On the printing balance of offset fountain solution

Lithography uses the immiscible properties of oil and water to keep the fountain solution and ink on the surface of the printing plate relatively balanced during the printing process to achieve a good copy of the original. In actual production operations, factors such as personnel quality, process, equipment, raw and auxiliary materials, environmental temperature and humidity will affect the printing balance.

The printing balance of fountain solution mainly includes the balance of fountain solution and ink in the printing state (motion state), the balance of the printing plate in the working state and the balance of the paper under the printing effect.

1 fountain solution

The fountain solution is an aqueous solution prepared by adding certain chemical components to the water to wet the plate surface during the offset printing process.

1) Purpose of using fountain solution Lithographic printing has three purposes: ① Form a water film that repels ink on the blank part of the printing plate to resist the infiltration of the ink on the blank into the blank part to prevent dirty plates. ② The electrolyte in the fountain solution reacts with the plate base exposed by abrasion to form a new hydrophilic layer to maintain the hydrophilicity of the blank part of the printing plate. ③Adjust the temperature of the ink. In general, the viscosity of ink changes rapidly with a small change in temperature. Experiments show that if the temperature rises from 25 ° C to 35 ° C. The viscosity of the oil volume drops from 50 Pa · s to 25 Pa · s, and the fluidity of the ink doubles. This will inevitably cause serious spreading of ink.

2) Commonly used fountain solution fountain solution is generally divided into three types: ordinary fountain solution, sprinkling fountain solution, non-ionic surfactant fountain solution.
The chemical components added to the ordinary fountain solution are phosphoric acid, phosphate, nitrate and other inorganic compounds, which have the characteristics of high surface tension, poor wettability, low cost and easy operation.
The alcohol fountain solution is added with ethanol, isopropanol or other alcohols, which has the characteristics of low surface tension and good wettability. It is suitable for lithographic printing machines equipped with an alcohol dampening system.
Non-ionic surfactant fountain solution is a new fountain solution developed in recent years. It is generally prepared by adding non-ionic surfactant to fountain solution containing other electrolytes. Non-ionic surfactants often use polyoxyethylene, polyoxypropylene ether, alkyl alcohol amide, etc. It has the characteristics of low surface tension, good wetting performance, stable chemical properties, etc. It is an ideal fountain solution for high-speed multicolor lithographic printing machines.

2 Balance of fountain solution and ink

Offset printing is based on the rule that oil and water are incompatible with each other to transfer ink. Theoretically, only when there is a strict boundary between the water film on the surface of the blank part of the plate and the ink film on the surface of the plate and text, the ink and ink will not infiltrate each other, that is, the water and ink balance is reached. However, both the printing plate, the water-filling roller, and the ink-filling roller transfer water and ink in the process of highly rolling with each other. The ideal static balance described above is difficult to achieve in a dynamic printing process.

1) Water-ink balance concept The water mixing curve of three different inks. The ordinate is the amount of water mixed in 100g of ink measured after mixing the wetting fluid and the ink. The abscissa is the mixing time of water and ink. Curve A represents the ink that is most likely to be mixed with water. Because the water content in the ink is too high, the ink head is too short, and the ink is difficult to output from the ink fountain of the printing press, so it is not suitable for lithographic printing; curve C indicates that it is the most difficult to mix in Ink with water, the ink is almost repulsive to water, the fountain solution squeezed into the ink during printing quickly separates from the ink, and attaches to the surface of the ink to form a water film, which hinders the ink The transfer to blankets, experience shows that the transfer performance of this ink is extremely poor, and it may make the prints bloom, and can not be used for lithographic printing; only the ink of curve B can make the printing go smoothly. The experiment proves that when the curve B The water content of the ink reaches 26% (that is, when 100g of ink contains 26g of water), the printing quality is the best.

2) Factors that affect the emulsification of ink in lithographic printing â‘  Wetting agent The fountain solution contains a certain amount of wetting agent. The wetting agent is also a surfactant. It can reduce the surface tension of the fountain solution. It also plays the role of accelerating ink emulsification. Gum arabic is a colloid with excellent hydrophilic properties. Although it is mainly used to rub the surface of the printing plate to protect the printing plate, a small amount of gum arabic is often added to the fountain solution, which is used to improve the fountain solution. Wetting degree, but it also increases the emulsification of the ink while increasing the dampening degree of the dampening plate.
â‘¡ The acidic fountain solution of the fountain solution should have a certain weak acidity, which is determined by the process technology. Due to the physical and chemical effects during printing, there are calcium salts deposited between the fountain solution, ink roller, printing plate, and blanket. Too much calcium salt will cause difficulty in ink transfer and imbalance in emulsification. The calcium salt mainly comes from the paint on the surface of the paper, the hardness of the water and the pigment of some inks (such as red ink). Excessively acidic fountain solution will accelerate the dissolution of ink pigments and paper coatings.
â‘¢ Excessive amount of fountain solution will accelerate the change of the ink from "water-in-oil" emulsification state to "oil-in-water" emulsification state. When this happens, the emulsification of the ink and the balance of the ink and fountain solution will be affected, and the balance will be lost.

