Forming method and forming apparatus

Details Forming method and forming apparatus Forming method and forming apparatus

1999-11-01

2001-12-04

Jenkins, Daniel J. (Department: 1742)

Powder metallurgy processes

Powder metallurgy processes with heating or sintering

Consolidation of powder prior to sintering

C419S023000, C425S078000

Reexamination Certificate

active

06325965

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a forming method and a forming apparatus. More specifically the present invention relates to a forming method and a forming apparatus in which a raw material powder comprising a fine powder or a granulated powder made therefrom is supplied into a cavity of a die, and the raw material powder supplied in the cavity is pressed to form a compact.
2. Description of the Related Art
In a press forming method, a die is formed with a cavity opening upward. A feeder box containing a raw material powder is placed above the cavity. The raw material powder is supplied by gravitational fall from the feeder box into the cavity. Then, the supplied raw material powder is pressed into a compact by an upper punch and a lower punch. An advantage of this method is that a compact of a high density can be obtained. According to the press forming method, in general, an amount of a binder to be used can be smaller than in an injection molding method or an extrusion molding method. Further, an amount of time required to perform a cycle of formation is also smaller. For these reasons, the press forming method is widely used.
When the press forming method is used to manufacture a small compact, the cavity of the die must have an area of an opening which is made accordingly and therefore considerably small, causing a difficulty that the raw material powder will not fall easily into the cavity. This is due to a phenomenon known as the bridging phenomenon, which is unique to a powder material. The bridging phenomenon makes unstable the amount of supply of the raw material powder into the cavity, making difficult to manufacture the compact satisfying a dimensional requirement. Further, the supply of the raw material powder into the cavity takes a longer time, increasing the amount of time required to perform the cycle of pressing operation.
In order to avoid the bridging phenomenon, there is employed a method of adding a binder to a powder thereby making a granulated powder having a greater grain diameter than the original powder grain (See Japanese Patent Laid-Open No. 8-20801, Japanese Patent Laid-Open No. 8-20802, and Japanese Patent Laid-Open No. 9-287001, for example). The granulated powder has a dramatically smaller contact area among granules, having a remarkably improved flowability. As a result, many small ceramic parts are now manufactured from the granulated powder, by using the press forming method.
On the other hand, a development is made also for a forming apparatus to avoid the bridging phenomenon, by utilizing a magnetic field or an ambient pressure difference, for example, in sucking the raw material powder into the cavity. Specifically, as a method of using the pressure difference, a proposal is made, in which the lower punch is quickly lowered when the feeder box comes above the cavity so as to create a partial vacuum within the cavity for sucking the raw material powder. In another proposal, the die is provided with a vent hole for sucking air from inside the cavity so that the raw material powder is supplied into the cavity under partial vacuum.
However, even if the granulated powder is used according to the former proposal, there is still a limit to catch up with further miniaturization of the parts, while there is a difficulty in further increasing a speed of the formation.
On the other hand, according to the latter proposal in which a relatively large pressure difference is created between inside and outside of the cavity for sucking the raw material powder, it is possible to quickly supply the raw material powder into the cavity. However, there is a narrow gap between the die and the lower punch from which a high pressure gas is discharged, allowing the raw material powder to build up in the gap. This can cause damage to the die when the lower punch is moved relative to the die, or cause seizure between the lower punch and the die. These problems interfere with continuous formation. Further, said sucking method can cause a fire accident, due to an excessive friction during the operation if the raw material powder is bound between the lower punch and the die and if the raw material powder is a rare-earth alloy powder.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide a forming method and a forming apparatus capable of supplying the raw material powder into the cavity at a high speed even if the compact to be formed is small, and the ratio of the area of the opening to the depth of the cavity is small due to a shape of the compact, and capable of performing an uninterrupted forming operation without such troubles as the seizure caused by a so-called powder clogging.
According to an aspect of the present invention, there is provided a forming method comprising: a powder supplying step of allowing a raw material powder to fall into a cavity of a die by bringing a raw material powder supplying means stored with the raw material powder above the cavity while an inside of the raw material powder supplying means and an inside of the cavity are each maintained at a pressure not greater than 10 kPa, the raw material powder having an average grain diameter of 0.1 &mgr;m~500 &mgr;m; and a press forming step of pressing the raw material powder supplied in the cavity into a compact.
According to another aspect of the present invention, there is provided a forming apparatus comprising: a die formed with a through hole for provision of a cavity, a raw material powder supplying means stored with a raw material powder having an average grain diameter of 0.1 &mgr;m~500 &mgr;m, allowing the raw material powder to fall into the cavity from above the cavity; a pressure maintaining means for keeping an inside of the raw material powder supplying means and an inside of the cavity each at a pressure not greater than 10 kPa at least while the raw material powder supplying means is above the cavity; and a press forming means for pressing the raw material powder supplied in the cavity into a compact.
The pressure inside the cavity is set to a value not greater than 10 kPa, because if the pressure in the cavity is greater than 10 kPa, the gas within the cavity is compressed by the raw material powder, and the pressure in the cavity is increased, reducing the falling speed of the raw material powder. In addition, the compact will not have a uniform density due to interference by the residual gas during the press forming. With the above arrangement, even if the ratio of the area of the opening to the depth of the cavity is small, the raw material powder can be supplied into the cavity smoothly and at a high speed. Further, since the inside of the raw material powder supplying means is also maintained at a pressure not greater than 10 kPa, there is virtually no pressure difference between the inside of the raw material powder supplying means and the inside of the cavity. Thus, the raw material powder falls from the raw material powder supplying means into the cavity solely by gravity. As a result, there is practically no case where the raw material powder enters the gap between the die and the lower punch as experienced when a big pressure difference is created between the two. Therefore, it becomes possible to perform an uninterrupted forming operation at a high speed, without such troubles as caused by the powder clogging.
Preferably, when the raw material powder is supplied into the cavity, the raw material powder supplying means is vibrated by activating a vibrating device, for example, provided in the raw material powder supplying means for activation at least while the raw material powder supply means is above the cavity. By vibrating the raw material powder supply means, even if the area of the opening of the cavity is small, it becomes possible to avoid the bridging phenomenon of the raw material powder, and to supply the raw material powder into the cavity at a high speed. Therefore, it becomes possible to form the compact even with less interruption and at a higher speed.
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