Analysis of the reasons for the printing resistanc

Analysis of the cause of the printing resistance of positive PS version IV

v. the influence of coating photosensitive solution

the photosensitive solution used in the positive PS version mainly includes photosensitive agent, film-forming agent, solvent and dye, as well as some additives such as plasticizer, stabilizer, etc. In this composition, the main factors that affect the printing resistance are photosensitizer, film-forming agent and deep sleep plasticizer

1. The influence of photosensitive agent

in the composition of photosensitive solution, the content of photosensitive agent should be between 3 ~ 6%. If the content is less than 3%, the wear resistance and alkali resistance of the photosensitive layer will be greatly reduced. When the content is more than 6%, the alkali resistance of the photosensitive layer is too strong, so that after normal exposure and development, the photosensitive agent still remains in the sand mesh, making the blank part highly sensitive to fat and causing dirt

secondly, the proportion of photosensitive agent and film-forming agent should be appropriate. The recommended proportion is: photosensitive agent: film-forming agent =1:16 ~ 1:2.0. Within this proportion range, the formed photosensitive layer can be guaranteed to have sufficient wear resistance, alkali resistance and smooth development

2. Effect of film-forming agent

in the photosensitive solution, the content of film-forming agent is about 6 ~ 8%. High content will affect the stability of the photosensitive layer; If the content is too low, the printing resistance and sensitivity of the printing plate will decrease

3. Effect of solvent

the amount of solvent directly affects the thickness of the photosensitive layer. If the photosensitive coating is too thin, the lipophilicity and printing resistance of the graphic part of the printing plate will be reduced. According to the latest positive PS plate industry standard Hg/t269495 issued by the Ministry of chemical industry, the quality of the photosensitive layer per unit area should be controlled at the same force value point as the standard tensile sample on the dial of the tensile testing machine. Compared with 1.60 ~ 2.50% g/m2, matched with this thickness, the amount of solvent should generally be controlled between 80 ~ 90%

4. Influence of plasticizer

the addition of plasticizer is conducive to reducing the brittleness of the touch key operation mode of the photosensitive layer and improving the wear resistance of the photosensitive layer in printing

above, the factors affecting the printing resistance of the positive PS plate in the production process are discussed. Through these production steps, we can get the positive printing plate (structure is shown in Figure 2)

Part II the influence of the use process of the printing plate on the printing resistance 8. The loading of the machine is automatically controlled by the computer and PLC electrical system

there are many factors affecting the printing resistance of the printing plate in the use process. Here, some main factors are discussed in depth from two aspects

I. impact of printing

the printing process of positive PS version is:

exposure → development → finishing → plate baking treatment → curing treatment

1. Impact of exposure

to understand the impact of exposure on printing resistance, we must first understand the chemical reactions occurring in the exposure process and the changes in the photosensitive layer of the printing plate (as shown in Figure 3). The positive PS photosensitive layer is a photosensitive substance of o-diazo-naphthoquinone, which is insoluble in alkali, but becomes an alkali soluble indocarboxylic acid compound after exposure

the amount of exposure will directly affect the decomposition of o-diazo-naphthoquinone (lipophilic layer) and the formation of printing carboxylic acid compounds (hydrophilic substances), that is, the change of lipophilicity and hydrophilicity of printing plates

overexposure will cause the dots of the image part of the printing plate to shrink, and the lipophilicity of the graphic part will be weakened, and the wear resistance will be reduced

if the exposure is insufficient, some lines of the image will be heavier than the original, the dot area will be expanded, and the hydrophilicity will be weakened, causing dirt

there are two reasons for inaccurate exposure:

① the density of the original text is too low or too high

exposure time is inaccurate. Usually, the density of the original: the transmission density in the graphic area cannot be less than 2.8, and the transmission density in the blank area cannot be less than 0.05

find a suitable exposure time. After a buffer transition period of nearly 1 year, each batch of PS plates can be tested with Bruner test strip or Fuji gray scale ruler. From the perspective of the printing ladder, the whitening of the printing plate is controlled at level 3.5 ~ 4.5, and the whitening of the printing plate is controlled at level 4.5 ~ 5.5. If the distance between the light source and the printing plate changes during the actual printing operation, the exposure time should be adjusted accordingly according to the law that "the exposure time is proportional to the square of the distance between the light source and the printing plate"