Note: Descriptions are shown in the official language in which they were submitted.
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POWDER MIXTURE FOR POWDER METALLURGY AND
METHOD OF MANUFACTURING SAME
TECHNICAL FIELD
[00011 This disclosure relates to a powder mixture for powder metallurgy, and
particularly to a powder mixture for powder metallurgy which can be ejected
from a die with less force at the time of compaction and suppresses die
galling.
In addition, this disclosure relates to a method of manufacturing the powder
mixture for powder metallurgy.
BACKGROUND
[00021 Powder metallurgy includes forming a raw material powder mainly
composed of an iron-based powder into a green compact using a die, and
subjecting the green compact to sintering to obtain a sintered part. In order
.. to improve the formability at the time of compaction, it is usual to add a
lubricant to the raw material powder and to make the lubricant adhere to the
surface of the die used in the compaction. If no lubricant is used, the
iron-based powder contained in the raw material powder is in direct contact
with the die, which increases friction. As a result, a desired green density
cannot be obtained at the time of compaction, and a large force is required
for
ejecting the green compact from the die after the compaction.
[00031 For these reasons, various lubricants are used in green compacting.
Examples of the lubricant include metal soaps such as lithium stearate and
zinc stearate, and amide-based lubricants such as ethylenebis stearamide.
[00041 Furthermore, JP 2005-330547 A (PTL 1) proposes using a graphite
powder to improve the lubricity. When the surface of the iron-based powder
is covered with graphite, the lubricity of the surface of the iron-based
powder
is improved. In addition, direct contact between the iron-based powder and
the die is avoided by interposing graphite therebetween, so that die galling
is
prevented.
CITATION LIST
Patent Literature
[00051 PTL 1: JP 2005-330547 A
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SUMMARY
(Technical Problem)
[0006] As proposed in PTL 1, using an iron-based powder which is covered
with a graphite powder can reduce friction at the time of compaction and
reduce the force required for ejecting the compact from the die. However, it
was found that the powder mixture of PTL 1 has the following problems.
[0007] PTL 1 uses fluid dispersion, in which graphite and a binder are
dispersed in water or an organic solvent, to cover the surface of an iron-
based
powder with a graphite powder, and therefore it requires production
equipment capable of handling liquid raw materials. In particular, it is
necessary to provide an apparatus for recovering and processing spent solvent.
[0008] In addition, PTL 1 uses a binder to adhere the graphite powder to the
iron-based powder. As a result of investigating the powder mixture obtained
with the above-described method, however, it was found that the binder is also
present on the surface of the graphite powder adhering to the iron-based
powder. Due to the presence of the binder on the powder surface, the
flowability of the powder mixture cannot be sufficiently improved.
[0009] It could thus be helpful to provide a powder mixture for powder
metallurgy which is extremely excellent in flowability, can be ejected from a
die with less force, and suppresses die galling at the time of compaction. In
addition, it could be helpful to provide a method of manufacturing the powder
mixture for powder metallurgy without using any solvent.
(Solution to Problem)
[0010] We engaged in intensive studies on the above problems and made the
following discoveries.
[0011] (1) In the case where a raw material powder, a graphite powder, and a
binder are simultaneously mixed, the surface of the graphite powder is also
covered with the binder, and it is impossible to uniformly cover the outermost
surface of the raw material powder with graphite.
[0012] (2) By covering the surface of a raw material powder with a binder and
then with a fine graphite powder, it is possible to prevent the surface of the
graphite powder from being covered with the binder. In this way, it is
possible to obtain a powder mixture for powder metallurgy which is extremely
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excellent in flowability, can be ejected from a die with less force at the
time of
compaction, and suppresses die galling.
100131 The present disclosure is based on the above discoveries and has the
following primary features.
