In the printing process, there are many factors that cause the color difference of spot color ink production, and these factors are discussed separately below.
1. The effect of paper on color
The effect of paper on the color of the ink layer is mainly reflected in three aspects.
(1) Paper whiteness: Papers with different whiteness (or with a certain color) have different effects on the color of the printing ink layer. For the same kind of whiteboard paper, 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 brightness above 70, the effect is particularly obvious, causing the spot color ink The ratio varies greatly. Therefore, in actual production, paper with the same whiteness should be selected for printing to reduce the influence of paper whiteness on the printing color.
(2) Absorption: When the same ink is printed on paper with different absorption under the same conditions, it will have different printing gloss. The structure of the paper determines that there are irregularities and pores formed by plant fibers on the surface of the paper. In order to obtain good uniformity and smoothness on 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 ink absorption capacity of the paper surface. The different absorption capacity will inevitably make the color of the printing ink layer different. Compared with uncoated paper, the black ink layer will appear dull and dull, and the color ink layer will drift, the color performance of cyan ink and magenta ink is the most obvious.
(3) Glossiness and smoothness: The glossiness of the printed matter depends on the glossiness and smoothness of the paper. The surface of printing paper is a semi-gloss surface, especially coated paper.
On color prints, when light strikes the surface of the paper at an incident angle of 45 °, about 4% of the light is reflected off. This is the first-layer surface reflection. The remaining incident light passes through the ink layer, after being selectively absorbed by the ink, and then 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 surface of the first layer is specularly reflected, which is not easy to 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 on the surface of the first layer will be diffusely reflected. At this time, the color we see is the mixed color produced by the main color light and the reflected light on the surface of the first layer. Because it contains white light components, the saturation of the main color light is reduced, so people feel that the color becomes lighter when observing the printed product, the density value decreases when the density meter is measured, and the brightness increases.
2. The effect of surface treatment on color
The surface treatment methods of packaging products mainly include coating (glossy film, matt film), glazing (covering gloss oil, matt oil, UV gloss oil), etc. 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. The physical change is mainly reflected in the increase of specular reflection and diffuse reflection on the surface of the product, which has a certain effect on the color density. When coated with gloss film, covered with gloss oil and UV oil, the color density increases; when covered with matte film, covered with gloss oil, the color density decreases. The chemical changes mainly come from a variety of organic solvents contained in the laminating adhesive, UV primer, and UV oil, which will change the color of the printing ink layer.
3. The effect of lightener on color
The desalting agent is a colorless and transparent substance in the form of ointment, which mainly plays the role of diluting the color in the spot color printing. The amount of desalting agent added is different, and the effect on the hue is also different, especially blue is the most obvious.
In the process of preparing spot color ink, the shearing force and pressure of the ink leveler and ink spreader on the ink are smaller than those on the printing press. There is no need to add a desalting agent during printing, but when adding a desalting agent to make a color card with ink, an even spot color ink display card can be played. 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 lightening agent changes the distribution state of the pigment in the ink, so that the ink absorbs, refracts and reflects the light. Changes, resulting in color difference, this color difference is caused by the difference in the system.
4. The impact of the difference in dry back density
The ink just printed is still wet and has a density difference from the dry state. The phenomenon that the wet color density is greater than the dry color density is called the dry-out density phenomenon. This is because the ink layer just printed has a certain leveling, 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 dull and dull than when it was just printed.
Because the dry color density is generally measured after printing is dried for 30 to 60 minutes, this brings difficulties to the measurement and control of the spot color density.
A density meter with a polarizer device can eliminate the light generated by the 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 ink layer. For coated paper, the measured density difference is 0.05 to 0.15, and for uncoated paper, the measured density difference is 0.1 to 0.2. Different colors have different chromatic aberrations, 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 higher than the density value of the standard color sample, so that it can play a controlling role.
5. Impact of system differences
The process of making a color card with a leveling ink meter and an ink spreader is a "dry printing" process, without water participation, and printing is a "wet printing" process, and wetting liquid is involved in the printing process, so in the offset printing ink must inevitably occur The emulsification phenomenon of water encapsulation, because the emulsified ink changes the distribution state of the pigment particles in the ink layer, it will inevitably produce color difference, and the printed product will appear dull and not bright.
In addition, the stability of the ink used in the preparation of spot colors, 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 press, the amount of water applied during printing, etc. will also have different effects on the color difference. Impact.
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