Gray balance control issues throughout the print workflow

The process of producing visually neutral gray scales from three types of printing process inks involves a problem of gray balance, and the gray balance can be confirmed by films and proofs during the color separation process. In the proofing and printing process with reference to the SWOP density standard, the dot sizes of the three printing primary ink dots of the printing gray are generally different, and the cyan dot dots are usually larger than the dot dots of the yellow and the color dots. In the proofing and printing production process, it is necessary to control the enlargement of the dots and maintain the same dot size as the original film in order to maintain the gray balance.

The GATF/SWOP proofing control bar has the following values:

Gray Balance Black Cyan Yellow 75% 75% 63% 63%
50% 50% 39% 39%
25% 25% 16% 16%

SWOP proofing test strip

Total Area Coverage (TAC)

This parameter specification specifies the total percentage content of cyan, color, yellow, and black in the black area of ​​the printed image. The theoretically darkest area should be printed with 400% of the maximum total ink. However, in four-color wet-overprinting, this situation can cause problems in the actual production operation of the printing press. The following problems will mainly arise:

1. Inverse transfer of ink - The ink is transferred from the printed web to the next printing unit of the inking system, thereby contaminating the ink on these printing units.

2. Ink drying problem - The temperature in the drying unit is unusually high, evaporation of a large amount of solvent, may also remove a large amount of moisture in the paper, resulting in blisters on the surface of the paper.

3. Difficulties in overprinting - The ink printed first refuses to print other inks on its top so that the ink after printing cannot be properly transferred. The post-printed ink can be transferred to the unprinted paper surface and cannot be transferred to the inked surface. The greater the coverage of the ink, the more pronounced the problems caused by poor ink overprinting.

According to press reports, the maximum TAC value that can be printed in the half tone region is 300%, and only one color can be printed in the field. Under this condition, black objects can be successfully copied. In some small image areas with no important details, a slightly higher percentage of TAC may also be acceptable. However, in this case, the TAC value of even a small area (often not important) should be below 325%. The TAC value should be detected in the heaviest (ie, darkest) area of ​​the film or electronic file, and each color should be read under the same actual conditions. The measuring points taken by the film inspectors of the publisher and their measurement methods should all be determined.

Gray Component Replacement (GCR) and Undercolor Removal (UCR)

In a traditional three-color process, copying a saturated color tends to produce chromaticity from two main subtractive colors, and a third color increases the gradation. Although this separation method has been quite successful, there are still some inherent difficulties. First, if this gray component cannot be precisely controlled on the press, color will lose its balance. Second, when replicating complex colors such as brown, purple, dark red, flesh, and gray, any slight change in the color of the printing process will cause some obvious changes. So if there are conflicting colors in the online object of the same press, obtaining the necessary color balance in one area will result in color shift in another area.

GCR is a color substitution technique that replaces some or all of cyan, color, and yellow in the separation process with gray. Therefore, the GCR process does not need to rely on the third main color to gray out or darken the color, but mainly uses the black ink to form the general outline and partial details of the image, which greatly reduces the possibility of color change. Smaller ink coverage also means less problems, and changes in the black printing unit can only cause a hue shift, which is much better than the change in hue due to other changes in the ink. Therefore, the black ink has the effect of controlling the color stability. For example, the cyan ink of the reddish wood region, the color ink of the green region, and the yellow ink of the violet region may be replaced with black ink.

Traditionally, prepress service providers often use UCR to reduce the total printed ink in the neutral shadow area. UCR has the least influence on the grayscale and shadow depth, and has a greater influence on the second color.

The recommended safety margin for GCR is now between 50% and 80%. The 50% GCR setting is to remove 50% of the ash component normally printed by the color ink and to add an equal amount of black ink compensation. When using a GCR of 80% or higher, prepress service providers and printing clients must pay particular attention to the following points: When using these GCR levels, the percentage of color dots on the film is relatively small, limiting the color when editing variation range. At the same time, the higher UCA will also reduce the gloss of dark shadows and black areas, and reduce the thickness while also reducing the detail. UCA increases the amount of color in the graphics black area.

The minimum TAC value that SWOP recommends to use in the neutral black area (as a function of the dot value of the black printing unit) is as follows:

The minimum TAC value for GCR.

Percentage of dots on the black printing unit Minimum coverage of the three-color area (Neutral gray must be maintained).