Analysis on the printing balance of offset fountai

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Analysis of the printing balance of offset fountain solution

lithography uses the immiscibility of oil and water to keep the fountain solution and ink on the surface of the printing plate relatively balanced in the printing process, so as to achieve a good copy of the original. In the actual production operation, personnel quality, process, equipment, raw and auxiliary materials, environmental temperature and humidity and other factors will affect the printing balance

the printing balance of fountain solution mainly includes three aspects: the balance between fountain solution and ink in the printing state (moving state), the balance with printing plate in the working state, and the balance with paper in the printing effect

1 fountain solution

fountain solution is an aqueous solution prepared by adding some chemical groups to water to wet the printing plate during offset printing

1) the purpose of using fountain solution

the purpose of using fountain solution in lithography is three: ① form a water film that repels ink in the blank part of the printing plate, so as to resist the infiltration of ink on the text to the blank part and prevent dirty version. ② The electrolyte in the fountain solution reacts with the exposed plate base due to wear to form a new hydrophilic layer and maintain the hydrophilicity of the blank part of the printing plate. ③ Adjust the temperature of the ink. Generally, the viscosity of ink will change sharply with the slight change of temperature. The experiment shows that if the temperature rises from 25 ℃ to 35 ℃. The viscosity of the oil volume decreases from 50pA s to 25Pa s, and the fluidity of the ink doubles. This will inevitably lead to the serious spread of ink

2) common fountain solutions

fountain solutions are generally divided into three types: ordinary fountain solutions, fine fountain solutions and non-ionic surfactant fountain solutions

the chemical components of ordinary fountain solution include inorganic compounds such as phosphoric acid, phosphate and nitrate, which have the characteristics of high surface tension, poor wettability, low cost and easy operation

alcohol fountain solution is added with ethanol, isopropanol or other alcohols, which has the characteristics of low surface tension and good wettability. It is suitable for lithographic printing machines equipped with alcohol wetting system

nonionic surfactant fountain solution is a new type of fountain solution developed in recent years. It is generally prepared by adding nonionic surfactant to fountain solution containing other electrolytes. Nonionic surface active agents that make full use of resources often use polyoxyethylene, polyoxypropylene ether, alkyl alcohol amide, etc. It has the characteristics of low surface tension, good wetting performance and stable chemical properties. It is an ideal fountain solution for high-speed multicolor lithography press

2 balance between fountain solution and ink

offset printing uses the law of oil-water immiscibility to transfer ink. Theoretically, it is believed that only when there is a strict boundary between the water film on the surface of the blank part of the printing plate and the ink film on the surface of the graphic part of the printing plate, the ink and water will not infiltrate each other, that is, the ink and water balance will be achieved. However, the printing plate, the water roller and the inking roller transfer water and ink in the process of high operation and mutual rolling. The above ideal static balance is difficult to achieve in the dynamic printing process

1) water ink balance concept

the water mixing volume curve of three different inks. The ordinate is the water mixing volume in 100g ink measured after mixing the wetting liquid and ink, and the abscissa is the mixing time of water and ink. Curve a represents the ink that is most easily mixed with water. Due to the high water content in the ink, the silk head of the ink is too short, and the ink is difficult to output from the ink bucket of the printing machine, so it is not suitable for lithography; Curve C represents the ink that is most difficult to mix with water. The ink is almost repellent to water. The fountain solution squeezed into the ink in the printing process quickly separates from the ink and attaches to the surface of the ink to form a layer of water film, which hinders the transfer of the ink to the blanket. Experience shows that the transfer performance of this kind of ink is extremely poor, and it may make the printed matter bloom, and it cannot be used for lithography; Only the ink of curve B can make the printing go smoothly. The experiment shows that when the water content of the ink of curve B reaches 26% (that is, 26G water in 100g ink), the printing quality is the best

2) factors affecting ink emulsification in lithography

① wetting agent

wetting agent contains a certain amount of wetting agent. Wetting agent is also a surfactant, which can not only reduce the surface tension of wetting solution, but also accelerate the emulsification of ink. Arabic gum is a kind of colloid with excellent hydrophilic property. Although it is mainly used to wipe the surface of the printing plate and protect the printing plate under the load of 1.86mpa, a small amount of Arabic gum is often added to the fountain solution to improve the wetting degree of the fountain solution. However, it not only improves the wetting degree of the fountain solution, but also increases the emulsification of the ink

② acidity of fountain solution

fountain solution should have a certain weak acidity, which is determined by the process technology. During printing, due to physical and chemical effects, calcium salts are deposited between fountain solution, ink roller, printing plate and blanket. Excessive calcium salts will cause ink transmission difficulties and emulsion imbalance. Calcium salt mainly comes from the paint on the surface of paper, the hardness of water and the pigment of some inks (such as red ink). Too strong acidic fountain solution will accelerate the dissolution of ink pigments and paper coatings

③ amount of fountain solution

excessive fountain solution will accelerate the ink from water in oil emulsification to oil in water emulsification. In this case, 1 When the machine stops automatically, the emulsification of ink and the balance between ink and fountain solution will be affected and lose their due balance

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