Online inspection technology is improving newspaper printing

The most important thing in newspaper printing is the printing quality of newspapers. This is not only the basis for customer satisfaction, but also an important technical means for newspapers to succeed in commercial trade. In recent years, with the popularity of color newspaper printing, advertising editions have become an indispensable part of newspaper layout. The continuous expansion of the advertisement layout has brought about good economic benefits for the newspapers, but also put forward higher requirements for the printing quality of the entire newspaper. As a result, newspaper printers inevitably face increasing pressure and challenges, and they have to face up to the ever-increasing demands and demands for improving print quality, especially the print quality of advertisements. Therefore, in order to cope with this change, many newspaper printing centers have begun to turn their attention to the advanced printing equipment with the functions of automatic detection and control adjustment commonly used in magazine printing.

Based on the growing pressure from newspapers to improve the printing quality of newspapers, when newspapers invest in printing new equipment and new technologies in newspapers, they first think of using closed-loop printing quality control systems. In the existing printing equipment using a print quality closed-loop control system, not only the color control and adjustment in the printing process but also the quality control of the printing plate production process can be completed. However, for newspaper printing, the closed-loop control of print quality in newspapers is difficult to achieve due to the variety of paper paths and paper paths in newspaper printing.

In the past several years, on-machine detection systems like paper-path-side control and paper tension detection have been gradually implemented. The latest development in newspaper printing is the deployment of automatic detection and control systems on the printing production line to achieve detection and adjustment of printing colors. . The use of on-machine closed-loop control systems in newspaper printing is an important measure to improve the printing quality of newspapers. It can also bring predictable advantages in terms of reducing losses and reducing customer disputes. In this way, the printing company really has a quality assurance system, which can not only print high-quality prints that are consistent with the accuracy of the sample, but also greatly reduce the consumption of printing ink.

Image quality control through gray balance detection

In the short time, two kinds of image quality detection methods have been proved to be very effective and practical. One is to use the gray balance measurement and control strip for printing quality detection and control, and the other is to use a spectrophotometer or a CCD camera. Print images for direct spectral detection. The detection principle of these two methods is obviously different, in which the camera detects the distribution of printed colors in a specific area, rather than the density of the conventional detection, because the density cannot be directly measured on the printed image. When it comes to printing color deviations, spectral detection methods can provide the best detection results. However, CCD cameras can only provide RGB images, which must then be converted back to density values. The detection speed and results are somewhat different from the spectral detection methods.

Finnish manufacturers leading in the field of print on-line inspection technology have provided printing companies with the best printing and gray balance detection and control strip detection system. The system can print paper at speeds of up to 17 meters per second. Within a distance of 5 mm from the paper surface, image detection was carried out for a gray balance bar measuring approximately 1.7 mm wide on the cross section of the paper strip arranged in the newspaper layout. The method is to configure a series of diode detection heads for detecting RGB on the test cross section. The detection head device has no moving parts and is specially designed to detect newspaper printing under poor production conditions. Careful consideration must also be given when selecting LEDs. Detection LEDs need to perform better than conventional LEDs.

When designing a newspaper layout, the print quality control strips can be set horizontally or vertically. In addition to the yellow, cyan, and cyan test strips, the black strips must also be printed in a mesh to enable reliable color detection. However, even if such a detection value is not quite accurate, at least two measurement bars need to be arranged in this system. The test result is not immediately reflected on the printing equipment and printing paper tape, but is implemented on a printing console other than the printing equipment. Practice has confirmed that narrow-width control strips can achieve excellent cooperation with newspaper layouts.

Although newspaper printing companies initially doubted online print quality testing, they are now eager to adopt this new system, and newspapers and customers have gradually accepted the fine gray control strips that appear in newspaper layouts. The equipment cost for configuring a newspaper printing color detection system will increase by 1.7%. However, due to closed-loop control of gray balance control, the printing cost will also drop by 0.8% to 1.6%. It will also reduce printing losses and reduce disputes with customers.

Printing closed-loop quality control is actually sending a message to customers. They are also involved in the industrial production of newspaper printing. Their print production is based on a newspaper printing orientation system, while in another company it may be based on rotation. The production of gravure printing guidance system, and the quality of the products produced by different printing guidance systems are of course inconceivable. Experts believe that closed-loop control of printing quality is part of the customer-oriented concept. Since newspaper printing serves customers, of course, it also serves readers.

