Analysis and solution to the cause of dirty printing on offset printing plate layout
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Hanging dirty refers to the insufficient or incomplete water film on the surface of the blank part of the printing plate and the improvement of the lipophilic performance caused by other original traps. The direct contact between the ink layer on the surface of the ink roller and the printing plate surface causes the ink layer to break and the viscous phenomenon caused by ink transfer. Commonly known as the "dry version." A prominent feature of this dirty plate phenomenon is that the upper visceral position is fixed with the basis of absorbing ink. This kind of 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 time and quickly.
1, the printing plate problem
The printing plate has not been printed, and the layout is greasy, mainly caused by improper printing. If the greasy weight is heavy, the PS plate is because the underexposed blank part of the photolysis is not thorough. Or the development is insufficient, the blank part of the photosensitive layer is not washed. If the greasy light is only a single part, it may be as follows:
(1) The old figure remains on the original plate and traces;
(2) Sun drying factors, improper operation during general exposure, large glass or soft film of the printing machine is contaminated with dirt, dirty spots, insufficient exposure, and development is not transparent;
(3) The front plate of the baking plate is not thoroughly cleaned or the protective agent is not uniformly coated;
(4) Uneven rubber or dirty ink.
Overcome the method: If the plate is greasy, it must be reprinted. If the plate is greasy and produces a small part, the PS plate must be reduced in fat and some Arabic resin added to the dampening solution.
2. Improper pH control of dampening solution
The pH of the dampening solution is large, the hydrophilic colloid layer is slow to generate, and the inorganic salt layer of the blank part cannot be supplied in time, and the blank is divided into hydrophilic and oleophobic; the pH value is too small, and the plate sand is etched. So that the surface of the blank portion cannot be enough water to cause the oil to be dirty.
Overcome the method: When blending the dampening plate, it is necessary to strictly control the PH value, according to the type of printing plate, the thickness of the printing ink layer, the amount of dryness, the nature of the paper, the temperature of the workshop, the temperature and other actual conditions. Under normal circumstances, the pH of the dampening solution is: the PS version is about 4.5 to 6.
3, the ink roller aging
Since the ink roller is rolled under heat for a long time and is in contact with dampening solution and ink, it is prone to heat aging, the surface is dry and conjunctiva, the hydrophilicity is enhanced, the ink transfer performance and the adsorptivity are weakened, and the ink on the plate surface is not greasy. Adsorption in time, resulting in greasy printing plates;
Overcome the method: often remove the ink roller to scrub, polish, prevent aging, and maintain its ink-transfer performance.
4. The pressure between the water roller and the printing plate is too large.
The pressure between the water roller and the printing plate is too large, the velvet cloth loses its elasticity, and the friction increases, which causes the sanding of the plate to wear, which destroys the stability of the inorganic salt layer, and the blank portion is prone to oily dirt;
Overcome the 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 0.2 mm and a width of about 20 to 30 mm can be inserted between the water rollers or between the water roller and the printing plate at both ends and in the middle, with the hand feeling There is friction resistance, and it is better to feel that the steel sheet can be pulled out smoothly. In addition, the water roller should be scrubbed frequently, and the water roller velvet should be replaced in time after losing its elasticity.
5, improper use of water roller maintenance
The fleece sleeve of the water roller has good hydrophilicity after being degreased. However, due to improper use, the surface of the water roller cover is first contacted with the ink or the large amount of oil adsorbed on the surface of the water roller is not cleaned and washed in time. The suede gradually loses its original hydrophilicity, and the water absorption and water permeability are gradually weakened, resulting in the surface water. The ink balance is destroyed and greasy;
Overcome the method: pay attention to control the moisture of the layout to prevent the water roller from sticking to the ink due to the small water. Timely use the concentrated syrup pumice powder to remove the dirty ink on the surface of the water roller, and often wash the water roller to ensure good hydrophilic performance;
6, the pressure between the printing plate and the blanket cylinder is too large
Printing is inseparable from the pressure, but the pressure between the printing plate and the blanket cylinder is too large, which will cause friction, which will cause the sand to wear and the plate to be bright, which will destroy the hydrophilic layer of the plate to produce greasy. This kind of rubbing friction generally comes from two aspects. On the one hand, the distribution of the lining is unreasonable, resulting in a large amount of slippage between the rollers; on the other hand, it is compression.
Overcoming method: On the basis of ensuring that the imprint is sufficiently strong without laminating, the minimum pressure is uniformly used, the thickness of the initial mat is correctly calculated and measured, and the amount of compression is determined according to the properties of the liner.
7. The pressure between the printing plate and the inking roller is too large.
The inking roller has the most frictional times, and there are inks on the roller surface. Some ink filler particles act as 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, so that the sand is self-generated. Affected by wear and tear, he sometimes wears out in the ink painting department.
Overcoming method: Correct the pressure between the printing plate and the inking roller, and adjust the pressure of the inking roller by the steel piece by roller root, and make the pressure on both sides of the ink roller consistent. Also see the width of the ink stick: the general ink roller and plate pressure should be adjusted to 5 ~ 6mm.
8, paper factors
Due to the composition of the paper: fibers, fillers and sizing are not tightly organized, the surface strength of the paper is poor, during the printing process, the paper is subjected to the peeling force, the surface is easy to remove the powder, and a layer is formed on the surface of the blanket. And through the blanket into the ink roller, the water roller to the printing plate, the time is long, the accumulated paper wool, powder and filler and other impurities, the friction between the rollers is intensified, the rubber imprint is blurred and empty, and the surface of the printing plate is smeared. Wear and tear destroys the hydrophilic layer, causing oil to dirty.
Overcome the method: Add 6# varnish and debonding agent in moderation, but the varnish is easy to dilute the ink color and increase the fluidity. Care must be taken when using it. In order to prevent the dirty ink from being dirty, the plate solution can be properly added, and the blanket and the plate should be cleaned frequently.