Use of fountain solution

Offset printing is the use of the principle of incompatible ink, by providing a certain amount of water to the blank part of the printing plate, so that it has enough hydrophilic and oil-repellent ability to maintain the ink balance between the blank part and the graphic part. There is a saying: "If you have mastered the water, you will have mastered offset printing." It can be seen that the fountain solution plays a decisive role in the offset printing process. Here I combine personal work experience and talk about my understanding.

First, fountain solution PH value control

Two principles: The first is that the PH value cannot be too high. Because the acidity is weak, the hydrophilic adhesive layer is not easy to generate, the surface tension of the fountain solution drops sharply, and the water content of the ink becomes higher, which leads to problems such as ink emulsification and serious dot enlargement; at the same time, the ability of the fountain solution to clean the printing plate also becomes decline. The second is that the PH value can not be too low, otherwise it will affect the ink's own oxidation drying rate, and intensify the encroachment on the printing plate, corrode the blank part and the graphic part of the photosensitive material, so that the printing plate is resistant to printing force and may damage the alkali. Paper surface coating. Practice has proved that for the PS version, the pH value range of the fountain solution that best meets the printing technical requirements is 4.8-5.5.

Second, fountain solution dosage control

The use of fountain solution involves a variety of factors in the printing process.

1. The higher the ambient temperature and the machine temperature, the greater the consumption of dampening fluid. The heat source of the ink part of the printer is mainly the axial turbulent friction between the string water roller and the ink roller. The higher the temperature, the faster the fountain solution on the plate, the water roller, and the ink roller evaporates. Therefore, when the temperature is high, it is necessary to increase the amount of fountain solution.

2. Ink ink emulsion 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 drawing property is poor, and the transfer performance is poor. The oil-in-water emulsification is presented. The ink with small viscosity and good transfer performance has a relatively small amount of fountain solution, and vice versa. In addition, if the amount of ink is large, the amount of fountain solution should also be increased accordingly.

3. Printing plate area In general, the field version is slightly larger than the line version. The text version is slightly larger than the dot version.

4. The paper properties of the paper have a great influence on the correct use of the fountain solution. The pH of the paper is preferably neutral, but the paper actually has different acidity and alkalinity depending on the papermaking process. In general, the non-coated papers such as offset papers have a pH of 4.5 to 7.0 and are weakly acidic. The coated papers have a slightly alkaline pH of 7.0 to 9.0. The printer can adjust the fountain solution according to the nature of the actual paper. When the paper is acidic, the pH of the fountain solution can be larger. When the paper is alkaline, the pH of the fountain solution can be smaller.

The moisture content of the paper is large, and the amount of fountain solution is small; the moisture content of the paper is small, and the amount of fountain solution is larger. If the surface strength of the paper is low, that is, the resistance to powder dropping and hair loss is weak, paper powder and paper dust tend to pile up on the blanket and cause various problems. At this time, if the amount of fountain solution is slightly larger, the amount of paper powder and paper on the blanket can be effectively reduced, the time interval between washing the blanket can be greatly extended, and the number of scrubs in the middle can be reduced.

5. Printing speed When the printing speed increases, the amount of fountain solution should also increase.

In addition, the printing pressure, blanket hardness, plate type and other factors will also affect the pH and amount of fountain solution used. In short, the amount of fountain solution should be reduced as much as possible without causing blemishes on the blank part of the printing plate.

Third, why join Arab gum

Adding a suitable amount of hydrophilic limbs (Arabic gum or CMC) in the fountain solution is beneficial for repairing the hydrophilic adhesive layer on the surface of the printing plate, and maintaining the wettability of the blank part of the printing plate.

Fourth, the fountain solution caused by the common fault

If the dampening solution is too acidic, it will corrode the plate graphic, making the plate easier to get dirty and the ink drying time longer. If the acidity is too weak, the ink emulsification will be serious, the dots will be enlarged, and the imbalance of ink and ink will be dirty. Excessive water content in the layout will reduce the density of the printed product on the spot, and it will cause false printing on the screen. It will affect the dryness of the printed product and cause the back of the printed product to become dirty. The paper absorbs too much water and the fiber is overprinted. If the water content of the layout is too small, the dots will be blurred, causing stencil, dirty version and other phenomena.

V. Precautions

1. Regularly clean the ink on the water roller cover to maintain good water delivery performance.

2. Clean the water roller and water bucket regularly. When adding water to the tank, keep the water pipes open and prevent floc-like sediments from rising up and plugging the pipes.

3. When wetting the wet powder or wetting original drops, quantitative control is required and the pH value is measured with a precision test strip.

4. Keep the pH of the fountain solution relatively stable during the printing process.

5. During the printing process, look at the moisture. If the reflection of water is observed on the printing plate, it indicates that the moisture is large. In the ideal state, the reflection of water cannot be observed on the plate surface. At the same time, the surface of the printing plate is not on the dirty paste plate.

6. Use water with low hardness as much as possible to prevent insoluble calcium citrate precipitates from calcium ions and wetting powder in the water or citrate in the wetting stock solution, blocking the micropores on the surface of the ink roller and causing the ink roller to deink. .

7. After a few minutes of general downtime, the water in the layout will evaporate. Before starting printing, increase the amount of water supply.

8. When the machine is stopped for a long time, it is necessary to drain away the fountain solution in the bucket so as not to corrode the bucket.