Difficulties in ink and water balance: control of modern offset fountain solution
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In offset printing, the plate water is first wetted by the water on the printing plate, and the blank part of the printing plate is hydrophilic and adsorbs a certain amount of water. The graphic part is lipophilic and does not absorb moisture; the printing plate is inked by the plate roller. At this time, the graphic part will absorb a certain amount of ink. By embossing, moisture and ink are transferred to the paper through the plate cylinder and the blanket cylinder. The difficulty of this process is the balance of ink and water, and the fountain solution is the key.
The composition and function of dampening solution
The choice of dampening solution should be very careful, to check whether it has the following ingredients: buffer, plate protectant, wetting agent, preservative, desiccant. Biocides and anti-foaming agents, etc. The buffer mainly serves to stabilize the pH value of the dampening solution. Because the dampening solution is generally acidic, with the slowness; the pH of the dampening solution will fluctuate greatly, and the added amount is not suitable. Will affect the life of the plate (corrosion on the plate). In addition, it is difficult to dry the ink, especially an ink containing a metallic pigment.
During the printing process, the fountain solution is always in contact with the printing plate, and the non-image portion (inorganic salt layer) of the printing plate is continuously dissolved, so that the hydrophilicity of the printing plate is lowered, resulting in a dirty plate phenomenon. The plate protectant continuously replenishes the dissolved inorganic salt layer to maintain dynamic balance and ensure the stable hydrophilicity of the plate. The plate protectant also contains a protective sheet of glue, and there is no need to wipe the plate in the event of a temporary shutdown.
In the printing process, the less dampening solution is used as long as it is not dirty, so the wetting agent is very important. The wetting agent can reduce the surface tension of the dampening solution, so that the moisture is evenly distributed on the printing plate, and a good dampening effect can be achieved with less dampening solution. Generally, the amount of alcohol added in the traditional dampening solution is more than 15%, and the modern new dampening solution contains a wetting agent, which can greatly reduce the amount of alcohol.
Preservatives protect metal parts because dampening fluids are weakly acidic and corrosive to the metal parts of the press; desiccants promote drying of the ink; biocides prevent the growth of algae and microorganisms in the water circulation system. Avoid clogging of the tubing; anti-foaming agent prevents foam formation because the large amount of foam in the tank is not conducive to wetting the plate.
Modern new dampening solution should contain the above ingredients. In addition, the user should also test the hardness, conductance value and pH value of the water used in the factory in combination with the water quality and other conditions of the printing plant. The fountain solution manufacturer shall also provide the applicable dampening solution and related parameters according to the user's water quality conditions. These control parameters mainly include the amount of dampening solution stock solution, the amount of isopropanol (alcohol) added, the pH of the dampening solution and Conductivity, etc.
Use of fountain solution
The amount of dampening solution is generally controlled at 2% to 3%, but if the necessary measuring tools are missing, or if the automatic extraction device fails, the operator can only add it by experience, which triggers the run. Many problems in the control of the amount of liquid.
If the amount of dampening solution is too small, the printing plate is easy to be dirty, and the adjustment time is long; it is difficult to achieve the ink balance, it is difficult to obtain a satisfactory ink color effect, and the risk of machine corrosion is increased. If the amount of the fountain solution is too large, the ink is easily over-emulsified, and the ink color is difficult to control; the ink is smeared, the ink is scattered, the water bucket and the water roller are contaminated; the ink roller is not inked, and the ink is easily accumulated on the blanket.
In order to solve these problems, a standard measuring tool is required to manually add the dampening solution to ensure the quantitative addition; if there is an automatic adding device, it should be regularly inspected and maintained to ensure its normal operation.
When using modern new dampening solution, the amount of alcohol should be controlled within 10%. Generally, the operator thinks that the larger the amount of alcohol added, the less the dirty plate is, so the amount of alcohol is always too large. In fact, when the amount of alcohol is too high, the surface of the paper will be intensified, and the printed paper will be prone to paper paper powder; the ink will be easily emulsified and affect the quality of the printed product.
The pH and conductance values are usually used to reflect and monitor the condition of the fountain solution. The test proves that when the pH value is in the range of 4.8 to 5.3, the printing is the most stable, the printing plate has the longest life, the printing quality is the best, the ink balance is the most easy to control, and the pH value is determined not only by the amount of the fountain solution, but also by the paper. The effect of acidity and alkalinity. Conductivity should generally be controlled at (1400 ± 200) μs, the conductivity value is susceptible to water hardness, alcohol dosage, dust and ink mixture, so the conductivity value is allowed in 800 ~ 2000μs, but when the fountain solution looks obvious When it has been contaminated, it must be reconstituted.
In short, controlling the dampening solution during the printing process often has a multiplier effect.