In the printing process, due to the printing process and mechanical accuracy, printed products often have one or more defects, resulting in the appearance of waste printing products. How to detect these waste products is an important requirement in the printing process. At the same time, as the degree of automation of printing becomes higher and higher, the speed is faster and faster, and the corresponding inspection technology requirements are also higher and higher. The printed product inspection technology has undergone subjective visual evaluation from the beginning. To the objective evaluation based on a certain theory (including density detection method and chromaticity detection method), and now the development process of the online detection method based on image processing.

1 Traditional printing inspection technology

The traditional printing inspection technology includes subjective visual inspection, density inspection and chroma inspection.

The subjective visual method is to compare the printed matter with the standard printed matter directly or with the help of a magnifying glass, subjectively evaluate the quality of the printed matter. Although this method is simple and flexible, it is easily affected by the operator's experience and psychological, physiological and other factors, and its accuracy and stability are poor.

The density detection method and the chromaticity detection method use instruments to measure the density and chromaticity of printed products, and analyze and process the data using relevant theories to evaluate the quality of the printed products. Both of these methods can provide a theoretical basis for objectively evaluating print quality, overcome the instability of subjective visual methods, and have a wide range of applications in the printing field. However, the instruments used for density and chromaticity detection can only detect the area around 10mm2, so the measurement control strip must be added, and the information we require cannot be obtained directly from the printed screen. Moreover, relevant research shows that the theory based on the density method and the chromaticity method has certain inaccuracies and limitations.

The traditional printed matter inspection technology also has a common defect: they are all sampled after the printing is completed. Once the printing quality problem occurs, it is often hundreds of sheets, and may even reach thousands of sheets. Problems, resulting in waste of printed materials and man-hours, and increase production costs.

Due to the many shortcomings of traditional inspection technology, online printing inspection technology came into being.

2 Printed product online detection technology

1) Advantages of online detection technology for printed products

The online detection of printed matter is a fully automatic detection method suitable for high-speed printing formed by combining image processing technology and printing principles. Its advantages are mainly manifested in the following two aspects:

First of all, online inspection is more accurate and can detect very small defects. For example, we focus on inspecting a printed product. Only when the contrast color of the printed product is relatively strong, the human eye can find defects larger than 0.3mm, and the obtained inspection quality is difficult to maintain continuous stability. It is difficult for human eyes to detect defects in the same color system, especially in light-colored prints. On-line inspection can easily find defects with a size of 0.1mm, and only one gray level difference is needed.

Secondly, online inspection improves production efficiency. The quality evaluation of the online detection system is independent of the operator's subjective judgment. The adjustment during operation is faster and easier. Even the adjustment is completed in one step, and the detection speed is greatly improved. At the same time, when quality defects occur, the operator can solve the problems in the work in time according to the real-time report on the spot to reduce the scrap rate, and the manager can also track the production process based on the analysis report of the test results to improve Management benefits.

At the same time, the application of online detection system can also reduce the labor intensity of workers and improve working conditions. [next]

2) The principle of online detection of printed products

The principle of the online inspection system: the system uses a CCD camera lens to collect non-defective prints as standard images, then collects the images to be inspected on the printing production line, and transmits each frame of the images to be inspected to the scene for comparison and analysis with the standard image. Identify the image with the title of quality, so as to find the quality problem of the printed matter corresponding to the frame of the image; Finally, automatically adjust the corresponding printing parts to achieve online control of the quality of the printing.

In the specific implementation of printed product online inspection, it is generally divided into two steps: the first is the preparation for inspection, that is, the image of the qualified product is acquired to obtain the standard print image, also known as the template. The second is the actual inspection, that is, the image of the printed product to be inspected Standard templates are compared to determine the existence and location of defects based on the comparison results, and record the defect information.

Print defects mainly include two kinds of shape defects and color defects. Shape defects mainly refer to defects, foreign objects, stains, knives, etc. It represents the difference between the gray value of the defect image and the standard image on the image. The gray value of the defective image is compared with the standard image to determine whether the difference has exceeded the preset threshold range, you can determine whether the image has shape defects. Color defects mainly refer to the color of the printed product to be inspected and the standard printed product. There is a deviation. The color space conversion model can be used to obtain the chromaticity value CIE L * a * b * of the printed image, and the color deviation of the printed product can be evaluated by the color difference.

3 Research status at home and abroad

At present, the research on the online detection system of printed products has been very deep abroad, and a group of advanced industry manufacturers such as Heidelberg have developed their own online detection systems. The famous ones are:

(1) The Heidelberg company's computer printing control system is the CPC system. The CPC system is Heidelberg applied to lithographic printing machines, which is used to pre-adjust the amount of ink, remotely control the ink, remotely register and monitor the printing quality. The system includes ink volume and registration control device CPC1, printing quality control device CPC2, printing plate image reading device CPC3, registration control device CPC4 and data management system CPC5. CPC23 is newly launched by Heidelberg, the world's first online image control system for sheet-fed offset printing presses.

The PC23 system uses a specially designed high-resolution CCD detection scanning head to scan and sample the entire printed sheet, and transfer the sampled data to a special computer for processing, and compare and analyze with the preset value. When an error is found, the printer will be immediately prompt.

(2) The Print-Expert 4000 system developed by Vision-Experts in Germany. The system can use one or more color line scan cameras to capture the surface image of the printed product, and then compare it with the standard image stored in the system. Print defects are detected. The maximum system detection speed can reach 600m / min, and the detection width can reach 2000mm. The detection items include missed printing, color distortion, paste version, streaks, stains, dirty stains, uneven color and other printing defects.

(3) The Print vision 9000 print quality inspection system developed by the Israeli AVT company uses image processing technology to automatically detect and alarm various problems that occur in the printing process, such as color deviation, pulling, and overprinting.

In addition, there are companies such as Japan's FUTEC and Japan's TOKIMEC, which have EasyMax series products and Print-Pac systems.

The domestic printing quality online inspection technology is still in the research and development stage, but after continuous efforts, some achievements have also been made, and some molded inspection systems have been developed, mainly including:

(1) Daheng Image has developed a printing quality detection system. Its detection speed can reach up to 6m / s, which can detect the color density of the ink, line breaks, text defects, color depth and color registration accuracy. When printing is detected When there is a defect, an audible and visual alarm signal can be issued, and the type of error and its corresponding image information can be displayed on the screen, which can also be stored and viewed.

(2) The printing quality detection system SW-YP0001 developed by Wuhan Sanwei Guangzhiyang Electric Co., Ltd. is a fully automatic color line scan visual inspection system based on a machine vision platform, which can complete real-time detection, tracking, alarm, and information Statistics and other functions integrate high-speed automatic detection, measurement, resolution and positioning. It is mainly used to track and identify various printing quality defects such as flying ink, knife wire and foreign matter generated in the production process of high-speed printing machines. The system is currently a relatively advanced detection system in China.

In addition, there is the Eagle Eye "EE8000" detection system of Luoyang Shengrui Electromechanical Co., Ltd .; the printing automatic registration control system of Weinan Kesai Electromechanical Equipment Co., Ltd. In general, domestic enterprises are not very mature in this area. , Need to be further improved.

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