Characteristics and Application of Sheet-fed Offset Four-color Printing Overprinting Ink

At present, although the development of commercial rotary printing and flexographic printing is advancing by leaps and bounds, the amount of sheet-fed offset printing ink still accounts for about 65% of the total amount of ink. High-grade imported multicolor offset printing machines are used to print high-grade printing products and are high-grade offset printing inks. The main market; printing medium and high-grade products with domestic medium-speed multi-color printing presses is the main market for medium-to-high-end inks; in addition, flexible and fast short-run printing of commercial advertisements and office documents also occupies part of the market share. Most of the markets use high-efficiency four-color and five-color offset presses for color printing. The following is a brief discussion on the characteristics of four-color overprinting inks for sheetfed offset printing and the effect on printing.

Ink transparency

Transparency is one of the important indicators to be investigated for four-color printing. If the transparency of the ink is not satisfactory, some of the inks of the previous color will be covered, and color shifts may appear in the copied color screen or affect the overall vividness of the screen.

2. Ink viscosity

The viscosity of the ink is one of the important performance indicators that determine the amount of ink transfer of each color. The ink is transferred from one ink roller to another ink roller on the printing machine, and then transferred from the ink roller to the printing plate, blanket and printing materials in turn. In each transfer, the ink ink layer is peeled off, and the viscosity of the ink is the force that hinders the peeling of the ink ink film. The greater the viscosity, the greater the resistance of the ink layer to peeling, and the more uneven the ink transfer, the less ink is transferred to the next transfer surface, and the ink roller stacking phenomenon is more likely to occur. If the viscosity of the ink is too high, the interface between the ink layer and the printing material will be peeled off. If the surface strength of the paper is low, a paper-pulling failure may occur during printing; in the second color printing or post-color printing, the ink is not only not Will be transferred from the plate to the surface of the substrate, and will also stick to the surface of the paper has been printed on the ink layer, the ink separation occurs between two undried ink layer, this phenomenon often occurs in the four-color overlay . Therefore, in wet and wet four-color printing, the viscosity of each color ink must be adjusted. In order to make the overprinting proceed smoothly, the ink viscosity should follow the law of diminishing, ie:

T1≥T2>T3>T4 The lowest ink viscosity (T4) of the last color printing.

3. Ink resistance of ink

In the offset printing process, the constant contact between ink and water can cause emulsification and affect the printability and print quality of the ink. The emulsified ink exhibits a variety of shapes on the ink roller and printing plate. During the ink transfer process, the dampening solution is transmitted in the reverse direction during the printing process, and the water is transferred to the ink fountain by the rubber roller, resulting in emulsification of the ink in the ink fountain. The fluidity is degraded and deinking occurs. On the other hand, in the dispensing stroke, on the one hand, emulsification of severe ink will cause a stacking of ink, resulting in the inability of the ink at both ends of the ink roller to be taken away in time; on the other hand, the ink density is getting smaller and smaller. Increasing the amount of ink will cause a dirty version and increase the amount of water. The vicious cycle of the ink and water giant caused the water on the ink roller to evaporate after shutting down, and then turned on, the ink density on the paper suddenly increased, resulting in an increase in the rejection rate. The emulsification of severe inks can also lead to failures such as flocculation of the pigments and heaping up of ink. When the printing is not severely affected, it does not affect the reproduction of the dots of the printed products. As the viscosity and fluidity of the ink are damaged to varying degrees, resulting in an increase in dots, the edges of the dots are not clean, and the level and color saturation of the image are reduced. For example, in the four-color printing, the magenta ink and the yellow ink do not appear large red (yellowish phase) after overprinting. The fundamental reason is that the dots of the high-profile part of the yellow version increase, and the amount of ink in the dark tone may not increase. . In order to take care of high school to adjust some (skin color) outlets, we have to reduce the amount of ink, but this will affect the saturation of the magenta and yellow parts.

The emulsification of the ink is related to the water resistance of the ink. The ink has low viscosity and large fluidity, so the water resistance is poor, emulsification is easy, the ink balance of the printing is damaged, the overprint performance of the ink is affected, and the ink can be measured using the DUKE emulsification meter. The amount of emulsification and hence the water resistance of the ink.

4. High-speed transfer performance of ink

Increasing production efficiency is an important symbol of modern printing equipment. The speed of the imported printing press has reached 20,000 rpm. The ink must meet the needs of high-speed printing and must have good high-speed transferability. In addition, the four-color overprint ink must also have good stability.

High-speed transfer performance refers to the ink transfer to the surface of the printing plate through the ink fountain and a series of transfer rollers, and it is transferred from the printing plate to the rubber cloth, and then transferred by the rubber cloth to the paper. Under normal circumstances, as the printing speed increases, the ink's transfer performance will gradually decrease. Moreover, the transfer performance of different inks is also different, the ink with good transferability consumes a small amount of ink during printing, and ink with poor transferability consumes a large amount of ink in printing.

5. Ink color standard

In general, the four-color printing ink should meet the color standard of the standard four-color printing, that is, to comply with the allowable range of the IS2846 international standard. Such as Tianjin Toyo Ink Co., Ltd. THK offset four-color printing ink.

6. Environmental protection requirements

With the development of packaging printing and the enhancement of environmental awareness, many foreign-funded printing companies require ink production companies to provide ink environmental testing data, and four-color overprint inks that do not meet environmental requirements will be banned from use. Therefore, environmental protection four-color overprint ink is the direction of the development of modern ink.

In short, offset printing four-color printing ink not only has the general printability of offset printing ink, but also applies to high-speed multi-color printing to meet the needs of modern printing.

Source: 21st Century Fine Chemicals Network