Measurement principle and classification of coating thickness gauge

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

Coating thickness measurement has become an important part of processing industry and surface engineering quality inspection, and is a necessary means for products to meet excellent quality standards. In order to internationalize products, China's export commodities and foreign-related projects have clear requirements for coating thickness. The main methods for

measuring coating thickness are: wedge cutting method, light interception method, electrolysis method, thickness difference measurement method, weighing method, X-ray fluorescence method, β-ray backscattering method, capacitance method, magnetic measurement method and eddy current measurement method, etc. The first five of these methods are destructive testing, with cumbersome measurement methods and slow speed, and are mostly suitable for sampling inspection. X-ray and β-

ray methods are non-contact and non-destructive measurements, but the equipment is complex and expensive, and the measurement range is small. Because of the radioactive source, users must comply with radiation protection specifications. X-ray method can measure extremely thin coatings, double coatings, and alloy coatings. β-ray method is suitable for coating and substrate atomic number greater than 3. The capacitance method is only used to measure the thickness of the insulating coating of thin conductors.

With the advancement of technology, especially the introduction of microcomputer technology in recent years, the thickness gauges using the magnetic method and eddy current method have taken a step forward in the direction of miniaturization, intelligence, multi-function, high precision and practicality. The resolution of the measurement has reached 0.1 micron, and the accuracy can reach 1%, which has been greatly improved. It has a wide range of applications, a wide range, simple operation and low price. It is the most widely used thickness gauge in industry and scientific research. The

use of non-destructive methods does not damage the coating or the substrate, and the detection speed is fast, which can enable a large number of detection work to be carried out economically.

1. Principle of magnetic attraction measurement and thickness gauge

The size of the attraction between the permanent magnet (probe) and the magnetic steel is proportional to the distance between the two, and this distance is the thickness of the coating. Using this principle to make a thickness gauge, as long as the difference in magnetic permeability between the coating and the substrate is large enough, it can be measured. In view of the fact that most industrial products are stamped and formed by structural steel and hot-rolled and cold-rolled steel plates, magnetic thickness gauges are the most widely used. The basic structure of the thickness gauge consists of a magnet, a relay spring, a scale and a self-stop mechanism. After the magnet is attracted to the object to be measured, the measuring spring is gradually stretched behind it, and the pulling force gradually increases. When the pulling force is just greater than the suction force, the magnitude of the pulling force is recorded at the moment when the magnet is separated to obtain the coating thickness. New products can automatically complete this recording process. Different models have different ranges and applicable occasions.

The characteristics of this instrument are simple operation, durability, no power supply, no calibration before measurement, and low price, which is very suitable for on-site quality control in workshops.

2. Magnetic induction measurement principle

When the magnetic induction principle is adopted, the size of the magnetic flux from the probe through the non-ferromagnetic coating and into the ferromagnetic substrate is used to measure the coating thickness. The size of the corresponding magnetic resistance can also be measured to indicate the coating thickness. The thicker the coating, the greater the magnetic resistance and the smaller the magnetic flux. In principle, the thickness gauge using the magnetic induction principle can have the thickness of the non-magnetic coating on the magnetic substrate. The magnetic permeability of the substrate is generally required to be above 500. If the coating material is also magnetic, the difference in magnetic permeability with the substrate is required to be large enough (such as nickel plating on steel). When the probe with a coil wound on the soft core is placed on the sample to be tested, the instrument automatically outputs a test current or test signal. Early products used pointer-type meters to measure the magnitude of the induced electromotive force. The instrument amplified the signal and then indicated the coating thickness. In recent years, circuit design has introduced new technologies such as frequency stabilization, phase locking, and temperature compensation, and used magnetic resistance to modulate the measurement signal. Patented integrated circuits are also used, and microcomputers are introduced to greatly improve the measurement accuracy and reproducibility (almost an order of magnitude). Modern magnetic induction thickness gauges have a resolution of 0.1um, an allowable error of 1%, and a range of 10mm. Magnetic

principle thickness gauges can be used to accurately measure paint layers on steel surfaces, porcelain and enamel protective layers, plastic and rubber coatings, various non-ferrous metal electroplating layers including nickel and chromium, and various anti-corrosion coatings in the chemical and petroleum industries. [page]

