The color of the self-adhesive label may become darker or lighter. The standard for high-quality printed matter is: 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 hue and ink intensity of the sample. There are many factors that affect the color change of printed matter, roughly in the following aspects.

Post-press quality inspection

1. The ink is not resistant to light discoloration and fading

The color and brightness of the ink will change to different degrees under sunlight. There is no ink that is absolutely light-resistant and does not change the hue. Under the strong sunlight, the color of all inks will change to different degrees. This change can be divided into two types.

Fading: Under the action of sunlight and ultraviolet rays, the ink has poor light resistance, loses the original bright color, and the color becomes pale and gray. Especially for light-colored inks and four-color plates, the yellow and red colors fade faster, while cyan and black fade slowly.

Discoloration: Contrary to the fading of the ink color of the self-adhesive printed matter, the color changes darker and darker under the action of sunlight, and the color also changes. People call this kind of change discoloration.

2. The influence of emulsification on the color of the imprint

Offset printing plates are inseparable from wetting the blank part of the printing plate with a wetting fluid. Offset printing is watered first and then inked. Emulsification is unavoidable with water. After the ink is emulsified, the color will lighten, but the original color can be restored after the water evaporates. Therefore, the greater the water, the greater the amount of emulsification, which will cause discoloration. In particular, color and ink with completely different emulsification degrees are mixed together, and the phenomenon of discoloration is particularly prominent.

3. The relationship between the nature of paper and imprint copying

The surface smoothness of the paper. The smoothness of the paper surface is closely related to the imprint copying. The uneven paper surface often requires a lot of pressure to make the ink have good contact with it. For example, the ink viscosity, fluidity, and ink layer thickness are maintained at a certain amount. Increasing the pressure often increases the spreading area of ​​the print. At the same time, the concave parts of the paper are still in poor contact, such as: coated paper and newsprint printed on the same printing plate If the imprinting effect is quite different, the different copying effects can be clearly compared.

Absorbency of paper. The absorbency of the paper is also directly related to the copying effect. Usually, when the self-adhesive printing paper with loose texture, if the ink fluidity is large, the viscosity is small, the paper will absorb more ink layer binder, if the capillary The diameter of the hole is larger than the diameter of the pigment particles, so even the pigment will be sucked in, which reduces the saturation of the print. Need to appropriately increase the thickness of the ink layer. However, increasing the thickness of the ink layer will cause “spreading” at the moment of imprinting, which will affect the effect of print copying. Paper with low absorption can make most of the ink film appear on the surface of the paper, so that the imprinted ink layer has better saturation.

4.the impact of paper permeability on color

The high 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 part of the pigment particles will be penetrated into the paper at the same time, so the color and luster will have a lightening feeling. For this reason, use paper with rough surface and loose texture, and paper with high ink fluidity, pay attention to discoloration.

5.the influence of the heat intolerance of the pigment

In the drying process of the ink, the bright and fast-drying offset printing ink is mainly dried by oxidized conjunctiva. Before the offset printing ink is dried, there is a fixation stage. The oxidative polymerization of the ink is an exothermic reaction. If it dries too fast, it will release a lot of heat. If the heat is dissipated slowly, it will change the color of the heat-labile pigment. For example: the golden color ink becomes darker and loses its original luster.

During printing, the printed sheets are stacked in piles on the delivery table. Due to the excessive stacking, the printing ink in the middle part of the printed sheet is oxidized and polymerized to exothermic reaction, and the heat is not easy to dissipate. If the temperature is too high, the middle part discolors more.

6. The effect of dry oil on the discoloration of imprints

Light color inks are cool colors, light yellow, emerald green, turquoise and other intermediate color inks, do not use red dry oil, because red dry oil itself has a darker purple-red color, which will affect the color of light color ink.

The white dry oil looks white, but it is light brown after oxidizing the conjunctiva. If the amount of white dry oil is large, it may become yellowish-brown after the blot is dried, and the dark color inks such as blue, black, and purple are dried with red The oil color will not be greatly affected.

7. the impact of the ink’s non-alkali resistance on the color of the print

The pH value of the self-adhesive printing paper is 7, and the neutral paper is the most ideal. Generally, the ink made of inorganic pigments is relatively poor in acid and alkali resistance, and the organic pigments have better acid and alkali resistance, especially medium blue and dark blue inks. It will fade in contact with alkali. The color of the medium yellow turns into reddish when it meets alkali. The hot-stamped electrochemical aluminum foil and printed gold will turn into pale yellow when it meets alkaline substances. Business card production) When encountering alkaline adhesives, if the packaging and decoration printing products are packaging alkaline substances such as soap, soap, washing powder, etc., the alkali resistance and saponification resistance of the ink must be considered, and the peacock blue ink is resistant Alkaline is better.

8.the storage of self-adhesive printed matter and the discoloration factors during use

Most self-adhesive printing products will inevitably turn yellow when stored for a long time. The reasons are as follows.

The fiber in the paper contains a lot of lignin and changes color. For example, newspapers printed on newsprint are the easiest to turn yellow and become brittle.

Offset four-color dot overprinting of color self-adhesive printing products, most of which are exposed to sunlight, weather, wind and rain, outdoor high temperature erosion, etc., because the pigments are not light and heat resistant, and discolor or fade.