Printability control of the hottest fountain solut

2022-08-05
  • Detail

Printability control of fountain solution

offset printing uses ink and fountain solution to form graphics and text and blank parts on the printing plate, and uses the principle of oil-water incompatibility to realize the reproduction of graphics and text. As the saying goes, mastering water means mastering offset printing, which is enough to prove the importance of water (fountain solution) to offset printing. The pH value of the fountain solution is very low. You will find that many materials existed in the early days of the invention of the automobile - leather, wood, metal and the blank part of the cloth plate generate hydrophilic inorganic salts, which are necessary to keep clean, and have a great impact on the transfer effect of the ink. The acid (pH 7) or alkali (pH 7) of the fountain solution is not conducive to the normal progress of offset printing. Therefore, in order to give full play to the role of fountain solution in offset printing, it is necessary to strictly control the pH value of fountain solution, which is generally tested with test paper, indicator and acidity meter. In this paper, we mainly discuss the influence and control of pH value of fountain solution on printing plate quality, drying speed and interfacial tension

the effect of pH value of fountain solution can not be ignored

1 The plate base used in offset printing is metal aluminum, which is unstable in strong acid and weak acid media. Only when the fountain solution is weakly acidic can it cause slight corrosion on the surface of the aluminum plate and generate a hydrophilic inorganic salt layer. If the pH value of the fountain solution is too small (strongly acidic), the blank part of the printing plate will be deeply corroded, resulting in coarse sand holes, which is not easy to form a solid hydrophilic layer, so that there is more fountain solution in the blank part, which will affect the transfer of ink; Even corrodes the graphic part and destroys its combination with the plate base, causing the diazo compound layer of the hydrophilic layer to fall off, causing point damage and "plate dropping" phenomenon

at present, the plates in offset printing are generally photosynthesized PS plates. The blank part of the printing plate is generated after the diazo compound is removed by the weakly alkaline developer after photodegradation. The image and text part is a diazo photosensitive adhesive layer covered by the print film without light. When the pH value of the fountain solution is high, it will dissolve the diazo compounds in the picture and text part to achieve high-precision and wide-range measurement. The 3 closed-loop (force, deformation and displacement) control system will cause incomplete pictures and text and affect the printing quality. This is also one of the reasons for the decline of printing plate resistance, but it is often ignored

2. Affect the drying speed of ink

in order to shorten the drying time of ink, a certain amount of drier is often added. When the pH value of fountain solution is too low, it will react with lead (PB), cobalt (CO), manganese (MN) and other metal salts in the drier, making the drier invalid and prolonging the drying time. Practice has proved that when the pH value of common fountain solution drops from 5.6 to 2.5, the ink drying time will be extended from 6 hours to 24 hours; When the pH value of non-ionic surfactant fountain solution ("F" series wetting agent) drops from 6.5 to 4.0, the ink drying time will be extended from 3 hours to 40 hours

3. Influence on interfacial tension

the ink with dry vegetable oil as binder can generate fatty acid rcooh under the action of oxygen in the air. Rcooh has poor orientation ability at the interface and is in a free state. When the pH value of the fountain solution is high, the oh- in the fountain solution increases and forms rcoo- with rcooh in the fountain solution. The directional arrangement of rcoo- on the ink-water interface reduces the interfacial tension. The higher the pH value is, the more rcoo- is generated, and the worse the interfacial tension decreases. Figure 1 shows the relationship between the interfacial tension of water and fatty acids and the pH value of the fountain solution. It can be seen that when the pH value is less than 6, the interfacial tension is very stable, and when the pH value is greater than 6, the interfacial tension drops rapidly. Generally speaking, the lower the interfacial tension, the more serious the ink emulsification

it can be seen that the pH value of fountain solution cannot be a certain value, but changes within a range. It is generally considered that the pH value of PS plate fountain solution is between 5 and 6, that of gravure plate fountain solution is between 4 and 6, and that of multilayer metal plate fountain solution is between 5 and 7

targeted control of the pH value of the fountain solution

1 When the amount of desiccant added in the ink and the amount of pigment

desiccant is large, the drying speed is fast, the ink viscosity is increased, the ink particles become thicker, the fat sensitivity of the blank part of the layout is increased, and the version is easy to paste. Therefore, we should reduce the pH value of the fountain solution (increase the dosage of the original solution) to prevent Mr. Li from coming to Shenyang last year to do the waste recycling business. But pay attention to the dosage to avoid a vicious circle

different inks have different requirements for the pH value of fountain solution due to different properties such as pigment, oiliness, viscosity, fluidity and acid resistance. The general rule is yellow, green, black, magenta, light ink and dark ink

2. Surface strength and pH value of paper

when printing loose paper, due to the high viscosity of the ink, the paper falls off powder and hair seriously, which increases the wear of the printing plate and is easy to get dirty. At this time, the pH value of the fountain solution should be properly reduced to supplement the hydrophilic inorganic salt layer and clean the dirty oil. When printing paper with high surface strength, especially coating paper, the pH value shall be increased (reduce the dosage of stock solution) to prevent yellowing and affect the product quality. Alkaline fountain solution is only suitable for printing on paper (acid paper). It was suggested that the pH value of the paper plus the pH value of the fountain solution should be equal to 13

3. Graphic structure and ink layer thickness

for printing plates that mainly focus on the field, lines and words, the ink layer is thick, the ink load is large, and the spreading is serious, so the pH value of the fountain solution should be reduced; The pH value of the fountain solution should be increased for the visual adjustment version based on graphics and text. For printing plates with both field, line, text and dots, the setting of pH value should be conducive to the stability of blank and graphic parts

4. The workshop temperature

is high, the ink viscosity decreases, the fluidity is large, and more free fatty acids will be decomposed, so the layout is easy to get dirty. Therefore, the pH value of fountain solution should be properly increased. When the machine speed is fast, the printing pressure is high, and the hardness of the blanket and liner is high, the pH value of the fountain solution should also be appropriately increased

the process of reducing or increasing the pH value of fountain solution does not occur independently in printing, and is often intertwined and restrained. Therefore, when adjusting the pH value of fountain solution, comprehensive analysis and careful consideration should be made, otherwise it will cause adverse effects

this article comes from the copyright of the Internet. The price of recycled waste plastics is much cheaper than the rising price of plastic raw materials in recent years. It belongs to the original author and is only for everyone to share and learn. If the author thinks that infringement is involved, please contact us and we will delete it immediately after verification

Copyright © 2011 JIN SHI