ferrite magnets are manufactured using injection
moulding techniques which are fully automated
and particularly suitable for high volume
production. Isotropic and anisotropic properties
can be obtained in parts with a wide variety of
shapes and sizes. One of the greatest advantages
of the injection moulding process is that a high
degree of shape complexity is possible, eg.
gears, snap fits and undercuts. It is also
possible to incorporate shafts, bushes and other
inserts into the process thereby greatly
simplifying magnetic assemblies.
Bonded ferrite materials are composite
materials consisting of ferrite powders
(BaO.6Fe2O3 or SrO.6Fe203) dispersed in an
engineering thermoplastic matrix. Polyamides are
the most commonly used as the matrix materials
in bonded ferrite compounds, but for
applications operating above 120ˇăC,
polyphenylene sulphide (PPS) can be used (up to
Tailoring of the desired magnetic performance
can be achieved by varying the proportion of
ferrite powder to polymer matrix. Very high
volume fractions of magnet powder are possible.
The magnetic temperature stability is
controlled by the ferrite powder, with
coefficients of Br = -0.19 %/ˇăC and HcJ = + 0.3
%/ˇăC. With very low temperatures there is a
risk of permanent de-magnetisation in magnet
systems with low working points. Corrosion
resistance of both the ferrite and matrix
material is good.
Processing begins with the controlled mixing and
dispersal of the ferrite powder into
molten thermoplastic matrix material. This
mixing is conducted on highly specialised state
of the art compounding equipment because of the
high volume fraction of filler being used. The
resultant pelletised compound is the feedstock
for the injection moulding process.
Injection moulding is carried out on
specially adapted machines. The process commonly
operates fully automatically with components
being collected and separated directly from the
moulding machine. Anisotropic properties can be
achieved in axial, radial, diametrical or
multipolar orientations during moulding with the
presence of the appropriate magnetic fields in
the mould tool.
Very precise dimensional control is achieved
in this process and the component should not
require further machining.
Components may be magnetized before final
inspection, packing and despatch.