Analysis of commonly used measurement techniques for film thickness of color-coated steel plate

The measurement of film thickness is divided into the measurement of dry film thickness and the measurement of wet film thickness. The measurement of dry film thickness is generally divided into magnetic thickness gauge method, micrometer method, metallographic microscope method and other non-magnetic thickness gauge methods, mostly off-line laboratory sampling plate measurement. The accuracy of these thickness measuring instruments is different, and there are many influencing factors; for example, the indication error of the magnetic thickness measuring instrument is ±1μm (DFT≦50μm), and due to the substrate roughness and structure, the external magnetic field and the geometry of the probe position ﹑Influenced by personnel skills and other aspects, the error accuracy is usually between 5% and 10%. The accuracy/process fluctuation ratio of the analysis results of the measurement system of the Benxi Iron and Steel Group is 27.60%. The system accuracy/tolerance The ratio is 21.34 %; the recognition power of the system is 5; and in the improvement analysis, it is found that most of the reasons for the errors exist in the operating norms and methods.


The micrometer method detects the thickness of the coating by measuring the difference between the thickness of the color plate before and after the coating is removed. The error of the digital display instrument is ±0.001mm. The thickness of the coating is measured differently, and the error of the instrument indication is ±2.5μm. The drilling destructive micro thickness gauge uses a drilling machine to drill a round hole with a certain taper in the color plate coating, observes the coating through an optical microscope, locates the coating interface, measures the horizontal distance and converts it according to the taper The thickness of the finished coating, the instrument indication error is ±0.5μm (the above error value is taken from the standard GB/T13448-2005).


Analysis of commonly used measurement techniques for color-coated steel sheet thickness with Figure 1


The measurement of wet film thickness is generally divided into two types: plate-shaped gear and disc-shaped wheel. Among them, the measurement principle of the plate gear type is that the end faces of the two outer teeth are at the same level, and the tops of the middle teeth are at different distances from the horizontal plane. When the instrument is pressed against the substrate under the wet film, one of the teeth can directly contact the wet film, and the height of the tooth that just touches the wet film layer is the wet film thickness. Although strictly speaking, this kind of wet film measurement is difficult to achieve accurate measurement, but because there are no other more accurate wet film thickness measuring instruments, these instruments are also widely used in production lines and laboratories. Usually, when performing more accurate wet film thickness measurements, use a special graduated microscope to check the integrity of the instrument, and wipe it clean with a special solvent and a soft, lint-free cloth after each use. The disc-shaped guide wheel thickness gauge uses three planes between two planes connected at the same level. When the planes on both sides touch the substrate under the wet film at the same time, the third plane is The wet film can be accessed vertically. Since the third plane has a height difference from the two planes on both sides, when the third plane just touches the wet film, the height difference is the thickness of the wet film. When measuring with this method, it should be noted that the instrument needs to be perpendicular to the surface to be measured. Due to the influence of the scale of the instrument itself, the accuracy of wet film thickness measurement is low.


From the above analysis of the dry film and wet film measurement system, it can be seen that it is very difficult to achieve accurate online measurement of the coating film, which requires the joint efforts of many factors such as complete instruments, the quality of operators, standardized operating methods, and perfect operating processes. Because many human factors are involved, there are large unforeseen errors in actual production management.


For more details, please refer to the full text of "Research on Film Thickness Accurate Measurement and Predictive Control Technology on Color Coating Production Line" (Xing Chang, Song Chengge)


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