Research on Binder of Binded NdFeB Magnets from
Injection Molding
Wan zhong Qin
Tianjin Vocational Institute, Tianjin 300410, China
Keywords: Binder; injection molding; NdFeB magnet ; mechanical property; magnetic property .
Abstract: The injection molding of magnetic powder could cost-effectively mass-produce net shape magnets. The
important influence of binder to the molding process and applied properties of injection molding magnet
was discussed. Based on the test and analysis of physical and mechanical properties of the three type
important binders, the effect of binder on the magnetic property, molding process property and mechanical
property of injection molding magnets were investigated. The injection molding bonded NdFeB magnets
with better combined properties were prepared. The research and develop directions of the binder were
Magnetic material is one of the important functional
materials. As third generation permanent magnetic
material, NdFeB rare-earth permanent magnetic
magnet has been extensively used in computer,
move-phone, acoustics equipment, electric motor
and so on; because of its small volume, small
weight, high magnetic property, high properties-
price ratio etc[1]. NdFeB magnets include sintered
NdFeB and bonded NdFeB. The production
technologys of bonded NdFeB magnet include
pressing molding, injection molding and extruding
molding mainly. Pressing molding has been used for
mass-production. The injection molding of NdFeB
magnets have many features such as accurate
dimensional tolerances, high productivity, low
production cost, combined molding and easy achieve
complicated geometries, so it has very good
developing prospects[2]. But the injection molding
of NdFeB magnets not only need to resolve the
oxidation and hardly flow problems in melt mixing
and injection molding of NdFeB magnetic powder,
but also to reduce wearing of NdFeB magnetic
powder with the screw and the mould, so lead to the
injection molding difficult[3-4]. At present, the
research on bonded NdFeB magnets prepared by
injection molding is in start stage. The properties of
three type binders and the effects of binder on the
molding process and applied properties of injection
molding bonded NdFeB magnets were investigated
in this test.
Bonded magnet is a compound material that consists
of permanent magnetic powder, binder and
additives. The magnetic property of magnet is
offered by permanent magnetic powder; but the
shape, size, strength, corrosion resistance and
fluidity for molding of magnet are offered by the
binder mainly; so it is very important to research and
develop the binder. The two main effects of binder
in injection molding magnet are: Make flowing
and turning of magnetic particle possible when
magnet molding. To bonding the magnetic
powder together effectively to offer shape and
mechanical property for magnet[5-6].
To see macroscopically, the injection molding
process of NdFeB magnet exactly is the flowing
process of magnetic particles. The fluidity of alloy
powder is bad which decided by it’s shape and
outward; but the high polymer binder possess of
good fluidity at liquid state and the flowing demand
of magnetic particle can be satisfied, furthermore it
has definite shape and mechanical property. The
viscosity of binder must be suitable. When the
viscosity is too high, although the magnetic
mechanical property is good, but the fluidity is bad
that will hinder magnetic particle from turning and
orienting and make the mold cavity filling failure,
then damage the magnetic property and outward of
magnet. When the viscosity is too low, the magnetic
powder will be separated from binder and lead to
injecting process failure. To see microcosmically,
bonding exactly is a mechanical linking of magnetic
powder and binder. Which include liquid state
binder moistens solid state magnetic powder and
forming physical and chemical bonding force
between them. So the properties which the binder
should possess as follow.
2.1 The Injection Technology Property
of Binder
The binder should possess excellent fluidity for
injection molding technology, so we can add stearate
or polyolefine to decrease the fluidity of injection
molding mix. The molding temperature of resin
must be suitable, and the space between melting
temperature and thermo-division temperature should
be great enough to ensure molding operation,
magnetic property, magnet quality and productivity.
2.2 The Physical and Chemical
Property of Resin Binder
The resin absorbs moisture in the moist environment
and lead to rare-earth permanent magnetic powder
corroded, so the easy absorb moisture resin need to
be modified to rise it’s corrosion resistance. Besides
the important employing property of binder such as
heat-resistance, environment-resistance, ageing-
resistance can be modified if necessary.
2.3 To Ensure the Size Accuracy of
Comparing with other forming technology, the
injection molding NdFeB magnet can achieve better
size precision, but the affect elements more and
complicated such as the shrinkage rate, fluidity,
expanding coefficient of binder, and the shrinking
rate with remarkable affect among them. The
difference of shrinkage rate for different binder is
great, large shrinkage rate benefit to raise density
and magnetic property of magnet, but small
shrinkage is better to ensure the size precision.
