Analysis of Color Difference of Spot Color Ink During Printing Process

In the printing process, there are many factors that cause spot color ink production chromatic aberration, and these factors are discussed separately below.

1. The effect of paper on color

The effect of paper on ink color is mainly reflected in three aspects.

(1) Paper whiteness: Paper with a different degree of whiteness (or a certain color) has a different influence on the color of the printing ink layer. For the same type of whiteboard paper, the whiteness is different, and the color difference of the printing ink layer is mainly reflected in the number of black ink components in the spot color ink, especially for the colors whose lightness is above 70, and the effect is particularly obvious, resulting in spot color ink. The ratio varies greatly. Therefore, in the actual production, the paper with the same whiteness should be printed as much as possible to reduce the influence of the whiteness of the paper on the printing color.

(2) Absorption: When the same ink is printed under the same conditions on paper with different absorbency, there will be different printing gloss. The structure of the paper determines that there are irregularities and voids formed by plant fibers on the surface of the paper. In order to obtain a good uniformity and smoothness of the paper surface, it is generally necessary to coat the surface of the paper with a coating of different thickness. The nature and thickness of the coating determine the absorbability of the ink on the surface of the paper. With different absorptive capacities, it is necessary to make the colors of printing ink layers different. Compared to coated paper, non-coated paper, black ink layer will appear dark, dull, and the color ink layer will drift, the color of the mix of cyan ink and magenta ink show the most obvious.

(3) Gloss and smoothness: The gloss of prints depends on the gloss and smoothness of the paper. The surface of the printing paper is a semi-glossy surface, especially coated paper.

In color prints, about 4% of the light is reflected when the light strikes the surface of the paper at an incident angle of 45°. This is the light reflected from the surface of the first layer. The rest of the incident light passes through the ink layer, is selectively absorbed by the ink, and is 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 is high, the reflected light on the surface of the first layer is a specular reflection and it is difficult to enter human eyes. The color observed at this time is basically the color reflected by the ink layer. If the surface of the paper is rough and the gloss is low, the diffused reflection will occur on the surface of the first layer. At this time, the color we see is the mixture of the main color light and the light reflected from the surface of the first layer. Since there is a white light component on the inside, the saturation of the main color light is reduced. Therefore, when people observe the printed product, the color becomes lighter, and when the density is measured with a densitometer, the density value decreases and the lightness increases.

2. Effect of surface treatment on color

The surface treatment methods of packaging products mainly include coating (bright film, matt film), coating (bright oil, matt oil, UV varnish) and so on. After the surface treatment of the printed products, there will be varying degrees of hue and color density changes. These changes are divided into physical changes and chemical changes. The physical change is mainly reflected in the specular reflection and diffuse reflection on the surface of the product, which has a certain influence on the color density. When covering the bright film, cover light oil and UV oil, the color density increases; when covering the matt film, cover the matte oil, the color density decreases. The chemical change mainly comes from the plastic film, UV base oil, and various organic solvents contained in the UV oil, which will change the color of the printing ink layer.

3. Effect of lighter on color

Absorption agent is a paste-like colorless and transparent substance, which mainly plays a role of diluting color in spot color printing. The amount of lighter added is different, and the influence on hue is also different, especially the blue color is the most obvious.

In the process of formulating spot-color inks, the leveling force and pressure generated by the uniform ink meter and the ink-extracting instrument are smaller than those on the printing press. There is no need to add a lighter when printing, but only use a lighter to create a uniform color ink display card. In the case of the same color density, there is a color difference between the color card and the printed product. This is because the addition of the diluting agent changes the distribution of the pigment in the ink and causes the ink to absorb, refract, and reflect light. As a result of the change, there is a color difference, which is caused by the difference in the system.

4. The influence of the dry density difference

For printed products that have just been printed, the ink is still wet and has a density difference when dry. The phenomenon that the wet color density is greater than the dry color density is referred to as a dry retreat density phenomenon. This is because the ink layer just printed has a certain leveling property, so the surface reflection is dominated by specular reflection, which looks bright and shiny. When the ink layer is in a dry state, the reflection of the surface is dominated by diffuse reflection, and the color is naturally dull and darker than when it was printed.

Since the dry color density is generally measured after printing for 30 to 60 minutes after drying, this results in difficulty in measurement and control of spot color density.

A densitometer with a polarizer device can eliminate light from the surface of the ink layer due to specular reflection, 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 dryness of the ink layer. For coated paper, the measured density difference is 0.05 to 0.15, and the non-coated paper has a measured density difference of 0.1 to 0.2. The color difference is also different for different colors, with the smallest yellow difference, the largest black color, and the blue and red colors in between. Therefore, when measured with such a densitometer, the measured value should be higher than the density value of the standard color sample, so as to play the role of control.

5. Effect of system differences

The process of making a color card with a uniform ink meter and an ink displayer is a “dry printing” process. No water is involved, and printing is a “wet printing” process. A wetting fluid participates in the printing process. Therefore, the ink must be generated in the offset printing. The emulsification phenomenon of the water-in-oil emulsified ink changes the distribution state of the pigment particles in the ink layer, and inevitably produces a color difference, and the printed product also appears to be dark in color and not vivid.

In addition, the stability of the ink used for the deployment of spot colors, the thickness of the ink layer, the accuracy of the weighing ink, the new and old differences in the ink supply area of ​​the printing machine, the speed of the printing press, and the amount of water during printing will also have different effects on the color difference. Impact