100141 1. A powder
mixture for powder metallurgy, comprising a raw
material powder, a binder, and a graphite powder, wherein
the raw material powder contains an iron-based powder in a content of
90 mass% or more of the raw material powder,
the binder is at least one selected from the group consisting of fatty
acid amide and copolymerized polyamide,
a melting point of the binder is 81 C or higher,
the graphite powder has an average particle size of less than 5 lam,
a ratio of mass of the binder (mb) to the sum of mass of the raw material
powder (mr) and mass of the graphite powder (mg), expressed as [mb / +
mg) x 1001, is 0.10 mass% to 0.80 mass%,
a ratio of mass of the graphite powder (mg) to the sum of mass of the
raw material powder (mr) and mass of the graphite powder (mg), expressed as
[mg / (m, + mg) x 1001, is 0.6 mass% to 1.0 mass%,
surface of the raw material powder is covered with at least a part of the
binder,
surface of the binder covering the surface of the raw material powder
is covered with at least a part of the graphite powder and,
surface of the graphite powder covering the surface of the binder is not
covered with any binders.
100151 2. The
powder mixture for powder metallurgy according to 1,
wherein the raw material powder contains at least one auxiliary raw material
selected from the group consisting of alloying powder and machinability
improvement powder.
100161 3. A
method of manufacturing a powder mixture for powder
metallurgy according to 1, consisting of:
first mixing where a raw material powder and a binder are mixed at a
temperature equal to or higher than a melting point of the binder to obtain a
powder covered with binder, and
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second mixing where the powder covered with binder and a graphite
powder having an average particle size of less than 5 lam are mixed at a
temperature equal to or higher than the melting point of the binder to obtain
the powder mixture for powder metallurgy, wherein
the raw material powder contains an iron-based powder in a content of
90 mass% or more of the raw material powder,
a ratio of mass of the binder (mb) to the sum of mass of the raw material
powder (mr) and mass of the graphite powder (mg), expressed as [mb +
mg) x 1001, is 0.10 mass% to 0.80 mass%,
a ratio of mass of the graphite powder (mg) to the sum of mass of the
raw material powder (mr) and mass of the graphite powder (mg), expressed as
[mg/ (m, + mg) x 1001, is 0.6 mass% to 1.0 mass%,
a melting point of the binder is 81 C or higher,
the binder is at least one selected from the group consisting of fatty
acid amide and copolymerized polyamide, and
in the powder mixture for powder metallurgy,
surface of the raw material powder is covered with at least a
part of the binder,
surface of the binder covering the surface of the raw material
powder is covered with at least a part of the graphite powder and,
surface of the graphite powder covering the surface of the
binder is not covered with any binders.
[0017] 4. The
method of manufacturing the powder mixture for powder
metallurgy according to 3, wherein the raw material powder contains at least
one auxiliary raw material selected from the group consisting of alloying
powder and machinability improvement powder.
(Advantageous Effect)
100201 The powder mixture for powder metallurgy of the present disclosure has
extremely excellent flowability. Therefore, it can be ejected from a die with
less
force at the time of compaction, and at the same time, the compaction can be
performed continuously without causing die galling. In this way, the yield
rate of
green compacts is improved, and high productivity can be realized. In
addition,
according to the manufacturing method of the present disclosure, the powder
mixture
for powder metallurgy can be manufactured without using any solvent.
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DETAILED DESCRIPTION
100211 The following describes the present disclosure in detail.
100221 The powder mixture for powder metallurgy of the present disclosure
contains
a raw material powder, a binder, and a graphite powder as essential
components.
Each of the components is described below.
100231 [Raw material powder]
The raw material powder is a powder containing an iron-based powder.
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The ratio of the iron-based powder in the raw material powder is 90 mass% or
more, and more preferably 95 mass% or more. On the other hand, the upper
limit of the ratio of the iron-based powder in the raw material powder is not
particularly limited, and may be 100 mass%. That is, the raw material
powder may consist only of an iron-based powder. However, from the
viewpoint of imparting various properties to a final sintered body, it is
preferable to use a mixed powder containing an iron-based powder and an
auxiliary raw material described later as the raw material powder.
