Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Heat and pressure simultaneously to effect sintering
Reexamination Certificate
2002-06-28
2003-08-26
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Heat and pressure simultaneously to effect sintering
C419S048000, C425S078000
Reexamination Certificate
active
06610246
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a sintering method and a sintering apparatus for use in the sintering method.
BACKGROUND TECHNOLOGY
A sintering apparatus is known which is of a type sintering a powder material by applying a current to a mold while applying pressure thereto. The present inventor has previously developed a sintering apparatus of a current-applying type, as shown in
FIGS. 19 and 20
, which comprises a cylindrical mold (e.g., made of carbon or graphite; an outer diameter: about 180 mm; coaxial length: about 60 mm)
102
for accommodating a powder material
101
, upper and lower punches
103
a
and
103
b
, respectively, disposed in the mold
102
so as to be movable for applying pressure to the powder material
101
filled in the mold
102
, and a pair of electrodes
104
a
and
104
b
for forming the powder material
101
into a sintered article by applying a current to the mold
102
from the side thereof (as indicated by the broken-lined arrow in
FIG. 20
) and applying heat to the powder material
101
.
As the sintering process apparatus of this current-applying type can lower a sintering temperature by elevating a pressure to be applied to the powder material
101
by means of the upper and lower solid punches
103
a
and
103
b
, respectively, the sintering process can reduce influences of the sintering temperature upon a rate of the oxidative loss of the mold
102
and the like as well as shorten the time for cooling the mold and the like after sintering, up to a level that can make the rate of the oxidative loss negligible. As a consequence, the sintering process of this type can control the oxidative loss in the mold
102
and the like and shorten a cycle time for sintering processes.
As a result of further extensive research on the sintering process conducted by the present inventor, it was found that the rise of the temperature at positions P
1
and P
2
, which are close to contact locations where the electrodes contact with the side surfaces of the mold, is faster than the rise of the temperature at positions P
3
and P
4
apart from the contact locations between the electrodes and the side surfaces thereof, and consequently that a local temperature difference may be caused to occur to some extent between the positions, resulting in the fact that a portion where the temperature rise is the highest (around the position P
1
) is caused to reach the sintering temperature faster than the other portions thereof while maintaining the entire state as it is. This may cause a partially non-sintered portion to be formed in a sintered article as a product (as shown in FIGS.
20
and
21
). It is further found that this tendency may become higher as the application of a current at the time of a rise is increased to a higher level in order to shorten the processing time. With this finding taken into account, the present inventor has come to recognition that improvements in this point are to be made in order to improve properties of a sintered article such as strength and the like.
The present invention has been completed on the basis of this finding and it has the object to form a sintered article so as to cause a local temperature difference to occur to the smallest possible extent at the time of sintering a powder material by applying a current thereto.
DISCLOSURE OF THE INVENTION
In order to achieve the object as described above, the present invention as claimed in claim
1
provides a sintering method comprising supplying heat to a powder material filled in a cylindrical mold for accommodating the powder material under pressurized condition by applying a current to the mold with whose surface a pair of electrodes are in contact, wherein:
positions of the pair of electrodes in which they are in contact with the side surfaces of the mold for applying the current are disposed so as to vary with time.
Preferred modes of this embodiment according to the present invention as claimed in claim
1
include modes of the embodiment according to the present invention as claimed in claims
2
to
7
.
In order to achieve the object as described above, the present invention as claimed in claim
8
provides a sintering method for sintering the powder material by applying a current thereto, thereto wherein the application of the current is partially suspended.
Preferred modes of this embodiment of the present invention as claimed in claim
8
include modes of the embodiment according to the present invention as claimed in claims
9
to
16
.
In order to achieve the object as described above, the present invention as claimed in claim
17
provides a sintering apparatus for sintering the powder material in a cylindrical mold for accommodating the powder material under pressurized condition, in which plural groups, each group composed of a pair of electrodes, are disposed in contact of the side surface of the mold and on the periphery of the mold in such a manner that a current is alternately applied to each group composed of a pair of electrodes.
In order to achieve the object as described above, the present invention as claimed in claim
18
provides a sintering apparatus for sintering the powder material around a cylindrical mold for accommodating the powder material under pressurized condition through plural groups, each group composed of a pair of electrodes disposed on the periphery of the mold, wherein each group composed of a pair of electrodes is allowed to alternately come into contact with the side surface of the mold and apply the current to the mold. Preferred modes of the embodiments of the present invention as claimed in claims
16
and
17
include modes of the embodiments of the present invention as claimed in claims
23
and
24
, respectively.
In order to achieve the object as described above, the present invention as claimed in claim
18
provides a sintering apparatus, comprising:
a pair of the electrodes disposed around a cylindrical mold for accommodating the powder material under pressurized conditions and for applying heat to the powder material by applying a current to the side surface of the mold;
current-application adjustment means for adjusting the application of a current to the pair of the electrodes from a power source; and
control means for controlling the current-application adjustment means to maintain the pair of the electrodes in a current-applying state with respect to the power source in a usual case, in which the current is being applied to the electrodes from the power source, and partially bring the pair of the electrodes in a current-suspending state with respect to the power source, in which the application of the current from the power source is being suspended. Preferred modes of the embodiment of the present invention as claimed in claim
18
include modes of the embodiments of the present invention as claimed in claims
19
to
24
.
The sintering method according to the present invention as claimed in claim
1
can positively apply heat (current) even to a portion of the mold where the temperature rises at a lower level because contact points between the pair of the electrodes and the side surface of the mold through which the current is applied are configured so as to vary with time by focusing on the fact that the temperature can rise to a higher level at the contact points between the electrodes and the side surfaces of the mold. This configuration of the sintering method can fail to cause a temperature difference to occur to the least possible extent in the mold during the sintering process and therefore provide a sintered article without causing the temperature difference to occur to the least possible extent during the sintering process.
The sintering method according to the present invention as claimed in claim
2
is configured such that three or more electrodes are disposed on the periphery of the mold in a spaced relationship apart from one another in a peripheral direction of the mold, and different two electrodes are optionally selected variably from such three or more electrodes, as an elapse of time,
Akane Co., Ltd.
Jenkins Daniel J.
Morrison & Foerster / LLP
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