With the development of society, people have higher and higher requirements on the quality of printed products. How to control the complicated image copying process and make the printing quality control to be digitalized and standardized is a general concern of producers, operators and consumers. From an international perspective, this is also an inevitable trend in the development of printing. Looking at the current status of China's printing industry, the data quality and standardization of printing quality control are still far away from the international developed countries. Therefore, how to improve and strengthen the digitalization and standardization of printing quality control technically and management is the inescapable responsibility of the majority of printing workers; it is also the key to whether the enterprise can survive in the fierce competition in the future.

The so-called digitization of printing quality control means that the necessary control factors for printing quality are measured and recorded in the form of data through the necessary test methods as the basis for quality control. The so-called standardization is to synthesize a series of laws that can guide production through a large amount of data, such as formulas, curves, charts, and data. At the same time, there are three standards for measuring the quality and standardization of printing quality control: first, good stability; second, good repeatability; third, high efficiency.

The purpose of printing is to reproduce the original as faithfully as possible. In order to achieve and maintain the high quality of multi-color printing, it is important to measure and control the amount of ink transferred from the yellow, magenta, cyan, and black ink rollers to the paper. The appearance of a halftone dot print depends on two factors: dot size and ink density inside the dot. The traditional detection method based on the density meter or colorimeter can only detect the area of ​​about 10 mm2, and cannot completely obtain the information we require directly from the printed screen. Therefore, a special color block (signal bar) must be printed on the printed sheet, so that the printing worker can control the printing quality. This method cannot know the ink supply volume and dot size change of each monochrome dot. In the use of test strips and signal strips, human eyes or simple test methods are used to obtain judgment results and serve as the basis for quality control in production. Because the quality of offset products is closely related to complex factors such as equipment, paper, ink, and environment, the use of traditional control methods is bound to be affected by many people's subjective factors. Since human subjective observations are not measurable, they cannot be made with good stability, good repeatability and high efficiency. At the same time, the printing speed of modern printing is nearly 10,000 sheets per hour. When the printing workers find problems and make adjustments based on the signal bar, bad prints have been formed in large numbers. Therefore, it is very important and far-reaching to find a correct, reliable and fast quality control method and develop an immediate automatic control system to ensure stable quality in the offset printing process, reduce printing costs, and improve printing quality and production efficiency.

Printing quality control is a very precise task. The current test strip and signal strip test methods have proved to be an effective quality control method, but because this quality control method mainly relies on the operator's long-term experience accumulation and subjective judgment, the repeatability and stability of the test results , Efficiency and reliability can not be guaranteed, let alone the data control and standardization of quality control. In recent years, microscopic image technology has achieved rapid development in the field of control. The microscopic image technology is used to collect, quantify and process the test strip and signal strip information on the sample and the product to obtain accurate detection data, thus laying the foundation for the automatic control of the test strip and signal strip in the offset printing production process.

In response to this problem, this paper proposes a digital technology advantage that uses the computer's speed, high repeatability, good stability, and is not affected by anyone's subjective factors to measure and process the signal bars of offset printing products. Using microscopic image processing methods to obtain a series of indicators consistent with the content of the signal bar control, so as to achieve real-time digital and standardized control of the quality of offset products, thereby improving production efficiency and reducing waste. At present, the application of microscopic image processing technology in the field of printing is becoming increasingly widespread. For example, the practical PrintVision 9000 and PrintVision 9000NT Automatic Defect Detection System developed by AVT in the United States adopt microscopic vision technology to solve problems that occur during the printing process, including color changes, overprinting, and brushing. , Stains and other defects are automatically detected and alarmed. American 3M company RSFISCH and others used the fast Fourier transform method to identify and classify the dot image on the screen generated by electronic screening, studied the density distribution inside the dot, and developed the dot based on the microdensity meter. Expansion and dot shape testing system; T.YUASA and H.MISHINA used two-dimensional Fourier transform to identify several patterns generated by moire in color printing, and found a method to control moire generation in order to better Control the quality of color reproduction; Zhao Wei used the edge recognition method to calculate the area of ​​the dots, and used this as a basis to modify the Newcastle equation. Other studies have used digital image processing techniques to measure ink coverage, average grayscale, dot area, form factor, and overprint accuracy. CHRISTINE BARRATTE of SWIDISH NEWSPRINT RESEARCH CENTER and others have developed a plate-reading system that automatically detects parameters such as dot size and density distribution in monochrome printed sheets of newspapers, and it has been put into use. With the entry of new technologies, the methods and techniques of printing quality measurement and control are constantly updated. The following discusses this technology in detail.

