In the process of printing and storage, the hue, lightness and saturation of the printed matter often change. Whether it is a single-color ink or two or more colors of ink, the color may become deeper under the influence of internal and external colors. May fade. The standard measure of high-quality prints is: the darkness of a batch of products should be consistent before and after, and the colors should be the same, and at the same time, they should be consistent with the ink hue of the proofs and the darkness of the ink. There are many factors that affect the color change of printed materials, and there are roughly the following aspects.
Discoloration and discoloration of inks that are not lightfast
Under the irradiation of sunlight, the color and brightness of the ink will change in different degrees. The ink that is absolutely light-resistant without changing the color tone is not. Under the strong sunlight, the colors of all inks will have different degrees of change. This change can be divided into two types.
1. Fading: Under the effect of sunlight and ultraviolet rays, the ink is poor in light resistance and loses its original bright color. The color becomes lighter and grayish. In particular, the color of yellow and red fades faster when light colors of ink and four-color cover printing are used, while cyan and ink fade more slowly.
2. Discoloration: In contrast to black fades in print inks, the color changes dark and the color changes during daylight. People call this change color change.
Effect of emulsification on the color of the imprint
Offset printing plates can not be separated from wetting the blank part of the printing plate with the wetting fluid. Offset printing is the first time after the water is applied, and water will inevitably appear emulsified phenomenon. After the ink emulsification, the color will be lightened, but it can restore the original color after evaporation of water, so the greater the moisture, the greater the emulsification, it will cause discoloration. In particular, inks with completely different degrees of emulsification are mixed together, and the phenomenon of discoloration is particularly prominent.
The relationship between paper properties and imprinting
1. The surface smoothness of the paper. The smoothness of the paper surface is closely related to the copying of the prints, and the unsmooth paper surface often requires a large pressure in order for the ink to have a good contact with it. For example, ink viscosity, fluidity, and thickness of the ink layer are kept in a certain amount. Increasing the pressure often increases the footprint area of ​​the imprint. At the same time, the low-lying areas of the paper are still in poor contact. For example, coated paper and newsprint are printed on the same printing plate. There are large differences in the imprinting effect, which can clearly contrast different reproduction effects.
2. Absorption of paper. The size of the paper's absorbency is also directly related to the effect of copying. Usually when printing loose texture paper, if the ink has large flowability and low viscosity, the paper will absorb more ink layer connecting material if the capillary hole is smuggled. Larger than the diameter of the pigment particles, then even the pigment can be inhaled, which reduces the saturation of the print. Need to properly increase the ink thickness. However, increasing the thickness of the ink layer will cause “spreading†at the moment of imprinting and affect the copying effect of the print. The absorptive paper can make most of the ink film appear on the surface of the paper, so that the ink layer has better saturation.
The effect of paper's permeability on color
The large permeability of the paper will make the thickness of the ink layer thinner, and the pores on the surface of the paper will be large, and some of the pigment particles will also be penetrated into the paper at the same time, so that the color has a feeling of lightening. For this reason, papers with a rough surface and a loose texture should be used, and papers with large ink flow should be noticed for discoloration.
Effect of pigments not heat-resistant
In the drying process of the ink, the bright fast-drying type printing ink is mainly based on the oxidized conjunctiva type drying. There is a fixing phase before the offset printing ink is dried. The oxidative polymerization of the ink is an exothermic reaction. When the drying is too fast, a lot of heat is released. If the heat is emitted slowly, the thermolabile pigment will be discolored. For example: gold color inks darken and lose their original luster.
During the printing, the sheet piled on the delivery table is piled up. Due to the excessive stacking, the ink in the middle part of the sheet ink oxidizes and exothermally reacts, heat is not easily dissipated, and the temperature is too high in the middle part.
Effect of Dry Oil on Imprint Discoloration
The light color ink belongs to the cold color, yellow, green, lake blue and other color ink, do not use red dry oil, because the red dry oil itself with a deep purple, will affect the color of lighter color ink.
White dry oil looks white, but after the oxide conjunctiva is light brown, if the amount of white dry oil is more, the imprinted dried cocoon will have the possibility of yellowish brown, while the dark-colored ink such as blue, black, purple with red dry Oil color will not be affected much.
The Effect of Insensitivity of the Ink on the Color of the Imprinting
Printed paper PH = 7, neutral paper is the best, generally inorganic pigments produced by the ink acid, alkali is relatively poor, organic pigments are better acid and alkali, especially in the blue, dark blue ink met alkali Will fade. The yellow color in the case of alkali becomes reddish, and the hot stamped electro-aluminum foil and printed gold will become pale yellow when it encounters alkaline substances. There is no gloss, and the paper is often weak and alkaline, and it is encountered in the late printing of binding. Alkaline adhesives, if the packaging and decorating printing products are packaging alkaline substances such as soap, soap, washing powder and other substances, we must consider the ink's alkali resistance, saponification resistance, better malachite blue ink alkali resistance.
Discoloration factors in the storage and use of printed materials
Most printing products will be yellowish for a long period of time due to the following reasons.
1. Fibers in paper contain many lignin and change color. For example, newspapers printed on newsprint are most likely to turn yellow and crisp.
2. Offset four-color dot-printing color printing products, most of the sun exposure, long days, wind and rain, outdoor high-temperature erosion, etc., because the pigment is not light, heat, and discoloration or fading.