Sintered NdFeB Magnet Selection Guide: A Comprehensive Guide from Performance to Application
As a leading representative of high-performance permanent magnetic materials, sintered NdFeB (Neodymium Iron Boron) is widely used in motors, sensors, medical devices, new energy vehicles, and other fields due to its excellent magnetic properties. However, different application scenarios have significant differences in requirements for magnet performance, temperature resistance, protection, and structural design. Based on over 20 years of industry experience, this guide provides a one-stop selection strategy from magnetic parameters, environmental adaptation, and structural design to cost optimization, helping you accurately match your needs.
I. Core Magnetic Properties: Decoding the “Soul of Magnets”
The performance of sintered NdFeB is determined by three core parameters, which should be prioritized according to application scenarios:
1. Residual Flux Density (Br)
– Definition: The magnetic field remaining in the magnet after removing the external magnetic field following saturation magnetization, measured in Tesla (T) or Gauss (G, 1T = 10,000G).
– Key Role: Directly determines the surface magnetic field strength and suction force, crucial for adsorption devices (e.g., magnetic tools, magnetic separators). For example, an N52 magnet (Br ≈ 1.48T) with a diameter of 10mm can generate a suction force of 30kg, while an N35 magnet (Br ≈ 1.23T) only provides about 18kg.
– Selection Tip: When prioritizing Br, confirm the magnet size simultaneously—small high-Br magnets (e.g., N50/N52) suit space-constrained scenarios like micro-motors, while larger magnets can balance cost and performance with medium grades (N38-N45).
2. Coercivity (Hc)
– Definition: Resistance to demagnetization, including磁感矫顽力 (Hcb) and intrinsic coercivity (Hcj), with the latter (unit: kOe or kA/m) being critical for high-temperature environments.
– Key Role: Higher Hcj means stronger resistance to demagnetization, especially under reverse magnetic fields or high temperatures. For example, automotive motors exposed to alternating stator fields require Hcj ≥ 20kOe (N42H grade), while toy motors only need Hcj ≥ 12kOe (N35).
– Common Misconception: Hcj ≠ temperature stability of Br! Insufficient Hcj can cause irreversible demagnetization at high temperatures (Br decrease is reversible below operating temperature, but insufficient Hcj leads to irreversible loss).
3. Maximum Energy Product (BHmax)
– Definition: The energy storage capacity of the magnet, measured in MGOe (1MGOe = 7.96kJ/m³), a core indicator for balancing volume and performance.
– Key Role: Higher BHmax allows smaller magnet sizes for the same magnetic performance. For example, drone motors requiring lightweight design use N50 (50MGOe), while industrial motors can reduce costs with N38 (38MGOe).
– Selection Formula: Use magnetic field simulation or empirical formulas (e.g., air-gap magnetic field B = Br × (L/D)/(1+2L/D), where L = magnet thickness, D = diameter) to back-calculate required BHmax based on space constraints.
II. Temperature Adaptation: Avoiding the “High-Temperature Demagnetization” Trap
The temperature resistance of sintered NdFeB is determined by grade suffixes (e.g., N35, N35M, N35H), requiring clarity on two critical thresholds:
– Operating Temperature (Tw): The long-term stable working temperature; exceeding it causes rapid Hcj decline and irreversible demagnetization.
– Curie Temperature (Tc): The critical temperature where magnetism is lost (N35 ≈ 310℃, N35SH ≈ 240℃; a 20%-30% safety margin is recommended for actual applications).
| Grade Series | Typical Grades | Tw (℃) | Tc (℃) | Applications |
| Standard | N35-N52 | ≤80 | 310-320 | Home appliances, toys, general sensors |
| Moderate Temp | N35M | ≤100 | 330 | Industrial motors, instruments |
| High Temp | N35H | ≤120 | 350 | Automotive EPS motors, servo motors |
| Ultra-High Temp | N35SH/N35UH/N35EH | ≤150/180/200 | 380+/400+/420+ | Automotive engines, high-temperature sensors, aerospace equipment |
Selection Essentials:
1. Calculate environmental temperature rise: Include ambient temperature, self-heating (I²R losses), and nearby heat sources (e.g., motor windings, PCBs); conduct thermal distribution tests or simulations.
2. Temperature coefficient compensation: Br has a coefficient of -0.12%/℃, Hcj -0.4%/℃. For example, N35H at 120℃ sees a ~4.8% Br reduction from 25℃; reserve magnetic performance margins.
