1.What are the measurement methods, advantages, and disadvantages of magnetic thickness gauges?
Principle: This is the most fundamental and widely used method. It utilizes the principle of magnetic induction. When the probe (with a built-in permanent magnet) approaches a ferromagnetic substrate (such as galvanized steel or aluminized zinc steel), the magnitude of the magnetic flux changes with the thickness of the non-magnetic coating (all organic coatings, such as polyester, PVDF, etc.). The instrument calculates the coating thickness by measuring this change.
Advantages:
Fast, convenient, and portable.
No damage to the coating surface.
Relatively inexpensive and widely applicable.
Disadvantages:
Only suitable for magnetic substrates such as steel; not suitable for non-magnetic substrates such as aluminum-magnesium-manganese and stainless steel.
Measurement accuracy is slightly affected by substrate thickness, surface curvature, and the substrate's own magnetism (e.g., the coating on aluminized zinc steel can affect magnetism), requiring calibration.
2.What are the measurement principles, advantages, and disadvantages of eddy current thickness gauges?
Principle: The high-frequency electromagnetic field generated by the probe induces eddy currents in a conductive substrate (such as aluminum, copper, or stainless steel). The intensity of the eddy currents is related to the distance between the probe and the substrate (i.e., the coating thickness). The coating thickness is determined by measuring the changes in the eddy currents.
Advantages:
Suitable for non-magnetic metal substrates, such as aluminum-magnesium-manganese plates.
Also non-destructive, fast, and portable.
Disadvantages:
Not suitable for magnetic substrates such as steel.
Accuracy is also affected by factors such as substrate conductivity and thickness.
Application Scenarios: Primarily used for measuring the coating thickness of aluminum-based color-coated coils.
3.What are the principles, advantages, and disadvantages of cross-sectional microscopy?
Principle:
A small sample is cut from the pre-coated steel roll.
The sample is inlaid and cured with resin.
The inlaid sample cross-section is finely ground and polished to create a mirror-like surface.
This cross-section is observed under a microscope, and the total thickness of the coating (including primer and topcoat) is measured directly using a ruler; the thickness of the primer and topcoat can even be measured separately.
Advantages:
Highest precision; the standard method for arbitration and scientific research.
The layered structure, uniformity, and adhesion to the substrate of the coating can be clearly observed.
Disadvantages:
Complex operation, time-consuming, and costly.
Permanent damage to the sample.
Requires specialized equipment and skilled operators.
4.What are the principles, advantages, and disadvantages of the micrometer measurement method?
Principle:
Measure the total thickness (T1) of the coated color-coated steel sheet using a high-precision micrometer.
Completely remove the coating from a small area using a chemical paint remover or mechanical methods (such as sandpaper) without damaging the substrate.
Measure the substrate thickness (T2) again at the same location using a micrometer after coating removal.
Coating thickness = T1 - T2.
Advantages:
The method is simple and intuitive, requiring no expensive equipment.
Disadvantages:
The accuracy is relatively low, affected by the micrometer's precision and the operator's technique.
It is a destructive test.
The paint removal process may be incomplete or damage the substrate, introducing errors.
5.What are the key precautions to take during measurement?
Calibration: Before using a non-destructive thickness gauge, zeroing and calibration must be performed on a substrate of the same material using a standard sheet. For example, when measuring a color-coated roll with an aluminum-zinc coated substrate, calibration must be performed on an uncoated aluminum-zinc coated sheet.
Substrate Effect: Magnetic thickness gauges are sensitive to the thickness and magnetism of the substrate. If the substrate is too thin (e.g., less than 1 mm), a sufficiently thick substrate of the same material should be used for support during measurement, or a probe specifically designed for thin substrates should be used.
Measurement Point Selection: Single-point measurements are not representative. Multiple points (usually at least 5 points) should be evenly selected on the surface of the plate according to standards (e.g., ASTM B499, ISO 2178) for measurement, and then the average value should be calculated. Special attention should be paid to the edges and center of the plate.
Surface Cleanliness: Measurement points must be clean and free of dust, oil, and other foreign matter.
Curvature Influence: When measuring curved surfaces, a small radius or dedicated probe should be used, and the effect of curvature on the reading should be considered.
Implementation Standards: Measurements should comply with relevant national or international standards, such as China's GB/T, the United States' ASTM, and international ISO standards. These standards provide detailed regulations on measurement methods, point selection, and result processing.