Analysis of the reasons for the color difference of spot color Printing ink

Spot color ink (Spot Color Ink) is relative to the traditional CMYK four-color ink. It refers to a new ink prepared by blending one or more than two inks and adding certain auxiliary materials. Make it suitable for printing needs and color requirements. The color gamut of spot color performance is largely beyond the range of RGB and CMYK color gamuts. Many colors that cannot be represented by CMYK four-color printing inks can be realized with spot color inks. Because spot color printing uses large-area solid color blocks to print colors, the color effect is good, and the color difference caused by dot overprinting and dot deformation is reduced. The color is stable and the color gamut is wide. Therefore, in order to achieve special artistic effects in cigarette labels, wine labels, and packaging products, spot color inks are widely used for printing. Spot color printing can not only solve the problems of four-color printing due to overprinting and ink balance, but also meet the quality requirements of innovative design, and the ink balance in the printing process is also relatively easy to control.

There are many factors that cause color difference in spot color ink printing, but the main ones are: material performance, ink properties, printing conditions, and surface finishing of printed matter. These factors will affect the color reproduction in the spot color printing process.

1. The influence of the printing material (paper) on the color of the ink

1. Whiteness of paper

The factors that affect the whiteness of paper are: ①The whiteness of pulp: an important influencing factor. ②Dye: The dye should be selected so that it does not affect its reflectivity near the dominant wavelength for measuring whiteness, that is, it does not affect the whiteness of paper. ③ Filler: depends on its type and grade. ④ Coating: The whiteness of the coated printing paper is determined by the whiteness of the base paper, the whiteness of the paint, and the amount of coating. Different whiteness and paper with a certain color (for example, some paper is bluish and the latter is yellowish), have different effects on the color of the printing ink layer. For the same white board paper, the whiteness is different, especially for the color with a higher lightness value, the impact is particularly obvious, resulting in a large difference in the ratio of the spot color ink. Therefore, in actual production, paper with the same whiteness should be selected for printing as much as possible to reduce the influence of paper whiteness on printing color.

2. Paper smoothness and gloss

From the perspective of the impact on the quality of printed matter, there is a very direct relationship between the glossiness of paper and the glossiness of printed matter. No matter what type of ink is used, the glossiness of printed matter increases with the glossiness of the paper itself. The glossiness of the paper is directly related to the ink absorption efficiency of the paper. The paper with high gloss can obtain higher printing density than the paper with low gloss on the paper surface with the same ink film thickness. Therefore, there is a close correlation between the gloss of printed matter and the gloss of the paper itself.

3. Ink acceptance and absorption properties of paper

Ink acceptance refers to the ability of the surface of the paper to receive the transferred ink at the moment of embossing on the printing press during the printing process. It is closely related to three aspects of paper performance: the ability of the paper surface to accept printing ink wetting; the ability of the paper surface to absorb certain ink components; the ability of the paper surface to fix and retain a uniform ink film. The ink absorption of paper occurs in a long period of time from the contact between the ink and the paper to the complete curing on the paper surface. It is related to the absorption of the low viscosity components in the ink by the pores of the paper and the absorption of some components in the ink to the paper. pertaining to endosmosis. Printing pressure, printing time, ink viscosity, and paper capillary radius all affect the paper's ability to absorb ink. When the same ink is printed on paper with different absorbency under the same conditions, there will be different printing densities. There is a pore structure formed by anisotropic multi-phase complex structural elements on the surface of paper. In order to obtain good uniformity and smoothness of the paper surface, it is generally necessary to coat the surface of the paper with different thicknesses of paint. The nature and thickness of the coating determines the ability of the paper surface to absorb ink. The difference in absorption capacity leads to differences in the color of the printed ink layer.

2. The influence of the properties of the ink on the color

The final effect of the printed matter is closely related to the physical and chemical properties of the ink, and the components of the ink determine the properties of the ink:

1. The size and dispersion of pigment particles

Although the role of ink film capillary is an important factor in forming the gloss of printed matter, the apparent size of pigment particles in the dispersed state is more important, which directly determines the state of ink film capillary.

Therefore, the pigment particles are small and well dispersed, which is conducive to the formation of a smooth ink film and is also very beneficial to improving the gloss of printed products.

2. The viscosity of the binder and the content of the pigment

According to the principle of interface chemistry, the capillary permeation velocity is inversely related to the liquid viscosity, that is, the permeation velocity decreases with the increase of viscosity. The capillary network structure formed between ink pigment particles is an important aspect that determines the gloss of printed matter. At the moment of embossing, the ink is pressed into the larger pores of the paper as a whole; after embossing, the binder begins to separate from the ink and penetrates into the smaller pores of the paper. Therefore, the size of the ink film capillary determines the separation amount of the binder. The role of capillary retention of the binder is much greater than that of printing pressure to press the binder into the pores of the paper.

