Why does the printed ink color change? How to solve it?
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Run solution stock supply
1. Cause analysis
The supply amount of the dampening solution is too large, so that the pH value of the dampening solution becomes small, causing separation of the ink (separation of the connecting material and the pigment), resulting in uncontrollable water consumption, causing discoloration of the ink color, and sometimes a dirty plate phenomenon, and increasing the amount of water. Dirty plate phenomenon is more serious; if the supply of dampening solution is less, the pH value of dampening solution will become larger, and the hydrophilic layer added to the blank part of the printing plate will be insufficient to resist the intrusion of ink, resulting in a large amount of water used in the printing plate. Version and color change of ink. For example, the printed product is a large-area gradient dot. During the printing process, the gradient dot is gradually pasted, the ink color is discolored, and it is considered to be a dirty version. After the water consumption is increased, the problem is not solved, so the machine is immediately shut down and cleaned with a cleaning agent. Version, measuring the pH value of the dampening solution is 6.0, adjusting the supply amount of the dampening solution, and lowering the pH value of the dampening solution to about 5.0, the problem is solved.
2. Exclusion method
Change the dampening solution, or adjust the supply of dampening solution to control the pH of the dampening solution to 5.0~5.5.
Alcohol addition ratio
1. Cause analysis
The proportion of alcohol added is too large, although it can reduce the surface tension of water, make the water film thinner, and have good wettability (ie, spreadability) on the surface of the printing plate, but it will cause the alcohol in the water to react with the colloidal solution, resulting in plasticity. The phenomenon causes the water to be large and the ink is discolored. On the contrary, the insufficient proportion of alcohol will cause the surface tension of the water to be high, and the wettability to the layout is poor. The amount of water must be increased to make the ink not dirty, so the ink color is discolored. . For example, a certain printing and packaging paper, because there is no attention to alcohol has been used up, resulting in a decrease in the proportion of alcohol by 6%, the phenomenon of dirty version, after increasing the amount of water, the dirty version disappears, but the color of the ink gradually changes, stop inspection, control window dampening It shows that the alcohol content drops below 6%, and the alcohol barrel is replaced to make the alcohol content reach 10%. When the printing is started, the water is reduced to the previous amount, and there is no dirty version and the ink color is normal.
2. Exclusion method
Control the water in the water tank so that the alcohol setting ratio is consistent with the actual value. The alcohol setting ratio is 10% to 14%; the isopropyl alcohol setting ratio is 8% to 13%.
Water hardness
1. Cause analysis
The hardness of water is high, and the calcium oxide, magnesium oxide and the like contained in the water are transferred to the surface of the ink roller together with the chemical components in the ink, paper powder and dusting, which makes the ink roller deinking and ink transfer uncontrollable, and these substances contain The fatty acid also causes the alkalinity of the ink to increase, which is likely to cause a paste phenomenon and cause discoloration of the ink. For example, long-term use of tap water with high hardness makes it easy to cause crystallized rollers to crystallize, causing the ink roller to crystallize. The printed products reflect a large amount of water and a thick water pattern (mainly when the printed version is low, if the alcohol ratio is low, This phenomenon occurs, causing the paste, dirty plates, printed products to be dark, and easy to block the water tank.
2. Exclusion method
The use of demineralized water reduces the amount of water, making the water grain thinner, less difficult to paste, and the gloss of the printed product is increased, and the crystallized surface of the ink roller is relatively reduced.
Dampening solution replacement time
1. Cause analysis
Due to the long-term recycling of the dampening solution, the microorganisms in the dampening solution may be excessive, causing the dampening solution to deteriorate, resulting in unstable water supply and discoloration of the ink. For example, when printing four open covers, the sample inspection found that the ink on both sides of the print was discolored. The inspection found that the layout was still very wet, and it was concluded that the water was too slow to cause the plate to evaporate slowly. Check the pressure of the water roller ink mark is normal, after reducing the water consumption, the middle dirty plate, after analysis, may be that the dampening solution has not been replaced for a long time, causing the dampening solution to deteriorate, causing the water film to be uncontrollable. After replacing the water in the tank, the printing ink is normal.
2. Exclusion method
Regularly change the dampening solution in the tank. Install a drain pipe at the water tank drain and connect the water tank to the machine's own water pipe to clean the water tank quickly. If the filter cloth cover is installed at the water return port to keep the water in the water tank clean, it can prevent the water pipe from being blocked for a long time.