The eternal theme of printing is the transmission and reproduction of colors. How to use different color control technologies to improve the quality of offset printing images commonly used in color restoration is a challenge faced by domestic companies. Based on summarizing the world's most advanced color control technology for offset printing images, this paper puts forward new perspectives for the development trend and characteristics of China's printing industry, and points out research directions for future color control technologies.
It is generally believed that offset printing is an inevitable and unstable printing method due to its printing principles. The "oil" in the ink and the "water" in the ink tank cannot always blend together as expected, so for printer manufacturers and consumer goods suppliers, it is necessary to continue to control the entire process with great effort process. For printers and print buyers, this means that only a very long verification process can finally reach a compromised state, that is, to achieve "commercially acceptable quality" rather than printing that can be foreseen in advance quality.
But this situation has changed. The current application of digital technology in the printing and publishing world 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 plate making process. Now the exposed version can be sure to get a completely consistent printing plate when remaking the plate after 3 months.
The current printing press already has very good control performance. On the printing press, the auxiliary plate-setting device will reduce the problems caused by the unevenness of the liner, and the temperature-controlled Cylinder excludes other uncertainties. Several common color control methods are as follows.
1 Color control method based on ICC files
Take the American Reader's Digest as an example. Their magazines have 48 editions in 19 languages ​​and are distributed in more than 60 countries around the world. There are 20 editions in Europe. In addition to magazines, they also distribute other products including books, such as music, videos and other special products. As their distribution and delivery methods expand, the company has correspondingly expanded the range of their printed products, including electronic media and traditional print media.
Image printing is a task for all printing sites that print readers' abstracts. It involves a lot of repetitive work and costs. Different printing sites make different versions of image quality control almost impossible to achieve, color management is especially a problem. Color quality and appearance differences can appear from inconsistent tone levels 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 all European versions of Reader ’s Digest. Including library files, product files, images and graphics. The company has now established a new workflow to ensure consistency between different media and printing houses. In different places in Europe, operators can use special URLs and passwords to enter this website to ensure printing consistency.
Colour Systems processes the RGB image input by the drum scanner. Each scan will use a scan correction ICC file (also called an ICC device connection file) to convert the color of the image into the CMYK color space of SRD (Selected Reader's Digest). When needed, the scanner performs regular maintenance and inspection according to the established control procedures and documents. In this way, the color and ink characteristics of the original input image and the final output image can be guaranteed, thereby ensuring color reproduction and matching on the printing press.
2 FM screening technology combined with chroma color control method
These changes have stimulated interest in FM screening and other screening technologies. For example, Agfa's Sublima researched and estimated the minimum dot size that a printing machine can replicate stably. If the dot size is 60μm, it can represent 150% pi plus 6% of the dots or 250lpi plus 8% of the dots. Under normal circumstances, this means that the printer is limited to a relatively low resolution. With Sublima, you can reduce the number of dots in the area less than 6% without trying to print smaller dots than the smallest dot of the printing press to achieve a higher profile. This effect is immediate, the printing machine can print more pleasantly than expected, the high-profile part appears brighter and the middle-tone layer is richer.
Creo takes a step forward in advancing the use of FM screening, but it not only emphasizes the effect of very small dots on print quality, it also promotes the application of balanced oil and water distribution due to the consistency of its dot size . This result is better, the printer can be controlled more consistently, the amount of ink is reduced, and the printing process can be better controlled. This is critical for long-form web offset printing, which is why FM screening is used in 40% of magazine printing in the United States. As Polestar set out to use Creo's Stacatto FM screening in its web offset printing workshop, FM screening also 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 it has added color closed-loop adjustment capability, that is, adaptive capability. Once the color closed-loop control system from QTI and GMI is set, 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 human intervention is required on the press. In contrast, GMI is a big step ahead in using chroma values ​​to achieve quantitative printing.
