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This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

      Dirty on PS plate is a common problem in offset printing production. This paper briefly analyzes PS plate from four aspects: structural characteristics, plate printing process, printing process and printing materials.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

Ordinary PS plate is composed of aluminum plate base, alumina layer and photosensitive adhesive layer. Before PS plate is coated with photosensitive solution, aluminum plate shall be pretreated, including electrolytic sand mesh, anodizing and hole sealing.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

1. Electrolytic sand mesh:This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

The purpose of electrolysis is to form sand mesh on smooth aluminum plate, so that the graphic part of the printing plate has a good adsorption foundation, not graphic

The part can be evenly wetted by water to form a closed water film.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

The mixed solution of HCI and HNO3 is mostly used as electrolyte for electrolytic sand mesh. Practice has proved that the fine, uniform and deep sand mesh is conducive to improve the resolution of printing plate, reproduce the fine level of picture and text, and establish a good adsorption foundation.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

2. Anodizing

The purpose of anodizing is to generate AI2O3 film on the surface of aluminum plate to improve the printing resistance and hydrophilicity of non graphic parts of the plate. The thicker the oxide film, the stronger its wear resistance. 

     However, if the thickness of the oxide film increases, the elasticity and rigidity of the film will decrease, making the film brittle and easy to crack in the process of high-speed printing, resulting in dirty printing plates.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

3. Hole sealing

      After electrolytic treatment, there will be uniform and deep sand grains on the plate base. If the photosensitive adhesive is directly coated at this time, the plate surface will be firmly adsorbed to the photosensitive adhesive and cannot be completely separated after exposure and development, so that the non graphic part of the printing plate is lipophilic and easy to get dirty during printing. This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

     Therefore, hole sealing treatment should be carried out to reduce the sensitivity of sand grains. Silicate solution is often used to fill the micropores on the aluminum plate base before coating the photosensitive solution.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

Influencing factors in printing process

1. Exposure

     The amount of exposure is the key to obtain the appropriate optical density on the PS plate. If the exposure is too large, the dot of the original plate will be reduced after printing, and the ink dot will be lost seriously, resulting in the decrease of lipophilicity and wear resistance of some graphic dots, which is not easy to show the gradual change of layers; This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

     If the exposure is too small, it is easy to cause incomplete light decomposition of the photosensitive resin, paste the dark tone and small white spots, and even cause difficulty in development, causing dirt on the non graphic part. The calculation formula of exposure is:This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

E=I·t

During exposure, the illuminance I and exposure time t shall be controlled according to the different printing requirements and product characteristics to obtain the appropriate exposure E.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

2. Development

      When the developer concentration is too high or the temperature is too high, it will accelerate the development and dissolve the photosensitive layer without light, so that the dot on the printing plate is reduced, the inking property is reduced, the graphics and text are virtual, and the plate will fall out in serious cases. This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

      If the developer concentration is too low or the temperature is too low, the development time will be prolonged, the development will not be complete, and the non graphic part of the printing plate will be dirty.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

Therefore, in the development process, the developer should be prepared according to the operating instructions of the PS plate manufacturer, and the development temperature should be controlled at 20-23 ℃.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

3. Printing environment

     The dirty marks on the glass of the plate printer, the dirt on the film, the adhesive tape used for plate making and the unnecessary regular lines that have not been removed will hinder the normal passage of light. If they are not found and repaired in time, they will cause incomplete development and dirt in the printing process.

