To improve the printing quality of color newspapers and make better use of its publicity, education, entertainment and other functions, the main problem to be solved is to improve the image clarity, tone level range, color saturation and inking uniformity. This article discusses the prepress process control of color newspaper production based on newspaper editing design and prepress platemaking theory, combined with the practice of color newspaper publishing and printing.
1 Edit design
Color newspaper printing has certain limitations in terms of overprint accuracy, color vividness, gradation and ink color uniformity. Editors and designers should make corresponding designs on the basis of understanding these technical characteristics, and try to avoid their technical limitations and maximize Give full play to its advantages.
1) When selecting pictures, editors should try to use pictures with strong contrast, high definition and rich levels to create a good condition for color separation and plate making.
2) Special attention should be paid when editors choose electronic drafts. The resolution of pictures on the Internet is often low, and the pixel resolution of pictures in the color plate making is required to be between 200dpi and 300dpi. Therefore, the pictures downloaded from the Internet should have higher accuracy; other electronic manuscripts should also pay attention to the accuracy requirements.
3) Due to the characteristics of newsprint, the dots in the highlight area are easy to be lost, the dots in the dark area are easy to expand and merge, and the vividness of the color is also low. Editors should use high-purity colors or high- and mid-tone areas as the main colors in design colors. It should be expected that the set colors will have the problems of underprint and color deviation of newsprint when printing.
4) Due to the limited overprint precision of the rotary machine, when designing colored text or reversed white characters, Song typeface and small-scale text should be avoided. The overlapping of small characters should not exceed two colors as much as possible.
5) The uniformity of the ink layer printed on the newsprint by the rotary press is limited by the conditions. The design should reduce the large-area flat screen, especially the multi-color overprinted flat screen is extremely difficult to print uniformly.
6) The take-up of the rotary machine is positioned by the pressure of the pressure roller, which is easy to be dirty at the middle seam. On the layout of the newspaper, try to avoid placing thick text and images on the seam of the newspaper.
2 Pre-press production
The newsprint and printing equipment used in color newspaper printing determine that color newspaper printing is different from general color printing. To stabilize and improve the printing quality of color newspapers, the pre-press production process control is very important.
1) Picture processing
â‘ The setting of scanning is to adapt to the technical characteristics of color newspaper printing. The setting of scanning should focus on improving the clarity and gradation effect of the picture, and pay attention to the following aspects.
? Reasonably set the scanning resolution. Generally speaking, the input resolution is between 200dpi and 300dpi, and should not be too high.
To reduce the loss of gradation, the tone range should be set to the maximum range during scanning, and the black and white field and tone curve should be preset according to the type of picture.
? To improve the sharpness, the picture should be sharpened, that is, the sharpening value should be adjusted according to the specific situation of the picture.
? Descreen the printed manuscript.
Black and white settings. The black and white fields are the starting point and end point of the image tone. Setting the black and white points correctly is the key to grasping the tone. Because the whiteness and ink density of newsprint are much lower than that of ordinary color prints, this makes the tone of color newspaper printing compressed to a smaller range. In order to improve the printing effect of the image, it is necessary to set the black and white fields appropriately according to the characteristics of the main body of the image, and intentionally compress the tone that does not affect the image area of ​​the main body, so as to increase the contrast.
For general color prints, in order to achieve color reproduction and retain the gradation of high-profile areas, the white field is usually set in the highlight area with a smaller range. In newsprint printing, small dots in highlights are easy to lose, and the paper itself has a grayscale of more than 5%. Too much emphasis on the level of bright tones will affect the overall tone range, making the picture gray. Therefore, the white field area should be properly expanded, and sometimes some bright tone levels have to be discarded in order to highlight the main body. At the same time, it should also be noted that the white field area cannot be too large to prevent a large area from being screened.
The richness of the picture is reflected in the dark tone. For general color prints, the darkest small area is selected as the black point in image processing. However, according to the characteristics of high dot expansion rate in newsprint printing and the easy appearance and gradation of dots, in order to take care of the overall effect of the image, some dark tone levels must also be appropriately discarded if necessary.