3. Extreme testing: Provide reports for 125℃/2000h or 200℃/500h tests to ensure long-term reliability.
III. Coating Protection: Armor for Harsh Environments
Sintered NdFeB is prone to oxidation from moisture, salt spray, chemical corrosion, or friction; coating selection must match environmental severity:
| Environment Type | Recommended Coating | Process Features | Spray Test (ASTM B117) | Typical Applications |
| Dry Indoor | Uncoated/Electrophoresis | Lowest cost, 5-10μm thickness | <24 hours | Transformers, speakers |
| Moist/General Outdoor | Zinc Plating (Zn) | High cost-performance, 8-15μm | 24-72 hours | Magnetic tools, home appliance motors |
| Salt Spray/Coastal | Nickel-Copper-Nickel (Ni-Cu-Ni) | Three-layer protection 20-30μm | 500-1000 hours | Marine equipment, automotive wheel sensors |
| High Temperature (≤180℃) | Epoxy Powder Coating | Good heat resistance, 50-100μm | Moisture resistance | Engine magnets, industrial high-temp sensors |
| Ultra-Thin/Precision Devices | Parylene | 1-5μm thickness, gapless coverage | Excellent chemical resistance | Medical implants, MEMS sensors |
| Wear-Resistant/High Hardness | Chromium Plating (Cr)/Teflon Coating | Surface hardness ≥500HV | Friction resistance | Mechanical transmission parts, magnetic connectors |
Customization Tips:
– Complex structures (multi-curved surfaces, blind holes) require vacuum coating or dip-coating to avoid uneven thickness;
– Salt spray tests should specify “initial rust time”而非”appearance-free time” (e.g., Ni-Cu-Ni allows minor edge rusting without main-body failure);
– Provide adhesion test reports (Cross-cut Test ISO 2409) to ensure coating durability under high temperature or vibration.
IV. Shape and Size: From “Standard Parts” to “Custom Design”
1. Basic Shape Selection:
– Disc/Cylinder: Axial magnetization, suitable for radial magnetic fields (sensors, suction cups), diameter φ1mm-φ300mm, thickness 0.5mm-50mm， maybe more. Up to the special situations；
– Block/Rectangular: Multi-directional magnetization (axial/radial/thickness), ideal for magnetic circuit design (motor stators, magnetic shielding), minimum size 1mm×1mm×1mm;
– Arc/Segment: Radial magnetization for motor rotors (e.g., EV drive motors), arc precision ±0.1°, the surface roughness Ra≤1.6μm;
– Custom Shapes: Machined to drawings, supporting features like counterbores, steps, and dovetail grooves; evaluate mold costs (simple shapes: $70-$280, complex shapes require CNC machining).
2. Size Optimization Principles:
– Axial Magnetization: Thickness (L) determines demagnetization resistance (L/D ≥ 0.5 for better stability);
– Radial Magnetization: The difference between outer and inner diameters (D-d) affects magnetic field strength (e.g., a ring magnet (D=20mm, d=10mm) has 1.5x stronger central field than a disc of the same volume);
– Pole Design: Multi-pole magnetization (4-pole, 8-pole) must match motor slot numbers; request magnetizing fixture designs to ensure pole uniformity (Toerlance≤±2%).
3. Processing Matching:
– Small sizes (<5mm): Precision wire cutting, tolerance ±0.02mm; large sizes: slicing + grinding, tolerance ±0.05mm;
– High surface roughness requirements (Ra≤0.8μm): Add polishing, increasing costs by 10%-20%;
– Magnetization directions support multi-axis (radial, axial, Halbach array); complex magnetization requires custom magnetizing fixtures.
V. Cost-Performance Balance: Whole-Life Cycle Cost Optimization
1. Performance Redundancy Control:
– Avoid “over-engineering”: Common magnetic adsorption place (e.g., magnetic brackets) don’t need N52; N40 suffices, reducing costs by 30%;
– Evaluate alternatives: Large magnets can use combining (combined small magnets) to reduce material waste, increasing scrap utilization to >90%.
2. Coating and Process Selection:
– Dry indoor environments: Skip multi-layer plating; single nickel plating (15μm) costs 1/3 of Ni-Cu-Ni;
– Complex shapes: Prioritize压制模具 (one-time mold cost $700-$2,100); batch ≥10,000 reduces unit cost by 50% compared to machining.
3. Testing and Certification:
– Provide basic reports for magnetic performance (gaussmeter), high-temperature (oven), and salt spray (corrosion chamber) tests; support third-party certifications (e.g., IATF 16949 for automotive);
– Prototype 3-5 samples to test consistency (Br deviation ≤±1.5%, Hcj ≤±3%) and avoid batch performance fluctuations.
VI. Customization Services: End-to-End Support from Requirement to Delivery
As a direct manufacturer, we offer one-stop customization:
1. Pre-Sales Consultation: Recommend 3-5 candidate grades and coatings based on application (motor power, temp, humidity), with cost comparisons;
2. Structural Design: Free magnetic circuit simulation (Maxwell software) to optimize size and magnetization direction, reducing material use by 20%-30%;
3. Prototyping & Production: Sample lead time 7-15 days, mass production 25-40 days; support low-volume trials ;
4. Quality Control: Batch inspection magnetic performance (gaussmeter + hysteresis graph), coating thickness , dimensional accuracy (CMM), with full traceability reports.
Conclusion
The core of selecting sintered NdFeB magnets lies in balancing “performance, environment, and cost.” By accurately analyzing magnetic parameters, quantifying temperature and protection needs, optimizing structure, and leveraging manufacturer customization, you can transition from “general selection” to “precise adaptation.” Contact us today for a tailored solution, ensuring magnet performance perfectly matches your application requirements.
(Note: Data is based on industry standards; specific parameters may vary by manufacturing process. For custom products, provide drawings or application details—we’ll respond with a plan within 24 hours.）