Using GATF Control Strip to Detect Offset Product Quality

The quality of offset printing products is closely related to the factors such as equipment, paper, and ink. Therefore, proper control of quality control methods is of great significance to ensure the quality of the printing process.

Everyone knows that the sampling inspection in production is an important part of quality control. So what method can be used to better achieve the purpose of accurate quality inspection and control? Using a signal strip to check the quality of controlled offset products is a convenient and quick method.

1. Check the quality of printed products with GATF quality control strips. This quality control bar is composed of two parts, namely diagonal lines and field bars. The sloping part corresponds to 70% of the dot area. The number of halftone screens, equivalent to 150 lines per inch. Based on the principle of lithography, that is, the layout first passes the fountain solution, and then receives the ink layer coating. If the fountain solution is excessive, the sloping part of the GATF control strip will have a phenomenon of “water accumulation”; When the amount of water on the layout is not enough, the ink on the upper part is better, while the lower part is easy to get dirty. These changes are difficult to observe directly with the naked eye and need to use a magnifying glass to see accurately. The GATF quality control bar can better test whether the proportion of water and ink in the layout product is appropriate. The GATF quality control bar is usually placed at the mouth of the printed product, and the ratio of water and ink on the printed page can be clearly seen, which facilitates control of the layout moisture.

2. Use the GATF star to control the amount of ink on the layout to ensure that the ink of the printed product meets the printing quality requirements. The method of identifying the quality is: when the dots are not deformed, ghosted, and the layout gives a moderate amount of ink, the middle of the star is white. If the layout is subjected to excessive ink, the center of the star mark on the printed sheet appears laps, 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. 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. If a dot appears ghosting, the central part of the star disappears. The incomplete outline looks like an "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. GATF stars are not only used to control the quality of offsets, but they can also be used to adjust photo platesetters. The method is: it is affixed to the document holder and corrected on the focusing glass. When the star mark is blurred, it indicates that the document holder or the focusing glass is inclined in a certain direction.

3, use the dot increase test strip to detect the layout. 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. In order to assess dot enlargement and contraction, the optical effect of the dot part must be determined. The densitometer is used to measure the density of the test strip in the field. The measured value is Dv, and then the density value DR of the mesh tone is determined.

The mesh tone value of the print product can be obtained from the Dv and DR density values ​​according to the following formula:

The value of the mesh tone of the print

FD=[1-(1/10 DR (mesh density))]/[1-(1/10 Dv (solid density))] × 100%

When testing the mesh density value, the DR values ​​of the high-, mid-, and dark-tone parts of the print test strip should be separately measured, and then the nettone value of the high-profile, middle-tone, and dark-tone parts of the print product should be determined by the above formula. . In fact, as long as the values ​​of Dv and DR are measured, the gradation value of the network can be read out from the calculation table without calculation. The difference between the FD on the Internet and the FF on the film is the value of the Internet.

4. Use a scale magnifying glass to measure the value of the mesh increase. If the detection instrument is incomplete, and the test strip and signal strip cannot be used due to the limitation of the print sheet size, a simple and quick method can be used. That is, the scale magnifying glass is used to determine the mesh expansion value, and then the printing quality is controlled according to the measurement result. The main method is: first use a 60-fold or 100-fold scale magnifying glass a to observe the specific dots of the corresponding graphic part on the printing plate and the printed sheet, and measure the area on the printing plate and the printed sheet or the number of rounds. The diameter of the dot or the length of the square dot of the dot, then the area is calculated respectively, and the relative ratio is obtained, which is used as a reference for the control of printing quality. The use of a scaled magnifier also allows the dot shape of the dot to be clearly displayed for quality control. If there is a test strip, use a scale magnifier to observe 50% or 75% of the mesh of the test strip, and the test results are better.

In short, due to the different types and uses of prints, the quality requirements are different, but the quality of printed products must meet the requirements of the market, which requires the strengthening of the quality control of the entire process, especially the process of sampling self-inspection is very important , It is related to the stability of batch product quality, and the application of the above-mentioned several kinds of printing quality control technology measures and methods are feasible to ensure the quality of offset printing products.

Reprinted from: Graphic Arts Forum