Analysis of the problem of insufficient printing r

2022-08-08
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Analysis of the problem of insufficient printing resistance of the printing plate II

III. the printing process is not standardized, resulting in insufficient printing resistance of the printing plate

the printing plate will be affected by many factors in the printing process, causing the printing plate to be damaged, reducing the focus on the research and development of key common technologies in the new material industry, reducing the printing resistance of the printing plate:

1 The printing plate is affected by the printing pressure: as we all know, the printing pressure refers to the mutual force between the imprints in the printing process, which inevitably brings compression deformation, speed difference and friction, destroys the original structure and adsorption performance of the plate and damages the printing plate. The first is the cylinder pressure, that is, the pressure between the plate cylinder and the rubber cylinder. If the pressure is adjusted improperly and the pressure is too large, it will aggravate the friction between the printing plate and the rubber, gradually wear the sand on the surface of the printing plate, damage the points of the graphic part of the printing plate, and reduce the printing resistance of the printing plate. Thirdly, the pressure between the ink rollers, that is, the pressure between the ink roller and the water roller, especially the pressure between the ink roller against the plate and the water roller against the plate and the printing plate. If the pressure between them and the printing plate is too large, the friction will increase, so that the points of the printing plate will be damaged, and there will be "plate drop", which will reduce the printing resistance of the printing plate

2. The influence of the use of fountain solution on the printing plate: we know that the use of fountain solution is unique to offset printing. Its purpose is to maintain the balance between the blank part and the graphic part, and to protect the blank part of the printing plate from the encroachment of ink, and not to expand it to the graphic part. In printing, we should first try to achieve a balance between water and ink. Generally, the ink film of the printing plate is 3 μ M, the water film is 1 μ M can completely prevent the expansion of the ink on the blank part. If the amount of printing plate fountain solution is too large, ink emulsification is easy to occur, the color density of imprinted ink cannot meet the requirements, and the corrosion of printing plate base is deepened; When the water is small, the printing plate will "paste" and "pile ink", which will affect the quality of printing products. At present, the main components of the commonly used fountain solution are "lithopone" and water mixed in a certain proportion. Whether the proportion of fountain solution is reasonable or not, and the pH value of fountain solution is too high or too low, will directly affect the printing resistance of printing plates. The pH value of the fountain solution is too high, and the blank part of the layout cannot form a fine and firm hydrophilic inorganic salt layer and a hydrophilic colloid layer adsorbed on it in time, and the unexposed diazo compounds in the graphics and text part of the PS version will be dissolved, resulting in incomplete graphics and text, weakening its strength and lipophilicity, reducing its service life and even making it impossible to print. If the pH value of the fountain solution is too low, the aluminum, the main component of the PS plate base, is unstable in strong acid or alkali solution, and the plate base will be corroded, and the blank part cannot form a stable hydrophilic inorganic salt layer. Therefore, the lipophilic base of the image and text part will be corroded and "embossed"

3. The surface of the substrate is rough, powdered, roughened, and there is sand in the paper. During the printing process, the suspender is placed on the frame of the horizontal experimental machine. Some substances will increase the pressure, accelerate the wear of the graphic part of the printing plate, and reduce the printing resistance of the printing plate

4. Some inks have coarser pigment particles, such as gold and silver powder inks, coloring media and white inks. Because they use the coarse media as the pigment carrier, the friction coefficient between the printing plate and the blanket will increase in the transmission process of the ink, accelerating the wear of the image and text parts, and producing "stencils"

5. The influence of insufficient residual ink layer in the graphic part of the printing plate. To keep enough residual ink layer in the graphic part of the printing plate is to prevent the graphic foundation from directly contacting with the fountain solution, so as to protect the graphic part of the printing plate. If the remaining ink layer in the image and text part is insufficient for a long time, the photosensitive layer will be directly worn or corroded by acid and alkali, which is easy to cause the "stencil" phenomenon and greatly reduce the printing resistance of the printing plate

