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During the printing process or in the use of printed products after the end of printing, whether the ink color on the surface of the printed matter can last for a long time or stay faded is also an important indicator of the quality of printed matter.
The cause of fading and discoloration of various printed products is mainly the performance of the ink, and there are also problems with the improper arrangement of the printing ink process. In the following, we will discuss some factors that affect the discoloration of ink from six aspects.
1. The ink fades due to light resistance.
Light resistance is an important indicator for evaluating the properties of inks, and many inks will change color under sunlight. The lightfastness of printing inks mainly depends on the pigments used in the inks. If the pigment used undergoes a chemical reaction or crystal form transformation under the influence of light, the ink will discolor or fade. The general experience in production is as follows.
a) Light color ink fades and discolors severely after long-term irradiation (Y, M, G), and dark color ink (C, B, K) is slow.
b) Use ink with good light resistance as much as possible when adjusting the ink.
c) When adjusting light-colored ink, attention should be paid to the light resistance after diluting.
d) Outdoor advertising should pay attention to the light resistance of the ink.
e) Phthalocyanine blue is lightfast than light lake blue and peacock blue.
f) Gray can be obtained by adding black ink and phthalocyanine blue to white ink.
g) Emerald green can use monochromatic phthalocyanine green (the color can be brighter by adding white ink), if not enough yellow, can add bright resin yellow. (If you use peacock blue and chrome yellow for a long time, it will fade and yellow).
! When making products for outdoor use (outdoor advertising, etc.), the color sequence of printing should be considered. Try to print Y and M first, and then print C and Bk, because C and Bk have better light resistance, which will have a certain blocking effect on the fading of inks with poor light resistance.
2. Discoloration during ink drying:
In the printing process, the ink color just printed is relatively dark. After a period of time, the ink color will become lighter after the blot is dried. Mainly due to the penetration and oxidative drying of the ink during the drying process, the discoloration is formed, especially the ink based on penetration drying (such as the rotary ink for newspaper printing). The thickness of the ink layer changes significantly after drying, and the ink layer becomes thin. Lighten, the ink color should be controlled darker in printing. The main reason for this result is that when the ink layer is thick, the number of light refractions is relatively large, so that the reflected light is relatively saturated. On the contrary, when the ink layer is thin, the number of light refractions is reduced, of course, the light exits The saturation will decrease.
During the printing process, the experienced master will slightly adjust the color of the printed product to be darker than the sample color when checking the color. This is out of consideration.
3. PH value of paper The paper is weakly alkaline. The ideal paper PH value should be 7, which is neutral. Due to the improper process of adding caustic soda (NaOH, sulfide, chlorine pulping process in the papermaking process, the paper PH value may be acidic or alkaline). These acid-base properties are mainly derived from the residue of paper pulp.
a) The acidity and alkalinity of the paper have a great influence on the printing process and the color durability of the printed matter.
b) The ink has poor alkali resistance. The chrome yellow ink is faded when encountering alkaline paper, and the medium blue is faded when encountering alkaline paper.
c) Especially gold-silver ink anodized aluminum foil, it will lose its original luster when encountering alkaline substances, the golden color will gradually change to bronze, silver ink will become black, and the adhesion will decrease.
d) The alkalinity of the paper comes from the papermaking process itself and the subsequent binding production. If the binder used in the binding process contains alkaline substances, the alkaline substances will penetrate into the paper.
e) Select raw materials to analyze the PH value and the effect of PH value on the ink (including gold, silver, aluminum).
4. Heat resistance of ink.
The heat resistance of the ink is 120 degrees, the ink is rubbed on the machine, and the temperature will increase (especially when the dry oil is added to the ink, the oxidation polymerization reaction will also emit a lot of heat), which requires the ink to have Very good heat resistance. When printing gold and silver ink, be careful not to stack the prints too high.
Some printed materials require mandatory drying when printing (such as iron printing ink, hose ink, thermosetting ink), or when the printed product needs to be heated for special purposes, the pigment must be able to withstand high temperatures without discoloration.
5. The amount of dry oil
a) The amount of red and white dry oil should not exceed 5%. Generally, it is better to control it at 5%. The color of golden light red ink will become darker when it meets red dry oil.
b) The color of red dry oil itself is red, and the addition of ink will have a certain effect on the hue of the ink, especially when printing light-colored ink.
It is also out of consideration for the influence of the ink color, so the use of dry oil should be cautious in the printing process, and remember not to add it blindly.
6. Discoloration of the printing process
a) UV coating. When the ink is dried, the effect of UV glazing on the color of the ink is not very large, but when the ink at the part of the glazing oil is not completely dry, due to the solubility of the benzene, phenol, alcohol, ether and other components in the UV ink to the ink , Will make the ink change color, or even fall off.
b) Polishing with solvent varnish. Since the solvent content of the solvent-based varnish is 50%, the drying time needs about 2 to 3 minutes, which has a greater influence on the color of the ink during the drying process.
c) Waterborne coating. Generally speaking, the influence of water-based glazing on the ink is not great, but if the glazing starts when the ink is not dry, it will also cause the ink to float.
Before applying UV varnish or solvent-based varnish, apply a layer of water-based varnish on the surface of the ink to make the bottom layer, the effect is more ideal. The function has two points: one is that the water-based varnish can protect the ink from contact with UV varnish and solvent-based varnish; one is that the floating liquid in the water and oil fills the pores of the paper and has adhesion, so that the UV glaze layer is flat And adhere firmly.
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