     At the same time, if the PS plate is not properly stored and irradiated by natural light, the photosensitive layer will have a slight photochemical reaction, resulting in fog on the plate and incomplete development, resulting in dirty non graphic lipophilic.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

     Maintain a good printing environment, use safety lights in the printing room, and do a good job in the preservation of film and PS version, which can effectively reduce the impact of environmental factors on development quality.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

4. Baking plate

       In order to improve the printing resistance of the printing plate, it is sometimes necessary to bake the plate. Incomplete dirt removal before baking the plate or dirt falling on the plate when wiping the baking solution will cause dirt, and too high baking temperature will also cause dirt. Therefore, the layout should be carefully checked before baking to ensure that there is no dirt. The baking temperature is 180-200 ℃, and the baking time is 3-5 minutes.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

Influencing factors in printing process

1. Damage to printing plate caused by mechanical wear

      In the process of high-speed printing, due to the effect of printing pressure, the sand mesh of the non graphic part of the printing plate is worn, resulting in the decrease of surface energy and water storage, resulting in the lipophilic and dirty non graphic part; This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

     Due to wear, the lipophilic hydrophobicity of the graphic part will change inversely, which will aggravate the ink emulsification and cause the pattern plate. Therefore, on the basis of ensuring good reproduction of pictures and texts, we should use as little printing pressure as possible.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

     In addition, excessive printing pressure not only destroys the foundation of the graphic part of the printing plate and the sand mesh of the non graphic part, but also reduces the printing resistance of the printing plate.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

2. Form of mechanical wear

(1) Wear between printing rollers

Improper selection of rubber drum lining leads to large deformation. In the printing process, the linear speed difference on the surface of the three rollers becomes larger and intensifies the wear of the printing plate.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

The blanket is not cleaned frequently, and the paper falls off wool, powder or peels and accumulates on the blanket, resulting in local bulge and wear of the printing plate; Improper patching of the damaged part of the blanket will also wear the printing plate.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

(2) Wear of plate water roller and ink roller

      During the printing process, the wear on the printing plate surface caused by the axial movement of the plate water roller and the plate ink roller shall be eliminated as far as possible. Attention shall also be paid to keeping the water roller and the fountain solution clean and avoiding ink sticking on the water roller flannelette to the greatest extent. This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

     Otherwise, in the process of contact with the printing plate, the lipophilicity of the blank part of the printing plate will increase, resulting in dirt on the blank part.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

Influence of printing materials

1. Impact of paper

If the printing suitability of the paper is poor, such as serious hair and powder loss, or impurities in the paper, the wear of the printing plate will be increased in the printing process.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

      The ideal pH value of printing paper should be neutral, but at present, alkaline pulping is mainly used in China, and NaOH is used as cooking agent, so most papers are slightly alkaline. If the alkalinity is too large, it will neutralize with the acidic fountain solution of the non graphic part of the printing plate.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

      which will weaken the acidity of the hydrophilic adhesive layer of the non graphic part. If it cannot be supplemented in time, it will inevitably lead to the decline of the hydrophilicity of the non graphic part.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

2. Influence of ink

     Ink consists of pigment, binder and auxiliary materials. Printing ink has high requirements on the dispersion of pigment particles, that is, the size of pigment particles. Generally, the dispersion should be less than 1 μ m。 If the pigment particles are large, it will aggravate the wear of the printing plate during the ink transfer processm.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

     Destroy the hydrophilic layer of the non graphic part, resulting in dirty, and the pigment will accumulate locally, causing plate pasting. If there is too much dry oil in the ink and the ink drying speed is too fast, there will also be accumulation on the printing plate, resulting in plate pasting.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

3. Influence of fountain solution

     Offset printing relies on the principle that the non graphic part of the printing plate is hydrophilic and the graphic part is lipophilic, and the ink transfer is realized according to the principle of oil-water repulsion. The alumina film formed during plate base treatment is amphoteric oxide. This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

     If the fountain solution is acidic or alkaline, it will increase the corrosion of printing plate sand and cause dirty printing plate. Therefore, in the printing process, the pH value of the fountain solution should be controlled between 5-6, and the replenishment amount of the fountain solution should be controlled so that the content of the fountain solution in the emulsified ink is 15% – 26%, so as to ensure the balance of water and ink.This Paper Analyzes the Causes of Dirty PS Version From Four Aspects

The reason why the PS version is dirty is not single, but the result of comprehensive factors. Understanding the possible causes of dirty PS version will help users find out the root cause of the problem through analysis and reasonably reduce and eliminate the problem.

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