It is generally believed that due to the principle of offset printing it is an unavoidable and unstable way of printing. The "oil" in the ink and the "water" in the ink tank cannot always be combined as well as one would expect. Therefore, for printer manufacturers and consumer product suppliers, they must continue to struggle to control the entire process. process. For printers and buyers of prints, this means that they can only reach a compromised state in a fairly long calibration process, that is, to achieve "commercially acceptable quality" rather than print that can be foreseen in advance. quality.
However, this situation has changed. The current application of digital technologies in the printing and publishing industries is the key to their success. Digital technology makes color quality control and consistency control no longer a very technical thing. For example, CTP reduces the uncertainties in the process of plate making. The now-exposed version can be determined to have exactly the same plate when it is re-plated after 3 months.
The current press has very good control performance. In the printing press, the auxiliary loading device will eliminate the problems caused by the unevenness of the gasket, and the temperature-controlled roller excludes other uncertainties. The common color control methods are as follows.
1 Color control method based on ICC file
Taking readers’ digests in the United States as an example, their magazines are available in 48 editions and 19 languages. They are distributed in more than 60 countries around the world and there are 20 editions in Europe. In addition to magazines, they also distribute other products including books, such as music, video, and other special products. With the expansion of their distribution and delivery methods, the company has accordingly expanded the range of their printed products, including electronic media and traditional print media.
Image printing is the task of all print sites that print Reader's Digest. It involves a lot of repetitive work and costs. Different printing sites make different versions of image quality control almost impossible to achieve, and color management is especially a problem. Differences in color quality and appearance range from inconsistent tone to severe color deviations. For this reason, they adopted a set of Colour Systems to avoid similar problems. This is a website dedicated to managing readers' digests of all European versions of documents. Including library files, product files, images and graphics. The company has now established a new workflow to ensure consistency between different media and printing plants. In different parts of Europe, operators can access this site using special URLs and passwords to ensure consistency in printing.
Colour Systems processes the RGB image input from the drum scanner. Each scan uses a scan-correcting ICC file (also called an ICC device connection file) to convert the color of the image into the CMYK color space of the SRD (selected Reader's Digest). When needed, the scanner does frequent maintenance and inspections based on established control procedures and documentation. This ensures that both the input original image and the final output image color and ink characteristics are up to the standard, thus ensuring color reproduction and matching on the press.
2 FM screening technology combined with chroma color control
These changes have aroused people's interest in FM screening and other screening technologies. For example, Agfa's Sublima has studied the size of the smallest dot that a printer can copy in a stable manner. If the dot size is 60μm, it can represent 6% of the 150lpi screens or 8% of the 250lpi screens. In the usual case, this means that the press is limited to a relatively low resolution. With Sublima, it is possible to achieve a higher profile part without trying to print less than the minimum dot of the press, but to reduce the number of dots in the area of ​​less than 6%. Such an effect is immediate and the printing press can print network cables that are more surprising than expected, with the high-profile part appearing brighter and the middle tone being more abundant.
Creo takes a step forward in advancing the use of FM screening, but it not only emphasizes the impact of very small dots on print quality, it also promotes the application of balanced distribution of oil and water due to the consistency of its dot size. . This result is better, the printer can be controlled more consistently, the amount of ink used can be reduced, and the printing process can be better controlled. This is critical for the long version of web offset printing, which is why 40% of US magazines use FM screening for printing. As Polestar began to use Creta's Stacatto FM screening in its web offset printing shop, FM screening began to be used in British magazine printing.
Between 1970 and 1980, register control on web printing presses was a very high technology, and now there is an increase in the ability to adjust the color closed loop, that is, the ability to adapt. Once the color closed-loop control system from QTI and GMI is set up, it can ensure that the same color is maintained throughout the printing process. As the ultimate owner of QTI, QuadGraphics is trying to ensure that no artificial intervention is required on the press. In contrast, GMI is taking a big step forward in using colorimetric values ​​to achieve quantitative printing.
3 spectrophotometer color control method
GMI (Graphics Microsystems Inc., Graphics Microsystems Inc.) added a "Press Analysis System" to its ColorQuick color loop tool. This system records a large amount of data obtained from spectrophotometers and stores them in a standard SQL database. The analysis tool can track the part of the printing press during the printing of a print. It can also track the printing machine's printing time for any customer within 1 week, 1 month or any specific time period or during this period of time. Health status.
This system can generate dot enlargement graphs and densities for any period of time. Calculations can monitor the difference between the original set target color and the color measured on the press. When used in reverse, the replica predicts the performance of the press, and these characteristics can be taken into account when making the plate. In other words, by setting a target color, the printer can read these colors and copy them out.
Printing simply monitors these data to ensure that the press is working properly. Publishers can also be provided with these data that indicate the operating status of the press. Since GMI uses a spectrophotometer, it can measure the value of the color and then determine how to adjust it by evaluating the neutral gray generated by the three-color overprint. Since neutral gray is the most sensitive color, any deviation from the set criteria is immediately discovered and can be corrected before affecting the overall balance of the print.
Even if the printer is operating at the correct density, the color may not be correct, which is why the density control is now turned to the color control on the press. This allows the printer to achieve gray balance on the monitor when the correct density value is reached. US publishers have stated that their goal is to omit all hard copy proofs, which is the last largest market for digital processes. They look forward to valuable advice on digital printing.
Due to the vast size, sending hardcopy proofs by express delivery is a time-consuming and costly matter. If a printer wants to match a set color value, visual assessment of the proof is not necessary. The main purpose of this is to reduce waste. In general, more than 1,000 sheets will be wasted when printing.
The place of attraction for printers is the introduction of multi-point control printing presses, which is what RR Donnelley is studying. All operations are performed under the same standard and the same prints will be produced. Similarly, the attraction for consumer companies is that they can provide them with a wealth of information about the status of their products printed on the press.
However, installing such a spectrophotometer on a sheet-fed press is quite rare. Even such control systems are installed on the most advanced offset presses and they are not widely used in markets such as labels and cartons which are critical for color fidelity requirements. The printer has not believed that it is reasonable to install the various devices, and now almost no printer buyers demand such a crowded analysis system.
However, there is still the problem of improving the consistency of color on printing presses. The ICC file has its place, but it turns out to be difficult to apply. In order to create an ICC document, the printer needs to print a set of test specimens of various color patches and measure them one by one. Through a series of long readings, a very accurate image was obtained to form the ICC file for the day. In fact, for most printers, the work was too heavy to be practical.
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