Analysis of the cause of printing color difference of packaging spot color ink
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
Spot Color Ink is a relatively new CMYK four-color ink. 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 largely beyond the gamut range of RGB and CMYK. Many colors that CMYK four-color printing ink cannot display can be realized with spot color ink. Because the spot color printing uses large-area 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.
Third, the impact of the printing process on color
1. Ink layer thickness and ink balance
Control of the hue of the ink during printing is primarily achieved by controlling the thickness of the ink layer. When the thickness of the ink layer changes by about 0.1 μm, a chromatic aberration of ΔE = 1.5 to 4.5 NBS is caused, and this effect is apparently large. The ink color reproduction of offset printing is closely related to the ink balance. Under the same amount of ink, the small water and water will affect the color depth. Properly mastering the ink balance, it is important to pay special attention to the pH of the dampening fluid. The principle of the amount of water used is that the amount of water supplied is as small as possible.
2, printing pressure
The size of the printing pressure has a great influence on the extent to which the ink is transferred 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 printing plate The ink spreads to the blank part outside the graphic, and the ink transfer rate is not improved, but also brings other disadvantages, and is also unsuitable for printing. Prints with thick ink layers, sharp images, rich tone and good color reproducibility can be printed only within the appropriate printing pressure range.
3, printing speed
The increase in printing speed will shorten the contact time between printing surfaces, the dot will be imaginary, the ink transfer rate will be lowered, and the printing quality will be degraded. In order to ensure sufficient 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 back density difference
On the same paper, the density of the sample just printed is higher, and after a period of time, the density value decreases as the drying smoothness of the ink decreases. The printed image exhibits a different hue due to the difference in density values before and after drying of the ink layer. The ink that has just been printed, the ink is still in a wet state, and there is a difference from the dry state. This is because the ink layer just printed has a certain leveling property, so the surface reflection is mainly specular reflection, which looks bright and lustrous. When the ink layer is in a dry state, the surface reflection is mainly diffuse reflection, and the color is naturally duller than when it is just printed. Therefore, the ink color of the general printed proof is slightly deeper than the standard proof, but how much depth should be used for data control. When proofing, the density values of the newly printed colors are measured. When printing, refer to these density values for printing, so that two The ink color is close to the same. A densitometer with a polarizing filter can eliminate the light generated by the specular reflection on the surface of the ink layer. The measured wet density is very close to the dry density, and the measured density value is little affected by the wet and dry of the ink layer.
Fourth, the effect of surface finishing of printed matter on color
The 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 surface; increase the gloss and artistic sense of the printed matter; And beautify the role of printed matter to enhance the value of printed matter. The surface finishing methods of packaging products mainly include glazing (printing varnish, coating varnish, matt oil, UV varnish, water varnish, etc.), waxing, filming (bright film, matt film) , bump embossing, bronzing, etc. After the surface treatment of the printed matter, there will be varying degrees of change in hue and saturation, ie, density. These changes can be divided into physical and chemical changes. Physical changes are mainly reflected in the increased specular and diffuse reflectivity of the product surface. In general, when the light film is coated, the varnish is coated with the varnish, and the color density is increased; when the matte film is coated and the matte oil is applied, the color density is lowered. The chemical changes mainly come from the organic solvents contained in the film glue, water-based varnish and UV varnish, which combine with the ink particles on the printed matter to change the color of the ink layer.
Fifth. Conclusion
In short, in packaging and printing, there are various reasons for the chromatic aberration caused by the spot color. It is necessary to analyze the specific problems for different reasons in actual production, try to control the deviation to the minimum range, and take corresponding countermeasures for specific reasons. Packaging products that satisfy customers' satisfaction. As long as the measures are properly done during the color matching and printing process, the color can be better controlled. Mainly for the deployment of spot colors and standardization and data management of the printing process, while keeping the production environment relatively stable.