[NextPage]
3 Spectrophotometer color control method
GMI (Graphics Microsystems Inc) has added a "Press Analysis System" to its ColorQuick color closed-loop tool. This system records a large amount of data obtained by the spectrophotometer and stores it in a standard SQL database. The analysis tool can track part of the printing process of the printing press during printing, and it can also track the printing press for 1 week, 1 month, or any specific time period or printing for any customer during this time period. Health status.
This system can generate dot expansion maps and densities in any period of time. Through calculation, the difference between the originally set target color and the color measured on the printing press can be monitored. When applied in reverse, the replica can predict the performance of the printing press, and these characteristics can be taken into account when making the plate. In other words, set a target color, the printer can read these colors and copy them out.
Printing only needs to monitor these data to ensure that the printing press is operating normally. It is also possible to provide publishers with these data references that indicate the operating status of the printing press. Because GMI uses a spectrophotometer, it can measure the value of color, and then know how to adjust it by evaluating the neutral gray produced by the three-color overprint. Since neutral gray is the most sensitive color, any deviation from the set standard is immediately discovered and can be corrected before affecting the overall balance of the printed product.
Even if the printer operates at the correct density, the color may be incorrect, which is why the density control is now shifted to the color control on the printing press. Only in this way can the printer achieve gray balance on the monitor when the correct density value is reached. American publishers stated that their goal is to omit all hard copy proofing, which is the last largest market for digital processes. They look forward to valuable suggestions regarding digital printing.
Due to the vast size, sending hard copy proofs by express is a time-consuming and costly affair. If a printer wants to match the chroma value that has been set, then visual evaluation of the proof is not necessary. The main purpose of this is to reduce waste. Generally, more than 1,000 sheets will be wasted when printing.
The attraction for printers is the introduction of a multi-point control press method, which is also the direction RR Donnelley is studying. All operations are performed under the same standard, and the same printed matter will be produced. Similarly, what attracts consumer companies is that they can be provided with a lot of information about the printing status of their products on the press.
However, it is very rare to install such a spectrophotometer on a sheet-fed printing press. Even such a control system is installed on the most advanced offset presses, and they are not widely used in markets such as labels and cardboard boxes where color fidelity requirements are critical. Printers have not believed that it is a reasonable cost to install each device, and there are few printer buyers who require such a crowding analysis system.
However, the problem of improving color consistency on the printing press still exists. The ICC file has its place, but it has proved difficult to apply. In order to create an ICC file, the printer needs to print a set of test samples of various color blocks, and then measure one by one. Through a series of long readings, a very accurate image average is obtained to form the ICC file for that day. In fact, for most printers, this work is too heavy to be practical.
4 Density value color control method
However, there are other ways. What attracted the attention of Polestar, QuebecorWorld, Scotland's George Stewart was Germinate's Press Signature product. Print according to the target density value, and then measure the tone on the signal bar. The Press Signature software will not only provide the dot increase, but also provide the reason why the incorrect dot expansion occurs on a specific printing job, and give Suggested correction method. Once the correction is appropriate, the dot expansion curve will be set in the ideal place, so that the printer can print the print with predictable quality under the correct density value. All measurement work only has the position of the signal bar. With the help of Harlequn RIP, all data transmission is automatic, which saves the cumbersome work of typing data.
The system provided to George Stewart by NCS can ensure that all printing machines (regardless of their age) can print the same quality products. In this way, in a short period of time, a job can be distributed to different printing machines without affecting the consistency of printing quality. Regardless of the type, location and culture of the printing press used for printing, the printing structure is exactly the same. Some manufacturers say that their purpose is to achieve real-time data replication, real-time inventory and usage control, so they chose to install Press Signature on their presses.
The attitude of some company's production department managers to this Press Signature system can be described by the word "ecstatic". Especially for those who need to print the same kind of products in different places around the world, in order to ensure the reputation of printing companies and customers, it is necessary to control the image quality. They believe that this system is a treasure trove of quality control.