3) Measures to ensure the balance of water and ink â‘  During the printing process, the printing plate must have a solid graphic and blank foundation to maintain the stability of lipophilicity and hydrophilicity.
â‘¡The surface tension of ordinary fountain solution is larger, the wetting performance is poor, and the supply amount should be larger; the surface tension of alcohol fountain solution is lower, the wetting performance is good, and the water and ink balance can be achieved with less fountain solution The surface tension of non-ionic surfactant fountain solution is also low, and the supply of fountain solution should also be less than that of ordinary fountain solution.
â‘¢ Master the principle of less water and thicker ink, assuming that the blank part of the layout is not stained, try to apply the smallest amount of water to achieve a good transfer of ink. Large water causes excessive ink emulsification, and the ink layer cannot be thick. From the point of view of the transfer process of offset ink and ink, in a single water supply and ink supply, a total of three times the mixing and emulsification of ink and water, it is impossible to maintain a strict boundary between water and oil. Therefore, the ink-water balance in offset printing can only be a relative concept, and the perfect water-ink balance does not exist. As long as the optimal balance is reached, the ideal print can be printed.
â‘£For smooth paper, the water supply can be slightly smaller, and rough paper can be slightly larger; when the machine runs at a fast speed, the water can be slightly smaller, and at low speeds, the water can be larger.
⑤Environmental temperature and humidity cannot be ignored. In the printing process, the moisture of the layout is distributed directly and indirectly. While the moisture of the layout meets the needs of ink and ink balance during printing, most of the moisture is distributed to the space. The higher the ambient temperature, the faster the distribution.
â‘¥Control the pH value of fountain solution at about 4.5-5.5. In addition, since the pH value of the offset printing paper surface has a great influence on the pH value of the fountain solution, it is best to measure the pH of the paper. When the pH of the paper is too low, the pH of the fountain solution should be raised slightly; conversely, if the pH of the paper is too high, the pH of the fountain solution should be appropriately lowered. According to the relevant information and practice, when the paper pH value is 9, the fountain solution pH value is 4; when the paper pH is 8, the fountain solution pH value is 5 is better.
⑦ Master the pressure of the water and ink roller. Too little pressure will cause poor water supply and ink supply. Too much pressure will easily damage the printing plate. [next]

3 Balance of fountain solution and printing plate

1) The effect of the amount of fountain solution on the printing plate.
The balance of ink and fountain solution does not mean that the relationship between printing plate and fountain solution is balanced. When the amount of fountain solution is too large, excessive ink emulsification will occur, and excessive fountain solution will obviously appear on the surface of the printing plate. This is a This is an obvious situation. However, when the fountain solution is too much, the ink will not immediately emulsify excessively. This is because the excessive fountain solution will be transferred to the ink roller after the printing plate is absorbed. At this time, there is too much dampening solution on the printing plate, and the amount of the printing plate and the dampening solution normally needs to be unbalanced. At this time, the water splash phenomenon will appear on the surface of the printing plate. If the amount continues to increase, excessive ink emulsification will occur.
If the amount of fountain solution is too small, it can also cause imbalance between the printing plate and fountain solution. The most obvious is that the inorganic salt layer on the blank part of the printing plate surface cannot be stably enriched and protected. The metal aluminum is exposed and the dirty version phenomenon occurs .
A large number of experiments and accumulated experience have shown that: during normal printing, if the thickness of the ink layer of the graphic part of the plate is 2 to 3 μm, and the thickness of the blank water film is about 0.5 to 1 μm, the volume of fountain solution contained in the ink The score is 15 to 26%, and the maximum is not more than 30%, basically achieving the water-ink balance of offset printing.

2) The influence of the acidity of the fountain solution on the printing plate From the point of view of the composition of the fountain solution, it is mainly acids and salts. During the printing process, due to the result of the chemical reaction, the inorganic salt protective layer on the surface of the printing plate can be timely Get compensated. During the printing process, the surface of the printing plate is rubbed by the water roller, and the inorganic salt protection layer on the surface of the printing plate will be damaged to varying degrees. It needs to be supplemented by acids and salts in the fountain solution. If the dampening acidity is too strong or too weak, it will make this compensation abnormal. This situation is actually the imbalance between the printing plate and fountain solution.
The common failures of the surface of the printing plate due to the unsuitable acidity of the fountain solution and the imbalance with the printing plate are summarized as follows:
It is generally believed that the pH of the fountain solution of the PS version is preferably between 4.5 and 5.5.

