In the production of offset printing, dirt on the PS plate is a frequent problem. This article only briefly analyzes the structural characteristics of the PS plate, the printing process, the printing process and the printing materials.
Structural features of PS version
The ordinary PS plate is composed of an aluminum plate base, an alumina layer and a photoresist layer. Before the PS plate is coated with photosensitive liquid, it is necessary to pre-treat the aluminum plate, including three steps of electrolytic sand mesh, anodizing and sealing.
1. Electrolytic sand
The purpose of electrolysis is to form sand on the 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 moisture, thereby forming a closed water film layer.
Electrolytic sand meshes mostly use a mixture of HCI and HNO3 as an electrolyte. Practice has proved that fine, uniform and deep sand meshes are beneficial to improve the resolution of the printing plate, can reproduce the fine layers of graphics, and establish a good adsorption foundation.
2. Anodizing
The purpose of anodizing is to generate AI2O3 film layer on the surface of aluminum plate base to improve the printing plate's printing resistance and the hydrophilicity of non-graphic parts. The thicker the oxide film, the stronger its wear resistance. However, if the thickness of the oxide film increases, the elasticity of the film will decrease, the rigidity will increase, and the film will become brittle, and it will easily crack during high-speed printing, resulting in dirty printing plates.
3. Sealing
After the electrolytic treatment, there will be uniform and deep sand on the plate base. If the photoresist is directly coated at this time, the plate will be too strongly adsorbed on the photoresist, and it cannot be completely detached after exposure and development, making the printing plate The graphic part is oleophilic and easy to get dirty when printing, so it should be sealed to reduce the sensitivity of sand. The sealing process usually uses silicate solution to fill the micropores on the aluminum plate before coating the photosensitive solution.
Influencing factors in the printing process
1. exposure
The amount of exposure is the key to obtaining a suitable optical density on the PS version. If the exposure is too large, the original dots will shrink after printing, and the ink dots will be seriously lost, causing the lipophilicity and wear resistance of some dots in the text. Decreased, it is not easy to show the gradation of the layer; too small exposure, easy to make the photoresist of the photoresist decomposed completely, dark spots and white spots stagnate, and even cause development difficulties, causing dirty parts of non-graphics. The formula for calculating the amount of exposure is:
E = I · t
During exposure, depending on the printing requirements and product characteristics, the illuminance I and the exposure time t should be controlled to obtain an appropriate exposure E.
2. development
When the concentration of the developer is too high or the temperature is too high, the development will be accelerated, and it will dissolve the photosensitive layer that has not been seen, which will reduce the dots on the printing plate, reduce the inking performance, and make the picture and text false. phenomenon. If the concentration of the developer is too low or the temperature is too low, the development time will be prolonged, and the development will not be thorough, resulting in the dirty part of the printing plate.
Therefore, in the development process, the developer should be prepared in accordance with the instructions of the manufacturer of the PS plate, and the development temperature should be controlled at 20 to 23 ℃.
3. Exposure environment
Dirty marks on the glass of the printing machine, dirt on the film, tape used for imposition and unremoved excess rule lines, etc. will hinder the normal passage of light. If it is not found and repaired in time, it will cause incomplete development and further printing. In the process, it caused dirt.
At the same time, if the PS plate is improperly stored and exposed to natural light, the photosensitive layer will undergo a slight photochemical reaction, which will cause fog on the plate surface and incomplete development, resulting in non-graphic and dirty oil.
Maintain a good printing environment, use safety lights in the printing room, and do a good job of preserving the film and PS version, which can effectively reduce the impact of environmental factors on the development quality.
4. Baked version
In order to improve the durability of the printing plate, sometimes baking is required. Incomplete dirt removal before baking or dirt on the plate surface when rubbing the baking liquid will cause dirtying. Excessive baking temperature will also cause dirtying . Therefore, carefully check the layout before baking the plate to ensure that there is no dirt. The baking temperature is 180 ～ 200 ℃, and the baking time is preferably 3 ～ 5 minutes.
Influencing factors in the printing process
1. Damage to the printing plate caused by mechanical wear
In the high-speed printing process, due to the effect of printing pressure, the non-graphic parts of the printing plate are worn, resulting in a decrease in surface energy and reduced water storage, causing the non-graphic parts to become oily and dirty; the graphic parts are worn due to wear, The lipophilicity and hydrophobicity will reversely change, which will increase the emulsification of the ink and cause patterning. Therefore, on the basis of ensuring good reproduction of graphics and text, as little printing pressure as possible should be used.
In addition, excessive printing pressure not only destroys the foundation of the graphic part and non-graphic part of the printing plate, but also reduces the printing plate durability.
2. Forms of mechanical wear
(1) Wear between printing cylinders
The rubber roller lining is not properly selected, and the amount of deformation is large. During the printing process, the linear velocity difference on the surface of the three rollers becomes larger, which aggravates the wear of the printing plate.
The blanket is not cleaned frequently, and the paper fluffs, powders or peels off the paper and accumulates on the blanket, causing local protrusions and abrasion of the plate; improper patching of the wounded area of ​​the blanket makes the local protrusions also wear the plate.
(2) Abrasion of plate water roller and ink roller
During the printing process, the abrasion of the plate surface due to the axial movement of the plate water roller and the plate ink roller should be eliminated as much as possible. Care should also be taken to keep the water roller and fountain solution clean to avoid the sticking of the water roller flannel to the greatest extent. Ink. Otherwise, in the process of contact with the printing plate, the lipophilicity of the blank portion of the printing plate will increase, thereby causing the blank portion to become dirty.
The impact of printed materials
1. Impact of paper
If the printability of the paper is poor, such as serious lint and powder, or the paper contains impurities, the printing plate will be more abraded during the printing process.
The ideal PH value of printing paper should be neutral, but at present, China mainly uses alkaline pulping and uses NaOH as a cooking agent, so most papers are slightly alkaline. If the alkalinity is too large, it will neutralize the acid fountain solution of the non-graphic part of the printing plate, which will weaken the acidity of the hydrophilic gel layer of the non-graphic part. If it cannot be replenished in time, it will inevitably cause the non-graphic part to be hydrophilic. Sexual decline.
2. Influence of ink
The ink consists of pigments, binders and auxiliary materials. The dispersion degree of printing ink pigment particles, that is, the requirements for the size of pigment particles are relatively high, and in general, the dispersion degree should be less than 1 μm. If the pigment particles are large, the abrasion of the printing plate will be aggravated during the ink transfer process, and the hydrophilic layer of the non-graphic part will be damaged, resulting in dirty, and the pigment will also be locally accumulated, causing paste. If there is too much oil in the ink, the drying speed of the ink is too fast, and it will also accumulate on the printing plate, causing paste.
3. The effect of fountain solution
Offset printing relies on the non-graphic part of the printing plate to be hydrophilic and the graphic part to be oleophilic, according to the principle of oil-water repulsion to achieve ink transfer. The aluminum oxide film layer formed during plate treatment is an amphoteric oxide. If the fountain solution is acidic or alkaline, it will increase the corrosion of the printing plate sand and cause the printing plate to become dirty. Therefore, in the printing process, the pH value of the fountain solution should be controlled between 5 and 6, and the amount of fountain solution should be controlled so that the content of the fountain solution in the emulsified ink is 15% to 26% to ensure the ink. balance.
The cause of the dirt on the PS version is not single, but the result of comprehensive factors. Understanding the possible causes of the dirty version of the PS version will help the user to find the source of the problem through analysis and reasonably reduce and eliminate the problem when encountering such problems.
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