Solve the problem of dirty printing plate offset printing
Dirty hanging refers to the insufficient or incomplete water film on the blank part of the printing plate and the improvement of the lipophilic performance caused by other original traps. The ink layer on the surface of the ink roller and the plate surface are in direct contact and the ink layer is broken due to the ink layer. Commonly known as "dry version". The outstanding feature of this dirty plate phenomenon is that the fixed position of the upper dirty plate has the basis for absorbing ink. This process failure is easier to find on the printed matter. The key is to find out the cause of the failure and find a reasonable solution in a timely manner.
1 Printing plate problem
The printing plate is greasy before printing, which is mainly caused by improper printing. If the greasiness is heavy, the PS version is due to insufficient exposure. The photodecomposition of the blank part is not complete. Or the development is insufficient, and the photosensitive layer of the blank part is not washed. If the greasy lightness is only an individual part, it may be the following reasons:
(1) There are traces of old pictures remaining on the original version;
(2) Exposure factors, such as improper operation during general exposure, dirt, dirty spots, underexposure, opaque development, etc. on the large glass or film of the exposure machine;
(3) The surface of the plate is not thoroughly cleaned or the protective agent is unevenly coated before baking;
(4) Uneven glue or dirty ink.
Solution: If the printing plate is heavy and greasy, it must be reprinted. If the printing plate is light and greasy and produces individual parts, the PS plate must be reduced in fat, and some Arabic resin should be added to the fountain solution.
2 Improper pH control of fountain solution
The pH value of the fountain solution is large, the formation of the hydrophilic colloid layer is slow, and the inorganic salt layer of the blank part can not be energized in time, and the blank is divided into hydrophilic and oleophobic; the pH value is too small, and it has corrosion on the printing plate sand. , So that the surface of the blank part cannot have enough water to cause oil stains.
Method of overcoming: PH value should be strictly controlled during the deployment of dampening plate. According to the type of printing plate, the thickness of the ink layer of the printing piece, the amount of drying economy, the nature of the paper, the temperature of the workshop, the temperature and other actual conditions to master. Under normal circumstances, the PH value of the fountain solution is: about 4.5 to 6 for the PS version.
3 Ink roller aging
Ink rollers are prone to thermal aging due to rolling for a long time under heat and contact with fountain solution and ink, dry film formation on the surface, enhanced hydrophilicity, weakened ink transfer performance and absorption, and floating ink on the surface, greasy cannot Absorbed in time, resulting in greasy printing plates;
Overcoming method: often remove the ink roller to scrub, polish, prevent aging, and maintain its ink-friendly ink transfer performance.
4 Excessive pressure between water roller and printing plate
The pressure between the water roller and the printing plate is too large, the flannel cloth loses its elasticity, and the friction increases, which makes the plate surface trachoma wear, destroying the stability of the inorganic salt layer, and the blank part is prone to oil dirt;
Overcoming method: Reasonably adjust the pressure between the water roller and the printing plate. A thin steel plate with a thickness of about 0.15 to 0.2mm and a width of about 20 to 30mm can be inserted between the water roller or both ends and the middle between the water roller and the printing plate. There is friction resistance, and I feel that the steel sheet can be pulled out smoothly. In addition, the water roller should be scrubbed frequently, and the water roller flannel should be replaced in time after it loses its elasticity.
5 Improper use and maintenance of water roller
The flannel cover of the water roller has good hydrophilicity even though it is degreased. However, due to improper use, the surface of the water roller sleeve first contacts the ink or the large amount of oil adsorbed on the surface of the water roller is not cleaned and dried in time. The suede gradually loses its original hydrophilicity, and the water absorption and water transfer are also gradually weakened, resulting in water on the plate surface. 2. The ink balance is destroyed, resulting in greasiness;
Overcoming method: Pay attention to control the water content of the layout to prevent the water roller from sticking ink due to the small water. Dip the pumice powder with concentrated potion in time to remove the ink on the surface of the water roller, and clean the water roller frequently to ensure good hydrophilic performance;
6 Excessive pressure between plate and blanket cylinder
Printing is inseparable from the pressure, but the pressure between the printing plate and the rubber cylinder is too large, it will produce friction, make the trachoma wear, brighten the plate surface, destroy the hydrophilic layer of the plate surface and produce greasy. This kind of grinding friction force generally comes from two aspects. On the one hand, the unreasonable distribution of the lining leads to a large amount of slip between the rollers; on the other hand, it is compression.
Overcoming method: On the basis of ensuring that the imprint is strong enough without layering, use the minimum pressure uniformly, correctly calculate and measure the thickness of the initial cushion, and at the same time, determine the amount of compression according to the nature of the liner.
7 Excessive pressure between printing plate and inking roller
The inking roller rubs the most times, and there is ink on the surface of the roller. Some ink filler particles play the role of abrasives, especially the particles of chrome yellow ink are relatively coarse. When the pressure between the printing plate and the inking roller is large, the friction is large, making the sand Suffering from wear and tear, he sometimes showed signs of wear.
Overcoming method: correct the pressure between the printing plate and the inking roller, and the steel plate can be used to adjust the inking roller pressure on a roller-by-roller basis and make the pressure on both sides of the ink roller consistent. You can also see the width of the ink bar: Generally, the pressure of the inking roller and the printing plate should be adjusted to 5 ~ 6mm.
8 Paper factor
Due to the composition of the paper: fibers, fillers, sizing and other tissues are not tight, and the surface strength of the paper is poor. During the printing process, the paper is subject to peeling force, and the surface is easy to peel off and fluff, forming a buildup layer on the surface of the blanket , And into the ink roller and water roller to the printing plate through the blanket, after a long time, the accumulated paper wool, powder and filler and other impurities, intensified the friction between the rollers, making the rubber imprint blurred and empty, and the surface of the printing plate is frosted Abrasion damages the hydrophilic layer, causing oil to become dirty.
How to overcome: Add 6 # ink and debonding agent in an appropriate amount, but the ink is easy to dilute the ink color and increase the fluidity. You must be cautious when using it. In order to prevent dirty ink, the fountain solution can be added properly, and the blanket and printing plate should be cleaned frequently when used.
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