Color Difference Analysis of Special Color Ink Printing of Packaging Products

Spot color ink (Spot Color Ink) is relative to the traditional CMYK four-color ink, which means that one or more than two inks are blended together, and certain auxiliary materials are added to prepare a new ink, and 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. Many colors that CMYK four-color printing inks cannot display can be achieved with spot color inks. Spot color printing uses large-area solid color blocks to print colors. The color effect is good, reducing the color difference caused by dot overprinting and dot deformation, stable color, and wide color gamut. 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 water-ink balance, but also meet the quality requirements of innovative design, and the water-ink balance in the printing process is relatively easy to control.

There are many factors that produce spot color ink printing chromatic aberration, but mainly include: material properties, ink properties, printing conditions, surface finish of printed matter. These factors will affect the color reproduction in the spot color printing process.

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

1. Paper whiteness

The factors that affect the whiteness of paper are: â‘  The whiteness of pulp: the most important influencing factor. â‘¡Dye: Select the dye so that it does not affect its reflectivity near the main wavelength of measuring whiteness, that is, it does not affect the whiteness of the paper. â‘¢ Filler: depends on its type and grade. â‘£Paint: The whiteness of coated printing paper is determined by the whiteness of base paper, whiteness of coating and coating amount. Papers with different whiteness and a certain color (such as blue paper or yellow paper) 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 a higher lightness value, the effect is particularly obvious, resulting in a large difference in the ratio of spot color inks. Therefore, in actual production, paper with the same whiteness should be used for printing to reduce the influence of whiteness on the printing color.

2. Smoothness and gloss of paper

From the point of view of the impact on the quality of printing, there is a very direct relationship between the gloss of the paper and the gloss of the printed matter. Regardless of the type of ink, the gloss of the printed matter increases as the gloss of the paper increases. The gloss of the paper is directly related to the inking efficiency of the paper. A paper with a high gloss can obtain a higher printing density at the same ink film thickness as a paper with a lower gloss. Therefore, there is a close correlation between the gloss of the printed matter and the gloss of the paper itself.

3. Ink acceptance performance and absorption performance of paper

Ink acceptance performance refers to the ability of the surface of the paper to receive transferred ink at the moment of imprinting on the press during the printing process. It is closely related to the three aspects of paper performance: the ability of the paper surface to receive 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 paper's ability to absorb ink occurs over a long period of time from the contact of the ink with the paper until it is fully cured on the surface of the paper. It absorbs the low-viscosity components in the ink and certain components of the ink to the paper Related to internal osmosis. 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 absorption under the same conditions, there will be different printing densities. There is a pore structure formed by anisotropic and multi-phase complex structural elements on the paper surface. In order to obtain good uniformity and smoothness on the paper surface, coatings of different thicknesses are generally applied on the paper surface. The nature and thickness of the coating determine the ink absorption capacity of the paper surface. The different absorption capabilities result in differences in the color of the printed ink layer.

Second, the effect of ink properties on color

The final effect of the printed matter 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 pigment particles and their degree of dispersion

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

2. The viscosity of the connecting material and the content of the pigment

According to the principle of interface chemistry, the capillary penetration rate is inversely related to the viscosity of the liquid, that is, the penetration 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 printed matter. At the moment of imprinting, the ink is pressed into the larger pores of the paper as a whole; after imprinting, the connecting material starts 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 connecting material. The capillary retains the connecting material much more than the printing pressure presses the connecting material into the paper pores.

3. Drying time

The rapid drying of the ink film on the paper surface can reduce the amount of the connection material penetrating into the paper pores, thereby improving the gloss and smoothness of the ink film. The high gloss of the ink film means that the surface has a relatively high degree of specular reflection, so that in most cases, the human eye can see that the white light reflected by the surface is much less than the ink film with low gloss, which makes the color saturation more high. Therefore, the printed products with high gloss look more colorful and full. The printing ink layer of spot color ink is mostly translucent or opaque. When selecting the ink formulation, the influence of the transparency of the ink layer on the spot color printing should also be considered according to the actual situation.

Third, the impact of the printing process on color

1. Ink layer thickness and ink balance

The control of the ink hue in the printing process is mainly achieved 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 color difference of ΔE = 1.5 to 4.5 NBS. Obviously, this effect is great. The ink color reproduction of offset printing is closely related to the balance of water and ink. Under the same ink volume, large water and small water will affect the color depth. Correctly mastering the balance of water and ink, it is very important to pay special attention to the PH value of the wetting fluid. The principle of water consumption is to use as little water 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 the ink is insufficient and it is not suitable for printing; when the printing pressure is too large, due to the Ink spreads to blank areas other than pictures and text, and the ink transfer rate is not only not improved, but also brings other disadvantages, and is also not suitable for printing. Only within the proper printing pressure range can the printed matter with thick ink layer, clear image, rich tone and good color reproducibility be printed.

3. Printing speed

Increasing the printing speed will shorten the contact time between the printing surfaces, make dots false, reduce the ink transfer rate, and reduce the printing quality. 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. Poor dry density

The density of the sample just printed on the same paper is higher, and the density of the sample after a period of time will decrease as the drying smoothness of the ink decreases. Because the density values ​​of the ink layer before and after drying are different, the hue of the printed image is also different. The ink just printed is still wet, and there is a difference between it and dry state. This is because the ink layer just printed has a certain leveling, so the surface reflection is mainly specular reflection, which looks bright and shiny. When the ink layer is in a dry state, the surface reflection is mainly diffuse reflection, and the color is naturally dull and dull than when it was just printed. Therefore, the ink color of the general printed sample is slightly darker than the standard sample, but how much deeper should be controlled by data. During the proofing, the density value of each color just printed is measured. During printing, referring to these density values ​​to print, you can make two The ink colors of these people are nearly the same. The density meter with polarizing color 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.

4. The effect of surface finishing of printed matter on color

Surface finishing is to properly process the surface of the printed matter to improve the light resistance, heat resistance, water resistance, abrasion resistance, folding resistance and chemical resistance of the printed matter surface; increase the gloss and artistic sense of the printed matter; protect the printed matter And beautify the role of printed matter, increase the value of printed matter. The surface finishing methods of packaging products mainly include glazing (printed varnish, coating hood varnish, matt varnish, UV varnish, water-based varnish, etc.), waxing, and laminating (glossy film, matt film) , Embossing, stamping, etc. After these surface treatments, the printed matter will have different degrees of hue and saturation changes, that is, differences in density. These changes can be divided into physical changes and chemical changes. The physical changes are mainly reflected in the increased specular and diffuse reflection capabilities on the product surface. Generally speaking, the color density increases when the gloss film is coated, coated with varnish and UV varnish; the color density decreases when the matte film is coated and coated with matte varnish. The chemical changes mainly come from the organic solvents contained in the film 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 and printing, there are various reasons for the color difference of spot colors. It is necessary to analyze the specific problems of different causes in actual production, try to control the deviation to the minimum range, and take corresponding countermeasures for the specific reasons. 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.

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