Causes and elimination of dirt on the blank part o

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Causes and elimination of dirty blank part of printing plate

during the printing process, the phenomenon of dirty blank part of printing plate often occurs, which not only reduces the printing resistance of printing plate, but also affects the printing quality. The causes of its formation and its elimination methods are analyzed and discussed for reference

I. dirty printing

1. Insufficient exposure

insufficient exposure will cause the photosensitive layer of the blank part of the printing plate to not be completely decomposed after the action of ultraviolet light, and will not be well-known during development. It will also be further expanded and completely dissolved in the alkaline developer, which will cause dirt on the product weight of the traditional process. The decomposition degree of the photosensitive layer is in direct proportion to the light intensity and the length of exposure time, so the selection of printing light source and the mastery of exposure time are important factors to determine the exposure amount. Different originals and PS versions from different manufacturers have different exposure times. We can use the continuous gray scale or Bruner test strip to determine the accurate exposure time

the selected printing light source should have the following characteristics:

① the illumination of the layout should be uniform, so that the four corners and the central part of the plate are clear, and the small points can be aligned

② the calorific value of the light source shall be small to protect the photosensitive layer from temperature

③ the light source spectrum is in the range of 350 ~ 460nm. ④ No environmental pollution or harmful gas emission. ⑤ Low price, simple operation and long service life. Common printing light sources include xenon lamp, metal halogen lamp, efficient and fast printing lamp, etc

2. Insufficient development

the causes of insufficient development are:

① insufficient development time

② insufficient developer concentration

③ the developer temperature is low. Different precoated photosensitive lithography plates have different alkali resistance and different requirements for developer concentration, so sometimes it is necessary to mix according to the concentration specified by the PS plate manufacturer. The temperature of the developer should be controlled between 20 ℃ and 251 kinds of materials to promote bone regeneration. The development time is determined by the gray scale or test strip, generally about 1 minute

3. Baking version. Baking plate is a kind of technology to improve the printing plate printing resistance. However, before baking the plate, it must be thoroughly cleaned and coated with protective agent, and baked at a constant temperature of 230 ~ 250 ℃ for 5 ~ 8 minutes, otherwise the photosensitive layer will be carbonized and dirty

II. Dirt caused by improper adjustment of the machine during printing

1. The pressure between the printing plate and the blanket is too high, so that the oxide film in the blank part is worn, the sand mesh becomes shallow, and the adsorption is weakened, resulting in dirty. Therefore, on the premise of ensuring that the imprint is solid without spreading, the minimum pressure can be uniformly used, which can be adjusted by pressing the fuse. The ideal indentation thickness is generally 0.1mm

2. The pressure between the water roller, inking roller and the printing plate is too high, the friction is enhanced, and the sand mesh is worn and chemically damaged, thus reducing the hydrophilicity of the blank part and getting dirty. At this time, the steel insert is used to adjust the pressure between them. Take J2108 machine as an example: the indentation between the inking roller and the printing plate is 4 ~ 6mm, and the indentation between the inking roller and the printing plate is 5 ~ 6mm

III. others

1. When printing, there are dirty marks on the PS plate, the film base and the glass of the printing machine, which hinder the light decomposition and get dirty. They can be removed by the "scattering film double exposure method", but the exposure time should not be too long

2. The ink layer of the printing plate is too thick, resulting in the spreading of the ink layer, making the blank part of the printing plate less clear and dirty. We should make the ink layer thicker when the printing field or paper structure is loose and rough, and the higher the number of dots and lines, the thinner the ink layer

there are many and complex reasons for dirty blank parts of PS version, but as long as we carefully analyze and strictly control them, we can prevent them before they happen. (by shenshifeng)

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