Guide: The uniform ink color and bright colors of printed products are the basic requirements of the quality of packaging products, and the color difference is a common problem in the quality of printed products. There are many reasons for the chromatic aberration in printing. There are bad factors in process technology and raw materials. The following share some bad factors that cause color difference in printing, the content is for the reference of purchasing friends:
Print color difference
Chromatic aberration, that is, in the printing process, the color of the printed product and the sample plate are often inconsistent, or the color mixing failure after the pattern screen is overprinted, the lighter will affect the product quality, and the serious will cause the entire batch of products to be scrapped. Color difference, also called deviation, is one of the typical quality defects in the printing process. Therefore, a correct understanding and understanding of the relationship between these printing conditions and ink color is of great significance for accurately controlling the quality of printing ink color and preventing the occurrence of color difference in printing.
1
Pressure discomfort
Printing pressure is one of the conditions for ink transfer. Since the surface of the printing plate cannot be absolutely flat, the surface of the paper will inevitably have fine unevenness and uneven thickness. If the printing pressure is insufficient or uneven, the ink layer will be easy to print. Uneven shades appear.
The ideal printing process is based on the “three flats”, that is, the printing plate surface and the substrate and the lining surface are required to reach a relatively flat condition, so that a thinner ink layer can be used to balance the printing pressure to obtain the printed product. More uniform ink color. When the printing pressure is insufficient or uneven, and there is a bad contact between the lining body and the printing plate surface, it is necessary to increase the amount of ink delivery to meet the visual effect of the ink on the plate surface, but this will not only increase the consumption of ink , It is easy to produce printing color difference and printing quality problems.
Therefore, to prevent the occurrence of color difference in printing, the basic conditions of printing “three flats” should be grasped, and uniform and constant pressure should be used for printing. If you are printing large-area plate products using the letterpress process (using a circular or flattening machine), it is best to use a metal-type backing material, so that the material has better flatness and firmness, and the printing pressure is relatively good. It is stable and helps prevent the color difference caused by pressure variation.
2
Bad ink roller
The quality of the rubber roller plays a crucial role in printing processes such as lithography and letterpress, and whether the printed product can obtain a uniform ink color.
There are generally three types of ink transfer rollers, ink distribution rollers, and inking rollers on the machine that prints color prints. Among them, the ink transfer roller and the inking roller have a greater impact on the intensity of the 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 body eccentricity, over-hard colloid, and poor colloidal finish, it is easy to cause uneven or unstable ink brushing during the printing process, which makes the printing ink color of each printed sheet inconsistent.
Therefore, printing color products should use better quality rubber rollers to ensure that the ink applied on the plate surface in each printing cycle remains basically the same. If printing large products, the rubber roller should be cleaned in time during printing to make the paper dust trapped on the rubber roller. Impurities can be removed to ensure the uniform production area of the ink and prevent the occurrence of color difference in printing.
On the other hand, the contact position of the ink roller and the iron roller should be adjusted. Generally, the automatic printing machine is equipped with several sets of inking devices to achieve uniform ink delivery. However, if the rubber roller and the iron roller are in poor contact, it will inevitably cause insufficient and uneven ink transfer, and uneven ink color on the horizontal layout of the printed product. In this regard, the shaft heads at both ends of the rubber roller should be adjusted to make the rubber roller and the iron roller reach a uniform contact condition. After adjustment, a 5-wire steel sheet can be inserted between the rubber roller and the iron roller for drawing inspection. When filming, there is an obvious sense of resistance as the degree.
In addition, it is necessary to adjust the contact position between the inking roller and the printing plate surface. If the inking roller is adjusted too high, the layout will not get uniform and sufficient ink coating amount, and it is easy to produce printing marks due to the inking roller slipping on the layout.
If the inking roller adjustment is too low, the ink layer on the layout will be dragged by the tip of the scraper printing plate, resulting in a color difference between the first and the darker ink color of the printed product, that is, the ink color of the gripping part is lighter, and the ink color of the trailing part is too thick. The inking roller and the printing plate surface should be adjusted to be parallel and even in contact. Only by accurately adjusting the contact position between the inking roller and the plate surface can the ink color of the printed product be uniform and consistent.