3. Drying time

The rapid drying of the ink film on the paper surface can reduce the amount of the binder penetrating into the pores of the paper, thereby improving the gloss and smoothness of the ink film. The high gloss of the ink film means that the degree of specular reflection on the surface is relatively high, so that in most cases, the human eye can see that the white light reflected by the surface is much less than that of the ink film with low gloss, making the color saturation more high. Therefore, printed products with high gloss look more vivid and full. Most of the printing ink layers of spot color inks are translucent or opaque. When selecting ink formulations, the influence of the light transmittance of the ink layer on spot color printing should also be considered according to the actual situation.

3. The influence of the printing process on the color

1. Ink layer thickness and ink balance

The control of the ink hue in the printing process is mainly realized by controlling the thickness of the ink layer. When the thickness of the ink layer changes by about 0.1 μm, it will cause a chromatic aberration of △E=1.5-4.5NBS. Obviously, this effect is very large. The ink color reproduction of offset printing is closely related to the balance of ink and water. In the case of the same amount of ink, large water and small water will affect the depth of color. There is no need to repeat the importance of correctly grasping the balance of water and ink, and paying special attention to the pH value of the wetting liquid. The principle of water consumption is to use as little water supply as possible without getting dirty.

2. Printing pressure

The size of the printing pressure has a great influence on the degree of ink transfer from the printing plate to the paper. When the printing pressure is insufficient, the transfer of ink is not sufficient and it is not suitable for printing; when the printing pressure is too high, due to the ink on the printing plate The ink spreads to the blank part other than the graphic, and the ink transfer rate is not only not improved, but also brings other disadvantages, which is also not suitable for printing. Only within the appropriate printing pressure range can the printed matter with thick ink layer, clear image, rich tone and good color reproducibility be printed.

3. Printing speed

The increase of the printing speed will shorten the contact time between the printing surfaces, the dots will become weak, the ink transfer rate will decrease, and the printing quality will decrease. In order to ensure the full contact of the printing surface and improve the printing quality, it is required to control the printing speed under a certain printing pressure to obtain a stable ink layer thickness.

4. Difference in drying density

For the same paper, the sample just printed has a higher density, but after a period of time, the density value of the sample will decrease as the ink dries and the smoothness decreases. Due to the different density values ​​before and after the ink layer is dried, the color tone of the printed image is also different. For the printed matter just printed, the ink is still in a wet state, and there is a difference from the dry state. This is because the newly printed ink layer has a certain leveling property, so the surface reflection is mainly specular reflection, which looks bright in color and good in gloss. When the ink layer is in a dry state, the surface reflection is dominated by diffuse reflection, and the color is naturally duller than when it was just printed. Therefore, the ink color of the general printing proof is slightly darker than the standard proof, but how much is darker should be controlled digitally. When proofing, measure the density value of each color just printed. When printing, refer to these density values ​​to print. The ink color of the two is close to the same.

A densitometer with a polarizing filter can eliminate the light generated by the specular reflection on the surface of the ink layer. The measured wet color density is very close to the dry color density, and the measured density value is little affected by the dryness of the ink layer.

4. The influence of the surface finishing of the printed matter on the color

Surface finishing is to properly process the surface of the printed matter to improve the light resistance, heat resistance, water resistance, wear resistance, folding resistance, and chemical resistance of the printed matter; to increase the gloss and artistic sense of the printed matter; to protect the printed matter And beautify the role of printed matter, improve the value of printed matter. The surface finishing methods of packaging products mainly include glazing (printing varnish, coating varnish, matt oil, UV varnish, water-based varnish, etc.), waxing, and lamination (glossy film, matte film) , embossing, bronzing, etc. After the printed matter undergoes these surface treatments, there will be varying degrees of changes in hue and saturation, that is, differences in density. These changes can be divided into physical changes and chemical changes. Physical changes are mainly reflected in the increase of specular reflection and diffuse reflection capabilities on the surface of the product. Generally speaking, when the glossy film is applied, the gloss oil and UV varnish are applied, the color density increases; when the matte film is applied, and the matte oil is applied, the color density decreases. The chemical changes mainly come from the organic solvents contained in the coating glue, water-based varnish, and UV varnish, which will combine with the ink particles on the printed matter to change the color of the ink layer.

V. Conclusion

In short, in packaging printing, there are various reasons for the color difference of spot colors. It is necessary to analyze the specific problems of different reasons in actual production, try to control the deviation in the minimum range, and take corresponding countermeasures for specific reasons. Production Produce packaging products that satisfy customers. As long as proper measures are taken in the color matching and printing process, the color can be better controlled. It mainly regulates the allocation of spot colors and standardizes and digitizes the printing process, while keeping the production environment relatively stable.