1. Basic principles and methods of microscopic image processing

The so-called microscopic image processing is to use the microscopic equipment to enlarge the image details that are difficult for human eyes to distinguish clearly during image data acquisition, so as to obtain an enlarged image. Then use a series of image processing methods and means to process the microscopic image to obtain the expected index value, and compare with the traditional test results to find the correlation between the two, so as to replace the traditional artificial with digital technology Control methods that are difficult to predict and control.

1. The basic process of microscopic image processing

2. The basic principles and basic algorithms of image processing

General images cannot be processed directly by computer, so first of all, all kinds of image signals (such as photos, graphics, X-ray photos, etc.) should be converted into digital images, that is, the digitization of images.

A monochrome image can be regarded as a two-dimensional continuous function, and its brightness is expressed as a continuous function of position. The digital image in the computer is represented by a matrix. Different processing is to use different algorithms to operate on this image matrix. From the two-dimensional continuous function to the matrix involves taking function values ​​(brightness) at different spatial positions as samples, and using a set of integer values ​​to represent the quantization process of these samples. This is the image sampling and quantization process, that is, the digitization process. The digitized image is divided into many small areas, which are called pixels. The value of each pixel is expressed by a luminance value or a gray value. For black and white images, there is only binary information, namely "black" and "white". In a computer, "1" indicates image information, and "0" indicates non-image information.

The original image is input into the computer through the image input device and becomes a discrete digital image, which can be expressed as a matrix as follows:

For a halftone micrograph, it can be represented.

The "black" area represents the dot image, and the "white" area represents the non-graphical portion. It can be seen from the figure that the percentage of this dot can be regarded as 13/25 = 50%, and the ratio of length to width is 1: 1.

In the process of image processing, such pixel matrix is ​​calculated and processed. Common image processing methods include image smoothing, sharpening, image segmentation, binary processing, edge tracking, and so on.

2. Offset printing quality control system

In the offset printing quality control system, the microscopic image is the measurement and control strip on the printing sheet, the proofing sample sheet or the printing plate. Use image magnification equipment, such as micro camera or micro camera, to magnify the measurement and control strip by 60 or 100 times, and then use digital equipment to collect the binary information of the image into the computer at a certain frequency; use image processing software to image The information is processed to obtain a predetermined series of index values. Through calculation and comparison, the control information of the printing quality is obtained, and then the information is fed back to the control devices such as the water supply device, the ink supply device, the pressure adjustment device, the sleeve rotation adjustment device of the printing machine through the computer, so as to adjust the printing conditions in time And change.

The development of hardware has provided us with great convenience and made it practical. At present, the image magnification and digitization can be completed with one device, and the computer has become the most common device in printing plants. For the lithographic offset printing quality control system, the image processing method involved is very simple and common, so the implementation of image processing software is supported by a high-speed computer, and real-time calculation and control are not difficult. A practical offset printing quality control system.

3. Use microscopic images to control the printing quality of offset printing

1. Ink and water balance control of printed page

The GATF quality control strip is usually placed at the bite of the printed product. You can clearly see the proportion of ink on the printed surface, which is convenient for controlling the moisture on the printed surface. This quality control bar is composed of two parts, namely the slash and the solid bar. If the fountain solution is excessive, the oblique line of the GATF control bar will produce "water accumulation"; on the contrary, if the amount of water on the plate is insufficient, the upper part of the ink is better, and the lower part is easy to get dirty. In the traditional printing quality inspection process, these changes are also difficult for the naked eye to directly observe, and a magnifying glass is needed to see accurately. In microscopic image processing, the concept of soiling can be described by the increase of the local "black" in the binary image. When specifically detecting this index, the information on the printed sheet can be compared with the information of the measurement and control strip. If water accumulates on the printed product, the value of the "black" color on the microscopic image is greater than the black value of the corresponding part on the proof sheet or on the printing plate; otherwise, if the amount of printed product is insufficient, the "black" "The value of the color is less than the black value of the corresponding part on the proof sheet or on the printing plate. In actual production, the calculation results of the microscopic image and the amount of water on the printing plate can be strictly quantified, so that the GATF quality control strip can be used to accurately check whether the water-ink ratio of the flat printing product is appropriate.

