Dirty printing plate is a common fault, but in a sense it is also a typical fault. In the production process, a detailed analysis must be carried out according to the signs and parts of the plate with dirt, to find out the cause of the failure, and take effective measures to prevent and solve it. Generally speaking, the dirty belt can be divided into floating dirty, dirty oil, paste and so on.
Dirty
Dirty dirt refers to a phenomenon in which a non-graphic part of a relatively large range and a relatively low density becomes dirty. There is no fat-sensitive foundation for its dirty parts. Because of this, the dirty parts of the non-graphic parts can be seen on the printed sheet, and may or may not be visible on the printing plate. Observed from the surface of the printed matter, the dirty spots sometimes float everywhere. It is possible that the dirty spots of the previous and next sheets are located at different positions. In addition, most of the printed materials with floating dirt generally have faint and dark ink.
The main cause of scumming is the ink emulsification caused by the imbalance of water and ink. In addition, there are factors such as oil paper quality, water, ink roller and printing pressure.
The specific causes and treatment methods are as follows:
(1) The ink has insufficient moisture resistance and is emulsified in the wetting fluid.
Treatment method: For soft or thin ink, increase the consistency of the ink, and add moisture-resistant ink to the short-filament ink that is easy to soak.
(2) The emulsifier is precipitated from the paper coating, causing scumming.
Treatment method: change paper or harden ink.
(3) In multi-color printing, if all colors of ink are scummed, it may be a paper problem. If only one-color or two-color inks are smeared, and the other ink colors are printed clearly, it indicates that the ink or printing plate is a problem. Monochrome machine has a similar phenomenon.
Treatment method: change paper, harden ink or change ink.
[Case 8-6]
A factory uses a four-color offset printing machine to process a batch of 250g /? Color boxes. Judging from the on-site printed matter, all four colors have floating stains, and the black stains are the most. After the shutdown, the plate is wiped off with a plate cleaner and disappears, but the floating stains appear again after the power is turned on.
[case study]
Like the above example, can it be considered a paper problem? Although the four colors are all dirty, but the degree is not the same, so it can not be determined that the paper is a problem, so how to check it?
(1) Check the ink viscosity, stringiness, and ink delivery.
(2) Check the pressure of water roller and ink roller.
(3) Check whether the water-ink balance reaches the best state (observe whether the water content of the layout is too large)
(4) Check whether the parameters of the water tank are normal, including temperature, alcohol concentration, conductivity value, PH value, etc.
(5) Clean the water tank and replace the wetting fluid.
Follow the above procedures to check carefully and draw conclusions.
Dirty dirt is often caused by the mass voiding of printed materials. Such quality accidents often occur during packaging and printing. Operators must pay attention.
[Case 8-7]
A factory of plastic concave co-printing a batch of cigarettes, the background color is red, of which the pinyin is pinyin. In order to make the pinyin conspicuous after the indentation, the ink is finally overprinted with white ink. When the operator prints the red The overall appearance of floating dirt makes the empty white text reddish, and finally the batch of live parts is scrapped by one third.
[Expert Tips]: Examples like this should not strictly occur. This is mainly due to the operator ’s lack of quality awareness, and it does not matter that the red version is a little dirty, and the white ink can cover it, but the result proves that the judgment was wrong.
Dirty oil
Dirty oil means that the blank part of the printing plate should not feel fat, so that the ink adheres to it, and the plate surface is dirty. The main reason for the fat sensitivity of the blank part of the printing plate: the water content of the blank part is insufficient or the single wet liquid is weak in acidity, or other reasons destroy the hydrophilic salt layer of the blank part of the printing plate.
The biggest characteristic of oil dirt is that the dirty part is relatively fixed, and the dirty part is not easy to wipe off. In addition, it can be removed by adding water and rubbing lotion and peach glue, but the oil dirt that damages trachoma can not be eradicated.
