U.S. patent application number 14/181982 was filed with the patent office on 2014-08-21 for powder mixture.
This patent application is currently assigned to Hitachi Chemical Company, Ltd.. The applicant listed for this patent is Hitachi Chemical Company, Ltd.. Invention is credited to Teruo TESHIMA, Tadayuki TSUTSUI, Masaki YANAKA.
Application Number | 20140230603 14/181982 |
Document ID | / |
Family ID | 51305265 |
Filed Date | 2014-08-21 |
United States Patent
Application |
20140230603 |
Kind Code |
A1 |
YANAKA; Masaki ; et
al. |
August 21, 2014 |
POWDER MIXTURE
Abstract
A powder mixture of the present invention is characterized in
that it comprises: an iron powder or an iron-based alloy powder,
wherein a graphite powder is adhered to a surface of the iron
powder or the iron-based alloy powder by a binding agent containing
a polyolefin wax; and a negatively-charged powder to be mixed with
the iron powder or the iron-based alloy powder, and consisting of
an iron powder and/or an iron-based alloy powder treated for
negatively charging. This enables keeping the good adhering
property of a graphite powder in a mixed powder and high
flowability as a mixture of the raw material powder.
Inventors: |
YANAKA; Masaki;
(Matsudo-shi, JP) ; TSUTSUI; Tadayuki;
(Matsudo-shi, JP) ; TESHIMA; Teruo; (Nasu-gun,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hitachi Chemical Company, Ltd. |
Tokyo |
|
JP |
|
|
Assignee: |
Hitachi Chemical Company,
Ltd.
Tokyo
JP
|
Family ID: |
51305265 |
Appl. No.: |
14/181982 |
Filed: |
February 17, 2014 |
Current U.S.
Class: |
75/255 |
Current CPC
Class: |
B22F 1/0062 20130101;
B22F 1/0003 20130101; B22F 1/0014 20130101; B22F 2003/023 20130101;
B22F 1/0081 20130101; B22F 3/004 20130101 |
Class at
Publication: |
75/255 |
International
Class: |
B22F 1/00 20060101
B22F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 18, 2013 |
JP |
2013-028703 |
Claims
1. A powder mixture comprising: an iron powder or an iron-based
alloy powder, wherein a graphite powder is adhered to a surface of
the iron powder or the iron-based alloy powder by a binding agent
containing a polyolefin wax; and a negatively-charged powder to be
mixed with the iron powder or the iron-based alloy powder, and
consisting of an iron powder and/or an iron-based alloy powder
treated for negatively charging.
2. The powder mixture according to claim 1, wherein the polyolefin
wax is at least one selected from the group consisting of
polyethylene wax and polypropylene wax.
3. The powder mixture according to claim 1, wherein the treatment
for negatively charging is coating treatment with at least one
selected from the group consisting of an alkylsilane,
dialkylsilane, methacrylsilane, alkylhalide silane, and
hexaalkyldisilazane.
4. The powder mixture according to claim 1, wherein the maximum
particle diameter of the negatively-charged powder is within the
range from 1 to 15 .mu.m.
5. The powder mixture according claim 1, wherein an additive amount
of the negatively-charged powder is from 0.02 to 0.5 parts by mass
relative to 100 parts by mass of the iron powder or the iron-based
alloy powder.
6. The powder mixture according to claim 1, wherein the powder
mixture further comprises an auxiliary raw material powder, and the
auxiliary raw material powder is adhered together with the graphite
powder by the binding agent to the surface of the iron powder or
the iron-based alloy powder.
7. The powder mixture according to claim 1, wherein the powder
mixture further comprises an auxiliary raw material powder, and the
auxiliary raw material powder is contained in a form as being
adhered together with the graphite powder by the binding agent to
the surface of the iron powder or the iron-based alloy powder as
well as in a form of free powder.
8. The powder mixture according to claim 1, wherein the powder
mixture further comprises an auxiliary raw material powder, and the
auxiliary raw material powder is contained in a form of free
powder.
9. The powder mixture according to claim 6, wherein a part of the
auxiliary raw material powder is an auxiliary raw material powder
treated for negatively charging, and the negatively-charged powder
consists of the auxiliary raw material powder treated for
negatively charging instead of the iron powder and/or the
iron-based alloy powder treated for negatively charging.
10. The powder mixture according to claim 6, wherein a part of the
auxiliary raw material powder is an auxiliary raw material powder
treated for negatively charging, and the negatively-charged powder
further contains the auxiliary raw material powder treated for
negatively charging in addition to the iron powder and/or the
iron-based alloy powder treated for negatively charging.
11. The powder mixture according to claim 1, wherein the powder
mixture further comprises a lubricant powder for compacting, and
the lubricant powder for compacting is contained in a form of free
powder.
12. The powder mixture according to claim 7, wherein a part of the
auxiliary raw material powder is an auxiliary raw material powder
treated for negatively charging, and the negatively-charged powder
consists of the auxiliary raw material powder treated for
negatively charging instead of the iron powder and/or the
iron-based alloy powder treated for negatively charging.
13. The powder mixture according to claim 8, wherein a part of the
auxiliary raw material powder is an auxiliary raw material powder
treated for negatively charging, and the negatively-charged powder
consists of the auxiliary raw material powder treated for
negatively charging instead of the iron powder and/or the
iron-based alloy powder treated for negatively charging.
14. The powder mixture according to claim 7, wherein a part of the
auxiliary raw material powder is an auxiliary raw material powder
treated for negatively charging, and the negatively-charged powder
further contains the auxiliary raw material powder treated for
negatively charging in addition to the iron powder and/or the
iron-based alloy powder treated for negatively charging.
15. The powder mixture according to claim 8, wherein a part of the
auxiliary raw material powder is an auxiliary raw material powder
treated for negatively charging, and the negatively-charged powder
further contains the auxiliary raw material powder treated for
negatively charging in addition to the iron powder and/or the
iron-based alloy powder treated for negatively charging.
Description
TECHNICAL FIELD
[0001] The present invention relates to a powder mixture used as a
raw material powder in a powder metallurgy process, particularly in
a die pressing method. In more detail, the present invention
relates to the powder mixture in which the powder is prevented from
segregation and the flowability of mixed powder is excellent.
BACKGROUND ART
[0002] As illustrated in FIG. 4, manufacture of a sintered machine
part by the powder metallurgy process is performed by filling with
raw material powder the cavity formed by a die hole 11 of a die 10
and a lower punch 20, followed by compacting the raw material
powder between the lower punch 20 and a upper punch 30 to form a
compact (so called the die pressing method) and heating to sinter
the compact obtained in a sintering furnace. Such a die pressing
method has an advantage of not only forming the sintered machine
part in a near net shape but also allowing mass production of
products with the same shape after a die for pressing is once
manufactured, thereby lowering the manufacture cost. Therefore, the
die pressing method is utilized in various fields.