3. Eddy current measurement principle The

high-frequency AC signal generates an electromagnetic field in the probe coil. When the probe is close to the conductor, an eddy current is formed in it. The closer the probe is to the conductive substrate, the greater the eddy current and the greater the reflected 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. Since this type of probe is specifically used to measure the coating thickness on non-ferromagnetic metal substrates, it is usually called a non-magnetic probe. Non-magnetic probes use high-frequency materials as coil cores, such as platinum-nickel alloys or other new materials. Compared with the magnetic induction principle, the main difference is that the probe is different, the signal frequency is different, and the signal size and scale relationship are different. Like the magnetic induction thickness gauge, the eddy current thickness gauge has also achieved a high level of resolution of 0.1um, an allowable error of 1%, and a range of 10mm.

In principle, the thickness gauge using the eddy current principle can measure non-conductive coatings on all conductive bodies, such as paint, plastic coatings and anodized films on the surfaces of aerospace and aircraft, vehicles, home appliances, aluminum alloy doors and windows, and other aluminum products. The coating material has a certain conductivity, which can also be measured through calibration, but the conductivity ratio of the two is required to be at least 3-5 times different (such as chrome plating on copper). Although the steel substrate is also a conductor, it is more appropriate to use the magnetic principle for such tasks.

IV. Measurement of coating thickness on plastic products

1. Precautions when measuring the coating on plastic products using the transfer method

Measurement of coating on plastic products. When using ultrasonic waves to measure, there is often no good sound wave reflection surface due to the dissolution of the coating and the substrate, which leads to measurement failure or serious deviation of the reading. If the cutting method is used, there are also many inconveniences in use and difficulty in reading.

Therefore, portable electronic product manufacturers currently generally use the transfer method to measure the coating on plastic products.

After years of practice, we recommend that you first cover the product with several small strips of polyester film of standard thickness (about 50 microns, our company has special customized supply), and then press the two ends with paper-based masking tape, leaving the middle part. Put the product into the spraying line for normal spraying and baking.

After the product is completed, remove the polyester film with the paint film, use an iron zero plate (or aluminum zero plate) as the substrate, and use a magnetic induction (or eddy current method) film thickness meter to measure the coated and uncoated parts respectively. The difference between the two is the coating thickness. Since the same zero plate is used, the electromagnetic induction reference point will not change, thus ensuring that the measurement benchmark remains unchanged and the measurement is accurate. In addition, especially because the difference method is used, 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 the different surface roughness, convex and concave deformation, thickness changes, etc. of each iron sheet or aluminum sheet will cause the electromagnetic induction reference point to change, which may cause the measurement error to increase and the repeatability to be poor. It needs to be avoided.

2. Other problems in coating thickness measurement

a. After the coating is baked and cured, there is a slow process of continued curing. Due to conditions, the factory generally takes down the product before measuring. If measured again after one or two days, the thickness will often be 1~3 microns less.

b. As for the thickness measurement by slice method, it is very important to prepare the slice. Since both the substrate and the coating are relatively soft, the deformation during cutting may cause the coating to squeeze and invade the substrate or vice versa, resulting in a larger or smaller reading.

In addition, the paint spraying process is that the liquid is dotted onto the substrate, and then it is leveled, cross-linked, and film-formed and solidified. The cross section must have an undulating surface, and it looks like an orange peel. The transfer method for thickness measurement is to randomly select points on the plane, or take the average, and the slice method is to read the readings visually under a microscope on a straight line section at the cutting point. The readings of the two will be different due to different point selection methods and positions. However, the two have mutual reference value, and can be analyzed and compared to find out the reasons. (end)