â‘¡ Level correction The correction level curve is to compensate the tone compressed due to the setting of the black and white field, expand the dark tone, avoid the dark dots to merge, and play the role of increasing the contrast and adjusting the ratio of high, medium and low tones. Ensure the correct restoration of the subject level and increase the stereoscopic effect of the image. The correction of the image hierarchy should be analyzed and processed according to its tone type. It requires the operator not only to have experience, but also to have the ability to understand and artistically process the image.
Color correction. Some images have a color cast, and sometimes in the process of setting the black and white field and gradation compensation on the image, it will cause the change of the color tone. In order to improve the color reproduction effect of the image and adapt it to the printing conditions of newsprint, color correction must also be performed.
The color correction should be processed according to the gray balance, and the color purity should be properly processed according to the characteristics of the newsprint itself with grayscale and color deviation, and the purity of the printed color is reduced.
For pictures with a certain color as the basic color, the basic color can be exaggerated and given adequately, and the amount of the opposite color can be appropriately reduced, such as red in cyan, green in (magenta) red, and blue in yellow to strengthen it. purity.
â‘¢ The printing ink is set to control the total ink volume. Due to the limitations of newsprint printing, this parameter cannot be too large, otherwise serious ink accumulation will occur. After printing tests, it is recommended to set this parameter to 240%, that is, the sum of the four inks does not exceed 240%.
GCR (gray component replacement) and UCR (background color removal). GCR is to remove the color of the gray area synthesized by the cyan, magenta, and yellow inks, and use black ink to replace the neutral gray of this part; UCR is to remove the dark part of the gray part synthesized by the three primary colors of ink, and use black ink to instead. Both of these methods are to reduce the total amount of ink during printing to reduce the phenomenon of stacking ink. Because UCR is sometimes difficult to control on printing presses, GCR is generally used. After the printing test, it is recommended to set the parameter to Medium, that is, the part with the tone lower than 20% is not replaced by black.
Black ink control. This parameter controls the maximum amount of black ink after color separation. After printing test, it is recommended to control the amount of black ink below 85%.
Ink color set. This parameter records the characteristics of the ink used for printing. The setting suitable for newspaper printing is SWOP, which is the web offset printing specification (newsprint).
Expanded outlets. This parameter records the dot expansion in printing. The setting of this value will affect the image display in Photoshop, making it look similar to the printing effect. In addition, the dot expansion of the four inks used in printing may be inconsistent, so the dot expansion values ​​of cyan, magenta, yellow, and black inks can also be set separately. [next]
2) Picture output
At present, most domestic printing presses still follow the traditional process of using color separation films as masters for assembling and printing plates. The quality of color separation films affects the quality of printing plates and the printing quality of color newspapers.
Output software RIP settings. RIP, full name raster image processor, has hardware RIP and software RIP, currently commonly used is software RIP. The role of RIP in the color desktop publishing system is very important. It is related to the quality and speed of the output, and even the operating environment of the entire system. It can be said that it is the core of the color desktop publishing system. The main function of RIP is to interpret various images, graphics and texts in the layout of the computer into dot matrix information that can be recorded by the printer or imagesetter, and then control the output device to record the image dot matrix information on the output medium.
The Founder Century RIP of Peking University Founder is commonly used by domestic newspaper printing plants. The following are some explanations for the setting of the Founder Century RIP used for color film output film:
â‘ The dot shape color newsletter mainly uses the rotary offset printing method. It is recommended to use the round dots of the default setting of Founder Century RIP. Founder Century RIP uses the latest technology for the generation of circular dots, which further improves the output quality of circular dots. For character pictures, it is not recommended to use square dots, because square dots will jump at 50%, affecting the restoration of skin tone.
â‘¡The linear phototyper has different dot expansion ratios for different films. Without linearization, the film expansion rate of the film should be kept within 10%. If the film cannot meet this requirement, it is best to replace the film. After the linearization is completed, it is necessary to confirm that the film dots are expanded within 1%. Only in this way, the output film can meet the requirements of color printing and printing.
â‘¢ Screen printing line number / output accuracy The paper printed by the newspaper is newsprint, and the drying of graphic prints is mainly permeation drying, so the screen printing line number is controlled between 87 lpi and 120 lpi. If the screen printing line number is too high, it will not only improve the image quality On the contrary, it will cause the dark tone to merge and affect the printing quality.
