CMYK color mode for printing and color separation (bottom)

Second, separation settings (Separation Setup)

When scanning in, the image is generally RGB color mode. When it is converted to CMYK mode, certain conversion rules need to be followed, that is to say, there needs to be a group of color rules.

1. Separation Type

In the printing industry, "Gray Component Replacement" or "Under Color Removal" UCR (Under Color Removal) color separation processes are commonly used.

a. GCR is also known as a "non-color structure" process. That is, the non-colored components in the picture (synthesized by the polychromatic color) are all formed by black ink printing. For monochrome and inter-color, it does not work. People call the black edition a "long version."
Due to the lack of black ink blackness today, the density is lower in the darker portion of the image. This is the case if the color ink is set in the shadow of the achromatic component. This process is called the Under Color Addition (UCA).

In the GCR method, the amount of ink to be replaced by the complex color, dark color, and neutral gray ink is less affected, while the screen composed of the primary color and the intermediate color locally is affected by the fluctuation of the ink amount to cause color cast. Therefore, when using the GCR method, the process requirements are relatively high.

b. UCR is also referred to as a "color structure" of a modern, traditional printing process. The achromatic and color components in the picture are all composed of the three primary colors of CMY. Since the non-color component blackness of the three primary colors is not enough, the amount of black ink is increased in the dark tone portion, and the amount of three primary color inks is reduced at the same time. People often refer to black editions as "short-versions" or "contours."

To sum up the two color separation methods, the UCR method is better than the GCR method from the perspective of the printing and dyeing process. This is mainly to reduce the non-uniformity in the screen printing process and the unevenness of the dye amount in the printing and dyeing process. According to the color effect, GCR is more delicate than UCR. Therefore, it is better to use GCR under the premise of relatively safe technology. In addition, GCR is usually used for coated materials, and the dye has a relatively small color. UCR is used for non-coating materials (printing fabrics mostly belong to this category). In this way, different separation methods should be selected for different fabrics.

Under normal circumstances, pictures made with UCR and GCR are unrecognizable to the naked eye. At the beginning, it is advisable to use the UCR method for a test to obtain other process parameters.

2. Black Generation

This item is only valid for GCR mode. Since the GCR mode replaces one or all of the C, M, and Y colors with black, the degree of substitution is selected by the item. From the printing point of view, Medium is preferred. For printing and dyeing, when the black is selected to be larger, the C, M, and Y colors will be reduced, and the effect of the softer tone will be more favorable to the clearness of the screen.

3. Black Ink Limit

Control the total amount of black ink. For printing, 90% to 100% are used; for printing, it is possible to select a lower value (eg, 70% to 80%) than the printing parameters. If you find that the selected black ink is excessive, you can lower the black ink limit.

4. Total Ink Limit

The total amount of ink indicates the total amount of C, M, Y, and K. If the ink has a limit, the remaining value is the total amount of oil for C, M, and Y. In printing, the values ​​of C, M, Y, and K are generally not less than 300%. For printing and dyeing, I think it is between 250% and 300%, because the amount of ink absorbed by printing and dyeing silk is greater than the amount of ink absorbed by printed paper. Therefore, the total amount of ink can be set lower for printing and dyeing.

5. UCR Amount

For the GCR method, the black ink cannot satisfy the dark part of the image alone (black ink purity is not enough). In this case, it is necessary to add color ink to the dark part of the image of the non-color component. It only works on complex colors and does not work on primary colors or inter-colors. For printing, this item is generally not used, so it is not necessarily used for printing and dyeing.
From the analysis in this section, we can see that the color separation settings are related to the printing and dyeing process. To make Photoshop's color separation settings in line with the printing and dyeing process, you can make some choices in the parameters of this column, look at the sample effect, and obtain experience and data from the sample to promote it.

Third, in the RIP (Raster Image Processor) software to compensate printing and dyeing process

Although the image seen on the screen has achieved satisfactory results, the actual sample is not satisfactory. In addition to some of the above parameters to meet the printing and dyeing process as much as possible, in addition to the linearity of the film when the film is released, the expansion rate of the screen when making the screen, and the tension of the printing and dyeing, etc., will cause changes in the printing point, so it is necessary to make up. There are actually many ways to compensate. The commonly used method is: First pass the test to measure the dot enlargement rate of each step on the actual sample, and then add a bit of gain in the aforementioned Photoshop software, and also make up a bit when RIP rastering, making the actual sample on the three The stimulus value is as consistent as possible on the screen. Therefore, in order to obtain the correct printing and dyeing parameters, this experiment should be started.

1) Adjust the focus of the photo-setter so as to reduce the expansion or contraction of the photo-setter's light spot, affecting the expansion rate of the film and the appearance of interference fringes.

2) Adjust the film linearity. Different films have different characteristics such as development and fixing, so that light spots of the same size will have different sizes when they are imaged on the film. In order to compensate for the expansion rate of the various tone points on the film, compensation parameters should be used in RIP.
3) Compensation for dot expansion in the printing process. In the previous description, we have defined a point expansion of 35% to 40%, which is only compensated for 50% of the tone. The compensation for each point is determined by Photoshop's default compensation curve. In other words, there is a certain difference between this compensation and reality. In addition, if the actual expansion rate of the 50% tone is greater than 40%, then photshop will be powerless. At this time we can only make one more compensation in the RIP.

Another method is to define the point gain in Photoshop as 0. The dot expansion of each tone is compensated in RIP. The compensation value is the difference between the measured value and the set value of the sample.

4) RIP parameter selection. For printing requirements, each color requirement can represent 0 to 255 levels. Printing and dyeing do not need to have such a high requirement, and generally can be controlled between 64 and 128 levels.

Screen number: For screen printing and dyeing requirements, the screen mesh number is between 98 and 118 meshes/cm (250 to 300 meshes/inch). For the printing and dyeing process, the number of meshes to be screened is in the range of 24 to 39 mesh/cm (60 to 100 mesh/inch). In order to meet a certain number of gradations, a 60-destination 600dpi photo-taking output machine can reach up to 101 levels.
Screening angle: The screen angles for the C, M, Y, and K color versions are recommended to be 15 degrees, 75 degrees, 90 degrees, and 45 degrees, respectively. If there is a conflict between silk and silk-screen patterns and color patterns, then the angle of the cloth pattern and the number of meshes of the screen can be adjusted (under normal circumstances, the number of meshes should not change).

Author/Yang Cheng (Hangzhou Tianhong Electronics Co., Ltd.) Shou Qihong (Zhejiang Institute of Silk Engineering)