Talking about the establishment of printing copy curve of offset press
The printing copy curve is a curve describing the relationship between the density or dots of each copy link from the original to the printed product. In this paper, the GATF star and 12-level gray scale are used as tools for observing dot gain and solid density control during the printing process. By measuring the percentage of dots at all levels in the three main process links of output, plate making, and printing, a printing replication curve is established.
In the printing work, the dot is the basic printing unit that constitutes a continuous tone image, and the tone, gradation, contrast, sharpness, and color of the printed image are all expressed through the change of the dot size, so the dot reproducibility is good or bad. Directly affect the quality of printing. At the same time, the evaluation of printing quality and quality monitoring and management in the process of printing operations are also mainly achieved through the evaluation of dot reproduction.
Printing Copy Curve (Printing Copy Curve) is a curve used to describe the hierarchical relationship between the manuscript, printing negatives, printing plate and printed matter in the entire copy process from printed manuscript to printed product. The printing replication curve is established by measuring the percentage or density of dots in each process, so the exact reproduction degree of the printed dots is closely related to the accuracy of the printing replication curve. The dot reproducibility is good, the dot increase value is within the specified range, and the printed manuscript can be well copied on paper or other substrates. In this paper, through offset printing experiments, quantify the change of dots in the entire process from the original to the finished product, and establish a printing replication curve. This curve can be used to analyze and evaluate the standard of operation of each process, that is, the performance of the existing printing equipment, process conditions and printing Operator skill level, at the same time, the curve has great guiding significance for printing the same or similar orders in the future, indirectly improving the production efficiency of the enterprise.
1. Experimental materials
Single-sided 115g coated paper, quick-drying magenta ink, developer, positive image PS plate (specification 460 × 395mm), Phoebe imagesetter, PS version automatic development machine, Win500 monochrome offset press, SBK-B iodine gallium lamp Plate machine, T6CR color reflection densitometer, WYSK-40 × 40 times reading microscope with light source.
2. Establishment of experimental monitoring objects
Using GATF star mark, it is easy to visually observe the ink volume and dot changes during the printing process. The GATF star is composed of 36 black wedge-shaped lines arranged at an equal interval distance within a circle. Such arrangement characteristics have various imprinting defects such as "dot expansion" and "ghosting" in printing. Can be clearly reflected. At the same time, the 12-level gray ladder is used to directly test the dot data of all levels on the printing negatives, printing plates and printed materials with a density meter, and use this data to establish a printing replication curve.
3. Sampling
In the printing operation, when the ink supply and water supply system of the printing machine are evenly balanced, the paper is accurately conveyed and positioned accurately, the pressure between the plate roller and the rubber roller, and the rubber roller and the paper are appropriate and It is determined that the various levels of the manuscript are well reflected in the printed matter and the sampling begins. Sampling methods for printed products generally have two methods of sampling according to a certain time interval and a certain number of printed sheets. After the selected manuscript in this article is separated by color, the layer on the C version is rich, and its density or dot change can reflect the quality of the printing quality well. The specific sampling method is: in 100 C-prints, one sample is taken every 10 sheets for measurement, 10 sets of data can be obtained, and then the average value is obtained and used as the measured value.
4. Experimental results and data analysis
â‘ According to the experimental data obtained by the dot and density test of the four processes of manuscript, printing negative film, printing plate and printed matter in the experiment.
â‘¡Establishment of printing copy curve
The shape of the two curves in the first quadrant is similar, but the tone density of the original is reproduced by the color separation process, and the dot percentage increases. This is mainly due to the fact that during the processing of the film development process, the processing time of the development, fixing, and water washing processes is not accurately controlled, so the dot increase phenomenon occurs. But in general, the dots generated by the obtained film are real and uniform, with large contrast, high contrast and high density in the graphic and non-graphic parts.
The curve in the second quadrant indicates that the film is transferred to the PS version after exposure and printing. The shapes of the two curves are very similar, indicating that after the film has been subjected to the printing process, all levels of the image are well reproduced on the printing plate, but compared with the standard curve, the percentage of dots and density on the printing plate are overall reduced In small cases, this is mainly caused by the accidental error of the operation during the printing process, which causes the exposure to be less than the standard exposure.
Similarly, the curves of the third and fourth quadrants show that the C color of the original image has a good performance on the printed matter, only because of the accidental errors in the previous two processes, which caused the experimental printing replication curve to deviate from the ideal. Print copy curve.
â‘¢Increase value of outlets
In the printing process, the percentage of dots at 50% and 75% is the key to printing control, and their increased value is also an important basis for evaluating the increased value of dots.
The 50% dot increase value recommended by China's printing industry standards stipulates that the range of the dot increase value at 50% of the C version of fine print is 8% to 20%.
In summary, when the entire printing and copying process performs standardized operations and tries to overcome accidental errors as far as possible, when the ink supply and water supply system of the printing machine are evenly balanced, the paper conveying system accurately conveys and accurately positions the printing plate The pressure between the roller and the rubber roller, and the rubber roller and the paper is appropriate and the print quality obtained at a constant time must be a true copy of the original. The printing copy curve has strong practicality. Through the establishment of the printing copy curve and the analysis of the operation of the entire printing process, you can know how to obtain excellent quality under the existing printing equipment, process conditions and printing operator skills. Prints. This has great guiding significance for printing the same or similar orders in the future, and can improve the production efficiency of enterprises.
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