Prepare electronic documents for screen printing (below)

V. Separation barriers

Most CMYK documents received by screen printers from customers are produced according to the standard of offset printing. The dot expansion parameter file is shown in Figure 1A. The number of lines at the lower tone is less than or equal to 50 lines/inch. Such a file is more suitable for screen printing; when the number of lines is greater than or equal to 65 lines/inch, the dot enlargement is changed to a more commonly used parameter file for screen printing (eg, 1B), which makes the image suitable in the midtone region. Compensation.

When a printing plant receives a file stored in CMYK mode, the color separation parameters of the file have been determined, such as screen angle, dot gain, gray balance, and so on. In most cases, the parameters attached to the document are set according to the offset printing process. If these documents are directly printed, the number of lines of the output silk screen image is only 50 lines or less. For some applications, such as wide output or POP display pictures, this number of lines is not enough. This is mainly because such products often need to be viewed from a distance. Therefore, it is required that the outlets be large enough to maintain the original tone range of the image. Only in this way can they use the dot expansion files set in the offset printing process.

But when we increase the number of lines to 65 lines/inch or higher, the trouble comes. Even with the fine mesh and UV inks used, the dots at highlights and shadows can be difficult to attach. At this time, with the increase of the number of lines, the dot enlargement file is converted from the universal offset parameter file as shown in FIG. 1A into a parameter file more suitable for screen printing. FIG. 1B. The higher the number of lines, the larger the effect of the dot enlargement profile, and the greater the difference in the appearance of the printed matter.

In order to solve this problem, it is necessary to ask the customer for two files whose patterns are CMYK and RGB, respectively. No matter what software the file is made of (possibly Photoshop or it may be captured by a high-end scanner), it is not easy to modify the separation parameters without damaging the file information. Almost all color separation software will lose some digital color information when converting a large color gamut RGB file into a CMYK file with a smaller color gamut. During the conversion process, ink color settings, total ink volume limits, dot gain, gray balance, and UCR/GCR parameters need to be determined. However, the setting of these parameters is usually based on the offset printing process and they do not match the screen printing ink. Therefore many values ​​are useless.

But if we receive the RGB file, we can modify the separation parameters before converting the file to CMYK mode according to some specific applications. One of the benefits of this is that it guarantees the consistency of color between CMYK files made by various printing methods (such as offset printing, screen printing, etc.).

If you do not get the original RGB file, you can convert the CMYK file back to an RGB file and then set the correct parameters for screen printing on it. However, the final image may not be as accurate as the original RGB image. This is because the black color was added during the initial conversion from RGB to CMYK, and the black color is converted back to the RGB image. When the color separation is finally transferred to CMYK, These blacks are all brought into the final image.

Screen printing also requires screen angles, screen lines, and dot shapes that are different from other printing methods. Ignoring the screen angle setting often produces annoying moire. For a relatively large number of screen lines such as 50 lines/inch, if the number of meshes is relatively small, such as 355 lines/inch, then the traditional offset printing angle (Y=0°, C=15°) is used. , M=75°, K=45°) Yes. At this time, the outlets are large enough that the intersection between the mesh and the outlets does not cause much trouble.

However, with the increasing number of lines (smaller dots), it is necessary to rotate the entire angle away from the traditional offset dot angle. This is mainly because the angle of the thread with the silk screen is almost the same as the angle of the black and black dots. It is easy to produce moire. In most cases, these angles are modified during the RIP interpretation and photocopying stages that are output to the film after separation.

The most common correct angles suitable for screen printing are Y=7.5°, C=22.5°, M=82.5°, and K=52.5°. This set of values ​​can be used in 95% of screen printing operations, even for applications with a large number of lines. In addition, if you use smaller diameter wire, the number of wires is not much, it can greatly increase the chance of success.

When rotating the screen's angle away from the default angle of the imagesetter, pay attention to turn off the screen filter. Screen filter is a subroutine often used in RIP software. It can avoid conflicts between various design elements deployed in the output file.

