What is the printing process? What are the reasons that affect the color?

The printing process is divided into three stages:

Pre-press → pre-print work generally refers to photography, design, production, typesetting, and output film proofing.

Intermediate printing → refers to the work in the middle of printing and the process of printing finished products by printing machine;

Post-press → refers to the post-printing work, which generally refers to the post-processing of printed matter, including gluing (laminating), UV, oiling, beer, bronzing, embossing, mounting, binding and cutting. , mainly used for publicity and packaging printed matter.

The reasons why printing affects color are:

First, the pressure discomfort caused by printing color difference

Printing pressure is one of the conditions for ink transfer. Because the printing plate surface cannot be absolutely flat, the paper surface inevitably has slight concave-convex shape and uneven thickness. If the printing pressure is insufficient or uneven, the ink layer is prone to uneven shading. The ideal printing process is on the basis of "three levels", that is, the printing plate, substrate and lining are required to reach a relatively flat state, so as to balance the printing pressure through the thin ink layer and obtain a relatively uniform ink color on the printed matter. When the printing pressure is insufficient or uneven and the contact between the liner and the printing plate is poor, it is necessary to increase the input amount of ink to meet the visual effect of the printing plate, but this will not only increase the consumption of ink, but also easily lead to the quality problems of printing color difference and dirty printed matter. Therefore, to prevent printing color difference, it is necessary to grasp the basic conditions of "three levels" of printing and use uniform and constant pressure for printing. If it is a large-area printing plate product, it is better to use embossed technology (using a circular flattening machine or a flat flattening machine), so that the smoothness and firmness of the material are better and the printing pressure is more stable, which is conducive to preventing printing color difference caused by pressure change.

Second, the printing color difference caused by poor ink roller.

The quality of cots plays an important role in lithography, relief printing and other printing processes, and whether the printed products can obtain uniform ink color. There are generally three kinds of printing machines for printing color printed matter: inking roller, inking roller and inking roller, among which the inking cots and inking rollers have great influence on the shade of ink. The elasticity, viscosity, concentricity, hardness and surface smoothness of the ink roller largely determine the ink quality of printed matter. If the inking roller has poor elasticity, insufficient viscosity, eccentric roller body, hard colloid and poor colloid finish, it is easy to cause uneven or unstable ink brushing in the printing process, so that the ink color of each paper is inconsistent. Therefore, good quality cots should be used for printing color products to ensure that the ink coated on the page remains basically unchanged in each printing cycle. If a large number of products are printed, the cots should be cleaned in time to make paper scraps stick to the cots. It can remove impurities, ensure the uniform source of ink and prevent printing color difference. On the other hand, the contact position between the ink roller and the iron roller should be adjusted. Automatic printing machines are generally equipped with several sets of ink stringing devices to realize uniform ink supply. However, if the rubber roller is in poor contact with the inking iron roller, it will inevitably lead to insufficient and uneven ink transfer, which will lead to uneven ink color in the horizontal layout of printed matter. Therefore, the shaft heads at both ends of the cots should be adjusted to make the contact between the cots and the iron rollers even. After adjustment, 5-wire steel plate can be inserted between rubber roller and iron roller for traction inspection. When pulling the steel plate, there should be obvious resistance. In addition, it is necessary to adjust the contact position between inking roller and printing plate. If the inking roller is adjusted too high, the layout will not be evenly and fully coated with ink, and it is easy to produce printing streaks because the inking roller slips on the layout. If the inking roller is set too low, the ink layer on the layout will be dragged by the scraper printing plate, resulting in the color difference phenomenon of the printed matter, that is, the ink color at the mouth is light and the ink color at the dragging tip is thick. The inking roller and printing plate should be adjusted to a parallel and uniform contact state. Only by accurately adjusting the contact position between the inking roller and the printing plate can the ink color of the printed matter be uniform.

Thirdly, the printing color difference caused by poor ink supply device

When the center of Mo Dou iron roller is bad or has wear deformation, Mo Dou steel plate also has wear deformation, and the mechanism for controlling the rotation of the ink fountain roller is defective, which makes the ink feeding unstable in the printing process and also causes printing color difference. Therefore, corresponding measures should be taken to prevent printing color difference. In addition, we should pay attention to the reasonable ink feeding device. There are two ways to adjust the amount of ink in Mo Dou. First, adjust the gap between the Mo Dou steel sheet and the ink transfer (ink feeding) iron roller; But by adjusting the rotation radian of the ink transfer iron bar. In the case of thin ink, it is not advisable to adjust the gap between steel sheet and iron bar too large to avoid ink leakage and uneven ink color. When adjusting the ink supply, generally, the rotation radian of the ink transfer iron rod should be controlled within the adjustment range of 1/2, so that when the ink concentration changes in the printing process, the ink supply can be flexibly adjusted to ensure the color uniformity of printed ink and prevent the occurrence of printing color difference problems.