The printed matter has oil stains. If the cause is traced, it is that the relevant parts related to the printing plate and the friction are suspect. example:
(1) The pressure from printing plate to blanket cylinder is too high.
(2) The pressure of the inking roller is too high.
(3) The printing plate is not tensioned during the installation.
(4) The ink's fat sensitivity and the amount of additives are inappropriate.
(5) The PH value of the wetting fluid is too large.
(6) The blanket is aging and lacks elasticity. [next]
Three paste version
The paste version refers to the lines and lines in the printed graphics and text, between the dots and the dots, and where there should be no ink color in the highlights, it is dirty, but sometimes it belongs to the category of floating dirt, and sometimes it belongs to the category of oil dirt.
In the printing process, it is often judged according to the following conditions:
(1) The location of the stain is determined, and the ink color is dark, it must be oil.
(2) The position with the stain often changes, and the ink color is lighter, it must be floating.
If the types of dirty plates are distinguished, the corresponding faults can be dealt with accordingly.
Four dirty, paste, pattern failure analysis
In the printing process, except for the ink factor, dirty, paste, and pattern plates are mostly of varying degrees, destroying the blank part of the printing plate and dividing the hydrophilic layer, but the manifestation is different. The specific factors are;
(1) The acidity of the wetting fluid is too weak. The acidity of the wetting fluid is too weak, the formation of the hydrophilic salt layer is slow, and the inorganic salt layer of the blank part cannot be supplied and replenished in time, which reduces the hydrophilicity and oleophobicity of the blank part. The acidity is too strong, which may corrode the grit of the printing plate, destroy the hydrophilic salt layer, and easily make the plate surface shine under pressure friction, resulting in dirty, sometimes flower plate.
(2) The aging of the inking roller makes the printing plate greasy and the stencil inking roller has aging. The hydrophilic performance is strong and the performance of transferring ink is poor. During mechanical rolling, water is dispersed in the ink in the form of fine droplets. The principle of incompatibility makes the ink on the inking roller emulsified, reduces the viscosity of the ink, and reduces the adhesion. The ink can easily fall from the inking roller to the blank part of the printing plate, causing the hydrophilic salt layer to produce lipophilicity, resulting in greasy and Graphic version of the flower version.
(3) The pressure between the water roller and the printing plate is too high, which causes the printing plate to produce oil stains and the printing plate contains the acidic wetting liquid. The water roller is driven by the pressure and depends on the surface friction force. In the middle, due to a certain linear speed difference between the compression deformation of the water roller and the plate cylinder, the water roller and the plate have surface friction, the sand is damaged, and the stability of the inorganic salt layer is also destroyed, making the blank part susceptible to grease and Produce a dirty version. Therefore, the contact pressure between the water roller and the printing plate must be adjusted reasonably.
(4) The excessive pressure between the inking roller and the printing plate makes the blank part dirty, and the graphic part is printed with the ink roller and the printing plate rubbing the most times, and there is ink on the roller, and some ink filler particles act as an abrasive, causing the printing The blank part of the edition is dirty, and the picture and text parts are flowered.
(5) The pressure of the inking roller on the plate surface is too small, which causes the pattern plate. When the inking roller is too small on the plate surface, the ink spread on the plate surface is reduced accordingly, and the ink on the plate graphic is less, and there is not enough ink on the plate surface. In the case of a layer, the graphic part is subject to wear, and the pattern will be caused after a long time.
(6) When the pressure between the plate cylinder and the blanket cylinder is too large, it will cause the plate to become dirty, and the pressure between the plate cylinder and the blanket cylinder will exceed the specified value. With the squeezing force, the printing plate sands gradually smoothed to reduce the water storage capacity, and began to produce paste plates. After printing to a certain amount, the sand plates were polished, the printing plates were shiny, and the dot foundations were also damaged, and flower patterns appeared again. . This shortcoming from the beginning paste version to the final flower version seriously affects the printing quality. At this time, the printing pressure and the liner must be readjusted.
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