[0003] The raw material powder used in the die pressing method is
prepared by selecting a main raw material according to the
properties desired for a machine part targeted and by adding an
auxiliary raw material powder to the main raw material powder to
mix. For example, in a machine part for structure the raw material
powder is prepared by using as the main raw material powder an iron
powder or an iron-based alloy powder, and adding graphite powder
and an auxiliary raw material powder such as copper powder, nickel
powder or a small amount of iron-based alloy powder with different
component from the main raw material powder as needed, as well as
adding a lubricant for compacting such as zinc stearate and mixing
them. In the raw material powder, a machinability improving powder
such as magnesium silicate type mineral powder or sulfide powder is
also used as the auxiliary raw material powder as needed in order
to improve the machinability of machine parts.
[0004] Incidentally, the raw material powder is generally stored in
a hopper 40, from which the powder is conveyed by free fall under
gravity through a hose 50 into a feeder 60 with the open bottom. As
the feeder 60 moves over the die 10 to the position above the
cavity formed of the die hole 11 and the lower punch 20, the cavity
is filled with the raw material powder from the open bottom of the
feeder 60. Therefore, the raw material powder requires the high
flowability in order to keep smooth filling properties as well as
to prevent variation in the amount of powder to be filled.
[0005] As described above, the raw material powder is prepared by
adding the graphite powder and the auxiliary raw material powder as
needed to the main raw material powder and mixing them so that the
powder mixture is constituted with the powders different in size,
shape, and specific gravity. As segregation of the graphite powder
and the auxiliary raw material powder occurs, the powder has
variation in composition causing change of dimension and large
variation in properties such as strength and the like to yield
defective products. Therefore, it is desired to prevent the
graphite powder and the auxiliary raw material powder from
segregation. Particularly since the density of graphite powder is
smaller than the density of the iron powder or the iron-based alloy
powder as the main raw material powder, the graphite powder with a
smaller specific gravity is likely to scatter upwards when the
powder slides in the hopper 40 caused by filling, so that it is
strongly required to prevent the graphite powder from
segregation.
[0006] With regard to such a problem of segregation various
proposals are offered such that segregation is prevented by melting
the lubricant for compacting to adhere the graphite powder and the
auxiliary raw material powder to the surface of the main raw
material powder (such as Patent Literature 1) or by adding a binder
component to adhere the graphite powder and the auxiliary raw
material powder to the surface of the main raw material powder
(such as Patent Literature 2).
CITATION LIST
Patent Literature
[0007] [Patent Literature 1] JP Hei01-219101 A [0008] [Patent
Literature 2] JP Hei02-217403 A
SUMMARY
Technical Problem
[0009] In various proposals above graphite powder and the auxiliary
raw material powder such as copper powder, nickel powder, and the
like is not sufficiently adhered to the surface of the main raw
material powder, and even when the graphite powder and the
auxiliary raw material powder are sufficiently adhered to the
surface of the main raw material powder, the flowability of the raw
material powder is reduced. Therefore, it is desired to develop the
raw material powder for powder metallurgy, in which the graphite
powder and the auxiliary raw material powder are sufficiently
adhered to the surface of the main raw material powder and the
flowability of the raw material powder is high. From such a
background the purpose of the present invention is to provide a
powder mixture in which segregation of graphite powder is prevented
and the powder properties such as flowability are excellent.
Solution to Problem
[0010] In order to solve the problem above the present inventors
carried out extensive study and found that the adhesive property is
excellent in the powder obtained as follows.
(1) First, in order to adhere a graphite powder and the auxiliary
raw material powder to the surface of the main raw material powder,
a binding agent (polyolefin wax) and the main raw material powder
are agitated and temperature is elevated to the melting point or
higher of the polyolefin wax to coat the surface of the main raw
material powder with the melted polyolefin wax. (2) Next, powder is
produced by adding the graphite powder and the auxiliary raw
material powder and lowering temperature to the melting point or
lower of the polyolefin wax while agitating to adhere the graphite
powder and the auxiliary raw material powder to the surface of the
main raw material powder through the polyolefin wax.
[0011] However, the flowability of the raw material powder in which
the graphite powder and the auxiliary raw material powder are
adhered to the surface of the main raw material powder by the
melting and mixing method above is reduced. The present inventors
investigated a cause of reduction of the flowability and obtained
the findings as follows.
[0012] That is, as schematically illustrated in FIG. 3 binding
agent 4 is delaminated from parts of the surface of main raw
material powder 1, graphite powder 2, and auxiliary raw material
powder 3 in some of powder when agitated and mixed, making it
difficult to completely coat the main raw material powder 1 with
the binding agent 4. Incidentally, as illustrated in FIG. 4 while
the cavity is filled with the raw material powder fed from the
hopper 40 through the hose 50 and the feeder 60, the raw material
powder flows in contact and friction with each other therewhile as
well as parts of the raw material powder are conveyed in contact
and friction with the inner wall of the hopper 40, the inner
surface of the hose, and the inner wall of the feeder.
[0013] FIG. 2 illustrates the state of the electrification by
friction thereat (Each letter and numeral is same as those in FIG.
3). Since the polyolefin wax contained in the binding agent 4 is a
substance which is likely to be negatively charged and has
electrical insulation properties, the polyolefin wax is negatively
charged on the surface thereof by friction when conveyed. Copper
powder used as the auxiliary raw material powder is also a
substance which is likely to be negatively charged as well as
binded in the electrically insulated state to the iron powder or
the iron-based alloy powder through the polyolefin wax with
electrical insulation properties so that the surface of copper
powder exposed at the top is negatively charged by friction when
conveyed.
[0014] On the other hand, the iron powder or the iron-based alloy
powder as the main raw material powder is likely to be positively
charged by friction when conveyed and the exposed surface is
positively charged by friction when conveyed. As illustrated in
FIG. 2(a) it is considered that the surface of the negatively
charged polyolefin wax of other powder or the surface of the
negatively charged copper powder are electrically attracted to the
exposed part of the positively charged iron powder or the
iron-based alloy powder to agglutinate, thereby reducing the
flowability.
[0015] Then, the present inventors consider that as illustrated in
FIG. 2(b) addition of the negatively-charged powder 5 with a high
negative charge causes selective attraction of the
negatively-charged powder with a high negative charge to the
exposed part of the positively charged iron powder or iron-based
alloy powder preventing other powder from attraction and preventing
agglutination as well as further improving the flowability of the
raw material powder by a repulsive force between the
negatively-charged powder with a high negative charge remained
without electrical attraction and each powder, and complete the
present invention by generating this effect in the test.
[0016] A powder mixture of the present invention based on the
findings above is characterized in that a negatively-charged powder
consisting of an iron powder and/or an iron-based alloy powder
treated for negatively charging gets mixed (addition and mixing)
with the iron powder or the iron-based alloy powder, wherein a
graphite powder is adhered to the surface of the iron powder or the
iron-based alloy powder through a binding agent containing a
polyolefin wax.