Spectral detection of printed images

Printing equipment R&D and manufacturing companies have been working hard on on-line inspection and control of printing presses, hoping to achieve direct inspection and control of image printing quality on high-speed presses. They see the image as being composed of symbols, just as in the normal image color detection, the printed image on the tape is considered to be composed of one dot and one dot. In the detection, the detected image CMYK data and printing paper type data are input into the spectrum, and the full-tone density of each color is defined as the control value of the image quality.

In order to ensure the calibration of the testing equipment before testing, it is very necessary to measure the full-tone area of ​​the first color printing and the second color printing at the beginning of printing, and the whiteness of the paper is calibrated and calculated as a white dot. The corresponding actual detection value is spectrophotometric. Instrument provided. The principle of spectral detection is that the bright color in the visible spectral region is absorbed, while the pure black in the infrared region is also absorbed. The result is identified by comparing the output of the gray level produced by bright color or black color.

This detection method can also be applied to print spot color measurements. Measured values ​​obtained from the printed image part of the screen are printed and the on-line color control system can be used to calculate the corresponding value of the full tone and compare it with the standard full-tone reference value. However, an important prerequisite for such measurements is that the color registration of printed images must be 100% accurate.

The idea of ​​image spectrum detection system originated from two years ago, but only one year later, the detection system has basically taken shape. This year, it has realized the installation of printing companies. This detection system can also be used to measure the density of printed images on the machine. Match. It is said that next year will design a closed-loop control system for a printing paper road, and will begin to provide customers with a complete detection system.

Closed-loop quality control in plate making

A print quality assurance system must include all areas involved in the printing process, such as the computer-to-print platemaking process. The problem with the current CTP link is that the plate speed has hindered the effective quality control of the printing plate because the printing speed required by newspaper printing is too fast.

Today, the CTP plate making speed can reach 74 sheets/hour. With the current rapid CTP system, manual inspection of each printing plate is almost impossible. With manual inspection methods, problems in production cannot be identified quickly enough. For example, some platemaking failures cannot be seen on the surface of the printing plate, but they will actually affect the service life of the printing plate. Therefore, the manual detection method does not match the closed-loop control of the printing process, and it also does not match the industrial production of newspaper printing.

The platemaking machine should be well integrated with the printing production plan, and the plate should be exposed before the printing is about to start, instead of being stored in advance after a large number of plate making. Some CTP companies have already begun to cooperate with Agfa, equipped with Agfa inspection systems for plate quality control, to ensure continuous monitoring of all process parameters during the platemaking process, and to monitor all process links through closed-loop control systems, such as platemaking machines. , plate washers, punchers and cutting machines, closed-loop control system is by no means an isolated or partial solution.

When the plate is punched, a camera is used to take a picture, and the taken image is re-evaluated during plate making. According to the evaluation result, the CTP system exposes a digital block and an analog block in the area of ​​6 mm×4 mm on the plate, and uses the digital block. A standard value of plate quality, gradation, and problematic image values ​​is generated, and the simulation block is positioned on the optical path of the laser beam to provide pre-processing for evaluating the characteristics of the plate making laser so that the plate can be completely hardened.

Future Prospects: Comprehensive Detection of Printed Images

Experts are researching and exploring and preparing to use fully automated closed-loop detection and control systems in future newspaper printing. At present, pilot printing companies have installed a CCD camera system for video density measurement. They believe that if the detection points are added to the printed image, and the corresponding wetting airflow is controlled, the printing color and the multi-paper path overprinting can be grasped, the problem of virtual image in video image detection can be avoided, and the printed image detection technology can be implemented in a closed-loop control system. Play a bigger role.

The closed-loop control system marks the real-time control and adjustment of different parameters of the printing plate making process and the printing process, which is of great benefit to the future improvement of the quality of printed matter and will be applied more widely. Printing closed-loop control is only part of the overall solution for newspaper printing, but it marks that newspaper printing is moving toward new types of industrial production.

Zhao Zhiqiang et al

Reprinted from: Digital Printing