FAQ – Bonded Magnet Materials

Is Bonded Nd-Fe-B material an alternative for ceramic magnets when greater performance is needed?

Bonded Nd-Fe-B material produces almost twice as much flux density as anisotropic ceramic magnets (grades 5 – 8), so substituting Bonded Nd-Fe-B material is a fairly safe way to extract greater performance from a device. Resistance to demagnetization for Bonded Nd-Fe-B material is also twice that of ceramic, so the Bonded Nd-Fe-B part can be half as thick as the ceramic magnet without a loss of performance. If possible, the steel parts onto which the Bonded Nd-Fe-B magnet is mounted should be twice as thick to carry the additional flux efficiently.

What is the highest bonded Sm-Co energy product? What temperature can it handle?

New grades of bonded Sm-Co are being added periodically. One popular grade is 17, with an energy product of 16.5 MGOe, Br=8.75 kG, Hc=6.5 kOe, and Hci=11 kOe.  The functional temperature limit will be set by the thermoplastic binder used to make the magnet, around 400 °F.

What is the maximum service temperature of Bonded Nd-Fe-B  material?

The maximum would be 150 °C (300 °F), because the bonder holding the magnet together softens. You might be able to push slightly higher than this, but consult a Dexter Applications Engineer first.

What is the minimum thickness that a bonded magnet can be pressed to?

Answer As a general rule, figure on 0.090″ [approximately 2 mm].

What kind of uniformity can we expect from Bonded Nd-Fe-B material?

Magnetic uniformity often exceeds the MMPA standard of +/-8% but every job is different so don’t use this for every case. Within a specific lot of material, the uniformity will be very good, but lot-to-lot variations will at a minimum meet the MMPA standard.

What manufacturing process variables affect bonded material uniformity?

Pressure & density variations and wear of the tool. The magnetizing fixture can be a factor as well.

What tolerances can be held on a pressed bonded Nd-Fe-B part? For the initial run for a new tool, figure on +/-.005″. We have to see how pieces will “spring-back” after ejection from the die. After the characteristics of the new tooling is figured out, the tolerances can shrink to +/-.002″.

Why are injection molded magnets weaker than compression molded magnets?

Compression molded magnets have much more magnet material in them than injection molded magnets, so they are inherently stronger. Compression molded magnets are made from a mixture with a high concentration of magnet powder and a small amount of epoxy as a binder. This mixture is then compressed to a high density before curing the epoxy. Injection molded magnets are made by mixing the magnet powder with a plastic that is liquid enough to inject into a mold, and pressures are lower, so injection molded parts have a much lower concentration of magnetic powders and are therefore weaker.

Why should I convert to bonded Nd-Fe-B magnet material?

Bonded Nd-Fe-B magnets can increase performance or reduce the size of devices made from anisotropic ceramic (grades 5 – 8) magnets. The material provides almost two times the flux and resistance to demagnetization. Substituting bonded Nd-Fe-B magnets then gives 4 times greater energy density if substituted directly, or the bonded Nd-Fe-B based device can be significantly smaller than its ceramic counterpart. Bonded Nd-Fe-B material is also isotropic, so it can be magnetized in any desired direction. The magnets are made from a compression molded material, so it can be molded to finished size as opposed to ceramic, which suffers shrinkage, and must be ground after pressing and firing. Some sintered Sm-Co devices are inefficient and could perform as well at a lower cost by substituting a pressed-to-size bonded Nd-Fe-B part.