In the process of decorative paper printing, there are many factors that lead to the color difference of ink production. The following are the discussion of these factors。
1.Effect of paper on color
The influence of paper on ink layer color is mainly reflected in three aspects.
(1) Paper whiteness: Paper with different whiteness (or with certain color) has different influence on the color appearance of printing ink layer. For the same decorative base paper, the whiteness is different, and the color difference of the printing ink layer is mainly reflected in the black ink composition of the pulp, especially for the color with a brightness of more than 70, which results in a great difference in the color ink ratio. Therefore, in order to reduce the influence of paper whiteness on printing color, the paper with the same whiteness should be selected in actual production.
(2) Absorption：The same ink under the same conditions printed on different absorbent paper, there will be different printing gloss. The structure of paper determines the existence of concave convex and pore formed by plant fiber on the surface of paper. In order to obtain good uniformity and smoothness of paper surface, different thickness of coating should be coated on the paper surface. The properties and thickness of the coating determine the absorption capacity of the paper surface to the ink. Different absorptive capacity makes the color of printing ink layer different. Compared with the coated paper, the black ink layer of the non-coated paper will be gray and lusterless, and the color ink layer will drift, the color of the blue ink and magenta ink is the most obvious.
(3) Glossiness and smoothness: The glossiness of decorative paper depends on the glossiness and smoothness of the paper. The surface of printing paper belongs to semi-gloss surface, especially coating paper.
In color printing, when the light hits the paper surface at 45 ° incidence angle, about 4% of the light will be reflected off, which is the first layer of surface reflected light. The rest of the incident light passes through the ink layer, after the selective absorption of the ink, it is reflected through the ink layer, enters the human eye, and is perceived by the human eye, which is the color we observe.
If the glossiness and smoothness of the paper are high, the reflected light on the first surface is specular reflection, which is not easy to enter the human eye. The color observed at this time is basically the color reflected through the ink layer.
If the surface of the paper is rough and glossy, the reflected light on the first surface will diffuse. At this time, the color we see is a mixture of the main color light and the reflected light of the first layer surface. Because it contains white light component, so it reduces the saturation of the main color light, so when people observe the finished decorative paper, they feel that the color becomes lighter. When using densitometer to measure, the density value decreases and the lightness increases.
2.Effect of surface treatment on color
The surface treatment methods of printed decorative paper mainly include dip coating (melamine, urea formaldehyde glue), polishing (gloss oil, matte oil, 宝丽,华丽, PU, UV varnish), etc. Printed decorative paper after these surface treatments, there will be varying degrees of color changes and color density changes. These changes are divided into physical changes and chemical changes. The physical change is mainly reflected in the increase of specular reflection and diffuse reflection on the surface of the product, which has a certain impact on the color density. Chemical changes mainly come from a variety of organic solvents contained in these glues, which will change the color of the printing ink layer.
3.Effect of diluent on color
Diluent is a kind of colorless transparent substance in the form of oil paste, which mainly plays the role of diluting color in decorative paper printing. The effect of different diluents on the hue is also different, especially the blue color.
4.Effect of dry regression density difference on color
Just printed decorative paper, ink is still in the wet state, and there is a density difference between the dry state. The phenomenon that the wet color density is greater than the dry color density is called the dry fading density phenomenon. This is because the newly printed ink layer has levelling property, so the surface reflection is mainly specular reflection, which looks bright and glossy.
When the ink layer is in a dry state, the surface reflection is mainly diffuse reflection, and the color is naturally dimmer than when it is just printed.
It is difficult to measure and control the color density of decorative paper because the dry color density can not be measured until 30-60 minutes after printing.
The density meter with polarizer can eliminate the light produced by specular reflection on the ink surface. The measured wet color density is very close to the dry color density, so the measured density is not affected by the dry and wet ink layer.
For coated paper, the measured density difference is 0.05~0.15, and that of non-coated paper is 0.1~0.2. The chromatic aberration of different colors is also different, yellow is the smallest, black is the largest, blue and red are between the two. Therefore, when measuring with such a densitometer, the measured value should be higher than the density value of the standard color sample, so as to play a control role.
5.Effect of water difference on color
In the water-based decorative paper printing, because of the participation of water, it is a “wet printing” process. And in the “wet printing” process, the PH of water is required.
Therefore, the emulsification of ink and water is inevitable in the printing of decorative paper. The emulsified ink changes the distribution of pigment particles in the ink layer, it is inevitable to produce color difference, so the printed products will appear dark and not bright.
In addition, the stability of the ink used in the preparation of the paste, the thickness of the ink layer, the accuracy of the ink viscosity, the difference between the old and the new ink supply part of the printing machine, the speed of the printing machine, and the water supply during printing will also have different effects on the color difference.