The human eye can distinguish about 256 levels. In order to ensure that the output film can reach 256 levels, it needs to be (output accuracy ÷ screen line number) 2 = 256
That is, the output accuracy ÷ the number of screen lines = 16
That is to say, under the current newsprint and printing process conditions, the output accuracy of the phototypesetting machine for newspaper output film is recommended to be set between 1270 dpi and 1800 dpi.
â‘£Gray level
Increasing the gray level of the grid can improve the output quality of the gradient, but the maximum should not exceed 1024 gray levels, because after the level is higher than 1024, the quality improvement has been invisible to the naked eye, but it has a greater speed of RIP influences. Of course, while increasing the gray level of the grid, the "RIP parameter / picture quality" option must be set to "fine", otherwise the settings related to the gray level will not work.
The quality of the film. Color separation film is the final product of image scanning and text grouping, and it is also the basis for printing and printing. The on-site density of the film output by the imagesetter is between 4.0 and 4.2, and the best output quality can be obtained. The lowest output density should not be less than 3.5; the bottom ash density is less than 0.04. The main factors affecting film density are: film type, exposure intensity, development time, development temperature, etc. Each time you replace a different batch of film, change the development or fixing solution, change the development time, temperature, you need to re-output the test film, and reset the exposure intensity and linearization curve. For imagesetters that have been in use for a long time, different densities may occur at different locations on the same film. This problem is due to the aging of the laser tube and is not controlled by RIP.
3 Spell-out process
1) Imposition
In the case where the advertising manuscript cannot be fully digitized and the laser phototype separation is mainly based on the four-fold, the process of imposition is essential. The imposition should be accurately positioned and fitted without errors. For the color newspaper printing, due to the limitation of the multi-color overprinting of the rotary press, the registration of the imposition is required to be more accurate. Newspapers are highly politically-oriented and time-sensitive products. If mistakes, miss-spells, or reverse order are made, they will cause incalculable losses. Therefore, in this process, in addition to solving the technical problems, it is more important to improve the operator's sense of responsibility and strengthen the responsibility system for checking the printed samples.
2) Exposure
Exposure printing is the last link in the prepress process and a key process in the quality control of color newspaper printing. The quality of the exposure directly affects the quality of newspaper printing. There are many factors that affect the quality of printing, according to the characteristics of color printing, on the basis of correct exposure, we should focus on solving the problem of improving the PS printing durability.
According to the characteristics of small print tone range and high dot expansion rate of color newspapers, it is particularly important to improve the quality of dots by correctly controlling the exposure and achieving the cooperation of development time, chemical concentration and temperature. In this regard, it is necessary to achieve quantitative control of various variable factors, so that the photosensitive part of the PS version forms a good graphic area and "sturdy" dots through normal chemical reactions.
Newspaper printing generally focuses on "long version" work. Newsprint paper has rough paper quality and the accumulation of paper wool is likely to cause problems such as grinding and dirtying. In printing, in order to reduce plate changes and improve timeliness, the quality of the PS version itself is higher. The PS plate should be selected with high printing durability, good photosensitive layer stability, uniform coating, high plate strength and uniform thickness. In addition, moderate roasting can also improve the PS printing durability.
When using a medium-sized machine or a mainframe for printing, the positioning of the PS plate is slightly deviated, and it is difficult to register part of the layout. Therefore, the cutting size error of the PS version, the slight error of the bending operation and the upper version operation will affect the accuracy of the overprint, and special attention should be paid. In addition, the quality of the PS version itself is also high. The PS plate should be selected with high printing durability, good photosensitive layer stability, uniform coating, high plate strength and uniform thickness.
With the development of prepress technology, direct plate making (CTP) technology has become an inevitable trend for the development of prepress technology for newspaper printing. It eliminates the phototypesetting, punching, imposition and other links in the traditional plate-making process, reduces the variable factors that affect the quality, and plays an important role in improving the printing quality and printing time. . It should be noted that while introducing and implementing new process technologies, new factors affecting quality will be encountered, and the process management of traditional plate-making will also change accordingly. It is necessary to timely study and establish the new process conditions Management style.
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