The color change of printed matter
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In the process of printing and storing, the hue, brightness and saturation 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. May fade. The high-quality quantity of printed matter is measured as follows: the ink color of a batch of products should be consistent before and after, and the color should be bright, and at the same time, it should conform to the ink color hue and the darkness of the ink color of the sample. There are many factors that affect the color change of printed matter, and there are roughly the following aspects.
First, the ink is not resistant to light discoloration and fading
Under the illumination of sunlight, the color and brightness of the ink will change to different degrees. There is no ink that is absolutely resistant to light without changing the color tone. Under the strong sunlight, the color of all inks will change to different degrees. This change can be divided into two types.
1. Fading: Under the action of ultraviolet light from the sun, the ink has poor light resistance, and the original bright color is lost, and the color becomes light gray. Especially for light-colored inks and four-color plates, the yellow and red colors fade faster, while the cyan and ink colors fade more slowly.
2. Discoloration: Contrary to the black fading of printed ink, the color changes dark under the influence of sunlight, and the color changes. People call this change discoloration.
Second, the effect of emulsification on the color of the print
Offset printing plates are inseparable from the use of dampening fluid to wet the blank part of the printing plate. Offset printing is first applied to the water after the water is applied, and the water is inevitably emulsified. After the ink is emulsified, the color will be lightened. However, after the water evaporates, the original color can be restored. Therefore, the larger the moisture, the larger the emulsified amount, which causes discoloration. In particular, the color inks with completely different degrees of emulsification are brought together, and the phenomenon of discoloration is particularly prominent.
Third, the relationship between the nature of paper and imprinting
1. The surface smoothness of the paper. The smoothness of the surface of the paper is closely related to the reproduction of the print. The uneven paper surface often requires a large pressure to make the ink have good contact with it. If the ink viscosity, fluidity, and thickness of the ink layer are kept constant, increasing the pressure often increases the area of the print spread. At the same time, the low concave portion of the paper is still in poor contact. For example, coated paper and newsprint are printed on the same plate. The effect of the blotting is greatly different, and the different copying effects can be clearly compared.
2. Paper absorbency. The absorption of paper is also directly related to the reproduction effect. Usually, when printing loose paper, if the ink has large fluidity and low viscosity, the paper will absorb more ink layer connecting materials, if the smuggling of capillary holes Above the diameter of the pigment particles, the pigment is also inhaled, which reduces the saturation of the print. It is necessary to appropriately increase the thickness of the ink layer. However, increasing the thickness of the ink layer causes "spreading" at the moment of imprinting, which affects the effect of imprinting. The paper with small absorption enables the majority of the ink film to appear on the surface of the paper, so that the imprinted ink layer has good saturation.
Fourth, the impact of paper permeability on color
The large permeability of the paper will reduce the thickness of the ink layer, and the pores on the surface of the paper will be large, and some of the pigment particles will be infiltrated into the paper at the same time, so that the color has a feeling of lightening. For this reason, it is necessary to pay attention to discoloration when using a paper having a rough surface and a loose texture, and a paper having a large ink flowability.
Fifth, the effect of the pigment is not heat resistant
In the drying process of the ink, the bright and fast drying type printing ink is mainly based on oxidized conjunctival type drying. There is a fixation phase before the offset 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 dissipated slowly, the heat-resistant pigment will be discolored. For example: the golden light color ink darkens and loses its original luster.
In the printing, the stacking of the sheets on the delivery table is too large, and the stacking ink in the middle part is oxidized and polymerized to exothermic reaction, the heat is not easily dissipated, and the temperature is too high to discolor in the middle part.
Sixth, Effect of dry oil on the discoloration of imprint
Light color inks are cool colors, light yellow, emerald green, lake blue and other color inks, do not use red dry oil, because the red dry oil itself has a deep purple red, it will affect the color of the light color ink.
White dry oil looks white, but it is light brown after oxidized conjunctiva. If the amount of white dry oil is too much, the blot will have a yellowish brown color, while the dark color inks such as cyan, black and purple will be dry. The color of the oil will not be greatly affected.
Seventh, the effect of ink alkali resistance on the color of the imprint
The printed paper has a PH value of 7, and a neutral paper is ideal. The inks produced by general inorganic pigments are poor in acid and alkali resistance, and the organic pigments are resistant to acid and alkali, especially in medium blue and dark blue inks. It will fade. In the case of medium yellow, the color of the alkali becomes reddish. The hot-printed electro-aluminized aluminum foil and the gold-plated material will become pale yellow when exposed to alkaline substances, and the paper will be weakly alkaline and often have a lot of paper in the printing. Alkali-containing adhesives, if the packaging and decoration printing products are packaged alkaline substances such as soap, soap, washing powder, etc., the alkali and saponification resistance of the ink should be considered, and the peacock blue ink has good alkali resistance.
Eighth, Discoloration factors during the storage and use of printed materials
Most printed products are stored for a long time, which is inevitably yellowish, for the following reasons.
1. The fibers in the paper contain more lignin and discolor. For example, newspapers printed on newsprint are most likely to become yellow and crisp.
2. Offset four-color dot-printed color printing products, most of which are exposed to sunlight, for a long time, wind and rain, outdoor high temperature erosion, etc., because the pigment is not resistant to light, heat, and discoloration or fading.