Color difference analysis of spot color printing i

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Color difference analysis of spot color ink printing for packaging products spot color ink is a new ink prepared by mixing one or more kinds of inks together and adding certain auxiliary materials to meet the printing needs and color requirements, which is relative to the traditional CMYK four-color ink. The spot color gamut largely exceeds the gamut range of RGB and CMYK. Many colors that cannot be presented by CMYK four-color printing inks can be realized with spot color inks. Spot color printing uses a large area of field 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 in cigarette labels, wine labels and packaging products 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 easy to control

there are many factors that cause the color difference of spot color printing ink, but they mainly include: material performance, ink properties, printing conditions, and 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

the factors affecting paper whiteness are: ① whiteness of pulp: the most important factor. ② Dye: select the dye so that it does not affect the reflectivity near the main wavelength for measuring whiteness, that is, it does not affect the whiteness of the paper. ③ Filler: depending on its type and grade. Yicheng Xinneng announced that ④ coating: the whiteness of coated printing paper is determined by the whiteness of base paper, coating whiteness and coating amount. The paper with different whiteness and certain color (such as some paper is blue and the latter is yellow) has 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 higher lightness value, which has a significant 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 glossiness of paper

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, the glossiness of printed matter increases with the improvement of the glossiness 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 acceptance performance and absorption performance of paper

ink acceptance performance refers to the ability of the paper surface to absorb all creatinine produced by a person weighing 60 kg within 1 hour when it is imprinted on the printer during the printing process, and then receive and transfer ink. 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 the ink and the paper to the complete solidification on the paper surface. It is related to the absorption of low viscosity components in the ink by the paper pores and the penetration of some components in the ink into the paper. 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, it will have different printing density. The paper surface has a pore structure formed by anisotropic multiphase complex structural elements. 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 determines the ink absorption capacity of the paper surface. Different absorption capacity leads to different color of printing ink layer

II. Influence of ink properties on color

the final effect of printed matter is closely related to the physical and chemical properties of the ink. The composition of the ink determines the properties of the 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 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. Viscosity of binder and pigment content

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 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 separation amount 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

rapid drying of the ink film on the paper surface can reduce the amount of connecting material penetrating into the paper pores, thus improving the gloss and smoothness of the ink film. The high gloss of the ink film means that the specular reflection degree of the surface is relatively high. In most cases, the human eye can see that the white light reflected by the surface is much less than that of the low gloss ink film, which makes the color saturation higher. Therefore, high gloss prints look more colorful and full. The printing ink layer of spot color ink is mostly translucent or opaque. When selecting the ink formula, the influence of the light transmittance of the ink layer on spot color printing should also be considered 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 realized 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 balance of ink and water. Under the same ink volume, large water and small water will affect the color depth. It is very important to correctly master the ink water balance and pay special attention to the pH value of the wetting solution. The principle of water consumption is to use as little water supply as possible in case of no dirt

2. Printing pressure

the printing pressure has a great impact on the degree of ink transfer from the printing plate to the paper and does not coincide with the main needle. When the printing pressure is insufficient, the ink transfer is insufficient and is 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 within the appropriate printing pressure range can we print a print with thick ink layer, clear image, rich tone and good color reproducibility

3. Printing speed

with the increase of printing speed, the contact time between printing surfaces will become shorter, the dot will become empty, the ink transfer rate will be reduced, and the printing quality will be reduced. 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. When a newly printed print uses a conduit, the ink is still in the wet state, and there is a difference between the wet state 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 glossy. When the ink layer is dry, the surface reflection is mainly diffuse reflection, and the color is naturally dimmer than when it was just printed. Therefore, generally, the ink color of the printed sample sheet is slightly deeper than that of the standard sample sheet, but the actual depth should be controlled by data. During proofing, the density values of each color just printed should be measured. During printing, the ink color of the two samples can be close to the same by referring to these density values

the densitometer 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 is little affected by the dry and wet ink layer

IV. The Influence of surface finishing on the color of printed matter

surface finishing is the appropriate processing of the surface of printed matter to improve the light resistance, heat resistance, water resistance, wear resistance, folding resistance and chemical resistance of the surface of printed matter; 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, printed matter will have different degrees of hue and saturation changes, that is, density differences. These changes can be divided into physical changes and chemical changes. Physical changes are mainly reflected in the increase of specular and diffuse reflection capabilities on the product surface. Generally speaking, the color density increases when covering the bright film, coating the bright oil and UV light 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 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 a word, there are various reasons for the color difference of spot color in packaging and printing. 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 proper measures are taken in the process of color matching and printing, its color can be well controlled. It mainly manages the spot color allocation and the printing process in a standardized and data-based manner, while maintaining a relatively stable production environment

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