In recent years, a large number of spot printings have been used in the printing of paper packaging products. Spot color printing is characterized by a large area of color blocks in the eye-catching, shelf display effect is better; reduce the color dots due to dot overprint, dot distortion, the color is relatively stable, relatively easy to identify. Below with industry peers to discuss the application of spot color ink and color control techniques.
First, the detection of spot colors
At present, most of the packaging and printing companies in China are lagging behind in the measurement and control of spot colors. Most of them rely on the experience of the workers' masters to deploy spot color inks. The disadvantage of this is that the proportion of spot ink is not precise enough, the deployment time is longer, and subjective factors have a greater impact. Some powerful large packaging and printing companies have adopted a spot color ink system to manage them.
Spot color ink system consists of computer, color matching software, spectrophotometer, analytical balance, even ink meter and ink display. The system uses the company's frequently used paper and ink parameters into the database, the use of color matching software for customer-supplied spot colors for computer automatic color, use the spectrophotometer to measure its CIELab value, density value, △ E, which can be achieved Color data management with ink.
The common spectrophotometers on the current market include the American X-Rite and the Swiss Gretag. Different spectrophotometers use different calculation methods to measure color differences, which will bring different tolerances.
The color space described by the color tolerance is divided into three types: box tolerance (CIELab), fan tolerance (CIELCH), and spherical tolerance (CMC2:1). The smaller the volume of the color difference space region described by the tolerance, the higher the accuracy of the color.
The color difference results calculated according to different tolerance methods are somewhat different from the human eye's evaluation of color. The accuracy of the box-like tolerance is 75%, the accuracy of the fan-shaped tolerance is 85%, and the accuracy of the spherical tolerance is 95%. It can be seen that different spectrophotometers use different tolerance methods, and the accuracy of color description is also different.
It can be seen that the boxed tolerance (CIELab) is used in the national standard, and its color description has an accuracy of 75%. In other words, in actual production, even if the value of △E measured using a spectrophotometer is zero, there is still a 25% deviation in color. Because the human eye is more sensitive to blue, this difference is most evident in blue. For those customers who have higher print requirements than national standards, this small deviation must be overcome. Therefore, in the preparation of spot color inks, the use of human eye observation and the use of spectrophotometric measurements combined to solve this problem.
Many factors, such as paper properties, printed surface finishing, diluting agents, dry withdrawal density, and system differences, can affect the color control of spot color printing.
Second, the factors that affect the color difference of spot colors
In the printing process, there are many factors that cause spot color inks to produce color differences. These factors are discussed separately below.
1, the paper's impact on color
The effect of paper on ink color is mainly reflected in three aspects.
(1) Whiteness of paper: 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 kind of whiteboard paper, the whiteness is different. 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 colors with a lightness above 70, resulting in a significant effect, resulting in spot color ink. The ratio is very different. 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 good uniformity and smoothness of the paper surface, it is generally necessary to apply a coating of different thickness to the surface of the paper. The nature and thickness of the coating determine the absorbability of the ink on the surface of the paper. According to the ability to absorb, it is necessary to make the color of the printing ink layer different. Compared to coated paper, non-coated paper, black ink layer will appear gray, dull, and the color ink layer will have a drift, the color of the mix of cyan ink and magenta ink show the most obvious.
(3) Glossiness and smoothness: The glossiness 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, when light strikes the surface of the paper with an incident angle of 45°, about 4% of the light is reflected off, which is reflected light on the surface of the first layer. The rest of the incident light passes through the ink layer and is selectively absorbed by the ink and then reflected through the ink layer. It 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 relatively high, the reflected light on the surface of the first layer is a specular reflection, and it is not easy 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 mixed color of the main color light and the light reflected from the surface of the first layer. Since the white light component is contained inside, the saturation of the main color light is reduced. Therefore, when people observe the printed product, the color becomes lighter. 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 these prints are processed on the surface, 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 the light film, cover oil and UV oil cover, 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, the 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 ink, the evenness and pressure of the ink distributor and the ink distributor are smaller than those of the printer. There is no need to add a lightening agent when printing, but only adding a lightening agent when creating a color card with ink can be used to create a uniform spot 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 impact of dry retreat density difference
For the freshly printed matter, the ink is still wet, 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 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. It 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 drying for 30 to 60 minutes after printing, this results in difficulty in measurement and control of spot color density.
A densitometer with a polarizer device eliminates 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 that it can play the role of control.
5, the impact 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 process of "wet printing." A wetting liquid participates in the printing process, so the ink must be in the offset printing. The phenomenon of emulsification of water-in-oil occurs. Since the ink after emulsification changes the distribution state of the pigment particles in the ink layer, a color difference is inevitable, 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.
Third, the control of spot colors
To sum up, in order to ensure that the color difference of the same batch or different batch of products meets the national standards and customer requirements, we have carried out such control of spot colors during the printing process.
1, making color cards
Firstly, according to the color standard provided by the customer, the proportion of spot color ink is given by the computer color matching system; the ink sample is then recalled, and the ink sample of different density is displayed by the uniform ink meter and the ink exhibition instrument; then according to national standards (or Customer) to the range of requirements for color difference, using spectrophotometer to determine the standard, shallow limit, deep limit, production of printing standard color card (color difference exceeds the standard need to be further amended). Half of the color card is a common color sample, and the other half is a surface-treated color sample, which is used for the convenience of quality inspection.
2, verify the spot color
Considering that paper is the main factor influencing the color difference, the actual printing paper should be used to "show" the color samples before each printing, and micro corrections should be made against the color cards to eliminate the influence of the paper.
3, printing control
During the printing, the leader uses the printing standard color card to control the thickness of the special color ink layer, and at the same time assists the density density meter to measure the main color density value and the Bk value to overcome the difference in ink dry and wet color density.
In short, in the process of packaging and printing, there are various reasons that lead to color differences in spot colors. We must analyze specific problems in actual production, solve problems, and try to minimize deviations within the minimum range. Customer-friendly packaging prints.
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