3
Defective ink supply device
When the center of the ink fountain iron roller is poor or there is abrasion deformation, the ink fountain steel plate also has abrasion deformation, and the mechanism that controls the rotation of the ink fountain roller has defects, which makes the ink delivery in the printing process unstable and also causes the printing color difference. , Should take corresponding measures to deal with it in order to prevent the occurrence of color difference in printing.
In addition, attention should be paid to the reasonable adjustment of the ink supply device. The ink in the ink fountain should be increased or reduced in an overall manner. There are two adjustment methods. One is to adjust the gap between the ink fountain steel sheet and the ink transfer (ink supply) iron roller; and It is achieved by adjusting the rotation arc of the ink transfer iron rod. When the ink is relatively thin, it is not advisable to adjust the gap between the steel sheet and the iron rod too large, so as to avoid ink leakage and cause uneven ink color.
When adjusting the amount of ink, generally the rotation arc of the ink transfer iron rod should be controlled within 1/2 of its adjustment range, so that when the ink density changes during the printing process, the amount of ink can be flexibly adjusted to ensure The ink color of the printed product is uniform and consistent to prevent the occurrence of color difference problems.
4
Poor dampening factor
If the lithographic printing process is not suitable for dampening, it is also prone to printing color aberration. For example, when the water roller has eccentricity, because of its inconsistent contact with the plate surface, it is easy to cause differences in the size of the plate surface water supply.
In addition, if the water roller melt sleeve is used to age or absorb ink and impurities, it will affect the water absorption of the water roller and the uniform transfer of the fountain solution.
Furthermore, if the water hopper device leaks, 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 the ink on the layout, and cause differences in the ink color of the printed product. Therefore, the dampening system should maintain a good body, the ink roller should be cleaned frequently, and the aging roller fleece should be replaced to better prevent the printing color difference.
5
Bad ink
The thickness and viscosity of the printing ink layer are not easy to produce color difference in printing. When printing, only a moderate and uniform printing ink layer can better guarantee the ink quality of the printed product and effectively prevent the occurrence of printing stickiness. If the printing ink layer is thicker, its viscosity will increase accordingly, which will easily cause the phenomenon of powder and fuzzing on the paper surface during printing, which will affect 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 light, and the visual effect of the printed product will be poor.
On the other hand, due to the thixotropic characteristics of ink, as the running time of the printing press increases and the printing speed increases, the friction coefficient of the machine increases, and the temperature of the ink will gradually increase, and its viscosity will also be correspondingly increased. Decrease, so that the printing ink color will be printed lighter and lighter, so that the ink color is inconsistent before and after the batch of products, and the color difference is formed.
Therefore, these factors must be fully considered when adjusting the ink, and the appropriate ink concentration must be adjusted according to the climate and 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 ink viscosity decrease is inevitable. The ink color of the printed product is often thicker, and then gradually faded. In this way, the ink delivery can be gradually increased by appropriately adjusting the rotation angle of the ink transfer iron rod. , So that the ink colors printed before and after are basically the same. When blending intermediate and multi-color inks, try to avoid mixing with inks with a large specific gravity difference, so as to avoid the ink in the ink fountain due to the large specific gravity difference during printing, causing “delamination” (ie precipitation and suspension) phenomenon to cause printing Chromatic aberration.
In order to maintain the continuous and even transfer of ink, an ink knife should be used to agitate the ink in the ink fountain frequently. In addition, during the printing process, the ink volume in the ink fountain is obviously reduced and thick, it is easy to make the gap between the ink fountain steel sheet and the ink transfer iron roller become smaller, which reduces the ink output and makes the ink color of the printed product lighter.
In order to prevent this from happening, pay attention to always add 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 ink color balance of the printed product, the contact between each rubber roller and the iron roller should not be too tight, and the rubber roller is not easy to be hard, so as to minimize the influence of frictional heat on the thixotropy of the ink , To reduce the ink fade phenomenon.
