Inking system

The main components of a lithographic offset press include the paper feeding system, the damping system, the inking system, and the drying system. 

Equipped with a fountain and ink rollers, the inking system is like the dampening system. Depending on the quantity of ink required, the ink can be supplied to the press fountains via pipes originating from a centralized pumping station, or from point-of-use drum pumps at each press, or even by hand. These inks tend to be very viscous, which makes them rather difficult to be poured. 

The ink delivered across the plate from the ink fountain is controlled by a series of blades or ink keys, which limit the volume of ink in discrete areas of the plate to match image requirements. On leaving the ink fountain, the ink is metered through a series of rollers before being applied to the plate as a thin film. 

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Precipita.tlon. An ink may also be caused to dry by precipitation of its binder rather than by evaporation of solvent. This can be accompHshed by a dding a diluent, such as water in the form of steam or humidity, to a hygroscopic solvent ink system, which causes the solubiHty of the resin in the ink film to decrease sharply and causes it to precipitate when its tolerance for the diluent is reached. Eurther drying is accompHshed by absorption of the solvents into the stock and then by evaporation. Another form of precipitation setting is the quick-set mechanism. This utilizes resins held in solution in a relatively poor solvent, by means of a small amount of an exceUent solvent (called a sweetener) blended with it. When the ink film is printed on the paper, an amount of the solvents is absorbed reducing the content of the sweetener solvent to a point which causes the resins to precipitate and the ink to set. 

Another secret-ink system used by the Germans during the war involved the chemical reactions between lead nitrate and sodium sulfide. As any high school student should know, solutions of these compounds are colorless, but form a black precipitate of lead sulfide when mixed. A spy can inscribe a message with the lead solution, and its recipient can read the missive by spraying it with the sodium sulfide solution. 

Oxidized carbon blacks may contain up to 15 wt% oxygen. They are strongly hydrophilic. Some of them form colloidal solutions spontaneously in water. In polar printing ink systems, lacquers, and coatings, a better wettability and dispersibility is achieved through surface oxidation [4.26], thus reducing binder consumption. 

Source represents the emission of radiant energy, which may be per unit time or area or both. In the cgs system of units, radiant energy is measured in ergs and in the inks system it is measured in joules. Other radiometric units evolve as listed in Table 16.1. 

The first five chapters of the book focus on fundamental aspects of printing technologies, pigments and ink formulations and, and interactions of the inks with the substrates. The next six chapters focus on actual inkjet inks formulations and raw materials, by discussing the main groups of inks waterborne, solvent-based, and UV inks. The last five chapters present rmique ink systems and functional inks, such as those for obtaining 3D structures or printed electronic devices. 

Of the various approaches taken while using a bi-component ink system, this is the most common one as reflected in the patent ht-erature (see Fig. 12). This system is based on the fact that several polymers are soluble in aqueous solution under certain conditions (e.g., pH value, presence or absence of metal ions) and will precipitate as soon as the chemical environment in the ink is changed. An acrylic polymer will be soluble in the ink at an elevated pH, hence it will instantaneously precipitate upon its reaction with a polyvalent cation which is present at the surface of the substrate. The result of this reaction will be a gelled ink droplet which will be practically immobilized. The schematic reaction mechanism is shown in Fig. 13. 

Improving print quality via this mechanism deals with a dispersed ink system. The dispersion is stabilized by dispersants, which are surface active agents attached to the surface of the dispersed particles. The reactant which is present on top of the substrate ("reaction solution") is capable of breaking the state of dispersion of the ink (and, mainly the pigment dispersion or dissolution), making it practically immobihzed. ... 

Piezo drop-on-demand print head s principle of operation is relatively simple in comparison to CIJ. The jetted fluid is held in balance between a shght negative pressure in the ink system, and capillary tension in small nozzles. Each nozzle is attached to a small chamber from which ink is ejected on demand when a piezo crystal coupled to that chamber flexes on apphcation of voltage. While the principle of operation is simple, producing a viable print head is not. The print heads themselves are complex constructions and have specific requirements on the fluid to enable rehable operation. 

In another microemulsion system, ° the colorant is incorporated into the ink as an aqueous pigment dispersion-based inkjet ink composition by formulating the ink to comprise at least one aqueous pigment dispersion and a microemulsion with at least one water-insoluble organic compound, one hydrotropic amphiphile, and water. This ink system was reported to improve waterfastness and bleed control, "" providing a fast drying ink. 

PAPILLOUD s and baudraz d, Migration tests for substrates printed with UV ink systems in aqueous simulants . Progress Organic Coatings, 2002 45 231-237. 

The foregoing results illustrate the complexily of these sterns and emphasize the importance of studying the ultraviolet light-cured ink system in addition to its individual components. 

The ideal ink system has to meet the requirements listed at the beginning of this chapter with instantaneous drying. The latter can already be achieved with a polymerised ink system used in conjunction with UV drying. Although UV systems are expensive to install, with an additional ink cost, their advantages usually outweigh any disadvantages. 

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