Color difference analysis of packaging spot color ink printing
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Spot color ink is a relatively new ink for four-color inks. It refers to a new ink blended with one or two kinds of inks and added with certain auxiliary materials. Adapt to the printing needs and color requirements. The color gamut of the spot color is far beyond the range of color gamut of rgb and cmyk. Many colors that can not be displayed by cmyk four-color printing ink can be realized with spot color ink. Because the spot color printing uses a large area of solid color block printing color, 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 tobacco labels, wine labels and packaging products, special color inks are widely used for printing. Spot color printing can not only solve the four-color printing problems due to overprinting and ink balance, but also achieve the quality requirements of innovative design, and the ink-and-wash balance in the printing process is relatively easy to control.
There are many factors that produce the color difference of the spot color ink, but mainly include: the properties of the material, the nature of the ink, the printing conditions, and the surface finish of the printed matter. These factors can affect the color reproduction during the spot color printing process.
First, the influence of printing materials (paper) on the color of ink
1, paper whiteness
The factors affecting the whiteness of the paper are: 1 Whiteness of the slurry: the most important factor. 2 Dyes: The dye should be selected so that it does not affect its reflectance near the dominant wavelength of whiteness measurement, that is, it does not affect the whiteness of the paper. 3 filler: depending on its type and grade. 4 Coating: The whiteness of the coating printing paper is determined by the whiteness of the base paper, the whiteness of the coating, and the amount of coating. Papers with different whiteness and a certain color (if some paper is blueish and yellowish) have different effects on the color of the printed ink layer. For the same kind of whiteboard paper, the whiteness is different, especially for the color with higher brightness value, the effect is particularly obvious, and the ratio of the spot color ink is very different. Therefore, in the actual production, paper with the same whiteness should be used as much as possible to reduce the influence of paper whiteness on the printing color.
2, the smoothness and gloss of the paper
From the impact on the quality of printed matter, there is a very direct relationship between the gloss of the paper and the gloss of the print. Regardless of the type of ink, the gloss of the print increases with the gloss of the paper itself. The gloss of the paper is directly related to the inking efficiency of the paper. Paper with a high glossiness can achieve a higher printing density when the paper with a lower glossiness has the same ink film thickness on the paper surface. Therefore, there is a close correlation between the gloss of the print and the gloss of the paper itself.
3, paper ink acceptance and absorption performance
Ink acceptance performance refers to the ability of a paper surface to receive transferred ink as it is imprinted on a press during printing. It is closely related to the three aspects of paper performance: the ability of the paper surface to withstand the wetting of the printing ink; the ability of the paper surface to absorb certain ink components; the ability of the paper surface to hold and retain a uniform ink film. The paper-to-ink absorption performance occurs over a long period of time from ink to paper contact to complete solidification on the paper surface. It absorbs the low-viscosity components of the ink with the capillary pores of the paper and some components of the ink. Internal osmosis is related. Printing pressure, printing time, ink viscosity, and paper capillary radius all affect the paper's ability to absorb ink. The same ink will have different printing densities when printed on paper with different absorbency under the same conditions. There are pore structures formed on the surface of the paper by anisotropic multi-phase complex structural elements. In order to obtain good uniformity and smoothness of the paper surface, coatings of different thicknesses are generally applied on the surface of the paper. The nature and thickness of the coating determine the ability of the paper surface to absorb ink. The difference in absorbency results in a difference in the color of the printed ink layer.
Second, the effect of the nature of the ink on the color
The final effect of the print is closely related to the physical and chemical properties of the ink. The composition of the ink determines the nature of the ink:
1, the size of the pigment particles and the degree of dispersion
Although the role of the ink film capillary is an important factor in forming the gloss of the print, the apparent size of the pigment particles in the dispersed state is more important, which directly determines the state of the capillary of the ink film. Therefore, the pigment particles are small and well dispersed, which is advantageous for forming a smooth ink film, and is also very advantageous for improving the gloss of the printed matter.
2. The viscosity of the binder and the content of the pigment
According to the interface chemistry, the capillary permeation rate is inversely related to the viscosity of the liquid, that is, the permeation rate decreases as the viscosity increases. The capillary network structure formed between the ink pigment particles is an important aspect that determines the gloss of the print. At the moment of imprinting, the ink is integrally pressed into the larger pores of the paper; after imprinting, the binder begins to separate from the ink and penetrate into the smaller pores of the paper. Therefore, the size of the ink film capillary determines the amount of separation of the binder. Capillary retention of the binder is much more effective than 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 which penetrates into the paper pores, thereby improving the gloss and smoothness of the ink film. The high gloss of the ink film means that the specular reflection of the surface is relatively high, so that the human eye can see that the white light reflected by the surface is much less than the ink film with low glossiness in most cases, so that the saturation of the color is more high. Therefore, the print with high gloss looks more vivid and full. The printing ink layer of the spot color ink is mostly translucent or opaque. When selecting the ink formula, the influence of the light transmittance of the ink layer on the spot color printing should be considered according to the actual situation.