The role and control of offset fountain solution (one)
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Offset printing is a process that uses oil and water incompatibility to complete printing. First, supply water to the printing plate, and then supply ink to the printing plate, so that the printing plate has ink and water, and the blank portion has water and no ink. However, the water used to wet the plate is not ordinary water, but a chemical substance, an electrolyte liquid, which is a fountain solution. It is thus known that the performance of fountain solution is one of the key factors determining the quality of offset products. The performance of the fountain solution is compatible with the printing machine, paper, ink, etc. to print high-quality products, so that the printed matter is bright and vivid, the layer is clear, the dots are smooth, no enlargement, no reduction, blank parts are not dirty, and the print is dry normal.
First, the main role of dampening solution
(1) Forming a water film on the blank portion of the printing plate
A uniform water film is formed in the blank portion of the printing plate, and the ink repellency and hydrophilicity of the blank portion of the printing plate are the main functions of the fountain solution.
(2) Supplementing the hydrophilic layer damaged during the printing process
When printing, the roller and the ink roller rotate to rub the printing plate, causing the hydrophilic layer on the printing plate to be worn. At the same time, the paper has more or less paper powder and paper hair falling off during the printing process. Further deepen the wear and damage the hydrophilic layer. Thus, if there is no electrolyte for the fountain solution, during the printing process, the metal layer of the printing plate is continuously chemically reacted to form a new inorganic salt hydrophilic layer, and we cannot guarantee that the blank portion is not dirty and the printing cannot be completed.
(3) reducing the surface temperature of the printing plate
At the time of printing, since the ink roller moves at a very high speed, its temperature rises and the temperature of the ink also rises. At high temperatures, the viscosity of the ink is lowered and the fluidity is increased. Thus, when printing, the ink spreads rapidly, which tends to cause the dot to expand and affect the printed matter. However, in the printing process, the dampening solution is used, which can effectively reduce the temperature of the printing plate, and can reduce such problems.
Second, the classification of dampening solution
The dampening solution is generally divided into: 1 ordinary dampening solution; 2 sprinkle moisturizing plate solution; 3 non-ionic surfactant dampening solution.
The chemical components added to the ordinary dampening solution are inorganic compounds such as phosphoric acid, phosphate, nitrate, etc., which have high surface tension and poor wettability, but are low in cost and easy to handle.
Alcohol dampening solution is added with ethanol, isopropyl alcohol or other alcohols. It has low surface tension and good wettability. It is suitable for lithographic printing presses equipped with alcohol wetting system.
The characteristics of alcohol are volatile, which can speed up the removal of heat from the printing plate and reduce the dirtyness of the plate. After adding alcohol to the water, the surface tension of the water can be reduced, so that the water can be quickly and evenly distributed, and the book is distributed on the printing plate. Easy to achieve ink and water balance, when printing, the water consumption is small, and the accuracy of overprinting is improved. The amount of alcohol is generally controlled between 12% and 20%. Too low will make the plate dirty, and the temperature of alcohol is generally controlled between 4 and 12 °C.
Nonionic surfactant dampening solution is a new dampening solution developed in recent years. It is generally prepared by adding a nonionic surfactant to a dampening solution containing other electrolytes. As the nonionic surfactant, polyoxyethylene, polyoxypropylene ether, and alkyl alcohol amide are often used. It has the characteristics of low surface tension, good wetting performance and stable chemical properties. It is an ideal dampening solution for high-speed multi-color lithographic printing presses. However, if the control is not good, ink emulsification may occur.