Quality Control GRACoL, GRACoL7 and G7

It is one of the dreams of many printing companies to establish a standardized printing production environment to improve the overall quality of printed materials. The development of various printing standards and the development of digital inspection and control technologies are all aimed at achieving this goal. Standardized and standardized production environments (systems) and corresponding detection methods are a hot topic for many research and production organizations. In this regard, GRACoL of the United States has made useful attempts and achieved very good results.

GRACoL (General Recommendations for Applications in Offset Lithography) is an application and promotion organization for offset printing technology in the United States and is affiliated to the International Digital Enterprise Alliance (IDEAlliance). GRACoL developed a standard set of methods for testing and implementing print production data in accordance with the ISO 12647-2 offset printing standard in China. In June 2006, GRACoL7 released the latest version of the implementation method.

Based on extensive experiments, GRACoL7 has extracted a series of new concepts and corresponding parameters for print inspection. By measuring and calculating these parameters, it finally obtains some data. Based on these results, it can adjust the RIP output calibration curve of CTP platesetter or imagesetter. The printing plant, on the basis of ensuring the repeatability of the system, further improves the tone and color reproduction quality of the printed matter.

GracoL7 test principle and implementation steps

1. Related concepts and parameters

GRACoL has proposed many new concepts and parameters for print testing and judgment, including: Neutral Print Density Curve (NPDC), Highlight Range (HR), and Shadow Contrast (SC). Highlights concepts such as Contrast HC (Highlight Contrast). Among them, the neutral printing density curve NPDC and the brightening range HR are the two most important concepts.

The NPDC contains both CMY and K monochromic package printing, reflecting the relationship between the measured neutral density and the percentage of original dots on a neutral gray scale, since the neutral gray density is an absolute measure. The value of the dot enlargement value is a relative quantity related to the density of the field, so this concept can guarantee the consistency of the measurement and the apparent sense of the tone scale and density.

Dot enlargement is a very important parameter in the field of printing control printing in China, but there are some defects in the operation comparison and so on. For this reason, GRACoL has also proposed the parameter of bright tuning range. The highlight range has replaced the dot expansion as a reflection in the proposed method. The overall tone of the print and gray balance parameters. HR needs to measure 4 times (2 times for CMY and K) and calculate 2 times; while the traditional dot expansion needs to measure at least 12 times for each color plate 3 times, and because HR is an absolute measurement value and dot gain is A relative value label exhibition, so the HR reflect the print contrast and visual brightness is more reliable than dot gain. among them:

HR_cmy=ND(50c, 40m, 40y)-ND(paper)HR_k=ND(50k)-ND(paper)

Darkness Contrast SC and Brightness Contrast HC is a parameter that quickly detects the correctness of neutral density curves in dark and bright tuning, where:

SC_cmy=ND(75c, 66m, 66y)-ND(paper)SC_k=ND(75k)-ND(paper) Darkness contrast depends on the dynamic range of the print, and can be calculated by IDEAlink curve software, bright contrast and dynamics Range-independent, is a constant:

HC_cmy=ND(25c, 19m, 19y)-ND(paper)=0.25HC_k=ND(25k)-ND(paper)=0.22 The introduction of these parameters brings many benefits to print quality control:

(1) The calculation methods of these parameters and dot gain rates are different. Lithographers, who are independent of the density of the field, thus directly reflect the visual brightness and make the subjective and objective quality evaluation more consistent

(2) It has nothing to do with the screening parameters, so no matter what kind of dot shape or number of screen lines can be compared under the same standard

(3) Due to the reduction of necessary measurement data, the efficiency of the printing press is greatly improved by adjusting the printing results of the measurement results.

2. Preparation

(1) According to the instructions of GRACoL7, about 6,000 to 10,000 sheets of paper are required for the test and calibration of the printing press, and the paper and ink must meet the GRACoL7 neutral density standard for GRACoL7 test sheets specified in ISO 12647-2. standard.

(2) Print layout. Layout text is the main test object of this test, and its necessary layout content includes:

a. Two sets of GRACoL7 P2P color deco printers, respectively, inverted 180 degrees layout;
b. A set of GRACoL7 grey balance guides;
c. Two IT8.7/4 characterization targets, reverse 180 degree layout;
d. about 1cm wide three-color overprint gray balance color bar: 50c, 40m, 40y;
e. Monochrome black and gray balance color bar about 1cm wide: 50k;
f. Necessary printed strips and subjective evaluation images.

(3) Clear all the calibration curves on the photoprinting or CTP platesetter (but ensure that the exposure system is in a normal stable working state) and measure the dot size on the plates produced in this state.

(4) Ensure that the performance of various color measuring equipment and observation equipment is stable, and then print the document under the normal operating conditions of the printing press.

3. Printing

GRACoL recommends that the printing color sequence be k-cm-y; the actual density that should be achieved in the printing process is c = 1.45 ± 0.10, m = 1.45 ± 0.10, and y = 1.0 ± 0.07 for personalized printing, k = 1.65 to 1.9.

4. Generate neutral printing density curve stickers printing

After the proofs are dried, measure the monochrome black and three-color overprint black ash scales on the P2P color scale respectively. To eliminate the error, measure the two sets of color patches patterned with 180 degrees each other, and then take the measurement results as average values. The measurement results are plotted on the GRACoL self-made coordinate chart and then the lithography is printed. Then the nearest point of the preset curve and the maximum density point of the drawn curve and some inflection points of the curve are found. According to the mapping rules, the RIP calibration curve is found in the figure. The keypoint value is entered into the RIP calibration curve.

5. Generate gray balance curve

The gray balance curve is obtained from the gray balance guide and the GRACoL7 plot. The process is as follows: First, a spectrophotometer is used to measure the patch color value at the gray balance guide portion C50 on the printed sample sheet. If the gray level of the center patch reaches a standard (a=0,b=-2), then The gray balance reduction of the output device is very accurate in Shanghai printing, and there is no need to establish a new gray balance curve. If the gray balance point is not a preset central color block, then the output device needs the correction of the gray balance curve.

If gray balance calibration curve is needed, firstly make CMY tri-color curve on the GRACoL7 graph, print the branch digitally on the basis of the reference line, find out the difference of gray balance achieved by three colors, and find the gray balance correction in RIP. The data is then entered into the RIP. Finally, use the newly added RIP calibration curve to reprint the test target plate and check whether the corresponding parameters are up to standard.

6. Conclusion

This article briefly describes the procedures for testing and calibrating the printing system specified by GRACoL7 in Beijing Screen Printing. The core mechanism of this method is to adjust the output calibration curve of CTP or CTF in order to achieve the purpose of improving the quality of the final printed matter; implementation of this test and calibration process has The two preconditions cannot be ignored. First, all output devices must be stable during the entire plate-making and printing process, and must have repeatable output. Otherwise, the parameters obtained will have no meaning for actual production. Second, the materials used must comply with ISO standards. The relevant standards regulate the printing of anti-counterfeit labels because this is the basis for measurement, calculation, and judgment.