Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making composite or hollow article
Reexamination Certificate
2000-10-27
2002-12-03
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making composite or hollow article
C419S005000
Reexamination Certificate
active
06488887
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a molding method for fabricating a metal composite compact according to a metal powder injection molding (MIM) process by integrating two compacts of the same type, or of different types, of material.
2. Description of the Related Art
In recent years, metal powder injection molding (MIM) has been used as a method for fabricating a metal compact. According to this method, metal powder is mixed with a binder, to give fluidity, and is subjected to injection molding. Almost all the binder is removed by heating or the like from the compact in a degreasing step, and the compact is heated to a higher temperature to sinter the metal powder in a sintering step thereby to produce the desired product.
Also, a sintered metal composite compact can be fabricated by integrating a plurality of sintered compacts of the same type or different types of material using the MIM method. In such a case, as shown in
FIGS. 6A and 6B
, a first compact
91
fabricated in advance is inserted in a die
8
. A second compact
92
of a material identical to or different from the first compact
91
is integrated with the first compact
91
and molded thereby to produce a metal composite compact. The integrated compact is later degreased and sintered to produce the aforementioned sintered metal composite compact.
However, the metal composite compact described above poses the problem that the concentration of the binder contained in the molding material is liable to increase in the boundaries of a plurality of compacts and a sound joining boundary cannot be obtained after sintering. Specifically, at the boundary surface S in
FIG. 6B
, a high binder concentration portion
918
is formed in the surface of the joining surface of the first compact
91
, as shown in FIG.
7
. On the other hand, the surface of the joining surface of the second compact
92
is also formed with a high binder concentration portion
928
. The boundary is obtained in which the two high binder concentration portions
918
,
928
join each other.
Specifically, the molding material used for the MIM method is a mixture of metal powder and a binder, and is fluidized by heating it to a predetermined temperature to liquefy the binder. In the fluidized state of the molding material, the binder has a higher fluidity than the metal powder.
As a result, as shown in
FIG. 8
, at the time of injection molding, the high binder concentration portion
951
of the molding material
95
blows out from the central portion of the flow path to the forward end and flows back around the side surface. Thus, the surface of the molded compact is formed with a layer high in both fluidity and binder concentration, which layer also remains in the boundary surfaces of the two compacts. The high binder concentration portion which first solidifies is subjected to a shearing force F by the internal flow on the side surface in contact with the die
8
along the direction of the flow. Therefore, the thickness of the high binder concentration portion on the side surface is thinner than that on the forward end portion.
In the case where the molding material is degreased while the high binder concentration portion remains on the boundary surfaces of a plurality of the compacts, a depression may be formed due to the loss of the binder. In such a case, a normal joint may not be obtained in the subsequent sintering process.
Also, according to the MIM method, a compact of a comparatively complicated shape can be fabricated. In the case where different types of material are integrated or a compact formed of a material of the same type is geometrically too complicated, however, the injection molding may not be accomplished at one time. In such a case, the two compacts are injection molded individually and integrated in a given step of the fabrication process.
According to a method in which a sintered compact is produced by integrating a plurality of parts of the same material or different types of material using the conventional MIM, a plurality of sintered compacts formed separately from each other are joined through an appropriate process such as welding. A welding step added after sintering, however, leads to an unstable quality and a higher cost due to an increased number of steps.
Another conventional method is an insert molding method such as disclosed in Japanese Unexamined Patent Publication (Kokai) NO. 3-232906, in which a first compact prepared separately is placed in a die and a second compact is molded by injection.
In this method, the first compact is required to be fabricated separately in advance, and therefore the number of steps is increased.
Also, in the case where the first compact is placed in a die, the size of the recess into which the first compact is inserted is required to be larger than the size of the compact, resulting in a lower dimensional accuracy. This is by reason of the fact that in the case where the recess and the compact have the same size, the compact is cut off or broken when inserted into the recess, leading to a higher rejection ratio.
Japanese Unexamined Patent Publication (Kokai) No. 7-90312 discloses still another method in which the die is formed with partitions and the injection molding is carried out in cavities sequentially for integration while moving the partitions.
In this case, a compact having a three-dimensional complicated shape which the MIM method is primarily intended for is very difficult to mold, and the realization of such a molding greatly complicates the die structure and the cost becomes higher.
In the case where the compact to be formed is small, it may be that the space for partitions cannot be secured or a sprue, a runner or the like cannot be arranged in the die.
Also, it is very difficult to obtain a plurality of compacts at a time without resolving the problem of a very complicated die structure and a higher cost.
Further, the provision of a multiplicity of sliding portions for moving the partitions shortens the life of the die.
Furthermore, as the result of adding the partitions, a multiplicity of sectional die surfaces are transferred to the compact and a superior surface cannot be obtained. Another problem is that a high dimensional accuracy cannot be secured due to the effect of the clearance unavoidably existing in the sliding parts of the partitions.
Japanese Unexamined Patent Publication (Kokai) No. 4-346604, on the other hand, discloses a method in which an inserted core is removed after forming a first compact and a second compact is molded by injection into the cavity formed by the core. Also, the second compact is molded by injection after the portion of the first compact to be in contact with the second compact is maintained at 20 to 70° C.
For the temperature of the outer surface of the first compact to be maintained at 20 to 70° C., however, it is necessary to wait until the temperature drops after injection molding in an ordinary injection molding machine, resulting in a very low productivity.
Also, the molding process using a core consumes a considerable length of time for mounting and demounting the core. Since the core is cooled once removed, it is necessary to wait until the core temperature increases to a predetermined level before continuing the injection molding, also resulting in a low productivity.
SUMMARY OF THE INVENTION
The present invention has been developed in view of the aforementioned problems, and a first object thereof is to provide a method of fabricating a metal composite compact in which the forming of a high binder concentration portion can be suppressed at the boundary surfaces of a plurality of compacts made of materials of different types or a material of the same type.
A second object of the invention is to provide a method of fabricating a metal composite compact including an integration of a plurality of compacts of the materials of different types or a material of the same type with a superior dimensional accuracy at low cost.
According to a first aspect of the invention, there is
Arai Tsuyoshi
Kayano Hisashi
Makino Isao
Mimura Eiji
Denso Corporation
Harness Dickey & Pierce PLC
Mai Ngoclan
LandOfFree
Method of fabricating metal composite compact does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of fabricating metal composite compact, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of fabricating metal composite compact will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2991591