The role and control of offset fountain solution (two)

The role and control of offset fountain solution (two)

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Third, the control of dampening solution

There are three aspects to the control of dampening solution.

1. The pH value of the dampening solution is controlled.

2. The dosage of dampening solution is controlled.

3. Concentration control of dampening solution.

(1) pH control of dampening solution

The pH value indicates the strength or acidity of the solution, that is, the 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 stage of the pH is 10 times. That is, PH = 5 is 10 times lower than PH = 6 pH. PH = 5 is 100 times separated from PH = 7 pH. That is 10 × 10 = 100.

Generally, the pH of the PS plate dampening solution is controlled between 5 and 6, which is weakly acidic.

The effect of fountain solution pH on printing:

Excessive acidity and low pH:

1 Destroy the hydrophilic protective layer of the blank part of the printing plate, the surface of the plate is corroded by acid, and the printing is easy to print.

2 The printing plate is partially eroded, making the dots smaller, and in serious cases, the plates are lost.

3 Destroy the oxidative conjunctiva properties of the ink, so that the drying time of the ink is prolonged, and if it is serious, the ink will not dry. Experiments have shown that if the pH drops from 5.6 to 2.5, the drying time of the ink is extended from the original 6 hours to 24 hours.

4 Accelerate the emulsification of the ink, affecting the normal transmission of the ink layer.

5 caused the ink to be dull and dull.

When the acidity is too weak and the pH is too large:

The hydrophilicity of the blank part of the printing plate is weakened, so that the ink of the graphic part is spread, which is easy to cause paste.

It is difficult to generate a protective layer in the blank portion of the printing plate, which causes the blank portion of the printing plate to be dirty.

3 Accelerate the emulsification of the ink, so that the contrast of the printed image is reduced, the printing effect is flat, light, and the color is not bright.

The graphic part of the 4 positive image PS version is easily dissolved, resulting in incomplete graphics and lack of strokes.

The general method for determining the pH of the dampening solution is:

1. Oily ink, the acid value of the dampening solution is larger, that is, the PH value is smaller, otherwise it is easy to dirty.

2. The dark ink is more acidic than the dampening solution of the light ink, that is, the PH value is smaller.

3. At the time of printing, a thicker product of the imprinted ink layer requires a more acidic dampening solution than a thin one.

4. The dampening solution used in the field version has a higher acidity than the screen version, and the thick wire version has this relationship compared to the fine wire.

5. In the printing workshop with high temperature and humidity, since the ink is more easily hydrolyzed, the acidity of the fountain solution should be appropriately increased.

6. Printing products with poor paper quality require more acidity of the dampening solution.

7. Printing paper is generally neutral or weakly alkaline, which is good for ink drying. However, the pH of the paper will be less stable, sometimes too acidic or too alkaline, which will adversely affect the drying and color of the ink. Therefore, the printer needs to adjust the pH of the dampening solution according to the acidity and alkalinity of the paper. When the paper is acidic, the pH of the dampening solution is larger and more alkaline. When the paper is alkaline, the pH of the dampening solution is smaller and the acidity is stronger. Keep the ink dry and bright by acid-base balance.

8. The pH value of the dampening solution should be predicted and set before printing, and should be adjusted in time according to the specific conditions during printing. To ensure product quality.

Dampening solution PH value measurement:

It is generally accurate to measure the pH of the dampening solution with a pH meter. The electrode of the pH meter is enclosed in a glass tube. When immersed in the fountain solution, the ion charge in the fountain solution senses the electrode in the glass tube. The charge induced in the electrode produces a small current in the circuit in the pH meter. This current is amplified and digitized and then compared to the value already stored in the electronic memory in the pH meter to determine its value. Finally The result is displayed on the LCD panel. PH meters are sold in the chemical instrument market.

(2) Control of the amount of dampening solution

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

  1. The emulsification of the ink consumes water. The slight emulsification of the ink helps to quickly balance the ink and water, while the larger the amount of ink, the greater the amount of water needs to be balanced.

  2. The paper absorbs the amount of water consumed. Since the fiber characteristics and smoothness, uniformity, sizing degree, water content, and expansion ratio of various papers are different, the water absorption amount of the paper at the time of printing is different. Generally, non-coated paper absorbs more water than coated paper.

3. During the printing process, the water is consumed by the surface of the water bucket, the water stick, the printing plate, and the like.

4. Squeeze and throw the amount of water.

Offset determines the conditions of water consumption

When printing, how much fountain solution should be used to achieve the best printing results, this is a dynamic balancing process, because many factors will change during the printing process, regardless of the printing plate, ink, paper, they will change , thus affecting the amount of dampening solution used. Therefore, the printing staff can only determine the correct dosage according to the printing environment at that time. Generally consider the following aspects:

  1. The area and distribution of the layout part of the layout. The graphic part of the printing plate has a large area and a large amount of water.

  2. The thicker the printing ink layer, the larger the water consumption, the thinner the ink layer, and the smaller the water consumption;

3. The paper fiber structure is loose, the surface is rough, the paper has strong water absorption performance, and the water consumption is large. Coated paper, cardboard and other structures are dense, the surface is smooth, the water absorption is small, and the water consumption is small.

4. The ink has good water resistance, high viscosity, small emulsification value and less water consumption. The ink has poor water resistance, small viscosity, large emulsification value, and large water consumption.

5. The higher the speed of the offset press, the more water it consumes.

6. The ambient temperature is high, the humidity is low, and the air circulation is large, and the amount of water consumed is large.

7. The better the quality of the plate, the better the sand treatment, and the smaller the water supply.

  8. The printing pressure of the offset printing is large, and the consumption rate of the fountain solution is high.

  9. The higher the hardness of the blanket, the greater the consumption of dampening solution.

During the offset printing process, the blank portion of the printing plate is always kept at a certain thickness and the water film is balanced with the ink in the graphic portion. This is the ink balance control. Generally, the thickness of the ink layer in the printing plate is 2 to 3 μm, and the thickness of the water film in the blank portion is about 0.5 to 1 μm, and the volume of the fountain solution contained in the ink is 15% to 26%. Basically, the ink and water balance is achieved.

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

When the amount of fountain solution is too large:

1 The ink balance is destroyed, the ink is accelerated and emulsified, and the text and text transmission cannot be performed normally, and malfunctions such as “deinking” and “flower version” occur;

2 The printing plate has too much water, the printing part is not normal, the image is tarnished, the color of the printing is poor, the level of the tone is poor, and the phenomenon of “flat and light” appears;

3 affecting the printing performance of the paper, on the one hand, changing the ink absorbing performance of the paper surface, causing the coating and adhesive on the surface of the paper to be affected, causing hair loss and powder falling phenomenon; on the other hand, excessive water content of the paper may cause the paper to expand and deform. , affecting the quality of overprinting.

When the amount of fountain solution is too small:

1 The blank part of the printing plate is not affected by the proper amount of dampening solution, so that some hydrophilic layers are affected, and in severe cases, it will be eroded, causing the ink to be stained in the blank part of the printing plate.

2 Paper will be pulled, the surface of the printed product will be dirty, the printing effect will be poor, and the tone level will be blurred.

Different amounts of water should be used for printing according to different prints. When replacing the printing plate, the water consumption should be reduced to a slight paste of the printing plate, and in the process of registering the color correction, gradually increase the amount of water until the printing plate is not pasted, so as to achieve the printing with the smallest amount of water, avoiding the water. Ink, so as to improve the quality of printing.