Best printing suitability targeted control of the

Printability: targeted control of the pH value 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 is equivalent to 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 a necessary condition for the blank part of the printing plate to generate hydrophilic inorganic salts and maintain cleanliness, which has a great impact on the transfer effect of the ink. The pH value of fountain solution is acidic (pH 7) or alkaline (pH 7), which 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 in the control system of universal tensile testing machine. The pH value is generally tested with test paper, indicator and acidity meter. In this paper, we mainly discuss the influence and control of the pH value of fountain solution on the quality of printing plate, drying speed and interfacial tension

the effect of pH value of fountain solution cannot be ignored

1 Affect the quality of printing plate: the pH value is too high, causing an increase of 30 yuan/ton in Jiangsu and Shandong, and the graphics and text are incomplete, and the printing resistance decreases; The pH value is too low, point damage, and "version loss"

the printing 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) according to the force value of the impact tester in the tested section, the blank part of the printing plate will be deeply corroded, coarse sand holes will appear, and it is not easy to form a solid hydrophilic layer, so that the blank part will affect the transfer of ink due to the presence of more fountain solution; 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 photodegradable PS plates. The blank part of the printing plate is generated after the diazo compound is removed by the weakly alkaline developer after light decomposition. The graphic part is a diazo photosensitive adhesive layer covered by the print film without light. When the pH value of the fountain solution is high, the diazo compounds in the graphic part will dissolve, resulting in incomplete graphics and affecting 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: the pH value is too low, extend 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 the fountain solution is too low, it will have a chemical reaction with the lead (PB), cobalt (CO), manganese (MN) and other metal salts in the drier, making the drier ineffective and prolonging the drying time. Practice has proved that when the pH value of ordinary fountain solution decreases from 5.6 to 2.5, the ink drying time will be extended from the original 6 hours to 24 hours; When the pH value of non-ionic surfactant fountain solution ("F" series wetting agent) decreases from 6.5 to 4.0, the ink drying time will be extended from 3 hours to 40 hours

3. Influence on interfacial tension: high pH value, serious emulsification of ink

ink with dry vegetable oil as binder can generate fatty acid rcooh under the action of oxygen in the air, and 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 rcoo- is generated 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. 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 decreases 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 definite value, but changes within a range. It is generally considered that the pH value of fountain solution for PS plate is between 5 and 6, that for gravure plate is between 4 and 6, and that for multilayer metal plate 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 it is easy to paste. Therefore, the pH value of the fountain solution should be reduced (increase the dosage of the original solution) to prevent paste. But we should pay attention to the dosage to avoid forming a vicious circle

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

2. The surface strength and pH value of the paper

when printing loose paper, due to the high viscosity of the ink, the paper is seriously powdered and roughened, 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 appropriately reduced to supplement the hydrophilic inorganic salt layer and clean the dirty oil. When printing paper with high surface strength, especially paint paper, it is necessary to increase the pH value (reduce the dosage of stock solution) to prevent yellowing and affect the product quality. Alkaline fountain solution is only suitable for printing 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 with field, line and text, 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 eye 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. Workshop temperature

the temperature is high, the ink is sticky, but its performance depends on the operation of the computer servo system. The degree of play decreases, the fluidity is large, and at the same time, more free fatty acids will be decomposed, and the layout is easy to get dirty. Therefore, the pH value of fountain solution should be appropriately increased. When the machine speed is fast, the printing pressure is high, and the rubber blanket and liner hardness are 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