Talking about the basic composition of gold, silver and ink
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First, the basic composition of offset gold ink
Copper powder (aluminum paste) 30% to 50%
Offset gold oil 50% to 70%
Mix the copper gold powder (aluminum silver paste) and the gold-plated oil to form a gold-silver ink.
Second, the cause of the discoloration of the printing gold and preventive measures
1. The pH of the paper. Due to improper handling in the pulping and papermaking process, such as organic acid in the pulp or residual chlorine in the bleaching process, the paper will be acidic, and the alkaline filler and coloring will make the paper alkaline. If the paper is acidic, the acid paper will act to inhibit the drying of the oxidized conjunctiva, so that the drying speed is slow; while the paper with too much alkalinity, the alkaline substance in the printing process will be continuously transferred to the water hopper. The original weak acid plate is neutralized, which affects the pH value of the normal dampening solution, causing the remaining ink to discolor and deteriorate, and in addition, the ink is emulsified and caused to be dirty. Therefore, the selected paper has a PH value of 7, which is neutral, which is ideal for avoiding discoloration.
2. The pH of the dampening solution is balanced with the ink. Using the offset printing of the lipophilic and non-image parts of the printing plate, the acidic substance in the dampening solution can play the role of cleaning the surface of the plate, but the acidity is too strong and the color of the gold is easily changed. Loss of metallic luster, resulting in printing plate, slow-drying, dirty back and other quality problems; and the acidity is too weak, the printing plate dot text dirty, paste version. Therefore, the pH of the general dampening solution is controlled to be 5-6. The ink balance is the minimum range in which the emulsification amount of the ink is controlled within 25% under the premise that the printing plate is not dirty or paste-free. Because the water is too large, it is easy to cause the ink color is too light, the gold ink is dull, the color is not bright, the gold ink is emulsified, the paper is stretched and deformed, and the overprinting is inaccurate. The moisture is too small, so the surface is dirty and greasy. And there are problems such as parallel paste, so it is very important to master the ink and water balance. In addition, the intermediate roller must be separated from the ink path and the water system, which is also an indispensable way to avoid it.
3. Printing speed. The printing speed is too fast, it is easy to cause intense rubbing between the ink rollers, and the heat generated causes the ink to change color. The general experience is to control at 5000 sheets/hour.
4. Oxidation of copper gold powder and aluminum silver paste. Silver powder and gold powder have the disadvantage of easy oxidative discoloration, so the unused gold and silver powder must be kept in airtight storage as much as possible, and the storage time should not be too long.
5. Adjust the pH of the gold oil. Both silver powder and gold powder have the property of being easily oxidized. Therefore, the gold-adjusting oil used must be a quick-drying fast-solidifying, high-viscosity resin oil with low oxidizability and neutral pH. In order to avoid long-term storage, oxidation is preferred. With the self-adjusting method of instant adjustment, the oxidation prevention discoloration can be further improved.