Causes of discoloration of printed gold in printing process
In modern printing, in addition to the bronzing (silver) process, offset printing gold (silver) printing process is also used by many manufacturers. Gold printing (silver process has the characteristics of fine and firm imprinting, good metal texture, strong hiding power, high floating power, etc., can show a strong metallic luster, and the cost is low. Generally, China printing company can directly choose gold (silver) The finished product of color offset printing ink can be printed in two ways: copper-gold powder (aluminum-silver paste) and offset-printing gold oil self-adjusting salty gold and silver ink.
However, in the offset printing gold printing process, if unskilled technicians use this printing process, it will often cause the phenomenon of discoloration of the printed gold, resulting in a big difference between the printed product and the actual high-quality product, or it is not at all similarly, there is no doubt that this result must be redone, so a lot of manpower, financial resources and time will be wasted. In order to avoid this situation, we must find out the reason so that this situation will not recur. So what are the reasons for the discoloration of printed gold?
1. The PH of the paper
Due to improper handling in the pulping and paper making process, such as the presence of organic acids in the pulp or residual chlorine during bleaching, it will make the paper acidic, while adding alkaline fillers and coloring materials 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, which will slow the drying speed; while the paper with too strong alkalinity will continuously transfer the alkaline substance during the printing process to the hopper liquid. The originally weakly acidic fountain solution is neutralized, which affects the normal pH value of the fountain solution, causing the remaining ink to be discolored and deteriorated. In addition, it will emulsify the ink and cause floating. Therefore, the pH value of the selected paper is 7, which means that it is neutral, which can effectively avoid discoloration.
2. Oxidation of copper-gold powder and aluminum-silver paste
Silver powder and gold powder have the disadvantage of being easy to oxidize and discolor. Therefore, unused gold and silver powder must be kept in airtight as far as possible, and the storage time should not be too long.
3. Printing speed
If the printing speed is too fast, it is easy to cause intense abrasion between the ink rollers, and the heat generated will change the color of the ink. The general experience is to control it at 5000 sheets per hour.
Fourth, the pH of gold oil
Both silver powder and gold powder have the property of being easily oxidized, so the gold adjusting oil used must be a fast-drying, fast-curing, high-viscosity resin oil with low oxidation and neutral pH. In order to avoid oxidation due to long-term storage, it is best to use instantaneous The self-adjusting method of instant adjustment can further improve the anti-oxidation and discoloration.
5. The pH value of fountain solution and the balance of ink
Utilizing the principle of lipophilic and hydrophobic, non-graphic and non-graphical offset printing, the acidic substance in the fountain solution can clean and remove dirt on the plate surface, but too strong acidity can easily cause discoloration of the printed gold. Loss of metallic luster, causing quality problems such as printing plate pattern, slow drying of imprints, and dirty back side; while the acidity is too weak, and the printing plate dot graphics become dirty and stale. Therefore, the PH value of the general fountain solution should be controlled at 5-6. The balance of ink and water is to use as little water as possible on the premise that the printing plate can not be dirty and paste, so that the emulsification amount of the ink is controlled within the minimum range of 25%. Too much water can easily cause the printing gold ink color to be too light, the gold ink is dull, the color is not bright, the gold ink is emulsified, and the paper is stretched and deformed to cause inaccurate overprinting. And there are problems such as parallel paste printing, so it is very important to master the balance of ink and wash.
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