Before any magnet leaves the factory, it has to pass a series of checks. Whether you buy a hundred or a hundred thousand, the same three groups of tests stand behind every shipment: appearance and dimensions, magnetic performance, and coating and durability.
Magnetic performance — the certified numbers
The headline numbers on a datasheet — Br, Hcj, Hcb and (BH)max — come from the full B-H curve, measured on samples from each batch in a closed-circuit tester (GB/T 3217 / IEC 60404-5). Because that test needs a full-size sample and a big machine, factories certify the batch with it, then use faster relative checks for day-to-day control:
- Surface field — a gaussmeter reading at a fixed point on the face. Fast, but position-sensitive.
- Flux — a coil and fluxmeter reading of the total flux through a defined area.
- Magnetic moment — a Helmholtz-coil measurement of the whole part. The most repeatable of the three, and the only one that works well on thin and odd-shaped parts.
Magnetic checks are usually done on a sampling basis — a few parts per batch — while cheap, fast checks like moment may run on every part for large orders.
Dimensions and appearance
Every part is checked for size with calipers, micrometers or go/no-go gauges, and inspected by eye or camera for chips, cracks and plating defects like pinholes or rust spots.
Coating and durability
Coating thickness is measured non-destructively with XRF. Corrosion resistance is proven with salt spray (coated parts, pass/fail) and HAST/PCT weight-loss tests (bare substrate, mg/cm²). For holding applications, a pull force test on a steel plate shows the force your product will actually feel.
One habit worth keeping: a grade name only states minimums. The batch test report shows what really shipped — read that, not the label.
Read on: dimensions and tolerances → · how the B-H curve is tested →