Characteristics and application of the most popula

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At present, although the development of commercial rotary printing and flexography is advancing by leaps and bounds, the consumption of sheet fed offset printing ink still accounts for about 65% of the total consumption of ink. Printing high-grade prints with high-quality imported multi-color offset printing machines is the main market of high-grade offset printing ink; Printing medium and high-end products with domestic medium speed multi-color printing machine is the main market of medium and high-end inks; In addition, flexible and fast commodity advertising and short edition printing of office documents that make money in the middle price difference also occupy part of the market share. In this market, high-efficiency four-color and five color offset printing machines are mostly used for chromatic printing. The following is a brief discussion on the characteristics of four-color overprint ink for sheet fed offset printing and its influence on printing

1. Ink transparency

transparency is one of the important indicators to be investigated in four-color overprint. If the ink transparency is not ideal, it will cover part of the ink printed in the previous color, make the copied color picture appear color deviation or affect the brightness of the overall picture

2. Viscosity of ink

the viscosity of ink is one of the important performance indicators that determine the transfer amount of each color of ink. The ink is transferred from one ink roller to another on the printing machine, and then from the ink roller to the printing plate, blanket and substrate in turn. In each transmission, the ink layer will peel off, and the viscosity of the ink is the force that prevents the ink film from being peeled off. The greater the viscosity, the greater the anti stripping force of the ink layer, the more uneven the ink transfer, the less ink transferred to the next transfer surface, and the more prone it is to the phenomenon of ink stacking on the ink roller. If the ink viscosity is too high, it will cause the interface between the ink layer and the printing material to peel off (see Figure 1a). If the surface strength of the paper is low, it is easy to have a rough drawing fault during printing (see Figure 1b); As a result, in the second color printing or post chromatic printing, the ink will not be transferred from the printing plate to the surface of the substrate, but also stick the printed ink layer on the paper surface, that is, the ink separation occurs between two non dried ink layers. This phenomenon often occurs in four-color chromatic printing. Therefore, in the wet press four-color printing, we must adjust the viscosity of each color ink. In order to make the overprint work smoothly, the ink viscosity should follow the law of decreasing, that is:

T1 ≥ T2 T3 T4

the ink viscosity (T4) of the last color printing is the lowest

3. Water resistance of ink

in the offset printing process, the continuous contact between ink and water will cause emulsification and affect the printing suitability and printing quality of ink. Emulsified ink shows a variety of forms on the ink roller and printing plate. During the inking process, because the fountain solution is transmitted in the reverse direction during the printing process, and water depends on the rubber roller to transmit to the ink bucket, the ink in the ink bucket is emulsified, the fluidity becomes poor, and deinking occurs; In the distribution process, on the one hand, the ink with serious emulsification will produce ink piling, so that the ink at both ends of the ink roller can not be taken away in time; On the other hand, the ink density is getting smaller and smaller, and increasing the amount of ink will cause dirty versions, but in reality, you have to increase the amount of water. The vicious cycle of ink and water leads to the evaporation of water on the ink roller after shutdown. When the machine is restarted, the ink density on the paper suddenly increases, causing the scrap rate to rise. Serious emulsification of ink will also lead to pigment flocculation, ink stacking and other failures. If it is not serious, it will not affect the printing process, but affect the point reproduction of prints. Because the viscosity and fluidity of the ink are damaged to varying degrees, resulting in the increase of dots, the edge of dots is not smooth, and the level and color saturation of the image are reduced. For example, in the four-color printing of low-energy building projects in China, including venues such as the Hamburg Pavilion of the Shanghai WorldExpo, and the demonstration project of the model road science and technology innovation park in Nanjing Gulou District, magenta ink + yellow ink cannot appear bright red (yellowish) after overprint. The fundamental reason is that the dot of the high-profile part of the yellow plate increases, and the ink volume of the dark part cannot go up. In order to take care of the high and middle tone part (skin color), we have to reduce the ink volume, but this will affect the saturation of magenta + yellow

the emulsification of ink is related to the water resistance of ink. If the viscosity of ink is small and the fluidity is large, the water resistance is poor and it is easy to emulsify, which destroys the water ink balance of printing and affects the overprint performance of ink. Using Duke emulsification tester, the maximum emulsifying amount of ink can be measured, and the water resistance of ink can be determined. As shown in Figure 2, compared with ink 2, ink 1 has stronger anti emulsifying ability

4. High speed transfer performance of ink

improving production efficiency is an important symbol of modern printing equipment. The speed of the imported printing machine has reached 20000 rpm. In order to meet the needs of high-speed printing, the ink must have good high-speed transfer performance. In addition, the four-color overprint ink must also have good stability

high speed transfer performance refers to the transfer performance of the ink transferred to the surface of the printing plate through the ink bucket and a series of ink transfer rollers, from the printing plate to the rubber cloth, and then from the rubber cloth to the paper. Generally, the mechanical properties of aircraft parts and so on will gradually decrease with the increase of printing speed. Moreover, the transfer performance of different inks is also different. The ink with good transfer performance consumes less ink in printing, while the ink with poor transfer performance consumes more ink in printing. The relationship between printing speed and transfer efficiency is shown in Figure 3

5. Ink color standard

generally, the four-color overprint ink should meet the color standard of standard four-color printing, that is, the allowable range of iso2846 international standard. For example, the color data of THK offset four-color printing ink of Tianjin Toyo Ink Co., Ltd. are shown in Table 1

6. Environmental protection requirements

with the development of packaging and printing and the enhancement of environmental protection awareness, many foreign printing manufacturers require ink manufacturers to provide ink environmental protection test data. Four color overprint inks that do not meet environmental protection requirements will be banned. Therefore, environmental friendly four-color overprint ink is the development direction of modern ink

in a word, offset four-color overprint ink should not only have the printability of general offset ink, but also be suitable for high-speed multicolor printing to meet the needs of modern printing

reprinted from: Printing Technology

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