As an indispensable auxiliary material in the traditional offset printing process, the influence of the fountain solution on the printing quality is very important. The fountain solution has a function of forming a uniform water film on the blank portion of the printing plate, supplementing 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 ink balance during the offset printing process. Here, this article will discuss how to control the process parameters of the alcohol fountain solution and the precautions for using the alcohol fountain solution.
The main process parameters of alcohol fountain solution
During the use of alcohol fountain solution, the following process parameters should be noted.
The pH value is one of the important parameters of the alcohol fountain solution, affecting the ink transfer effect. The pH is too high or too low to be suitable for printing.
When the pH is low, on the one hand, due to the high acidity, the corrosion of the metal plate material will be exacerbated, the combination of the graphic part and the metal plate base will be destroyed, the graphic part will fall off, and the phenomenon of “off the plate” will occur; on the other hand, The alcohol fountain solution reacts with the driers in the ink (lead, cobalt, manganese, etc. salts), thus delaying the drying time of the ink and causing the back of the printed product to pick up dirt. When the pH is high, on the one hand, the slightly alkaline alcohol dampening solution will have a certain degree of dissolution of the diazo compounds in the photo-degradable graphic part, resulting in the loss of the graphic part, resulting in a "flower pattern" phenomenon; On the one hand, it will make the ink seriously emulsified, which is not conducive to printing.
Under normal circumstances, the pH of the alcohol fountain solution should be controlled at 4.5 to 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, water is added to the alcohol fountain solution. When the pH value is too high, fountain solution is added to the alcohol fountain solution.
Conductivity is an indicator of the conductivity of the 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, paper powder and ink are mixed in the alcohol fountain solution due to the paper falling powder, hair removal, and the influence of the ink, thereby affecting the electrical conductivity and gradually increasing the electrical conductivity. When the electrical conductivity exceeds the standard range, alcohol The wetting effect of the fountain solution deteriorates, resulting in stencil and de-inking. At this time, the tank must be cleaned and the alcohol fountain solution must be replaced. The standard alcohol fountain solution conductivity is 800 ~ 1200μs/cm, the exact value to be determined according to the specific circumstances of the printing.
For the same type of alcohol fountain solution, the water conductivity in the different places is not the same, which is related to the degree of softness and hardness of water (the amount of Ca2+ and Mg2+ in the water). Before we formulate the alcohol fountain solution, we should first measure the conductivity of the water, the 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 8 mmol/L.
The ink balance largely depends on the concentration of the alcohol fountain solution. Most of the alcohol fountain solutions currently on the market are buffered dampening fluids. When the concentration changes greatly, the pH does not change significantly. However, changes in alcohol fountain solution concentration will lead to the occurrence of its constituent contents. Change, resulting in changes in surface tension, ink emulsification, ink balance, etc., and thus affect the wetting effect, resulting in decreased quality of the print. Therefore, when using alcohol fountain solution, its concentration must be strictly controlled. At present, conductivity is generally used to reflect changes in alcohol fountain solution concentration and to control the addition and release of fountain solution. The modern offset printing press control system supports dampening original liquid automatic control function, and automatically controls the addition and release of fountain liquid by measuring conductivity.
4. The quality of alcohol/isopropanol
The quality of alcohol is the key to influencing the wetting effect of alcohol fountain solution. Many printing companies use edible alcohol or industrial alcohol in the fountain solution. The main component is ethanol. This type of alcohol fountain solution is not easy to reach the ink balance during printing. , The amount of addition is large. If using this alcohol fountain solution, the alcohol concentration is more than 12%, its wetting effect is not ideal, we must consider whether there is a problem with the quality of alcohol. Therefore, it is recommended that isopropyl alcohol is used in the fountain solution, and the ink balance is easily achieved during printing. The amount of addition is small, generally 8% to 12%, and the better the performance of the fountain solution, the smaller the amount of isopropyl alcohol added. When purchasing isopropanol, ensure that the purity of isopropanol is above 95%, otherwise it will result in insufficient isopropanol content and affect its wetting effect.
The influence of the temperature of the alcohol fountain solution on printing is mainly reflected in two aspects: First, the temperature must be stable, the temperature change will cause the temperature of the ink to change, which will cause the instability of the printing quality; secondly, it directly affects the volatilization rate of alcohol. Affects the stability of the alcohol concentration in the alcohol fountain solution, thereby affecting the print quality. Therefore, for the alcohol fountain solution, the temperature should be controlled within a fixed range. If the temperature is too high or too low, it is not conducive to ink transfer. Generally, the temperature should be controlled at 8 to 12°C (tank temperature).
Precautions during use of alcohol fountain solution
In order to improve the quality of printed products, the following points should be noted during the use of alcohol fountain solution.
(1) Determine the alcohol fountain solution formula or fountain fountain brand according to the characteristics of the printed product. There will be a chemical reaction between different brands of alcohol fountain solution, so the brand of alcohol fountain solution should not be changed once it is determined.
(2) Measure the conductivity of the water and use soft water to formulate the alcohol fountain solution. If the hardness of the water is too high (greater than 15 mmol/L), the water must be softened.
(3) According to the requirements of the fountain solution, alcohol fountain solution is prepared according to different proportions, and the pH and conductivity of the alcohol fountain solution are measured at different proportions, and a curve is drawn to ensure that the alcohol fountain solution corresponds to different concentrations. The pH and conductivity are within the standard range.
(4) Each time using the same concentration of alcohol fountain solution for printing, in order to facilitate comparison of which concentration of alcohol dampening solution wetting effect is better, so as to facilitate the use of alcohol fountain solution next time.
(5) In actual printing, the conductivity of different concentrations of alcohol fountain solution can be measured through experiments, and the relationship curve between conductivity, pH value and concentration can be drawn, and then within the standard range, the quality of the printed product can be determined. Good or bad to find the best conductivity and pH (usually measured after adding alcohol, because the conductivity will drop after adding alcohol, the pH will increase), and record the best print quality, the alcohol dampening The concentration, pH, and conductivity of the liquid are used as the reference standard for the preparation of alcohol fountain solutions in the future.
(6) During the printing process, the temperature of the alcohol fountain solution should be kept constant, and its concentration, pH value, and conductivity should be measured frequently. When the measured value exceeds the standard range, it should be adjusted in time. When it is not possible to change the alcohol fountain solution, it should be replaced once a week and the wetting system should be cleaned.
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