Special-shaped magnetic assemblies consist of non-standard magnets combined with metal parts, plastic components, shafts, housings, sleeves, adhesives, or other functional components.
Common designs include:
- Arc magnets;
- Sector magnets;
- Multipole magnetic rings;
- Rotor magnets;
- Gear-shaped magnets;
- Magnets with holes or slots;
- Thin-wall magnets;
- Integrated magnet-and-bracket structures.
Compared with standard block or cylindrical magnets, material selection for special-shaped magnetic assemblies is more complex. Engineers must consider not only magnetic force, but also operating temperature, demagnetization risk, corrosion resistance, dimensional accuracy, mechanical strength, manufacturing method, production volume, and total cost.
1. Determine the Magnetic Requirements First
Magnetic material should not be selected only according to surface magnetic flux density. Surface flux is influenced by magnet dimensions, measurement distance, air gap, number of poles, magnetization direction, and surrounding ferromagnetic materials.
Two magnets made from the same material can produce very different surface flux readings if their geometries or magnetic circuits are different.
More reliable design parameters include:
- Remanence, Br;
- Intrinsic coercivity, Hcj;
- Normal coercivity, Hcb;
- Maximum energy product, (BH)max;
- Demagnetization curve;
- Recoil permeability;
- Temperature coefficient;
- Permeance coefficient.
Thin, flat, or open-circuit special-shaped magnets may have a relatively low permeance coefficient and can therefore be more vulnerable to self-demagnetization. In these cases, coercivity and the shape of the demagnetization curve are especially important.
For motor rotors, robotic actuators, and similar assemblies, finite element analysis should be used to evaluate air-gap flux density, back electromotive force, torque, torque ripple, local reverse magnetic fields, and temperature distribution.
Selecting the highest magnet grade does not automatically create the best magnetic assembly.
2. Select Materials According to Operating Temperature
As magnet temperature increases, remanence and coercivity generally decrease. If the operating point crosses the knee of the demagnetization curve, irreversible magnetic loss may occur. The original magnetic performance may not recover completely after the magnet cools.
Engineers should distinguish among:
- Ambient temperature;
- Actual magnet temperature;
- Continuous operating temperature;
- Short-term peak temperature;
- Temperature during overload or fault conditions.
Heat generated by motor windings, bearings, eddy currents, and surrounding metal components can cause the magnet temperature to be significantly higher than the ambient temperature.
| Magnetic material | General magnetic performance | Temperature capability | Corrosion characteristics | Special-shape capability | Typical applications |
| Sintered NdFeB | Very high | Depends strongly on grade | Usually requires coating | Can be machined into arcs, sectors, and drilled shapes | High-power motors, robots, wind turbines |
| Injection-molded NdFeB | Moderate | Determined by powder and binder | Relatively good | Excellent; suitable for integrated molding | Precision rotors, sensors, and 3C products |
| Bonded SmFeN | Moderate to high | Often limited by binder | Relatively good | Excellent | Multipole rings, compact motors, precision assemblies |
| Sintered SmCo | High | Excellent | Good | Machinable but brittle | High-temperature motors, aerospace, sensors |
| Ferrite | Relatively low | Stable | Excellent | Suitable for sintering or injection molding | Appliance motors, speakers, economical assemblies |
| AlNiCo | Moderate to low | Excellent high-temperature stability | Good | Suitable for casting complex structures | Instruments, sensors, high-temperature equipment |
These are general material characteristics. The actual allowable temperature and magnetic performance must be verified according to the specific grade, dimensions, magnetic circuit, and operating conditions.
3. Complex Shapes Require Suitable Manufacturing Processes
Sintered NdFeB offers very high magnetic performance, but it is a hard and brittle material. Thin walls, sharp corners, deep holes, narrow slots, and high length-to-diameter ratios can increase the risks of chipping, cracking, and dimensional variation.
If a complex shape must be produced entirely through grinding, electrical discharge machining, or wire cutting, material utilization may decrease while machining cost increases.
Injection-molded magnets combine magnetic powder with a thermoplastic resin. They can be formed into thin-wall, gear-shaped, multipole, or insert-integrated structures in a single molding operation.
Important advantages include:
- High shape freedom;
- Good dimensional accuracy;
- Stable production consistency;
- Reduced secondary machining;
- The ability to mold around shafts or inserts;
- Suitability for high-volume production;
- Flexible multipole magnetization.
