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Since traditional offset printing relies on the principle of ink-water repellence, the use and control of fountain solution in offset printing is a key part of printing. Although waterless offset printing has been developed to a great extent, but waterless printing is still not widely popularized today, how to control the indicators of fountain solution in offset printing and use fountain solution more reasonably, It is still an important topic that we need to discuss.
Let us see what role wetting fluid plays in offset printing:
First, the wetting liquid forms a uniform water film on the blank part of the printing plate to resist the infiltration of the ink on the graphic into the blank part.
Secondly, due to the contact and friction between the blanket cylinder, water roller and ink roller and the printing plate, as well as the de-powdering and lint of ink and printing materials, the friction on the surface of the printing plate is caused, which causes the abrasion of the hydrophilic salt layer on the surface of the printing plate. The electrolyte in the fountain solution repairs the damaged hydrophilic layer to form a new hydrophilic salt layer.
In addition, the fountain solution can also control the temperature of the printing surface during printing, and take away the heat generated during the friction between the machine parts and the shearing of materials such as ink.
The fountain solution also promotes the emulsification of the ink in the printing process (guaranteeing a certain emulsification value of the ink has a very important effect on the good transmission of the ink in the printing), and quickly achieves the balance of water and ink.
Below we will further discuss the control of fountain solution in production.
1. PH value of fountain solution
In the offset printing process, due to the printing pressure of the printing process and some other abrasion reasons, it is easy to destroy the hydrophilic oxide layer on the surface of the PS plate, so the fountain solution needs to ensure a certain acidity in order to form a new hydrophilic salt Layer, repair the damaged hydrophilic layer in time. In addition, in the printing process to ensure that the fountain solution has a certain weak acidity, it can also play a role in removing the oil on the blank part of the plate. Practice has proved that for the PS version, the PH value range of the fountain solution that best meets the printing technical requirements is 5 to 6.
When the PH value of the solution is too low, it will not only play a role in repairing the printing plate, but will corrode the printing plate in a vicious manner, and cannot form a stable hydrophilic salt layer, the combination of the graphic part of the printing plate and the metal plate base It may also be damaged, thereby reducing the printing durability of the printing plate. In addition, the fountain solution will react with the drier in the ink, thereby delaying the drying of the ink during the printing process, causing the back of the printed product to smudge, and affecting the overprint rate of the ink.
If the PH value of the solution is too large, the ink-receptive layer of the graphic part will be dissolved first, resulting in a decrease in the printing durability of the printing plate; in addition, the basic groups in the solution will react with the fatty acids present in the ink, resulting in very active Strong hydrophilic groups, such a large increase in hydrophilic groups will cause serious emulsification of the ink during the printing process.
The PH range given above is not an absolute range. The situations encountered in actual production are varied, and the PH value of the fountain solution is not constant. That is to say, in the printing process, we should make some appropriate adjustments to the PH value of the fountain solution according to the actual situation.
1. The effect of paper acidity and alkalinity on fountain solution. The PH value of the paper is preferably neutral, but in fact, due to different papermaking processes, the acidity and alkalinity of various papers are also different. Generally speaking, the pH value of non-coated paper such as offset paper is 4.5-7.0, which is weakly acidic; the pH value of coated paper is 7.0-9.0, which is weakly alkaline, and the printing worker can adjust the dampening according to the nature of the paper used. Adjust the fluid accordingly. When the paper is acidic, the PH value of the fountain solution can be higher (when printing offset paper, the PH value can be controlled between 5.5 and 6.5 according to the situation), and when the paper is alkaline, the PH value of the fountain solution can be smaller ( Generally, when printing on coated paper, the pH value of fountain solution can be controlled at 4.8 ～ 5.0, and it can be reduced appropriately according to the situation).
2. When the thickness of the ink layer is increased, the ink in the graphic part will inevitably spread to the blank part, which is prone to "dirty version"; when the amount of desiccant in the ink is increased, the viscosity of the ink increases and the particles become thicker. The adhesion is increased, and dirty plates are prone to occur; when printing products with low surface strength, due to the serious phenomenon of powder and fluff, the hydrophilic salt layer of the plate surface is easy to be worn; when the ambient temperature is high, More free fatty acids will be separated from the ink, which will react with the OHˉ in the fountain solution, causing serious emulsification of the ink. In the above situations, we should pay attention to lower the pH value of fountain solution to increase the acidity of the solution.
Second, the amount of fountain solution
The amount of fountain solution in the printing process also needs to be well controlled. The principle of controlling the amount of fountain solution is to reduce the amount as much as possible on the premise that the blank part is not dirty and the ink does not show serious emulsification.
If the amount of fountain solution is too small, it will obviously not protect the blank part of the printing plate, which will blur the dots and cause the phenomenon of paste and dirty plates.
