Polymer-bonded magnets are valuable for many sensor applications that require the production of unique and reproducible field profiles, not necessarily fields with the highest strength.
In vehicles, for example, a polymer-bonded device can be used to magnetize a rotating soft magnetic gear-wheel. The total field differs, depending on whether a tooth is or is not above the magnet. From this sequence of large and small fields, rotation speed can be extracted. The detailed measurement of the rotation rate determines each wheel’s rotation speed; the data, in turn, then activates the anti-lock brake system (ABS) and tire pressure control systems.
The magnetic field is measured by a sensor such as a Hall, Giant MagnetoResistance (GMR), or Tunneling Magneto-Resistance (TMR) device. An automotive application like the determination of wheel rotation requires the field to be maximized in one direction and in orthogonal directions — as small as possible, to guarantee that the highly sensitive GMR and TMR sensors are not saturated by the external field.
Such a requirement can only be fulfilled with a magnet that is shaped in complicated, complex ways. Tailored magnetic shapes, materials, fields, and orientations support new designs that allow higher manufacturing tolerances for a range of detection technologies, including in-vehicle sensors.
The Role of 3D Printing
3D printing enables a customization of magnets  . To transform a commercial 3D printer to a small-sized magnets manufacturer, however, requires the replacement of the filament. The polymer must be switched with a mainly magnetic material.
Researchers at the Magnetic Sensing and Materials laboratory at TU Wien, a university based in Vienna, Austria, used a compound of isotropic neodymium iron boron (NdFeB) particles inside a PA11 matrix. The magnet printer employed specially produced filaments of the magnetic microgranulate, joined by a polymer binding material.
The NdFeB parts are initially demagnetized; each particle has approximately zero net magnetization. Scanning Electron Microscope (SEM) images show in Fig. 1b the size and shape of the NdFeB particles. The spherical particles have a diameter of approximately 50±20 μm. The compound exhibits a remanence Br of 400 mT and a coercivity of Hcj of 630 kA/m.
The 3D printer, a commercially available fused deposition modeling (FDM) device, then uses the filament and manufactures the structure, layer by layer, into a meltable thermoplastic. The filament, heated above its softening point, is extruded through a movable nozzle. The object is built up in layers on the printer bed’s already solidified material. Finally, an object is obtained, built out of magnetic material very similar to that of standard polymer-bonded magnets.
The maximum building size of the printer is an object measuring 220 × 210 × 164 mm (L×W×H), with a layer height resolution of 0.05 — 0.3 mm. The 0.4-mm-diameter nozzle is fed with filaments. Fig. 1a shows the 3D printer setup. After the printing process, the magnet must be magnetized by an external magnetic field — a pulse coil with a force of 4 T.
The Vienna-based researchers fabricated a magnet used in a speed/wheel sensor application  . By employing a pyramidal-shaped cavity within the top part of the magnet (as seen in Fig. 2a-b), the team achieved the application’s specific magnetic requirements: a large field in one direction and small field in the other. A picture of the printed magnet is shown in Fig. 2b.
The 7 × 5 × 5.5 mm (L×W×H) sensor featured a layer height of 0.1 mm. A comparison of the measured stray field (Fig. 2a) and a simulated field assuming perfect magnetic properties is shown in Fig. 2c; conformity is achieved between the printed and simulated magnet.
The 3D printing system can extrude two different filaments, which enables a gradual change in magnetic properties. Hence, a continuous change from a magnetic material to a non-magnetic is possible as a function of space.
As an example, Fig. 3a shows magnetic parts (grey regions) printed directly next to a non-magnetic polymer. The measured field is shown in Fig 3b. Such capability allows the creation of magnets that generate, for example, a linear decaying field along the magnet.
A linear decaying field assists position sensing applications that require an increased linear range. Position sensing — essential for robots to control position, orientation, or motion of the robotic joints — also supports structural health monitoring in bridges or buildings to monitor stress and potential cracks.
In future work, the researchers plan to locally magnetize, or even align, particles during the solidification process. Magnets can then be generated with properties that cannot be produced by other means. Additionally, new designs can be employed to create Halbach arrays or undulators — high-energy physics technologies that require local and varied magnetization directions to bend electronic beams.
The capability of printing magnets with complex shapes and internal magnetization profiles will open the path for new magnetic designs and applications that do not exist yet, since they are impossible to fabricate with state-of-the-art methods.
This article was written by Dieter Süss, Head of the Christian-Doppler Advanced Magnetic Sensing and Materials laboratory at University of Vienna (Vienna, Austria). For more information, Click Here.
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