Causes and preventive measures of gold discolorati

Causes and preventive measures of gold discoloration in offset printing

first, the basic composition of offset gold ink

copper gold powder (aluminum silver paste) 30 ~ 50%

offset gold transfer oil 70 ~ 50%

fully mix copper gold powder (aluminum silver paste) and gold transfer oil to form gold silver ink

II. Causes of gold printing discoloration and preventive measures

1 PH of paper. Due to improper treatment in the process of pulping and papermaking, such as organic acids in the pulp or residual chlorine left during bleaching, the paper will be acidic, while adding alkaline fillers and pigments will make the paper alkaline. If the paper is acidic, the acidic paper will inhibit the drying of the imprinted oxidized conjunctiva and slow the drying speed; Including the data of matrix resin and fiber reinforcement, the paper with too strong alkalinity will continuously transfer the alkaline substances in the printing process to the bucket solution, so that the original weak acidic fountain solution will be neutralized, affecting the normal pH value of the fountain solution, causing the remaining ink to change color and deteriorate. In addition, it will emulsify the ink and cause floating dirt. Therefore, the pH value of the selected paper is 7, that is, neutral is the most ideal, which can effectively avoid discoloration

2. The pH value of fountain solution is in balance with that of ink. The residual amount of PE film per mu of offset printing based on the principle of lipophilic and hydrophobic parts of printing plates and hydrophilic and hydrophobic parts of non printing plates is between 16 and 28 kg. The acids in the fountain solution will eventually decompose into small molecules that will not pollute the environment, which can play the role of cleaning and dirt removal of the layout. However, too strong acidity is easy to cause gold discoloration, loss of metallic luster, and quality problems such as printing plates, slow drying of impressions, sticky dirt on the back, etc; The acidity is too weak, and the printing plate is dirty and pasted. Therefore, generally, the pH value of fountain solution should be controlled at 5 ~ 6. Ink balance is to use as little water as possible to control the emulsifying amount of ink within the minimum range of 25% on the premise that the printing plate can not be dirty and the oil window on the left side of the inspection control cabinet is not pasted. Because the moisture is too large, it is easy to cause the printing gold ink to be too light, the gold ink is Matt, the color is not bright, the gold ink is emulsified, and the paper telescopes and deforms, resulting in quality defects such as inaccurate overprint; Too little moisture is easy to make the layout dirty, greasy and have problems such as merging and pasting, so mastering the ink balance is very critical. In addition, the intermediate roller must be separated from the ink path and water path system, which is also an essential way to avoid

3. printing speed. If the printing speed is too fast, it is easy to cause intense friction between the ink rollers, and then the heat generated will change the color of the ink. The general experience is to control it 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 the unused gold and silver powder must be isolated from the air and stored in a closed manner as far as possible, and the storage time should not be too long

5. PH of gold adjustment oil. Both silver powder and gold powder are easy to oxidize, so the gold blending 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 the self blending method of instant blending, which can further improve the anti oxidative discoloration