Spot color printing control skills
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In recent years, in the printing of paper packaging products, a large number of spot color printing has been adopted. The characteristics of spot color printing are: large-area solid color blocks are dazzling, and the shelf display effect is good; the color difference caused by dot overprinting and dot deformation is reduced, the color is stable, and easy to recognize. Let's discuss the application and color control techniques of spot color inks with industry peers.
First, the detection of spot colors
At present, most domestic packaging and printing enterprises are relatively backward in the measurement and control of spot colors, and most rely on the experience of workers to deploy spot colors. The disadvantage of this is that the ratio of the spot color ink is not accurate enough, the deployment time is long, and subjective factors have a great influence. Some powerful large-scale packaging and printing companies have adopted a spot color ink system to manage them.
The spot color ink distribution system consists of a computer, color matching software, spectrophotometer, analytical balance, ink level meter, and ink meter. Using this system, the paper and ink parameters frequently used by the company are included in the database, and the color matching software is applied to the computer to automatically color the spot color provided by the customer, and the CIE is measured by the spectrophotometer. Lab value, density value, △E, which can realize the data management of spot color ink distribution.
The spectrophotometers currently on the market are available in the US X-Rite and Swiss Gretag brands. Different spectrophotometers use different calculation methods when measuring chromatic aberration, which will bring different tolerances.
The color difference space region described by the color tolerance is divided into box tolerance (CIE Lab), fan tolerance (CIE LCH), and spherical tolerance (CMC). 2:1) 3 species. The smaller the volume of the chromatic aberration space described by the tolerance, the higher the accuracy of the color.
The color difference results calculated according to different tolerance methods are different from the human eye's evaluation of color. The accuracy of the box-like tolerance is 75%, the accuracy of the fan-like tolerance is 85%, and the accuracy of the spherical tolerance is 95%. It can be seen that different spectrophotometers have different accuracy for color description due to different tolerance methods.
It can be seen that the box-like tolerance (CIE) is used in the national standard Lab), and its description of the color, the accuracy is 75%. That is to say, in actual production, even if it is zero, the ΔE value measured by the spectrophotometer still has a 25% deviation. Since the human eye is more sensitive to blue, this difference is most evident in blue. For customers who have higher requirements for print than national standards, this small deviation must be overcome. Therefore, in the preparation of spot color inks, the combination of human eye observation and spectrophotometer measurement is generally used to solve this problem.
Paper quality, print surface finishing, lightening agent, dry density, system differences and other factors will affect the color control of spot color printing.
Second, the factors affecting the color difference of the spot color
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: Paper with different whiteness (or a certain color) has a different effect on the color of the printed 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 black ink component in the spot color ink, especially for the color with a brightness of 70 or more, the influence 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) Absorbency: 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 coatings of different thicknesses on the surface of the paper. The nature and thickness of the coating determine the ability of the paper surface to absorb ink. Different absorption capacities make it necessary to make 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 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 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 matte oil is applied, the color density is lowered. The chemical changes mainly come from the laminating glue, the UV base oil, and various organic solvents contained in the 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-cutting ink is dispensed with a light-removing agent, a uniform spot color ink color display card can 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 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 ink. 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, the amount of water in the printing, etc., will also have different chromatic aberrations. Impact.
Third, the control of spot colors
In summary, in order to ensure that the color difference of the same batch and different batches of products meets the national standards and customer requirements, we have such control over the spot color in the printing process.
1. Making a color card
Firstly, according to the color standard provided by the customer, the proportion of the spot color ink is given by the computer color matching system; then the ink sample is adjusted, and the color samples of different densities are displayed by the ink level meter and the ink stick meter; then according to the national standard (or Customer) For the range of requirements for color difference, use the spectrophotometer to determine the standard, shallow limit, and deep limit, and make a printed standard color card (the color difference exceeds the standard and needs further correction). 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, as shown in the drawing.
2. Verify spot color
Considering that paper is the main factor affecting chromatic aberration, it is necessary to use the actual printing paper to "show" the color sample before each printing, and make minor corrections against the color card to eliminate the influence of the paper.
3. Printing control
The printing machine uses a printing standard color card to control the thickness of the spot color ink layer, and at the same time assists in measuring the main density value and Bk value of the color by the density meter to overcome the dry and wet color density difference of the ink.
In short, in packaging and printing, there are various reasons for the chromatic aberration caused by the spot color. It is necessary to analyze and solve the problem for different reasons in actual production, try to control the deviation to a minimum, and produce a package print that satisfies the customer.