Today, in the printing and packaging industry, more and more manufacturers have the economic ability to purchase all kinds of advanced printing and post-press processing equipment. With the upgrading of printing equipment, all kinds of high-end printed products are also increasing. Looking at domestic printing companies, some of them already have advanced and similar equipment and have the same production technology capabilities, but the result is similar products and companies lacking competitiveness.
How the printing house can maintain a leading position in the same industry and then obtain reasonable profits depends on whether the enterprise itself can provide non-homogeneous features. For example: unique new technology and good and stable printing quality. However, unique new technologies and good and stable quality are difficult to measure with money. Moreover, in terms of quality, for a company to have a good and sustainable overall quality for a long time, it must have a reasonable and perfect quality management system.
Whether it is a manufacturer or an end user, everyone wants to get good and stable quality. In theory, the quality of a good printed product should be an agreement that the manufacturer and the end user have negotiated and confirmed on a particular printed product. There must be a clear, quantifiable and reasonable description. Although there are certain standards for the definition of “quality†of brush products in China, the standards are different because of the different prints, and it is more likely that the test results will be subject to the actual test of the printed products because of the specific executors. Too many external factors and human factors make it difficult to achieve a unified and quantifiable stable standard. How can we achieve a uniform, quantifiable and stable quality standard? People in the industry are eager to establish such a quality standard as soon as possible, and are 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 actively working on the use of image processing systems to digitize visual information, to realize "seeing" and "cognition" of print quality through computers, and to perform high-speed, high-precision, and 100% real-time detection completely eliminates human errors and establishes a unified and quantifiable inspection standard. At present, BOBST in Switzerland, PROIMAGE in the United States, DAC and TOKIMEC in Switzerland can provide fully automatic printing quality inspection equipment.
Automatic printing quality inspection equipment
1. Working principle
Most of the inspection systems used in the automatic printing quality inspection equipment use high-definition, high-speed camera lenses to take standard images first, and then set certain standards on this basis; then take the inspected images, and then compare the two. The CCD linear sensor converts the change in light quantity of each pixel into an electronic signal. After comparison, as long as the detected image is different from the standard image, the system considers the detected image to be a non-conforming product. The various errors generated during the printing process are only the difference between the standard image and the detected image for the computer. Defects such as smudges and chromatic aberrations are included.
2. Detection accuracy
The earliest used for the detection of printing quality is the technology of comparing the gray level of the standard image with the detected image. Now the more advanced technology is based on the RGB three primary colors for comparison. What is the difference between automatic machine inspection and human eye inspection? Taking the human eye as an example, when we concentrate on a printed product, if the contrast of the printed product is strong, the smallest defect that can be found by the human eye is the obvious contrast, not less than 0.3mm; but it depends on the human Ability is difficult to maintain sustained and stable visual effects. But to put it another way, if you are looking for defects in prints of the same color system, especially if you are looking for quality defects in a light color system, the defects that the human eye can detect need at least 20 gray levels. The automated machine can easily find defects with a size of 0.10mm, even if there is only one gray level difference between the defect and the standard image.
But in terms of actual use, even the same full-color contrast system has different ability to distinguish color difference. Some systems can detect defects with large variations in contours and chromatic aberrations, while others can identify very small defects. For white cardboard and some simple-style printed materials, such as the Japanese KENT cigarette label and the US Marlboro cigarette label, simple detection may be sufficient. Most domestic printed products, especially various labels, have many characteristics. There are too many flash elements, such as gold and silver cardboard, hot stamping, embossing or glazing prints, which requires that the quality inspection equipment must have sufficient ability to find very small grayscale differences, perhaps 5 grayscales The level difference may be a stricter gray level difference. This is crucial to the domestic label market.
Accurate comparison between the standard image and the image of the printed matter to be inspected is a key issue of the inspection equipment. Normally, the inspection equipment collects the image through the lens. In the middle part of the lens range, the image is very clear, but the image at the edge may produce false images. The detection result of the ghost part will directly affect the accuracy of the entire detection. From this point of view, if only the contrast of the full-frame area is not suitable for some fine prints. If the resulting image can be subdivided again, for example, the image is divided into 1024dpi × 4096dpi or 2048dpi × 4096dpi, the detection accuracy will be greatly improved, and at the same time, because the edge of the ghost image is avoided, the detection result is more stable.
3. Detection parameters
After the quality inspection parameters are set, will the use of high-speed, automated quality inspection equipment lead to an over-standard scrap rate? Increasing the scrap rate will undoubtedly lead to lower profits. But in fact all quality standards are relative, such as generally unacceptable minor defects, if it is in some specific areas (glue, paste area) is acceptable. In the case of manual testing, an appropriately relaxed and flexible standard can be automatically formed. How can the rational and data-based testing be completed? A more reasonable approach is for the printer and the end user to reach an agreement to adopt regional and graded quality inspection standards for printed products. This is only a hard and fast rule, but it must be operable, acceptable and quantifiable. And these standards must be all kinds of data set by the user, such as the area to be detected, the detection level of each area, and the detection standard executed by each detection level.
First of all, we partition the printed matter being tested, as shown in Figure 2. In the picture, A is the most strict detection area-the most important part of the printed matter (trademark, product); B is the strict detection area-the important part (product); C is the ordinary detection area-background; D is the ignore area-concealed Parts (gluing wings, paste parts)
Then, set each detection level. This is very critical. It will directly affect the ability to detect defects and the scrap rate. Therefore, customers will inevitably request an acceptable and reasonable tolerance range, including setting acceptable and defect size. Range tolerance, acceptable, grayscale tolerance of defects.
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