The color reproduction of packaging and printing

In the actual production of color box packaging products, color restoration problems often occur, such as neutral gray cast, etc., which affects the quality of the product. This is in addition to the problems of the equipment itself and the technical level of the production personnel. The key problem is that the company has not Can carry out effective color management and reasonable application of ink.

The concept and necessity of color management

Color management is to manage people's perception of color, the purpose is to achieve the conversion between different colors in the entire image copying process to ensure that the appearance of the image color from input display to output matches as much as possible, and finally reaches the original Harmony with the color of the replica.

Image replication needs to go through multiple stages such as image acquisition, processing, color separation, and printing. At each stage, the color information will be expressed according to the color rendering principle and color description characteristics of the currently used equipment. Different scanning and display devices will have different colors for the same manuscript. In addition, the use of RGB three-color display and four-color printing inks has different color expression capabilities, and different application software to convert RGB data to CMYK also lacks consistency, using different paper and ink, The prints obtained are also different. Therefore, it is necessary to control the color reproduction on various devices and media in the production system through color management technology.

Content of color management

The establishment of the device's color profile (Profile) is the core of color management. The description file has the representative color characteristics of each device, such as the chromaticity characteristic curve, output color gamut characteristic curve, etc. The color management system uses these representative The characteristic color features enable the matching and conversion of the color space of each device.

For color management, a series of prescribed operating procedures must be followed to achieve the desired effect. Color management is divided into three steps in principle, referred to as "3C", namely calibration (Calibration), characterization (Characterization) and conversion (Conversion).

Calibration: In order to ensure the stability, reliability and sustainability of the color information transmission process, it is required to calibrate the input, display and output devices to ensure that they are in a standard working state. Input correction includes correction of brightness, contrast, black and white fields of scanners, etc. To ensure that for the same original, no matter when it is scanned, it should be able to obtain the same image data.

By setting the brightness, contrast, color temperature of the display and the gamma value of the entire display system, it can accurately display colors. Output calibration includes the calibration of printers, imagesetters, printing presses and proofers. After correcting its characteristics, the device can be output according to the standard characteristics of the factory.

Characterization: Recording the corrected characteristics of all equipment is the process of characterization. These characteristic description files are the bridges from the device color space to the standard device-independent color space (PCS).

Conversion: Based on the calibration of the system equipment, using the device description file, using the standard device-independent color space as the medium, to achieve the correct conversion between the color space of each device. Because the color gamut of the output device is narrower than the color gamut of the original, scanner, and display, the color gamut needs to be compressed during color conversion. The color gamut compression has the absolute color method, relative color method in the ICC protocol, Highlight four methods: saturation method and sensory method.

Implementation of color management

Generally speaking, for a packaging and printing company to do color management, it needs a standard ambient light source; high-quality color characteristic file generation software; advanced color management system; standard measuring instruments and test tools; standard color target ; Standard printing management; standard remote digital proofing color management; standard scan reduction, digital camera color management; standard display calibration and screen soft proofing. In the specific implementation process of color management, the following aspects should be done: Carefully complete the standardization, standardization and data production management of each process in the printing process, which is the premise and foundation of applying ICC color management.

Enterprises should strictly formulate quality standards for fine products, intact standards for raw materials and equipment such as paper and ink, standards for measuring tools and environmental light sources, and develop standardized management documents, etc., and require everyone to strictly follow the standards. Monitors, soft proofs, ICC files of printers, various parameters of printing machines, etc. should be standardized and quantified within the standards and specifications. In all places where data can be expressed, through testing methods, summarize and summarize the data that can guarantee quality. The three complement each other and depend on each other to ensure stable high-quality and efficient production.

The pre-press image processing technology is the foundation and key link of print copy quality. The display should be professional. The scanning of originals and the processing of digital originals should be guided by the four basic standards of copy quality (gray balance, tone reproduction, color correction, sharpness enhancement), and the parameters of the four standards should be standardized through experiments. . After scanning the file, it is necessary to adjust and process the defects of the non-adapted manuscript to make the printed layer richer and achieve an effect superior to the manuscript. For enterprises printing in different places, the profile management of remote digital proofing can ensure that the colors of digital proofs in the two places are consistent. [next]

Choice of color management system

At present, there are many color management systems on the market, such as Adobe's Photoshop color management system, Apple's ColorSync color management system, Kodak's color management system, etc. Different color management systems have different color management capabilities for devices and have their specialties. Therefore, companies must first determine which devices need to obtain consistent colors in order to choose.

Secondly, it is necessary to determine the self-description file in the color management system and whether it supports the creation of a description file in an open system; if it is a closed system, the manufacturer must create a description file, so you need to determine whether you can afford to create a description file The cost required. According to the company's own situation, the system's performance, device supportability, scalability, compatibility and ease of use should be compared and tested. The same color manuscript can be used during the test to observe which system is in the specific Under the operating environment, you can get the most consistent effect with the original.

