Analysis of the reasons why PS plate is dirty in t

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Analysis of the reasons for the dirt on PS plate in offset printing

in offset printing production, the dirt on PS plate is a common problem. This paper only makes a brief analysis from four aspects: the structural characteristics of PS plate, the printing process, the printing process and the printing materials

structural characteristics of PS version

ordinary PS version is composed of aluminum plate base, aluminum oxide layer and photosensitive adhesive layer. Before the PS plate is coated with photosensitive solution, the aluminum plate should be pretreated, including electrolytic grit, anodic oxidation and hole sealing

1. Electrolytic grit

the purpose of electrolysis is to form grit on a smooth aluminum plate, so that the graphic part of the printing plate has a good adsorption foundation, and the non graphic part can be evenly wetted by water, so as to form a closed water film

electrolytic grit mostly uses the mixture of HCI and HNO3 as the electrolyte. Practice has proved that fine, uniform and deep grit is conducive to improving the resolution of printing plates, reproducing the subtle levels of graphics and texts, and establishing a good adsorption foundation

2. anodizing

the purpose of anodizing is to generate an AI2O3 film on the surface of the aluminum plate base to improve the printing resistance of the plate and the hydrophilicity of the non graphic part. The thicker the oxide film is, the stronger its wear resistance is. However, if the thickness of the oxide film increases, the elasticity of the film will decrease and the rigidity will increase, which will make the film brittle and easy to crack in the process of high-speed printing, resulting in dirt on the printing plate

3. Sealing holes

after electrolytic treatment, there will be uniform and deep sand meshes on the plate base. If the photosensitive glue is directly coated at this time, the plate surface will be too firmly absorbed by the photosensitive glue, and cannot be completely separated after exposure and development, making the non graphic part of the printing plate lipophilic and easy to get dirty during printing. Therefore, sealing holes should be carried out to reduce the sensitivity of sand meshes. Silicate solution is often used to fill the pores on the aluminum plate base before coating the photosensitive solution

influencing factors in the process of printing

1. Exposure

the amount of exposure is the key to obtain the appropriate optical density on the PS version. If the exposure is too large, the dots of the original version will shrink after printing, and the ink dots will be lost seriously, resulting in the lipophilicity and wear resistance of some dots in the image and text, which is not easy to show the gradual change of levels; If the exposure is too small, it is easy to cause incomplete light decomposition of the photosensitive resin, paste the dark dots, and even cause difficulties in development, causing dirt on the non graphic parts. The calculation formula of exposure is:

e = I · T

exposure under slow and uniform loading, the illumination I and exposure time t should be controlled according to the different printing requirements and product characteristics, so as to obtain the appropriate exposure E

2. development

when the concentration of the developer is too high or the temperature is too high, it will accelerate the development and dissolve the photosensitive layer without light, so that the dots on the printing plate are reduced, the inking property is reduced, the graphics and texts are blurred, and the plate drop phenomenon will occur in serious cases. If the developer concentration or temperature is too low, the development time will be prolonged and the development will not be complete, resulting in dirty non graphic parts of the printing plate

therefore, in the development process, the developer should be prepared according to the instructions of the PS version manufacturer, and the development temperature should be controlled at 20 ~ 23 ℃

3. printing environment

the dirt on the glass of the printing machine, the dirt on the film, the tape used for making up and the unnecessary regular lines that have not been removed will hinder the normal passage of light. If not found and repaired in time, it will cause incomplete development, and then cause dirt in the printing process

at the same time, if the PS version is improperly stored and exposed to natural light, a slight photochemical reaction will occur in the photosensitive layer, resulting in fog on the version, incomplete development, and dirty non graphics and text

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 the development quality

4. Baking version

in order to improve the printing resistance of the printing plate, it is sometimes necessary to bake the version. Incomplete dirt removal before baking the version or dirt falling on the version when smearing 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

influencing factors in the printing process

1. Damage of mechanical wear to the printing plate

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, causing the non graphic part to be oily and dirty; Due to wear and tear in the graphic part, the lipophilic hydrophobicity will change in reverse, which will aggravate the emulsification of the ink and cause the stencil. Therefore, on the basis of ensuring the good reproduction of pictures and texts, we should use as little printing pressure as possible

in addition, excessive printing pressure not only destroys the foundation of the graphic and non graphic parts of the printing plate, but also reduces the printing resistance of the printing plate

2. The form of mechanical wear

(1) wear between printing cylinders

the rubber cylinder liner is improperly selected and has a large amount of deformation, resulting in a large linear speed difference on the surface of the three cylinders during the printing process, which intensifies the wear of the printing plate

the blanket is not cleaned frequently, and the paper falls off wool, powder or peels and accumulates on the blanket, causing local bulge and wearing the printing plate; If the damaged part of the rubber blanket is improperly patched, the local bulge will also wear the printing plate

(2) wear of water roller and ink roller

during the printing process, the wear of the printing plate surface caused by the axial movement of water roller and ink roller should be eliminated as much as possible. Attention should also be paid to maintaining the cleanness of water roller and fountain solution, so as to avoid oil sticking on the flannelette of water roller. The demand for plastics in the fields of packaging, automobile, agriculture and construction is increasing day by day. 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

influence of printing materials

1. Influence of paper

if the printing suitability of paper is poor, such as serious hair and powder loss, or the paper contains impurities, the wear of the printing plate will be increased in the printing process

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 the 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, 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 cause the hydrophilicity of the non graphic part to decline

2. Influence of ink

ink is composed of pigments, binders and auxiliary materials. Printing requires high dispersion of ink 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 in the process of ink transfer, which can be widely used in packaging, construction and other industries, damage the hydrophilic layer of the non graphic part, cause dirt, and the pigment will accumulate locally, causing paste. If there is too much dry oil in the ink and the ink dries too fast, it will also accumulate on the printing plate, causing a paste

3. The 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 ink transfer is realized according to the principle that oil and water repel each other. The aluminum oxide film formed during plate base treatment is amphoteric oxide. If the fountain solution is acidic or alkaline, it will increase the corrosion of the printing plate sand and cause dirt on the printing plate. Therefore, in the printing process, the pH value of the fountain solution should be controlled between 5 and 6, and the replenishment amount of the fountain solution should be well 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

the reason why the PS version is dirty is not single, but the result of comprehensive factors. Understanding the reasons that may lead to dirty PS version will help users find out the root cause of the problem through analysis, and reasonably reduce and eliminate the problem

information source: printing technology

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