Powder metallurgy processes – Forming articles by uniting randomly associated metal particles – Consolidation of powders
Reexamination Certificate
2001-06-19
2003-12-02
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Forming articles by uniting randomly associated metal particles
Consolidation of powders
Reexamination Certificate
active
06656417
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates for example to a punch for pressing a powder such as a rare-earth alloy powder into a compact, for manufacture of a magnet used in a voice coil motor, a powder pressing apparatus and a powder pressing method using the above punch, a compact formed by such a pressing method as above, a sintered body and a voice coil motor using the sintered body.
2. Description of the Related Art
Referring to FIG.
14
and
FIG. 15
, description will cover a conventional method for pressing the rare-earth alloy powder into a compact
8
(see FIG.
16
). The compact
8
is sintered and then used as a magnet for the voice coil motor and so on.
In order to manufacture the compact
8
, a tolling
1
for pressing operation as shown in
FIG. 14
is used. The tolling
1
includes a die
3
having a through hole
2
, a lower punch
4
to be inserted into the through hole
2
in advance, and an upper punch
5
to be inserted into the through hole
2
. The lower punch
4
is provided with an upper surface having a center portion formed with a generally arcuate projection
4
a
and two edge portions each formed with a flange-like projections
4
b.
The upper punch
5
has a lower surface formed with a recess
5
a.
Each of the lower punch
4
and the upper punch
5
is made of a hard alloy such as cemented carbide, and for protection from chipping and cracking, each tip of the projection
4
b
and an edge portion
5
b
is chamfered by 0.8 mm.
When pressing, first, the lower punch
4
is lowered to form a cavity
6
in the through hole
2
, and the cavity
6
is fed with a rare-earth alloy powder
7
.
Then, the rare-earth alloy powder
7
in the cavity
6
is pressed between the lower punch
4
and the upper punch
5
while being oriented by a magnetic field. The pressing operation to the rare-earth alloy powder
7
is continued until the two edge portions
5
b
of the upper punch
5
are about to contact the corresponding projections
4
b
of the lower punch
4
(until a gap between the punches becomes about 1 mm for example) as shown in
FIG. 15
in order to form a shape as close as of a final product.
As a result, a compact
8
as shown in
FIG. 16
is obtained.
The compact
8
is formed to have a generally arcuate section, including an upper surface
8
a
formed by the recess
5
a
of the upper punch
5
, a lower surface
8
b
formed by the projection
4
a
of the lower punch
4
, slopes
8
c
formed by the projection
4
b
of the lower punch
4
, and end surfaces
8
d
formed by a wall of the through hole
2
.
As shown in
FIG. 16
, the compact
8
has a problem that cracks A develop along border portions
8
e
between the upper surface
8
a
and the slope
8
c.
Causes of the crack A will be described.
As shown in
FIG. 14
, when the rare-earth alloy powder
7
is fed into the cavity
6
, layers of a marking material B having a color different from that of the rare-earth alloy powder
7
were inserted at a predetermined interval, and then the pressing operation was made. Then, as shown in
FIG. 15
, the gap between layers of the marking material B was found to be very narrow between the projection
4
b
of the lower punch
4
and the edge portion
5
b
of the upper punch
5
. This indicates that the rare-earth alloy powder
7
has a much higher density between the projection
4
b
of the lower punch
4
and the corresponding edge portion
5
b
of the upper punch
5
than in other portions. This is presumably that when being pressed, the rare-earth alloy powder having a poor flowability was sandwiched between a tip of the projection
4
b
of the lower punch
4
and a tip of the edge portion
5
b
of the upper punch
5
, became unable to move, and was compressed into such a high density at the border portion
8
e.
Therefore, pressure exerted to the compact
8
is relieved when the compact
8
is taken out of the through hole
2
of the die
3
, allowing a highly compressed portion such as the border portion
8
e
to expand more significantly, often developing cracks or fractures. A similar problem occurs in a sintering process.
If the rare-earth alloy powder
7
is pressed in a strong magnetic field not smaller than 0.5 MA/m oriented in a direction indicated by an arrow C (longitudinally of the projection
4
b
) shown in
FIG. 16
, grains of the magnetized rare-earth alloy powder
7
repel against each other, making the powder density higher in a region closer to a perimeter of the cavity
6
than in a center portion of the cavity
6
, increasing further the density near the projection
4
b.
Further, if the rare-earth alloy powder
7
fed into the cavity
6
is wiped flush at an upper edge of the cavity
6
by a lower edge of feeder box (not shown), region to be pressed by the projection
4
b
is fed with an excess amount of the rare-earth alloy powder
7
than needed. Since the rare-earth alloy powder
7
does not have enough flowability, the density in this region after the pressing becomes higher than in other regions. Therefore, in these cases, the compact
8
becomes more apt to develop the cracks or fractures in the border portion
8
e
when taken out of the cavity
6
.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide a punch, a powder pressing apparatus and a powder pressing method capable of preventing the cracks and fractures from developing in the product, thereby improving productivity.
Another object of the present invention is to provide a compact manufactured by the above method.
Still another object of the present invention is to provide a sintered body and a voice coil motor using the sintered body.
According to an aspect of the present invention, there is provided a punch used for pressing a rare-earth alloy powder, comprising a punching surface for pressing the rare-earth alloy powder. The punching surface has an edge portion including a projection having a tip chamfered by a width not greater than 0.5 mm.
According to another aspect of the present invention, there is provided a powder pressing apparatus comprising the above described punch having the tip chamfered by the width not greater than 0.5 mm, and a die having a through hole for insertion by the punch.
According to another aspect of the present invention, there is provided a powder pressing method using the above punch having the tip chamfered by the width not greater than 0.5 mm and a die having a through hole for insertion by the punch. The method comprises a first step of feeding a rare-earth alloy powder into a cavity formed in the through hole, and a second step of pressing the rare-earth alloy powder fed into the cavity, by using the punch.
According to still another aspect of the present invention, there is provided a compact manufactured by the above described powder pressing method.
The rare-earth alloy powder has a sharp-edged grain, and has poor flowability. Therefore, even if the rare-earth alloy powder is vibrated for example at the time of pressing, the rare-earth alloy powder cannot move smoothly in the cavity during the pressing, making difficult to achieve uniformity of the compact density. However, according to the present invention, by decreasing the width of the chamfered portion, which is a portion formed at the tip of the projection provided at the edge portion of the punching surface, to 0.5 mm or smaller, thereby reducing the amount of the rare-earth alloy powder pressed and clogged by the edge portions, the flowability of the rare-earth alloy powder around the edge portion is improved. Therefore, when a pressing force is applied to the rare-earth alloy powder at the tip of the projection of the punching surface at the time of pressing, the rare-earth alloy powder moves to a region of a lower density along the slope of the projection, without stagnating at the tip of the projection. Therefore, it becomes possible to obtain a compact having a uniform density, making possible to prevent the cracks and fractures caused by the non-uniform density.
According to an aspect of the
Kidowaki Shinji
Kohara Seiichi
Armstrong Westerman & Hattori, LLP.
Mai Ngoclan
Sumitomo Special Metals Co. Ltd.
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