Plastic and nonmetallic article shaping or treating: processes – Forming electrical articles by shaping electroconductive... – Conductive carbon containing
Reexamination Certificate
2001-01-25
2003-05-06
Staicovici, Stefan (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming electrical articles by shaping electroconductive...
Conductive carbon containing
C264S109000, C264S334000, C425S078000, C425S345000, C425S353000, C425S356000
Reexamination Certificate
active
06558594
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a method and an apparatus for powder compression molding for manufacturing cathode pellets for batteries.
2. Description of Related Art
Molding of powder mixture pellets is generally accomplished with a rotary type compression molding apparatus, wherein a rotary disk carries at its circumferential edge a number of molding units arranged at equal intervals, and as the rotary disk rotates, the molding units successively perform molding actions by means of cam members.
FIG. 8
shows one example of prior art arrangement for one such molding unit mounted on a rotary disk. A center pin
34
passing through a lower plunger
35
is fixed to the rotary disk
31
with a fixing pin
44
. An upper plunger
36
has a cylindrical cavity to receive therein the top end of the center pin
34
. The upper plunger
36
and the lower plunger
35
are respectively pressed in molding directions with an upper pressure roller
39
and a lower pressure roller
38
at predetermined locations on the rotary disk.
FIGS. 9A
to
9
E illustrate the process steps for molding ring-like pellets with the molding unit of FIG.
8
.
FIG. 9A
illustrates an initial state where a die
33
, the lower plunger
35
, and the center pin
34
are flush with each other at the top after the previous pellet has been unloaded. When the lower plunger
35
is lowered from its initial position, an annular space for compression molding is formed between the die
33
and the center pin
34
as shown in FIG.
9
B. The annular space is then filled with a powder mixture
42
. As a feed shoe
43
runs along the top sides of the die
33
and the center pin
34
located flush with each other, an excess of the powder mixture
42
is removed to measure out a predetermined amount to be molded into one pellet
40
. This is followed by a step where the upper plunger
36
is lowered and the lower plunger
35
is lifted up as shown in
FIG. 9D
, by which the powder mixture
42
in the annular space is compressed from upper and lower sides, thus forming the pellet
40
. The pellet
40
is then unloaded upwardly from the die
33
by the upward movement of the lower plunger
35
as shown in FIG.
9
E and taken out as a compression molded product.
Such conventional procedure of compression molding has, however, a drawback that the powder mixture
42
when being supplied into the annular space between the die
33
and the center pin
34
is likely to produce a bridge across the very small gap between the center pin and the die, particularly when a thin pellet
40
having a small diameter is formed. Because of the bridges frequently formed, it is difficult to constantly supply a given amount of the powder mixture
42
, resulting in variations in the weight of pellets
40
.
In view of the stringent requirements for uniform and high electrical performance of batteries in recent years, it is extremely important to reduce variations in the battery capacity which is directly determined by the amount of pellets forming the active material of battery. Therefore, it is essential to ensure that each pellet is molded from a predetermined, constant amount of powder mixture, so that pellets are formed with as little variation as possible in their weight and volume.
Another problem associated with the conventional molding apparatus is that the components constituting the cam mechanism for driving the plungers are subject to great stress, and particularly small components, such as the fixing pin
44
in
FIG. 8
for fixing the center pin
34
, which cannot withstand the stress, often break and must be replaced. For the same reasons the sliding surfaces between the upper and lower plungers and the pressure rollers, and the cams which come to frequent engagement therewith suffer severe abrasion. These are all because the pellet
40
is tightly stuck to the center pin
34
and the die
33
by the pressure given during the compression molding, and a great amount of force is required to push the formed pellet out of the die
33
.
Also, since the powder mixture is compressed from the upper and lower sides, the adhesion between the inner side of the resultant pellet and the center pin and that between the outer side of the pellet and the die
33
is very strong. In order to unload the pellet with the lower plunger
35
without damaging the pellet
40
, it is the normal practice to provide a tapered surface at a relatively wide angle to the center pin
34
and to provide, correspondingly, an inverted tapered surface to the die
33
. Therefore, the resultant pellets
40
are tapered on both inner and outer sides. The amount of the powder mixture contained in one battery is therefore reduced by these tapered portions.
Under the circumstances, several ring-like, tapered pellets are contained in one cell as shown in FIG.
10
. As shown, one cell
61
normally contains four ring-like pellets
40
made from a powder mixture consisting of positive electrode active materials because of the small height of pellets. Negative electrode active material is contained within the cylindrical bore hole formed by these ring-like pellets arranged vertically upon one another in a cylindrical cell case, with a separator
64
interposed therebetween.
With such structure, there is still room for more active material to be filled because of the taper as mentioned above. Also, the tapered surfaces of pellets create slight gaps between themselves and the separator
64
and the cell case
62
as can be seen from
FIG. 10
, because of which smooth flow of electric current is obstructed. Moreover, since the cells need to be filled with four pellets each, the production line involves a considerable number of operations, resulting in low efficiency and high cost. Therefore, it is extremely desirable that pellets for batteries be formed with a greater height and with as little taper as possible.
Usually, ring-like pellets produced by the compression molding machine as described above are transferred by belt conveyors or parts feeders to a next step in which they are loaded into cell cases. However, since the molded pellets tend to be broken at their edges during transportation, those steps are hardly carried out at a high speed. Specifically, the pellets for alkaline manganese dry cells are composed of a mixture of manganese dioxide and graphite with a binder, and these are extremely fragile and liable to chipping even with a slight shock. Also, the entire system including the compression molding machine, conveyors, and automatic loaders is bulky and not adaptable for mass production at high speed.
These problems have previously been addressed by packing a powder mixture in a cell case and compression-molding it within the cell case, as disclosed in U.S. Pat. Nos. 3,577,842 and No. 3,729,281. With such methods, while a strong bond is obtained between the case and the shaped mold of the powder, the density of the mold thus shaped is relatively low, because it is molded within the confined space of cell case from which air cannot escape, and because the powder is compressed from only one direction, i.e., from the open end side of the cell cases. Also, because the center pin inherently has a flat top end, there is a problem that powder sticks thereto. Moreover, the center pin must have a tapered surface for facilitating removal from the shaped mold, which presents the disadvantages described above.
BRIEF SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide an improved powder compression molding method and an apparatus for producing high and hollow cylindrical pellets, of which inner side is not tapered at all and of which outer side is substantially not tapered.
Another object of the present invention is to improve measurement precision of powder mixture to be molded into pellets, so that resultant pellets have as little variation as possible in weight.
It is yet another object of the present invention to provide a compact rotary type powder compression moldi
Hattori Shigeharu
Kouda Minoru
Nakatsuka Saburo
Sanukiya Toshio
Takebayashi Hiroshi
Matsushita Electric - Industrial Co., Ltd.
Staicovici Stefan
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