Title:
Production method for sintered part
Kind Code:
A1


Abstract:
A production method for a sintered part comprises preparing a metal powder and a binder composed of a thermoplastic resin and a wax, mixing the metal powder and 40 to 60 vol. % of the binder with respect to the metal powder into a mixed powder, and heating and kneading the mixed powder into a raw material. This production method further includes supplying a predetermined amount of the raw material in a hole of a die and compacting the raw material into a green compact having a predetermined shape by pressing the raw material by a punch. This production method further includes ejecting the green compact from the hole of the die, removing the binder from the ejected green compact by heating, and sintering the green compact by heating so as to diffusion bond particles of the green compact. The compacting is performed by pressing at a moving rate U of the punch, which is not more than a rate calculated from the following equation (1). In this case, ΔP (Pa) is pressing power of the punch, μ (Pa·s) is viscosity of the raw material, L (m) is a length of the green compact, and De (m) is a corresponding tube diameter.


U=ΔP/(32μ×LDe2 (1)




Inventors:
Okahara, Masahiro (Matsudo-shi, JP)
Ishijima, Zenzo (Matsudo-shi, JP)
Kusano, Mitsuo (Tokyo, JP)
Suzuki, Kazuya (Tokyo, JP)
Hirano, Toru (Tokyo, JP)
Application Number:
12/320969
Publication Date:
11/26/2009
Filing Date:
02/10/2009
Assignee:
HITACHI POWDERED METALS CO., LTD. (Matsudo-Shi, JP)
HITACHI INDUSTRIAL EQUIPMENT SYSTEMS CO., LTD. (Tokyo, JP)
Primary Class:
International Classes:
B22F3/12; B22F1/00
View Patent Images:



Primary Examiner:
ZHU, WEIPING
Attorney, Agent or Firm:
OLIFF PLC (ALEXANDRIA, VA, US)
Claims:
What is claimed is:

1. A production method for a sintered part, comprising: preparing a metal powder and a binder composed of a thermoplastic resin and a wax; mixing the metal powder and 40 to 60 vol. % of the binder with respect to the metal powder into a mixed powder; heating and kneading the mixed powder into a raw material; supplying a predetermined amount of the raw material in a hole of a die; compacting the raw material into a green compact having a predetermined shape by pressing the raw material by a punch; ejecting the green compact from the hole of the die; removing the binder from the ejected green compact by heating; and sintering the green compact by heating so as to diffusion bond particles of the green compact, wherein the compacting is performed by pressing at a moving rate U of the punch, which is not more than a rate calculated from the following equation (1):
U=ΔP/(32μ×LDe2 (1) based on ΔP (Pa) as pressing power of the punch, μ (Pa·s) as viscosity of the raw material, L (m) as a length of the green compact, and De (m) as a corresponding tube diameter.

2. A production method for a sintered part, comprising: preparing a metal powder and a binder composed of a thermoplastic resin and a wax; mixing the metal powder and 40 to 60 vol. % of the binder with respect to the metal powder into a mixed powder; heating and kneading the mixed powder into a raw material; supplying a predetermined amount of the raw material in a hole of a die; compacting the raw material into a green compact having a predetermined shape by pressing the raw material by a punch; ejecting the green compact from the hole of the die; removing the binder from the ejected green compact by heating; and sintering the green compact by heating so as to diffusion bond particles of the green compact; wherein the die is made of a magnetic die material, the die has a forming surface at the hole, and the die is provided with a cooling device for running a cooling medium at the inside along the forming surface and is provided with a high-frequency induction heating device around the cooling device, wherein the raw material supplied in the hole of the die is heated by heating the hole by the heating device, wherein the punch is drive controlled by a servo device in the compacting, and wherein the green compact is cooled by cooling the hole of the die by the cooling device after the compacting, and then the ejecting is performed.

