Automatic Printing Fault Detection System and Its Technology Research
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In the past few years, the detection system has developed rapidly in the processing industry. Today, 100% automatic fault detection technology has been realized. Today, you can hear the word "fault detection" in almost all industries. Used in the printing and packaging industry, it helps us identify all flaws in printed products and can be applied to different printing methods.
In terms of printing quality inspection, countries such as Europe, America and Japan have made great progress. As early as the 1970s, there were already manufacturers who were actively working on the use of image processing systems to digitize visual information, and to achieve "seen" and "cognition" of print quality through computers, and to produce high-speed, high-precision prints. 100% real-time detection, completely eliminate human error and establish a unified and quantifiable test standard. At present, we can provide automatic print quality inspection equipment such as BOBST in Switzerland, PROIMAGE in the US, DAC in Japan and TOKIMEC in Japan.
1. Type
Currently, there are two basic types of detection systems for fully automatic printing fault detection technology, namely 100% sampling type and 100% comprehensive type.
The 100% sampling system refers to repeatedly selecting a part of the printing in one time, and the part is monitored 100%, and the repetition time can be from several seconds to several minutes. The 100% versatile system monitors each part of the product processed on the press 100%. Both systems use two camera technologies that are diametrically opposed: progressive (using linear CCD cameras) and block-by-block scanning (using a typical CCD camera). The 100% comprehensive inspection system based on block-by-block scanning is relatively technically updated, and is better on some packages than progressive scan technology.
The 100% sampling system only detects faults seen by the camera at a particular moment in a particular print location. Specifically, the 100% sampling type inspection system does not have the ability to find faults on the entire print. They have the advantage of being able to accurately monitor certain specific properties of the processing quality, such as color, overprint, streaking and repetitive faults. And 100% comprehensive detection technology can help you find the lack of the entire print, but the investment of this system is more than three times the usual type, the customer must prove that there is a real need, the investment can have a tangible return. Before the actual purchase of the inspection system, the printing manufacturer must do some investigation work. It is necessary to investigate the types and causes of the printing waste currently being faced, and select the appropriate detection system according to different reasons, so that the right medicine can cure the chronic disease.
2. Working principle
The fully automatic printing fault detection system is a new type of auxiliary equipment designed to proactively search for possible problems on printed pages during the printing process. The fully automatic printing fault detection system can help the operator to monitor the quality of the product in the printing process, ensuring that every product printed is a good product. The detection system used by it mainly uses a high-definition, high-speed camera lens to capture a standard image, and then sets a certain standard; then, the detected image is taken, and then the two are compared. The CCD linear sensor converts the change in the amount of light of each pixel into an electronic signal. If the detected image is different from the standard image after comparison, the system considers the detected image to be a non-conforming product, as shown in Figure 1 below. Various errors in the printing process are only different for the computer after the standard image is compared with the detected image, such as smudges, ink dot chromatic aberrations and the like.
Among them, the 100% sampling system uses a similar technique to the roller observation system. In fact, this type of inspection system only adds additional software to the drum viewing system. A video camera is attached to an engine-mounted beam and equipped with a flash device. In a series of printing processes, the system can be scanned continuously, moving from one marked position to another every time the flash is taken. During this time, the markers for each location are scanned, but only a small portion of the markers are captured by the camera after flashing. The printed image of each mark is compared to the main image in memory. Any difference can be displayed on the screen for reporting to the operator and a warning is issued. The system is very good for the observation of the roller image, because after all, the operator can see the image in a small area every time. However, for automatic quality inspection systems, it seems that sometimes it is possible to pick out the problems in printing, sometimes not. In other words, this system works intermittently.
The 100% comprehensive system uses a completely different approach. It still requires a beam with an engine and several cameras are permanently mounted across the length of the drum. These cameras are usually of the line scan type. This means that instead of taking a rectangular snapshot of the print with a flash, the camera can sample a sample of the length of the roll at a time through a continuous illumination system. As the roll rolls and the printing is completed, the system samples the next line of samples. The retrieved sample information is continuously compared to the main image stored in memory. The main image is composed of printed information having the same length as the print and a width of the same length of the drum. Once the detection system detects a certain fault, they are not only matched in the length direction of the drum, but also in synchronism with the running speed of the printing press, it can always detect the quality of the pixels at the corresponding position.
In addition, the fault information detected by the fully automatic printing fault detection system can be used as a warning and alarm, as well as for fault reporting and data collection. Information is typically collected into a database and accessed through an interface into an existing PC network to analyze the data from the printer. The screen shows the speed and the location of the fault in the lateral direction of the drum, as well as the type of fault that is captured. Scroll bars help operators view early data and previous data. At the end of the print, the print report is automatically created.
