In the process of water offset printing, it is important to control the balance of water and ink, which is closely related to whether the blot is normally transferred, the depth of the ink color, the accuracy of the color registration, the dryness of the paper, and whether it is sticky, etc., which is the key to ensuring the stable quality of the product .
There are many factors that affect the quality of water offset printing products, and it is more difficult to control them in actual production. Based on my accumulated work experience, the author summarizes some quality control points and shares with the reader.
Paper inspection
During the printing process, the results of various factors will be reflected on the paper, so the inspection of the paper is very important and necessary. The inspection of the paper in the printing operation can generally be divided into two stages of starting printing and normal printing.
1. Start inspection at the printing stage
â— Check the paper ink color
At the beginning of printing, the operator should take out the paper frequently, and start a comprehensive inspection from the grip of the paper to both sides and in the direction of dragging. If there are quality problems on the paper, such as ink color depth, oil stains, etc., it always starts from the mouth and gradually expands to a little bit. Therefore, during the inspection, the line of sight should scan the entire screen for quality problems, and check whether the ink color meets the proofs. When the ink color of the entire screen or local area does not meet the sample, it should be adjusted in time. The adjustment method cannot be limited to the adjustment of the ink volume, and the control of the amount of water should also be considered.
â— Check the paper size
After checking the ink color of the paper, you should quickly check whether the paper T-line meets the requirements. Especially for paper with printed text on both the front and back sides, you should also check whether the size of the T-line and the mouth of the front and back sides are the same.
â— Inspection of imprinted dots
The dot is the most basic unit that constitutes the color of the printed product. If the dot is obviously deformed and expanded, it will directly affect the color restoration and copy effect. Therefore, you can use the magnifying glass to compare the difference between the dotted lines of the printed product and the proofs, and observe whether the dotted lines of the imprint are empty, whether there are obvious enlargements, and quality problems such as vertical and horizontal deformation and ghosting.
In addition to checking the ink color, specifications and prints of paper, various quality defects that are easy to produce in the production of enterprises are also the focus of inspection at the beginning of the printing stage.
2. Inspection at the normal printing stage
â— Check paper overprint and ink color
Entering the printing stage, as the ink is gradually in a balanced state, the focus of the operator's inspection is still paper overprint and ink color. Generally, the two sides of the paper can be compared with the middle part, and compared with the papers extracted in the previous few times. In addition, you should also pay attention to whether there is any change in the moisture content of the layout. As for the inspection of rules, you can take 2 to 30 sheets of paper from time to time, collide the mouths of them and spread them out, and observe whether the cross rule lines are consistent and the positioning of the front and side rules is accurate.
â— Pay attention to the influence of powder, paper wool, etc.
After a period of continuous printing, paper powder, paper wool, and fine ink skin and other dirt in the ink tend to adhere to the printing plate or blanket, resulting in thick hair on lines or blots. In this case, stop the ink and wipe the paper. In order to prevent the ink color depth change caused by the shutdown, it is generally necessary to continue printing after putting 8 to 10 sheets of offset paper. If too much powdery dirt adheres and affects the quality, the printing plate and blanket should be cleaned.
Water and ink control
In the process of water offset printing, it is very important to control the balance of water and ink. It is closely related to whether the print is normally transferred, the depth of the ink color, the accuracy of the color registration, the dryness of the paper, and whether it is sticky, etc., which is to ensure the stable quality of the product. One of the keys. During the printing process, the color depth of the printing ink is directly related to the water supply and the water content of the plate. However, in production practice, we cannot simply rely on reducing the amount of water supply to improve the ink color, because the water is too small, the water and ink balance is lost, and it is easy to make the blank part sticky. Therefore, in the printing process, while adjusting the amount of ink supply, it should be considered whether the amount of water supply is appropriate, and timely and correctly determine whether the ink is balanced. Experienced operators often control the water supply volume to the smallest possible range and keep the water supply volume and ink volume in a relatively stable state on the premise of ensuring that the printing plate is not sticky. In this way, it can ensure that the ink color of the paper is consistent before and after, and the printing operation is stable.
1. Control of water supply
It is necessary to properly control the amount of water supply to achieve the balance of water and ink during the printing process, and to correctly identify the water content of the layout is a prerequisite. For the identification of the moisture content of the board, there is no fixed value so far, so it cannot be mechanically specified. In production practice, the size of the water is often identified by the operator visually inspecting the intensity of the reflected light on the layout. This visual method can play a certain role in identifying the water content of the plate surface, because a certain thickness of the water film can fill up the sand of the printing plate, reduce diffuse reflection and increase the amount of reflected light. However, this method also has certain limitations, because it is limited by various reasons such as the ratio of the area of ​​the graphic part of the layout and the area of ​​the blank part, the plate material, the intensity of the light, and the angle of the observation position. The water film cannot make the plate surface have the same amount of reflected light. Therefore, in daily printing operations, in addition to the above-mentioned visual inspection method, you can also observe from the following points. If the following occurs, it means that the plate surface moisture is too large. ①Use an ink knife to shovel ink on the ink roller, leaving small water drops on the ink knife; ② There are small water drops on the ink transfer roller, and there are also water drops in the ink fountain tank; ③ The layout often appears dirty or after a long shutdown There are still no signs of dryness; ④ The imprint is empty, the mouth is blotchy and wavy, and the ink is dull and dull; ⑤ The paper is curled and weak, and it is uneven; ⑥ There are small drops of water on the rubber roller; There is a big gap between the printing paper and the ink color of the paper before shutdown.