100241 [Iron-based powder]
The iron-based powder is not particularly limited, and may be any
powder. Examples of the iron-based powder include iron powder (so-called
pure iron powder) and alloyed steel powder. The alloyed steel powder is
preferably at least one selected from the group consisting of: pre-alloyed
steel
powder (completely alloyed steel powder) obtained by pre-alloying an
alloying element during smelting; partial diffusion-alloyed steel powder
obtained by partially diffusing and alloying an alloying element in an iron
powder; and hybrid steel powder obtained by further partially diffusing an
alloying element in a pre-alloyed steel powder. Note that the "iron-based
powder" here refers to a metal powder having an Fe content of 50 mass% or
more, and the "iron powder" refers to a powder consisting of Fe and inevitable
impurities.
100251 The method of producing the iron-based powder is not limited, and an
iron-based powder produced with any method may be used. Examples of the
iron-based powder that can be suitably used include atomized iron-based
powder produced by atomization, and reduced iron-based powder produced by
reduction.
100261 The average particle size of the iron-based powder is not particularly
limited, but preferably 70 i_tm to 100 jim. Note that the particle size of the
iron-based powder is a value measured with the dry sieving method based on
JIS Z 2510: 2004 unless otherwise specified.
100271 [Auxiliary raw material]
The auxiliary raw material is not particularly limited, and may be any
material such as one generally used as an auxiliary raw material in powder
metallurgy. The auxiliary raw material is preferably at least one selected
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from the group consisting of alloying powder and machinability improvement
powder. Generally, the alloying powder may be a metal powder. The metal
powder, for example, is preferably at least one selected from the group
consisting of Cu powder, Ni powder, and Mo powder. The machinability
improvement powder may be MnS, for example. The ratio of the auxiliary
raw material in the raw material powder is 10 mass% or less.
[0028] [Binder]
The surface of the raw material powder is covered with at least a part
of a binder. The binder may be anything as long as it can adhere a graphite
powder to the surface of the raw material powder. For example, it may be at
least one selected from the group consisting of fatty acid amide such as fatty
acid monoamide and fatty acid bisamide, and organic resin. Among the
above, it is preferably an organic resin, and more preferably at least one
resin
selected from the group consisting of copolymerized polyamide, polyurethane,
and polyethylene.
[0029] Addition amount of binder: 0.10 mass% to 0.80 mass%
When the addition amount of the binder is less than 0.10 mass%, the
binder cannot sufficiently cover the surface of the raw material powder.
Therefore, the addition amount of the binder is 0.10 mass% or more. On the
other hand, when the addition amount of the binder exceeds 0.80 mass%, the
binder also covers the surface of the graphite powder, which lowers the
flowability. Therefore, the addition amount of the binder is 0.80 mass% or
less. The addition amount of the binder here is defined as the ratio of mass
of the binder (mb) to the sum of mass of the raw material powder (mr) and
.. mass of the graphite powder (mg), i.e. [mb/(mr + mg) x 100]. In other
words,
when the total mass of the raw material powder and the graphite powder is 100
parts by mass, the mass of the binder is 0.10 parts by mass to 0.80 parts by
mass.
[0030] The binder is preferably in the form of powder. When the average
particle size of the binder is less than 5 Jim, the cost for grinding the
binder to
such a size increases and the raw material cost increases. Therefore, from
the viewpoint of reducing the cost, the average particle size of the binder is
preferably 5 1.1m or more. On the other hand, when the average particle size
of the binder exceeds 100 txm, the time required for uniformly mixing the
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binder with the raw material powder increases, which decreases the
productivity. Therefore,
from the viewpoint of further improving
productivity, the average particle size of the binder is preferably 100 gm or
less.
100311 When the melting point of the binder is 60 C or higher, it is possible
to prevent the flowability of the powder mixture from being lowered even, for
example, in the summer season with a high temperature. Therefore, the
melting point of the binder is preferably 60 C or higher. On the other hand,
when the melting point of the binder exceeds 160 C, the time and energy
required for heating the powers to a temperature equal to or higher than the
melting point increase, which decreases the productivity. Therefore, from
the viewpoint of further improving productivity, the melting point of the
binder is preferably 160 C or lower.
100321 [Graphite powder]
The surface of the binder covering the surface of the raw material
powder is covered with at least a part of a graphite powder. In other words,
the surface of the raw material powder is covered with a graphite powder via
the binder. When the surface of the iron-based powder is covered with a
graphite powder via the binder, the lubricity of the surface of the iron-based
powder is improved. In addition, direct contact between the iron-based
powder and a die is avoided by interposing graphite powder therebetween, so
that the iron-based powder does not adhere to or deposit on the surface of the
die, which leads to less die galling.