3) Factors influencing the balance between printing plate and fountain solution â‘  Commonly used in the production of blankets are air cushion blankets and ordinary blankets. When they are in contact with the plate, due to the certain adsorption performance of the blanket surface, the printing Part of the fountain solution attached to the surface of the plate is transferred to the surface of the blanket. If some of the fountain solution is lost on the surface, it needs to be replenished in time, and the amount of replenishment will be balanced and unbalanced.
â‘¡Water roller The water roller is the main component that directly contacts the printing plate and supplies the fountain solution at the same time. Too much contact pressure with the printing plate will damage the image part. The fountain solution cannot coat the printing plate evenly and easily. "Flower version" appears. If the contact pressure is too light, the fountain solution on the surface of the printing plate is not properly coated, resulting in a coating-like situation, and the phenomenon of "patterning" obviously appears on the surface of the printing plate.
③ Graphic area of ​​printing plate The amount of fountain solution required on the surface of the printing plate is directly related to the graphic area. The larger the graphic area, the smaller the blank area; the smaller the graphic area, the larger the blank area. In general, the amount of fountain solution required for a printing plate with a large graphic area is less, whereas the amount of fountain solution is greater.
â‘£Ink layer thickness The thickness of the ink layer of the printed matter has a certain relationship with the amount of fountain solution on the surface of the printing plate. There are two ways to increase the thickness of the printing ink layer: one is to keep the amount of fountain solution unchanged and increase the amount of ink; one is to keep the amount of ink unchanged and reduce the amount of fountain solution. The reason for these two approaches is the same. They increase the ink content under the "water-in-oil" condition, and reduce the content of fine water droplets in the ink on the plate surface, so as to achieve the purpose of increasing the thickness of the ink layer.

4 Balance of fountain solution and paper

During the lithographic printing process, after the transfer of the fountain solution through the printing plate and blanket, a certain amount of fountain solution must be transferred to the paper. After the paper absorbs the fountain solution, it will expand and contract, affecting the quality of the overprint. Therefore, when we understand their balance problem, we should first clarify the following relationships:

1) The relationship between the deformation of the paper and the water content The paper is a thin layer of material interwoven with natural plant fibers. The plant fibers include cellulose, hemicellulose, lignin and other components. Due to the special structure of the paper, the paper not only absorbs water when in contact with the paper, but also has the ability to absorb moisture from humid air and to dehumidify into dry air.
After a dry fiber is completely wetted, the increase in diameter is about 30% and the increase in length is about 1% to 2%. Since the paper fiber absorbs water, the increase in diameter and length has such a difference, so this deformation must be different in length and width due to the direction of the paper fiber. In the printing process, it is difficult to adjust the graphic size of the printing plate or blanket along the axis of the cylinder, but the graphic size along the circumference of the cylinder, that is, the printing direction, can be adjusted by adding pad paper under the printing plate or blanket. Therefore, in printing, the wide side of the paper and the printing direction are often taken to be parallel, that is, the longitudinal thread paper is used for printing, without affecting the printing performance of the paper.

2) The relationship between the acidity of fountain solution and paper. Paper is generally weakly acidic, and a few coated papers (such as coated paper) are weakly alkaline. The relationship between fountain solution and paper is mainly the acidity of fountain solution to paper Impact. When the fountain solution is too acidic, some components of the paper surface paint and filler will ooze and be suspended in the fountain solution, and some will accumulate on the surface of the blanket. This phenomenon not only causes the printing to be dirty, but also the blanket to be dirty. It will have a negative impact on paper organization and printing ink absorption and ink drying. The impact of the dampening solution's excessive acidity on acidic paper and alkaline paper is almost the same. When the fountain solution is too acidic, the impact on the paper is much smaller by comparison.

3) The relationship between the amount of fountain solution and paper. The amount of fountain solution directly changes the water content of the paper, which affects the degree of paper expansion and contraction and the accuracy of product overprinting.
If the amount of fountain solution is too large, it is easy to change the ink absorption performance of the paper surface, and the coating and adhesive on the paper surface will also be affected, which will cause the paper to fluff and powder, and delay the drying time of the ink. In addition, excessive moisture content of the paper will cause the paper to expand and contract, affecting the quality of the overprint.
If the amount of fountain solution is too small, although it will not have a significant impact on the paper, the paper surface will appear dirty, blurred tone and other disadvantages.
Water resistance is an important technical index in the performance of offset paper, and the water absorption performance is different for different paper types. Compared with the smoothness of the paper surface, the greater the smoothness, the less water absorption, and the smaller the smoothness, the greater the water absorption. The coated paper surface is smoother than the offset paper surface, and the water absorption is smaller than the offset paper; from the paper surface sizing degree to compare, apply Large gluing degree means low water absorption, small sizing degree means high water absorption, offset paper sizing degree is larger than that of sticker and writing paper, and its water absorption is smaller than that of sticker and writing paper; compared with paper structure, the structure is close The paper has low water absorption, and the paper with loose structure has high water absorption.

In the offset printing process, the most obvious problem is the relationship between the fountain solution and the ink and the printing plate. The amount of fountain solution used is determined by the amount of ink and printing plate required, not by the moisture content of the paper. . In order to ensure that the paper content does not exceed the allowable amount as much as possible, while meeting the ink and printing plate requirements, we strive to achieve a balance between the fountain solution and the paper.

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