Digitalization and prospect control of color management of cigarette label printing

Color management in the printing process occupies an important position in the entire color management system, because the quality of work at this stage is directly related to the quality of batch products, especially in the field of cigarette label printing with stability as the first requirement. Poor control, ranging from batch scrapping to customer complaints and returns, and the direct impact on their reputation.

1. The importance of data standards

The so-called standardization is the maximum degree of standardization. The same process conditions, different locations, different times, and different operating staff to print, the effect should be completely consistent. This requires the measurement and control of process parameters to stabilize the process conditions. This requires us to determine the key processes in the printing process according to the characteristics of our company. All the processes in the printing process must be well controlled to achieve the purpose of professional production.

In traditional printing production, employees' thinking is perceptual, empirical, and disorderly, often adopting the method of masters and apprentices to impart skills. In the thinking habits of some teachers, it is always believed that printing production relies on the experience passed on from generation to generation. It is unclear and unclear. If you have more experience, you will know it naturally. In fact, this is an unscientific and rigorous approach. Modern printing requires many steps to complete together, to maximize cost savings, improve production efficiency and ensure stable printing quality, it requires a scientific and strict system and process.

The standardization and standardization of enterprises can form strong productivity. A research institute in London, UK has done a lot of long-term data statistics, and concluded that 13 of the economic growth in Europe is due to standardization, which is much higher than the economic drive of patented technology. We must be keenly aware that the rational age of the printing industry has arrived. In recent years, the business strategy alliance formed by a number of printing industry suppliers and academic groups has promoted a unified "job identification format", both by establishing standard "output" and "input" information formats, thereby controlling The effective operation of the production process has become the consensus of "standardization, digitization and standardization" in the industry, and is known as a milestone in the standardization of the printing industry.

Standardization is indeed conducive to printing quality control, effectively reducing costs and improving service levels. The printing production process is long and complicated. The control of printing quality lies in the whole process. Any process link is not handled properly, the skill level of the operators in each link is uneven, and the operation is not standardized. It is impossible to produce high-quality printing products. For each production process, set up control points and control elements to control whether the process meets the standard requirements as the basis for the next process.

Second, the color data of cigarette pack products

For printing companies, while formulating the color quality standards for printed products, the quality standards of the printing materials (ink and paper) must be formulated and effectively controlled to achieve the required print quality. The tolerance range of the color of the printed product and the paper can adopt the same control standards as the user of the printed product (cigarette enterprise); or, according to the production conditions of the printing company, formulate relatively strict control standards. The standard chromaticity coordinate value of the target color of the printed product is the same; or, the data obtained by measuring the ink color sample obtained by scraping the specified ink using an automatic ink scraper is used as the standard chromaticity coordinate value of the ink. In this way, the incoming ink does not need to be printed on the machine, and it can be easily detected and controlled. The color difference of the printed product is a comprehensive performance of all influencing factors. Therefore, for the quality control of the incoming ink, a relatively strict qualification tolerance must be formulated Range, so that the printed product obtained after the color difference changes caused by other influencing factors after printing can meet the final quality requirements.

1. Formulation of target color standard chromaticity value

Cigarette trademark color quality management can use spectrophotometer and color data processing system. The minimum measurement caliber is 3mm; therefore, the actual measured area of ​​the color measured on the cigarette trademark must be greater than 3mm to make sense; it must be greater than 5mm to facilitate the staff to detect and obtain good repeat accuracy.

After the cigarette trademark is wrapped, it will be covered, and will not affect the sizing area, and print 5-8mm diameter detection color blocks for all colors that need to be controlled.

Sampling 20 to 100 or more proofing trademarks, after visual observation by the standard-setting department with human eyes, meets the design color requirements and the difference between the proofs within the width of the human eye proofing trademark as a standard color sample.

Carry out chromaticity measurement and statistical calculation for each color of the above standard color samples, and calculate the average value of L, a, and b as the standard chromaticity value of the color in the trademark; or use the target color in the color quality control software In the setting function, the average value measurement sets the standard chromaticity value.

