Note: Descriptions are shown in the official language in which they were submitted.
1075567
FIEI.D OF TIIE INVENTION
This invention relates to a method for the manufacture
of permanent magnets by comminuting a substance of high magnetic
crystal anisotropy to a powder, mixing the powder with a binder,
applying a magnetic field to the mixture and treating the
mixture by the application of pressure and vibration to produce
a molded body.
BACKCROUND OF THE INVENTION
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Methods of the above kind for the manufacture of
permanent magnets are known, for example, from Swiss Patent
Specification 516,217, especially Column 4, paragraphs 2, 3 and 4
They lead to comparatively mechanically stable molded bodies. -
secause of the high viscosity of the resln mixture, however,
the alignment of the particles in the magnetic field is greatly
impeded. The high viscosity results from the high proportion
of the mixture consis~in~ of a very finely pu]verized
magnetizable substance.
Subsequent impregnation of an already manufactured
compressed molded body with particular solvents or epoxide resins -
such as is described in Swiss Patent Specification 500,573,
has also not proved very successful in practice, since, on
the one hand, it cannot be carried out for small bodies
(e.g., lx 1 x 1 mm) and, on the other hand, it is not economical
for larger parts, especially for mass production.
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SUMMARY OF THE PRESENT INVENTION
~ It is a principal object of the invention to provide
a method for the manufacture of permanent magnets which does not
suffer from the disadvantages of known methods of manufacture
and is superior to these in technical and/or economic respects.
According to the present invention there is provided a
method for the manufacture of permanent magnets comprising the
steps of comminuting a substance of high magnetic crystal
anistropy to form a powder mixture, mixing the powder mixture
with a binder, and applying a magnetic field to the resultant
mixture with the binder and processing the resultant mixture in
a mold to produce a molded body while using pressure and vibratior ,
the pressure and vibration being simultaneously applied during
the molding process. The binder is either composed of a material
which softens with increase in temperature above a predetermined
temperature or of a material which hardens with increase in
temperature above a predetermined temperature. The mold is held
at a temperature below the predetermined temperature for
softening when a binder which softens with increasing temperature
is used. The mold is held at a temperature above the predetermin~ d
temperature for hardening when a binder which hardens with
temperature is used.
For a better understanding of the present invention,
reference is made to the accompanying drawing and detailed
description of various embodiments of the invention, while the
scope of the invention will be pointed out in the amended claims.
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BRIEF DESC~IPTION OF THE DRAWING
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The drawing represents, in partially schematic
representation, apparatus for performing the method of the
present invention.
DETAIL_D DESCRIPTION OF THE INVENTION
The apparatus shown in the drawing comprises essentially
a press consisting of an upper press ram (upper die) 1, a press
cylinder (mold) 2 and a lower press ram (ejector) 3, the rams 1
and 3 being operatively coupled with air-pressure cylinders 4
and 5, respectively. The upper press ram 1 is constructed as
an ultrasonic resonator, which is fed by an ultrasonic generator 6
The press cylinder 2 is surrounded by a magnetizing coil 7. With
the exception of the magnetizing coil, the construction of the
apparatus corresponds more or less to a known plastics ultrasonic
welding machine.
The powder mixture consisting of magnetic powder and
binding medium is filled loosely into the mold 2. Suitable
binding media are plastics powders, such as,for example,
PPS (polyphenyl sulfide, melting point 290C, density 1.34 g/cm3),
PA-11 (poIyamide 11, melting point 185C, density 1.04 g/cm3),
PE (polyethylene, melting point about 110 - 110C, density
0.91 ).94 g/mm3) or polyester thermoplastics, such as that
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~075567
marketed under the Trade Mark "Ultradur" by BASF (melting point
225 - 260C, density 1.29 - 1.37 g/cm3). All the plastic
powders named above are commercially available and can be
supplied in different gra-in sizes. Additionally, low melting-
point metals or metal alloys, with melting points not exceeding
300C, for example, pure tin, Wood's metal and the like can be
employed as binding media. Practically all magnetic materials
known in the present state of the art, such as are described,
for example in Swiss Patent Specification No. 616,217, with
grain sizes between 5 and 50 microns, are suitable for use as
the magnetic powder.
