1. Printing color difference caused by pressure discomfort

Printing pressure is one of the conditions for ink transfer. Since the surface of the printing plate cannot be absolutely flat, subtle unevenness on the paper surface is inevitable. In the case of uneven and uneven thickness, if the printing pressure is insufficient or uneven, the printing ink layer is prone to uneven shades. The ideal printing process is based on the "Three Levels", that is, the printing plate surface, the substrate and the lining surface are required to be relatively flat, so that a thin ink layer can be used to balance the printing pressure, so that the printed matter can be obtained More uniform ink color. When the printing pressure is insufficient or uneven, and there is poor contact between the lining body and the printing plate, it is necessary to increase the amount of ink to meet the visual effect of inking on the plate, but this will not only increase the consumption of ink , and it is easy to produce printing color difference and smudge quality problems on printed products. Therefore, to prevent the occurrence of printing color difference, we should grasp the basic conditions of "three levels" of printing and use uniform and constant pressure for printing. If you are printing a large-area plate product using the letterpress process (using a circular flattening or flattening machine), it is best to use a metal backing material. In this way, the material has better flatness and solidity, and the printing pressure is relatively high. Stable and helpful in preventing printing color differences caused by pressure variations.

2. Printing color difference caused by defective ink rollers

In lithographic, letterpress and other printing processes, the quality of the rubber roller plays a vital role in whether the printed product can obtain uniform ink color. Machines that print color prints are generally equipped with three types of ink transfer rollers, ink distribution rollers and ink application rollers. Among them, the ink transfer rollers and ink application rollers have a greater impact on the shade of ink. The elasticity, viscosity, roundness, hardness and surface finish of the ink roller largely determine the ink quality of the printed product. If the inking roller has poor elasticity, insufficient viscosity, roller eccentricity, excessively hard colloid, and poor colloid finish, uneven or unstable ink application may occur during the printing process, resulting in inconsistent printing ink colors on each sheet. Therefore, printing color products should use better quality rubber rollers to ensure that the ink applied to the layout remains basically the same in each printing cycle. If printing a large number of products, the rubber roller should be cleaned in time during printing to remove paper dust remaining on the rubber roller. Impurities are removed to ensure uniform production of ink and prevent printing color differences. On the other hand, the contact position between the ink roller and the ink-crossing iron roller should be adjusted. Generally, automatic printing machines are equipped with several sets of ink-crossing devices to achieve uniform ink delivery. However, if there is poor contact between the rubber roller and the inking iron roller, it will inevitably lead to insufficient and uneven ink transfer, resulting in uneven ink shades in the horizontal layout of the printed product. In this regard, the shaft heads at both ends of the rubber roller should be adjusted to achieve uniform contact between the rubber roller and the iron roller. After adjustment, a 5-wire steel sheet can be inserted between the rubber roller and the iron roller for pull inspection. There will be an obvious sense of resistance during the film. In addition, the contact position between the inking rubber roller and the printing plate surface must be adjusted. If the ink roller is adjusted too high, the layout will not be evenly coated with sufficient ink, and printing marks will easily occur due to the ink roller slipping on the layout. If the ink roller is adjusted too low, the ink layer on the printing plate will be dragged to the tip by the scraper printing plate, resulting in a color difference phenomenon in which the ink color of the printed product is lighter at first and darker at the end, that is, the ink at the mouth is lighter and the ink at the tip is thicker. The ink roller and the printing plate surface should be adjusted into a parallel and even contact state. Only by accurately adjusting the contact position between the ink roller and the printing plate can the uniform ink color of the printed product be ensured.

3. Printing color difference caused by defective ink feeding device

When the ink fountain iron roller has poor circularity or wear and deformation, the ink fountain steel plate also appears wear and deformation, and the ink fountain roller rotation control There are defects in the mechanism, which makes the amount of ink input unstable during the printing process and also causes printing color difference. Corresponding measures should be taken to deal with this to prevent the occurrence of printing color difference. In addition, attention should be paid to the reasonable and accurate ink feeding device. The ink in the ink fountain must be fully increased or reduced in delivery volume. There are two adjustment methods. One is to adjust the gap between the ink fountain steel plate and the ink transfer (ink feeding) iron roller; It is achieved by adjusting the rotation arc of the ink-transmitting iron rod.

When the ink is thin, it is not advisable to adjust the gap between the steel plate and the iron rod too large to avoid ink leakage and uneven ink color. When adjusting the amount of ink supply, the rotational arc of the ink transfer iron rod should generally be controlled between 1/2 of its adjustment range. This allows the ink supply amount to be flexibly adjusted when the ink concentration changes during the printing process to ensure The ink color of the printed product is uniform and consistent, preventing the occurrence of printing color difference problems.

