K100%, CMYK All 100% & Rich Black
K100% Is Processed as Overprint
Full-color printing is expressed with ink that is applied and overlaid in the order K→C→M→Y . When much ink is printed on paper, the paper might expand or contract, and, as a result, "registration" or plate alignment might start to be inaccurate at that time. If colors become misaligned or "out of registration," a slight gap occurs between the colors. With small letters that are designated as K100% (CMY all set as 0%), the slight gap becomes conspicuous, so generally "black overprint" is automatically processed on design elements designated as K100%. This influences the minimum required registration. Because a lot of letters or objects are designated as K100% in actual printed matter, it is difficult to process them all as overprint by hand, and so human error cannot be avoided when creating designs. To counter this, overprint is generally carried out automatically at the RIP processing stage on the output side. Overprint reduces the burden on the designer and ensures that beautiful prints are made. There are, however, unintentional mistakes and failures due to overprinting.
The following introduces some examples of such failures and solutions the designer can take to prevent these. The illustrations below show the final printed result. (These are similar to Multiply in Photoshop.)
Note: Overprint is not applied to the K100% part of images (bitmap data). It is applied only to graphics (vector data) and is automatically processed in accordance with our RIP processing rules. Note, however, that K100% that is influenced by transparency or effects might not be processed as overprint.
Of the four color inks, K (black) is printed first followed by cyan (C) with the part of the cyan plate underneath the overlapping section of the black plate not printed or "knocked out." However, a slight gap is likely to occur because of misregistration, so K100% must be processed as overprint. (See Picture 2.) Other colors are processed as "trapped." Trapping is the process of compensating for this slight gap by printing extra small amounts of overlapping color where printed design elements meet. This process hides white gaps of paper that might occur when it is printed.
Note: Trapping is not applied to the K100% part of images (bitmap data). It is applied only to graphics (vector data) and is automatically processed in accordance with our RIP processing rules. Note, however, that data that is made with complicated objects or transparency or other effects might not be processed as trapped.
In this example, the overprint process (one technique of printing plates) is applied. A slight gap caused by misregistration has not arisen because overprint prevents this. However, because the thickness of each color ink on the paper is about 1.1 μ (micron=0.001 mm), black as a background might slightly "rise up" or come to the foreground, making it more dominant and visible.
※1.1 μ is the thickness of standard ink.
When a design contains a large area of K100% that stands out as in Picture 2, letters become less conspicuous. One way to avoid this phenomenon is to set up color as "C60％ / M40％ / Y40％ / K100％*," for example, to give K100% or "plain black" more "depth." Black with varying increased amounts of CMY density is called "rich black."
*These are the densities for CMYK recommended by GRAPHIC for making rich black.
Rich Black, CMYK All 100%
The following explains important points to bear in mind about rich black explained above.
Sharper or deeper black is created by making rich black by adding varying amounts of CMY to K100%. When the density of each color is increased to its maximum of 100% as in "C100％ / M100％ / Y100％ / K100％," this is called "CMYK all 100%." However, since a lot of ink is printed on the paper with this color setup, it will be hard or it will take a long time for the ink to dry or this may cause bleed-through. Also, thinner paper cannot be printed on. So, try to avoid this color setup.
It has already been explained that, with designs made with a high density of ink overall, colors are more likely to become misaligned or "out of registration," resulting in a slight gap between the colors. This also applies to rich black. Printing might turn out like the picture on the left in some cases. Be careful of placing small white letters or thin lines on a rich black background. The printed result might not be as intended like this picture due to misregistration.
While black is flexible because it can be defined in a variety of ways by varying amounts of CMY, people generally think of black as the black displayed on computer monitors. This black, however, becomes a different color when it is printed. Use a moderate amount of rich black at only necessary parts of your design, and your product will be better.
This example occurs only rarely. You could, for example, be creative and make an interesting design using different densities of rich black and K100% but this has its pitfalls. The Kanji character "黒" meaning "black" placed on a rich black background is processed as overprint in our RIP. As a result, the character melts back into the rich black background and vanishes. Add a small amount of a different color, C5% for example, to K100% in this case.
Overprint Failure 1 (portrait)
K100% is processed as overprint. When the hair is made of K100% only, objects behind such as the person’s head or background can be seen through K100%. (The figure on the left below is an extreme example for illustrative purposes only.) Our recommended setup is a rich black consisting of C60% / M40% / Y40% / K100%. The color setup of the hair in the figure on the right below is rich black. Rich black is effective when designs contain a wide black areas.
Overprint Failure 2 (letter with white outlines)
When a black letter with white borders is placed on a background and part of the black letter is set as only K100% and processed as overprint, the background or object behind the black letter can be seen through the K100%. (Figure on the left below) This seen-through area can be knocked out by adding another color, e.g. C ink, of at least 1% in addition to K100%. The black letter in the foreground is knocked out. (Figure on the right below) This phenomenon can also be resolved by making the same white color in the [Stroke] color setup with color [Fill] set as the background of the black letter with white borders.
Failure of K100% on Image
The following introduces an unintentional phenomenon that will occur due to overprinting. K100% is processed as overprint by our printing plate making system. The following explains how the following design will turn out in the final printed result.
The yellow flower image was placed in Adobe Illustrator, a box set to K100%（C0% / M0% / Y0% / K100%) was placed over the image and letters were typed in the box.↓
As a result, part of K100% is processed as an overprint and the background image can be seen through K100%. To avoid the phenomenon above, set rich black or C60% / M40% / Y40% / K100%. (The image on the right is an extreme example for illustrative purposes only.)