3. A production method for a sintered part, comprising: preparing a metal powder and a binder composed of a thermoplastic resin and a wax; mixing the metal powder and 40 to 60 vol. % of the binder with respect to the metal powder into a mixed powder; heating and kneading the mixed powder into a raw material; supplying a predetermined amount of the raw material in a hole of a die; compacting the raw material into a green compact having a predetermined shape by pressing the raw material by a punch; ejecting the green compact from the hole of the die; removing the binder from the ejected green compact by heating; and sintering the green compact by heating so as to diffusion bond particles of the green compact, wherein the die is made of a magnetic die material, the die has a forming surface at the hole, and the die is provided with a cooling device for running a cooling medium at the inside along the forming surface and is provided with a high-frequency induction heating device around the cooling device, wherein the raw material supplied in the hole of the die is heated by heating the hole by the heating device, wherein the punch is drive controlled by a servo device in the compacting, and the compacting is performed by pressing at a moving rate U of the punch, which is not more than a rate calculated from the following equation (1):
U=ΔP/(32μ×LDe2 (1) based on ΔP (Pa) as pressing power of the punch, μ (Pa·s) as viscosity of the raw material, L (m) as a length of the green compact, and De (m) as a corresponding tube diameter, and wherein the green compact is cooled by cooling the hole of the die by the cooling device after the compacting, and then the ejecting is performed.

4. The production method for the sintered part according to claim 1, wherein the compacting is performed at a moving rate U of the punch, which is not less than 80% of a rate calculated from the equation (1).

5. The production method for the sintered part according to claim 3, wherein the compacting is performed at a moving rate U of the punch, which is not less than 80% of a rate calculated from the equation (1).

Description:

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a production method for a sintered part using a powder metallurgical method, and specifically, the present invention relates to a production method for a small sintered part having a thin wall portion approximately 0.01 to 0.2 mm thick or having a convex portion.

2. Background Art

Powder metallurgical methods are broadly classified as die pressing methods and injection molding methods. In the die pressing method, a raw powder is supplied in a hole of a die, and the raw powder is compacted into a green compact by pressing by a punch. This green compact is sintered into a sintered compact. In the injection molding method, a raw powder and a large amount of a binder are mixed and are kneaded into a raw material having flowability, and the raw material is injected into a gap of a mold by pressing so as to obtain a green compact. This green compact is heated so as to remove the binder and is then sintered.

In the die pressing method, in order to ensure flowability of the raw powder and lubricity between the raw material and the die, a lubricant of approximately not more than 1 mass % may be mixed with the raw powder. Since the amount of the lubricant is small, the lubricant is easily removed by volatilizing in an early stage of the sintering step, and a degreasing step can be performed for a short time. In the die pressing method, a raw powder is filled into a die such that the raw powder is dropped from a powder feeding device into a cavity formed by the die assembly and a lower punch. The powder feeding device is called a feeder (powder box). In this method, a certain degree of unevenness in the filling of the raw powder inevitably occurs. In production of a small product having the above-described small portion, this unevenness is not within an acceptable range. When the raw powder is filled in a small gap formed in a die in order to obtain the above small portion, a raw powder having small particle sizes must be used. In this case, the flowability of the raw powder is decreased, and the fillability of the raw powder is decreased, whereby the raw powder cannot be reliably supplied.

In the injection molding method, a green compact having a shape of an undercut or the like, which cannot be formed by the die pressing method, can be formed. In this case, in order to securely obtain the flowability of a raw material, 30 to 70 vol. % of a binder, such as a thermoplastic resin, is mixed and is kneaded with a raw powder. Therefore, a green compact includes a substantial amount of the binder, and a step for removing the binder takes a long time. For a thin wall portion having a thickness of approximately 0.1 to 0.3 mm, a cavity of a mold is too small, whereby a metal powder is not easily uniformly supplied into the cavity. In the injection molding method, a raw material is injected into a mold through a gate and a runner. Therefore, when a gap of a mold to be filled with a raw material is small, the raw material must be injected into the gap at high pressure. However, a high-pressure device for the injection molding method is not practical, because the metal powder and a binder may be separated from each other, and burrs because of the mold may be formed. Alternately, improving of the flowability of a raw powder is investigated by increasing a binder, but increasing of a binder leads to increase in dimensional shrinkage after sintering, whereby a sintered compact may be deformed. Accordingly, the minimum limit of the thickness that can be practically injection-molded may be 0.5 mm.

