impression line is the line of contact between the blanket cylinder and the impression cylinder. Traditional blankets are not compressible. Under pressure, we must try our best to restore the original shape and produce friction on the printing plate. This friction will not only shorten the service life of the printing plate, but also cause the phenomenon of network expansion called ghosting. Like the traditional leather 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 compressive properties on the impression line. This design adapts to greater squeezing force and improves ink transfer performance.
Due to the large lining range of the compressible rubber cloth, the lining allows a large tolerance. In fact, this is very important, because improper pressure will cause too many dots to expand. Choosing the thinnest lining thickness required will control the expansion of the network. Although the lining of the compressible blanket is very tolerant, we recommend that it not exceed 0.004-0.006 inches.
Relationship between water and ink volume and network expansion
On the offset press, the inking device not only applies ink to the printed image, but can also perform other functions, such as ink leveling, to improve mobility, and receive wetting fluid to emulsify the ink. The above factors will affect the network expansion. The thicker the ink layer provided by the ink roller system, the more non-directional dots expand.
The wetting system applies a water-based wetting fluid to the printing plate. Its function is to quickly and completely separate the image and non-image parts on the printing plate. The operator must maintain a balance between water supply and ink supply. Too much water supply may cause the viscosity of the ink to decrease, thereby increasing the degree of network expansion.
Although electronic pre-press technology, laser image output technology and optical density measurement technology provide some means for controlling the expansion of dots, the complexity of the lithographic printing process and the ever-changing technological changes will still cause the annoyance of dot expansion for printers, so Understanding how dot expansion occurs, where it occurs, and how to control it will certainly help printers obtain high-quality prints.
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