As a revolutionary control method, G7 has a great advantage in the CTP era. Although originally designed for commercial offset printing, it can be used in any CMYK imaging process in the current practice and has been successfully tested to more printing processes, including coated and non-coated paper offsets. , newsprint printing, gravure printing, flexographic printing, dye sublimation printing, inkjet printing, and xerographic printing, and so on.
When it comes to G7, it is impossible to say that the GRACoL standard is a set of operation guides and detailed specification manuals developed for the high-quality commercial offset printing by the American Offset Printing Commercial Printing Organization (GRACoL) combined with the practice of CTP for many years. The standard is named GRACoL standard and GRACoL7 is the latest version of GRACoL. G7 is a method for correcting printing and printing presses according to the GRACoL7 standard. G represents the gray value to be corrected, and 7 represents 7 basic colors defined in the ISO 12647-2 printing standard. , ie CMYKRGB. G7 achieves the accurate reduction of color by controlling the chromaticity and neutral gray of CMYKRGB. From the visual characteristics of the human eye, a series of neutral gray density curves (NPDC) are defined for printing and copying, which can realize different materials. "The same appearance printing."
Quality control parameter
G7 proposes several novel quality control parameters, and in quality control, the most important are the following two.
1. HR Brightness Range: The HR Brightness Range is mainly used to evaluate the performance of bright and dark parts of printed matter. It has replaced the dot expansion to become the main parameter reflecting the overall tone and gray balance of printed matter. There are two main methods for evaluating HR, namely CMY ash (50C, 40M, 40Y) and black ink monochrome black (50K). In order to facilitate calculation and analysis, HR is mainly expressed by the value of neutral gray density (ND), and the value of ND can be calculated from the Y value in CIE XYZ value.
The formula for calculating ND: ND = log10 (100/Y) (0<Y≤100)
HR_CMY can be obtained by subtracting the neutral gray density of the three-color gray block from the neutral gray density of the paper. The formula is: HR_CMY= ND(50C, 40M, 40Y) - ND(paper). The general print has an HR_CMY of 0.54.
HR_K can be obtained by subtracting the neutral gray density of the paper from the neutral gray density of the measured monochromatic gray block. Calculation formula: HR_K = ND (50K) - ND (paper). The general print has an HR_K of 0.50.
2. Neutral ash density curve: The neutral ash density curve (NPDC) is another new concept proposed by G7. This curve shows the relationship between the measured density of the neutral gray scale and the original. Because NPDC is an absolute value, and the printed dot gain curve (TVI) is a relative value, NPDC can better ensure density matching and contrast matching between different devices.
G7 defines a total of two kinds of NPDC, that is, the gray scale of the CMY three-color superimposed and the gray scale of the black ink. The NPDC correction compares the printed gray scale with the reference scale and then calculates the RIP correction value as a percentage to change the printing state to bring the NPDC to the desired shape. Curve correction can be drawn on the drawing by manual calculation or automatically calculated by the IDEA link curve software.
Correction method
The G7 calibration process consists of three main steps, equipment calibration, linearization printing, and gray balance correction.
Equipment Calibration: Equipment calibration mainly includes two parts: CTP correction and printer calibration.
1. CTP correction: CTP correction is divided into stability test of CTP plate condition and linearization correction of CTP plate output. To carry out the stability test of CTP plateting conditions, firstly, the parameters such as HP value, conductivity, temperature and dot percentage of CTP developer in a certain period are monitored, and then the influence of these parameters on the output of the dot is statistically analyzed to judge the CTP rush. Whether the version condition is in a stable state. When the CTP plate condition is in a stable state, the linearization correction of the CTP plate output can be performed. The linearization correction process of the CTP plate output is as follows: First, the test version without any correction is output and the data is measured, and then the measurement data is used to generate the plate linearization compensation curve, and then the compensation curve is used to output the test plate again and perform data measurement. After the CTP linearization is completed, the error of the outlet output is required to be within ±0.5%.
2. Printing machine calibration: The stability and repeatability of the printing press is an important prerequisite and application basis for realizing the G7. Therefore, it is necessary to calibrate the press to ensure that it achieves optimal results before performing the G7 test. The press and its consumables should be properly adjusted, including blankets, linings, fountain solutions, and more. The printing machine calibration factors include ink balance, printing pressure, registration control and ink drying, and the working state of the printing machine can be quantified by the relevant properties of the printing proof, such as solid density, printing contrast, overprinting rate, dot gain, registration accuracy. Wait. The easiest way to determine if a press's performance meets the test requirements is to print a flat screen.
