Application of Computer Microscopic Image Processing in Offset Quality Inspection (Part 2)

Third, the use of microscopic images to control the quality of offset printing

1. Print layout ink balance control

The GATF quality control bar is usually placed at the mouth of the printed product, and the ratio of the ink on the printed page can be clearly seen, which makes it easier to control the layout moisture. This quality control bar is composed of two parts, namely diagonal lines and field bars. If the fountain solution is excessive, the sloping part of the GATF control strip will produce a “water accumulation” phenomenon; conversely, if the amount of layout water is insufficient, the upper part of the ink will be better in ink, while the lower part is easy to get dirty. In the traditional printing quality inspection process, these changes are also difficult to observe directly by the naked eye and need to use a magnifying glass to see accurately. In microscopic image processing, the concept of dirtiness can be described by the increase in the "black" local area in a binary image. When specifically checking this index, the information on the printed sheet can be compared with the information on the measurement and control strip. If the printed product is filled with water, the "black" value on the microscopic image is greater than the black value of the corresponding portion on the proof or on the plate; conversely, if the amount of the printed product is insufficient, the "black" on the microscopic image The value of the color is less than the black value on the proof or the corresponding part of the plate. In actual production, the rigorous quantification experiments can be performed on the calculation results of the microscopic image and the amount of water on the printing plate, so that it is possible to use the GATF quality control bar to accurately test whether the ratio of the typographical product layout ink is appropriate.

2. Control of the ink volume of printed products

Using the GATF star to control the amount of ink on the layout, you can ensure that the print inks meet the printing quality requirements. When the dots are not distorted, ghosted, and the layout gives a moderate amount of ink, the center of the star is white. In a binary image, the total amount of "0" in this part is more than the total amount of "1". If the layout is subject to excessive ink, a large black circle appears on the center of the star mark on the printed sheet, and the larger the black circle, the more ink is indicated. On the other hand, if the ink on the layout is not enough, the blank circle at the center of the star will enlarge and the dot will shrink. Therefore, in actual production, the ratio between the total amount of "0" at the star position and the total of "1" can be established, and the supply of ink can be judged very accurately.

If the dots are laterally deformed, the black circle in the center expands into a duck shape in the longitudinal direction. If the dots are longitudinally deformed, the black circle in the center of the star will extend horizontally in the shape of a duck egg. Create two indicators at the center of the star, one representing the amount of lateral ink, that is, how many “black” pixels are in the horizontal coordinate, and the other indicator is how much the vertical “black” pixels are. The ratio of the two indicators can reflect the deformation of the outlets, and the results reflected are extremely accurate. In the same way, if the network appears ghosting, the central part of the star will disappear, and the incomplete outline will resemble the "8" shape. The "8" shape expands laterally, and the ghost image is generated vertically. If the "8" shape expands longitudinally, ghosting appears horizontally. If there is a black dot in the center of the interstellar space, it indicates that the ink on the layout is too large.

3. Detection of dot gain

In the lithography process, the amount of ink has a great influence on the dot gain. Both dot size and ink thickness affect the mesh tone value. The dot gain test strip provides test segments for field and mesh tones for testing. The increase in dots indicates that the amount of ink on the layout is too large; otherwise, it indicates that the amount of ink is not enough. In traditional testing methods, in order to assess dot gains and densities, densitometers are used to determine the percentage density of the test bar's field density converted into dots. Since it is necessary to determine the optical effect of the dot part, the influence of various factors on the optical effect of the dot will inevitably affect the result of the dot percentage. With the use of microscopic image processing technology, it is possible to directly lock a single dot and count the number of pixels in the dot, which allows very accurate calculation of dot percentages. In fact, as long as the number of dot pixels on the printed sheet and the number of dot pixels on the printing plate are measured, the difference between them is the increased value of the printed sheet. Depending on the number of different screens, the calculation of the point increase or decrease can be accurate to one-ten thousandths of a square centimeter. Establishing a series of corresponding values ​​between the dot change and the ink supply system can quickly achieve accurate ink supply in actual production.

4. Gray balance detection and control

The Bruner test strip has introduced three generations of test strips. Its test content has been continuously enriched and improved, and the pre-press and post-printing process indicators have been better integrated. But its essence is also to measure the dots on the test strip on the printing plate and printed sheet. The negative effect control of gray balance is mainly to detect 25%, 50%, 75% dot gains on the printing plate or printed sheet. The method is completely consistent with the microscopic image processing method described above, but it only describes the change of the index. Correspondingly, a one-to-one correspondence between the measurement and processing results of the image processing and the description index is established, and the purpose of using the microscopic image processing to control the offset printing quality is achieved.

In short, due to the different types and uses of prints, quality requirements vary. However, the quality of printed products must meet the requirements of the market, which requires the strengthening of the data and standardized quality control of the entire process. The use of microscopic image processing to control the quality of offset products, especially the on-line sampling self-test in the printing process is very important. It is related to the stability of batch product quality. The above-mentioned several kinds of printing quality control technology measures and methods, together with the high precision, high repeatability and high consistency of computer microscopic image control technology, are the guarantees for the excellent quality of offset printing 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.

Author: Wang Xiaohong Shanghai Publishing and Printing College of Higher