How to correctly set prepress system parameters for color newspapers

The modern press and publishing industry has already transitioned from the traditional letter-to-plate printing to the era of computer laser photocopying, especially the emergence of color desktop publishing systems, which has made the printing industry experience a remarkable revolution. In recent years, there have been more and more colorful newspapers. Its colorful, photo-realistic, and innovative ideas have given readers a powerful appeal, satisfying readers' demand for high levels of enjoyment. However, the current quality of color printing paper is uneven, some colorful, natural, structured, overprinting accuracy; some are dull, low clarity, blurred images. The reason for the low printing quality is that, in addition to overprinting and ink unevenness in the printing process, improper prepress processing, especially the prepress system parameters, are also an important factor.

At present, almost all manufacturers use the Founder or Huaguang color photocopying system. Photo processing is done on photoshop software. Setting system parameters correctly is of great significance. To set the correct prepress system parameters in photoshop software, you must first understand the characteristics of color newspaper printing:

Newsprint printing, due to its fast speed, large quantity, and relatively low requirements for printing materials, determines its uniqueness in image reproduction tones, ink coloration, dot change, and traditional leaflets due to different printing speeds and printing materials. There are many differences in offset printing.

1. The whiteness of newsprint is low. Whiteness is a physical quantity used to characterize the reflection of light on the surface of a sheet of paper. Most newsprint papers have low whiteness, so on the one hand, printing will cause color cast due to paper color cast; on the other hand, it will cause problems such as large print gray scale, small image contrast, and insufficient saturation.

2. Newsprint has low smoothness. Newsprint is much rougher than other papers, especially coated paper, and has a low degree of smoothness. The reduction of printed color pictures is based on dots as the basic unit, and the ink has a low transfer rate on rough newsprint paper, so the restored dots are not complete, and the ink is relatively unsturdy.

3. Newsprint has poor dimensional stability. The main ingredient of newsprint is pulp, which has high impurity content, poor water resistance, and poor paper dimensional stability. In color printing, it is easy to cause image misalignment and overprint errors. This is a fatal weakness of newsprint, and it is also an important factor in printing that is extremely difficult to control and seriously affects print quality.

4. Newsprint is highly absorbent. The texture of the newsprint is loose, and the pores between the fibers are larger, so the absorption of the paper is large. Together with the expansion of the ink on the paper, the dark spots of the image can easily cause the dots to be stenciled and adjusted. Concurrently, it caused a serious loss of image tone levels.

5. Printed dot gains are large. Due to the printing pressure, the blanket on the printing press will be deformed, which will cause dot expansion. Due to the use of newsprint paper and penetrating-drying inks for newspaper printing, the dot gain value is about 20% to 30% higher than that of coated paper.

6. Printing inks have a small color gamut. Due to the particularity of the paper, color printing has a smaller color gamut than that on coated paper, that is, the number of colors that can be reproduced is small, and the reproducibility is poor.

Due to these characteristics, color newspapers suffer from quality problems such as overprinting, color blemishes, short tone levels, and uneven ink. Based on many years of practice and with reference to relevant information, the author summarized how to correctly set the prepress system parameters to enhance the quality of color newspaper printing.

-,scanning

1. The scanning resolution should be twice the number of hanging lines. If the imager 120IPi network cable output, the input resolution when scanning should be 240Ipi.

2. Make color corrections and adjustments in RGB and make fine detail adjustments in CMYK.

3. Properly handle: Due to the large gray level of newsprint, the image should be properly illuminated. Also, because the contrast of the newsprint printing image is small, the contrast is appropriately increased for a document with a small contrast, the gradation reproduction range is appropriately compressed, the network point is appropriately reduced in high-profile, and the gradation value of the halftone is reduced in the dark tone. Saturation can be properly processed to improve the purity of color.

Second, separation settings

Printing ink settings: In the Photoshop menu, select Edit/Color Settings from the CMYK drop-down menu of custom CMYK (Figure 1): pop-up custom CMYK window (Figure 2):

A: Inks colors. In the "Printing Ink Settings", the "ink color" is first set, and one of the Swop (web offset specification) settings is usually selected.

B: Dot Gain. The dot gain is also called the dot gain compensation. The dot gain value indicates that the dot gain is 50% in the middle tone. Since the expansion of the dots during printing is spread along the edge, the edges of the dots at the 50% point are the longest, and the enlargement value is the largest. The dots are enlarged to compensate for the value after the separation, and the middle tone is adjusted more to ensure that the printed image is not too dark. As the gain of the dot gain increases, the corresponding data obtained from the color separation are reduced. These reductions are to compensate for the expansion of outlets at the time of printing. If the dot gain value is larger at the back, then the data will be smaller when the color is separated, so that the printing can be just right.

C: Separation options (color separation type GCR, UCR). The GCR is an achromatic structure (Gray Component Replacement); UCR is Under Color Removal.

The GCR allows black ink to be used instead of color inks in the full tone range. Black Generation provides 5 processing methods for the user to choose from what tone to start the black substitution, namely, None (no black substitution), Light40% (no black substitution is replaced in the part with tone lower than 40%), and Maximum (All black substitutes) If the low-key portion of the image occupies a large area, a partial ash component should be appropriately substituted to reduce drag and gradation loss caused by overprinting during the printing process.

D: The black version produces the black tone. Custom black version of the range curve (Figure 3):

The black tone allows the user to choose the black version of the tone change. The customization in the black tone bar allows the user to draw the black version of the image itself, including the starting point, the maximum amount, and the degree of change in the middle section. Photoshop software made a lot of research on the tone of the black version, because the black version has an extremely important role in image reproduction. The black color is added to the YMC three primary colors, which makes the dark tone level of the printed image generally increase by a density of 0.4 to 0.8, that is, the overall contrast of the image is opened, and the level and density of the thick color portion are increased, so that The image is more three-dimensional. There are the following rules in the custom black edition: If it is a rich and dark mixed-color manuscript of color and achromatic half, the black version can be used from bright tuning. The starting point is between 20% and 30% of the cyan version. The darkest tone Between 80% and 90%, the intermediate tone needs to be changed according to the specific image; if it is an achromatic, color-assisted manuscript, the black version needs to be used from the bright tuning part, and the darkest part can also be increased to 100%. Outlets.
E: Total Ink Limit. Indicates the maximum value of the sum of four-color dots. About 260% of newspapers often print, this value is determined directly by the printing process, if the network is enlarged when the printing is serious, the value must be taken small, otherwise, the dark field will stick to death, the level will be lost.

F: Black Ink Limit. Different types of originals should have different values. In general, the maximum value taken by the electricity separation process is 70%. In color newspaper printing, we often take 85% because this data can ensure that the black field of the printed crystal is sufficiently dark, that is, there is enough contrast.

G: UCA (Under Color Addition) Amonunt background color gain. It is mainly to increase the amount of data of CMY at the dark tone.

After setting the separation parameters, it is also necessary to process the originals in photoshop according to different manuscripts. A fundamental rule must be followed in the processing: ensuring the gray balance in the copying process. Gray balance is to superimpose a certain number of cyan, magenta, and yellow inks in accordance with printing characteristics to obtain a visually neutral gray color. At this time, it is called achieving gray balance. At the same time, the percentages of blue, yellow, and yellow dots are called gray balance data. It can be said that regardless of platemaking or printing, as long as the gray balance of the ink is carefully controlled, it is considered half the problem.

The above is the author's experience, I hope to be able to help the majority of prepress workers, if there are errors, please criticize and correct me.