Linear printing
The linear printing is carried out in order to obtain the printing state of the printing machine after reaching the corresponding technical parameters, and the CIE LAB value of the monochrome solid ink CMYK and the two-color overprinting ink RGB specified by ISO 2846-1 can be realized. Since each ink contains three target values ​​L*, a* and b*, and there is only one solid density value, it is easier to measure the CMY solid density value with a densitometer and control the solid density as the target density in the print control. . When CIE LAB and density values ​​conflict, the former takes precedence. After reaching the calibrated amount of solid ink, check the TVI of each CMYK swatch. The main purpose is to ensure that the printing status is normal, that is, all ink TVI must be smooth and smooth.
In the linear printing stage, gray balance and NPDC are not important, but the uniformity of the whole sample should be achieved as much as possible. The color difference of CMYKRGB should be within the tolerance range (<5), and the density of the bite area should be uniform (error ± 0.05) Within), the registration error is within 0.01mm.
Gray balance correction
G7 mainly controls the print quality by controlling the gray balance, so the gray balance correction is crucial. There are two ways to adjust the gray balance. One is to adjust the device itself, such as adjusting the amount of ink, and the other is to create a CMY RIP color separation curve. When performing offset press calibration, it is best not to use the RIP separation curve because most offset printing equipment gray balance errors are unstable or not uniform. Conversely, for more stable equipment, such as proofing systems or inkjet printers, the RIP separation curve can be used for calibration because the gray balance of these devices is relatively stable.
The gray balance is controlled during the production process by measuring the HR patches (50C, 40M, 40Y) to ensure that the corrected equipment is still in gray balance. Measuring gray balance also requires SC patches (75c, 66m, 66y) and HC patches (25c, 19m, 19y) to find small errors in the reproduction of the image table in the running process, but because of the inherent instability of the device itself, these The error is difficult or impossible to correct. Therefore, in actual production, the standard Lab value requirements for HR, SC and HC patches should not be too harsh.
Nowadays, G7 certification has become a symbol of high-quality printing. G7 certification seems to indicate that the company has certain quality control capabilities and is in line with international standards. It regulates printing production operations, improves printing quality and enhances operators. And the professional quality of technical management personnel also has an important role in promoting. Perhaps, we should make the G7 more violent based on the understanding of the G7.
As a revolutionary control method, G7 has a great advantage in the CTP era. Although originally designed for commercial offset printing, it can be used in any CMYK imaging process in the current practice and has been successfully tested to more printing processes, including coated and non-coated paper offsets. , newsprint printing, gravure printing, flexographic printing, dye sublimation printing, inkjet printing, and xerographic printing, and so on.
When it comes to G7, it is impossible to say that the GRACoL standard is a set of operation guides and detailed specification manuals developed for the high-quality commercial offset printing by the American Offset Printing Commercial Printing Organization (GRACoL) combined with the practice of CTP for many years. The standard is named GRACoL standard and GRACoL7 is the latest version of GRACoL. G7 is a method for correcting printing and printing presses according to the GRACoL7 standard. G represents the gray value to be corrected, and 7 represents 7 basic colors defined in the ISO 12647-2 printing standard. , ie CMYKRGB. G7 achieves the accurate reduction of color by controlling the chromaticity and neutral gray of CMYKRGB. From the visual characteristics of the human eye, a series of neutral gray density curves (NPDC) are defined for printing and copying, which can realize different materials. "The same appearance printing."
Quality control parameter
G7 proposes several novel quality control parameters, and in quality control, the most important are the following two.
1. HR Brightness Range: The HR Brightness Range is mainly used to evaluate the performance of bright and dark parts of printed matter. It has replaced the dot expansion to become the main parameter reflecting the overall tone and gray balance of printed matter. There are two main methods for evaluating HR, namely CMY ash (50C, 40M, 40Y) and black ink monochrome black (50K). In order to facilitate calculation and analysis, HR is mainly expressed by the value of neutral gray density (ND), and the value of ND can be calculated from the Y value in CIE XYZ value.
The formula for calculating ND: ND = log10 (100/Y) (0<Y≤100)
HR_CMY can be obtained by subtracting the neutral gray density of the three-color gray block from the neutral gray density of the paper. The formula is: HR_CMY= ND(50C, 40M, 40Y) - ND(paper). The general print has an HR_CMY of 0.54.
HR_K can be obtained by subtracting the neutral gray density of the paper from the neutral gray density of the measured monochromatic gray block. Calculation formula: HR_K = ND (50K) - ND (paper). The general print has an HR_K of 0.50.