2. Master the relationship between ink and wash
In the printing preparation process, after finishing the work of installing the plate, loading paper and adding ink in the ink fountain, we must first apply ink on the water and then proofread the color. When adjusting the moisture, the actual size and distribution of the graphic area of ​​the layout, the thickness of the printed ink layer, the nature of the printing paper, the performance of the ink, the type of the printing plate, the speed of the machine, and the ambient temperature of the workshop should be considered. During color correction, if you feel the ink color is light, you can of course adjust the amount of ink used, but you can't ignore the influence of moisture, and you should consider both ink and water. Generally speaking, for PS version, smooth paper, the moisture can be slightly smaller; for rough paper, the moisture can be slightly larger. When the machine is running at high speed, the moisture can be slightly smaller; for low-speed machines, the moisture can be slightly larger. In addition, environmental conditions and temperature and humidity can not be ignored. Since the moisture on the layout is distributed directly and indirectly, in addition to meeting the needs of printing ink balance, most of the moisture will be distributed into the air. Therefore, the higher the ambient temperature, the faster the distribution. Sometimes when the oily ink is added with gum in the fountain solution, the paper is rough, the fiber structure is loose, and it is easy to shed hair and powder. After adding the auxiliary agent to the ink, the amount of the ink is not appropriate, and the original water-ink relationship will change.
The fluidity of the ink is also affected by temperature. When the temperature is low, the ink fluidity is relatively poor. When the temperature of the machine rises, the fluidity of the ink becomes better. Therefore, with the same adjustment amount of the ink fountain screw, the ink lowering amount will be different. In order to maintain the consistency of the ink color before and after the paper as much as possible, attention should be paid to the change in the amount of ink under the ink. During the printing process, new ink can be gradually replenished to keep the fluidity of the ink relatively stable as much as possible.
For the balance of water and ink, we cannot simply look at the amount of both. Because the water used for offset printing is not pure water, but an aqueous solution containing a certain amount of electrolyte and hydrophilic colloid, forming a hydrophilic colloid system. The ink is not a completely non-polar oil. In addition to the binder, it also contains pigments, seasonings and other substances to form a water-repellent colloid system. Therefore, the relationship between water and ink is not only the relationship between water and oil, but also the relationship between hydrophilic colloid and water-repellent colloid. Water and ink are basically immiscible at rest, but under the influence of emulsifier and external air temperature conditions and pressure, the two will be mixed to some extent. This emulsification phenomenon more or less runs through the entire printing process, the key is that the amount of emulsification should be controlled within a certain range.
3. Control the pH of fountain solution
During the printing process, not only the amount of fountain solution but also the pH of the fountain solution should be paid attention to, and the PH value should be maintained at 5 to 6, because the PH value is too low or too high will cause some adverse results.
If the PH value is too low, it will cause problems such as corrosion of the printing plate, deinking of the ink roller, and slow drying of the blot. This is because the base of the PS version is aluminum, and the hydrophilic blank part is an aluminum oxide layer. The aluminum oxide layer will not be corroded under weakly acidic conditions, but it will be corroded under strong acidic conditions. It is that the blanks are eroded. If the corrosion is further intensified, the metal base of the graphic part will be damaged, causing the lines and dots on the layout to fall off. The acidic fountain solution reacts with the surface of the metal ink roller to form a hydrophilic salt layer on the surface, thereby forming ink roller deinking, affecting the normal transfer of ink. If the fountain solution is too acidic, the desiccant in the ink will change, and the drying effect will be lost. As a result, the ink is not easy to dry, and the blot is easy to smudge.
If the PH value is too high, it will cause problems such as dirty plates, graphic loss, and ink emulsification. This is because the hydrophilic layer of the blank part of the printing plate may be damaged due to friction during the printing process, which requires an acidic solution to react with the base metal in time to form a hydrophilic salt layer to supplement the damaged part. If the PH value of the fountain solution is too high, this process of repairing the damage cannot occur or occurs more slowly, and the phenomenon of getting dirty. The graphic part of the PS version is a hardened photosensitive resin. This material will be dissolved in the alkaline solution, causing the graphic to fall off. The higher the PH value of the fountain solution, it will also free the fatty acid radical ions of the ink. It is an anionic surfactant, which can reduce the surface tension of the fountain solution and the ink and cause the emulsification of the ink to increase.
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