Magnet Types & Processes
Bunting-DuBois specializes in manufacturing magnets and technical assemblies using a variety of processes. The most significant feature of our bonded magnet processes is our ability to precisely target the magnetic performance and utilize net-shaping techniques, allowing us to achieve even the most complicated shapes. We give you exactly what you need to achieve the level of performance that is needed for your specific application. We offer a number of different performance levels, but in some cases a precise blend is created to give you the most effective solution.
Processes, Performance & Shaping
Based on techniques used in ceramics, powder metal and pharmaceuticals, Compression Molded requires the lowest level of non-magnetic binder, thus producing the highest magnetic performance in bonded magnets. The use of compression bonded is almost exclusively restricted to rare earth alloy products such as Neodymium Iron Boron (NdFeB).
The shaping limitations of compression molding are found in the need to eject a part of the tool. The most common shapes are 2-D rings, but compression molded is also available in disks, blocks and segments. The binders used in compression molding are typically epoxy resins, which have good dimensional stability and resistance to chemicals. Some are able to withstand temperatures in excess of 250°C.
The maximum energy product achievable is typically 12 MGOe for Isotropic product, while anisotropic (aligned with a field during molding) grades of samarium high cobalt give even higher performance and enhanced temperature stability.
Injection Molded is ideal when additional precision or complexity of shape is required. It is also used where insert or over-molding is beneficial. This technique is used for high volume production because of the relatively high tooling and equipment cost.
Binders include a range of nylons, polyphenylene sulfides and other polymers for operating temperatures excess of 180°C. The loading of magnetic powder that can be incorporated is lower than compression molding, resulting in lowered magnetic properties.
For ferrite powders, the upper limit is approximately 2MGOe, while isotropic NdFeB is up to 6MGOe and bonded isotropic NdFeB is up to 12MGOe.
Injection Molded can offer significant savings and improvements in precision for applications where magnet components are molded directly on to shafts or where additional assembly operations can be removed from a process.
Magnet Manufacturing Process
There are several processes for making magnets, but the most common method is called Powder Metallurgy. In this process, a suitable composition is pulverized into fine powder, compacted and heated to cause densification via “liquid phase sintering”. Therefore, these magnets are most often called sintered magnets. Ferrite, Samarium Cobalt (SmCo) and neodymium-iron-boron (neo) magnets are all made by this method. Unlike ferrite, which is a ceramic material, all of the rare earth magnets are metal alloys.
How SmCo and Neo Magnets are Made
Suitable raw materials are melted under vacuum or inert gas in an induction melting furnace. The molten alloy is either poured into a mold, onto a chill plate, or processed in a strip caster – a device that forms a thin, continuous metal strip. These cured metal “chunks” are crushed and pulverized to form a fine powder ranging from 3 to 7 microns in diameter. This very fine powder is chemically reactive, capably of igniting spontaneously in air and therefore must be protected from exposure to oxygen.
There are several methods for compacting the powder and they all involve aligning the particles so that in the finished part all the magnetic regions are pointing in a prescribed direction. The first method is called axial or transverse pressing. This is where powder is placed into a cavity in a tool on the press and punches enter the tool to compress the powder. Just prior to compaction, an aligning field is applied. The compaction “freezes-in” this alignment. In axial (parallel) pressing, the aligning field is parallel to the direction of compaction. In transverse (perpendicular) pressing, the field is perpendicular to the compaction pressure. Because the small powder particles are elongated in the direction of magnetic alignment, transverse pressing yields better alignment, thus a higher energy product. Compacting powder in hydraulic or mechanical presses limits the shape to simple cross-sections that can be pushed out of the die cavity.
A second compaction method is called isostatic pressing wherein a flexible container is filled with powder, the container is sealed, an aligning field is applied, and the container is placed into the isostatic press. Using a fluid, either hydraulic fluid or water, pressure is applied to the outside of the sealed container, compacting it equally on all sides. The main advantages to making magnet blocks via isostatic pressing is that very large blocks can be made – frequently up to 100 x 100 x 250 mm and since pressure is applied equally on all sides, the powder remains in good alignment producing the highest possible energy product.
