Offset printing is a relatively complicated production process. Ink, fountain solution, printing plate, blanket, substrate, printing pressure and other factors in the offset printing process will directly affect the printing process and printing effect. The consistency and stability of print quality has always been the center of attention in the industry. In actual production, in order to obtain the best printing quality, we must first adjust the printing process to the best working conditions, and then set the quality control threshold, and finally through repeated measurement and adjustment in the printing process to maintain the quality and consistency of the printed parts.

1. Current status of evaluation research on print quality

The evaluation methods of print quality include subjective visual method, density detection method and chromaticity detection method.

The subjective visual method is a completely subjective, but also the simplest method. Directly compare the printed matter with the standard sample sheet to make a judgment on the quality of the printed sheet. However, it is greatly influenced by the observer's experience, psychology and other factors. There are too many variable factors that cannot be quantitatively described, which affects the accuracy and reliability of the evaluation.

Density measurement is actually a measure of the amount of reflected or transmitted light, and is a reflection of the relative value of the same ink saturation. The instrument used in this method is cheap and widely used. However, when measuring the color surface, it can only obtain the relative amount of a certain primary color ink in printing, but cannot indicate the hue of the color being measured, and cannot contact the color metering system to describe the color. Its use has certain restrictions.

The chromaticity detection method first determines the spectral reflectance within the debugging range, and then realizes the adjustment and control of the amount of ink according to the spectral reflectance or some chromaticity characteristic parameters derived therefrom. The measurement result is directly related to the visual perception of the human eye. More intuitively shows the color of printing. At present, the commonly used method is that the printing press operator or quality inspector extracts some printed sheets, compares these printed sheets with the printed samples by visual comparison, and measures the quality control strip printed at the same time as the printed image to make an evaluation of the printed sheet quality. These control bars are generally placed on the trailing part of the printed sheet. Although parameters such as solid density, overprint rate, dot expansion, and contrast of various colors of ink can be obtained, they cannot completely display the overall printing situation of the printing screen. Their judgment on the printing quality is subjective, and the customer's requirements are often over compensated, which is not conducive to the control and management of the printing quality.

Although the online printing screen detection recently developed can detect the overall printing of printed sheets, it is limited by the speed of computer image processing and printing format, and is currently mainly used for small-format bill printing. Based on this, use a spectrophotometer to measure the chromaticity of the printed sheet to obtain the color development of the detection point set on the printed sheet, which is more feasible and effective as the basis for printing control.

Most of the printing industry uses the CIE1976L * a * b * uniform color space and its corresponding color difference formula. The advantages of this color space are: L *, a *, b * are consistent with the human eye's vision. When the color difference of the color is greater than the visual recognition threshold and smaller than the adjacent two-level color difference in the Munsell system, it can be better The psychological feeling effect of reflecting the object color. The use of chromatic aberration to evaluate the similarity of printed matter is consistent with the visual characteristics of the human eye, and a good evaluation effect can be obtained. The color difference can comprehensively reflect the color condition, and can effectively control the color quality condition of the printed product. Generally,> 5 can clearly detect the color difference between the test sample and the standard sample.
(L, a, b are the data of standard proofs, L, a, b are the data of test proofs) [next]

2. Experimental design and data collection

In this paper, samples are obtained through normal printing, and 200 consecutive printed sheets (from the 18th sheet after paper printing) are selected as the test object. Considering that the color of the printing screen is greatly affected by the ink area of ​​the printing press, several test points are set in different ink areas and positions of the printing screen, the chromaticity value is measured, and then statistical analysis is performed. This paper selects 9 representative test points from different locations for comparative analysis.

â‘  Experimental instrument.

175g coated paper, Tianjin Coswood quick-drying bright four-color offset sheetfed ink, Airfa laser phototypesetting film, Huaguangyang graphic PS version, Japanese screen Screen FT-R3050 phototypesetting machine, Shanghai Nanren Xingda W- PS-880 automatic developing machine, Shanghai Nova SBK1150-GM printing machine, Heidelberg M04K printing machine, printing color sequence is KMCY, round dots, screen line number 200L / In, screen angle: C15 °, M45 °, Y90 ° , K75 °, X-Rite 528 spectrodensitometer.

â‘¡Data collection environment.

The X-Rite528 spectrodensitometer was used to measure the chromaticity value of the selected test color patch on the printed sheet with a 2 ° field of view and CIE1976L * a * b * chromaticity space under the D65 light source.

