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Paper is the most important substrate for printing. The printability of paper will directly affect the quality of the printed matter. There are great differences in surface gloss, surface strength, whiteness, absorption, smoothness, and elasticity of different papers. In the same printing conditions, the same ink printed on different paper will often produce different color effects. In actual operation, the printing characteristics of the paper should be fully grasped, and the printing parameters should be determined according to the properties of the paper in order to improve the quality of the printed matter.
1 Determine the number of hanging screens according to the paper performance
The performance of paper is an important factor affecting the number of hanging screens. The surface properties such as the smoothness of the paper determine the height of the screen.
Coated paper or high-grade matte whiteboard paper has high surface smoothness and can reproduce finer dot. Therefore, the number of hanging lines is relatively high, generally it can be set to 200-250 lpi, and high-grade imported coated paper can even be as high as 300 lpi.
The surface of the offset paper is rougher than the coated paper, and the number of hanging lines can be between 133 and 175 lpi.
The printability of the light-coated paper lies between the coated paper and the offset plate, and the number of the hanging lines can be around 133 to 175 network lines.
The printability of light paper is equivalent to offset paper, and the dot enlargement ratio during printing is large, and the number of hanging lines can be between 100 and 150 lpi.
The surface of newsprint is more rough and uneven, too small dots will form broken edges, or completely fall in the sunken place, so you should use larger dot printing, and the number of hanging lines can be between 80 and 133 lpi.
2 Determine black and white calibration before printing according to paper properties
In the high-light areas of the printed image, generally, 1% to 3% of the dots are not printed (or dots are missing), that is, the highlights of the print are basically formed by the color of the paper. If the whiteness of the paper is different, the color brightness and saturation of the highlight portion of the screen will be affected, thereby affecting the color contrast of the screen. Similarly, the texture of the paper is also very relevant, offset paper is a porous material, printing ink is easily absorbed by the fiber, so that the printing surface produces a high degree of light scattering, the ink film density is reduced, the black is not black, poor contrast and level .
a. The poor surface properties of offset paper will directly affect the color and level of printing ink, even if the number of better offset paper hanging lines does not exceed 150 lpi. In view of these characteristics, the black and white field calibration value, white field can be small area network (C = 1%, M = 1%, Y = 2%, K = 0%), the black field can be calibrated as C = 55 ~ 65 %, M, Y55 ~ 60%, K75 ~ 80%, the total amount of ink superimposition is also smaller, 240 ~ 270.
b. Coated paper surface has good smoothness, whiteness, and light reflection ability. The ink printed on the coated paper has small diffusion, dot type is good, the dot enlargement rate can be controlled at about 15%, and the color is good and saturation. Well, bright colors, can better reproduce the level and contrast. CTP printing plate high-light can print 1% (2 to 3% can be printed when the film is released). The total darkness of overprinting dots can reach 350 or so, and the number of hanging lines can be as high as 300 lpi or more. The density contrast can reach 2.0. Therefore, for coated paper, the black scale calibration is roughly C=90 to 95%, M=Y=80 to 90%, and K=75 to 80%. If there is no white point on the image, you can use paper color as the reference point for adjusting the white field.
3Pre-press production adjusts the expansion value of the dot according to the paper performance
When the ink is transferred to the surface of the paper under pressure, the ink spreads around, creating dot gains that make the entire image darker. Obviously, different dot gain effects will occur for different types of paper. The stronger the paper absorbency, the larger the dot gain value. Specifically, newsprint is the largest and coated paper is the smallest. The enlargement of the dot formed by the paper performance generally presents an expansive law of enlargement. In the process of plate making, the printability compensation must be made for the effect of dot gain.
The dot gain value is different for different sizes of dots. Generally, the value of the high-profile and dark-tone network increase is small, and the halftone dot gain value is larger. When the coated paper is printed, the dot gain is about 5 to 10%, the offset paper is about 15 to 20%, and the newsprint is about 20 to 30%.
(to be continued)
Analysis of the Hanging Net Process Points Used in Book Printing (Chinese)
4 Suitability of printed materials