Six-color printing in screen printing (under the first part)

(A Six-Color Cure For Screen-Printed Halftones, Part 1) The Theory and Illustration of the Six-Color Printing Process Let us first carefully study the two-tone process through a tone reproduction curve. People engaged in prepress work may have a deeper understanding of the traditional gamma curve, which reflects how the halftone dots on the film are converted into printed dots. When the printed dots accurately replicate the percentage of halftone dots on the film, that is, the input is equal to the output, the gamma curve is a 45[deg.] diagonal (Figure 1). ****Figure 1: Screen-printed gamma curve***** Dots are transferred from the film to the substrate for accurate copying. The 45° diagonal line in the figure indicates that there is no dot gain or loss in the entire area of ​​the image. ******************************** This tone curve is barely available in realistic screen printing. In most cases, we must deal with the loss of high-profile regional outlets, maintain the relative stability of midtone transfer, and compress and expand the network of more than 75% of shadowed regions. The typical curve in four-color overprint screen printing should be an “S” curve. The finer the number of halftone screen lines, the greater the “S” outward expansion. The typical result of four-color screen printing is an "S" curve, which indicates that the high-profile site is missing, and the dark tone site is expanded. The goal of six-color overprinting is to turn the high-profile and dark-sounding curves into a nearly straight line. The stable transmission site of the curve is midtones. Here, the input and output of the dot are generally in a straight line, and the unstable parts are the toe (high profile) and the shoulder (dark tone) of the profile. These areas change very quickly and are difficult to adjust in printing. However, we can use this information to develop a more stable and repeatable six-color overprint. With a six-color overprint scheme, it must be known that the reproduction of tones and colors are two related factors. The first factor to consider is the ink density or strength used. The correctness of this value is directly related to the second factor - how close the visible reflection value is to the value in the actual image. For example, if we print with a standard magenta ink with a density of 1.15, the 20% printed dot area is very close to the 20% tone optical reflectance. However, if the ink density increases to 1.45, the actual reflectance in the 20% dot area is darker than we expected—close to 40% dot. Halftone dot values ​​are closely related to ink density, which is one of the reasons why printers spend a lot of energy on density control. If one or both are incorrect, the final color will be biased. The severity of this deviation is proportional to the number of overprint colors used. Therefore, deviations in four-color printing may make the whole product messy. The use of traditional cyan and magenta halftones in four-color printing has a problem in that when the midtones remain relatively stable, high-profile information is lost and dark tone information is enlarged. Can we compensate by combining the errors at both ends of the tonal range? The only answer is to separate the tonal range. The purpose of the two-tone printing is to change the density or color intensity of the high- and dark shades of cyan and magenta in order to maximize the stability of the gamma curve to pass straight lines. This sounds very technical, but it is not. To achieve this goal, we only use halftones in different ways. The principle that we rely on is to adjust the dot size by changing the intensity (density) of the color. We reduced the color intensity of the magenta ink to 25% of the normal case in the high profile area, which means that 100% of the magenta dots have the same reflectance as the 25% regular density ink dot. In other words, in order to get a 25% reflection, we need to increase the size of the high-profile magenta dot to four times the original. (The typical light cyan and light magenta color densities measured by the densitometer ranged from 0.15 to 0.25.) Further thought, the 5% dot just enough to print had a 1.25% tone reflection and was a very bright color. . It allows us to have more control over the color, because every 1% dot gain is lost and the reflection value changes by only 0.25%. It minimizes the loss of high-profile sites and prevents color drift. In actual screen printing, 16% of the dot area change only caused a 4% change in reflectance. Within this threshold, each person engaged in color overprint jobs can easily work. Another advantage of using this light-colored ink is that if a 5% print dot appears moiré, it is not easily seen on a white background of the paper. The use of this process by personnel who perform printing on very large areas of very soft colors and very high light areas can clearly feel its advantages. At the dark end of the tonal range, we doubled the density of cyan and magenta (between 1.45-1.60). This means that 50% dark green or dark magenta printing dots have a dot equivalent to 100% standard ink (density 1.15). In other words, we can greatly reduce the size of the largest dot to the original 75%, and can achieve darkness and depth of darkness. We can also reduce the difficulty of maintaining 75% of the dots, so we can greatly enhance the details and resolution of the shadows, and successfully achieve the original difficult to achieve images, such as black feathers. If the process is so good for cyan and magenta treatments, why don't we divide the yellow and black into two tones? The reason is simple: Yellow is a color with no contrast, because it is relatively shallow in itself, and its influence is mainly in the bright tone region of the tonal range. The darker cyan and magenta inks used in the six-color overprinting have less yellowish bias than the conventional inks and tend to be blue. Unlike four-color printing, black is used to compensate for yellow and increase dark tone density. Bi-tonal cyan and magenta in six-color overprinting actually reduces our need for black ink and may not be needed under certain circumstances. The addition of six-color inks separates the magenta and cyan colors so that we can make the tone curve of the screen printing more stable, and will not bend too much. We can control the loss of high-profile nodes and expand the tone levels to enhance detail. Since the size of the 1/2 standard dot is used at the shadow tone, or four times the dot size is used in the highlight tone to obtain the same hue as the four-color print, this results in a greatly reduced color shift. Since we have already explained the basic theory of this color reproduction method and discussed its advantages in screen printing, the following is the time when it is officially used. In the second part we will look at all the issues in production and the different technologies we can use to make copies. There are other issues, such as gray balance, color channel separation, color separation, selection of halftone screen lines and screen angle, proofing, use of DCS2.0 output, printing color sequence, ink and register lines, etc. the second part.