â‘¢Statistical analysis of print quality.

Through comparative analysis, after selecting standard samples, the chromaticity values ​​of the selected test patches on 200 consecutive sheets are measured, and the color difference values ​​are calculated one by one, and finally the dot plot (dynamic) analysis and probability distribution (static) statistics Analysis (limited to space, only the test results of 3 points are listed here). [next]

3. According to the above data and analysis chart, the following analysis results can be obtained:

①Overall, in this study, from the official printing of the 17th sheet, the printing process reached a stable state (the number 1 in this article is the 18th sheet after the official printing). Except for a very few (8 out of 600 points) singular points, the color difference ΔE of the three test points and the corresponding points of the standard sample fluctuates within a small range, less than 5, to meet the general printing needs. Within the 95% probability range, the color difference distribution range of test point 1 is [0, 3.3942], the color difference distribution range of test point 2 is [0, 3.1799], and the color difference distribution range of test point 3 is [0, 1.9070 ]. The color fluctuation of continuous printing sheets at the same test point may be caused by the dynamic balance of ink, printing pressure, paper moisture deformation, vibration of printing machinery and other factors.

②The test results on 200 continuous prints show that the average color difference of test point 1 is 1.8180 and the distribution variance is 0.9553; the average color difference of test point 2 is 1.3222 and the distribution variance is 1.1259; the average color difference of test point 3 is 1.1355 and the distribution variance is 0.4676; No. 1 test point, within ± 3σ distribution range, statistical probability is 86.37%; ± 2.58σ, statistical probability is 75.08%; ± 2σ, statistical probability is 53.86%; No. 2 test point, within ± 3σ distribution range Statistical probability is 96.60%; ± 2.58σ, statistical probability is 92.00%; ± 2σ, statistical probability is 79.48%; test point 3, within ± 3σ distribution range, statistical probability is 71.62%; ± 2.58σ, statistical probability is 56.02%; ± 2σ, the statistical probability is 33.42%; it can be seen that the color difference of each test point is distributed in a bell shape, which does not conform to the normal distribution law.

â‘¢ There are two singular points (color difference> 5) of test point No.1, which occurred on No.26 (color difference is 5.1595) and No.148 printing sheet (color difference is 5.1187); the human eye can clearly see the color difference. It can be seen from the analysis chart that the test block is darker and redder than the standard sample. | 5.1187- | = 3.3007> 3 = 30.9553, which is a gross error and should be deleted; there are 6 singular points on the 2nd test point, which occurred on the 26th (color difference is 6.6116), 27 (color difference is 5.4212), No. 28 (color difference 5.0997), No. 29 (color difference 6.6607) and No. 149 printed sheets (color difference 5.5939); Nos. 26 to 29, the test points on these sheets are greener than the standard, indicating that the amount of cyan ink is larger . Compared with the standard sample, the test point on the 149th sheet has a color difference of 5.5939, and there is a sudden large fluctuation, | 5.5939- | = 4.2717> 3 = 31.1259, and it basically corresponds to the test point 1 and the fluctuation quickly disappears. It belongs to gross error and should be deleted; No singular point occurs at test point No. 3.

â‘£ Comprehensive analysis of the color rendering of the three test points, it is not difficult to find: the average color difference and distribution variance of test point 3 are the smallest, indicating that its color consistency and stability are better than the other two points; this may be The position on the printed sheet is related. The first and second test points are located in different ink areas in the direction of the mouth of the printed sheet, which is greatly affected by the ink key adjustment. Correspondingly, the color difference from the corresponding point on the standard sample sheet will be larger. The third test point and the first test point are in the same ink area, located in the middle of the printed sheet. Theoretically, the printing color is greatly affected by the ink area, and the color development in the same ink area should be consistent. However, as can be seen from the above distribution diagram, although the third test point and the first test point are in the same ink area, its color rendering effect is better than that of the first test point, and it is stable, indicating that the printing Under the system, the color rendering at different locations in the same ink zone is different, and the printing effect in the middle of the printed sheet is better than the gripper.

4 Conclusion

In this paper, through the actual printing experiment, the color of the printed matter at different positions on the printed matter and the same position on the printed sheet are analyzed and found that the color of the different areas of the printed matter is different from the color on the printed sheet. On the location. They all have a bell-shaped distribution, but they do not conform to the normal distribution law. The distribution characteristics of each test point are different, and the printing quality of the middle part of the printed sheet is better than the gripper position.

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