Caused by the color difference of the spot color ink during the printing process
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In the printing process, there are many factors that cause the color difference of the spot color ink to be produced. These factors are discussed separately below.
1. The effect of paper on color
The influence of paper on the color of the ink layer is mainly reflected in three aspects.
(1) Paper whiteness: Papers with different whiteness (or a certain color) have different effects on the color of the printed ink layer. For the same kind of whiteboard paper, the whiteness is different. The color difference of the printing ink layer is mainly reflected in the black ink component in the spot color ink, especially for the color with a brightness of 70 or more, the effect is particularly obvious, resulting in the spot color ink. The ratio varies greatly. Therefore, in actual production, paper with the same whiteness should be used as much as possible to reduce the influence of paper whiteness on printing color.
(2) Absorptivity: When the same ink is printed on paper with different absorbency under the same conditions, it will have different printing luster. The structure of the paper determines the presence of irregularities and pores formed by plant fibers on the surface of the paper. In order to achieve good uniformity and smoothness of the paper surface, it is common to apply different thicknesses of paint 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 absorption capacity necessitates a difference in the color of the printed ink layer. Non-coated paper Compared with coated paper, the black ink layer will appear gray and dull, and the color ink layer will drift, and the color of the cyan ink and magenta ink will be the most obvious.
(3) Gloss and smoothness: The gloss of a print depends on the gloss and smoothness of the paper. The surface of the printing paper is a semi-gloss surface, especially coated paper.
In color prints, when light is incident on the surface of the paper at an incident angle of 45°, about 4% of the light is reflected off. This is the first layer of reflected light. The remaining incident light passes through the ink layer and is selectively absorbed by the ink, and then reflected through the ink layer to enter the human eye and be perceived by the human eye. This is the color we observe. If the gloss and smoothness of the paper is high, the reflected light on the surface of the first layer is specularly reflected and does not easily enter the human eye. The color observed at this time 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 surface of the first layer will be diffusely reflected. At this time, the color we see is the mixed color of the main color light and the reflected light from the surface of the first layer. Since the white light component is contained therein, the saturation of the main color light is lowered, so that the color is perceived to be light when the printed product is observed, and the density value is lowered and the brightness is increased when measured by a densitometer.
2. The effect of surface treatment on color
The surface treatment methods of packaging products mainly include film (bright film, matt film), glazing (cover varnish, matt oil, UV varnish). After these surfaces are treated, the prints will have varying degrees of hue change and color density change. These changes are divided into physical and chemical changes. The physical changes are mainly reflected in the increase of specular reflection and diffuse reflection on the surface of the product, which has a certain influence on the color density. When the light film, the varnish and the UV oil are covered, the color density increases; when the matte film or the matt oil is applied, the color density is lowered. The chemical changes mainly come from various organic solvents contained in the coating rubber, UV base oil and UV oil, which change the color of the printing ink layer.
3. The effect of the fading agent on color
The lightening agent is a colorless transparent substance in the form of a paste, which plays a role in coloring in the spot color printing. The amount of the fading agent added is different, and the effect on the hue is also different, especially the blue color is the most obvious.
In the process of preparing the spot color ink, the ink and the ink spreader produce less shear force and pressure on the ink than on the printing machine. It is not necessary to add a light-removing agent during printing, but only when a color-reducing ink is dispensed with a light-removing agent can a uniform spot color ink color display card be produced. In the case of the same color density, the color of the color card and the printed matter is chromatic aberration. This is because the addition of the etchant changes the distribution state of the pigment in the ink, so that the absorption, refraction and reflection of the ink occur. The change, and thus the chromatic aberration, which is caused by the difference in the system.
4. The effect of the difference in dry recoil density
Just printed out, the ink is still in a wet state, and there is a density difference with the dry state. The phenomenon that the wet color density is greater than the dry color density is called the dry back density phenomenon. 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.
Since the dry color density is generally measured after drying for 30 to 60 minutes, this makes it difficult to measure and control the spot color density.
The densitometer with the polarizer device can eliminate the light generated by the specular reflection on the surface of the ink layer, and the measured wet density is very close to the dry density, so that the measured density value is not affected by the wet and dry of the ink layer. For coated paper, the difference in measured density is 0.05 to 0.15, and the difference in measured density of non-coated paper is 0.1 to 0.2. The color difference is different for different colors, the yellow difference is the smallest, the black is the largest, and the blue and red are somewhere in between. Therefore, when measured with such a densitometer, the measured value should be appropriately higher than the density value of the standard color sample in order to play a controlling role.
5. The impact of system differences
The process of making the color card with the ink meter and the ink stick is a “dry printing” process, without water participation, and the printing is a “wet printing” process, and the wetting liquid participates in the printing process, so the ink must be generated in the offset printing. The emulsification phenomenon of water inclusion, the emulsified ink changes the distribution state of the pigment particles in the ink layer, and the color difference is inevitably produced, and the printed product appears to be dark and not bright.
In addition, the stability of the ink used to match the spot color, the thickness of the ink layer, the accuracy of the weighing ink, the old and new differences in the ink supply area of the printing press, the speed of the printing machine, and the amount of water applied during printing will also have different chromatic aberrations. Impact.