Effective color management is the guarantee of the entire reproduction quality, but the final product is obtained through ink. As one of the five elements of printing, ink has an important influence on the printing process and quality. Correct and reasonable use and deployment of ink is the key to ensure accurate color reproduction.

Ink and ink use problems

Composition and performance of water-based ink: Water-based ink is processed by water-soluble resin, high-grade pigments, solvents and additives. The hue depends on the nature of the pigment. Generally, the pigment is required to have a bright color, proper coloring power and hiding power, good resistance and high dispersion. In order to obtain good printability of ink, it is often necessary to adjust the fluidity, dryness and viscosity of the ink with various ink additives according to different situations. There are many factors that determine the quality of ink and wash. Improper use will bring a series of quality problems. Attention should be paid to analysis when purchasing and using, to judge its pros and cons, and make adjustments according to the situation to ensure its excellent printability .

In printing, the ink has a certain viscosity is necessary to maintain normal transfer and transfer of ink. Therefore, the viscosity of high-grade ink is generally controlled at about 20 ± 5 seconds, and the difference in viscosity will cause the difference in the printing color. In the actual production process, the pH value of the ink on the machine can be adjusted or controlled between 8.0 and 9.5 according to the difference between the load and the temperature.

The allocation of ink: The ink allocation mainly refers to the color allocation of the ink. In the printing of packaging products, the color used in ink is mainly spot color solid flat coating, to achieve the dual effect of visual and anti-counterfeiting.
These different spot colors cannot be overprinted by cyan, magenta, yellow, and black, and often cannot be used directly from the original ink. It must be obtained by technical staff by adjusting the original ink in different proportions according to the production status. .

The color adjustment of the ink is based on the principle of subtractive color method, that is, any color can be mixed and mixed using different proportions of the three primary colors, and the three basic attributes of hue, lightness and saturation are considered. The color matching should be based on the actual printing process, ink layer thickness, printing substrate and other conditions to determine the ink used, and consider the hue, gloss, tinting power, hiding power and other factors of the ink itself.

Color matching process

Analyze the color of the original: Toning is done according to the color sample of the original. In order to reduce the blindness of color matching, the color of the original must be analyzed first to determine the hue and proportion of the primary color ink. The analysis of the original manuscript is mainly a combination of instrument color measurement and human eye color comparison. It is relatively simple and practical to determine the proportion of tri-primary ink by printing chromatography. The content of tri-primary ink and white ink can be calculated according to the percentage of primary color dot area marked in the chromatography.

Color Matching: According to the calculated ratio, a small amount of primary color ink is weighed for blending. Note that the ink color is always compared with the original color. When the two colors are close, the adjusted ink can be used to compare the scrape on the scraping paper, and so on, until the two colors are completely consistent. When comparing colors, the ink should be uniform, and the thinner the ink layer, the better. In order to ensure the accuracy of the color hues, the paper should be the same as the paper used for printing. The adjustment of brightness and saturation is achieved by adding white ink and black ink. When adding white and black inks, special attention should be paid to the accuracy of weighing.

After the prepared ink meets the original requirements, the batch ink can be adjusted in proportion. The amount of ink used in a batch of jobs is usually determined by the type of paper, the image area, the number of prints, and the amount of wear, and the principle of more is not less.

Record: After the preparation is completed, the information of the ratio of each color ink for ink adjustment, ink adjustment oil, dry oil, ink type, quantity, manufacturer and other information will be registered in detail, so that it can be used as a reference for future ink preparation.

Other precautions: When analyzing manuscripts and color comparisons, it is best to use a standard light source or a light source close to sunlight. At the same time, pay attention to the color comparison after the ink film is dried to reduce visual errors. Try to use color close to the primary color ink or other single color ink when matching colors. Too much color ink is used for color matching, the worse the brightness and saturation, which affects the printing effect. Therefore, the fewer the number of inks in color matching, the better. It is better to choose the ink products of the same manufacturer.

During color mixing, we must fully understand the daily printing speed and printing pressure of our factory equipment, and control the ink viscosity. The color shade of the printed products under different printing speeds and pressures will be different, so the inks prepared at low speeds must be checked again with the original after the printing speed is turned to the normal speed to avoid color difference and waste.

After the ink is transferred out, if there is an error between the color sample printed on the machine and the original, it should be corrected in time. When changing the color, the ink fountain and the ink roller should be cleaned to avoid discoloration caused by unclean cleaning. The spot color ink remaining after printing can still be adjusted according to the hue of the new manuscript, but the saturation is often gray, especially the remaining ink retrieved from the ink fountain may have been emulsified or the printability has changed.

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