4. Printing chromatic aberration caused by poor dampening factors

If the dampening process is not suitable in the lithographic printing process, printing chromatic aberration is also prone to occur. For example, when the water roller is eccentric, due to inconsistent contact with the layout, it is easy to cause differences in the amount of water supply to the layout. In addition, if the water roller sleeve is aged or absorbs ink or impurities, it will affect the water absorption of the water roller and the uniform transmission of fountain solution. Furthermore, if water leakage occurs in the water bucket device, it will also affect the uniform transportation of moisture on the printing plate. The existence of these undesirable conditions will destroy the uniform and stable transfer of ink on the layout, causing differences in the ink color of the printed product. Therefore, the dampening system must be maintained in good condition, the ink rollers should be cleaned frequently, and the aged water roller velvet covers should be replaced to better prevent printing color differences.

5. Printing color difference caused by poor ink

The process conditions show that improper thickness and viscosity of the printing ink layer can easily produce printing color difference. When printing, only a moderate and uniform printing ink layer can better ensure the ink quality of the printed product and effectively prevent the occurrence of printing smear problems. If the printing ink layer is thicker, the viscosity will increase accordingly, which will easily cause powder loss and nap on the paper surface during printing, thereby affecting the uniformity of the ink color of the printed product. However, if the printing ink layer is too thin and the ink viscosity is too small, the ink color of the printed product will appear lighter and the visual effect of the printed product will be poor. On the other hand, due to the thixotropic nature of the ink, as the running time of the printing machine increases and the printing speed increases, the friction coefficient of the machine increases, the temperature of the ink will gradually increase, and its viscosity will also increase accordingly. Decrease, so the printing ink color will become lighter and lighter, making the ink color of the front and back of the batch of products inconsistent, resulting in color difference. Therefore, these factors must be fully taken into account when adjusting ink. The appropriate ink concentration must be adjusted according to the climate temperature conditions and the characteristics of the printing machine to reduce the chance of color difference during printing and make the printing ink color relatively stable. However, in the actual printing process, the decrease in ink viscosity is inevitable. The ink color of the printed product is often thicker at the beginning of printing, and then gradually fades. In this way, the amount of ink can be gradually increased by appropriately adjusting the rotation angle of the ink transfer iron rod. , so that the ink color of the front and back printing is basically consistent. When blending secondary color and secondary color inks, try to avoid mixing inks with large specific gravity differences to avoid "stratification" (i.e. sedimentation and suspension) of the ink in the ink fountain due to large differences in specific gravity during printing, causing printing problems. Color difference. In order to maintain the continuous and even transmission of ink, the ink knife should be frequently used to stir the ink in the ink fountain. In addition, during the printing process, because the amount of ink in the ink fountain is significantly reduced and thicker, it is easy to reduce the gap between the ink fountain steel plate and the ink transfer iron roller, thereby reducing the ink output and making the printed ink color lighter. In order to avoid this situation from happening, attention should also be paid to frequently adding ink to the ink fountain during production, so that a certain amount of ink is always maintained in the ink fountain. In order to prevent the ink viscosity from changing too much and affecting the balance of printed ink colors, the contact between each rubber roller and the iron roller should not be too tight, and the rubber rollers should not be too hard to minimize the impact of frictional heat on the thixotropy of the ink. , Reduce the phenomenon of ink fading.

6. Printing color difference caused by poor substrate material

In daily printing process, we can find that the difference in substrate material is also one of the reasons for printing color difference, such as the gloss of the substrate Differences in degree and hue will cause printing color aberration problems. Taking embossed aluminized paper as an example, differences in the hue of the aluminized paper surface will cause inconsistencies in the printing color rendering effect. Therefore, when printing color products, substrates with consistent gloss and hue should be used as much as possible. If objective factors exist, and the same batch of products uses two materials with different colors and glossiness, attention should be paid to separate printing, and the ink hue and printing pressure should be adjusted in the process to make the printing ink colors of the two materials basically the same. To prevent printing color difference.

