How to solve the error of color separation
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Excellent print quality begins with excellent color separation.
Screen printers often complain about the quality of the color separation at the output center. They are very headaches against the unpredictable results of the machine, and the cost increases a lot if the color separation error occurs. The complaints are mainly focused on the appearance of color shifts, dark circles and other problems.
Where is the reason? First, the output center is responsible for this; secondly, the ink manufacturer is also responsible for the color that is not expected. The discussion of these issues can be described as a cliché. This article analyzes some common color separation problems and provides a correct solution.
Here, we did not attribute the color separation problem to prepress producers or ink suppliers, but to find other reasons. Through a better understanding of the relationship between the original, film, template and printing workshop, the resolution of the color separation problem has greater confidence and confidence, and get more profits from it.
There is an old saying, "Grow melons, melons, and beans." The same is true for color separation. If the original manuscript or scanned image is not closed, the result will only get worse. Common problems are:
1. Low file resolution;
2. Originally stored in JPEG format;
3. The image has been transferred from RGB mode to CMYK mode;
4. The image is based on an inappropriate ICC color profile;
5. The image is scanned from the printed matter;
6. QuarkXPress or Illustrator documents are populated with a large number of EPS files with different attributes;
7. The files placed are from different software versions and the program uses different color management parameters.
All of this puts you at a disadvantage, because it's hard to determine which factor is causing the quality of the color separation to drop. This leaves too many uncertainties in the management of production processes.
There is no clear answer to the resolution of these problems. Printers may have to say that they have to work according to the principle of what to do, although this is the truth, but if this is resigned, don't complain too much about the problems that arise, and it is impossible to get the best product quality. It may be possible to edit and correct each file at a cost that is costly, but often it does not help.
If the image quality that you own is really not very good, and you want to get satisfactory quality, how is it good?
When a digital file is passed for color separation, it should have the correct resolution and the correct format that matches the company's standards. The process shop must then re-edit and correct the file. Perhaps the corrected image does not look good on the surface, but the resulting color separation data is correct and matches the color registration requirements.
In addition, you must know what is good quality. When receiving the color separation film, you must clearly understand what is a high quality color separation film and what is a low quality color separation film. This is one aspect that screen printers often overlook.
Before you image the positive film on the screen, it is necessary to measure the data of the positive film to make sure they are correct. The following are the minimum principles for quality assurance:
1. The minimum Dmax of the black solid area is 3.75 (requires transmission density measurement);
2. The minimum and maximum printable dots are defined in the control area that can be measured (requires transmission density meter measurement);
3. The point shape is complete. This can be judged by looking at the 50% dot (middle) to see if it is oval or diamond shaped. If it is round or square, there is a problem (requires a small magnifying glass 10 times or more);
4. The corner of the net should meet the requirements. They can be checked with a colored or Beta screen angle determinator.
5. The number of halftone lines must meet the requirements. Use a magnifying glass to count the number of lines at 50% of the dots. You can purchase a 10x small magnifying glass with a calibrated crosshair to measure the number of lines within 0.1 inch, then multiply by 10 to get lpi;
6. The final image should match the size requested by the customer.
The reason for the special requirements for these six factors is that they often make mistakes. The number of lines, corners, and dots often conflicts with the products that many service companies provide to customers, which means that they often have to be specially specified to change the default settings of RIP to meet the requirements. These instructions are easy to ignore.
The problem with the correct size comes from redefining the output size, or directly magnifying the intermediate file to the final file size. It is necessary to check the size before printing and printing. Image size Due to the color separation from different versions of the image, the size often varies, and the wrong version of the file is inadvertently passed in until the last time it is discovered by the customer, which is bound to cause huge losses.
Problem analysis
When an improper color separation image is sent to the pressroom, the most common complaint heard is that the printed color is inconsistent with the proofing color or the entire image is good but there is a color shift in the image. These problems are difficult to find the cause. The fundamental problem is that the primary color of the image separation does not match the printing color of the printer.
It is a good idea to refer to the SWOP value, which is the standard ink color value for the offset industry. These color values are often used by ink manufacturers and color colorants for screen printing color separation. However, since the SWOP standards are based on offset printing, they cannot be simply used for screen printing.
The ideal way to create a color separation is to use a RGB image with a better color balance. The chrominance value (Lab or Yxy) is used as the intermediate conversion for RGB to CMYK conversion.
The measurement requires a colorimeter or a reflection densitometer. Although these devices are currently much cheaper, only a few printers own these instruments. Without these tools, you can rely on the data and color labels provided by the ink manufacturer to match the correct color value.
If you get a CMYK file from the customer, the image has been converted to CMYK mode. You can be sure that these values will not match the printed results, and there will be a color shift. Usually converting RGB to CMYK is done in Photoshop and then converted to CMYK that matches the ink. Doing so will lose some of the color, but it is closer to the color that can be printed.
In the proofing, the analog proofing is used. When comparing the solid color blocks, you will print the colors that match the proofing solid color blocks. If the colors do not match, the printed products must also not match the proofing products.
One of the main reasons for the color mismatch is that the output of the analog proof is performed according to the preset simulated 20% dot gain. That is to say, the color tone used for proofing has been increased by a certain amount to compensate for the dot gain in printing. If the ink matches the solid color in intensity or density, then the effect on the print is at least 20% darker.
This is why a reflection densitometer is needed to ensure that the printed value is correct. It is also one of the main reasons for not using analog proofing in screen printing. This method is simple but accurate. If you are printing a coarser halftone (65 line inches or less), then using analog proofing is a better approach. With this resolution, the screen-printed dots expand closer to the proofing effect. When it exceeds 85 line inches, the reliability of proofing decreases as the number of lines increases.
When the color appears locally partial color, that is to say, most of the image is good but the brown or gray is not correct, it may be a problem in the color separation, so that the ink color does not match the color separation value. At this point, you need to go back to the beginning of the process and push forward to find the correct color information.
The core of the color separation problem is that Y+M+C is not equal to neutral gray. That is, it does not reach the gray balance, it is the most important part of the color separation. If the gray balance is out of balance, it is impossible to get a better image.