[0017] When the powder mixture comprises an auxiliary raw material
powder, the powder mixture is characterized in that a
negatively-charged powder consisting of at least one selected from
the group consisting of an iron powder, an iron-based alloy, and an
auxiliary raw material powder treated for negatively charging gets
mixed (addition and mixing) with the iron powder or the iron-based
alloy powder, wherein the graphite powder and the auxiliary raw
material powder are adhered to the surface of the iron powder or
the iron-based alloy powder through a binding agent containing a
polyolefin wax.
[0018] A preferred aspect of the powder mixture in each of the
present invention is that the polyolefin wax is at least one
selected from the group consisting of polyethylene wax and
polypropylene wax.
[0019] A preferred aspect of the powder mixture in each of the
present invention is that the treatment for negatively charging is
the coating treatment with at least one selected from the group
consisting of an alkylsilane, dimethylsilane, octylsilane,
methacrylsilane, fluoroalkylsilane, and hexamethyldisilazane.
Further, a preferred aspect is that the maximum particle diameter
of the negatively-charged powder is within the range from 1 to 15
.mu.m, and an additive amount of the negatively-charged powder is
from 0.02 to 0.5 parts by mass relative to 100 parts by mass of the
iron powder or the iron-based alloy powder.
[0020] A preferred aspect of the powder mixture in each of the
present invention is that when an auxiliary raw material powder is
comprised, the auxiliary raw material powder is adhered through the
binding agent to the surface of the iron powder or the iron-based
alloy powder together with the graphite powder, but an aspect in
which parts or all of the auxiliary raw material powder are
contained in a form of free powder is valid. Also, a preferred
aspect of the powder mixture in each of the present invention is
that when a lubricant powder for compacting is comprised, the
lubricant powder for compacting is contained in a form of free
powder.
Advantageous Effects of Invention
[0021] Since in the present invention the powder mixture is a
mixture wherein the graphite powder is adhered to the surface of
the main raw material powder by using the binding agent containing
a polyolefin wax with excellent adhering properties, segregation of
graphite powder in the powder mixture can be consistently
prevented. At the same time, agglutination of the powder mixture by
electrification of the polyolefin wax can be prevented by addition
of the negatively-charged powder to the mixed powder yielding the
high flowability.
[0022] The disclosure of the present application relates to the
main subject described in Japanese Patent Application No.
2013-28703 applied on Feb. 18, 2013, and the content of such
disclosure thereof is incorporated herein by references.
BRIEF DESCRIPTION OF DRAWINGS
[0023] FIG. 1 is a schematic view illustrating an example of a
powder mixture of the present invention.
[0024] FIG. 2 is a schematic view illustrating the state of the
electrically charged powder mixture. FIG. 2(a) shows the case in
which the powder mixture is only a powder in which the graphite
powder 2 and the auxiliary raw material powder 3 are adhered to the
surface of the main raw material powder 1 with the binding agent 4,
and FIG. 2(b) shows the case in which a negatively-charged powder
is added to the powder in which the graphite powder 2 and the
auxiliary raw material powder 3 are adhered to the surface of the
main raw material powder 1 with the binding agent 4.
[0025] FIG. 3 is a schematic view illustrating the state of powder
prepared by the melting and mixing method.
[0026] FIG. 4 is a view illustrating the outline of a compacting
machine in the compacting process by the die pressing method.
DESCRIPTION OF EMBODIMENTS
[0027] In the present invention, since the powder mixture is a
mixture in which a graphite powder is adhered to the surface of the
main raw material powder by using the binding agent containing a
polyolefin wax with excellent adhering properties, segregation of
the graphite powder in the powder mixture can be consistently
prevented. At the same time, agglutination of the powder mixture by
electrification of the polyolefin wax can be prevented by addition
of the negatively-charged powder to the mixed powder yielding the
high flowability.
[0028] In the present invention, since a general type of materials
is used as the polyolefin wax or a coating material for the
treatment for negatively charging respectively, its operation is
easy. Also, in the present invention, the effect of preventing
segregation above and high flowability can be consistently obtained
as illustrated in Tables 1 and 3.
[0029] Further in the aspect comprising an auxiliary raw material
powder in the present invention, the specification in which all or
parts of the auxiliary raw material powder are adhered to the
surface of the main raw material powder together with the graphite
powder, the specification in which the rest is contained in a form
of free powder, and further the specification in which all of the
auxiliary raw material powder are contained in a form of free
powder can be valid. Also, in the present invention, the aspect
comprising a lubricant powder for compacting has the specification
in which the lubricant powder for compacting is contained in a form
of free powder.
[0030] In the present invention the negatively-charged powder is to
be mixed with the main raw material powder, and consists of the
iron powder and/or the iron-based alloy powder treated for
negatively charging. In an aspect comprising the auxiliary raw
material powder such an aspect can be accepted as parts of the
auxiliary raw material powder are treated for negatively charging
and the negatively-charged powder consists of the auxiliary raw
material powder treated for negatively charging instead of the iron
powder and/or the iron-based alloy powder treated for negatively
charging, and such an aspect can be accepted as the
negatively-charged powder further contains the auxiliary raw
material powder treated for negatively charging in addition to the
iron powder and/or the iron-based alloy powder treated for
negatively charging.
[0031] These aspects demonstrate a detail of each invention above
for confirmation.
[0032] Hereinafter, after an optimal mode of the present invention
will be described, its usefulness will be demonstrated using
examples.
[0033] The schematic view in FIG. 1 illustrates an example of the
structure of a powder mixture in the present invention. The powder
mixture in this figure comprises main raw material powder 1
consisting of an iron powder or an iron-based alloy powder,
graphite powder 2, auxiliary raw material powder 3 consisting of
copper powder, lubricant powder for compacting 6,
negatively-charged powder 5, and binding agent 4.
[0034] The graphite powder 2 and the auxiliary raw material powder
3 are adhered to the surface of the main raw material powder 1 with
the binding agent 4 containing the polyolefin wax. The
negatively-charged powder 5 and the lubricant powder for compacting
6 are not binded to the main raw material powder 1, but exist in a
form of free powder.
[0035] The binding agent 4 herein as the first required property
needs to keep graphite powder adhered to the main raw material
powder from the conveying stage to the compacting stage. Therefore,
it becomes important for the binding agent to have not only a high
adhering force but also strength high enough for resisting against
the vibration when conveyed. For example, the binding agent
(lubricant) in the patent literature 1 is brittle, and while it can
temporarily adhere graphite powder to the surface of the main raw
material powder, it is easily delaminated by vibration when
conveyed so that segregation of the graphite powder cannot be
prevented.
[0036] The polyolefin wax used as the binding agent in the present
invention has a high adhering force as well as a certain level of
ductility, and has strength high enough for resisting against the
vibration when conveyed.
[0037] The binding agent as the second required property needs to
be easily decomposed when heated for sintering in sintering of a
compact and to have no adverse effect on the sintered compact.
Also, since the binding agent is a substance to be disappeared in
this way, it is desired to be inexpensive as much as possible. In
this point, the polyolefin wax used as the binding agent in the
present invention is relatively simple in structure and inexpensive
as found from, for example, polyethylene wax and polypropylene wax
as well as is easily decomposed on heating to disappear, and has no
adverse effect on the sintered compact.
