Magnetics Business & Technology - Summer 2012 - (Page 6)

FEATURE ARTICLE By Thomas schliesch, head of ReseaRch & developmenT • max BaeRmmann GmBh Injection Molded Magnets for Electrical Machines Injection molded magnets are applied as rotor and sensor elements for electrical machines in high quantities especially in automotive industry as well as in areas like household appliances, computer components or medical systems. There are specific advantages of injection molded magnets, which effect these high demands. One is that even most complicated geometries can be produced easily. On the other hand, a high variety of field shapes can be manufactured by use of anisotropic magnetic powders or by skillfully designed magnetizing coils. An additional advantage is the relatively high output per time interval, often by use of multi-cavity systems, optimized cycle times and a high grade of automation. The following article will provide some insights into injection molded magnets for electrical machines, especially into materials, manufacturing processes, application systems and magnetic design methods. Injection molded magnets consist of magnetic powders of the commonly known hard magnetic types like Strontium or Barium Ferrites, NdFeB, SmCo or SmFeN. As polymers, usually different grades of polyamides are employed that can link to a high amount of magnetic powder without loosing too much of their flowability. For very high temperature applications PPS based polymers are used instead of polyamides. The amount of magnetic powder being dissolved in the polymer matrix ranges from 50 percent to 65 percent by volume in most cases. Besides the two main components (magnetic powder particles and polymers), small amounts of additives are included in the compound compositions. Their tasks are to improve the particle to polymer adhesion, particle dispersion, orientability as well as temperature stability. Manufacturing is mainly done on commercially available injection molding machines that are usually assembled with units for automatic handling of shafts or plastic inserts to be overmolded, for packing the magnets or for automatic quality control. There are two main lines of manufacturing processes and materials. Products that are most important, at least by the volume of manufactured magnets, are made of hard magnetic Ferrites. In most cases they are oriented and magnetized directly in the mold. Such magnets are usually called pole oriented or field oriented. By implementing assemblies of sintered permanent magnets around the cavities, fields can be applied to the melt, which are strong enough to orient the magnetic particles and to magnetize them into saturation at least at those locations where it is physically required. Magnetic fields may not only be applied by permanent magnetic assemblies but also by use of DC coils. A few Ferrites also need to be magnetized by a pulse discharge processes. The second main line of manufacturing is the production of rare earth magnets. In most cases such magnets are isotropic Materials and Manufacturing Four cavity tool for injection molding pole oriented rotor magnets for stepper motors. ones, some magnets can also be made with unidirectional or multidirectional anisotropy. Injection molded rare earth magnets demand some know how in the design of pulse discharge coils, whereas pole oriented magnets are mainly challenging in regard to the magnetic design of the mold. Types of Magnets and Their Application One of the oldest but still very important applications of injection molded magnets for electrical machines are rotors for stepper motors. As these rotors do not demand a high flux density they can be easily made of Ferrite magnets with pole orientation under alternating circumferential fields. Often these magnets include overmolded metallic shafts or shafts made of polymers. Those systems are used in most cases for positioning purposes in the automotive industry. Demands for rotors for brushless DC motors have increased to high volumes during the last decades. Here, both pole oriented Ferrites and Rare Earth magnets are used, depending on the level of torque and effectiveness that have to be reached. Typical applications are pump motors in automotive industry, motors for fans in household appliances or driving systems for computer peripherals. In water pump systems, it is common use to mold fan elements directly with the magnets. Such magnets are mainly made of PPS bonded Ferrites due to their good temperature resistance and stability against fluids and corrosion. Magnetic rotors for DC motors do not only act as torque producing elements but often provide magnetic signals for commutation by the same field, which interacts with the armature. In other cases, separate magnetic traces www.MagneticsMagazine.com 6 Magnetics Business & Technology • Summer 2012 http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2012

Magnetics Business & Technology - Summer 2012
Editor's Choice
Injection Molded Magnets for Electrical Machines
The Process of Developing and Testing a New Transformer
Rare Earths & Corporate Social Responsibility
Research & Development
RE-use, RE-duce, RE-cycle, RE-place
Magnets • Materials • Measurement
Application • Component Developments
Industry News
Rare Earth Crisis Dissected: Securing the Future
Marketplace
Advertising Index
Spontaneous Thoughts: The End of Axial Pressing?

Magnetics Business & Technology - Summer 2012