100331 Average particle size of graphite powder: less than 5 p.m
Generally, the particle size of a graphite powder used in powder
metallurgy is about 5 11M to 201.tm. On the other hand, an iron-based powder
generally has an average particle size of about 70 1..tm to 80 1.1.m and at
most
about 250 jim. When the particle sizes of the graphite powder and the
iron-based powder are in such a relationship, it is difficult to uniformly
cover
the surface of the iron-based powder with the graphite powder. In the
present disclosure, the average particle size of the graphite powder is less
than
5 [tm in order to uniformly cover the surface of the raw material powder
containing the iron-based powder with the graphite powder. On the other
hand, the lower limit of the average particle size of the graphite powder is
not
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particularly limited. However, if the particle size is too small, the energy
required for grinding is increased, which is economically disadvantageous.
Therefore, the average particle size of the graphite powder is preferably 100
nm or more.
[0034] Addition amount of graphite powder: 0.6 mass% to 1.0 mass%
When the addition amount of graphite is less than 0.6 mass%, the
graphite powder cannot sufficiently cover the outermost surface of the
iron-based powder, and the surface of the iron-based powder is exposed.
Therefore, in order to sufficiently obtain the effect of graphite powder
covering, the addition amount of the graphite powder should be 0.6 mass% or
more. On the other hand, although the graphite powder is finally consumed
by carburizing at the time of sintering to improve the properties of the
sintered body such as the strength, the properties of the sintered body are
deteriorated when the addition amount of the graphite powder exceeds 1.0
mass%. Therefore, the addition amount of the graphite powder is 1.0 mass%
or less. The addition amount of the graphite powder here is defined as the
ratio of mass of the graphite powder (mg) to the sum of mass of the raw
material powder (mr) and mass of the graphite powder (mg), i.e. [mg / (mr +
mg) x 100].
[0035] [Manufacturing method]
Next, the method of manufacturing the powder mixture for powder
metallurgy will be described. The manufacturing method according to one
embodiment of the present disclose includes: first mixing where a raw
material powder and a binder are mixed at a temperature equal to or higher
than the melting point of the binder to obtain a powder covered with binder,
and second mixing where the powder covered with binder and a graphite
powder having an average particle size of less than 5 pm are mixed at a
temperature equal to or higher than the melting point of the binder to obtain
a
powder mixture for powder metallurgy.
100361 If the binder and the graphite powder are mixed in advance, the
viscosity of the binder increases. As a result, it is difficult to uniformly
cover the surface of the raw material powder with the binder. Therefore, the
surface of the iron-based powder is covered with the binder first and then
with
the graphite powder. In this way, it is possible to uniformly cover the
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surface of the raw material powder only with the binder. In view of the
above, it is preferable to add only the binder to the raw material powder and
mix them in the first mixing. Furthermore, it is preferable, without adding
any more binder, to add only the graphite powder to the raw material powder
covered with the binder (the powder covered with binder) and mix them in the
second mixing.
[0037] In addition, if the surface of the raw material powder is covered with
the binder and the graphite powder simultaneously, the surface of the graphite
powder is also covered with the binder, so that the effect of graphite powder
covering cannot be sufficiently obtained. Therefore, the graphite powder
covering is performed after the binder covering to prevent the surface of the
graphite powder from being covered with the binder. In other words, in the
powder mixture for powder metallurgy obtained with the method of the
present disclosure, the surface of the raw material powder is uniformly
covered with the graphite powder adhered via the binder. In addition, the
binder is hardly exposed on the surface of the raw material powder particle
and the graphite powder is on the outer side, so that the flowability and the
ejectability at the time of molding are excellent.
[0038] The mixing means of the first mixing and the second mixing is not
particularly limited, and any means may be used, such as various known
mixers. From the viewpoint of facilitating heating, it is preferable to use a
high-speed bottom stirring mixer, an inclined rotating pan-type mixer, a
rotating hoe-type mixer, or a conical planetary screw-type mixer.