[0038] The binding agent as the third required property needs to
possess the property that the graphite powder can be easily adhered
to the surface of the main raw material powder. In this point,
since the polyolefin wax has a low melting point and can be easily
melted, it is possible to perform melt mixing at low temperature to
adhere the graphite powder to the surface of the main raw material
powder.
[0039] The polyolefin wax used as the binding agent increases the
adhering properties and strength with an increase of its molecular
weight. From this viewpoint, the polyolefin wax with the weight
average molecular weight Mw of 1000 or more is preferred. On the
other hand, since the melting point and the decomposition
temperature are increased with a higher molecular weight, the
polyolefin wax with the weight average molecular weight Mw of
400,000 or less is preferably used. From these facts, for example,
polyethylene wax (weight average molecular weight Mw is preferably
within a range of 1,000 to 40,000, and more preferably within a
range of 1,000 to 10,000), polypropylene wax (weight average
molecular weight Mw is preferably within a range of 1,000 to
40,000, and more preferably within a range of 10,000 to 35,000) and
the like among the polyolefin waxes are preferably used. As
polyethylene wax and polypropylene wax two types or more of waxes
with the different weight average molecular weight are preferably
mixed for use, since the binding agents are not decomposed at once
when sintered but decomposed stepwise.
[0040] As the polyolefin wax with the above weight average
molecular weight, commercially available products can be used or
products appropriately prepared by arbitrary production method can
be also used.
[0041] The binding agent preferably consists of the polyolefin
wax.
[0042] Since the polyolefin wax used as the binding agent needs to
disappear when sintered and to have no adverse effect on the
properties of a sintered compact, the amount used is an amount good
enough to adhere graphite powder to the surface of the iron powder
or the iron-based alloy to be used as the main raw material powder.
For example, since the specific gravity of the binding agent is
small, its use in a large amount results in an increase of the
relative amount of the binding agent contained in the compact so
that the density of the compact is reduced thereby and the
compactability of the raw material powder is lowered. Therefore,
the amount of the polyolefin wax used as the binding agent should
be adjusted depending on the additive amount of the graphite
powder, and is preferably adjusted for use to 10 to 80 parts by
mass relative to 100 parts by mass of the graphite powder as the
additive amount.
[0043] Incidentally, when the auxiliary raw material powder is
adhered to the main raw material powder together with the graphite
powder, the amount of the polyolefin wax used is preferably
adjusted for use to 10 to 80 parts by mass relative to 100 parts by
mass of the graphite powder and the auxiliary raw material powder
which are adhered to the main raw material powder.
[0044] The additive amount of the graphite powder is preferably
from 1 to 2.5 parts by mass relative to 100 parts by mass of the
iron powder or the iron-based powder as the main raw material
powder. When the powder mixture of the present invention is used
for structural part, the additive amount of the graphite powder is
preferably from 1 to 1.2 parts by mass relative to 100 parts by
mass of the iron powder or the iron-based powder as the main raw
material powder.
[0045] Preferably the surface of the main raw material powder is
completely coated with the polyolefin wax, but it is difficult to
obtain such a state and parts of the surface of the main raw
material powder are exposed. In the present invention, since the
iron powder or the iron-based alloy powder is used as the main raw
material powder, parts of the surface of the iron powder or the
iron-based alloy powder are exposed.
[0046] In the powder of which the surface is coated with the
polyolefin wax for adhering the graphite powder, the surface of the
polyolefin wax is negatively charged by friction when conveyed.
Also, since the polyolefin wax has the high insulation property,
the state of the surface of the polyolefin wax becomes negatively
charged. On the other hand, since the iron powder or the iron-based
alloy powder of which parts of the surface are exposed is
positively charged by friction when conveyed, an electrically
attractive force exerts on the surrounding powder coated with the
polyolefin wax which is negatively charged.
[0047] In the present invention, the negatively-charged powder is
added in a form of free powder in order to prevent the powder
coated with the polyolefin wax from electrically attracting each
other. Since the negatively-charged powder is a free powder
not-adhered, it is electrically attracted to the exposed part of
positively charged iron powder or iron-based alloy powder to cover
the exposed part of the iron powder or the iron-based alloy powder.
Therefore, the powder coated with the polyolefin wax to which the
negatively-charged powder is adsorbed has the whole surface thereof
being negatively charged, preventing the powder coated with the
polyolefin wax from electrically attracting each other.
[0048] Further, excess of negatively-charged powder which is not
electrically attracted to the exposed part of the iron powder or
the iron-based alloy powder exerts the electrically repulsive force
on the powder coated with the polyolefin wax, of which the whole
surface is negatively charged, thereby improving the flowability of
the powder mixture.
[0049] Incidentally, when the surface of the iron powder or the
iron-based alloy powder is completely coated with the polyolefin
wax, the powder coated with the polyolefin wax above does not
electrically attract each other, but even in this case, by addition
of the negatively-charged powder, an electrical repulsive force
between the negatively-charged powder and the powder coated with
polyolefin wax acts and the flowability of the powder mixture can
be improved.
[0050] In order that the negatively-charged powder expresses the
effect to actively attract to the exposed part of the iron powder
or the iron-based alloy powder, a powder with a larger negative
charge than the polyolefin wax charged by friction is used. For
this purpose, an alkylsilane, dialkylsilane, methacrylsilane,
alkylhalilde silane, and hexaalkyldisilazane have high negative
electrification so that the powder of which the surface is coated
with at least one among these compounds is preferably used.
[0051] Alkyl groups of the above compounds include alkyl group with
carbon numbers 1 to 10, and preferably include methyl group, ethyl
group, propyl group, butyl group, pentyl group, hexyl group, octyl
group and the like.
[0052] The alkylsilane is not specifically limited, but includes
methylsilane, ethylsilane, propylsilane, butylsilane, pentylsilane,
hexylsilane, octylsilane and the like.
[0053] The dialkylsilane is not specifically limited, but includes
dimethylsilane, diethylsilane, dipropylsilane and the like.
[0054] The methacrylsilane is not specifically limited, but
includes organosilane compounds having acryloyloxy group or
methacryloyloxy group and the like.
[0055] The alkylhalilde silane is not specifically limited, but
includes fluoromethylsilane, difluoromethylsilane,
chloromethylsilane, fluorochloromethylsilane, fluoroethylsilane and
the like.
[0056] The hexaalkyldisilazane is not specifically limited, but
includes hexamethyldisilazane, hexaethyldisilazane and the
like.
[0057] Since dimethylsilane, octylsilane, methacrylsilane,
fluoroalkylsilane and hexamethyldisilazane have high negative
electrification, a powder of which the surface is coated with at
least one among these compounds is preferably used as the
negatively-charging powder. A powder coated with octylsilane or
hexamethyldisilazane is particularly preferable.