In view of the above circumstances, a forming method having the advantages of the die pressing method and the injection molding method has been suggested in Japanese Patent Application Publication of Laid-Open No. 2006-344581. In this forming method, a binder is added to a raw powder in a greater amount than that used in the typical die pressing method so as to obtain a raw material, and the raw material is compacted by a die pressing method. The invention disclosed in Japanese Patent Application Publication of Laid-Open No. 2006-344581 relates to an electrode for a cold cathode fluorescent lamp. This invention provides a production method including preparing a metal powder composed of one of Mo and W, and a binder composed of a thermoplastic resin and a wax, mixing the metal powder and 40 to 60 vol. % of the binder with respect to the metal powder into a mixed powder, and heating and kneading the mixed powder into a raw material. This production method further includes supplying a predetermined amount of the raw material in a hole of a die and compacting the raw material into a green compact having a cylindrical portion and a bottom by pressing the raw material by a punch. This production method further includes ejecting the green compact from the hole of the die, removing the binder from the ejected green compact by heating, and sintering the green compact by heating so as to diffusion bond particles of the green compact. As a result, a small sintered part having a small portion of a cylindrical portion 0.1 to 0.2 mm thick is produced. In Japanese Patent Application Publication of Laid-Open No. 2006-344581, the compacting may be performed by heating the raw material to a temperature of not less than a softening point of the thermoplastic resin, and the ejecting may be performed by cooling the green compact to a temperature of not more than the softening point of the thermoplastic resin and not less than a softening point of the wax.

The invention disclosed in Japanese Patent Application Publication of Laid-Open No. 2006-344581 is preferably used in a production of small sintered parts having a thin wall portion approximately 0.1 to 0.2 mm thick or having a convex portion. In this case, the compacting is performed by heating the raw material to a temperature of not less than the softening point of the thermoplastic resin, and the ejecting is performed by cooling the raw material to a temperature of not more than the softening point of the thermoplastic resin and not less than the softening point of the wax. Therefore, a forming cycle of this production method takes a long time. One forming cycle of the above die pressing method is performed through only steps of filling of a raw material, compacting, and ejecting. In contrast, one forming cycle of the invention disclosed in Japanese Patent Application Publication of Laid-Open No. 2006-344581 is performed through steps of supplying a raw material, heating the raw material, compacting, cooling the compact, and ejecting. Accordingly, compared to the die pressing method, one forming cycle of this invention has more steps, and therefore, it takes longer. Therefore, for mass production, shortening of the forming cycle is important.

SUMMARY OF THE INVENTION

In view of these circumstances, an object of the present invention is to provide a technique for improving mass production by shortening the forming cycle disclosed in Japanese Patent Application Publication of Laid-Open No. 2006-344581.

The present invention is based on findings obtained from investigations regarding characteristics of raw material that is heated and is fluidized. The essential feature of the present invention is that shortening of a compacting time and a forming cycle is achieved by improving a die assembly so as to heat a raw material before compacting and to reduce the time for cooling a green compact after compacting.

