Analysis on the factors causing the color differen

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Analysis of factors causing the color difference of spot color ink in the printing process

in the printing process, there are many factors causing the color difference of spot color ink production. These factors are discussed below

1. The influence of paper on color

the influence of paper on the color of ink layer is mainly reflected in three aspects

(1) Paper Whiteness: paper with different whiteness (or with a certain color) has different effects on the color appearance of the printing ink layer. For the same kind of white board, the whiteness is different, and the color difference of the printing ink layer is mainly reflected in the amount of black ink in the spot color ink, especially for the color with a lightness of more than 70, 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 influence of paper whiteness on printing color

(2) absorbency: when the same ink is printed on paper with different absorbency under the same conditions, it will have different printing gloss. The structure of the paper determines that when the stress reaches the highest point, there are bumps and pores formed by plant fibers on the paper surface. In order to obtain good uniformity and smoothness on 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 capacities inevitably lead to different colors of the printing ink layer. Compared with coated paper, the black ink layer will appear gray and Matt, and the color ink layer will drift. The color mixed by cyan ink and magenta ink is the most obvious

(3) gloss and smoothness: the gloss of printed matter depends on the gloss and smoothness of the paper. The surface of printing paper belongs to semi gloss surface, especially coated paper. For example, data editing

on color printing, when the light shines on the paper surface at a 45 ° incidence angle, about 4% of the light will be reflected, which is the reflected light on the first surface. The rest of the incident light passes through the ink layer, after selective absorption of the ink, it is reflected through the ink layer, enters the human eye, and is perceived by the human eye. This is the color we observe. If the gloss and smoothness of the paper are high, the reflected light on the first surface is specular reflection, which is not easy to enter the human eye. At this time, the observed color is basically the color reflected through the ink layer. If the paper surface is rough and glossy, the reflected light on the first surface will diffuse. At this time, the color we see is the mixed color produced by the main color light and the reflected light on the first surface. Because it contains white light, the saturation of the main color light is reduced, so people feel that the color becomes lighter when they observe the print. When they measure with a densimeter, the density value decreases and the brightness increases

2. The influence of surface treatment on color

the surface treatment methods of packaging products mainly include film covering (bright film, matte film), glazing (cover bright oil, matte oil, UV light oil), etc. After these surface treatments, prints will have varying degrees of hue changes and color density changes. These changes are 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 has a certain impact on the color density. The color density increases when covered with light film, light oil and UV oil; When covered with Matt Film and Matt oil, the color density decreases. Chemical changes mainly come from film covering glue, UV primer and various organic solvents contained in UV oil, which will change the color of the printing ink layer

3. The effect of diluent on color

diluent is a kind of colorless transparent substance in the form of ointment, which mainly plays the role of diluting color in spot color printing. The influence on hue varies with the amount of diluent added, especially blue

in the process of preparing spot color ink, the shear force and pressure produced by the ink homogenizer and ink expander on the ink are smaller than those on the printing press. It is not necessary to add a diluent when printing, but only when adding a diluent when making a color card with ink, can a uniform spot color ink color display card be made. When the color density is the same, there is a color difference between the color card and the color of the printed matter. This is because the addition of diluent changes the distribution of pigments in the ink, so that the absorption, refraction and reflection of light by the ink have changed, resulting in a color difference, which is caused by the difference of the system

4. The influence of the difference of drying density

for the newly printed print, the ink is still in the wet state, and there is a density difference between it and the dry state. The phenomenon that the wet color density is greater than the dry color density is called the dry fading density phenomenon. 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

since the dry color density can only be measured after 30 ~ 60 minutes of drying after printing, it is difficult to measure and control the spot color density

the densimeter with polarizer device 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, so that the measured density value is not affected by the dry and wet of the ink layer. For coated paper, the measured density difference is 0.05 ~ 0.15, and for non coated paper, the measured density difference is 0.1 ~ 0.2. The color difference of different colors is also different. The difference between yellow is the smallest, black is the largest, and blue and red are between the two. Therefore, when measuring with such a densimeter, the measured value should be appropriately higher than the density value of the standard color sample, so as to play the role of control

5. Influence of system differences

the process of making color cards with ink homogenizer and ink spreader is a "dry printing" process, without the participation of water, while printing is a "wet printing" process, with the participation of wetting liquid. Therefore, in offset printing, the emulsification of ink in water will inevitably occur, which will bring unnecessary losses to the enterprise. The emulsified ink will inevitably produce color differences because it changes the distribution state of pigment particles in the ink layer, The printed products also appear gray and not bright

in addition, the stability of the ink used to prepare the spot color, the thickness of the ink layer, the accuracy of weighing the ink, the old and new differences in the ink supply area of the printer, the speed of the printer, and the amount of water added during printing will also have different effects on the color difference

source: fine chemicals in the 21st century

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