Control of spot color printing of the hottest pape

2022-08-14
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Control of spot color printing of paper packaging products

in recent years, spot color printing has been widely used in the printing of paper packaging products. Spot color printing is characterized by: large area of on-site color block, eye-catching color, good shelf display effect; The color difference caused by dot overprint and dot deformation is reduced, and the color is stable and easy to identify. Next, we will discuss the application of spot color ink and color control skills with our peers in the industry

I. detection of spot color

at present, most domestic packaging and printing enterprises are relatively backward in the measurement and control means of spot color, and most rely on the experience of workers to deploy spot color ink. The disadvantage of this is that the proportion of spot color ink is not accurate enough, the deployment time is long, and subjective factors have a great impact. Some powerful large packaging and printing enterprises have adopted spot color inking system to manage them

spot color inking system is composed of computer, color matching software, spectrophotometer, analytical balance, ink homogenizer and ink spreading instrument. With this system, the parameters of paper and ink often used by the company are included in the structure and operation method database of concrete pressure testing machine. The spot color provided by the customer is automatically matched by computer with color matching software, and the CIE LAB value, density value and △ e are measured by spectrophotometer, so that the data management of spot color ink matching can be realized

at present, two brands of spectrophotometer are common in the market: American X-Rite and Swiss Gretag. Different spectrophotometer adopts different calculation methods when measuring color difference, which will bring different tolerances

the color difference space area described by color tolerance is divided into box tolerance (CIE LAB), fan tolerance (CIE LCH) and spherical tolerance (CMC 2:1). The smaller the volume of the color difference space area 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 box tolerance is 75%, that of fan tolerance is 85%, and that of spherical tolerance is 95%. It can be seen that different spectrophotometers have different accuracy in color description due to different tolerance methods

it can be seen that the box tolerance (CIE LAB) is used in the national standard, and its accuracy of color description is 75%. In other words, in actual production, even if the △ e value measured by the spectrophotometer is zero, there is still a 25% deviation in color. Because the human eye is more sensitive to blue, this difference is most obvious in blue. For those customers whose requirements for printed matter are higher than national standards, this small deviation must be overcome. Therefore, in the preparation of spot color ink, the combination of human eye observation and spectrophotometer measurement is generally used to solve this problem

many factors, such as paper performance, print surface finishing, thinning agent, drying density, system differences, will affect the color control of spot color printing

second, factors affecting spot color difference

in the printing process, there are many factors that lead to the production of spot color ink color difference. These factors are discussed below

1. The influence of paper on color

the influence of paper on the color of ink layer is mainly reflected in three aspects

(1) Paper Whiteness: paper with different whiteness (or with a certain color) has different effects on the color appearance of the printing ink layer. For the same kind of white board, 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 lightness of more than 70, which has a particularly obvious impact, resulting in a great difference in the proportion of spot color ink. Therefore, paper with the same whiteness should be selected for printing in actual production 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 gloss. The structure of the paper determines that there are concave convex and pores formed by plant fibers on the surface of the paper. In order to obtain good uniformity and smoothness of the paper, it is generally necessary to coat the surface of the paper with coatings of different thickness. The nature and thickness of the coating determine the ink absorption capacity of the paper surface. Different absorption capacities inevitably lead to different colors of the printing ink layer. Compared with coated paper, the black ink layer will appear gray and Matt, and the color ink layer will drift. The color mixed by cyan ink and magenta ink is the most obvious

(3) gloss and smoothness: the gloss of printed matter depends on the gloss and smoothness of the paper. The surface of printing paper belongs to semi gloss surface, especially coated paper

on color printing, when the light shines on the paper surface at an incidence angle of 45, about 4% of the light will be reflected, which is the reflected light on the first surface. The rest of the incident light passes through the ink layer, after selective absorption of the ink, it is 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 first surface is specular reflection, which is not easy to enter the human eye. At this time, the observed color is basically the color reflected through the ink layer. If the paper surface is rough and glossy, the reflected light on the first surface will diffuse. At this time, the color we see is the mixed color produced by the main color light and the reflected light on the first surface. Because it contains white light, the saturation of the main color light is reduced, so people feel that the color becomes lighter when they observe the print. When they measure with a densimeter, the density value decreases and the brightness increases