Each image placed in the layout usually includes the gradation information of the image itself, which is embedded in the entire design file. This means that each image has its own dot shape, angle, and line number. The screen filter ensures that all image elements can be output with the same net corner, dot shape, and line number. Unfortunately, such screen filters are usually used only at standard output angles for the offset process, and rarely support applications with less than 65 lines per inch.

To turn off the screen filter, first open each of the deployed files to ensure that the screen angle and tone information are the same. Then turn off the screen filter on the imagesetter RIP. If you are exporting color separations at the output company, you need to explain the situation to them.

Figure 2 - Image adjustments that describe the appearance of pre-and post-adjustment CMYK digital images when preparing documents for screen printing. The leftmost side is FIG. 2A, which shows the effect of the image displayed on the screen according to the offset setting, and FIG. 2B (the middle) shows that the total ink restriction and the gray component substitution (GCR) are adjusted on the screen according to the requirements of the screen printing. Effect. The effect of the image printed according to the adjusted separation file is shown in FIG. 2C (rightmost).

Sixth, digital file proofing - how to screen printing proofing image

Another challenge faced in the screen printing process is how to proof the image to be printed. Because silk screen images with 65 lines/inch or less have similar color and tone characteristics as offset prints, traditional overprint proofs, Chromalins, or Matchprints proofing methods are ideal.

These proofs will exhibit dot characteristics on the separation film. It is expected that the physical dot gains appearing on the printing press will be manifested in the form of optical dot enlargement (higher color density) on the proofs. Since most proofing systems are designed for offset printing, the standard optical dot gain is approximately 20%, which means that the color on the proof looks 20% darker than the actual print. Since the dot gain parameter file is the same, we can use an optical approximation to represent the physical dot gain that may occur in screen printing.

In the process of matching the four-color ink density with the color image on the proof sheet, the screen printing factory often encounters troubles. This process usually matches the color intensity value of the field CMYK on the proofs. As a result, the final screen printing image will be 40% darker than the original, of which 20% will occur on the proof and the other 20% will occur on the physical dots on the printing press.

For halftone dots with a line number of 65 lines/inch or higher, the accuracy of the proofs has been shaken by changing the dot gain parameter file. Screen dots on the screen printing of finer lines will lose some of the highlights and shadows, and will show greater expansion in the 1/4, middle and 3/4 tone than the proofs.

Applications that face this situation are compact discs, art prints, and some highly detailed prints that require close viewing. If you want to apply traditional analog proofing methods to these images, the highlights and shadows will be printed too brightly, and the midtones of the image will be printed too dark. The overall visual effect is a large shift in the non-color (such as gray, light brown, brown and brown), the contrast of the image increases,

If you have a precise dot gain parameter file for the press, the proofing system should be adjusted to match this file. But even so, because of the different imaging processes of some proofing devices, it is impossible to produce completely consistent results. For example, some proofing devices form images that are continuous, fine-grained inkjet and sublimation processes. Neither of them can display the structure of the halftone dot, which greatly limits the application in the field of screen printing.

A real solution is to make proofing images on screen printers. If the job is a large-volume print, this is a very feasible solution. It is the only way to produce products that meet customer expectations.

in conclusion

In order to satisfy the customer's needs as much as possible, it is necessary to control the preparation of documents, even if the customer or designer has provided corresponding electronic documents. The first thing to do is to train the production and sales personnel in the company so that they understand what is incompatible with production in the electronic documents provided by the customer, how to correct them, and why they are corrected.

In addition, they should continue to communicate with customers so that they can produce more useful documents. For example, the factory can establish internal parameter specifications or guidelines for customers to understand our process. You can also organize some seminars on document preparation to explain the differences between screen printing and other types of printing processes. Finally, when the modifications are completed and the production is completed, feedback should also be provided to the designers and agents who create the image files, pointing out the problems found, and suggesting how they will optimize in the future when preparing files for screen printing.

There are many efforts made by enterprises to modify documents. For this purpose, they need to develop a standard program for checking and correcting documents. Usually referred to as “preflight”, it helps to quickly find problems and simplify document preparation. process. When each document arrives on the shop floor, it needs to go through a “pre-inspection” procedure instead of a few days or weeks later. This ensures that if problems are found in the documents, it will immediately communicate with the customer so as not to delay the delivery date.




Source: Bison