In the printing process, gold (silver) products have the advantages of fine and solid imprint, good metal texture, strong hiding power, high floating force, strong metallic luster and low cost. However, in the printing process, the failure of gold (silver) ink discoloration often occurs. Let's analyze the reasons for the discoloration of gold (silver) ink and propose preventive measures.
(1) pH of paper
In the pulping and papermaking process, if the pulp contains organic acids or chlorine remains during bleaching, the paper will be acidic; and the addition of alkaline fillers and colorants will make the paper alkaline. If the paper is acidic, it will inhibit the imprinted oxidized conjunctiva on the surface and slow down the drying speed; if the paper is too alkaline, the alkaline material will be continuously transferred to the water bucket during the printing process, which is weakly acidic The neutralization of the plate solution affects the pH value of the fountain solution, which causes the remaining ink to become discolored and deteriorated, and also emulsifies the ink, resulting in the phenomenon of scumming. Therefore, the pH value of the paper is 7, that is, it is neutral, which can effectively avoid the discoloration of gold (silver) ink.
(2) pH value of fountain solution and water-ink balance
In the offset printing process, the acidic substance in the fountain solution can clean the plate surface, but too strong acidity can easily cause the gold (silver) ink to change color, make it lose its metallic luster, and also cause the printing plate to bloom and the print is not easy to dry. , The back of the printed matter is dirty and other quality failures; and if the acidity is too weak, the printed screen mesh tone graphics are easy to get dirty and paste. Therefore, it is appropriate to control the pH value of the fountain solution at 5 to 6.
The water-ink balance is to use as little water as possible under the premise that the printing plate is not dirty and not pasted, and the amount of ink emulsification is controlled within a range of less than 25%. Because the water supply is too large, the gold (silver) ink is easy to be emulsified, which causes the ink color to be too light, the ink layer is dull, and the color is not bright when printing the gold (silver) ink; and the water supply is too small, it is easy to make the layout Dirty, greasy, and mixed-up paste. Therefore, it is very important to control the ink-water balance.
(3) Printing speed
If the printing speed is too fast, it is easy to cause strong friction between the ink rollers, and the heat generated will discolor the gold (silver) ink. Generally, it is more appropriate to control the printing speed of gold (silver) ink for offset printing at 5000 sheets / hour.
(4) Oxidation of copper gold powder and aluminum silver paste
Copper-gold powder and aluminum-silver paste are easy to oxidize and change color, so unused copper-gold powder and aluminum-silver paste must be kept in airtight and sealed, and the storage time should not be too long. In addition, the gold oil used in the preparation of gold (silver) ink must be a quick-drying, fast-setting, high-viscosity resin oil with low oxidation and neutral pH. In order to avoid the oxidation and discoloration of gold (silver) ink for long-term storage, it is best to adjust it immediately.
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