[0058] The treatment for negatively charging such as coating with
the alkylsilane is performed by a mixing process comprising the
steps of agitating alkylsilane and the like and the powder to be
treated for negatively charging, and elevating temperature to the
melting point or higher of the alkylsilane and the like. This
mixing process allows coating of the surface of the powder to be
treated for negatively charging with the melted alkylsilane and the
like followed by lowering temperature to the melting point or less
of the alkylsilane and the like to form the solidified film of the
alkylsilane and the like on the surface of the powder to be treated
for negatively charging.
[0059] Incidentally, the negatively-charged powder interfering the
sintering and providing an adverse effect on the sintered compact
is not preferred. From this point, the powder to be treated for
negatively charging above is at least one among an iron powder and
an iron-based alloy powder. Since the main raw material powder is
the iron powder or the iron-based alloy powder, the powder to be
treated for negatively charging consisting of at least one among
the iron powder and the iron-based alloy powder is easily diffused
together with the main raw material powder mutually to unite as
well as does not provide an adverse effect on the structure of the
metal in the sintered compact. Use of the powder with the same
component as the main raw material powder, as the powder to be
treated for negatively charging is herein preferred since there is
no adverse effect on the structure of the metal in the sintered
compact.
[0060] Also, the specific surface area of the negatively-charged
powder is increased with a finer size of powder as well as its mass
becomes smaller making easier attraction to the exposed part of the
positively charged iron powder or the ion-based alloy powder above,
that is, the main raw material powder 1. Also, as the
negatively-charged powder becomes large in size, it likely disturbs
the sintering property.
[0061] Therefore, the maximum particle diameter of the
negatively-charged powder is preferably 15 .mu.m or less. On the
other hand, as the particle diameter becomes too small, the
negatively-charged powder added in a form of powder is likely
penetrated into the space between the die and the punch so that
scuffing of the die easily occurs. Therefore, the minimum particle
diameter of the negatively-charged powder is preferably 1 .mu.m or
more.
[0062] The average particle diameter (D50) of the main raw material
powder is preferably within the range from 50 to 150 .mu.m, and
more preferably within the range from 50 to 120 .mu.m. The average
particle diameter (D50) of the auxiliary raw material powder is
preferably within the range from 50 to 100 .mu.m, and also smaller
than that of the main raw material powder. The average particle
diameter (D50) of the primary particle diameter of the graphite
powder is preferably within the range from 1 to 50 .mu.m, and more
preferably within the range from 1 to 10 .mu.m.
[0063] As the main raw material powder, the auxiliary raw material
powder, and the graphite powder with the above particle diameter
respectively, commercially available products can be used, or
products obtained by preparing by arbitrary production method and
then selecting with sieve or the like can be also used.
[0064] Further, the additive amount of the negatively-charged
powder is preferably from 0.02 to 0.5 parts by mass relative to 100
parts by mass of the iron powder or the iron-based alloy powder
(main raw material powder). This is due to the fact that as the
additive amount of the negatively-charged powder is below 0.02
parts by mass, it is difficult to obtain its effect consistently,
and also as the additive amount is above 0.5 parts by mass, the
ejection pressure of a pressurized powder-compact is substantially
increased after compacting the powder.
[0065] The auxiliary raw material powder is a powder having the
effect of solid solution strengthening of the base of the sintered
compact by diffusing into the base of the sintered compact formed
with the main raw material powder, the effect of improving the
strength of the sintered compact by forming a compound to reinforce
the base of the sintered compact, the effect of improving the
properties such as hardenability of the base of the sintered
compact by diffusing into the base of the sintered compact formed
with the main raw material powder, the effect of improving the
strength of the sintered compact by activating and promoting the
sintering, and the effect of improving the properties such as wear
resistance of the sintered compact and machinability of the
sintered compact by dispersing into the sintered compact.
[0066] Also, when the iron powder or the iron-based alloy powder is
used as the main raw material powder, as the auxiliary raw material
powder, for example, graphite powder, copper powder or copper alloy
powder such as copper-tin alloy powder, nickel powder, molybdenum
powder, iron-based alloy powder (in this case a small amount of the
iron-based alloy powder with a different component from the main
raw material powder) such as iron-phosphorous alloy powder and the
like, various hard phase-forming powders, magnesium silicate type
mineral powder, calcium fluoride powder, sulfide powder, and the
like are used. The amount of the auxiliary raw material powder used
is 30 parts by mass or less, preferably 20 parts by mass or less,
and further preferably 10 parts by mass or less relative to 100
parts by mass of the main raw material powder.
[0067] Such an auxiliary raw material powder is preferably adhered
to the surface of the iron powder or the iron-based alloy powder
similarly to graphite powder as the auxiliary raw material powder 3
in FIG. 1. In particular, when the auxiliary raw material powder
with the lower specific gravity such as the exemplified magnesium
silicate type mineral powder, calcium fluoride powder, and sulfide
powder in part as compared to the iron powder or the iron-based
alloy powder to be the main raw material powder are used, or when
the powder with a smaller particle diameter is used as the
auxiliary raw material powder, segregation of the powder is likely
to occur similarly to graphite powder so that the auxiliary raw
material powder are preferably adhered with the graphite powder to
the surface of the iron powder or the iron-based alloy powder to be
used as the main raw material powder.
[0068] On the other hand, when the powder with the close specific
gravity to the iron powder or the iron-based alloy powder and
having a certain level of particle size is used as the auxiliary
raw material powder, segregation of the powder is unlikely to occur
so that the powder can be added in a form of free powder. In this
case, since the auxiliary raw material powder is not required to be
adhered, the amount of polyolefin wax can be reduced by the amount
for the auxiliary raw material powder.
[0069] Therefore, when the auxiliary raw material powder consists
of a plural kind of powders and parts of the powder are likely to
segregate, it is preferred to adhere only parts of the auxiliary
raw material powders which are likely to segregate to the surface
of the iron powder or the iron-based alloy powder and use the rest
of the auxiliary raw material powders which resist segregation in a
form of free powder, since the amount of the polyolefin wax used is
reduced to only the requirement.
[0070] When the auxiliary raw material powder is used as described
above, the auxiliary raw material powder treated for negatively
charging can be used as the negatively-charged powder instead of,
or in addition to at least one among the iron powder and the
iron-based alloy powder used as the negatively-charged powder. This
is due to the fact that when the auxiliary raw material powder is
used as a negatively-charged powder, it is easily diffused together
with the main raw material powder to unite as well as it does not
provide an adverse effect on the structure of the metal in the
sintered compact.
[0071] The polyolefin wax is generally used as the lubricant, but
in the powder mixture of the present invention the polyolefin wax
is used as the binding agent so that its function as the lubricant
for compacting is low. Therefore, when a die wall lubrication
method (external lubrication method) is used, the method comprising
the steps of applying a powder lubricant or a liquid lubricant to
the surface of the die wall and then performing the compacting, the
powder mixture can be used as it is. On the other hand, when the
die is used without applying the powder lubricant or the liquid
lubricant to the surface of the die wall for the compacting, a
mixed lubrication method (internal lubrication method) in which the
lubricant powder for compacting is mixed with the raw material
powder is preferably used.
