Analysis of the cause of dirty on PS plate in offset printing
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In offset printing, the PS plate is a common problem. This article only briefly analyzes the structural features, printing process, printing process and printing materials of the PS plate.
PS version of the structural features
The ordinary PS plate is composed of an aluminum plate base, an aluminum oxide layer, and a photosensitive adhesive layer. Before the PS plate is coated with the photosensitive liquid, the aluminum plate is pretreated, including three steps of electrolytic sanding, anodizing and sealing.
1. Electrolytic sand
The purpose of electrolysis is to form a sand mesh on a smooth aluminum plate, so that the graphic portion of the printing plate has a good adsorption base, and the non-image portion can be uniformly wetted by moisture to form a closed water film layer.
The electrolytic sand mesh uses a mixture of HCI and HNO3 as the electrolyte. It has been proved that the fine and uniform sand depth is beneficial to improve the resolution of the printing plate, reproduce the fine layer of the graphic and establish a good adsorption basis.
2. Anodizing
The purpose of anodizing is to form an AI2O3 film layer on the surface of the aluminum plate to improve the printing durability of the printing plate and the hydrophilicity of the non-image portion. The thicker the oxide film, the stronger the wear resistance. However, if the thickness of the oxide film layer is increased, the elasticity of the film layer is lowered, the rigidity is increased, the film layer becomes brittle, and cracking occurs during high-speed printing, resulting in dirty printing plates.
3. Sealing
After 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 firmly adsorbed by the photosensitive adhesive, and it cannot be completely detached after exposure and development, so that the plate is not detached. The graphic part is lipophilic and easy to be dirty when printed. Therefore, sealing treatment is required to reduce the sensitivity of the sand. The sealing treatment mostly uses a silicate solution to fill the micropores on the aluminum plate base before coating the photosensitive liquid.
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 plate. If the exposure is too large, the original dot will be reduced after printing, and the ink dot will be lost seriously, causing the lipophilicity and abrasion resistance of the image dot. Decrease, it is not easy to show a gradual gradient; the exposure is too small, it is easy to make the photorefractive decomposition of the photosensitive resin, the darkness is white, and even the development is difficult, causing the non-image part to be dirty. The formula for calculating the amount of exposure is:
E=I·t
When exposing, the illuminance I and the exposure time t should be controlled according to the printing requirements and product characteristics to obtain a suitable exposure amount E.
2. development
When the concentration of the developer is too high or the temperature is too high, the development will be accelerated, and the photosensitive layer that does not see light will be dissolved, so that the dot on the printing plate is reduced, the ink repellency is lowered, the image is falsified, and when it is serious, the plate is lost. phenomenon. If the developer concentration is too low or the temperature is too low, the development time will be prolonged, the development will be incomplete, and the non-image portion of the printing plate will be dirty.
Therefore, in the development process, the developer should be prepared according to the manufacturer's instructions for the PS plate, and the development temperature should be controlled at 20 to 23 °C.
3. Printing environment
Dirty marks on the glass of the printing machine, dirt on the film, tape used for imposition, and unnecessary rule lines that are not removed will hinder the normal passage of light. If not found or repaired in time, the development will be incomplete and further in printing. Causes filth during the process.
At the same time, the PS plate is improperly stored, and it is exposed to natural light. The photochemical layer will undergo a slight photochemical reaction, which will cause fog on the plate surface, and the development will not be thorough, resulting in non-graphic oily and dirty.
Maintain a good drying environment, use safety lights in the printing room, and save the film and PS plate, which can effectively reduce the influence of environmental factors on the development quality.
4. Baked version
In order to improve the printing durability of the printing plate, it is sometimes necessary to bake the plate. Before the baking plate is dirty or dirty, when the baking solution is applied, dirt will fall on the plate surface, which will cause viscera. If the baking temperature is too high, it will cause viscera. . Therefore, carefully check the layout before baking to ensure that there is no dirt. The baking temperature is 180-200 ° C, and the baking time is preferably 3 to 5 minutes.
Influencing factors in the printing process
1. Mechanical wear damage to the plate
In the high-speed printing process, due to the printing pressure, the sand of the non-image part of the printing plate is subject to wear, resulting in a decrease in surface energy and a decrease in water storage, causing non-image parts to be oleophilic and dirty; The oleophilic hydrophobicity will undergo a reverse change, which will aggravate the emulsification of the ink and cause a flower plate. Therefore, on the basis of ensuring a good reproduction of the graphic, a small printing pressure should be used as much as possible.
In addition, excessive printing pressure not only destroys the base and non-image parts of the graphic part of the printing plate, but also reduces the printing durability of the printing plate.
2. Mechanical wear form
(1) Wear between printing cylinders
The rubber roller lining is improperly selected and has a large deformation amount. During the printing process, the line speed difference on the surface of the three rollers becomes large, which aggravates the wear of the printing plate.
The blanket is not cleaned, the paper is detached, powdered or peeled off and piled up on the blanket, causing local bulging and wearing of the printing plate; improper adhesion of the blanket bruise causes the partial bulging to wear the plate.
(2) Wear by the plate water roller and the ink roller
During the printing process, the wear of the plate surface should be eliminated as much as possible due to the axial movement of the plate water roller and the plate roller, and the water roller and the fountain solution should be kept clean to avoid sticking on the water roller cloth to the utmost extent. Ink. Otherwise, during the contact with the printing plate, the lipophilicity of the blank portion of the printing plate may rise, thereby causing the blank portion to be dirty.
Influence of printed materials
1. Paper effect
If the printing quality of the paper is poor, such as the phenomenon of severe hair loss and powder, or the paper contains impurities, the printing plate will wear more.
The ideal pH value of printing paper should be neutral, but at present, China mainly uses alkaline pulping and NaOH as cooking agent, so most papers are slightly alkaline. If the alkalinity is too large, it will neutralize the acidic fountain solution in the non-image part of the printing plate, so that the acidity of the hydrophilic gel layer in the non-image part is weakened. If it cannot be replenished in time, it will inevitably cause the non-image part to be hydrophilic. Sexual decline.
2. Ink effect
The ink consists of a pigment, a binder and an auxiliary material. Printing has a high degree of dispersion of ink pigment particles, that is, a requirement for the size of the pigment particles, and the dispersion should generally be less than 1 μm. If the pigment particles are large, the wear on the printing plate will be intensified during the ink transfer process, and the hydrophilic layer of the non-image portion will be destroyed, resulting in viscera, and the pigment will also locally accumulate, causing the paste. If there is too much dry oil in the ink, the ink will dry too fast, and it will accumulate on the printing plate, causing the paste.
3. Effect of dampening solution
Offset printing relies on the non-image part of the printing plate to be hydrophilic, and the graphic part is lipophilic, and the ink transfer is realized according to the principle of oil and water repelling. The aluminum oxide film formed during the substrate treatment is an amphoteric oxide. If the dampening solution is acidic or alkaline, it will increase the corrosion of the printing plate and cause the plate to become dirty. Therefore, in the printing process, the pH value of the dampening solution should be controlled between 5 and 6, and the amount of dampening solution should be controlled so that the content of dampening solution in the emulsified ink is 15% to 26% to ensure ink. balance.
The reason for the dirty version of the PS version is not single, but is the result of a combination of factors. Knowing the reasons that may cause the PS version to be dirty will help users to find out the root cause of the problem by analyzing and correcting and eliminating the problem.