2. The influence of surface treatment on color

the surface treatment methods of packaging products mainly include film covering (bright film, matte film), glazing (cover bright oil, matte oil, UV light oil), etc. After these surface treatments, prints will have varying degrees of hue changes and color density changes. These changes are divided into physical changes and chemical changes. Physical changes are mainly reflected in the increase of specular reflection and diffuse reflection on the product surface, which has a certain impact on the color density. The color density increases when covered with light film, light oil and UV oil; When covered with Matt Film and Matt oil, the color density decreases. Chemical changes mainly come from film covering glue, UV primer and various organic solvents contained in UV oil, which will change the color of the printing ink layer

3. The effect of diluent on color

diluent is a kind of colorless transparent substance in the form of ointment, which mainly plays the role of diluting color in spot color printing. The influence on hue varies with the amount of diluent added, especially blue

in the process of preparing spot color ink, the shear force and pressure produced by the ink homogenizer and ink expander on the ink are smaller than those on the printing press. It is not necessary to add a diluent when printing, but only when adding a diluent when making a color card with ink, can a uniform spot color ink color display card be made. When the color density is the same, there is a color difference between the color card and the color of the printed matter. This is because the addition of diluent changes the distribution of pigments in the ink, so that the absorption, refraction and reflection of light by the ink have changed, resulting in a color difference, which is caused by the difference of the system

4. The influence of the difference of drying density

for the newly printed print, the ink is still in the wet state, and there is a density difference between it and the dry state. The phenomenon that the wet color density is greater than the dry color density is called the dry fading density phenomenon. This is because the newly printed ink layer has a certain leveling property, so the surface reflection is mainly specular reflection, which looks bright and shiny. When the ink layer is dry, the surface reflection is mainly diffuse reflection, and the color naturally looks dimmer than when it was just printed

since the dry color density can only be measured after 30 ~ 60 minutes of drying after printing, it is difficult to measure and control the spot color density

the densimeter with polarizer device can eliminate the light generated by 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 of the ink layer. For coated paper, the measured density difference is 0.05 ~ 0.15, and for non coated paper, the measured density difference is 0.1 ~ 0.2. The color difference of different colors is also different. The difference between yellow is the smallest, black is the largest, and blue and red are between the two. Therefore, when measuring with such a densimeter, the measured value should be appropriately higher than the density value of the standard color sample, so as to play the role of control

5. The influence of system differences

the process of making color cards with ink homogenizer and ink expander is a dry printing process, without water participation, while printing is a wet printing process, with wetting liquid participation in the printing process. Therefore, in offset printing, the ink must have the emulsification phenomenon of water in oil. The emulsified ink will inevitably produce color difference because it changes the distribution state of pigment particles in the ink layer, and the printed products will also appear gray and not bright

in addition, the stability of the ink used to prepare the spot color, the thickness of the ink layer, the accuracy of weighing the ink, the old and new differences in the ink supply area of the printer, the speed of the printer, and the amount of water added during printing will also have different effects on the color difference

III. spot color control

to sum up, 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 carried out such control on spot color in the printing process

1. Making color cards

first, according to the color standard samples provided by customers, the proportion of spot color ink is given by computer color matching system; Then pull out the ink sample, and display the color samples with different densities with the ink homogenizer and ink expander; Then, according to the required range of color difference in national standards (or customers), determine the standard, shallow limit and deep limit with a spectrophotometer, and make a printing standard color card (the color difference exceeds the standard and needs further correction). Half of the color cards are ordinary color samples, and the other half are surface treated color samples, which is convenient for quality inspection, as shown in the attached figure

2. Verify spot color

considering that paper is the main factor affecting the color difference, the color sample should be displayed on the actual printing paper before each printing, and slightly corrected against the color card to eliminate the influence of paper by establishing an index database

3. Printing control

during printing, the collar machine uses the 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 with a densimeter to overcome the difference between the dry and wet color density of the ink

in short, in packaging and printing, there are various reasons for the color difference of spot color. It is necessary to analyze the different reasons in actual production, solve the problems, and try to control the deviation within the minimum range to produce packaging prints that satisfy customers

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