Talk about color print quality inspection technology

In the printing process, because of the printing process and the mechanical precision, print defects often occur in one way or another, resulting in the appearance of defective printed products. How to detect these waste products is an important requirement in the printing process. At the same time, with the increasing degree of automation of printing, the speed is getting faster and faster, and the corresponding detection technology requirements are also getting higher and higher. The print inspection technology has undergone an initial subjective visual evaluation. To the objective evaluation based on a certain theory (including density detection method and colorimetric detection method), and then to the current development process of online detection based on image processing.

1 Traditional print inspection technology

Traditional print inspection techniques include subjective visual inspection, density inspection, and colorimetric detection.

Subjective visual inspection refers to the human comparison of prints and standard prints, either directly or with the aid of a magnifying glass, to subjectively assess the quality of prints. Although this method is simple and flexible, it is susceptible to the operator's experience and psychological, physiological and other factors, and the accuracy and stability are poor.

The density detection method and the colorimetric detection method use the instrument to measure the density and the color of the printed product, and use the relevant theory to analyze and process the data, and then evaluate the quality of the printed product. Both of these methods can provide a theoretical basis for objectively evaluating the quality of printed products, overcome the instabilities of subjective visual inspection methods, and have a wide range of applications in the printing field. However, the instruments used for density and colorimetric detection can only detect areas around 10mm2. Therefore, it is necessary to add measurement and control strips to obtain the information we require from the print screen completely and directly. Moreover, related research shows that there are certain inaccuracies and limitations in the theory based on density and colorimetry.

The traditional printing inspection technology also has a common defect: they are all after the completion of the printing of the sample inspection, once the print quality problems, it is often several hundred, may even reach thousands, can not be found in the printing process Problems, resulting in waste of printed materials and working hours, increase production costs.

Due to many deficiencies of the traditional detection technology, on-line printing inspection technology came into being.

2 Printing Online Inspection Technology

1) Advantages of Online Detection Technology for Printed Products

The on-line detection of printed matter is a kind of automatic detection method suitable for high-speed printing formed by combining the image processing technology with the printing principle. Its advantages are mainly manifested in the following two aspects:

First, on-line detection is more accurate and can detect very small defects. For example, if we concentrate on inspecting a certain printed product, only when the contrasting color of the printed product is relatively strong, the human eye can find defects larger than 0.3 mm, and the quality of the obtained test is difficult to maintain continuous stability. In the same color system, especially in light-colored printed matter, the quality of the printed matter is detected, and it is difficult for the human eye to find defects. On the other hand, on-line inspection can easily detect 0.1mm defects, and only one gray level difference is required.

Second, on-line inspection improves production efficiency. The quality assessment of the on-line inspection system is independent of the operator's subjective judgment. In the running process, regulation is implemented more quickly and easily, and adjustment is completed in one step. The detection speed is greatly improved. At the same time, in the event of a quality defect, the operator can promptly resolve problems that occur during the work according to the real-time reports on the site, reduce scrap rates, and managers can also track the production process based on analysis reports of test results. Management benefits.

At the same time, the application of on-line inspection systems can also reduce the labor intensity of workers, improve working conditions and so on.

2) principle of online detection technology of printed products

On-line detection system principle: The system uses CCD camera lens to collect non-defective prints as standard images, and then collects the image to be checked on the printing production line, and transmits each frame of the collected image to the scene for comparison and analysis with the standard image. Find out the image with quality as the subject, and find the quality problem of the print product corresponding to the frame image; finally adjust the corresponding printing part automatically to realize the online control of the quality of the printed matter.

In the specific implementation of on-line detection of printed products, it is generally divided into two steps: First, test preparation, that is, through the image acquisition of qualified products, to obtain standard print image, also called the template is followed by the actual detection, the image and standard of the printed matter to be examined. The template is compared to determine the presence or absence of a defect and the location of the defect based on the comparison result, and the defect information is recorded

Printing defects mainly include shape defects and color defects. Two kinds of shape defects mainly refer to foreign material stains, such as knife wire. It expresses the difference between the gray value of the defect image and the standard image. The value is compared with the standard image, and it is judged whether the difference has exceeded the preset threshold range, it can be judged whether the image has a shape defect. The color defect mainly means that there is a deviation in the color of the printed product from the standard print. The color space can be used. The conversion model obtains the colorimetric value CIE L*a*b* of the print image, and uses the color difference to evaluate the color cast of the printed product.

3 Research Status at Home and Abroad

At present, the research on the on-line detection system for printing products is very deep in foreign countries. A group of advanced industry manufacturers such as Heidelberg have developed their own on-line inspection systems. Among them are:

(1) The computer printing control system of Heidelberg, Germany, ie, the CPC system CPC system, is a scalable system applied to lithographic printing presses by Heidelberg for pre-adjusting the ink remote control to the ink remote control register and monitoring the printing quality. Including the ink volume and register control device CPCl, the print quality control device CPC2, the plate image reading device CPC3, the register control device CPC4 and the data management system CPC5. CPC23 is a newly launched online image control system of sheetfed offset press.

The PC23 system uses a specially designed high-resolution CCD detection scan head to scan and sample the entire print sheet, and the sampled data is sent to a dedicated computer for processing and comparison with a preset value. When errors are found, the printer will be immediately executed. prompt.

(2) The Print-Expert 4000 system developed by Vision-Experts of Germany. The system can use one or more color line scanning cameras to capture the surface image of the printed product and compare it with the standard image stored in the system. The defect detection speed of the system can be up to 600m/min, and the inspection width can reach 2000mm. The inspection items include printing defects such as color distortion of the offset printing plate, smearing of stains, and uneven coloration.

(3) The Print vision 9000 print quality inspection system developed by Israel's AVT company uses image processing technology to automatically detect and alarm various defects that occur in the printing process, such as miscoloring and overprinting.

In addition, companies such as FUTEC in Japan and TOKIMEC Co., Ltd. in Japan have EasyMax series products and Print-Pac systems.

Domestic on-line quality inspection technologies for printing products are still in the research and development stage. However, after continuous efforts, some achievements have also been made. Some formative inspection systems have been developed, mainly including:

(1) Daheng Image has developed a print quality inspection system with a detection speed of up to 6m/s. It can detect broken ink line, text missing, color depth, and color registration accuracy when detecting print defects. Can send out sound and light alarm signals, and display the error type and corresponding image information on the screen, and can also store and view.

(2) The print quality inspection system SW-YP0001 developed by Wuhan 3D Guangyang Electric Co., Ltd. is a full-automatic color line array scanning visual inspection system based on the machine vision platform. It can complete real-time detection, tracking, alarm, and information. Statistics and other functions, set high-speed automatic detection, measurement, resolution, positioning in one. It is mainly used to track and identify various printing quality defects such as ink knife filaments and foreign materials generated during the production process of high-speed printing presses. The system is currently a relatively advanced detection system in China.

In addition, there is Hawkeye "EE8000" detection system of Luoyang Shengrui Electromechanical Co., Ltd.; the printing automatic register control system of Gannan Kesai Electromechanical Equipment Co., Ltd. In general, the products of domestic enterprises in this area are not yet mature. Need to be further improved.