2.4 Effects of Binder to Injection
Molding NdFeB Magnet Properties
According to the magnetic property of ideal
permanent magnet, the magnetic property of bonded
magnet can be expressed as follow:
Br = P · A · μ
(BH)max = Br
/ 4
In the expressionBr —Br of bonded magnet
(BH)max—(BH)max of bonded magnet
—Br of ideal magnetdensity is 100%
orienting degree is 100%
P —percentage volume of magnetic powder in
bonded magnet
A —orienting degree
Show in the expression, the magnetic property
of bonded permanent magnet decided by the density
and orienting degree of magnet. The binder is no-
magnetic material and binder and pore decrease the
magnet density, and decrease the magnetic property
of magnet certainly. So the content of binder should
be as low as possible when it satisfied the
technology and employ property.
The injected magnet ensure certain pressure
when forming, if the compression density of pure
magnetic powder is dp(g/cm3) and the real density is
dt(g/cm3), then the volume percentage of magnetic
powder is Vf :
Vf = dp / dt×100%(Vo.%)
The volume content of magnetic powder in
magnet should below this value, and the remained
space should be filled by binder. In fact the content
of binder is more because the fluidity of compound
needs to be considered sufficiently. This section
must be in two
2.5 Effects of Binder to Injection
Molding Nd FeB Magnet
Mechanical Properties
The mechanical property of bonded magnet is
decided by the type and content of binder. Although
increase magnetic powder loadage can rise magnet
magnetic property, but lead to the mechanical
property decrease. Besides adding sufficient binder,
we need to modify the binder to ensure the
mechanical property of magnet. For example add
plasticizing agent, strengthening agent, coupling
agent to gain high strength, good toughness and
pretty outward appearance.
2.6 The Main Component of Injection
Molding NdFeB Magnet
The binder is different according to the uses of
injection molding magnet. Generally it is high
polymer binder consist of a few composition that
include resin, coupling agent, plasticizing agent,
strengthening agent, stabilizing agent, antioxidant,
burning resistance agent etc.. Resin and coupling
agent are necessary compositions, the others can be
selected according to the technology and property.
Resin is the basic and most important
composition of binder, generally it is synthetic high-
molecular compound, include resinoid and
thermoplastic resin. Only very rarely do we find that
resinoid is used for binder because of it’s difficult
control of injection technology and difficult
reprocessing of the solidified pouring system
material. But the thermoplastic resin without
shortcoming above, and can gain fairly good
fluidity, accurate production size, fairly good
magnet property. Generally the resins can be used
for binder are: PA, PPS, PE, PBT, PP, PC and so on,
and PA and PPS are the most in use. All of them are
engineering plastics that with very low viscosity and
excellent injection molding property at melting state
and can ensure maximum loadge of magnetic
Coupling agent is a high-molecular additive that
one side of the molecular contain unit reacting with
NdFeB and the other side contain unit reacting with
binder, so the inorganic material and organic
material can be coupled by chemical bond. Besides
the NdFeB powder is extremely easy oxidized when
the temperature more than 150, then lead to
magnetic property decrease rapidly, so the magnetic
powder need anti-oxidation treatment. After the
surface treated by coupling agent, the property of
NdFeB magnetic powder can keep stabilization at
high temperature and 100% humidity environment,
and the anti-oxidation property of the magnet made
by the magnetic powder raised remarkably.
For the binder PA, heat-resistance PA, and PPS:
According to the ASTM methods, drying the binder
and injection molding the testing samples use
injecting machine and accurate mould, then testing
the physical and mechanical property of binder use
special equipments.
For injection molding magnet: The ferroxcube
magnetic powder particle is fine and no high
temperature oxidation problem, so the injection
molding is easy relativity. The MQP-B NdFeB
magnetic powder is adjusted on particle size, coated
with coating agent and surface treated with coupling
agent; then is mixed with drying binder, lubricant,
antioxidant according to different loadage of
magnetic powder in high speed mixer. And then the
compound is melted and mixed and extruded in the
double screw extruder and cutting. At last, injecting
mold appropriate property testing samples use
injecting machine and accurate mould. According to
the Chinese GB method, test physical property,
mechanical property and magnetic property of
magnet by special equipments.
4.1 Polyamide Resin Binder
Polyamide resin is a type of high-molecular
compound which contain many repeated amide unit
(-NH-CO-) in the molecular main chain and is
popular named PA. PA is made from
polyiondensation reaction of carbonic acid and
amine, or polymerization reaction of amide, so it can
be classified as many kinds according to the unit for
polymerization reaction, for example PA6, PA66,
PA12, PA6/66, PA6/12 etc.. PA is a thermoplastic
resin and with excellent strength, toughness, heat-
resistance property, wear-resistance, corrosion-
resistance property and so on, and is nonpoisonous.
So it is used as engineering plastics extensively.
