Control of fountain solution in modern offset printing

In offset printing, the water roller is wetted by the printing plate water, and the blank part of the printing plate is hydrophilic and absorbs a certain amount of water. The graphic part is oleophilic and does not absorb moisture; the printing plate is inked by the printing roller. At this time, the graphic part will absorb a certain amount of ink. Then by imprinting, moisture and ink are transferred to the paper by the plate cylinder and the blanket cylinder. The difficulty in this process is the balance of ink and water, and fountain solution is the key.

The composition and function of fountain solution

The choice of fountain solution should be very careful to check whether it has the following ingredients: buffer, plate protection agent. D. Preservatives, desiccants. Killer and anti-foaming agent. Slow; medium mainly to stabilize the fountain solution pH value, due to the fountain solution is generally acidic, with the amount of buffer added, the pH of the fountain solution will have greater fluctuations, the amount of added is not appropriate Will affect the life of the plate (corrosion of the plate). It also makes it difficult to dry the ink, especially inks containing metallic pigments.

During the printing process, the fountain solution is always in contact with the printing plate, and the non-graphical portion (inorganic salt layer) of the printing plate will be continuously dissolved, thereby reducing the hydrophilicity of the printing plate, resulting in a dirty plate phenomenon. The plate protection agent can continuously replenish the dissolved inorganic salt layer, maintain the dynamic balance, and ensure the plate's stable hydrophilicity. The printing plate protector also contains guard plate glue, and there is no need to wipe the plate in case of temporary stoppage.

In the printing process, the less the better the dampening solution is, the better the dampening agent is. Therefore, the wetting agent is very important. The wetting agent can reduce the surface tension of the fountain solution so that the water is evenly distributed on the printing plate, and a good wetting effect can be achieved with less fountain solution, as shown in FIG. 1 . The amount of alcohol in the traditional fountain solution is more than 15%, while the modern new fountain solution contains a wetting agent, which can greatly reduce the amount of alcohol.

Preservatives protect metal parts because the fountain solution is weakly acidic and corrosive to the metal parts of the press; desiccants promote ink drying; biocides prevent algae and microorganisms from growing in the water circulation system. Avoid blockages in the tubing; anti-foaming agents prevent the formation of foam because the bulk of the foam in the tank is detrimental to the wetting of the printing plate.

The modern new fountain solution should contain the above ingredients. In addition, the user should also choose the water hardness of the factory in combination with the water quality of the printing factory and other conditions. Conductance and pH are tested. The fountain solution manufacturer should also provide applicable fountain solution and related parameters according to the users' water quality conditions. These control parameters mainly include the amount of the fountain solution and the amount of isopropyl alcohol (alcohol) added. Fountain solution pH and conductivity.

Use of fountain solution

The addition amount of fountain solution is generally controlled at 2% to 3%, but if there is no necessary measurement tool, or if the automatic extraction equipment fails, the operator can often only add by experience, which leads to the run. There are many problems in the control of the dosage of the liquid.

If the amount of the fountain solution is too small, the printing plate may be dirty and the machine may take a long time to adjust. It is difficult to achieve ink balance, and it is difficult to obtain a satisfactory ink effect, which increases the risk of machine corrosion. If the amount of fountain solution is too large, the ink is easily over-emulsified and the ink is not easy to control; Ink flying phenomenon, pollution bucket and water roller; ink roller is not inked, the ink is easy to accumulate on the blanket.

In order to solve these problems, a standard measuring tool is required to manually add the fountain solution to ensure the quantitative addition; if there is an automatic adding device, it should be regularly checked and maintained to ensure its normal operation.

When using modern new fountain solution, the amount of alcohol added should be controlled within 10%. Generally, the operator will think that the greater the amount of alcohol added, the more difficult it is to contaminate the plate, so the amount of alcohol is always too large. In actual fact, when the amount of alcohol is too high, the dissolution of the surface of the paper will be intensified, and the printed paper is prone to paper powder. The ink is easily over-emulsified and affects the quality of the printed product.

The two parameters of pH and conductance are usually used to reflect the condition of the fountain solution and monitor it. Tests have proved that when the pH value is within the range of 4.8 to 5.3, the printing is the most stable, the printing plate has the longest service life, the quality of the printing product is the best, the ink and ink balance is the most easily controlled, and the pH value is determined not only by the addition amount of the fountain solution. , also affected by the acidity of the paper. The conductivity should generally be controlled at (1400 ± 200) us. The conductance value is sensitive to water hardness and alcohol consumption. Due to the influence of dust and ink inclusions, conductivity values ​​of 800-2000us are permissible. However, when the dampening solution seems to be contaminated, it must be reconstituted.

In short, controlling the well-printing solution during the printing process will often have a multiplier effect.