[0072] As the lubricant powder for compacting, the material
conventionally used can be used. For example, a fatty acid such as
stearic acid and the like, and a metal salt of the fatty acid such
as zinc stearate, lithium stearate and the like can be used. Since
the property of lubrication is reduced when provided in a melted
form, it is preferably provided in a form of free powder. Also, the
additive amount of the lubricant powder for compacting is within
the range from 0.1 to 1.5 parts by mass relative to 100 parts by
mass of the main raw material powder as conventionally used.
[0073] A material conventionally used in a powder metallurgy
process can be used as the lubricant powder for compacting, and the
particle diameter and the like thereof are not particularly
limited.
[0074] The powder mixture above can be manufactured, for example,
as follows. That is, the manufacture method is a method comprising
a process of adhering the graphite powder, or the graphite powder
and the auxiliary raw material powder to the surface of the iron
powder or the iron-based alloy powder through the binding agent
containing the polyolefin wax, and a process of adding the
negatively-charged powder consisting of the iron powder and/or the
iron-based alloy powder treated for negatively charging to the raw
material powder obtained in the process above and mixing them.
[0075] As described in detail, firstly a primary mixing process of
elevating temperature to the melting point or higher of the
polyolefin wax while agitating a mixture of the binding agent
containing the polyolefin wax and the iron powder or the iron-based
alloy powder, is performed. The surface of the iron powder or the
iron-based alloy powder is coated with the melted polyethylene wax
by the primary mixing process.
[0076] Next a secondary mixing process of adding graphite powder to
the powder mixture obtained in the primary mixing process at the
temperature of the melting point or higher of the polyethylene wax
and agitating them, is performed. The graphite powder is attached
by the secondary mixing process to the melted polyolefin wax which
coats the surface of the iron powder or the iron-based alloy
powder. The mixture is cooled from this state to the melting point
or lower of the polyolefin wax, thereby yielding the powder mixture
in which the graphite powder is adhered by the binding agent to the
surface of the iron powder or the iron-based alloy. The secondary
process can be performed such that after the primary mixing
process, the mixture is once cooled to form the powder mixture, and
then the powder mixture is reheated separately, but considering the
time required for cooling and energy loss for reheating, the
secondary mixing process is preferably performed such that the
graphite powder is successively added without cooling the mixture
after the primary mixing process.
[0077] After the secondary mixing process a tertiary mixing process
of adding the negatively-charged powder to the powder mixture
obtained at the temperature of the melting point or higher of the
polyolefin wax and mixing them, is performed. The
negatively-charged powder can be provided in a form of free powder
by performing the tertiary mixing process at the temperature of the
melting point or lower of the polyolefin wax. The tertiary mixing
process can be initiated when the temperature becomes the melting
point or lower of the polyolefin wax in the cooling step after the
secondary mixing process, but the negatively-charged powder can be
added and mixed at the ambient temperature after cooling the powder
mixture obtained in the secondary mixing process to the ambient
temperature. When the lubricant powder for compacting is added, the
lubricant powder for compacting can be provided in a form of free
powder by adding and mixing the lubricant powder for compacting in
the tertiary mixing process.
[0078] When the powder mixture comprises the auxiliary raw material
powder, the auxiliary raw material powder can be adhered to the
surface of the iron powder or the iron-based alloy powder by adding
and mixing the auxiliary raw material powder in the secondary
mixing process. Also, as the auxiliary raw material powder is added
and mixed in the tertiary mixing process, the auxiliary raw
material powder can be provided in a form of free powder.
EXAMPLES
Example 1
[0079] Iron powder (100 mesh, average particle diameter (D50): 75
.mu.m), electrolytic copper powder (200 mesh, average particle
diameter (D50): 45 .mu.m), graphite powder (325 mesh, average
particle diameter (D50): 10 .mu.m), and zinc stearate powder as the
lubricant powder for compacting were prepared as well as a
polyolefin wax (polyethylene wax) with the weight average molecular
weight of 8,000 was prepared. Iron powder of which the surface was
coated with an alkylsilane (octylsilane) and iron powder of which
the surface was coated with hexamethyldisilazane (maximum particle
diameter of each powder is 5 .mu.m) were prepared as the
negatively-charged powders.
[0080] The primary mixing process was performed by adding 0.5 parts
by mass of the polyolefin wax relative to 100 parts by mass of the
iron powder, feeding them into a Henschel mixer, and mixing them in
the mixer while elevating temperature to 130.degree. C. higher than
the melting point of the polyolefin wax (110.degree. C.) to coat
the surface of iron powder with the melted polyolefin wax. Next the
secondary mixing process was performed by adding a copper powder
and a graphite powder to the mixture while keeping the melted state
of the polyolefin wax so as to adjust the amount of the copper
powder and the graphite powder to 1.5 parts by mass and 1.0 part by
mass, respectively, relative to 100 parts by mass of the iron
powder, and mixing them to fully attach the copper powder and the
graphite powder to the iron powder by the melted polyolefin wax and
to uniformly disperse the powders. Thereafter, the mixture was
cooled to the ambient temperature while agitating to yield a
secondary mixture. The secondary mixture is a product in which 1.5
parts by mass of the copper powder and 1.0 part by mass of the
graphite powder are adhered with 0.5 parts by mass of the
polyolefin wax as a binding agent relative to 100 parts by mass of
the iron powder. The additive amount of the polyolefin wax is 50
parts by mass relative to 100 parts of the graphite powder
added.
[0081] A negatively-charged powder was added to the secondary
mixture obtained in a ratio (the addition rate is based on the part
by mass relative to 100 parts of iron powder) indicated in Table 1
as well as 0.8 parts by mass of the lubricant powder for compacting
was added to the mixture, which was mixed in a V-shape mixer to
prepare the powder mixtures of sample numbers of 01 to 17. For
comparison, 1.5 parts by mass of the copper powder, 1.0 part by
mass of the graphite powder, and 0.8 parts by mass of the lubricant
powder for compacting were added relative to 100 parts by mass of
the iron powder, which were mixed in the V-shape mixer to prepare
the powder mixture of sample number 18.
[0082] The amount of graphite powder attached, the flow rate as
flowability, and the ejection pressure (ejection pressure, MPa) of
these powder mixtures were measured.
