The fountain solution is an indispensable auxiliary material in the traditional offset printing process, and it has a crucial impact on the printing quality. The fountain solution has the functions of forming a uniform water film on the blank part of the printing plate, replenishing the damaged hydrophilic salt layer, lowering the surface temperature of the printing plate and cleaning the printing plate. At present, some printing companies still use alcohol fountain solution to control the water-ink balance in the offset printing process. During the use of alcohol fountain solution, the following process parameters should be noted.

1.pH value

The pH value is one of the important parameters of the alcohol fountain solution, which affects the transfer effect of the ink. If the pH value is too high or too low, it is not suitable for printing.

When the pH value is low, on the one hand, due to the high acidity, it will aggravate the corrosion of the metal plate, destroy the combination of the graphic part and the metal base, and cause the graphic part to fall off, and the phenomenon of "off-page" occurs; Alcohol fountain solution will react with the drier (lead, cobalt, manganese and other salts) in the ink, thereby delaying the drying time of the ink, causing the back of the printed matter to smudge. When the pH value is too high, on the one hand, the weak alkaline alcohol fountain solution will have a certain dissolution effect on the diazo compounds in the photodegradable graphic part, causing the graphic part to be lost, resulting in a "pattern" phenomenon; On the one hand, it will seriously emulsify the ink, which is not conducive to printing.

Under normal circumstances, the pH value of alcohol fountain solution should be controlled at 4.5 ~ 5.5, when it exceeds this range, it should be adjusted in time. The control method mainly adopts the method of increasing or decreasing the fountain solution. When the pH value is too low, add water to the alcohol fountain solution; when the pH value is too high, add the fountain solution to the alcohol fountain solution.

2. Conductivity

Electrical conductivity is an index to measure the conductivity of alcohol fountain solution. The higher the conductivity, the higher the ion concentration in the alcohol fountain solution and the stronger the conductivity. The conductivity actually measures the amount of dissolved salt in the alcohol fountain solution.

In the printing process, due to the effects of paper dusting, depilation and ink, paper dampener and ink will be mixed in the alcohol fountain solution, which affects its conductivity and gradually increases the conductivity. When the conductivity exceeds the standard range, alcohol The dampening effect of the fountain solution becomes poor, which will cause paste and deinking. At this time, the water tank must be cleaned and the alcohol fountain solution must be replaced. The standard alcohol fountain solution has a conductivity of 800 to 1200 μs / cm, and the exact value should be determined according to the specific printing conditions.

For the same kind of alcohol fountain solution, prepared with water from different places, the conductivity is also different, which is related to the degree of water hardness (how much Ca2 + and Mg2 + are in the water). Before preparing the alcohol fountain solution, we should first measure the conductivity of the water, its normal value should be 300 ~ 500μs / cm. If the conductivity of the water is too high, the water should be softened and its hardness should be controlled within 8mmol / L.

3. Concentration

The water-ink balance depends largely on the concentration of the alcohol fountain solution. At present, the vast majority of alcohol dampening solutions on the market are buffer dampening solutions. When the concentration changes greatly, the acidity and alkalinity will not change significantly, but changes in the concentration of alcohol dampening solution will cause the content of its components to occur. Changes, which cause changes in surface tension, ink emulsification properties, water-ink balance, etc., which in turn affect the wetting effect, resulting in a decline in print quality. Therefore, when using alcohol fountain solution, the concentration must be strictly controlled. At present, the conductivity is generally used to reflect the change of the alcohol fountain solution concentration, and the addition of the fountain solution is controlled. The modern offset press control system supports the automatic control function of the fountain solution, and automatically controls the addition and release of the fountain solution by measuring the conductivity.

4. The quality of alcohol / isopropanol

The quality of alcohol is the key to affecting the wetting effect of alcohol fountain solution. Many printing companies use edible alcohol or industrial alcohol in fountain solution, the main component of which is ethanol. This kind of alcohol fountain solution is not easy to achieve water-ink balance during printing. , The added amount is larger. If you use this alcohol fountain solution, the alcohol concentration exceeds 12%, and its wetting effect is not ideal, you must consider whether there is a problem with the quality of alcohol. Therefore, it is recommended to use isopropyl alcohol for fountain solution. It is easy to achieve water-ink balance when printing, and the amount of addition is small, generally 8% to 12%. The better the fountain solution, the smaller the amount of isopropyl alcohol. When purchasing isopropanol, the purity of isopropanol should be above 95%, otherwise it will lead to insufficient isopropanol content and affect its wetting effect.

5. Temperature

The influence of the temperature of the alcohol fountain solution on printing is mainly reflected in two aspects: first, the temperature should be stable, and the temperature change will cause the ink temperature to change, which will cause the instability of the printing quality; It affects the stability of the alcohol concentration in the alcohol fountain solution, which affects the printing quality. Therefore, for alcohol fountain solution, the temperature should be controlled in a fixed range, too high or too low temperature is not conducive to the transfer of ink, generally should be controlled at 8 ~ 12 ℃ (water tank temperature).

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