Third, fountain solution control

There are three aspects to the control of the fountain solution: 1. The pH of the fountain solution is controlled.
2, fountain solution dosage control.
3, fountain solution concentration control.

(A) Fountain solution PH value control

The pH value indicates the strength or alkalinity of the solution, ie pH. PH value is divided into 14 levels;
PH = 7 is a neutral solution;
PH < 7 is an acidic solution;
PH > 7 is an alkaline solution;
The pH between each level is separated by 10 times. That is, PH = 5 and PH = 6 pH 10 times. PH = 5 and PH = 7 pH 100 times. That is, 10 x 10 = 100.
General PS version of the fountain solution PH value is controlled between 5 to 6, which is weakly acidic.

The effect of dampening solution pH on printing:

If the acidity is too strong and PH is too low:
1 Destroy the hydrophilic protective layer on the blank part of the printing plate, corroded by the acid in the plate surface, and print the easy-to-print version.
2 Part of the printing plate graphic is eroded, making the dot smaller, resulting in a serious version.
3 Destruction of the ink's oxidized conjunctiva performance, so that the ink drying time is prolonged. In severe cases, the ink will not dry. Experiments have shown that if the PH value drops from 5.6 to 2.5, the ink drying time is extended from the original 6 hours to 24 hours.
4 accelerate ink emulsification, affect the ink ink layer of normal transmission.
5 Ink is dull and dull.
When the acidity is too weak and the pH is too high:
1 The blank part of the printing plate has weakened hydrophilicity, which spreads the ink of the graphic part and can easily cause paste printing.
2 It is difficult for the blank part of the printing plate to generate a protective layer, causing the blank part of the printing plate to be dirty.
3 Accelerate ink emulsification, reduce the contrast of the printed image, and the printing effect appears flat, light, and bright.
4 The graphic part of the positive PS version is easily dissolved, resulting in incomplete graphics and missing strokes.
The general method of determining the pH of the fountain solution is:
1. Large oily ink, the acid value of the fountain solution is slightly larger, that is, the PH value is smaller, otherwise it is easy to dirty the plate.
2. The dark ink is more acidic than the fountain ink of the light ink, ie the PH value is smaller.
3. At the time of printing, a product with a thick ink layer thickness needs a more acidic dampening solution than a thin one.
4. The acidity of the dampening solution used in the field version is higher than that of the cable version. This is also the relationship between the thick cable version and the thin cable.
5. In the printing workshops with high temperature and humidity, since the ink is more easily hydrolyzed, the acidity of the fountain solution must be appropriately increased.
6. Printed products with poor paper quality require increased acidity of the fountain solution.
7. Printing paper is generally neutral or slightly alkaline, which is good for ink drying. However, the pH value of the paper is also not very stable, and it is sometimes acidic or alkaline so as to adversely affect the ink drying and color. Therefore, the printer needs to adjust the pH of the fountain solution according to the acidity and alkalinity of the paper. When the paper is acidic, the pH of the fountain solution is larger and more alkaline. When the paper is alkaline, the fountain solution has a smaller PH value and is more acidic. Through the acid-base balance, keep the ink dry and colorful.
8. The pH of fountain solution should be predicted and set before printing. It should be adjusted according to the specific situation in printing. To ensure product quality.

Fountain solution PH value measurement:

It is generally more accurate to use a pH meter to measure the pH of a fountain solution. The electrode of the PH meter is enclosed in a glass tube and when immersed in the fountain solution, the ionic charge in the fountain solution induces the electrode in the glass tube. The charge induced in the electrode generates a small current in the circuit in the PH meter. This current is amplified and digitized and then compared with the value stored in the electronic memory in the PH meter to determine its value. Finally, The result is displayed on the LCD panel. PH meter is sold in the chemical instrument market.

(II) Control of dosage of fountain solution

The consumption of fountain solution: The consumption of offset fountain solution is mainly in the following aspects:

1. Ink emulsification consumes water. The slight emulsification of the ink contributes to the rapid ink-water balance, while the larger the ink volume, the greater the amount of water needed to balance.

2. Paper absorbs water consumption. Because the various fiber properties and smoothness, evenness, sizing, moisture content, stretch rate and other indicators are not the same, the paper's water absorption during printing is different. In general, non-coated paper absorbs more water than coated paper.
3. In the printing process, water is evaporated from the surfaces of water buckets, water bats, plates, etc. to the space to consume water.
4. Squeeze throws water.

Offset determines the conditions of water consumption

When printing, how much fountain solution should be used to achieve the best printing effect is a dynamic balancing process because many factors will change during the printing process. Regardless of the printing plate, ink, or paper, they will change. , thus affecting the amount of fountain solution used. Therefore, the printer can only determine the correct amount according to the printing environment at that time. Generally consider from the following aspects:
1. The area and distribution of the layout text. The plate area on the printing plate has a large area and a large amount of water.
2. The thicker the printing ink layer, the greater the amount of water used, the thinner the ink layer, and the smaller the amount of water used;
3. The paper fiber structure is loose, the surface is rough, the water absorption performance of the paper is strong, and the water consumption is large. Coated paper, cardboard and other dense structure, smooth surface, low water absorption, water consumption is small.
4. The ink has good water resistance, high viscosity, low emulsification value, and low water consumption. The ink has poor water resistance, low viscosity, large emulsification value, and large water consumption.
5. The higher the speed of the offset press, the more water is consumed.
6. High ambient temperature, low humidity, large air circulation, and high water consumption.
7. The better the quality of the plate, the better the sand treatment, and the smaller the amount of water.
8. Offset printing pressure is high and fountain solution consumption rate is high.
9. The higher the hardness of the blanket, the greater the consumption of fountain solution.

During the offset printing process, the blank part of the printing plate must always maintain a certain thickness of the water film and the graphic ink balance. This is the ink balance control. In general, the ink layer thickness of the printing plate is 2 to 3 μm, and the water film thickness of the blank portion is about 0.5 to 1 μm. The volume of the fountain solution contained in the ink is 15% to 26%. Basically achieved the ink and water balance.

Too much or too little fountain solution can adversely affect the quality of printed products.

When the amount of fountain solution is too large:

1 The ink-water balance is destroyed, the ink accelerates emulsification, the graphic transmission cannot be properly performed, and the "deinking" and "flower version" problems occur;
2 There is a large amount of water in the plate layout, the ink in the graphic part is not normal, the image imprint is dull, the color of the printed product is poor, the level of the tone is poor, and the phenomenon of "flat and light" appears;
3 affect the printing performance of paper, on the one hand to change the paper surface of the ink absorption performance, so that the surface of the paper coating and adhesives are affected, the occurrence of hair loss, powder off phenomenon; on the other hand, excess water content of the paper will cause the paper expansion deformation , affecting the quality of overprinting.

When the amount of fountain solution is too small:

1 The blank part of the printing plate can not be compensated by an appropriate amount of dampening solution, which will cause some hydrophilic layers to be affected. In severe cases, it will be eroded, and the blank part of the printing plate will be stained with ink.
2 The paper will be napped, dirt will appear on the surface of the print, the printing effect will be poor, and the tone level will be blurred.
When printing, different water consumption should be used according to different prints. When changing the printing plate, reduce the amount of water used until the printing plate is slightly pasted, and gradually increase the amount of water to the printing plate in the process of registering the color, so as to achieve the use of the smallest amount of water for printing, to avoid water Large ink, thereby improving the printing quality.

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