[0083] Among these measurements, firstly to determine the
attachment rate, the carbon content of a powder mixture (sample
number 01) in which the negatively-charged powder was not added,
was determined by the carbon content analysis using the infrared
absorption method after combustion in a high frequency induction
furnace defined in G1211 of JIS Standard. The measured carbon
content is a carbon content of the whole sample of the powder
mixture including the graphite powder not adhered but free (the
carbon content of a total of the carbon component in graphite
powder and wax). Next, to prevent effects of the graphite powder
which could not be adhered but free and negatively-charged powder
which was added in a form of free powder, the mixed powder was
classified by a 100 mesh sieve and a 200 mesh sieve to collect the
powder which was passed through the 100 mesh sieve but not through
the 200 mesh sieve (powder with the particle diameter of 75 to 150
.mu.m), and the carbon content of the powder collected was
determined by the carbon content analysis using the infrared
absorption method after combustion in a high frequency induction
furnace defined in G1211 of JIS Standard. The measured carbon
content is a carbon content of the graphite powder attached to the
iron powder through the wax (the carbon content of a total of the
carbon component in graphite powder and wax). The attachment rate
of graphite powder is determined as the ratio of the carbon content
in graphite powder attached to iron powder through wax to the
carbon content of a whole sample of the powder mixture including
the free graphite powder by using the measured carbon content of a
whole sample of the powder mixture and the measured carbon content
of the graphite powder attached to the carbon powder through
wax.
[0084] Measurement of the flow rate of the powder mixture was
performed by using the method for determination of flow rate
defined in Z2502 of JIS Standard.
[0085] To determine the ejection pressure, a die set of a floating
die type in which the die was supported by a spring and a lower
punch, and having the structure of which a load cell was
incorporated into the pressure receiving plate of the lower punch
was used, a cylindrical compact with a diameter of 11.3 mm and a
height of 10 mm was compacted at pressure of 700 MPa by using an
Amsler type universal test machine, the ejection force when
ejecting the compact from the die was measured, and the ejection
pressure was calculated by dividing the load pressure measured with
the outer circumferential area of the cylindrical compact. The
values are also shown in Table 1.
TABLE-US-00001 TABLE 1 Negatively-charged powder Iron powder Iron
powder coated with Attachment Flow Ejection Sample coated with
hexamethyl- rate rate pressure No. alkylsilane disilazane (%)
(Sec.) (MPa) 01 0.0 97 36 12.8 02 0.02 97 32 12.9 03 0.05 97 30
12.9 04 0.1 97 28 13.1 05 0.2 97 27 13.3 06 0.3 97 26 13.4 07 0.4
97 26 13.9 08 0.5 97 26 14.2 09 1.0 97 26 16.2 01 0.0 97 36 12.8 10
0.02 97 31 12.8 11 0.05 97 29 12.9 12 0.1 97 28 12.9 13 0.2 97 27
13.2 14 0.3 97 26 13.3 15 0.4 97 26 13.8 16 0.5 97 26 14.3 17 1.0
97 26 16.2 18 -- -- 30 34 18.1
[0086] Table 1 shows the results in which effects of the additive
amount of the negatively-charged powder were studied, and sample
numbers 01 to 09 are examples where the iron powder of which the
surface is coated with the alkylsilane (octylsilane) is used and
sample numbers 01 and 10 to 17 are examples where the iron powder
of which the surface is coated with hexamethyldisilazane is
used.
[0087] Sample number 18 in Table 1 is an example of the powder
mixture in which the powder is simply mixed conventionally without
use of the polyolefin wax. The attachment rate of the graphite
powder is 30%, low in value in the powder which is passed through a
100 mesh sieve but not through a 200 mesh sieve (powder with the
particle diameter within the range of 75 to 150 .mu.m).
Incidentally, the graphite powder with the attachment rate of 30%
is a graphite powder which is trapped in the depressed area of
irregular shaped iron powder. On the other hand, the powder mixture
sample of sample number 01 is a sample in which the graphite powder
is attached to the surface of the iron powder by the polyolefin
wax, and indicating that the attachment rate of graphite powder is
97%, high in attachment rate. However, the flow rate in the powder
mixture sample of sample number 01 is reduced relative to the flow
rate in sample number 18 in which powders are simply mixed
conventionally to form a powder mixture.
[0088] Powder mixture samples (sample numbers 02 and 10) in which
0.02 parts by mass of the negatively-charged powder is added to
such the powder mixture in which the graphite powder is attached to
the surface of the iron powder by the polyolefin wax improve the
flowability of the powder mixture and have a smaller flow rate as
compared to the powder mixture of sample number 18 in which the
powder is simply mixed conventionally. Also, the more the additive
amount of the negatively-charged powder increases, the more the
flowability of the powder mixture is improved and the smaller the
flow rate is. However, further improvement in the flow rate is not
observed even when the additive amount of the negatively-charged
powder to the powder mixture exceeds 0.3 parts by mass.
[0089] It is confirmed from the above results that the attachment
rate of graphite powder can be substantially improved by attaching
graphite powder to the surface of the iron powder using the
polyolefin wax but the flowability is reduced, and the reduction of
the flowability can be improved by adding 0.02 parts by mass or
more of the negatively-charged powder relative to 100 parts by mass
of the iron powder to the powder mixture in which the graphite
powder is attached to the surface of the iron powder using the
polyolefin wax, and in this case its flowability can be further
improved as compared to the powder mixture in which the powder is
simply mixed conventionally.
[0090] It is also confirmed that the effects of the
negatively-charged powder above can be similarly obtained when
either iron powder coated with the alkylsilane or iron powder
coated with hexamethyldisilazane is used as the negatively-charged
powder, and it is confirmed that similar effects can be obtained as
long as a negatively-charged powder is used.
[0091] Incidentally, in the powder mixture (sample number 01) in
which the graphite powder is attached to the surface of the iron
powder by the polyolefin wax, the ejection pressure is reduced to a
range of 70% of the ejection pressure of the powder mixture (sample
number 18) in which the powder is simply mixed conventionally,
because the wax works as a lubricant. As the negatively-charged
powder is added to the powder mixture in which the graphite powder
is attached to the surface of the iron powder by the polyolefin
wax, the ejection pressure is increased. It is also found that the
ejection pressure tends to increase with an increase of the
additive amount of negatively-charged powder, and in the samples
(sample numbers 08 and 16) of the powder mixture in which the
additive amount of negatively-charged powder is 0.5 parts by mass,
the ejection pressure is reduced to a range of 80% of the ejection
pressure of the powder mixture (sample number 18) in which the
powder is simply mixed conventionally. As the additive amount of
negatively-charged powder exceeds 0.5 parts by mass (sample number
17), the ejection pressure is reduced to a range of 90% of the
ejection pressure of the powder mixture (sample number 18) in which
the powder is simply mixed conventionally. It is found from the
results above that the additive amount of the negatively-charged
powder to the powder mixture in which the graphite powder is
attached to the surface of iron powder by the polyolefin wax is
preferably 0.5 parts by mass or less.
Example 2
[0092] In Example 2 the powder mixtures of sample numbers of 19 to
29 were prepared by using the iron powder, the copper powder, the
graphite powder, and the lubricant powder for compacting used in
Example 1 above as well as using the iron powder coated with an
alkylsilane (octylsilane) in the same manner in Example 1 except
that the polyolefin wax was changed to a wax (polyethylene wax)
with the weight average molecular weight indicated in Table 2. The
attachment rate, the flow rate, and the ejection pressure of the
powder mixture obtained were measured similarly to Example 1. The
results are shown in Table 2.