7. Printing color differences caused by machine defects

If a worker wants to do his job well, he must first sharpen his tools. Maintaining good performance of the machine is a reliable guarantee for improving the quality of printing ink. However, if there are bad conditions in the machine, such as the printing system is unstable, the pressure of each printed sheet is inconsistent, the paper feeding system is abnormal, and problems such as empty sheets and multiple sheets frequently occur during printing, it is also the most likely to cause printing problems. The appearance of color difference problems. Therefore, you should pay attention to the maintenance and upkeep of the equipment at ordinary times, and pay attention to the lubrication of key parts of the machine, such as the roller shaft and other parts, to prevent wear and tear, so that the imprinting mechanism can maintain good working condition and the printing pressure can remain constant. It is necessary to adjust the feeder and air pump devices so that the air path is smooth and unobstructed, and the printing sheet is sucked normally, and the paper feeding is ensured smoothly without empty sheets, so as to better prevent the occurrence of printing color difference.

8. Printing color difference caused by defective printing plate

The printing plate is one of the basic elements of the quality of image and text reproduction. During the printing process, the printing plate can only maintain no deformation or wear. , in order to better ensure the stability of the amount of ink transferred to the layout. During the printing process, if the dot layout is deformed due to pasting or wear, and the lines are worn and expanded, then the ink color of the printed product will inevitably deviate due to changes in the amount of ink absorbed by the layout. Therefore, when printing color products, attention should be paid to controlling the appropriate printing pressure and adjusting the contact pressure of the rubber roller and water roller on the printing plate to prevent excessive pressure from exacerbating the wear of the printing plate surface and causing ink color deviations in the products printed before and after. On the other hand, materials with good smoothness and resistance to powder and lint loss should be used for printing. This is also one of the process measures to effectively avoid wear and deformation of the layout during printing, and is conducive to better preventing the occurrence of printing chromatic aberration.

9. Printing color difference caused by poor lining

Dust is the medium for imprint transfer and has a great influence on the printing ink color. If the elasticity of the lining is too poor, as the number of printings continues to increase, it will gradually sink and deform and become difficult to reset. Then, the printing ink color on the front and back of the printed product will be inconsistent. On the other hand, if a layer of ink, paper scraps and other impurities adhere 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, you should choose a lining with a certain degree of elasticity, and clean the imprints or paper scraps on the surface of the blanket in a timely manner to prevent printing color differences caused by poor lining.

10. Differences in the color of printed products due to different light source characteristics and observation angles

Usually, the light sources used to identify the ink color of printed products include daylight (natural light), fluorescent lamps, incandescent lamps and other light sources. . The characteristics of the light sources they emit are different, and the colors of the emitted light are also different. For example, sunlight is mellow white, and the color observation is relatively more accurate. The light of a fluorescent lamp is white with bluish color, similar to sunlight. The light of incandescent lamps is red with a yellowish color, so it is difficult to accurately identify the color. Although daylight and fluorescent lamps are somewhat similar, the differences between the two should also be considered when identifying higher-quality color products. The influence of light on identifying ink color not only depends on the characteristics of the light source, but also has a lot to do with the intensity of the light and the angle of illumination. Under the same light source, the one that is farther away is weaker. The correct illumination angle should be that the incident angle is equal to the reflection angle. The light will inevitably be partially refracted when it shines on the sample, but it will not have much impact on the ink color. When identifying the ink color of the printed matter, the intensity of the light source should be used no matter day or night. and irradiation angle are the same. On the other hand, when identifying the ink color of printed products, especially those printed on aluminized paper, the color of the proofs should be compared during printing, and the color cast of the proofs should be correctly distinguished between those placed side by side for identification and those placed overlapping. For example, if the printed sample is placed on the standard sample Comparing colors separately will cause certain visual errors due to the large difference in viewing angles, which can easily lead to inaccurate ink color identification. When the ink color samples of the printing sample are overlapped and placed on the standard sample for comparison, the color identification is relatively accurate due to the small difference in viewing angles. The author has a deep understanding of this. For example, when I sent a label sample (the main label and the back label have the same background color) to a customer for confirmation, the customer thought that there was an obvious color difference in the background colors of the main label and the back label after looking at the standards placed side by side. , raised an objection.

The author stacked the back label on top of the main label and compared the colors and felt that the colors were relatively close, so the customer finally signed and confirmed. Process practice shows that differences in light source characteristics and observation feet will directly affect the accurate identification of colors. Therefore, products that require higher color quality should be printed during the day, and when comparing colors, multiple methods should be used. Observe and compare repeatedly from various angles to prevent improper ink control during the printing process, resulting in inconsistent ink colors of the front and back printed products.

In summary, a correct understanding of several factors that produce printing color differences, and a good control of the printing process and operating techniques, can not only effectively avoid the occurrence of this or that printing disadvantage, but also better Ensure the ink quality of color prints.