Specifically, according to a first aspect of the present invention, the present invention provides a production method for a sintered part. This production method includes preparing a metal powder and a binder composed of a thermoplastic resin and a wax, mixing the metal powder and 40 to 60 vol. % of the binder with respect to the metal powder into a mixed powder, and heating and kneading the mixed powder into a raw material. This production method further includes supplying a predetermined amount of the raw material in a hole of a die and compacting the raw material into a green compact having a predetermined shape by pressing the raw material by a punch. This production method further includes ejecting the green compact from the hole of the die, removing the binder from the ejected green compact by heating, and sintering the green compact by heating so as to diffusion bond particles of the green compact. The compacting is performed by pressing at a moving rate U of the punch, which is not more than a rate calculated from the following equation (1). In this case, ΔP (Pa) is pressing power of the punch, μ (Pa·s) is viscosity of the raw material, L (m) is a length of the green compact, and De (m) is a corresponding tube diameter.


U=ΔP/(32μ×LDe2 (1)

In the first aspect of the present invention, the compacting is preferably performed at a moving rate U of the punch, which is not less than 80% of the rate calculated from the equation (1).

According to a second aspect of the present invention, the present invention provides a production method for a sintered part for shortening a forming cycle. This production method includes preparing a metal powder and a binder composed of a thermoplastic resin and a wax, mixing the metal powder and 40 to 60 vol. % of the binder with respect to the metal powder into a mixed powder, and heating and kneading the mixed powder into a raw material. This production method further includes supplying a predetermined amount of the raw material in a hole of a die and compacting the raw material into a green compact having a predetermined shape by pressing the raw material by a punch. This production method further includes ejecting the green compact from the hole of the die, removing the binder from the ejected green compact by heating, and sintering the green compact by heating so as to diffusion bond particles of the green compact. In this case, the die is made of a magnetic die material, and the die has a forming surface at the hole. The die is provided with a cooling device for running a cooling medium at the inside along the forming surface and is provided with a high-frequency induction heating device around the cooling device. The raw material supplied in the hole of the die is heated by heating the hole by the heating device, and the punch is drive controlled by a servo device in the compacting. The green compact is cooled by cooling the hole of the die by the cooling device after the compacting, and then the ejecting is performed.

According to a third aspect of the present invention, the present invention provides a production method for a sintered part for shortening the time of the above compacting, the time of heating the raw material before the compacting, and the time of cooling the green compact after the compacting. This production method includes preparing a metal powder and a binder composed of a thermoplastic resin and a wax, mixing the metal powder and 40 to 60 vol. % of the binder with respect to the metal powder into a mixed powder, and heating and kneading the mixed powder into a raw material. This production method further includes supplying a predetermined amount of the raw material in a hole of a die and compacting the raw material into a green compact having a predetermined shape by pressing the raw material by a punch. This production method further includes ejecting the green compact from the hole of the die, removing the binder from the ejected green compact by heating, and sintering the green compact by heating so as to diffusion bond particles of the green compact. In this case, the die is made of a magnetic die material, and the die has a forming surface at the hole. The die is provided with a cooling device for running a cooling medium at the inside along the forming surface and is provided with a high-frequency induction heating device around the cooling device. The raw material supplied in the hole of the die is heated by heating the hole by the heating device, and the punch is drive controlled by a servo device in the compacting. The compacting is performed at a moving rate U of the punch, which is not more than a rate calculated from the equation (1) based on ΔP (Pa) as pressing power of the punch, μ (Pa·s) as viscosity of the raw material, L (m) as a length of the green compact, and De (m) as a corresponding tube diameter. The green compact is cooled by cooling the hole of the die by the cooling device after the compacting, and then the green compact is ejected.

In the third aspect of the present invention, the compacting is preferably performed at a moving rate U of the punch, which is not less than 80% of the rate calculated from the equation (1).

