Automatic print quality testing equipment and technology
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Today, in the printing and packaging industry, more and more manufacturers have the economic ability to purchase all kinds of advanced printing and finishing equipment, and with the improvement of printing equipment grades, all kinds of high-grade printed materials are also increasing. Looking at the domestic printing companies, some have advanced and similar equipment, and have the same production technology capabilities, but the result is the same product, the company lacks competitiveness.
The key to how a printer can maintain its leading position in the industry and obtain a reasonable profit is whether the company itself can provide non-homogeneous features. For example: unique new technology and good, stable print quality. However, unique new technologies and good and stable quality are difficult to measure with money. Moreover, in terms of quality, in order to make the company have a good and consistent overall quality for a long time, it must have a reasonable and complete quality management system.
Whether it is a manufacturer or an end user, everyone wants a good, stable quality. In theory, the quality of a good print should be an agreement between the manufacturer and the end user to negotiate and confirm a particular print, and must have a clear, quantifiable and reasonable description. Although there are certain standards in the definition of “quality” of brush products in China, the standards are different because of the different prints. It is more likely that the test results will be affected when the actual prints are detected because of the specific performers. Excessive external factors and human factors are difficult to achieve uniform and quantifiable stability standards. How can we achieve a unified, quantifiable and stable quality standard? The industry is eager to establish such a quality standard as soon as possible, and is actively looking for a reasonable and best detection method.
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 image processing systems, digitizing visual information, and achieving "seen" and "cognition" of print quality through computers, and high-speed, high-precision prints. 100% real-time detection, completely eliminate human error and establish a unified and quantifiable test standard. Currently available in automatic print quality inspection equipment are BOBST in Switzerland, PROIMAGE in the US, DAC in Japan and TOKIMEC in Japan.
Automatic print quality inspection equipment
1. Working principle
The detection system used in the automatic print quality inspection equipment is to first use the high-definition, high-speed camera lens to capture the standard image, and set a certain standard on this basis; then take the detected image and compare the two. 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. The various errors that occur during the printing process are only the difference between the standard image and the detected image for the computer, such as smudges, ink dot chromatic aberrations and other defects are included.
2. Detection accuracy
The earliest used for print quality inspection is the technique of comparing the standard image with the detected image. Now the more advanced technology 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 focus 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; 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 0.10mm 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 comparison between the standard image and the image of the printed product is the key problem of the detection device. 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-width 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 is greatly improved, and the detection result is more stable because the ghost of the edge portion is avoided.
3. Detection parameters
After the quality detection parameters are set, will the high-speed, automated quality inspection equipment lead to excessive scrap rate? 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, an appropriately 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.
First, we partition the printed matter to be detected. In the figure, A is the most strict detection area - the most important part of the printed matter (trademark, product); B is the strict detection area - important part (product); C is the ordinary detection area - background; D is the neglected area - concealed Part (upper wing, mouth part)
Then, each detection level is set. This is very critical and will be directly related to the ability to detect defects, and directly related to the scrap rate, so the customer will necessarily require an acceptable and reasonable tolerance range, including setting acceptable, defect size. Range tolerance, acceptable, defect grayscale tolerance.