This parameter specification specifies the total percentage content of cyan, magenta, yellow and black in the black area of the printed image. In theory, the darkest area should be printed with 400% of the maximum total ink. However, in four-color wet overprint, this situation will lead to problems in the actual production operation of the printing press. The following problems will occur:Grey Balance Control Problem

1. Reverse transfer of ink – the ink is transferred from the printed web to the next printing sheet of the inking system, thus polluting the ink on these printing sheets.

2. Ink drying problem – the temperature in the drying device is unusually high, which evaporates a large amount of solvent and may remove a large amount of moisture from the paper, resulting in blistering on the paper surface.Grey Balance Control Problem

3. Overprint difficulty – the ink printed first refuses to print other ink on its top, so that the ink printed later cannot be transferred properly. The post printed ink can be transferred to the unprinted paper surface, but not to the inked surface. The greater the ink coverage, the more obvious the problem caused by poor ink overprint.Grey Balance Control Problem

       According to the printing industry, the maximum TAC value that can be printed in the half tone area is 300%, and only one color can be printed on the spot. Black objects can be successfully copied under this condition. In some small image areas without important details, a slightly higher TAC percentage may also be acceptable. 

      However, in this case, the TAC value of even small areas (often unimportant) should be less than 325%. TAC value shall be detected in the heaviest (i.e. darkest) area of film or electronic document, and each color shall be read under the same actual conditions. The measuring points taken by the publisher’s film inspector and their measuring methods should be determined.Grey Balance Control Problem

Gray component replacement (GCR) and undercolor removal (UCR)

      In the traditional three color process, to copy a saturated color, the chromaticity is often generated by two main subtractive colors, and the third color is used to increase the gray level. Although this color separation method has been quite successful, there are still some inherent difficulties. First of all, if this gray component cannot be accurately controlled on the printing press, the color will lose balance. Grey Balance Control Problem

      Secondly, when copying complex colors such as brown, purple, crimson, flesh and gray, any slight change in the primary color of printing will cause some obvious changes. Therefore, if there are conflicting colors in the online object of the same printer, obtaining the necessary color balance in one area will lead to color offset in another area.Grey Balance Control Problem

      GCR is a color substitution technology, which uses gray to replace part or all of cyan, color and yellow in the color separation process. Therefore, the GCR process does not need to rely on the third main color to make the color gray or black, but mainly forms the general outline and some details of the image through black ink, which greatly reduces the possibility of color change. Grey Balance Control Problem

      Smaller ink coverage also means smaller problems, and the change of black printing sheet can only cause tone change, which is much better than the chromaticity change caused by other changes of ink. Therefore, black ink has the function of controlling color stability. For example, black ink can be used to replace cyan ink in mahogany area, color ink in green area and yellow ink in purple area.Grey Balance Control Problem

     Traditionally, prepress service providers often use UCR to reduce the total amount of printing ink in neutral shadow areas. UCR has the least influence on gray and shadow depth, and has a greater influence on the second color.Grey Balance Control Problem

      The recommended safety range of GCR is between 50% and 80%. The 50% GCR setting is to remove 50% of the gray component usually printed by color ink and increase the same amount of black ink compensation. When using GCR of 80% or higher, prepress service providers and printing customers must pay special attention to the following points: when using these GCR levels, the dot percentage of color on the film is relatively small, which limits the color change range during editing. Grey Balance Control Problem.Grey Balance Control Problem

      At the same time, higher UCA will also reduce the gloss of dark shadows and black areas, reduce the thickness and reduce the details. UCA increases the amount of color in the black area of the figure.Grey Balance Control Problem

The minimum TAC value recommended by swop in neutral black area (which can be used as a function of the dot value of black printing sheet) is as follows:

Minimum TAC value of GCR.Grey Balance Control Problem

The dot percentage of black printing sheet and the minimum coverage of three-color area (neutral gray must be maintained).Grey Balance Control Problem