Fourthly, the printing color difference caused by poor wettability.

If the lithography process is not suitable for dampening, printing color difference is also easy to occur. For example, when the water roller is eccentric, it is easy to make the water supply of the layout inconsistent because of its inconsistent contact with the layout. In addition, if the water roller sleeve ages or absorbs ink and impurities, it will affect the water absorption of the water roller and the uniform transfer of fountain solution. Moreover, if the bucket device leaks water, it will also affect the even transmission of water on the printed page. The existence of these bad conditions will destroy the uniform and stable transmission of layout ink and make the ink color of printed products different. Therefore, in order to keep the fountain system in good condition, the ink roller should be cleaned frequently and the aged water roller pile sleeve should be replaced to better prevent printing color difference.

Five, printing color difference caused by poor ink

The process shows that the thickness and viscosity of ink layer are not suitable, which is easy to produce printing color difference. When printing, only a moderate and uniform ink layer can ensure the ink quality of printed matter and effectively prevent the occurrence of printing dirt. If the printing ink layer is thick, its viscosity will increase accordingly, which will easily cause the phenomenon of powder removal and galling on the paper surface during printing, and then affect the ink color uniformity of printed matter. However, if the printing ink layer is thin and the viscosity of the ink is too small, the ink color of the printed matter will be light and the visual effect of the printed matter will be poor. On the other hand, due to the thixotropy of ink, with the increase of printing time and the acceleration of printing speed, the friction coefficient of the machine will increase, the temperature of the ink will gradually increase, and its viscosity will also decrease accordingly, thus making the printing ink lighter and lighter, leading to inconsistent ink colors before and after batch products, resulting in color difference. Therefore, these factors must be fully considered when preparing ink, and the appropriate ink concentration should be adjusted according to the climatic and temperature conditions and the characteristics of the printing machine, so as to reduce the chance of color difference in printing and make the printing ink color relatively stable. However, in the actual printing process, the viscosity of ink will inevitably decrease. Often the ink color of printed matter is relatively thick, and then it will gradually fade. In this way, by properly adjusting the rotation angle of the ink transfer iron bar, the throughput of ink can be gradually increased, so that the color of ink before and after printing is basically the same. When blending halftone and multicolor inks, we should try our best to avoid mixing inks with different specific gravity, and avoid the phenomenon of "layering" (i.e. precipitation and suspension) of the inks in Mo Dou due to the large specific gravity difference during printing, resulting in printing color difference. In order to keep the continuous and uniform transmission of ink, we should always use an ink knife to stir the ink in Mo Dou. In addition, in the printing process, because the amount of ink in the Mo Dou is obviously reduced, it is easy to reduce the gap between the Mo Dou steel sheet and the ink transfer iron roller, thus reducing the amount of ink output and fading the printed ink. In order to avoid this situation, we should also pay attention to adding ink to Mo Dou frequently during production, so as to keep a certain amount of ink in Mo Dou. In order to prevent the ink viscosity from changing too much and affect the balance of ink color, the contact between each rubber roller and the iron roller should not be too tight and the rubber roller should not be too hard, so as to minimize the influence of friction heat on the thixotropy of ink and reduce the fading phenomenon of ink.

Six, printing color difference caused by poor substrate material

In the daily printing process, we can find that the difference of substrate material is also one of the reasons for printing color difference, such as the difference of substrate gloss and hue, which will cause printing color difference. Taking embossed aluminized paper as an example, there is hue difference on the paper surface of aluminized paper, which will cause inconsistent printing color rendering effect. Therefore, printing color products should try to choose the substrate with the same luster and hue. If two materials with different colors and glossiness are used in the same batch of products under the existence of objective factors, attention should be paid to separate printing. By adjusting the ink hue and printing pressure, the printing ink colors of the two materials are basically the same, so as to prevent the occurrence of printing color difference.

Seven, the printing color difference caused by poor machine.

