The role and control of offset fountain solution (three)
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(3) Concentration control of dampening solution
The concentration of the fountain solution refers to the amount of the stock solution contained in the fountain solution. The stock solution is generally a wetting powder or a dampening solution.
In many printing plants, when using dampening solution, only the pH of the dampening solution should be controlled within the required range (pH 5.0-6.0), and the concentration of dampening solution is neglected. Since a part of the dampening solution contains more buffer, when the concentration of the dampening solution changes greatly, its pH (ie, pH value) does not change significantly. Therefore, it is unscientific to control only the pH value. .
Because the concentration of the dampening solution changes its chemical composition, which affects the ink and water balance. Changes in the concentration of the fountain solution also cause a change in the concentration of the surfactant, causing a change in surface tension. If the concentration in the fountain solution is too high, the ink will be emulsified and the blotting will be hindered. In the high-speed printing process, Mg(HCO3)2 and Ca(HCO3)2 in the dampening solution will be decomposed due to temperature factors, resulting in insolubilization. CaCO3 calcium carbonate and MgCO3 magnesium carbonate in water adhere to the water roller, which easily prevents water transfer and viscous plate failure. Therefore, it is necessary to soften the water before preparation. Therefore, the concentration of dampening solution is a key factor in the quality of the printed product. We must control the concentration of dampening solution while maintaining a stable pH (pH).
In order to make the printed images bright and clear, the blank layout is not dirty, and the concentration of the fountain solution must be properly adjusted according to the specific printing conditions to ensure that high-quality printed matter is printed.
1, paper properties and fountain solution concentration
When the printing paper is loose or contains dust in the paper, the printing ink often has paper and paper powder on the blanket due to the high viscosity of the printing ink, which will increase the friction of the printing plate and make the printing plate easy to get dirty. In this case, the concentration of the fountain solution should be increased appropriately. When printing paper with a high flatness and tightness, especially when printing coated paper and glass paper, the dampening solution should be appropriately reduced. concentration.
2, ink properties and fountain solution concentration
Due to the different properties of printing inks, the properties of viscosity, fluidity and acid resistance are also different, and the response to the concentration of dampening solution will be different. Therefore, the concentration of dampening solution should be adjusted as appropriate. In general, the concentration of the fountain solution is based on the principle that the concentration of the printed red ink is greater than the concentration of the printed black ink; the concentration when printing the black ink is greater than the concentration of the printed cyan ink; and the concentration when printing the dark ink is greater than The density of the printed light ink.
3, the amount of desiccant and the concentration of dampening solution
When printing the stacked color plate product, in order to accelerate the drying speed of the ink, it is necessary to add dry oil to the ink, which also makes the particles of the ink thicker and the viscosity increases, so that the fatiness of the blank portion of the printing plate is increased, so that the paste is easily generated. The version of the ill. Based on this situation, the amount of stock solution should be increased as appropriate.
4, printing plate and fountain solution concentration
Printed products are available in a wide range of layouts, from solid to mesh and to textured and textured. If the layout is full-line lines and text-based, the concentration of the fountain solution should be adjusted appropriately due to the larger ink-loading area. If all are the screen version, the concentration of dampening solution should be adjusted appropriately.
5, ambient temperature and dampening solution concentration
When the temperature of the printing environment rises, the molecular motion of the substance will accelerate, the fluidity of the ink will increase, and at the same time, more free fatty acids will be decomposed, which will make the printing plate easy to produce dirt. Therefore, when the temperature is high, the dampening solution should be appropriately increased. concentration.
6, printing speed and fountain solution concentration
Since the ink has thixotropic characteristics, when the printing speed is high, the ink roller of the printing machine has a large friction coefficient, and it is easy to generate heat to make the ink thin, so that the coloring power is lowered, and the ink color is light, and the ink amount needs to be increased accordingly to deepen. Printing ink, so the plate is prone to dirty. Therefore, when the printing speed is high, the concentration of the fountain solution should be appropriately increased.
7, if the printing pressure is high and the texture of the blanket and the liner is hard, it should also be considered to increase the concentration of the fountain solution. In order to achieve a better anti-dirty effect. The above indicates that the concentration of dampening solution should be comprehensively considered according to various specific conditions.
Determination of the concentration of dampening solution:
The concentration of the fountain solution can be determined by measuring the conductivity. It is proved by experiments that there is a linear relationship between the concentration change of the fountain solution and its conductivity change, that is, the higher the concentration, the greater the conductivity, so it should be changed. Find a certain value in the range as its optimal wetting effect value. We should use soft water when formulating dampening solution to ensure that its pH value is between 4.5 and 5.5, and the conductivity should be between 1200 and 600 us/cm.