6
Bad substrate material
In the daily printing process, we can find that the material difference of the substrate is also one of the reasons for the printing color difference. For example, the difference in the gloss and hue of the substrate will cause the printing color difference problem.
Taking embossed aluminized paper as an example, differences in the hue of the aluminized paper surface will cause inconsistencies in the color rendering effect of printing. Therefore, printing color products should try to choose substrates with consistent gloss and hue. If, in the presence of objective factors, the same batch of products uses two materials with different colors and gloss, you should pay attention to separate printing, and adjust the ink hue and printing pressure in the process to make the printing ink colors of the two materials basically the same. In order to prevent the production of color aberration.
7
Bad machine
If a worker wants to do his job well, he must first sharpen his tools. Maintaining the good performance of the machine is a reliable guarantee for improving the quality of printing ink. However, if the machine has bad conditions, such as the imprinting system is unstable, the pressure of each printed sheet is inconsistent, and the paper conveying system is abnormal, and the frequent occurrence of empty sheets, multiple sheets and other faults in printing, it is most likely to cause printing. The emergence of chromatic aberration problems.
Therefore, you should always pay attention to the maintenance and maintenance of the equipment, and pay attention to the lubrication of the key parts of the machine, such as the roller shaft and other parts to prevent abrasion, keep the imprinting mechanism in good working condition, and keep the printing pressure constant. It is necessary to adjust the feeder and air pump device so that the air path is unimpeded and the printed sheet is sucked and fed normally, and the paper is fed smoothly without empty sheets, so as to better prevent the occurrence of printing color aberration.
8
Bad 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 be maintained without deformation and wear in order to better ensure the stability of the ink transfer on the layout. During the printing process, if the dot layout appears to be pasted or deformed by abrasion, the lines will be worn and expanded. In this way, the ink color of the printed product will inevitably deviate due to the change in the ink absorption of the layout.
Therefore, printing color products should pay attention to controlling the appropriate printing pressure, and adjust the contact pressure of the rubber roller and water roller to the printing plate to prevent excessive pressure from aggravating the abrasion of the printing plate surface and causing deviations in the ink color of the products printed before and after. On the other hand, we should try our best to use materials with good smoothness, not easy to shed powder and lint for printing. This is also one of the technological measures to effectively avoid the abrasion and deformation of the layout during printing, which is beneficial to better prevent the production of color aberration.
9
Poor lining
The lining is the medium for imprinting transfer, which has a great influence on the printing ink color. If the lining elasticity is too poor, as the number of printing continues to increase, it is difficult to reset after it gradually sinks and deforms, then the printing ink color before and after 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, it will also cause deviations in the ink color of the printed product. Therefore, printing color products should choose a flexible lining, and clean the imprints or paper scraps on the surface of the blanket in time to prevent the printing color difference caused by poor lining.
10
Different light source characteristics and viewing angles
Usually, the light sources for identifying the ink color of printed products mainly include sunlight (natural light), fluorescent lamps and incandescent lamps.
They have different characteristics of the light source, and the color of the emitted light is also different. For example, the sunlight is mellow white, and the color observation is relatively accurate. The light of a fluorescent lamp is white with bluish, similar to daylight. The light of the incandescent lamp has a reddish color with a yellowish color, so it is difficult to accurately identify the color. Although daylight and fluorescent lamps are somewhat similar, the difference between the two should also be considered when identifying higher-quality color products.
The influence of light on the identification of ink color not only lies in the characteristics of the light source, but also has a great relationship with the intensity of the light and the angle of illumination. Under the same light source, those who are illuminated are weaker than those far away. The correct illumination angle should be that the incident angle is equal to the reflection angle. The light will inevitably be partially refracted on the sample, but it will not have much effect on the ink color. When identifying the ink color of the printed product, it should be at the intensity of the light source no matter it is during the day or at night. And the irradiation angle is the same.