Sharp tools can do a good job. Maintaining the good performance of the machine is a reliable guarantee to improve the quality of ink. However, if the machine has bad conditions, such as the unstable operation of the bronzing system, resulting in inconsistent pressure on each sheet, abnormal paper feeding system, and frequent failures such as blank paper and multiple sheets during printing, it is also most likely to cause printing color difference problems. Therefore, attention should be paid to the maintenance of equipment and the lubrication of key parts of the machine, such as the lubrication of roller shafts, to prevent wear and tear, so as to keep the stamping mechanism in good working condition and the printing pressure constant. It is necessary to adjust the Feida and air pump devices to make the air path smooth, the paper suction and feeding normal, the paper conveying smooth, and there is no blank paper, so as to better prevent the occurrence of printing color difference.

Eight, plate printing caused by poor color difference

Printing plate is one of the basic elements of graphic reproduction quality. In the printing process, only when the printing plate is not deformed and worn can the stability of ink transfer in the layout be better guaranteed. In the printing process, if the dot layout is distorted due to pasting or abrasion, and the lines are enlarged due to abrasion, the ink color of printed matter will inevitably deviate due to the change of ink absorption of the layout. Therefore, when printing color products, we should pay attention to control the appropriate printing pressure and adjust the contact pressure between rubber roller and water roller on the printing plate to prevent the printing plate from being worn out due to excessive pressure, so that the ink color of the products printed before and after will be deviated. On the other hand, try to use materials with good smoothness, and it is not easy to lose powder and hair. This is also one of the technological measures to effectively avoid the wear and deformation of the layout in printing, which is conducive to better preventing the occurrence of printing color difference.

Nine, printing color difference caused by poor lining.

Dust coating is the medium of imprint transfer, which has great influence on ink color. If the elasticity of the liner is too poor, with the increase of printing times, it will be difficult to reset after gradually sinking and deforming, so the ink color before and after printing will be inconsistent. On the other hand, if there is a layer of ink, paper scraps and other impurities attached to the surface of the lining during the printing process, the ink color of the printed product will also deviate. Therefore, when printing color products, we should choose flexible packaging, and clean up the imprint or paper scraps on the surface of the blanket in time to prevent printing color difference caused by poor packaging.

Ten, the characteristics of light source and different observation angles to identify the color difference of printed matter.

Usually, the main light sources for identifying the color of printed ink are sunlight (natural light), fluorescent lamp and incandescent lamp. The characteristics of the light sources they emit are different, so the colors of the light they emit are different. For example, if the sunlight is white, it is more accurate to observe the color. The light emitted by fluorescent lamps is white with blue, similar to sunlight. The light of incandescent lamp is red with yellow, so it is difficult to accurately identify the color. Although fluorescent lamps are somewhat similar to fluorescent lamps, the differences between them should also be considered when identifying high-quality color products. The influence of light on distinguishing ink color lies not only in the characteristics of light source, but also in the intensity of light and the angle of illumination. Under the same light source, the radiation is far and weak. The correct illumination angle should be that the incident angle is equal to the reflection angle. The light will inevitably refract on the proof sample, but it has little effect on the ink color. When identifying the ink color of printed matter, it should be carried out under the condition of the same light source intensity and irradiation angle for ten days or at night. On the other hand, when identifying the ink color of printed matter, especially aluminum-plated paper printed matter, we should correctly distinguish the color difference between parallel identification and overlapping identification. For example, if the printed matter is placed next to the standard sample for color comparison, there will be some visual errors due to the large difference in observation angles, which will easily lead to inaccurate ink color recognition. However, the ink sample blocks of printed samples are overlapped on the standard samples for comparison, and the color recognition is relatively accurate because of the small difference in observation angles. I have a deep understanding of this. For example, I sent a trademark sample (the background color of the main label and the back label is the same) to the customer for confirmation. After reading the standard samples placed side by side, the customer thought that the background color of the main label and the back label had obvious color difference and raised an objection. The author thinks that the color of the back label is close to that of the main label, so the customer finally signs it for confirmation. The process practice shows that the difference between the characteristics of light source and observation needle will directly affect the accurate color recognition. Therefore, products with high printing color quality should be printed in the daytime. When making color comparison, we should repeatedly observe and compare from all angles to prevent improper ink control in the printing process, resulting in inconsistent ink colors before and after printing products.

To sum up, a correct understanding of some factors that cause printing color difference and a good combination of printing technology and operation technology can not only effectively avoid printing disadvantages of one kind or another, but also better ensure the ink quality of color printed matter.