The reasons and factors that lead to the color difference of spot color ink are analyzed

(1) Paper whiteness: Paper with different whiteness (or with a certain color) has different effects on the color appearance of Printing Ink layer. For the same kind of whiteboard paper, the whiteness is different, and the color difference of printing ink layer is mainly reflected in the number of black ink components in spot color ink, especially for colors with lightness above 70, which has a particularly obvious influence, resulting in great difference in the proportion of spot color ink. Therefore, in actual production, paper with the same whiteness should be selected as far as possible to reduce the influence of paper whiteness on printing color.

(2) Absorbability: When the same ink is printed on paper with different absorbability under the same conditions, it will have different printing gloss. The structure of paper determines that there are concavities, convexities and pores formed by plant fibers on the surface of paper. In order to obtain good uniformity and smoothness of paper, it is generally necessary to coat the surface of paper with different thicknesses. The nature and thickness of the coating determine the ink absorption capacity of the paper surface. Different absorption capacity will inevitably make the color of printing ink layer different. Compared with coated paper, the black ink layer of uncoated paper will appear dull and dull, and the color ink layer will drift, especially the color mixed with cyan ink and magenta ink.

(3) Glossy and smoothness: The glossiness of printed matter depends on the glossiness and smoothness of paper. The surface of printing paper belongs to semi-gloss surface, especially coated paper.

In color printed matter, when the light strikes the paper surface at an incident angle of 45, about 4% of the light will be reflected, which is the first surface reflection light. The rest of the incident light passes through the ink layer, is selectively absorbed by the ink, and then reflects through the ink layer, enters the human eye and is perceived by the human eye, which 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 surface of the paper is rough and the gloss is low, the reflected light from the first surface will diffuse, and the color we see at this time is the mixed color produced by the main color light and the reflected light from the first surface. Because it contains white light, it reduces the saturation of the main color light, so people feel that the color becomes lighter when they observe the printed matter, and the density value decreases and the brightness increases when they measure it with a densimeter.

2. Influence of surface treatment on color
The surface treatment methods of packaging products mainly include laminating (bright film, matt film) and glazing (covering bright varnish, matt varnish and UV gloss varnish). After these surface treatments, the printed matter will have different degrees of hue change and color density change. 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 certain influence on color density. When coated with bright film, bright varnish and Uv Varnish, the color density increases; When coated with matt film and covered with matt varnish, the color density decreases. The chemical changes mainly come from various organic solvents contained in laminating adhesive, Uv Primer and UV varnish, which will change the color of printing ink layer.
3. The influence of reducer on color
Reducer is a colorless and transparent substance in the form of ointment, which mainly plays the role of diluting color in spot color printing. The influence of different adding amount of bleacher on hue is different, especially blue.
In the process of preparing spot color ink, the shearing force and pressure produced by ink homogenizer and ink developer are smaller than those produced by printing machine. There is no need to add a thinner when printing, but only by adding a thinner when making a color card with ink can a uniform spot color ink color display card be printed. In the case of the same color density, there is a color difference between the color card and the printed matter. This is because the addition of the thinning agent changes the distribution of pigments in the ink, which changes the absorption, refraction and reflection of light by the ink, resulting in color difference, which is caused by the difference of systems.

4. Influence of dry density difference
The ink of the newly printed product is still in wet state, which has a density difference from that of dry state. The phenomenon that wet color density is greater than dry color density is called 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 in a dry state, the surface reflection is mainly diffuse reflection, and the color is naturally dimmer than when it was just printed.
Because the dry color density can generally be measured after printing and drying for 30 ~ 60 minutes, it brings difficulties to the measurement and control of spot color density.

The densimeter with polarizer can eliminate the light generated by mirror reflection on the surface of ink layer, and the measured wet color density is very close to the dry color density, so that the measured density value is not affected by the wet and dry ink layer. For coated paper, the measured density difference is 0.05 ~ 0.15, and for uncoated paper, the measured density difference is 0.1 ~ 0.2. Different colors have different chromatic aberration, yellow has the smallest difference, black has the largest difference, and blue and red are in between. Therefore, when measuring with such a densitometer, the measured value should be slightly higher than the density value of the standard color sample, so as to play a control role.

5, the influence of system differences
The process of making color cards with an ink homogenizer and an ink developer is a "dry printing" process, without water, while printing is a "wet printing" process, with wetting liquid participating in the printing process.Therefore,emulsifying phenomenon of water-in-oil is bound to occur in offset printing, and the emulsified ink will inevitably produce color difference because it changes the distribution state of pigment particles in the ink layer, and the printed products will appear dull and dull.
In addition, the stability of the ink used for spot color blending, the thickness of the ink layer, the accuracy of weighing the ink, the difference between the old and new ink supply areas of the printing machine, the speed of the printing machine, and the amount of water supplied during printing will also have different effects on the color difference.