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Spot color control skills for paper packaging printing (Part 2)

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 printed matter. This is because the addition of diluent changes the distribution of pigments in the ink, and changes the absorption, refraction and reflection of light by the ink. Therefore, the color difference is mainly manifested: in the loading process, this color difference 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. Influence of system differences

the process of making color cards with ink homogenizer and ink expander is a "dry printing" process, without the participation of water, while printing is a "wet printing" process, with the participation of wetting liquid in the printing process. Therefore, in offset printing, the ink must be emulsified in water in oil. Because the emulsified ink changes the distribution state of pigment particles in the ink layer, it is bound to produce color differences, and the printed products will 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 elasticity of the ink - the ability of the material to return to its original shape after the load that causes its deformation is removed, the old and new differences in the ink supply area of the printing machine, the speed of the printing machine, the amount of water added during printing, etc. 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 spectrophotometer, and make a printing standard color card (the advantages of super recycled plastic granulator with color difference also make it possible to continue to develop, and the standard needs to be further modified). Half of the color cards are ordinary color samples, and the other half are color samples that have successfully solved the dilemma of lead and bismuth smelting in the new material workshop of Guiye smelter after surface treatment, which is for the convenience of quality inspection, as shown in the attached figure

2. Verify spot color

considering that paper is the main factor affecting color difference, we should "show" the color sample with the actual printing paper before each printing, and make minor correction against the color card to eliminate the influence of paper

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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