On the Establishment of Printing and Copying Curves of Offset Printing Machine

The print-copy curve is a curve that describes the relationship between the density of the original to the copying links of the printed matter or the dots. In this paper, GATF stars and 12 gray scales are used as tools for observation of dot gain and density control in the printing process. Print copy curves are established by measuring the percentages of all levels of hierarchical dots in the three main processes of output, plate making, and printing.

In printing jobs, dots are the basic printing units that form continuous tone images. The tone, level, contrast, sharpness, and color of the printed images are expressed by the size of the dots, so the dot reproducibility is good or bad. Directly affect the quality of prints. At the same time, the quality control and management of print quality evaluation and printing operations are also mainly achieved through the evaluation of dot reproduction.

The Print Copy Curve is a curve that describes the hierarchical relationship between originals, printing negatives, printing plates, and prints during the entire copying process from printed manuscripts to printed products. The print copy curve is established by measuring the percentage or density of halftone dots in each process step, so the exact reproduction degree of the print dot is closely related to the correctness of the print copy 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. Through offset printing experiments, this article quantifies the change of dots from the original to the finished printing process, and establishes a printing copy curve. This curve can be used to analyze and evaluate the degree of operation standards for various processes, ie, the performance of existing printing equipment, process conditions, and printing. Operator skill level, meanwhile, the curve has great guiding significance for printing the same or similar orders in the future, indirectly improving the production efficiency of the company.

Experimental material

Single side 115g coated paper, quick-drying magenta ink, developer, positive PS version (size 460 × 395mm), Phoebe imagesetter, PS version of automatic development machine, Win500 monochrome offset press, SBK-B iodine light Edition machine, T6CR color reflection densitometer, WYSK-40 x 40 times reading microscope with light source.

2. Establishment of experimental monitoring objects

Using the GATF star mark, it is easy to visually observe the amount of ink and dot change during the printing process. The GATF star mark is composed of 36 black wedge lines arranged at an equal interval distance and arranged within a circle. This sort of arrangement features various imprinting defects in printing such as "dot enlargement", "ghosting", etc. Can be clearly reflected. At the same time, with the use of a 12-step gray scale, densitometers were used to directly test the dot data at all levels of the printing plate, printing plate, and printed matter, and the data was used to establish a printing copy curve.

3. Sampling

In the printing operation, when the ink supply and water supply system of the printing press is uniform, the paper feeding system accurately feeds and accurately positions, and the pressure between the plate cylinder and the blanket cylinder, the blanket cylinder and the paper is proper and Heng Ding, all levels of the manuscript were well represented on the print and began sampling. Sampling methods for printed products generally follow two methods: sampling at certain time intervals and a certain number of printed sheets. After the manuscript selected in this article has been color-separated and filmed, the C-version is rich in layers, and its density or dot change can reflect the quality of the print. The specific sampling method is: In 100 C prints, one sample is taken every 10 sheets for measurement, and 10 sets of data are available, and the average value is taken as the measured value.

4. Experimental results and data analysis

1According to experimental data obtained from the experiment on the originals, printing plates, printing plates, and printed materials, the dots and densities were measured.

2 printing replication curve establishment

The two curves of the first quadrant are similar in shape, but the gradation density of the original is increased by the color separation process when the film is reproduced. This is mainly because in the process of the washing process, during the processing of the three stages of development, fixing, and washing, the control of the processing time is inaccurate, so the dot enlargement phenomenon occurs. However, overall, the dots generated by the film are real and uniform, and the contrast between the graphic part and the non-graphic part is large, the contrast is large, and the density is high.

The curve in the second quadrant indicates that the film was transferred to the PS plate by exposure printing. The shape of the two curves is very similar. It shows that all layers of the image are well reproduced on the printing plate after the printing process. However, compared with the standard curve, the percentages and density of the dots on the printing plate are reduced as a whole. In the small case, this is mainly caused by accidental errors in the operation resulting in exposures less than the standard exposure during the printing process.

Similarly, the curves of the third and fourth quadrants show that the various levels of the C color of the original image are well represented on the printed matter. However, due to accidental errors in the previous two processes, the resulting printed copy curve deviates from the ideal. Print copy curve.

3 dot gain

In the printing process, the percentage of dots at 50% and 75% is the key to printing control, and their increasing value is also an important basis for evaluating the dot gain value.

The 50% dot gain value recommended by China's printing industry standards stipulates that the dot gain value at 50% of the fine print C version is in the range of 8% to 20%.

In summary, when the entire printing and copying process carries out standardized operations and tries to overcome accidental errors as much as possible, when the ink supply and water supply system of the printing press is uniformly balanced, the paper feeding system accurately feeds and accurately positions, and the printing plate The quality of the prints obtained by the proper and constant pressure between the rollers and blanket, blanket and paper must be a true reproduction of the original. The printing and copying curve has strong practicality. Through the establishment of the printing and copying curve, and analyzing the operation of the entire printing process, we can know how to obtain excellent quality under the conditions of the existing printing equipment, process conditions and printing operator skills. Prints. This has a great guiding significance for printing the same or similar orders in the future, and can improve the production efficiency of the company.

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