Measurement principle and classification of the ho

2022-09-21
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The measuring principle and classification of coating thickness gauge

the covering layer that protects and decorates the material surface, such as coating, plating, coating, coating, chemically generated film, etc., is called coating in some national and international standards

coating thickness measurement has become an important link in the processing industry and surface engineering quality inspection, and is a necessary means for products to achieve excellent quality standards. In order to internationalize products, there are clear requirements for the thickness of coating in China's export commodities and foreign-related projects

the measurement methods of coating thickness mainly include: wedge cutting method, light cutting method, electrolysis method, thickness difference measurement method, weighing method, X-ray fluorescence method, β Ray backscatter method, capacitance method, magnetic measurement method and eddy current measurement method, etc. The first five of these methods are destructive testing. The measurement means are cumbersome and slow. They are mostly suitable for sampling inspection

x-rays and β X-ray method is non-contact nondestructive measurement, but the device is complex and expensive, and the measurement range is small. Due to the presence of radioactive sources, users must comply with the radiation protection specifications. X-ray method can measure extremely thin coating, double coating and alloy coating. β The X-ray method is suitable for the measurement of coatings and substrates with atomic numbers greater than 3. The capacitance method is only used when measuring the thickness of the insulation coating of thin conductors

with the increasing progress of technology, especially after the introduction of microcomputer technology in recent years, the thickness gauge using magnetic method and eddy current method has taken a step towards miniaturization, intelligence, multifunction, high precision and practicality. The resolution of the measurement has reached 0, and this 1 part can be added immediately 1 micron, the accuracy can reach 1%, which has been greatly improved. It has wide application range, wide measuring range, simple operation and low price. It is the most widely used thickness measuring instrument in industry and scientific research

the nondestructive method does not damage the coating or the substrate, and the detection speed is fast, which can make a lot of detection work economically

I. magnetic suction measurement principle and thickness gauge

the magnitude of the suction between the permanent magnet (probe) and the magnetic conducting steel is proportional to the distance between them, which is the thickness of the coating. Using this principle to make a thickness gauge, as long as the difference between the permeability of the coating and the substrate is large enough, it can be measured. Since most industrial products are formed by stamping structural steel and hot-rolled cold-rolled steel plates, magnetic thickness gauge is the most widely used. The basic structure of the thickness gauge is composed of magnetic steel, relay spring, ruler and self-stop mechanism. After the magnetic steel is absorbed with the measured object, the measuring spring will be gradually lengthened and the tension will gradually increase. When the tensile force is just greater than the suction force, the coating thickness can be obtained by recording the tensile force at the moment of magnetic steel separation. New products can automatically complete this recording process. Different models have different ranges and applications

this instrument is characterized by simple operation, durability, no power supply, no calibration before measurement, and low price. It is very suitable for on-site quality control in workshops

II. Principle of magnetic induction measurement

when using the principle of magnetic induction, the thickness of the coating is measured by the size of the magnetic flux flowing into the ferromagnetic substrate from the probe through the non ferromagnetic coating. The corresponding magnetoresistance can also be measured to indicate the coating thickness. The thicker the coating, the greater the magnetoresistance and the smaller the magnetic flux. The thickness gauge based on the principle of magnetic induction can have the thickness of non-magnetic coating on the magnetic conductive substrate in principle. Generally, the permeability of the base material is required to be above 500. If the coating material is also magnetic, the difference between the permeability and the substrate is required to be large enough (such as nickel plating on steel). When the probe around the coil on the soft core is placed on the tested sample, the instrument automatically outputs the test current or test signal. Early products used pointer type meter to measure the induced electromotive force. The instrument amplified the signal and then indicated the coating thickness. In recent years, new technologies such as frequency stabilization, phase locking and temperature compensation are introduced into the circuit design, and the measurement signal is modulated by magnetoresistance. It also adopts the patented integrated circuit and introduces a microcomputer, which greatly improves the measurement accuracy and reproducibility (almost up to an order of magnitude). Modern magnetic induction thickness gauge has a resolution up to that of magnetic induction thickness gauge_ Principle of eddy current measurement_ Principle of magnetic suction measurement and thickness gauge_ The thickness gauge based on eddy current principle reaches 0.1um, and the allowable error is up to 1%, and the measuring range is up to 10mm

the magnetic principle thickness gauge can be used to accurately measure the paint layer, porcelain and enamel protective layer on the steel surface, measure the plane plastic and rubber coating of the lower pressing plate, various non-ferrous metal electroplated coatings including nickel chromium, and various anti-corrosion coatings waiting for chemical and petroleum industry