3. Detection accuracy
The earliest technique used for quality inspection of printed matter is to compare the standard image with the detected image in grayscale. Now the more advanced one is based on the RGB three primary colors. What is the difference between fully automatic machine testing and human eye detection? Taking human eyes as an example, when we concentrate on a certain print, if the contrast color of the printed matter is strong, the smallest flaw that can be found by the human eye is a defect with a contrasting color of not less than 0.3 mm, but relying on human It is difficult to maintain a continuous, stable visual effect. In other cases, if a defect is found in a print of the same color system, especially in a light color system, the defect that the human eye can find requires at least 20 gray level differences. Automated machines can easily find defects of 0.1mm size, even if the defect is only one grayscale difference from the standard image.
However, in terms of practical use, even the same full-color contrast system has different ability to distinguish chromatic aberrations. Some systems are able to detect defects with large variations in contours and chromatic aberrations, while others can identify very small defects. For white cardboard and some simple-style prints, such as the Japanese KENT cigarette label, the US Marlboro cigarette label, the simple test may be enough, and most of the domestic prints, especially the various labels, have many characteristics. With too many flash elements, such as gold, silver cardboard, hot stamping, embossing or glazing, this requires the quality inspection equipment to have enough ability to find very small gray level differences, perhaps 5 gray levels The level difference, perhaps a more stringent one gray level difference. This is crucial for the domestic label market.
The accurate comparison between the standard image and the image of the printed product is a key issue in the inspection equipment. Usually, the detection device collects the image through the lens. In the middle part of the lens range, the image is very clear, but the image of the edge part may be virtual. Shadow, and the detection results of the phantom part directly affect the accuracy of the entire test. From this point of view, if only the contrast of the full area is not suitable for some fine prints. If the obtained image can be subdivided again, for example, the image is divided into 1024 dpi × 4096 dpi or 2048 dpi × 4096 dpi, the detection accuracy will be greatly improved, and the result of the detection is more stable because the ghost of the edge portion is avoided.
4. Detection parameters
After the quality inspection parameters are set, will the high-speed, automated quality inspection equipment lead to excessive scrap rates? The increase in the scrap rate will undoubtedly lead to a decrease in profits. But in fact all the quality standards are relative, such as the generally unacceptable minor defects, if it is acceptable in certain areas (sizing, eating parts). In the case of manual detection, a properly relaxed and flexible standard can be automatically formed. How does the rational and data-based detection complete the detection? It is more reasonable for the printer to reach an agreement with the end user to adopt a sub-regional and hierarchical quality inspection standard setting for the printed matter. This is only a rigid rule and must be a measurable standard that is operational and acceptable to all parties. Moreover, these standards must be various types of data set by the user, such as the area to be detected, the detection level of each area, and the detection standard performed by each detection level. For example, first we will divide the printed matter to be tested, such as dividing the printed matter into the most stringent inspection area – the most important part of the printed matter (trademark, product), the strict detection area – the important part (product), the common inspection area – the background, the neglected area. – concealed part (top rubber wing, paste part). Then, each detection level is set. This is very critical and will directly affect the ability to detect faults. It is also directly related to the scrap rate. Therefore, customers will inevitably require an acceptable and reasonable tolerance range, including setting acceptable and fault sizes. Range tolerance, acceptable, fault grayscale tolerance.
5. Advantages
5.1 continuous, proactive observation – 100% roller, 100% time.
5.2 Ensure that consistent quality is observed, regardless of operator experience or shift changes.
5.3 Improve productivity by optimizing the speed and quality level of the press.
5.4 Provide early warnings for upcoming faults.
5.5 The color is displayed on the screen to indicate the fault, and the nature and source of the fault can be quickly identified.
5.6 When printing expensive printed materials, maximize the amount of printing and reduce the amount of overprint.
5.7 reduce the waste of expensive printing materials, minimize its use, reduce costs, and improve customer turn-around rate.
5.8 Improve the company's image and make it competitive.
5.9 can be used in accordance with the requirements of the ISO9000 standard.
5.10 It is easy to produce short-term products with higher efficiency.
Helps make the most of existing equipment.
Today, fully automatic printing fault detection is a relatively mature technology, but with the further development of cameras and computer systems and the networking of workflows and the integration of data management functions, this technology is bound to New development, printing quality and efficiency will be greatly improved.