Talking about the Establishment of Printing and Copying Curve of Offset Press
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The print copy curve is a curve that describes the relationship between the density of the original to the copy of the print or the dot. In this paper, GATF star and 12-level gray scale are used as tools to observe the dot gain and solid density control in the printing process. The printing replication curve is established by measuring the percentage of dots at each level in the three main process steps of output, plate making and printing.
In the printing work, the dot is the basic printing unit that constitutes the continuous tone image, and the tone, gradation, contrast, sharpness and color of the printed image are expressed by the size change of the dot, so the reproducibility of the dot is good or bad. Directly affect the quality of printed matter. At the same time, the quality evaluation and management of the print quality and the quality of the print operation are also mainly achieved through the evaluation of the dot reproduction.
The Printing Copy Curve is a curve used to describe the hierarchical relationship between the original, the negative film, the printing plate and the printed matter during the entire copying process from the printed original to the printed product. The print replication curve is established by measuring the percentage or density of dots in each process step, so the exact degree of reproduction of the printed dots is closely related to the correctness of the print replication curve. The dot reproducibility is good, the dot gain value is within the specified range, and the printed manuscript can be well reproduced on paper or other substrates. In this paper, the offset printing process is used to quantify the change of the dot from the original to the finished product, and the printing replication curve is established. This curve can be used to analyze and evaluate the operating standard of each process, that is, the performance of existing printing equipment, process conditions and printing. The skill level of the operator, at the same time, the curve has great guiding significance for printing the same or similar orders in the future, and indirectly improves the production efficiency of the enterprise.
1. Experimental material
Single-sided 115g coated paper, fast-drying magenta ink, developer, positive-image PS version (size 460×395mm), Phoebe photo-setting machine, PS version automatic developing machine, Win500 monochrome offset printing machine, SBK-B iodine-gamma lamp Plate machine, T6CR color reflection densitometer, WYSK-40×40 times with light source reading microscope.
2. Establishment of experimental monitoring objects
The GATF star is used to facilitate visual observation of ink volume and dot changes during the printing process. The GATF star is composed of 36 black wedge-shaped lines arranged at equal intervals and arranged within a circumference. Such an arrangement characteristic, such as various dot defects in printing such as "mesh enlargement", "ghost imaging", etc. Can be clearly reflected. At the same time, using 12-level gray ladder ruler, the density point meter is used to directly test the dot data of the printing film, the printing plate and the printed matter at all levels, and the printing replication curve is established by using the data.
3. Sampling
In the printing operation, when the printing machine's ink supply, water supply system is evenly balanced, the paper feeding system is accurately fed and accurately positioned, the pressure between the plate roller and the rubber roller, the rubber roller and the paper is appropriate and Hengding, the various levels of the original are well reflected in the printed matter and then start sampling. Sampling methods for printed products are generally sampled in two ways at regular intervals and a certain number of sheets. After the selected originals in this paper are separated by color separation, the C version has a rich layer, and its density or dot change can reflect the quality of the printing. The specific sampling method is: in 100 C-prints, one sample is taken every 10 sheets for measurement, and 10 sets of data can be obtained, and the average value is obtained and used as a measured value.
4. Experimental results and data analysis
4.1 According to the experiment, the experimental data obtained from the dot and density test of the manuscript, the printing plate, the printing plate and the printed matter are shown in Tables 1, 2, 3 and 4:
Table 1 Transmission density and percentage of dots of the original
Table 2 Transmission density and dot percentage of C-film
Table 3 Reflectance density and dot percentage of PS plate
Table 4 Reflection density and dot percentage of C prints
4.2 printing replication curve establishment
The printed replication curve established by the experimentally measured data is shown in Figure 2:
Remarks: The dotted line is the theoretical printing replication curve, and the solid line is the experimentally established printing replication curve.
As shown in Figure 2, the experimentally obtained replication curve is very close to the theoretical replication curve, indicating that in the various processes of printing and replication, the dot gain is well controlled and the print quality is good.
The two curved shapes of the first quadrant are similar, but the dot density of the original has an increase in the dot percentage when the film is reproduced by the color separation process. This is mainly because during the processing of the filming process, during the processing of development, fixing, and water washing, the control of the processing time is not accurate, so the phenomenon of dot gain occurs. However, in general, the dots generated by the film are real and uniform, and the contrast between the graphic part and the non-image part is large, the contrast is large, and the density is high.
The curve of the second quadrant indicates that the film is transferred to the PS plate by exposure printing. The shapes of the two curves are very similar, indicating that after the film is subjected to the printing process, the various layers of the image are well reproduced on the printing plate, but the percentage and density of the dots on the printing plate are reduced overall compared with the standard curve. In small cases, this is mainly caused by accidental errors in the operation caused by the accidental error of the operation being less than the standard exposure.
Similarly, the curves of the third and fourth quadrants show that the various layers of the C color of the original image are well represented on the printed matter, only because of the accidental error in the previous two processes, which leads to the deviation of the experimentally produced print replication curve. Print copy curve.
4.3 dot gain value
In the printing process, the percentage of dots at 50% and 75% is the key to printing control, and their increase value is also an important basis for evaluating the increase value of the dot. In this experiment, the percentages of the dots of the PS plate and the printed products of 50% and 75% are shown in Tables 5 and 6.
Table 5 percentage of outlets in PS version
Table 6 Percentage of dots in printed matter
Table 7 dot gain value
The 50% dot gain value recommended by China's printing industry standard stipulates that the dot gain value of 50% of the fine print C version is 8% to 20%. As can be seen from Table 7, the dot gain of the printed product obtained in this experiment is increased. Values are within the limits set by industry standards.
In summary, when the entire printing and copying process performs standardized operations and tries to overcome accidental errors, when the printing machine's ink supply and water supply system are evenly balanced, the paper feeding system is accurately fed and accurately positioned, the printing plate The pressure between the roller and the rubber roller, the rubber roller and the paper is appropriate and the quality of the printed product must be the true reproduction of the original. The printing replication curve has strong practicability. Through the establishment of the printing replication curve and the analysis of the operation of the entire printing process, it can be known how to obtain excellent quality under the existing printing equipment, process conditions and printing operator skills. Printed matter. This has great guiding significance for printing the same or similar orders in the future and can improve the production efficiency of the enterprise.