ClassA Motor

Raw material

Sintered NdFeB Magnet

Melting & strip casting → Hydrogen decrepitation → Jet milling → Field orientation & pressing → Cold isostatic pressing → Sintering & ageing → Machining → Surface coating → Final inspection

✓ Expert reviewed All content on this page has been reviewed by a senior motor engineer.

Process flow

9 main process steps, with the equipment and instruments used at each step, their main suppliers, and the technical control points.

  1. 1

    Melting & strip casting

    Nd/Pr, Dy/Tb, Fe, B and additives are vacuum-induction melted and cast onto a water-cooled wheel into thin flakes.

    ⚙ Vacuum Induction Melting & Strip-casting FurnaceAutomated
    International
    ULVAC
    Mainland China
    Zhongbei Vacuum

    Technical control points:

    • ▪Alloy composition — Total rare earth per grade (typically 29–32 wt%), B ≈ 0.9–1.0 wt%Check: ICP-OES; every heat
    • ▪Flake thickness and microstructure — 0.2–0.4 mm; fine columnar grains, no α-Fe dendritesCheck: Micrometer and metallography; every heat
    • ▪Oxygen content of alloy — ≤ 300 ppmCheck: O/N analyser; every heat
  2. 2

    Hydrogen decrepitation

    Flakes absorb hydrogen and crack along grain boundaries into coarse powder, followed by dehydrogenation.

    ⚙ Hydrogen Decrepitation FurnaceAutomated
    International
    To be added
    Mainland China
    To be added

    Technical control points:

    • ▪Hydrogen absorption — Absorption to saturation (pressure stops falling)Check: Pressure / temperature log; every batch
    • ▪Dehydrogenation and residual hydrogen — Dehydrogenation at about 500–600 °C; residual H ≤ specCheck: Hydrogen analyser; every batch
    • ▪Inert-gas protection — Vessel O₂ ≤ 100 ppm during transferCheck: Oxygen analyser; continuous
  3. 3

    Jet milling

    Coarse powder is milled in nitrogen to fine single-crystal particles.

    ⚙ Jet Mill (N2 protected)Automated
    International
    Hosokawa Alpine
    Mainland China
    To be added

    Technical control points:

    • ▪Particle size — D50 ≈ 3–5 µm, narrow distribution (D90 / D10 ≤ about 4)Check: Laser particle-size analyser; every batch
    • ▪Oxygen content — Powder O ≤ 1,000 ppm (≤ 600 ppm for low-oxygen process); mill gas O₂ ≤ 50 ppmCheck: O/N analyser per batch; gas O₂ continuous
    • ▪Lubricant / antioxidant addition — Per recipe weight ratioCheck: Weighing record; every batch
  4. 4

    Field orientation & pressing

    Powder is aligned in a strong magnetic field and pressed into blocks inside a protective glove box.

    ⚙ Magnetic-field Orientation PressAutomatedSemi-automatic
    International
    To be added
    Mainland China
    To be added

    Technical control points:

    • ▪Alignment field — ≥ 1.5 T (typically 1.5–2.0 T) — determines BrCheck: Gaussmeter; every shift
    • ▪Green density — About 3.8–4.5 g/cm³, uniform within the blockCheck: Weight and dimensions; sampling per press cycle
    • ▪Glove-box atmosphere — O₂ ≤ 50 ppmCheck: Oxygen analyser; continuous
  5. 5

    Cold isostatic pressing

    Sealed green blocks are pressed isostatically to raise and even out density.

    ⚙ Cold Isostatic PressSemi-automatic
    International
    To be added
    Mainland China
    To be added

    Technical control points:

    • ▪Pressure and hold time — About 150–250 MPa, hold about 1 minCheck: Pressure log; every cycle
    • ▪Package integrity — No leaks (oil contact causes oxidation and contamination)Check: Visual; every block
  6. 6

    Sintering & ageing

    Blocks are vacuum-sintered at about 1,050–1,100 °C, then aged in one or two stages to develop coercivity.