6. The influence of printing machinery and equipment failure on printing resistance: part of the reason is closely related to the improper operation of the equipment. For example, due to the improper installation of the machine, the printing speed is too fast, and the printing pressure is too high, causing the machine vibration, which will accelerate the wear of the printing plate during printing. The shaft sleeves at both ends of the rubber drum are seriously worn, causing the rubber drum to jump during printing, which will inevitably cause the wear of the printing plate and reduce its service life

7. The rubber blanket is aged for a long time, the rubber hardens and loses elasticity, the grease sensing ability decreases, the ink transfer performance is poor, and the ink on the printed matter is light. Therefore, many printers believe that the pressure is small and increase the pressure between the rubber cylinder and the plate cylinder, causing a vicious cycle and accelerating the wear of the plate

IV. technological measures to improve the printing resistance of printing plates

1 Standardization in the plate making stage: carefully check the density of the film before printing, and flexibly adopt different exposure methods and exposure times according to the density of the film. In the printing stage, the exposure should be properly controlled, especially before each batch of PS version is used, it is best to conduct exposure experiments first to determine the appropriate exposure time of the batch of PS version. In addition, pay close attention to the vacuum pumping of the printing machine, neither too fast nor too slow, to ensure the vacuum pumping quality of the printing machine. In the development stage of printing, strictly control the length of development time, and pay attention to the ratio and temperature of developer. Otherwise, the phenomenon of "peeling" will appear after the protective glue is blown dry, which will affect the printing resistance. When baking the plate, pay attention to the correct mixing of baking solution and control the baking time

2. Pressure adjustment: carefully adjust the center distance of the drum, accurately measure the lining of the drum, carefully correct the pressure of the water roller and inking roller, so that the pressure of the drum, inking roller and inking roller is at the ideal pressure for a long time, and try to reduce the friction of the printing plate surface

3. Ensure the good flow performance of the ink, and keep enough residual ink layer in the graphic part of the printing plate. In the printing process, use driers reasonably to prevent early drying in the ink transfer process

4. Correct use of fountain solution: first, accurately prepare fountain solution, and the pH value should be mastered according to the characteristics of different plates. In addition, in the printing process, try to control the balance of water and ink, so that the fountain solution on the printing plate can be as small as possible, and the principle is that the printing plate is not dirty, so that the fountain solution does not occupy the image and text parts. After shutdown, the printing plate should not lean against the landing roller containing the fountain solution for a long time, so as to avoid the erosion of the fountain solution and damage the printing plate

5. Do a good job in the storage and management of PS version. Generally, the PS version should be stored in the warehouse for more than a week before delivery, and should be kept away from light and humidity. If the storage environment temperature is too high or too wet, the chemical molecules of the photosensitive layer are very active, which is easy to absorb moisture in the air, resulting in the photosensitive layer falling off and premature aging and deterioration. In the plate making stage 2, it is to promote the progress and development of plastic blending technology, new products of plastic additives and utilization technology. PS plate management mainly focuses on preventing the slow oxidation of plates due to light leakage, and adjusting the temperature and humidity of the workshop environment. Because the photosensitive layer of PS version will oxidize slowly under ordinary light, which will affect the final imaging effect of the photosensitive layer. At the same time, the storage period should be shortened as far as possible, and the storage period of printing plates should generally not exceed half a year

6. Wash the paper powder, sundries, sand, etc. accumulated on the rubber drum frequently, reduce the friction coefficient between the printing plate and the blanket, and reduce the probability of accelerated wear, which can effectively prolong the service life of the printing plate

7. When the printing plate is coated with hydrophilic colloid, pay attention to whether the concentration of colloid meets the requirements. The coating colloid should be uniform and of appropriate thickness, so as to ensure that the low-energy surface with graphics and texts and the high-energy surface of the blank part are covered, and the printing plate will not be damaged by the oxidation of air

we know that there are many other external and internal factors in printing that will affect the printing resistance of printing plates. For printing workers, maintaining a high sense, improving and perfecting various printing standards and technical levels, we will be able to fundamentally improve the printing resistance of printing plates and print high-quality and high-level prints

(author: Hu Dayong, Chen Xiaoyan, Zhou Shisheng

School of printing and packaging engineering, Xi'an University of Technology)

Guangdong printing, 2004, issue 4

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