Control and use of fountain solution
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Since traditional offset printing relies on the principle of ink repellent, the use and control of fountain solution in offset printing is a key part of printing. Although waterless offset printing has been greatly developed, but in the past, waterless printing has not been widely used, how to control the indicators of dampening solution in offset printing, and more rational use of dampening solution, It is still an important topic that we need to explore.
Let us see what role the wetting fluid plays in offset printing:
First, the wetting fluid forms a uniform water film in the blank portion of the printing plate to resist the infiltration of the ink onto the blank portion of the image.
Secondly, due to the contact and friction between the blanket cylinder, the water roller and the printing plate, and the abrasion of the ink and the printing material, the surface of the printing plate is rubbed, which causes the wear of the hydrophilic salt layer on the surface of the printing plate. The electrolyte in the fountain solution repairs the damaged hydrophilic layer and reforms the new hydrophilic salt layer.
In addition, the fountain solution can also control the layout temperature during printing, taking away the heat generated by the friction between the machine parts and the material such as ink.
The dampening solution also has a certain promoting effect on the ink emulsification during the printing process (guaranteeing a certain ink emulsification value, which plays a very important role in the good transfer of the ink in the printing), and quickly realizes the ink and water balance.
Below we will further discuss the control of dampening solution in production.
First, the pH value of the fountain solution
In the process of offset printing, due to the printing pressure of the printing process and some other wear reasons, it is easy to destroy the hydrophilic oxide layer on the surface of the PS plate, so that the fountain solution is required 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, the dampening solution is surely weakly acidic, and it can also remove the oil stain in the blank part of the layout. Practice has proved that for the PS version, the pH value of the dampening solution that best meets the printing technology requirements is 5-6.
When the pH of the solution is too small, it will not only play the role of repairing the printing plate, but will viciously corrode the printing plate, and will not form a stable hydrophilic salt layer. The combination of the graphic part of the printing plate and the metal plate base It is also possible to be destroyed, thereby reducing the printing durability of the printing plate. In addition, the dampening solution reacts with the drier in the ink, thereby delaying the drying of the ink during the printing process, resulting in smudging on the back side of the print and affecting the overprinting rate of the ink.
If the pH of the solution is too large, the ink-repellent layer of the image portion will be dissolved first, resulting in a decrease in the printing durability of the printing plate. In addition, the basic group in the solution will react with the fatty acid present in the ink to generate a very active agent. Strong hydrophilic groups, a large increase in such hydrophilic groups, can lead to severe emulsion emulsification during printing.
The range of pH values given above is not an absolute range. The conditions encountered in actual production are varied and the pH of the fountain solution is not constant. That is to say, in the printing process, it is necessary to make some appropriate adjustments to the pH value of the dampening solution according to the actual situation.
1. The effect of acidity and alkalinity on the dampening solution. The pH of the paper is preferably neutral, but in fact, the acidity and alkalinity of various papers are different due to different papermaking processes. Generally speaking, the non-coated paper such as offset paper has a pH value of 4.5 to 7.0, which is weakly acidic; the coated paper has a pH of 7.0 to 9.0, which is weakly alkaline, and the printing worker can apply the dampness according to the nature of the actual paper. The liquid is adjusted accordingly. When the paper is acidic, the pH of the dampening solution can be larger (when printing the offset paper, the pH can be controlled between 5.5 and 6.5 depending on the situation). When the paper is alkaline, the pH of the dampening solution can be smaller ( Generally, when printing the coated paper, the pH of the dampening solution can be controlled to 4.8 to 5.0, and can be appropriately reduced according to the situation).
2. When the thickness of the ink layer increases, the ink in the graphic part will diffuse to the blank portion, which is prone to “dirty plate”; when the amount of the desiccant in the ink increases, the viscosity of the ink rises and the particles become thicker, and the blank portion of the printing plate The adhesion is increased, and the dirty plate is prone to occur; when the printed surface is not high in strength, the phenomenon of falling off the powder is severe, so that the hydrophilic salt layer of the plate is easily worn; when the ambient temperature is high, More free fatty acids are separated from the ink, which will react with OHˉ in the fountain solution, resulting in serious ink emulsification. In the above cases, it is necessary to pay attention to appropriately lowering the pH of the dampening solution to increase the acidity of the solution.
Second, the amount of dampening solution
The amount of dampening solution used in the printing process also needs to be well controlled. The principle of controlling the amount of dampening solution is to minimize the amount of the ink in the case where the blank portion is not dirty and the ink does not undergo serious emulsification.