Pressed parts are packaged in “boats” for loading into a vacuum sintering furnace. The particular temperatures and presence of vacuum or inert gas is specific to the type and grade of magnet being produced. Both rare earth materials are heated to a sintering temperature and allowed to densify. SmCo has the additional requirement of a “solutionizing” treatment after sintering. After reaching room temperature, both materials are given a lower temperature tempering heat treatment. During sintering, the magnets shrink about 15-20% linearly. Completed magnets have a rough surface and only approximate dimensions. They also exhibit no external magnetic field.
Sintered magnets receive some degree of machining which can range from grinding them smooth and parallel, OD or ID grinding, or slicing block magnets into smaller parts. The magnet material is both brittle and very hard (Rockwell C 57 to 61) and requires diamond wheels for slicing and diamond or special abrasive wheels for grinding. Slicing can be performed with excellent precision often eliminating the need for subsequent grinding. All of these processes must be conducted very carefully to minimize chipping and cracking.
In some cases, the final magnet shape is conducive to processing with a shaped diamond grinding wheel such as arcs and bread loafs. Product in approximate final shape is fed past the grinding wheel which provides the precise dimensions. For lower volume manufacturing of these complex shapes, EDM machining is commonly used. Simple two-dimensional profiles, EDM is faster while more complex shapes using 3-5 axis machines run slower.
Cylindrical parts may be pressed-to-shape, usually axially, or core-drilled from block stock material. These longer cylinders, either solid or with an ID, can later be sliced to form thin washer-shaped magnets.
- Secure a trash bag. …
- Hold pins while sewing. …
- Corral paper clips. …
- Stick up kids’ cups. …
- Add removable pizzazz to a lamp shade. …
- Fix a drafty door. …
- Organize your makeup. …
- Store aluminum foil and plastic wrap on the fridge.
Magnets are essential to enabling technologies of medical equipment, such as MRI
machines, and also play a part in medical technology such as magnetic
switches, blood separators, magnets used to assist in extracting foreign
objects from patients, and motors used in surgical and dental devices.
A bit more thought reveals that many more magnets are lurking elsewhere
in their home. They are installed in anything containing an electric
motor, from the washing machine to the hair dryer, from the kitchen
blender to any appliance containing a cooling fan. Most loudspeakers and
microphones contain magnets. And if you own a modern car, it will
almost certainly be packed full of magnets, inside sensors measuring
liquid levels, monitoring wheel speed and seat-belt status, running
motors for the wipers, windows, sun-roof and starter, the pump for
windscreen washer, not to mention applications in the dashboard
instrumentation and loudspeaker system.
Although intricate magnet shapes can be produced from these alloys, the materials are best suited for simpler shapes. Holes, large chamfers or slots are more costly to produce. Tolerances are more difficult to hold on more complex shapes which are likely to result in flux field variations and potential physical stressing of the part in an assembly.
Machined magnets will have sharp edges which are prone to chipping. Coating around a sharp edge is also problematic. The most common method for reducing the sharpness is a vibratory hone, often called vibratory tumbling and done in an abrasive media. The specified rounding of the edge depends upon subsequent processing and handling requirements but is most often 0.005” to 0.015” (0.127 to 0.38 mm) radius
Neo magnets, which are prone to rusting or reacting chemically, are almost always coated. Samarium cobalt is naturally more corrosion resistant than neo, but does, on occasion benefit from coating. The most common protective coatings include dry-sprayed epoxy, e-coat (epoxy), electrolytic nickel, aluminum IVD, and combinations of these coatings. Magnets can also be coated with conversion coatings such as zinc, iron or manganese phosphates and chromates. Conversion coatings are generally adequate for temporary protection and can form an under-layer for epoxy coating or an over layer to enhance protection from aluminum IVD.