JLMAG Innovation’s injection-molded magnets are designed for applications requiring complex geometry, dimensional consistency, and component integration.
However, the polymer binder reduces the volume fraction of magnetic powder. Injection-molded magnets therefore generally have a lower energy product than sintered magnets. Material selection must balance magnetic performance against structural integration and manufacturing efficiency.
4. Mechanical Loads Determine the Assembly Method
Most permanent magnet materials have relatively good compressive strength but limited tensile, bending, and impact resistance. They should not be treated as conventional structural metals.
Magnets in high-speed rotors may experience:
- Centrifugal force;
- Thermal stress;
- Electromagnetic force;
- Vibration;
- Repeated acceleration and deceleration;
- Shock loads during assembly or operation.
Common fixing methods include:
- Structural adhesive bonding;
- Stainless-steel sleeves;
- Carbon-fiber retaining sleeves;
- Interference fitting;
- Grooves and mechanical locking;
- Injection overmolding;
- Insert molding.
The adhesive must be compatible with both the magnet coating and the substrate. Its shear strength, operating temperature, humidity resistance, oil resistance, curing conditions, and long-term aging performance should be verified.
For high-speed rotors, mechanical stress analysis and overspeed testing should be conducted using an appropriate safety factor. Adhesive strength alone should not be assumed to guarantee rotor safety.
5. Environmental Conditions Determine Coating Requirements
The rare-earth-rich grain-boundary phases in sintered NdFeB are chemically active. In humid, salt-spray, condensation, or chemically aggressive environments, unprotected magnets may corrode.
Common protection systems include:
- Nickel-copper-nickel plating;
- Electrophoretic epoxy coating;
- Zinc plating;
- Phosphating;
- Parylene coating;
- Composite protective coatings.
A coating should not be selected only according to appearance. If the component is adhesively bonded, the coating must provide suitable adhesion. If it is press-fitted, coating thickness, friction, and edge damage must be considered.
For exposure to salt spray, coolant, oil, cleaning agents, or chemicals, application-specific environmental testing is necessary.
SmCo, ferrite, and SmFeN powder have different corrosion characteristics from sintered NdFeB. Nevertheless, shafts, steel housings, adhesive layers, and interfaces may still become failure points. Corrosion resistance should therefore be evaluated for the entire assembly rather than only for the magnet.
6. Material Selection Process for Special-Shaped Assemblies
| Design stage | Parameters to confirm | Possible influence on selection |
| Magnetic definition | Air-gap flux, attractive force, torque, pole number | Material, grade, size, and magnetization pattern |
| Temperature analysis | Continuous temperature, peak temperature, cooling conditions | Hcj grade, SmCo, or heat-resistant binder |
| Structural design | Wall thickness, holes, radii, and tolerances | Sintered machining or injection molding |
| Environmental evaluation | Humidity, salt spray, oil, and chemicals | Material, coating, and sealing |
| Mechanical analysis | Speed, impact, vibration, and centrifugal force | Adhesive, sleeve, or mechanical locking |
| Production planning | Annual volume, tooling cost, and yield | Machining, compression molding, or injection molding |
| Verification | Magnetic flux, demagnetization, life, and reliability | Final grade and safety margin |
7. The Material Must Serve the Entire Magnetic Assembly
If the application requires very high magnetic energy within a limited volume, sintered NdFeB is usually the first material to evaluate.
If the assembly operates at a high temperature and requires stable magnetic performance, SmCo may be more suitable. If the geometry is complex, dimensional accuracy is important, and production volume is high, injection-molded NdFeB or SmFeN may provide greater manufacturing advantages.
For cost-sensitive applications with sufficient installation space, ferrite is often the more economical option. For high-temperature instruments and certain sensing applications, AlNiCo may also be considered.
JLMAG Innovation offers sintered NdFeB, injection-molded magnets, compression-molded magnets, SmFeN, SmCo, ferrite, AlNiCo, and different categories of magnetic assemblies.
For a special-shaped magnetic assembly project, the recommended process is to begin with the application conditions and magnetic circuit objectives. Engineers can then determine the appropriate material, grade, geometry, coating, magnetization direction, and assembly method.
Only by considering magnetic design and structural manufacturing together can a suitable balance be achieved among magnetic performance, reliability, manufacturability, and cost.