What if the amount of fountain solution is too large? Excessive moisture on the plate will reduce the solid density of the printed product and make the dots false; it will affect the dryness of the printed product and cause the back of the printed product to be dirty; the paper absorbs too much water and the fiber elongation is not accurate.
The amount of fountain solution used in the work is not static, and some appropriate adjustments should be made according to some specific situations:
1. Temperature: The higher the ambient temperature and the machine temperature, the greater the consumption of fountain solution. The heat source of the ink and ink part of the printing press is mainly the axial running friction between the water inking roller and the ink inking roller. The higher the temperature, the faster the fountain solution on the printing plate, water roller, and ink roller evaporates, and more fountain solution will seep into the ink. Therefore, when the temperature is high, the amount of fountain solution should be increased.
2. Ink: Ink emulsification is the main factor affecting the amount of fountain solution. The critical value of the ink receiving water is between 20% and 25%. When the critical value is exceeded, the pigment particles are in a free state, the viscosity of the ink is poor, the drawability is poor, the transfer performance is poor, and the oil-in-water emulsification is shown. For inks with low viscosity and good transfer performance, the amount of fountain solution is relatively small; otherwise the amount is large. In addition, if the amount of ink is large, the amount of fountain solution should be increased accordingly.
3. Graphic area of ​​the printing plate: Generally speaking, the field version consumes a slightly larger amount of fountain solution compared to the line version; the text version uses a slightly larger amount of fountain solution compared to the dot version.
4. Printing speed: when the printing speed is increased, the amount of fountain solution should be increased accordingly.
3. About alcohol fountain solution
Now because of the many advantages of alcohol fountain solution, its use has been quite popularized and promoted.
The main components of alcohol fountain solution are water, an appropriate amount of alcohol (or isopropyl alcohol) and fountain solution stock solution. The concentration of alcohol has a great influence on the use of fountain solution.
First, let's take a look at the role of alcohol in printing:
1. Reduce the surface tension of the fountain solution, so that it can enhance the wetting ability of the plate surface.
2. Increase the viscosity of the fountain solution so that it can easily pass through the roller gap.
3. Accelerate the emulsification speed of ink and fountain solution, while preventing excessive emulsification of ink.
4. At the same time of volatilization, it takes away heat and has a cooling effect.
Let's talk about the control of alcohol fountain solution from the aspects of alcohol concentration and temperature
In the use of fountain solution, the alcohol concentration first affects the surface tension of the fountain solution. The greater the concentration, the smaller the surface tension of the fountain solution. Experience has shown that it is more appropriate to control the concentration of alcohol at 10% to 15%. If the concentration of the fountain solution is less than 10%, the effect is not obvious; if it exceeds 30%, alcohol will flocculate with the gum in the fountain solution and destroy the fountain solution.
In the production process, it is generally necessary to set the minimum concentration value of alcohol. Whether it is automatic or manual measurement, good control is required, especially manual measurement control, pay attention to diligent measurement and diligent control. In addition, when there is gum arabic in the solution, the measured data will be very different from the actual data. For example, when the gum arabic concentration in the solution is 2%, when the actual alcohol concentration in the solution is 12%, the measured data is 5.5% will be displayed.
Since alcohol has good volatility, if the temperature cannot be controlled well, the alcohol concentration of the fountain solution will not be controlled well. In addition, the alcohol fountain solution can ensure the ideal high viscosity at a relatively low temperature, which is very beneficial for passing the water roller and wetting the printing plate, but the temperature of the fountain solution should not be too low, too low temperature will cause the temperature of the ink If it is reduced, the transfer ability will be reduced, so the temperature of the fountain solution should be well controlled during production. Generally, the water temperature of the fountain solution is controlled at 10-12 ° C in the circulation tank, and the temperature in the water bucket should be controlled at 14-15 ° C.
Control of fountain solution stock solution. For the addition of fountain solution, generally speaking, the conductivity method is more accurate. The concentration value of the fountain solution changes proportionally to the conductivity of the fountain solution. The greater the concentration of the fountain solution, the greater the conductivity of the fountain solution. Experiments show that it is reasonable to control the conductivity of fountain solution in the range of 700 ～ 1700μs / ?, and it can be adjusted differently according to different brands and models, such as Vegra 3300ph or Superdot fs-12 fountain solution. The conductivity should be controlled within the range of 1000 ～ 1300. In the measurement and control, pay attention to regularly clean the circulating water tank, because the foreign objects in the water tank (paper powder, ink, paint, dust, etc. entering the wetting fluid) will affect the accuracy of the measurement.
Above we have conducted some discussions on the control and use of fountain solution in offset printing. In actual work, many factors related to the use of fountain solution do not exist independently, but restrict each other. This requires us to make better judgment in production practice in order to better control.
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