III. principle of eddy current measurement

high frequency AC signal generates electromagnetic field in the probe coil, and eddy current is formed in it when the probe is close to the conductor. The closer the probe is to the conductive substrate, the greater the eddy current and the greater the reflection impedance. This feedback action represents the distance between the probe and the conductive substrate, that is, the thickness of the non-conductive coating on the conductive substrate. Because this kind of probe is specially used to measure the coating thickness on non-ferrous metal substrate, it is usually called non-magnetic probe. The non-magnetic probe uses high-frequency materials as the coil core, such as platinum nickel alloy or other new materials. Compared with the principle of magnetic induction, the main difference is that the probe is different, the frequency of the signal is different, and the size and scale relationship of the signal are different. Like the magnetic induction thickness gauge, the eddy current thickness gauge also achieves a high level of resolution of 0.1um, allowable error of 1%, and measuring range of 10mm

the thickness gauge based on the eddy current principle can measure the non-conductive coating on all conductors in principle, such as the paint, plastic coating and anodic oxide film on the surface of aerospace aircraft, vehicles, household appliances, aluminum alloy doors and windows and other aluminum products. The coating material has certain conductivity, which can also be measured through calibration, but the conductivity ratio of the two is required to be at least times different (such as chromium plating on copper). Although the steel matrix is also a conductor, it is more suitable to use the magnetic principle for such tasks

IV. measurement of coating thickness on plastic products

1. precautions when measuring coating on plastic products by transfer method

measurement of coating on plastic products. For example, when using ultrasonic measurement, there is often no good acoustic reflection surface due to the compatibility between the coating and the substrate, resulting in measurement failure or serious reading deviation. If the wedge cutting method is used, it is also inconvenient to use and difficult to read

therefore, at present, portable electronic product manufacturers generally use the transfer method to measure plastic ch, etc. to prepare the coating of plla-3 methylene carbonate (TMC) copolymer and methylcellulose (MC) mixed porous catheter stent adhesive products

after years of practice, we recommend covering the product with several strips of polyester film of standard thickness (about 50 microns, which our company has customized supply), and then pressing the two ends with paper-based masking tape to leave the middle part. Put the product on the spraying line for normal spraying and baking

after the completion of the product, remove the polyester film with paint film, use iron Zero plate (or aluminum Zero plate) as the substrate, and use magnetic induction (or eddy current method) film thickness gauge to measure the coated part and the uncoated part respectively. The difference between the two is the coating thickness. Because the same Zero plate is used, the electromagnetic induction reference point will not change, so as to ensure that the measurement benchmark remains unchanged and the accuracy of the measurement is guaranteed. In addition, especially because of the difference method, the errors of the instrument and the zero plate will be offset by subtraction. This greatly reduces the requirements for the accuracy of the zero plate and the instrument

some factories still use the method of pasting iron sheets and aluminum sheets. It should be noted that different surface roughness, convex and concave deformation, thickness changes, etc. of each iron sheet or aluminum sheet will cause changes in the electromagnetic induction reference point, which may increase the measurement error and poor repeatability. Attention should be paid to avoid

2. Coating thickness WDS ⑴ 00 liquid crystal digital display electronic universal experimental machine is also very popular! Welcome to inquire about other problems of experimental machine degree measurement

a. after the coating is baked and cured, there is a slow process of continuous curing. Due to the limitations of conditions, the factory usually takes off the product and measures it. If it is measured again after a day or two, the thickness will often be less than 1~3 microns

b. as for thickness measurement by sectioning, it is very important to prepare sections. Due to the soft substrate and coating, cutting deformation may cause the coating to squeeze and occupy towards the substrate direction or the substrate to the coating direction, resulting in larger or smaller readings

in addition, the paint spraying process is liquid dot sputtering to the substrate, and then leveling, cross-linking, film-forming and curing. The cross-section must be undulating on the surface, but it appears orange peel on the appearance. Thickness measurement by transfer method is to take random points on the plane, or take an average number again. Slicing method is to read microscopic visual readings on a straight section at the cutting place. The readings of the two will be different due to different point taking methods and positions. However, the two have mutual reference value, which can be analyzed and compared to find out the reasons

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