The amount of dampening solution is too small, it is obvious that it will not protect the blank part of the printing plate, which will make the dot blurred, causing paste and dirty plates.
If the amount of dampening solution is too large? If the moisture content of the plate is too large, the density of the printed product will decrease, and the dot will be false; the printing will be dry, which will cause the back of the printed product to be dirty; the paper will absorb too much water and the fiber will not be overprinted.
The amount of dampening 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 machine temperature, the greater the consumption of dampening solution. The heat source of the ink portion of the printing machine is mainly the axial turbulent friction between the water roller and the ink roller. The higher the temperature, the faster the dampening solution on the plate, water roller, and ink roller evaporates, and more dampening solution penetrates into the ink. Therefore, when the temperature is high, it is necessary to increase the amount of dampening solution.
2. Ink: Ink emulsification is the main factor affecting the amount of dampening 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 stringiness is poor, and the transfer property is poor, and the oil-in-water type is emulsified. For inks with low viscosity and good transfer performance, the amount of dampening solution is relatively small; otherwise, it is larger. In addition, if the amount of ink is large, the amount of dampening solution should be increased accordingly.
3. Printed graphic area: Generally speaking, the amount of dampening solution consumed is slightly larger than that of the line version; the text version is slightly larger than the dot version, and the amount of dampening solution is slightly larger.
4. Printing speed: When the printing speed is increased, the amount of dampening solution should also increase.
Third, about alcohol dampening solution
Nowadays, due to the many advantages of alcohol dampening solution, its use has been quite popularized and promoted.
The main components of the alcohol dampening solution are water, an appropriate amount of alcohol (or isopropanol) and a fountain solution. The concentration of alcohol has a great influence on the use of dampening solution.
First let's take a look at the role of alcohol in printing:
1. Reduce the surface tension of the dampening solution to enhance the wetting ability of the plate.
2. Increase the viscosity of the dampening solution so that it easily passes through the roller gap.
3. Accelerate the emulsification speed of the ink and dampening solution while preventing excessive emulsification of the ink.
4. While volatilizing, take away heat and have a cooling effect.
Here we talk about the control of alcohol dampening solution from several aspects such as alcohol concentration and temperature.
In the use of dampening solution, the first effect of the alcohol concentration is the surface tension of the dampening solution. The higher the concentration, the smaller the surface tension of the dampening solution. Experience has shown that it is a suitable concentration to control the concentration of alcohol at 10% to 15%. If the concentration of the dampening solution is less than 10%, the effect is not obvious; if more than 30% of the alcohol will flocculate with the colloid in the dampening solution, destroying the dampening 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 needed, especially manual measurement control, and attention should be paid to diligent measurement control. In addition, when there is gum arabic in the solution, the measured data and the actual data will be very different. For example, when the concentration of gum arabic in the solution is 2%, when the actual alcohol concentration of the solution is 12%, the data we measured will be Will show 5.5%.
Because alcohol has a good volatility, if the temperature is not well controlled, the alcohol concentration of the fountain solution will not be well controlled. In addition, the alcohol dampening solution can guarantee the ideal high viscosity at a relatively low temperature, which is very advantageous for passing the water roller and wetting the printing plate, but the temperature of the dampening solution can not be too low, too low temperature will cause the temperature of the ink. Lowering, the transfer capacity is reduced, so the temperature of the dampening solution should be well controlled in production. Generally, the water temperature of the fountain solution is controlled at 10 to 12 ° C in the circulation tank, and the temperature in the water tank should be controlled to 14 to 15 ° C.
Control of dampening solution stock solution. For the addition of dampening solution, it is generally more accurate to use the conductivity method. The concentration of the dampening solution is proportional to the conductivity of the dampening solution. The greater the concentration of the dampening solution, the greater the conductivity of the dampening solution. Experiments have shown that it is reasonable to control the conductivity of the fountain solution in the range of 700 to 1700 μs/cm. It can also be adjusted according to different brands and models, for example, Vegra 3300ph or Superdot fs-12 type fountain solution. The conductivity should be controlled to the range of 1000 to 1300. In the measurement and control, it is necessary to clean the circulating water tank regularly, because the foreign matter in the water tank (paper powder, ink, pigment, dust, etc. entering the dampening fluid) will affect the accuracy of the measurement.
Above we have discussed some aspects of the control and use of fountain solution in offset printing. In the actual work, many factors related to the use of dampening solution are not unique, but are mutually restrictive, which requires us to better judge in the production practice, in order to better control.