The Causes and Preventive Measures of Discoloration of Offset Printing Gold and Silver
1. The basic composition of offset printing ink
Copper gold powder (aluminum silver paste) 30 ～ 50 ％
Offset printing gold oil 70 ～ 50 ％
Fully mix copper-gold powder (aluminum-silver paste) and gold-adjusting oil to make gold-silver ink
2. Causes and preventive measures of discoloration of printing gold
1. The pH of the paper. Due to improper handling in the pulping and papermaking process, such as organic acids in the pulp or residual chlorine left 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, the acidic paper will play a role in inhibiting the drying of the imprinted oxidized conjunctiva, making the drying speed slow; while the paper with too strong alkalinity, the alkaline substances in the printing process will be continuously transferred to the water bucket. The weakening acid fountain solution is neutralized, which affects the normal pH value of the fountain solution, causing the remaining ink to change color and quality. In addition, it will emulsify the ink and cause floating. Therefore, the PH value of the selected paper is 7, that is, neutral is the most ideal, which can effectively avoid discoloration
2. PH value of fountain solution is balanced with ink. The offset printing with the principle of oleophilic hydrophobicity and non-graphical hydrophilicity and oleophobicity in the printing plate, the acidic substance in the fountain solution can play the role of cleaning and decontamination of the plate, but too strong acidity will easily cause the printing gold to discolor. Loss of metallic luster causes quality problems such as plate printing plates, slow drying of blots, and sticky dirt on the back side; while the acidity is too weak, the printing plate dots are 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 paste, so that the amount of ink emulsification is controlled within a minimum range of 25%. Because the moisture is too large, it is easy to cause the color of the gold ink to be too pale, the gold ink is dull, the color is not bright, the gold ink is emulsified, and the paper deforms and deforms to cause overprinting and other quality deficiencies; the moisture is too small, it is easy to make the layout dirty and greasy And there are problems such as parallel paste, so it is very important to master the balance of water and ink. In addition, the intermediate roller must be separated from the ink and water system, which is also an inevitable method of avoidance.
3. Printing speed. If the printing speed is too fast, it is easy to cause intense rubbing between the ink rollers, and the heat generated will cause the ink to change color. The general experience is that it is preferably controlled at 5000 sheets / hour.
4. Oxidation of copper gold powder and aluminum silver paste. Silver powder and gold powder have the disadvantage of easy oxidation and discoloration, so unused gold and silver powder must be kept as airtight as possible, and the storage time should not be too long.
5. Adjust the pH of gold oil. Both silver powder and gold powder have the property of being easily oxidized, so the gold oil used must be a quick-drying, fast-setting, high-viscosity resin oil with low oxidation and neutral acidity. In order to avoid long-term storage and oxidation, it is best to use With the self-adjusting method of instant adjustment, it can further improve the anti-oxidation discoloration.
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