According to the present invention, the present invention provides a production method for a sintered part. The production method includes preparing a metal powder and a binder composed of a thermoplastic resin and a wax, mixing the metal powder and 40 to 60 vol. % of the binder with respect to the metal powder into a mixed powder, and heating and kneading the mixed powder into a raw material. The production method further includes supplying a predetermined amount of the raw material in a hole of a die and compacting the raw material into a green compact having a predetermined shape by pressing the raw material by a punch. The production method further includes ejecting the green compact from the hole of the die, removing the binder from the ejected green compact by heating, and sintering the green compact by heating so as to diffusion bond particles of the green compact. In the present invention, a forming cycle including steps of supplying, compacting, and ejecting, is shortened, whereby the mass production of sintered parts is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows frame formats (drawings in the upper side) of compacting steps A to E relating to the present invention and shows a graph (graph in the lower side) indicating relationships between a lowered amount of an upper punch and a compacting load in the compacting steps.

FIG. 2 shows photographs of a green compact in each compacting step A to E.

FIG. 3 is a graph showing a correlation between an extruded amount of a thin wall portion and a load required for extrusion in compacting, which are indicated in terms of measured value and theoretical value.

FIGS. 4A and 4B are sectional views showing an embodiment for preferably performing the production method of the present invention, and the embodiment has a die assembly and a structure for obtaining one green compact by using the die assembly.

FIG. 5 is a control block diagram indicating a control system of the die assembly shown in FIGS. 4A and 4B.

FIG. 6 is a diagram showing an operation of the die assembly shown in FIGS. 4A and 4B.

FIG. 7 is a diagram showing the time required for one forming cycle of an embodiment.

FIG. 8 is a cross sectional photograph of a sintered part obtained in an embodiment.

PREFERRED EMBODIMENTS OF THE INVENTION

An embodiment of the present invention is described with reference to the figures hereinafter.

“Characteristics of Raw Material”

First, results of experiments for investigating movement of a raw powder in compacting are described.

A tungsten powder having an average particle diameter of 2 μm as a metal powder and a resin binder primary made of a polyacetal resin and a paraffin wax were prepared. The metal powder and 56 vol. % of the resin binder with respect to the metal powder were mixed and were formed into a cylindrical solid pellet having an outer diameter of 1.88 mm and a total length of 2.97 mm, whereby a raw material was prepared. A die including a circular hole having a diameter of 2.08 mm was mounted with a band heater around the outer circumference thereof, and a lower punch was slidably fitted to the hole of the die. This die was placed on an Instron-type testing machine (SHIMADZU AUTOGRAPH). An upper punch having a diameter of 1.68 mm and an upper punch having a diameter of 1.88 mm were prepared, and one of the upper punches was placed on the Instron-type testing machine so as to be coaxial with respect to the hole of the die, whereby a die assembly was formed. The raw material was supplied in the hole of the die assembly, and the die and the raw material were heated to 433 K by the band heater. Then, the raw material was compacted into a green compact having a cylindrical shape of 0.1 mm or 0.2 mm thick and a bottom by lowering the upper punch at a rate of 0.08 mm/s. Changes in compacting load in this case are shown in FIG. 1. The drawings in the upper side in FIG. 1 shows the compacting steps in the order of A to E, and FIG. 1 shows a reference numeral 31 indicating a pellet of a raw material, a reference numeral 32 indicating a die having a hole 32a, a reference numeral 33 indicating a lower punch, and a reference numeral 34 indicating an upper punch. FIG. 2 shows photographs of a green compact in each compacting step A to E.

As shown in FIG. 1, the relationship of the moved distance of the upper punch and the compacting load includes three stages of an initial deformation stage (A to B), a middle deformation stage (B to C), and a late deformation stage (C to E). In the initial deformation stage (A to B), the upper punch and the raw material are brought into contact with each other (A), and the raw material is deformed, as the upper punch is lowered, and contacts the wall surface of the hole (B). In the middle deformation stage (B to C), after the raw material contacts the wall surface of the hole (B), the raw material is further deformed and fills the hole (C). In the late deformation stage (C to E), after the raw material fills the hole (C), a thin wall portion is extruded (D), and compacting is finished (E). In each of the initial deformation stage (A to B), the middle deformation stage (B to C), and the late deformation stage (C to E), the compacting load was increased at a constant rate with respect to the moving distance of the upper punch. In comparing a green compact with a thin wall portion having a thickness of 0.1 mm and a green compact with a thin wall portion having a thickness of 0.2 mm, the load for forming the thin wall portion of the former green compact was greater than that of the latter green compact. In addition, the increasing amount of the load of the former green compact was increased as the moving distance of the upper punch was increased, which was greater than that of the latter green compact.