PA have the specific property of suck up
moisture because it contain the amide unit which is
close to moisture. The saturation moisture capacity
(at 23) of PA6, PA12, PA66, PA6/6T are 9.5%,
1.8%, 8.5%, 7% respectively. So the moisture
capacity of PA is need to pay special attention when
design magnet product. PA need to be modified
when used for injection molding magnet binder by
add additives, for example plasticizing agent,
strengthening agent, stabilizing agent, lubricant etc.
to modify the strength, toughness, heat-resistance,
fluidity, molding property etc.. The properties of PA
are shown in table 1.
Table1: General physical properties of polyamide resin.
In the production of injection molding permanent
magnet, use polyamide resin as binder have
excellent blending processing property, forming
property, physical property, magnetic property
compared with other thermoplastic resin.
Characteristic of injection molding NdFeB magnet is
shown in table 2.
Table 2: Characteristics of injection molding NdFeB
4.2 Heat-resisting Polyamide Resin
To raise the properties of bonded magnet, study on
permanent magnetic powder is important no doubt,
but research on binder polymer is fairly important
too. In the field of automobile, electrical appliances
and electronic industry, besides magnetic
characteristics, the magnet generally need to possess
high-level heat-resistance, durability, heat-impact
resistance, size stability, high-strength, forming
property, reprocessing property etc., and many of
these properties are related to binder. The bonded
permanent magnet is applying to automobile field
along with the high-property and low-cost change of
rare-earth permanent magnetic powder. In the
automobile engine room, the working temperature
can achieve to 130, so the magnet should can long
time working at -40~140. Only heat-resistance
polyamide can meet the demand above. Polyamides
include fat-family and benzenoid-family two types.
The benzenoid-family PA have higher heat-
resistance and strength than fat-family PA(include
PA6, PA12, PA66) because it contain more upright
and strong benzene molecular chain. The heat-
resistance of benzenoid-family PA is higher than
PA12 and lower than PPS. The testing result is
shown in table 3, T
is glass transition temperature,
is melting temperature, GF% is glass fiber
content added.
Table 3: Heat-characteristics of polyamide.
4.3 Poly Phenylene Sulfide Resin
Poly phenylene sulfide is a engineering plastic with
high property. At the initial developing stage, the
PPS was cross-linking type resin and too brittle to
use although it have many excellent properties. In
recent years, the new developed PPS is straight
chain type resin and changed the brittle property
remarkably. The typical property of PPS is shown in
table 4.
Table 4: Characteristics of PPS.
Comparing with other binder, the characteristics
of new-type PPS as follow: Good heat-resistance:
heat distortion temperature is more than 260 and
can be continuous using at 200~220 ℃;High
strength and rigidity: with high tensile strength,
flexural strength, flexural modulus; Excellent size
stability, linear expansion coefficient extreme low,
forming shrinkage small; Excellent corrosion
resistance: no solvent can dissolve PPS below 200
; Forming process property and fluidity fine.
The properties of injection molding magnet by
different binder are shown in table 5 (isotropic
ferroxcube). The new-type PPS resin have the sticky
and firm characteristic which the straight chain type
macromolecule peculiar. Comparing with the
injection molding magnet bonded by PA, the magnet
bonded by PPS has characteristics as follows:
The same magnetic feature; Hardness and
flexural modulus high; Excellent heat-resistance;
The same tensile strength and flexural strength;
Small linear expansion coefficient and forming
shrinkage; Small moisture capacity and heat
expansion coefficient. It is expected that more PPS
bonded injection molding magnet will be applied in
automobile because it has excellent heat-resistance.
Table 5: Property of injection molding magnet.
The application prospects of injection molding
NdFeB magnet is very promising and the magnet
output will increase rapidly. In view the important
function of binder, the research and develop
direction of binder should be concentrated on the
following aspects.
To develop binder with better bond strength
and fluidity. One of the main reason to decrease
bonded magnet magnetic property is the adding of
binder, so the increasing of (BH)max need to
develop better binder to reduce the binder content
and raising the orienting degree of magnetic particle.
To develop new type additive and related
treatment technology to raise the coupling property
of NdFeB magnetic powder and binder, and to
modify forming technology and property of injection
molding magnet.
Proper treating the magnetic powder with
physical or chemical method before mixing the
NdFeB magnetic powder and binder to modify
surface state of magnetic powder and its combing
force with binder. At the same time the magnetic
property is raised too.
To develop heat-resistance binder. Injection
molding NdFeB magnet applied in automobile will
be a vast market, but one of the key problems need
to be solved is the heat-resistance of magnet. The
heat-resistance is mainly decided by binder, so the
developing of heat-resistance binder provoke
extensive interest of related people.
This work was financially supported by the Tianjin
Natural Science Foundation (08JCYBJC14000).
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