Analysis of discoloration and elimination of printed matter
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In the printing process or in the use of printed products after printing, whether the ink color on the surface of the printed matter can be kept bright or not discolored is also an important indicator of the printing quality of the printed matter.
The reasons for the fading and discoloration of various printing products are mainly the performance problems of the ink, and also the problem that the printing ink process is not properly arranged. Below we will talk about some factors that affect the discoloration of ink from six aspects.
1. The ink fades because it is not resistant to light.
Light fastness is an important indicator for evaluating the properties of inks. Many inks will change color when exposed to sunlight. The light resistance of printing inks depends mainly on the pigments used in the inks. If the pigment used undergoes a chemical reaction or a crystal transformation under the action of light, discoloration or fading of the ink may occur. The general experience in production has the following points.
a) Light color inks fade after long-term exposure to light, discoloration (Y, M, G), dark ink (C, B, K) is slow.
b) Use inks with good light resistance when adjusting the ink.
c) When lightening the color ink, you should pay attention to the light resistance after diluting.
d) In outdoor advertising, attention should be paid to the lightfastness of the ink.
e) Phthalocyanine blue is lighter than pale lake blue and peacock blue.
f) Gray can be obtained by adding black ink and phthalocyanine blue to the white ink.
g) Green color can be used in monochrome phthalocyanine green (adding white ink color can be more beautiful), if it is not yellow, you 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.), consider the color sequence arrangement of printing. Try to print Y and M first and then print C and Bk. Because C and Bk have better light resistance, they will have a certain blocking effect on the light fading of inks with poor light resistance.
2. Discoloration during ink drying:
In the printing process, the ink that has just been printed is darker in color. After a while, the ink will fade after drying. Mainly due to the penetration of ink in the drying process and the cause of oxidative drying, especially the ink based on osmosis drying (such as printed rotary ink), the thickness of the ink layer changes after drying, the ink layer becomes thinner. Lighter, the ink should be controlled deeper in printing. The main reason for this result is that when the ink layer is thick, the number of times of light refraction is relatively large, so that the reflected light is relatively saturated. On the contrary, when the ink layer is relatively thin, the number of times of light refraction is reduced, and of course, the light is emitted. The saturation will decrease.
When the experienced master in the printing process corrects the color, the color of the printed product will be slightly adjusted to be darker than the color of the sample. This is for this reason.
3. Paper PH value
The paper is weakly alkaline. The ideal paper should have a pH of 7 and be neutral. Due to the need to add caustic soda during the papermaking process (the process of NaOH, sulfide, or chlorine pulping is improper, it is possible that the pH of the paper is either acidic or alkaline). These acid-base properties are mainly derived from the residue of the papermaking slurry.
a) The acidity and alkalinity of the paper has a great influence on the color durability of the printing process and printed matter.
b) The alkali resistance of the ink is relatively poor. The chrome yellow ink is fading in the case of a large alkaline paper, and the medium blue is fading in the case of a large alkaline paper.
c) Especially the gold and silver inks are gilded with aluminum. When exposed to alkaline substances, the original luster will be lost. The golden color will gradually become bronze, the silver ink will turn black, and the adhesion will decrease.
d) The alkalinity of the paper comes from the papermaking process itself and the subsequent binding. If the binder used in the binding process contains alkaline substances, the alkaline substance will seep into the paper.
e) Select the raw material to analyze the pH value and the effect of pH on the ink (including gold, silver, aluminum).
4. The heat resistance of the ink.
The heat resistance of the ink is 120 degrees, the ink is rubbed on the machine, and the temperature is increased (especially when the dry oil is added to the ink, the oxidative polymerization also emits a large amount of heat), which requires the ink to have Very good heat resistance. When printing gold and silver ink, be careful not to stack the printed product too high.
Some prints require mandatory drying (such as tin ink, hose ink, thermosetting ink) when printing, or when the printed product needs to be heated for special purposes, the paint 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 controlled at 5%, and the color of red light ink will be darkened when it encounters red dry oil.
b) The color of the red dry oil itself is red. Adding ink will have a certain effect on the hue of the ink, especially when printing light ink.
Also for the consideration of the influence of ink color, the use of dry oil should be cautious in the printing process, remember not to blindly add more.
6. Discoloration of the glazing process of printed matter
a) UV glazing. When the ink is dried, the effect of UV glazing on the ink color is not very large, but when the ink in the contact portion of the varnish is not completely dried, the benzene, phenol, alcohol, ether and other components in the UV ink dissolve the ink. Will cause the ink to change color or even fall off.
b) Solvent-based varnish glazing. Since the solvent content of the solvent type varnish is 50%, the drying time takes about 2 to 3 minutes, and the color of the ink is relatively large during the drying process.
c) Waterborne glazing. Generally speaking, the effect of water glazing on the ink is not very large, but when the ink is not dried, it will also cause the ink to float.
Before the UV varnish or the solvent varnish, the surface of the ink is first coated with a layer of water-based varnish, and the effect is ideal. There are two functions: First, the water-based varnish can protect the ink from contact with UV varnish and solvent-type varnish; first, the floating liquid in the water and oil fills the micropores of the paper and has adhesiveness, so that the UV glazing layer is flattened. And it is firmly attached.