After the powder mixture has been filled into the mold,
a pre-compression of the powder mixture is effected with the
help of the upper press ram 1, without the use of ultrasonic
energy but with the simultaneous application of a magnetic field
of not less than 5,000 Oersteds. After the withdrawal of the
upper press ram 1, the precompressed tablet is brought flush
with the upper margin of the mold 2 by actuation of the lower
press ram 3. The upper press ram 1 is again applied and the
tablet is compressed for about 1 second with the simultaneous
application of pressure (200 kp/cm2), ultrasonic ener~y
(amplitude of oscillation not less than 30 microns) and aligning
magnetic field. ~fter a cooling time of some 2 seconds, the
completed magnetic body may be removed from the mold by the
renewed actuation of the lower press ram 3.
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107S567
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It has been found that powder mixtures using polyethylene
as the binding medium lead to the best results. It flows-readily
into the mold, any surplus being squeezed out laterally adjacent
the upper press ram. The completed magnets may readily be
removed from the mold. Since polyethylene is a comparatively
soft plastics material, the completed magnets may likewise be
readily worked. However, powder mixtures using tin as the -
binding medium also lead to high quality magnet bodies, even
though the completed, pressed magnet bodies tend to stick to
the rams or mold, which may, however, be easily avoided by
appropriate choi~e of the materials from which the press tools
are made.
Relatively homogeneous magnetic bodies may be made up
to magnet heights of some 2 mm for metallic binding media and
some 3 to ~ mm for plastics materials. For greater heights,
difficulties arise, since it is difficult to produce a homogeneou
distribution of energy in the blank.
The method according to the invention, as has been
described above, is particularly suitable for the manufacture of
cobalt-rare earth magnets, but may, however, also be employed
for Al-Ni-Co magnets. The powder mixture, consisting of the
comminuted magnetic substance and solid or liquid binding medium,
is introduced into the mold cavity as a powder, a mass or a shape
tablet and is treated in the magnetic field with the assistance
of pres re and vibration, preferably ultrasonic vibration.
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For thermoplastic binding media, the mold temperature should bebelow the solidification (freezing) point of the thermoplastic
material. The mold is kept below the "solidification" point
in order to prevent melting of the resin particles before
application of pressure and vibration. Because of the oscillator Y
absorption of the thermoplastic in the ultrasonic range
(20 kHz - 40 kHz), the thermoplastic material melts. Similar
conditions apply for metals when used as the binding medium.
Here, the metal is brought to its melting point by the ultrasonic
energy.
On the other hand, for hardenable synthetic materials
(thermosetting materials), the mold temperature should be higher
than the hardening (setting or curing) temperature of the
thermosetting material. Through application of the ultrasonic
oscillation, the synthetic resin is heated and the hardening
process is thus initiated. In this case, the energy supplied by
ultrasonic vibration must be stopped immediately after the curing
process is completed. Otherwise, the already compacted and
solidified molded body would be destroyed again mechanically.
In addition to the introduction of heat and the change
in condition of the binding medium associated therewith, the
oscillatory treatment has the effect of making comparatively
high-viscosity mixtures more fluid, so that the alignment of the
particles in the magnetic field is considerably facilitated.
At the same time, the packing density of the màgnetic material
is increased.
10755~;7
The described method of manufacture of magnetic bodies
is particularly suitable for mass production because of the
short handling times, and provides high-quality magnetic bodies
that are easy to machine and to manufacture. The method of
manufacture is particularly simple if the binding medium and
the magnetic powder are not mixed from separate fractions
before filling into the mold, but magnetic powder with an existinc
coating of a binding medium is employed. Magnetic powder of
this kind is known from the aforementioned Swiss Patent
Specification 516,217, especially, Example 10 (acrylic binding
medium). However, magnetic powder provided with a coating of
tin or some other melting point metal may also be employed.
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While the invention has been particularly shown and
described with reference to the preferred embodiments thereof,
it will be understood by those skilled in the art that various
modifications and changes in form and detail may be made therein
without departing from the true spirit and scope of the present
invention.
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