[0039] The mixing temperature during the first mixing and the second mixing
is equal to or higher than the melting point (T.) of the binder used. In the
case of using a plurality of binders with different melting points, the
highest
one of the melting points of the plurality of binders used is taken as the Tm.
The mixing temperature is preferably T. + 20 C or higher, and more
preferably Tm + 50 C or higher. On the other hand, although the upper limit
of the mixing temperature is not particularly limited, it is preferably Tm +
100
C or lower, because a too high mixing temperature leads to problems such as
deterioration in production efficiency and oxidization of iron-based powder.
[0040] The powder mixture obtained as described above can be used to
produce a sintered body by powder metallurgy. The method of producing the
_
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sintered body is not particularly limited, and may be any production method.
However, the powder mixture for powder metallurgy is usually filled into a
die, compacted, optionally subjected to sizing, and then sintered. Generally,
the compacting is performed in a temperature range from room temperature to
180 C. In the case where it is particularly necessary to increase the density
of the green compact, warm forming can be employed so that the powder and
the die are both preheated and then subjected to forming. Furthermore, the
obtained sintered body may be optionally subjected to heat treatment such as
carburizing-quenching, bright quenching and induction hardening to obtain a
product (such as a mechanical component).
[0041] Moreover, in the present disclosure, either or both of an additional
auxiliary raw material and a lubricant may be optionally added to the powder
mixture for powder metallurgy after the second mixing. The additional
auxiliary raw material may be similar to the above-described auxiliary raw
material contained in the raw material powder. The lubricant is preferably a
lubricant which is not an organic resin, and more preferably at least one
lubricant selected from the group consisting of fatty acid, fatty acid amide,
fatty acid bisamide, and metal soap.
EXAMPLES
[0042] The composition and function effects of the present disclosure are
described in more detail below, by way of examples. Note that the present
disclosure is not limited to the following examples.
[0043] Powder mixtures for powder metallurgy were prepared according to
the following procedure. First, a raw material powder and a binder were
mixed by a high-speed bottom stirring mixer while being heated to a
predetermined mixing temperature (first mixing). The raw material powder
was a raw material powder containing iron powder (atomized iron powder
JIP301A manufactured by JFE Steel Corporation) as an iron-based powder and
Cu powder as an auxiliary raw material. The type of binder used, the
addition amount of each component, and the mixing temperature are listed in
Table 1.
[0044] Next, a graphite powder was further added to the high-speed bottom
stirring mixer, and mixed while being heated to the mixing temperature
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(second mixing). After completing the mixing, the obtained powder mixture
for powder metallurgy was discharged from the mixer. The graphite powder
was a commercially available graphite powder having an average particle size
listed in Table I.
[0045] For comparison, instead of adding a graphite powder in the second
mixing, the graphite powder was added in the first mixing in some
comparative examples. Furthermore, in some comparative examples (No.
17), the powers were mixed at room temperature without being heated in the
first mixing or the second mixing. In No. 17,
since the mixing was
performed without heating, the surface of the raw material powder was not
covered with the binder or the graphite powder.
[0046] Next, the powder mixtures for powder metallurgy thus obtained were
each subjected to flow rate measurement and green compact pressing
according to the following procedure.
[0047] (Flow rate)
The obtained powder mixture for powder metallurgy in an amount of
50 g was filled into a container having an orifice diameter of 2.5 mm, and the
time period from filling to discharging was measured to determine the flow
rate (unit: s/50 g). The other measurement conditions were in accordance
with JIS Z 2502: 2012. The flow rate is an index indicating the flowability
of the mixed powder at the time of die filling, and a smaller value of flow
rate
means better flowability of the mixed powder. In some
comparative
examples, the powder mixture for powder metallurgy did not flow and was not
discharged from the orifice.
[0048] (Pressing)
During the pressing, the powder mixture for powder metallurgy was
pressed using a die to obtain a green compact having a diameter of 11.3 mm
and a height of 11 mm. The pressure during the pressing was 686 MPa.
