Color Difference Analysis of Spot Color Printing of Packaging Products

Spot color ink (Spot Color Ink) is relative to the traditional CMYK four-color ink, refers to one or two or more inks together, and add certain auxiliary materials, formulated into a new ink, and Make it suitable for printing needs and color requirements. The color gamut of spot colors largely exceeds the gamut of RGB and CMYK. Many colors that cannot be represented by CMYK four-color printing inks can be achieved with spot color inks. Spot color printing uses a large area of ​​solid color printing, color effect is good, reducing the color dots due to dot overprint, dot distortion, color stability, wide color gamut. Therefore, in order to achieve special artistic effects, the use of spot color inks is widely used in cigarette labels, wine labels, and packaging products. Spot color printing can not only solve the problem of four-color printing due to overprint and ink balance, but also can meet the quality requirements of innovative design, and the ink balance during the printing process is also easier to control.

There are many factors that produce the color difference of spot color inks, but the main ones are: material properties, ink properties, printing conditions, surface finishing of printed products. These factors can affect the color reproduction in the spot color printing process.

First, the impact of printing materials (paper) on ink color

1. Whiteness of paper

The factors affecting the whiteness of paper are: 1 Whiteness of pulp: the most important influencing factor. 2 Dyes: The dye should be selected so that it does not affect the reflectivity near the dominant wavelength for measuring whiteness, ie it does not affect the whiteness of the paper. 3 filler: depends on the 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 coating amount. Papers with a different degree of whiteness and a certain color (for example, some of the paper is blue and the latter is yellowish) have different effects on the color of the printing ink layer. For the same kind of whiteboard paper, the whiteness is different, especially for colors with higher lightness values, and the ratio of spot inks varies greatly. Therefore, in the actual production should try to use the same white paper printing, in order to reduce the impact of paper whiteness on the printing color.

2. The smoothness and gloss of the paper

From the perspective of the impact on the quality of printed matter, there is a direct relationship between the gloss of the paper and the glossiness of the printed matter. Regardless of the type of ink, the glossiness of the printed matter increases with the glossiness of the paper itself. The gloss of the paper is directly related to the paper's ink efficiency. A paper with a higher gloss has a higher print density than a paper with a low gloss at the same ink film thickness. Therefore, there is a close correlation between the gloss of the printed matter and the gloss of the paper itself.

3. Paper's ink acceptance and absorption properties

Ink-receiving performance refers to the ability of a paper surface to receive transferred ink instantaneously on a press during printing. It is closely related to the three aspects of paper properties: the ability of the paper surface to accept 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's ability to absorb ink occurs from a time when the ink comes into contact with the paper and it completely solidifies on the paper. It absorbs the low-viscosity components of the ink and the components of the ink in the ink. Internal penetration is relevant. Printing pressure, printing time, ink viscosity, paper capillary radius will affect the paper's ability to absorb ink. When the same ink is printed under the same conditions onto papers with different absorbency, there will be different printing densities. There is a porous structure formed on the surface of the paper by anisotropic and heterogeneous complex structural elements. In order to obtain good uniformity and smoothness on the paper surface, it is generally necessary to coat different thicknesses of paint on the surface of the paper. The nature and thickness of the coating determine the absorbability of the ink on the surface of the paper. Absorption capacity is different, resulting in a difference in the color of the printing ink layer.

Second, the nature of the ink on the impact of 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 and dispersion of pigment particles

Although the role of ink film capillary is an important factor in the formation of print gloss, the apparent size of the pigment particles in the dispersed state is more important. It directly determines the state of the ink film capillary. Thus, the pigment particles are small and well dispersed, which is favorable for the formation of a smooth ink film, and is also very advantageous for improving the gloss of prints.