    ⚙ Vacuum Sintering & Ageing FurnaceAutomated
    International
    ULVAC
    Mainland China
    Zhongbei Vacuum
    ⚙ Hysteresisgraph (BH tracer)Test instrumentSemi-automatic
    International
    Magnet-Physik, Brockhaus Measurements
    Mainland China
    To be added

    Technical control points:

    • ▪Sintering temperature, time and vacuum — About 1,050–1,100 °C for 3–5 h; vacuum ≤ 10⁻² Pa; furnace uniformity ±5 °CCheck: Furnace log every batch; temperature uniformity survey periodic
    • ▪Sintered density — ≥ 7.5 g/cm³Check: Archimedes method; every batch
    • ▪Ageing — Two-stage (about 900 °C + 500 °C) with controlled gas quenchCheck: Furnace log; every batch
    • ▪Witness-sample properties — Per grade, e.g. N38SH: Br 1.22–1.26 T, Hcj ≥ 1,592 kA/m (20 kOe)Check: BH tracer; every batch
  7. 7

    Machining

    Blocks are sliced, ground or wire-cut into arc segments, then chamfered and cleaned.

    ⚙ Slicing, Grinding & Wire-cut MachinesAutomatedSemi-automatic
    International
    To be added
    Mainland China
    To be added
    ⚙ CMM, Roundness & Runout GaugesTest instrumentSemi-automaticManual
    International
    MitutoyoDongguan, HexagonGuangzhou, MarpossShenzhen+1 more →
    Mainland China
    To be added

    Technical control points:

    • ▪Dimensions and arc profile — Per drawing, typically ±0.05 mm; arc radius by profile gaugeCheck: CMM / projector first piece; SPC sampling
    • ▪Chipping and cracks — Edge chips within appearance standard (e.g. ≤ 0.5 mm); no cracksCheck: Visual; AQL sampling
    • ▪Orientation direction — Magnetization direction marked and correctCheck: Fluxmeter check on magnetized samples; per lot
  8. 8

    Surface coating

    Ni-Cu-Ni electroplating, zinc or epoxy coating protects the magnet from corrosion.

    ⚙ Electroplating / Coating LineAutomated
    International
    To be added
    Mainland China
    To be added

    Technical control points:

    • ▪Coating thickness — Ni-Cu-Ni 10–20 µm; Zn 5–12 µm; epoxy 10–25 µmCheck: XRF thickness gauge; every batch
    • ▪Adhesion — No blistering or peeling after thermal shock; cross-hatch for epoxyCheck: Thermal-shock / cross-hatch test; every batch
    • ▪Corrosion resistance — NSS per spec (e.g. ≥ 48 h for Ni); PCT 121 °C / 2 atm / 96 h, weight loss ≤ 5 mg/cm²Check: Salt-spray and PCT; qualification and periodic
  9. 9

    Final inspection

    Magnetic properties, dimensions and appearance are checked per lot.

    ⚙ Hysteresisgraph (BH tracer)Test instrumentSemi-automatic
    International
    Magnet-Physik, Brockhaus Measurements
    Mainland China
    To be added
    ⚙ CMM, Roundness & Runout GaugesTest instrumentSemi-automaticManual
    International
    MitutoyoDongguan, HexagonGuangzhou, MarpossShenzhen+1 more →
    Mainland China
    To be added

    Technical control points:

    • ▪Magnetic properties — Br, Hcj, (BH)max within grade (GB/T 13560)Check: BH tracer (IEC 60404-5); every lot
    • ▪Lot-to-lot consistency — Flux spread within ±2%Check: Helmholtz coil / fluxmeter; sampling
    • ▪Appearance — No coating defects, chips or cracksCheck: Visual; AQL sampling

Incoming inspection & approval tests

  • Magnetic properties (Br, Hcj, (BH)max)IEC 60404-5 / GB/T 3217
  • High-temperature irreversible loss (e.g. 150 °C × 2 h)Agreed per grade and duty
  • Coating neutral salt sprayISO 9227 / GB/T 10125
  • Pressure cooker test PCT / HASTe.g. 121 °C, 2 atm; check weight loss and powdering
  • Coating adhesion and thicknessThermal shock method, XRF thickness
  • Dimensions and chamfersIncoming inspection

Buyer FAQ

What do N35SH or N42UH mean?
The number is the maximum energy product in MGOe (35 ≈ 279 kJ/m³). The letters give the coercivity class and so the working temperature: none ≈ 80 °C, M 100 °C, H 120 °C, SH 150 °C, UH 180 °C, EH 200 °C. Motors usually need SH or higher.
Nickel, zinc or epoxy coating?
Ni-Cu-Ni is the general-purpose choice with good appearance and salt-spray life; zinc is cheaper but less durable; epoxy (e-coat) gives the best corrosion protection and insulation and suits bonded rotors. Specify the salt-spray hours and adhesion test.
What should I check on incoming magnets?
Dimensions and chamfers, coating thickness and adhesion, surface flux or flux per part, and periodic hysteresigraph tests of Br and HcJ — including HcJ at your maximum temperature. Ask which plant sintered the blocks.