The force required for ejecting the green compact from the die (ejection
force)
and the green density of the obtained green compact (average of the green
compact) were measured. Die
galling occurred in some comparative
example so that the compaction could not be performed.
[0049] The measurement results are listed in Table 1. As can be seen from
this result, the powder mixture for powder metallurgy satisfying the
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conditions of the present disclosure is extremely excellent in flowability,
can
be ejected from a die with less force, and suppresses die galling at the time
of
compaction.
,Z7
Table 1
CD
CA
Fast mixing Second mixing
Measurement result 41:)
Raw material powder Binder Graphite powder
Cu powder
Iron-based Graphite Mixing
(auxiliaty raw No. Green
Ejection
Average temperature
Flow rate Remarks
powder powder Melting Addition Addition
density force material) particle r
Type point amount *3 amount 02
sue C) (sI50g)
(81991) (MPa)
Addition Addition ( C) (mass%) (mass%)
Type amount . I amount *2 (PrI)
(mass%) (mass%)
1 301A 2 0.8 Copolymerized polyamide 116 0.4 .
4 170 30.0 7.09 16.1 Comparative example
2 301A 2 - Copolymerized polyamide 116 0.4 0.8
4 , 170 23.4 7.15 13.7 Example
3 , 301A 2 0.8 Copolymerized polyamide , 116 0.6
- 4 170 Not flowing 7.04 18.8 Comparative example
4 _ 301A 2 - Copolymerized polyamide 116 0.6 0.8 4
170 27.1 7.11 13.0 Example 0 I ,. 301A 2 0.8
Polyurethane 90 0.4 - 4 140 Not flowing 7.09 14.6
Comparative example u4 . o
6 301A 2 , Polyurethane 90 0.4 0.8 4
140 25.1 7.17 11.9 Example .c.
o
cn
7 301A 2 0.8 Polyurethane 90 0.6 4 140
Not flowing 7.04 16.5 Comparative example iv
-
I
1.
8 30IA 2 . Polyurethane , 90 0.6 0.8 4
140 28.5 7.11 10.9 Example -- o io
I
9 30IA 2 0.8 Polyethylene . 99 0.6 4 140
Not flowing 7.10 15.3 Comparative example .. - i 1-4
1-4
301A 2 .. Polyethylene 99 0.6 08 4 140
28.6 7.15 12.7 Example r
.
co
11 30IA Copolymerized polyainide 116 0.3 0.6
4 170 24.2 7.16 12.5 Example
12 301A 2 . Copolymerized polyamide 116 aos
0.8 4 , 170 Not flowing Not compacted due to galling
Comparative example
_ 13 301A 2 Copolymerized polyamide 116 1.0 0.8
.. 4 170 Not flowing 7.01 18.2 Comparative example _
_ _
14 301A 2 - Copolymerized polyarnide 116
0.6 03 4 170 Not flowing Not compacted due to
galling Comparative example
30IA 2 _ Copolymerized polyamide 116 0.6 1.2 4
170 27.4 7.05 10.8 Comparative example
_
16 301A 2 , Copolymerized polyamide 116 0.6
0.8 17 170 Not flowing Not compacted due to galling
Comparative example
17 301A 2 , Copolymerized polyamide 116 0.5
0.8 4 Room temperature 28.9 7.00 17.7 Comparative
example
/8 _ 301A 2 - Stearic acid amide 100 0.8 0.8
4 125 30.2 7.11 12.8 Example
19 30IA 2 - Erucic acid amide 81 0.8 0.8
4 110 31.2 7,13 12.1 Example
,. 301A 2 - Ethylenebis stearic acid amide , 145
0.8 0.8 4 160 28.8 7.12 13.9 Example
21 30IA 2 - Ethylenebis oleic acid amide 119 0.8
0.8 4 140 29.2 7.14 11.3 Example
_
.1 Mass of Cu powder / (mass of iron-based powder + mass of Cu powder 4-
mass of graphite powder) * 100
.2 Mass of graphite powder I (mass of iron-based powder + mass of Cu powder
+ mass of graphite powder) x 100
.3 Mass of binder / (mass of iron-based powder + mass of Cu powder + mass
of graphite powder) x 100