Repeat the above operations to formulate the target color tolerance range of L, a and b standard chromaticity values ​​for different trademarks and different colors.

2. Use the ΔECMC eligibility determination method to formulate the tolerance range

According to the product quality requirements, a reasonable commercial index difference value (different colors with consistent color quality requirements only need to set the same value) as the final color quality qualification judgment tolerance range.

3. Means of production process control

Generally, process technicians always want to maintain constant process parameters and technical methods for production. But in the actual production process, due to one or another factor change, it is often difficult to achieve this ideal production conditions. Especially for inks, due to the constraints of the chemical and pigment industries, it is impossible for inks produced in different batches to maintain the same quality level, especially the quality fluctuations between batches with long production dates. Therefore, before printing, it is necessary to adjust the specific process parameters on the basis of the original process standards for the actual production conditions such as material changes, so as to ensure that different batches of printed products obtain satisfactory and stable color quality.
Example 1: When the color saturation of the ink is slightly higher than the standard, and a new printing plate with relatively deep scores is used, the viscosity of the ink should be reduced or the printing speed should be increased to adjust the thickness of the ink layer of the printed product to reduce the printed product accordingly. Color saturation.

Example 2: When the ink color is consistent with the standard, and the paper is slightly yellowish. Then, the ink color of the printed product printed on a large area must be slightly yellowish. Therefore, the paper with relatively high whiteness must be replaced, or the yellow component of the ink color should be adjusted appropriately. In this way, the final quality requirements of the color of the printed matter can be achieved.

4. Quality control

During the printing process, due to the random errors of the printing equipment, the technical capabilities of the operator, the volatilization of the ink composition and the change in viscosity, the blockage of the printing plate, the fluctuation of the printing speed, the change in the drying temperature, and the change in the environmental temperature and humidity All will cause the color quality fluctuation of the same batch of printed products. Therefore, it is necessary to use a color difference meter to conduct timely sampling and testing of semi-finished products during the printing process, understand the color difference of the printed product and adjust the process parameters in time, and control the color quality of the printed product to fluctuate within the smallest change range. Thereby reducing the scrap rate of printed products and improving economic efficiency. According to whether the printing equipment is equipped with a slitting mechanism, a corresponding random sampling time shall be formulated to timely sample semi-finished products for printing.
5. Use color difference measurement to detect randomly sampled products for color difference analysis

Note: The color difference analysis in the quality control process is not a judgment of the conformity of printed products. If the sampling test results exceed the tolerance range of the qualification, the process parameters are adjusted to control, which will cause many unnecessary waste products. Therefore, it is necessary to adjust the process parameters in time according to the actual color deviation. Combined with the relevant content of the CIELAB system in the basic theory of color, the color difference bias analysis was performed on the above sampling test data. [next]

3. Prospect control of color management

A manuscript that includes pictures and text often has to be processed by many devices before it can be copied. These devices include input devices, graphics and image processing devices, and output devices. Since the digital prepress graphic information processing system is open, it is not limited to the equipment, materials and processes used. The diversity of color rendering characteristics of devices of various brands, types, and color characteristics increases the difficulty of accurately reproducing colors. In the transmission of graphic information in these devices, information loss will inevitably occur, so that the copied image and the original are far apart in terms of color, gradation and saturation, and even the entire image is completely unrecognizable. To copy the manuscript correctly and completely, there must be a mechanism to control the color conversion and transmission. This is color management.

Color management is abbreviated as CMS (ColorMangementSystem). It is first a question of color space, that is, which color space is used to control color. Monitors, digital cameras, scanners, etc. all work in the RGB color space; printers, proofers, printing machines, etc. all work in the CMYK color space. When the same image is output on these devices, the final color effect is entirely possible different. This is because they are in different color spaces, and there are differences in color expression.

Color management needs to be based on a color space that has nothing to do with any specific equipment, materials, and processes. At present, in the color management technology, the so-called color feature connection space is the chromaticity space of CIE1976Lab. The colors on any device can be converted to this space to form a "universal" description method, and then the color matching conversion is performed. . Within the computer operating system, the task of implementing color matching conversion is performed by the "color matching module", which is important for the reliability of color conversion and whether the colors match.