2. Neutral ash density curve: The neutral ash density curve (NPDC) is another new concept proposed by G7. This curve shows the relationship between the measured density of the neutral gray scale and the original. Because NPDC is an absolute value, and the printed dot gain curve (TVI) is a relative value, NPDC can better ensure density matching and contrast matching between different devices.
G7 defines a total of two kinds of NPDC, that is, the gray scale of the CMY three-color superimposed and the gray scale of the black ink. The NPDC correction compares the printed gray scale with the reference scale and then calculates the RIP correction value as a percentage to change the printing state to bring the NPDC to the desired shape. Curve correction can be drawn on the drawing by manual calculation or automatically calculated by the IDEA link curve software.
Correction method
The G7 calibration process consists of three main steps, equipment calibration, linearization printing, and gray balance correction.
Equipment Calibration: Equipment calibration mainly includes two parts: CTP correction and printer calibration.
1. CTP correction: CTP correction is divided into stability test of CTP plate condition and linearization correction of CTP plate output. To carry out the stability test of CTP plateting conditions, firstly, the parameters such as HP value, conductivity, temperature and dot percentage of CTP developer in a certain period are monitored, and then the influence of these parameters on the output of the dot is statistically analyzed to judge the CTP rush. Whether the version condition is in a stable state. When the CTP plate condition is in a stable state, the linearization correction of the CTP plate output can be performed. The linearization correction process of the CTP plate output is as follows: First, the test version without any correction is output and the data is measured, and then the measurement data is used to generate the plate linearization compensation curve, and then the compensation curve is used to output the test plate again and perform data measurement. After the CTP linearization is completed, the error of the outlet output is required to be within ±0.5%.
2. Printing machine calibration: The stability and repeatability of the printing press is an important prerequisite and application basis for realizing the G7. Therefore, it is necessary to calibrate the press to ensure that it achieves optimal results before performing the G7 test. The press and its consumables should be properly adjusted, including blankets, linings, fountain solutions, and more. The printing machine calibration factors include ink balance, printing pressure, registration control and ink drying, and the working state of the printing machine can be quantified by the relevant properties of the printing proof, such as solid density, printing contrast, overprinting rate, dot gain, registration accuracy. Wait. The easiest way to determine if a press's performance meets the test requirements is to print a flat screen.
Linear printing
The linear printing is carried out in order to obtain the printing state of the printing machine after reaching the corresponding technical parameters, and the CIE LAB value of the monochrome solid ink CMYK and the two-color overprinting ink RGB specified by ISO 2846-1 can be realized. Since each ink contains three target values ​​L*, a* and b*, and there is only one solid density value, it is easier to measure the CMY solid density value with a densitometer and control the solid density as the target density in the print control. . When CIE LAB and density values ​​conflict, the former takes precedence. After reaching the calibrated amount of solid ink, check the TVI of each CMYK swatch. The main purpose is to ensure that the printing status is normal, that is, all ink TVI must be smooth and smooth.
In the linear printing stage, gray balance and NPDC are not important, but the uniformity of the whole sample should be achieved as much as possible. The color difference of CMYKRGB should be within the tolerance range (<5), and the density of the bite area should be uniform (error ± 0.05) Within), the registration error is within 0.01mm.
Gray balance correction
G7 mainly controls the print quality by controlling the gray balance, so the gray balance correction is crucial. There are two ways to adjust the gray balance. One is to adjust the device itself, such as adjusting the amount of ink, and the other is to create a CMY RIP color separation curve. When performing offset press calibration, it is best not to use the RIP separation curve because most offset printing equipment gray balance errors are unstable or not uniform. Conversely, for more stable equipment, such as proofing systems or inkjet printers, the RIP separation curve can be used for calibration because the gray balance of these devices is relatively stable.
The gray balance is controlled during the production process by measuring the HR patches (50C, 40M, 40Y) to ensure that the corrected equipment is still in gray balance. Measuring gray balance also requires SC patches (75c, 66m, 66y) and HC patches (25c, 19m, 19y) to find small errors in the reproduction of the image table in the running process, but because of the inherent instability of the device itself, these The error is difficult or impossible to correct. Therefore, in actual production, the standard Lab value requirements for HR, SC and HC patches should not be too harsh.
Nowadays, G7 certification has become a symbol of high-quality printing. G7 certification seems to indicate that the company has certain quality control capabilities and is in line with international standards. It regulates printing production operations, improves printing quality and enhances operators. And the professional quality of technical management personnel also has an important role in promoting. Perhaps, we should make the G7 more violent based on the understanding of the G7.
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