Talking about reasonable control of using dampening solution
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Lithographic offset printing is based on the principle of incompatibility, wetting, adsorption and selective adsorption of oil and water. Ink balance is the most important principle of lithographic offset printing. Reasonable control and use of fountain solution is very important in lithographic offset printing operations. This article takes the alcohol-based fountain solution which is currently used more as an example. From the aspect of the composition and application method of the fountain solution, combined with the author's printing work practice experience, talk about how to properly control the use of dampening solution.
Water: The main component of dampening solution is water. The water quality conditions vary from place to place. Generally, the conductivity of soft water is about 0-225uS, and hard water is usually 450uS. the above. When the water quality is hard (mainly when the Ca + content is high), it usually leads to Ca.+ Contact with the resin in the ink to form saponified calcium. When combined with the acid in the acid fountain solution, a salt such as lemon calcium or calcium phosphate is produced. The saponified calcium has lipophilicity, which causes the ink to be deposited in unnecessary areas such as water rollers and inking rollers, causing the surface of the rubber roller to crystallize, resulting in poor water transfer and ink transfer; and the calcium salt is hydrophilic, which hinders the ink. The transfer and cause the ink roller to deink, so the dampening solution additive used when the water is hard water should contain the corresponding substances to eliminate the influence of excessive calcium ions.
Alcohol or alcohol substitute: The main role of alcohol in dampening solution is to reduce the surface tension of the aqueous solution. The volatility of the alcohol can reduce the emulsification of the ink, keep the color of the printed product bright and accelerate the drying of the ink on the substrate. Since isopropanol has a slower volatilization rate, the ability to lower the surface tension of water is stronger, while general industrial alcohol has a higher volatility and a lower ability to lower the surface tension of water, and the production quality using isopropanol printing is generally stable. At present, the quality and price of alcohol products on the market are quite different. When using, products with high purity and good texture should be carefully selected. Use isopropyl alcohol as much as possible, do not use industrial alcohol, reduce the dosage, propose the use effect and production quality, and reduce cost. Generally, the surface tension of tap water is about 72 dyne, and the surface tension of the offset ink is about 30-36 dyne. The surface tension of the dampening solution must be controlled to be the same as the surface tension of the ink, so that the two are balanced and stable. To control the degree of emulsification of the ink. Usually, the isopropyl alcohol content can be achieved when the content is 8-12%, and the alcohol content should not be too high, otherwise the ink will be seriously emulsified.
Wetting solution additives: The choice of dampening solution additives should be very careful. Consider whether these ingredients are included: acidic electrolytes, buffers, hydrophilic colloids, surfactants, preservatives, fungicides, defoamers, etc. A good dampening solution should have the following characteristics: (1) It is weakly acidic, has good buffering capacity and stable PH (2) can react with the non-image part of the printing plate to form a hydrophilic inorganic salt that is insoluble in water; (3) can spread on the non-image part of the printing plate, and has good wettability; (4) for the machine, Corrosive to printing materials such as printing plates; (5) Compatible with the structure of the dampening system of the offset printing machine, no precipitation, no bubble, no blocking of the circulation pipe; (6) Printing compatibility with ink and substrate Matching, in line with the quality requirements of printed products; (7) The fountain solution is preferably colorless and transparent, which is conducive to the coloring effect of color printing; (8) It is environmentally friendly and harmless to the health of the operators.
The amount of dampening solution additive should be kept stable. Standard measuring tools should be equipped for manual addition. The automatic tank adding device should be regularly checked and maintained to ensure the normal operation of the extraction equipment. The test conductivity is used to control the amount of dampening solution additive added. Firstly, the conductivity of the clean water should be measured. In practice, the conductivity of the clean water should be deducted. Since the alcohol or alcohol substitute is non-conductive, the conductivity will decrease after the addition. The conductivity should be measured after adding the alcohol. Practice has shown that the reference value of conductivity is generally higher than that of clean water (after adding alcohol) 800-1200uS, PH The value is controlled between 4.8 and 5.3, and the quality of the product is easy to control.
The use of dampening solution should be used reasonably according to the machine, water quality, printing materials and other conditions of each printing factory. At present, printing enterprises in the northern regions and coastal areas, as well as some printing plants that use groundwater, have a hard water quality and often have excessive calcium. High-quality fountain solution additives should be suitable for modern high-speed printing conditions, and have anti-calcium function for hard water, with excellent buffering capacity, PH The value can be stabilized at about 5.1, and it can effectively prevent calcium from water, paper and ink from accumulating on the rubber roller, keeping the dampening solution clean without precipitation, no bubble, and stable under hard water conditions.
Reasonable control of the use of dampening solution in the printing production process can prevent and solve a variety of printing problems, often with a multiplier effect, you may wish to try.