Screen printers often complain about the quality of the color separation film in the output center. They are very troubled by the unpredictable results of the row machines, and once the color separation goes wrong, the cost increases a lot. The complaints mainly focused on the color shift, dark tone and other problems.
Where are the reasons? 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 clichés. This article analyzes some common separation issues and provides a correct solution.
Here, we did not attribute the problem of separation to prepress makers or ink suppliers, but instead identified other reasons. Through a better understanding of the relationship between manuscripts, films, stencils, and printing shops, there is greater confidence and confidence in the resolution of color separation problems, and more profit from them.
To have an overall concept
There is an old saying, "Fruit and bean seed," and the separation is the same. If the original manuscript or scanned image does not pass, then the result can only get worse. Common problems include:
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. Images are scanned from prints;
6. QuarkXPress or Illustrator documents are loaded with a large number of EPS files that have different attributes;
7. The files put in come from different software versions, and the program uses different color management parameters.
All of this puts you in an unfavorable situation because it is difficult to determine which factor degrades the quality of the color separation film. This makes too many factors in the management of production processes uncertain.
There is no clear answer to the resolution of these issues. Printers may have to say that they have to work in accordance with the principle of what to do. Although this is true, if you do so, you should not complain too much about the problems that arise and you may not get the best product quality. It may be possible to edit and correct each file at a relatively high cost, but often this does not help.
If the quality of the image itself is not really good, but also want to obtain a satisfactory quality, what is good?
When a digital file is passed over for color separation, it should have the correct resolution and the correct format in accordance with company standards. Afterwards, the craft workshop must also re-edit and correct the document. Perhaps the corrected image does not appear ideal in appearance, but the obtained color separation data is correct and matches the color registration requirements.
In addition, you must also know what is good quality. When receiving the separation film, you must clearly understand what a high-quality color separation film is 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 plate, it is necessary to measure the positive data to make sure they are correct. The following are the minimum principles for ensuring quality:
1. The minimum Dmax of the black solid area is 3.75 (need to measure with a transmission densitometer);
2. Minimum and maximum printable dots are defined in the control area that can be measured (requires densitometer measurements);
3. The dots are complete. This can be judged by looking at the 50% dot (middle tone) to see if it is oval or diamond shaped. If it is round or square, there is a problem (a small loupe of 10 or more is needed);
4. Web corners must meet the requirements. They can be checked with a color or Beta screen angle determiner. Make sure to avoid 0°, 45°, or 90° as mesh angles. These angles are more prone to moire than other angles;
5. The number of halftone screen lines must meet the requirements. Use a magnifying glass to count the number of lines in 50% of the dots. You can purchase a 10x magnifying glass with a calibrated reticle to measure the number of lines in 0.1 inch, then multiply by 10 to get lpi;
6. The final image must be the same size as the customer's request.
The reason why these six factors are particularly required is that they often make mistakes. The number of lines, nets, and dots often conflicts with products that many service companies provide to customers. This means that they often must make special instructions to change RIP's default settings at launch to meet requirements. These instructions are easily overlooked.
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 dimensions before printing and printing. Image size due to the color separation from different versions of the image, so the size will often appear biased, and the wrong version of the file was inadvertently passed in until finally found by the customer, which is bound to cause great losses.
Problem analysis
When an improper separation image is sent to a print shop, the most common complaint that is heard is that the print color is inconsistent with the proof color or the entire image is good but the image has a partial color shift. These problems are difficult to find. The fundamental problem is that the primary colors of the image separation do not match the printer printing colors.
Referencing the SWOP value is a good approach, and SWOP is the industry standard ink color value for offset printing. These color values ​​are often used by ink manufacturers and color separators for screen printing separations. 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 good color balance. The chrominance value (L*a*b* or Yxy) is used as an intermediate conversion when making RGB to CMYK conversions.
Measurements require a colorimeter or a reflection densitometer. Although these devices are currently much cheaper, only a few printers own these instruments. Without these tools, the ink manufacturer's data and color patches can be used to match the correct color values.
If the CMYK file is obtained from the client, the image has been converted to the CMYK mode. You can be sure that these values ​​do not match the printed results, and the color shift will definitely occur. Normally converting RGB to CMYK is done in Photoshop and then converted to CMYK that matches the ink. Doing so will lose part of the color, but it is closer to the color that can be printed.
In proofing, you use simulated proofing. In contrast to the actual color blocks, you will print the colors that match the actual color blocks. If the colors do not match, the printed product must not match the proofing product.
One of the main reasons for color mismatch is that the output of analog proofing is based on a preset analog 20% ​​dot gain. In other words, the color tone used for proofing has been increased by a certain amount of compensation for dot gain in printing. If the ink matches the solid color in intensity or density, the effect on the print will be at least 20% darker.
This is why a reflection densitometer is needed to ensure that the print value is correct. At the same time, it is also one of the main reasons why screen printing does not need to simulate proofing. This method is simple but accurate. If you are printing a coarser halftone (65 lines/inch or less), then using mock proofs is a better method. Printed at this resolution, screen-printed dot gains close to proofing. When more than 85 lines/inch, the reliability of proofing decreases with the number of lines.
When the color appears to have a partial color cast, which means that the image is mostly good but brown or gray is incorrect, there may be a problem in the 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 out the correct color information.
The core of the separation problem is that Y+M+C does not equal neutral gray. That is, the gray balance is not achieved. It is the most important part of separation. If the gray balance is out of balance, it is impossible to get a good image.
Source: Wins
Resolution of wrong problems in color separation
Good print quality begins with good separation.