Canon built this Press Signature into a website so that printers can view the data of their test press operation records recorded by Press Signature. The purpose of this is to generate a graph showing the expanded area of ​​the dots relative to the ideal target, and feedback the correction value to that printing machine through the software. Once the suggested correction value is entered or modified, the printing plate exposure system will compensate the printing press and the curve will return to the required state. Any changes made will be reflected in the graphic feedback to the website.
No matter where the printing machine is in Europe, if they all adopt this system, then they will get the same high-quality printing effect, which ensures that Canon's printing quality can be measured. Soon, printers who print products for Canon will have this software system.
And this software is an opportunity for small printers or remote printers. Once they adopt this system, they set a standard process for objectively evaluating the printing quality, so that they can print high quality. The product. And this objectivity is the key to printing process control. Any operator using visual evaluation to make adjustments to the printing press can only get subjective changes. This change may be visually better, but it actually contains many other problems.
For example, if the magenta dot expansion is too small, the printer will compensate for more magenta ink, but may actually get a "dirty" color. Conversely, printing based on quantified data eliminates subjective problems. With this system, you can avoid complicated density measurement calibration and subjective evaluation, and the colors appear to be "cleaner", and the printable color tone has also been expanded a lot.
[NextPage]
5 Rotary offset printing image color control
At present, the color quality control methods and effects of sheet-fed offset printing machines in China are being developed quite maturely, but relatively speaking, the printing quality control of rotary printing machines is relatively lagging behind. With the continuous improvement of the requirements for printing quality, the corresponding color quality control methods and means must also keep up with the requirements of the times. This has promoted the research, development and use of the color control system of the rotary printing press. The appearance of the closed loop color control system (Closed Loop Color Control System) has made a good solution to the printing quality of the rotary press. Since the debut of the first-generation closed-loop color control system at Drupa 95, it is estimated that more than 300 units have been sold so far. Approximately 62% of the original rotary machine was transformed, and the remaining 38% was installed on the new rotary machine. Of all these machines, publishing and printing accounted for 49%, followed by commercial printing, which accounted for 29% of the total. Through investigation and analysis, the closed-loop color control system can not only improve the quality of rotary printing, but also improve efficiency, save preparation time and waste of pre-printing, and use and distribute labor more reasonably, thereby maximizing the benefits of printing enterprises.
The closed-loop color control system is relative to the open-loop control system that was originally widely used. Open-loop control means that when the printing machine is running continuously without stopping, the operator takes out the printed matter from the printing machine regularly or irregularly and puts it on the console, and then uses the instrument to measure or visually judge the printing quality. Make various adjustments to the printing press for different situations. The closed-loop color control system continuously scans and calculates the color test strip on the moving substrate through the imaging device, and feeds the data back to the central console of the printing press ink to adjust or correct the ink layer thickness. At this time, the printing press still keeps running continuously, and all testing and adjustments are done automatically by the machine without subjective intervention of the operator. In addition to maintaining consistency in color quality, this system also provides effective methods for real-time data collection for statistical process control and customer quality confirmation.
6 Color control technology of adaptive neural network combined with color difference
Combining the principle of adaptive neural network with color difference detection to control the color of offset printing images is mainly based on two considerations: 1) Adaptive neural network has self-learning, forming a closed-loop control structure in the process of offset printing process, so that high speed The printing process can adjust and maintain the printing machine in the optimal state in the shortest time to avoid waste and affect the construction period; 2) Color difference is a color measurement indicator that is independent of the device, so it can make the established adaptive nerve The network color control model can adapt to various printing models and substrates, and improve the adaptability of the color control technology. This color control technology is currently being studied.
7 Conclusion
All in all, printing under the same standards can improve the quality and consistency of the printed matter. If implemented properly, less waste and reprinting can be achieved. Judging from the users who currently use this system, this is no longer doubtful.
BNF HARDWARE CO., LTD , https://www.bnfhardware.cn