Six-color printing process theory

In the past 10 years, four-color overprinting has had a great impact on screen printing. Some complicated design software and new screen-making methods have emerged. Digital prepress technology and printing equipment have also been greatly improved and improved. Can accurately predict the quality of the product, the product's reproducible printing has also been guaranteed. However, due to the limitations of the four-color printing mode and the screen printing method itself, it is not possible to obtain a color that is completely consistent with the original.

In the past, color silk screen four-color overprint was completed by a monochrome printer. Only one color can be printed at one time, and the second color can be printed after drying, until all four colors are completely printed. This makes the control of the color very difficult to grasp. In recent years, due to the application of multi-color printing presses, the consistency and accuracy of color have been greatly improved. Initially there were two- and four-color printers, and now there are five-color and six-color printing systems.

Here, we will introduce the six-color overprint scheme in two parts.

The lack of four-color printing

Four-color overprinting is an economically and technically acceptable model. It is a universally accepted printing method that can also reproduce almost all colors. However, there are many technical defects in four-color overprinting.

Although image reproduction technology has greatly improved in recent years, four-color reproduction is still a less stable process, and it needs to control too many factors. What's more, it produces moire and dot gains in printing, resulting in the final image being either too flat or too contrasting. In addition, with the four-color process, some visible colors cannot be duplicated.

Therefore, four-color printing is difficult to carry out consistent quality control. On a press, the color may shift several times, even to a point where it is completely different. This is due to many environmental and physical factors, the main factors are: squeegee edge wear, printer temperature changes, screen tension changes, ink performance inconsistent (pigment accumulation and viscosity and rheological properties change) and so on.

The degree of color reproduction depends on the range of color gamut that the ink can print. From this perspective, four-color printing is also very limited.

The advantages of six-color overprint

Extending the traditional four-color overprinting to six-color overprinting is a subject worthy of close attention and careful consideration. The six-color overprinting process is based on the traditional four-color overprinting and is obtained by expanding the standard four-color unit. However, unlike the hexagonal spot color process, hexagonal spot colors are based on CMYK with orange and green colors, and the six-color overprints described here still contain only four CMYK colors.

There are no two new colors added to the six-color system, but instead of the standard cyan and magenta ones, they use their lighter and deeper series respectively. Six-color overprint is represented by CMYKLcLm, where C and M represent dark cyan and dark magenta, and Lc and Lm represent lighter colors of the two colors. There are also CMYKcm or CcMmYK.

Note that these names do not indicate the order of the colors; they merely indicate the hue of the colors used in the printing. This use of two different ink densities is called double-domain or two-color replication. It is a change in the new color reproduction method called "n-channel", where n is the number of colors used in printing.

There are several important advantages to using two-color extended overprint inks. The biggest advantage is that it greatly expands the scope of the color gamut. Epson and several other inkjet printers and ink manufacturers are using a process similar to this six-color overprint to develop inkjet printers in hopes of achieving more realistic image colors. Inkjet printing has become a more popular digital replacement for film processors, and the use of lower-range inkjet printers can produce extraordinary results.

By adjusting the intensity and density of the colors used in printing, we can improve the appearance. The appearance effect is enhanced because the use of two colors reduces the contrast between colors. For example, when we have a very bright magenta and print the upper detail with a darker magenta, under bright magenta it may lose the darker point and enhance the image. This method of achieving the overall image effect by printing halftone dots in the details is clearly continuous in tone. POP display and other large designs used for close-up look can get better results from this method.

With the six-color process, not only can the image clarity and resolution be improved, but also the degree of overtone smoothness is better. Some of the more difficult to obtain colors such as flesh, grilled food, water, sky, chrome, and similar metallic colors can all be obtained by adding light tones in two-tone printing.

Duotone printing also reduces the appearance of textures in overprinting, which on the one hand reduces the halftone dot pattern and on the other expands the range of reproduced tones. With six-color printing, coarse halftones appear less coarse, 50 lines/inch halftone looks like 100 lines/inch, and 65 lines/inch looks like 120 lines/inch or better.

Printing using the duotones of cyan and magenta also extends the dynamic range of the copied image. The dynamic range is the difference between the brightest area and the darkest area. In short, it makes the darker tone darker and brighter.

Most prints have a maximum dynamic range of no less than 2.0. This standard is rarely met in screen printing and is generally between 1.6-1.8. A 2.0 dynamic range means that 1% of the bright dots can be restored, and the copied ink can absorb 99% of the light and reflect only 1% of the light (compared to the screen-printed black, which can only fall to 97.5%. -98.4%).

However, the dynamic range of the eyes of the human eye can reach 4.0 or more (1/10000 light perception, or 99.99% black perception). This is why dark details can be seen in the visual world and cannot be achieved in printing. With the two-tone color mode, we can increase the printed dynamic range to 2.2 (99.4% of black). This does not seem to be a big deal, but we can see that the black level obtained is better than the four-color overprint.

With two-tone printing, we have increased the highlights and shadows in the tonal range. In the past, our process often lost some of the layers when printing lighter, softer or darker colors. Duotones have a relatively long tone because of their soft appearance, good depth of detail, and dark details.

With the advantages of two-tone printing for high light color control, we have achieved better control over previously uncontrollable high neutral neutral colors such as beige, sand, ice, crystal reflections, detergents, tan, and more. These colors all have very few third color components. Duotone printing allows us to use larger dots in these color areas. This allows for greater control of the environment and physical variables, more accurate color reproduction, and shorter visible color range.

There is also a benefit in the darker part. By using darker colors than the normal primary colors magenta and cyan, we can better control the third color that needs to enhance the surface color of the dark tone region. Its main advantage is that we don't need to use too much black to compensate for the darkness. By using less black ink in the third color component, rich colors such as red brown, walnut, dark red, chestnut, dark green, indigo, and the like can be maintained. In the four-color mode, the additional black must completely cover the hue components of these colors, presenting a flat, black color.

Using two-tone can compensate for difficulties in copying brighter and darker colors, and the color drift phenomenon is also better controlled. Color drift refers to the color characteristics that change the appearance when the color tone becomes darker.

The swimming pool is an appropriate example of color drift. Although the water in the entire pool is the same, for the observer, the deeper water is bluer than the shallower one, that is, the shallow water drifts toward the yellow. This phenomenon is related to the problem of light absorption and reflection, that is, shallow water and deep water have different absorption and reflection of light. In the same way, printing ink is also the same, so there is also a phenomenon of color drift.

In the example involving the problem of color reduction, some wavelength of light is absorbed and the reflected wavelengths constitute the color we feel. Light colors tend to be yellow, while dark colors tend to be blue.

The challenge in printing is that when we increase the ink color density (Dmax of the ink ink layer), the color is somewhat bluish. The higher the density, the darker the color, but it tends to shift away from the soft shades and drift to a distinct blue tone. On the contrary, if we reduce the color density of the ink, yellow drift will not show up in high-profile areas. Reducing the intensity of the color will show up in the dark tone, which shows a relatively flat color and loss of brilliance, with a certain yellow drift.