2. Ink volume control of printed products

Using the GATF star to control the amount of ink on the layout can ensure that the ink color of the printed product meets the print quality requirements. When the dots are not deformed, ghosted, or the amount of ink supplied by the layout is moderate, the center of the star is white. In the binary image, the total amount of "0" in this part is more than the total amount of "1". If there is too much ink on the layout during printing, a large black circle appears in the center of the star on the printed sheet, and the larger the black circle, the more the amount of ink. Conversely, if the amount of ink on the layout is insufficient, the blank circle at the center of the star expands and the dots shrink. Therefore, in actual production, by establishing the ratio between the total amount of "0" at the star mark and the total amount of "1", the supply of ink volume can be judged very accurately.

If the dots are deformed laterally, the black circle in the center expands vertically into a duck egg shape. If the dots are deformed longitudinally, the black circle in the center of the star expands laterally into a duck egg shape. Create two indicators at the center of the star, one represents the amount of horizontal ink, that is, how many "black" pixels are in the horizontal direction on the center coordinate, and the other is the number of vertical "black" pixels. The ratio of the two indicators can reflect the deformation of the dot, and the reflected result is extremely accurate. In the same way, if there is a ghost image in the dot, the central part of the star disappears, and the incomplete outline is like an "8" shape. The "8" shape expands horizontally, and ghosting occurs vertically; if the "8" shape expands vertically, the ghosting appears horizontally. If there is a black dot in the center of the interstellar, it means that the amount of ink on the layout is too large.

3. Detection of increase in outlet

In the lithography process, the amount of ink has a great influence on the dot increase. The dot size and ink layer thickness both affect the mesh tone value. The dot increase test strip provides a field and mesh level test section that can be used for testing. The increase of the dots indicates that the amount of ink on the page is too large; otherwise, it indicates that the amount of ink is insufficient. In the traditional test method, in order to assess the increase and decrease of the dots, a density meter is used to determine the solid density of the test strip and convert it into a percentage of dots. Since the optical effect of the halftone dot must be determined, the influence of various factors on the halftone dot optical effect will inevitably affect the results of the halftone dot. Using microscopic image processing technology, you can directly lock a single dot, count the pixels of the dot, and calculate the dot percentage very accurately. In fact, as long as the number of dots on the printed sheet and the number of dots on the printing plate are measured, the difference between them is the value at which the dot on the printed sheet increases. According to the different screen lines, the calculation of the increase or decrease of the dots can be accurate to one ten thousandth of a square centimeter. Establishing a series of corresponding values ​​between the dot change and the ink supply system, in actual production, you can quickly achieve accurate ink supply.

4. Gray balance detection and control

Brunel test strips have successively launched three generations of test strips. The test content is constantly enriched and improved, and the pre-press and post-press process indicators are better combined. But its essence is also measured by the dots on the test strips on the printing plates and printed sheets. The control of the negative effect of gray balance is mainly to detect the expansion of 25%, 50%, and 75% dots on the printing plate or printed sheet. The method is exactly the same as the microscopic image processing method introduced above, but only describes the change of the index. Correspondingly, the establishment of a one-to-one correspondence between the measurement and processing results of image processing and the description index achieves the purpose of using microscopic image processing to control the offset printing quality.

In short, due to the different types and uses of printed matter, the quality requirements are different. However, the quality of the printed matter must meet the requirements of the market, which requires the strengthening of data and standardized quality control throughout the process. The use of microscopic image processing to control the quality of offset printing products, especially the online sampling self-test during the printing process is very important. It is related to the stability of the quality of the batch of products. The above-mentioned process measures and methods of printing quality control, coupled with the high precision, high repeatability and high consistency of the computer microscopic image control technology, guarantee the excellent quality of offset products . In the specific implementation of this technology, it also involves the hardware and electrical parts of the circuit, this article will not repeat them.

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