So, how to realize the transmission of color in the "universal" color space and realize the lossless or minimal loss of color? This requires each set of equipment to generate a profile, that is, the color profile of the equipment. We know that when presenting and transmitting colors, various devices, materials, and processes will exhibit different characteristics. In color management, the colors presented on one device must be displayed on another device with high fidelity, which requires us to understand the color rendering characteristics of colors on various devices. Since the color space independent of the device has been selected, namely the CIE1976Lab color space, the color characteristics of the device are as follows: the corresponding relationship between the description value of the device and the chromaticity value of the "universal" color space, the corresponding relationship is The color description file for the device.

In color management technology, there are three types of device color characteristic description files. The first type is the scanner signature file, which provides the standard manuscripts of Kodak, Agfa, and Fuji and the standard data of these manuscripts. These manuscripts are input using the scanner. The difference between the scanned data and the standard manuscript data reflects the scanner The second type is the characteristic file of the display, which provides some software to measure the color temperature of the display, and then generates a color block on the screen, and the color block information reflects the characteristics of the display; the third type is the printing device Feature file, it also provides a set of software, the software generates a graphic containing hundreds of color blocks in the computer, and then output the graphic on the output device, if it is a printer, directly proof, the printing machine will first output the film 3. Proofing and reprinting, measuring these output images to reflect the characteristic file information of the printing device.

The generated profile, that is, the color characteristic file, its format is composed of the file header, the mark table, and the mark element data.

File header, which contains the basic information of the color characteristic file, such as file size, type of color management method, version of file format, device type, device color space, color space of characteristic file, operating system, device manufacturer , Color restoration target, original medium, light source chromaticity data, etc., the file header occupies 128 bytes. Mark table, which contains information about the name of the mark, storage location, and data size, but does not contain the specific content of the mark. The name of the mark occupies 4 bytes, and each item of the mark table occupies 12 bytes . Marking element data is to store various information required for color management at a prescribed location according to the marking table, and varies according to the complexity of marking information and the amount of marking data.

For the color characteristic files of equipment in printing enterprises, the operator of graphic information processing has two ways to obtain them.

The first way is when the device is purchased, the manufacturer provides the profile with the device, which can meet the general color management requirements of the device. When the application software of the device is installed, the profile is loaded into the system.

The second way is to use special profile production software, according to the actual situation of the existing equipment, to generate a suitable color feature description file, so that the generated file is usually more accurate and more in line with the user's actual situation. Because the state of equipment, materials and process flow will change or shift with time. Therefore, the profile needs to be re-created at intervals to adapt to the color response at the time.

Now, let's take a look at how colors are transmitted in each device.

First, for a manuscript with normal colors, scan and input it with a scanner. Due to the profile of the scanner, the color from the scanner (that is, the tristimulus values ​​of red, green, and blue) to the CIE1976Lab chromaticity is provided. Correspondence relationship of the space, so the operating system can obtain the chroma value Lab of the original color according to this conversion relationship.

The scanned image is displayed on the monitor screen. Since the system has mastered the correspondence between Lab chromaticity values ​​and the red, green, and blue drive signals on the display, the display does not directly use the chromaticity values ​​of the scanner's red, green, and blue, but from In the Lab chromaticity value of the original in the previous step, according to the conversion relationship given by the profile of the display, the display drive signals of red, green and blue that can correctly display the color of the original on the screen are obtained, and the display is driven to display the color. This ensures that the color displayed on the monitor matches the color of the original.

After observing the accurate image color display, the operator can adjust the image according to the color of the screen according to the customer's requirements. Since the profile of the printing device is included, after the image is separated, the correct color after printing can be observed on the display. When the operator is satisfied with the color of the image, the color is separated and stored. When color separation, according to the color conversion relationship carried by the profile of the printing equipment, get the correct percentage of dots. After RIP (raster image processor), record printing, printing, proofing, printing, you can get a printed copy of the original to complete the entire process.

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