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

According to the interface chemistry principle, the capillary permeation rate is inversely related to the viscosity of the liquid, ie, the permeation rate decreases as the viscosity increases. The capillary network structure formed between ink pigment particles is an important aspect that determines the gloss of prints. At the moment of imprinting, the ink is pressed into the larger pores of the paper as a whole; after imprinting, 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 amount of separation of the binder. The effect of capillary retention of the binder is much greater than the effect of printing pressure on pressing 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 pores that the binder penetrates into the paper, thereby increasing the gloss and smoothness of the ink film. The high gloss of the ink film means that the surface mirror reflection degree is relatively high, so that the human eye can see the surface reflected white light much less than the low gloss ink film in most cases, making the color saturation more high. So high gloss prints look brighter and fuller. The printing ink layer of the spot color ink is mostly semi-transparent or opaque. When selecting the ink formulation, the influence of the light transmittance of the ink layer on the spot color printing must also be taken into consideration.

Third, the impact of the printing process on the color

1. Ink layer thickness and ink balance

The control of 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, a color difference of ΔE=1.5 to 4.5 NBS is caused. Obviously, this effect is significant. Offset ink reproduction and ink balance are closely related. Under the same ink conditions, water and water will affect the depth of color. It is important to properly understand the ink-water balance and pay particular attention to the pH of the wetting fluid. The principle of water consumption is to keep the amount of water supplied as small as possible when it is not dirty.

2. Printing pressure

The size of the printing pressure has a great influence on the degree of transfer of ink from the printing plate to the paper. When the printing pressure is insufficient, the ink is not transferred enough and is not suitable for printing; when the printing pressure is too high, due to the printing plate The spread of ink to blank parts other than those in the graphic and graphic text shows that the ink transfer rate has not been improved, and it has also brought other ills. It is also not suitable for printing. Only in the appropriate range of printing pressure can print printed materials with thick ink layers, clear images, rich gradations, and good color reproducibility.

3. Printing speed

As the printing speed increases, the contact time between the printing surfaces will be shortened, the dots will become imaginary, the ink transfer rate will decrease, and the printing quality will decline. 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 in order to obtain a stable ink layer thickness.

4. Dry back density difference

On the same paper, the proof density just printed is higher, and after a certain period of time, the proof density decreases as the ink's dry smoothness decreases. Due to the different density values ​​of the ink layer before and after drying, the color tone of the printed image is also different. For the freshly printed product, the ink is still wet and has a difference from the dry state. This is because the ink layer just printed has a certain leveling property, so the surface reflection is dominated by specular reflection, which looks bright and shiny. When the ink layer is in a dry state, the reflection of the surface is dominated by diffuse reflection, and the color is naturally dull and darker than when it was printed. Therefore, the general print proofs are slightly darker than the standard proofs. However, how much deeper is the use of data control? When proofing, determine the density values ​​of various colors that have just been printed. When printing, refer to these density values ​​for printing. The color of the ink is nearly the same. The densitometer with polarizing filter can eliminate the light produced by the specular reflection on the surface of the ink layer. The measured wet color density and the dry color density are very close, and the measured density value is affected little by the wet and dryness of the ink layer.

Fourth, the impact of surface finishing on the color of printed matter

Surface finishing is the proper processing of the surface of the printed matter to improve the light resistance, heat resistance, water resistance, abrasion resistance, folding resistance, and chemical resistance of the surface of the printed matter; increase the gloss and artistic sense of the printed matter; and protect the printed matter And beautify the role of print, improve the value of print. The surface finishing methods of packaging products mainly include coating (printing bright oil, coating cover bright oil, matte oil, UV varnish, waterborne varnish, etc.), waxing, laminating (bright film, matt film) , embossing, bronzing and so on. After these surfaces are processed by the printed matter, there will be variations 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 mirror surface and diffuse reflectance of the product. In general, the color density increases when the bright film is coated, the bright gloss oil and the UV varnish are applied, and when the matt film is coated and the matte oil is coated, the color density decreases. The chemical changes mainly come from the organic solvents contained in the coating glue, the water-based varnish, and the UV varnish. They combine with the ink particles on the print to change the color of the ink layer.

V. Conclusion

In short, in the packaging and printing, the reasons that lead to color differences in spot colors are various. It is necessary to analyze specific problems in practical production for different reasons, try to minimize the deviations, and take corresponding measures for specific reasons. To make customer satisfied packaging products. As long as the proper measures are taken during color matching and printing, the color can be controlled better. Mainly to the deployment of spot colors and the printing process of standardized, data management, while maintaining a relatively stable production environment.