The Causes and Prevention of the Discolouration of Offset Printed Gold and Silver

In today's packaging and decorating and other printed matter, in addition to the use of gold (silver) technology, offset printing gold (silver) printing process is still a lot of manufacturers in the selection. The printed gold (silver) has the advantages of delicate and solid imprinting, good metal texture, strong hiding power, high floating force, strong metallic luster, and low cost. Offset printing companies can directly select gold (silver) color offset ink products or use copper powder (aluminum silver paste) and offset printing oil self-adjusting salted gold and silver ink in two ways.

First, the basic composition of offset ink

Copper powder (aluminum paste) 30～50%
Offset transfer oil 70~50%
Mix copper powder (aluminum silver paste) and gold oil into gold silver ink

Second, the causes of discoloration of India gold and preventive measures

1. The pH of the paper. Due to improper handling in the pulping and papermaking process, residual chlorine in the pulp with organic acids or bleaching will make the paper acidic, while adding alkaline fillers and pigments will make the paper alkaline. If the paper is acidic, the acidic paper will play a role in inhibiting the drying of the imprinted conjunctiva, which will slow the drying speed; while for alkaline paper, alkaline substances in the printing process will be continuously transferred to the fountain liquid. The original weakly acidic dampening solution is neutralized, which affects the normal pH of the fountain solution, causing the remaining ink to discolor and deteriorate. In addition, the ink will be emulsified and cause turbidity. Therefore, the pH value of the selected paper is 7, which means that the neutrality is the most ideal and can effectively avoid discoloration.

2. Fountain solution pH and ink balance. Using the printing and graphic printing part of the hydrophobic, non-graphical part of the hydrophilic oleophobic principle of offset printing, the acid in the fountain solution can be used to clean the layout of the dirty, but the acidity is too easy to cause the gold discoloration, Loss of metallic luster, resulting in printing plate version, imprinted slow dry, back sticky and other quality problems; and acidity is too weak, printing plate network graphics from the dirty, paste version. Therefore, the general fountain solution PH value is controlled at 5 ~ 6 is appropriate. The ink balance is the use of as little moisture as possible without the printing plate being dirty or paste-free. The emulsification amount of the ink is controlled within a minimum range of 25%. Because the water is too large, it is easy to cause the gold ink is too light, the gold ink is dull, the color is not bright, the gold ink is emulsified, the paper is stretched and deformed to cause overprinting and other quality defects; the water is too small, it is easy to make the layout dirty, and greasy As well as problems such as parallel stencils, it is critical to master the balance between water and ink. In addition, the intermediate roller must be separated from the ink and water system, and it is also an essential method to avoid it.

3. Printing speed. If the printing speed is too fast, it will easily cause intense grinding between the ink rollers, and the resulting heat will cause the ink to change color. The general experience is that the control should be controlled at 5000 sheets/hour.

4. The oxidation of copper powder and aluminum silver paste. Silver powder and gold powder have the drawback of being easily oxidized and discolored. Therefore, unfinished gold and silver powder must be kept as close to the air as possible, and the storage time should not be too long.

5. The pH of the oil. Both silver powder and gold powder have the property of easy oxidation. Therefore, the gold transfer oil used must be a fast-drying, fast-setting, high-viscosity resin oil with small oxidizing power and neutral pH, and it is preferred to use oxidation to avoid long-term preservation. With the self-aligning method, the anti-oxidation and discoloration can be further improved.

Source: Guangdong Printing