TABLE-US-00002 TABLE 2 Weight average Attachment Flow Ejection
Sample molecular weight rate rate pressure No. of polyolefin wax
(%) (Sec.) (MPa) 19 500 94 34 12.2 20 1,000 95 30 12.8 21 3,000 95
29 12.9 22 5,000 97 28 13.0 05 8,000 97 27 13.3 23 10,000 97 25
13.6 24 15,000 97 25 14.7 25 20,000 97 25 16.2 26 25,000 97 25 16.5
27 30,000 97 25 17.0 28 35,000 97 25 17.0 29 40,000 97 25 17.6 18
-- 30 34 18.1
[0093] Table 2 indicates that the powder mixture in which the
graphite powder is attached to the surface of the iron powder by
using the polyolefin wax has the higher attachment rate of the
graphite powder as compared to the powder mixture (sample number
18) in which the powder is simply mixed conventionally, regardless
of the weight average molecular weight Mw of the polyolefin
wax.
[0094] Incidentally, in the powder mixture sample (sample number
19) in which the weight average molecular weight Mw of the
polyolefin wax is 500, the wax coating the surface of the iron
powder is soft so that the attachment rate is somewhat lower and
the flow rate is in the same level as the powder mixture (sample
number 18) in which the powder is simply mixed conventionally. On
the other hand, in the powder mixture samples (sample numbers 05
and 20 to 29) in which the weight average molecular weight Mw of
the polyolefin wax is 1000 or more, the waxes coating the surface
of the iron powder become harder than the powder mixture sample of
sample number 19. Therefore, the attachment rate is improved and
the flowability is improved in each sample as compared to that of
the powder mixture (sample number 18) in which the powder is simply
mixed conventionally. It is found from these results that use of
the polyolefin wax with the weight average molecular weight Mw of
1000 or more is desired, since not only the attachment rate of the
graphite powder but also the flowability can be improved.
[0095] On the other hand, the ejection pressure tends to increase
with an increase of the weight average molecular weight Mw of the
polyolefin wax. In the powder mixture sample (sample number 29) in
which the weight average molecular weight Mw of the polyolefin wax
is 40,000, the ejection pressure is still lower in value than that
of the powder mixture (sample number 18) in which the powder is
simply mixed conventionally. However, as the weight average
molecular weight Mw of the polyolefin wax exceeds 40,000, it is
considered that the ejection pressure increases to the same level
to that of the powder mixture (sample number 18) in which the
powder is simply mixed conventionally. Therefore, considering the
ejection pressure, the polyolefin wax with the weight average
molecular weight Mw of 40,000 or less is preferably used.
Example 3
[0096] In Example 3 the powder mixtures of sample numbers of 30 to
37 were prepared by using the iron powder, the copper powder, the
graphite powder and the lubricant powder for compacting used in
Example 1 in the same manner in Example 1 except for using copper
powder (maximum particle diameter: 5 .mu.m) coated with an
alkylsilane (octylsilane) as the negatively-charged powder and
except that the blending ratio was followed as indicated in Table
3. The attachment rate, the flow rate, and the ejection pressure of
the powder mixture obtained were measured similarly to Example 1.
The results are shown in Table 3.
TABLE-US-00003 TABLE 3 Blending ratio mass part Lubricant Copper
powder Attachment Flow Ejection Sample Iron Polyolefin Copper
Graphite powder for coated with rate rate pressure No. powder wax
powder powder compacting alkylsilane (%) (Sec.) (MPa) 01 100 0.5
1.5 1.0 0.8 0.0 97 36 12.8 30 100 0.5 1.5 1.0 0.8 0.02 97 36 12.9
31 100 0.5 1.5 1.0 0.8 0.05 97 34 13.0 32 100 0.5 1.4 1.0 0.8 0.1
97 33 13.1 33 100 0.5 1.3 1.0 0.8 0.2 97 32 13.2 34 100 0.5 1.2 1.0
0.8 0.3 97 30 13.4 35 100 0.5 1.1 1.0 0.8 0.4 97 29 13.9 36 100 0.5
1.0 1.0 0.8 0.5 97 29 14.3 37 100 0.5 0.5 1.0 0.8 1.0 97 28 16.3 18
100 -- 1.5 1.0 0.8 -- 30 34 18.1
[0097] Table 3 shows the results in which the effects are studied
when the auxiliary raw material powder is used as the
negatively-charged powder, and sample numbers 30 to 37 are the
examples in which copper powder of which the surface is coated with
an alkylsilane (octylsilane) is used instead of iron powder treated
for negatively charging of Example 1.
[0098] Table 3 indicates that in the powder mixture sample (sample
number 30) in which 0.02 parts by mass of the negatively-charged
powder using the auxiliary raw material powder is added to the
powder mixture in which the graphite powder is attached to the
surface of the iron powder by the polyolefin wax (polyethylene
wax), the flowability of the powder mixture is improved and the
flow rate becomes smaller as compared to the powder mixture of
sample number 18 in which the powder is simply mixed
conventionally. Also, the flowability of the powder mixture is
further improved with an increase of the additive amount of the
negatively-charged powder and the flow rate is reduced. However,
further improvement in the flowability is not observed even when
the additive amount of the negatively-charged powder to a powder
mixture exceeds 0.3 parts by mass.
[0099] Also, addition of the negatively-charged powder using the
auxiliary raw material powder to the powder mixture in which the
graphite powder is attached to the surface of iron powder by the
polyolefin wax increases the ejection pressure. It is also found
that the ejection pressure tends to increase with an increase of
the additive amount of the negatively-charged powder. In the powder
mixture sample (sample number 36) in which the additive amount of
the negatively-charged powder is 0.5 parts by mass, the ejection
pressure is reduced to a range of 80% of the ejection pressure of
the powder mixture (sample number 18) in which the powder is simply
mixed conventionally, and as the additive amount of the
negatively-charged powder exceeds 0.5 parts by mass (sample number
37), the ejection pressure is reduced to a range of 90% of the
ejection pressure of the powder mixture (sample number 18) in which
the powder is simply mixed conventionally.
[0100] It is found from the results above that as iron powder
(similar component of main raw material powder) is replaced with
copper powder (component of auxiliary raw material powder) for the
powder to be treated for negatively charging, similar results can
be obtained.
INDUSTRIAL APPLICABILITY
[0101] Since the powder mixture of the present invention is a
powder mixture in which the segregation of the powder in the mixed
powder is prevented and the flowability of the mixed powder is
excellent and a sintered machine part can be manufactured by the
die pressing method without variation in quality, it is suitable
for manufacture of various sintered machine parts.
EXPLANATION OF REFERENCES
[0102] 1 main raw material powder [0103] 2 graphite powder [0104] 3
auxiliary raw material powder [0105] 4 binding agent [0106] 5
negatively-charged powder [0107] 6 lubricant powder for compacting
[0108] 10 die [0109] 11 die hole [0110] 20 lower punch [0111] 30
upper punch [0112] 40 hopper [0113] 50 hose [0114] 60 feeder
* * * * *