Color difference analysis of the most popular spot

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Color difference analysis of packaging spot color ink printing

spot color ink is a new ink prepared by mixing one or more kinds of inks together and adding certain auxiliary materials, which can meet the printing needs and color requirements, compared with the traditional CMYK four-color ink. Spot color performance gamut largely exceeds the performance gamut range of RGB and CMYK. Many colors that cannot be presented by CMYK four-color printing inks can be achieved with spot color inks. Spot color printing uses a large area of on-site color blocks to print color, which has good color effect, reduces the color difference caused by dot overprint and dot deformation, has stable color and wide color gamut. Therefore, spot color inks are widely used to print cigarette labels, wine labels and packaging products in order to achieve special artistic effects. Spot color printing can not only solve the problems of overprint and ink balance in four-color printing, 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 produce the printing color difference of spot color ink, but the main ones are: the performance of materials, the nature of ink, printing conditions, and the surface finishing of printed matter. These factors will affect the color reproduction in the process of spot color printing

I. Influence of printing materials (paper) on ink color

1. Paper whiteness

factors affecting paper whiteness include: ① whiteness of slurry: the most important influencing factor. ② Dye: select the dye so that its reflectivity will not be affected near the main wavelength of whiteness measurement, that is, the whiteness of the paper will not be affected. ③ Filler: depending on its type and grade. ④ Coating: the whiteness of coated printing paper is determined by the whiteness of base paper, coating whiteness and coating amount. Papers with different whiteness and certain colors (such as some papers are blue and the latter is yellow) have different effects on the color presentation of the printing ink layer. For the same kind of white board, the whiteness is different, especially for the color with high lightness value, which has a particularly obvious impact, resulting in a great difference in the proportion of spot color ink. Therefore, paper with the same whiteness should be selected for printing in actual production to reduce the impact of paper whiteness on printing color

2. Smoothness and gloss of paper

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

3. Ink receiving performance and absorption performance of paper

ink receiving performance refers to the ability of the paper surface to receive and transfer ink instantly during the printing process. It is closely related to three aspects of paper performance: the ability of the paper surface to accept the wetting of printing ink; 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 performance of paper occurs in a long time from the contact between ink and paper to complete solidification on the paper surface. It is related to the absorption of low viscosity components in ink by paper pores and the penetration of some components in ink into paper. Printing pressure, printing time, ink viscosity, paper capillary radius will affect the paper's ability to absorb ink. When the same ink is printed on paper with different absorbency under the same conditions, it will have different printing density. There is a pore structure formed by anisotropic polyphase complex structural elements on the paper surface. In order to obtain good uniformity and smoothness of the paper surface, it is generally necessary to coat the paper surface with coatings of different thickness. The nature and thickness of the coating determine the ink absorption capacity of the paper surface. Different absorption capacity leads to different colors of the printing ink layer

second, the influence of ink properties on color

the final effect of printing is closely related to the physical and chemical properties of printing ink. The composition of printing ink determines the properties of printing ink:

1. The size of pigment particles and their dispersion degree

although the role of ink film capillary is an important factor in forming the gloss of printing products, 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 prints

2. Viscosity of binder and content of pigment

according to the principle of interface chemistry, capillary penetration rate is inversely related to liquid viscosity, that is, the penetration rate decreases with the increase of viscosity. The capillary complex structure formed between ink and pigment particles is an important aspect to determine the gloss of printed matter. At the moment of imprinting, the ink is pressed into the larger pores of the paper as a whole; After embossing, the connecting material 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 separation amount of the connecting material. The capillary retains the connecting material, which is much more effective than the printing pressure to press the connecting material into the paper pores, and the wall part will produce a great negative wind pressure

3. Drying time

rapid drying of the ink film on the paper can reduce the amount of connecting 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 specular reflection degree of the high-end surface is relatively high, so that the human eye can see that the white light reflected by the surface is much less than that of the low gloss ink film in most cases, making the color saturation higher. Therefore, prints with high gloss look brighter and fuller. The printing ink layer of spot color ink is mostly translucent or opaque. When selecting the ink formula, we should also consider the influence of the light transmittance of the ink layer on spot color printing according to the actual situation

III. Influence of printing process on color

1. Ink layer thickness and water ink balance

the control of ink hue in the printing process is mainly achieved by controlling the thickness of ink layer. When the thickness of the ink layer changes by about 0.1 M, it will cause a color difference of △ e=1.5 ~ 4.5nbs. Obviously, this effect is great. The ink reproduction of offset printing is closely related to the ink balance. Under the same ink volume, large water and small water will affect the color depth. It is very important to correctly master the balance of ink and water, and pay special attention to the pH value of the wetting liquid. The principle of water consumption is to use as little water supply as possible when it is not dirty

2. Printing pressure

the size of printing pressure has a great impact on the degree of ink transfer from the printing plate to the paper. When the printing pressure is insufficient, the transfer of ink is insufficient and not suitable for printing; When the printing pressure is too high, because the ink on the printing plate spreads to the blank part outside the picture and text, the ink transfer rate is not only not improved, but also brings other disadvantages, which is also not suitable for printing. Only in the appropriate range of printing pressure can we print a print with thick ink layer, clear image, rich tone and good color reproducibility

3. Printing speed

the increase of printing speed will shorten the contact time between printing surfaces, make the dot appear 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. Dry and fade density difference

for the same paper, the density of the newly printed sample is higher, while after a period of time, the density value will decrease as the drying smoothness of the ink decreases. Due to the different density values of the ink layer before and after drying, the hue of the printed image is also different. For newly printed products, the ink is still in the wet state, and there is a difference between it and 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 and shiny. When the ink layer is dry, the surface reflection is mainly diffuse reflection, and the color naturally looks dimmer than when it was just printed. Therefore, generally, the ink color of the printed sample is slightly deeper than that of the standard sample, but the exact depth should be controlled by data. When proofing, measure the density values of each color just printed. When printing, refer to these density values to print, so that the ink color of both can be close to the same. The densimeter with polarization filter can eliminate the light generated by 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 very little affected by the dry and wet ink layer

IV. The Influence of printing surface finishing on color

surface finishing is the appropriate processing of the surface of printing to improve the light resistance, heat resistance, water resistance, wear resistance, folding resistance and chemical resistance of the surface of printing; Increase the luster and artistic sense of the printed matter; Play a role in protecting and beautifying printed matter, and improve the value of printed matter. The surface finishing methods of packaging products mainly include glazing (printing gloss oil, coating gloss oil, matte oil, UV gloss oil, water-based gloss oil, etc.), waxing, coating (gloss film, matte film), concave convex embossing, bronzing, etc. After these surface treatments, the printed matter will have different degrees of hue and saturation changes, that is, the difference of density. The unidirectional loading fatigue experiment can be divided into pulsating fatigue experiment and fluctuating fatigue experiment. 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 on the product surface, which is the safe installation and reflection ability to avoid chaos. Generally speaking, the color density increases when covering gloss film, coating gloss oil and UV gloss oil; When the matte 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 film covering 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 a word, in packaging and printing, there are various reasons for the color difference of spot color. It is necessary to analyze the specific problems of different reasons in actual production, try to control the deviation within the minimum range, and take corresponding countermeasures for specific reasons to produce packaging products that satisfy customers. As long as we take proper measures in the process of color matching and printing, we can better control its color. It mainly deals with the allocation of spot colors and the standardized and data-based management of the printing process, while maintaining a relatively stable production environment

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