As shown in FIG. 1, the load required for obtaining a predetermined cylindrical shape having a bottom was approximately 1.6 N in a case of forming a thin wall portion 0.2 mm thick, and was approximately 2.6 N in a case of forming a thin wall portion 0.1 mm thick. Therefore, the compacting can be performed at extremely low pressure, compared to that in a die pressing method. In this experiment, the cross sectional area of the upper punch was approximately 2.776 mm2 in the case of forming a thin wall portion 0.2 mm thick, and was approximately 2.217 mm2 in the case of forming a thin wall portion 0.1 mm thick. Accordingly, the compacting pressure of the upper punch is approximately 0.72 MPa in the case of forming a thin wall portion 0.2 mm thick, and is approximately 0.94 MPa in the case of forming a thin wall portion 0.1 mm thick.

In a backward extrusion of a bulk metal at a constant pressure, a portion of raw material, which contacts a container (die) and a wall surface portion of a punch, does not slide on the die, whereby extrusion pressure (compacting load) exhibits a constant value. On the other hand, in the above experiment, the compacting load required for extruding the thin wall portion in the late deformation stage (C to E) was increased at a constant rate in accordance with the increase in the moving distance of the upper punch. This movement of the above experiment is clearly different from that in a case of the backward extrusion of a bulk metal at a constant pressure.

In the late deformation stage in which a thin wall portion is extruded, the thin wall portion is formed while the raw material continuously slides on the die. Therefore, the frictional area is increased as the moving distance of the upper punch is increased, whereby the compacting load is increased. In this case, the raw material is semimelted and moves as a fluid after the raw material fills the hole (C), and therefore, the raw material slides on the die. Accordingly, deformation resistance of the mixture, that is, plastic deformation of a solid, is shown during the initial deformation stage to the middle deformation stage (A to C). In addition, rheological movement primarily depending on viscosity of the mixture, that is, flowability of the fluid, is shown during the extrusion of the thin wall portion (the late deformation stage: C to E).

Regarding the movement in the late deformation stage in a case of forming a thin wall portion having a thickness of 0.1 mm or 0.2 mm, load required for extruding the thin wall portion with respect to an extruded amount of the thin wall portion in compacting was calculated. In this case, a pressure drop equation (Hagen-Poiseuille equation) was used based on the above idea. The results are shown in FIG. 3, and the pressure drop equation is expressed by the following equation (2).


ΔP=32μ×L×U/De2 (2)

In this case, ΔP (Pa) is pressure drop, μ (Pa·s) is viscosity, L (m) is a length, and U (m/s) is a flow rate. In the present experiment, De (m) is a corresponding tube diameter and is a value of subtracting a diameter of an upper punch from a diameter of a hole of a die, and the value is twice the thickness t of a thin wall portion.

As shown in FIG. 3, for the correlation between the extruded amount of the thin wall portion and the load required for extruding the thin wall portion, the calculated values of the above-described pressure drop equation substantially corresponds to the measured values. According to this result, it was confirmed that a raw material, which is sufficiently heated to a temperature of not less than the melting point of the binder composition and is fluidized, can be used as a fluid.

According to the above experimental results, the following findings were obtained. When a raw material is heated to a temperature of not less than a softening point of a thermoplastic resin and is compacted, the raw material can be used as a fluid. In this case, the raw material is obtained by mixing a metal powder and a binder made of a thermoplastic resin and a wax into a mixed powder and by heating and kneading the mixed powder. In addition, load required for extrusion with respect to extruded amount at a small thin wall portion can be calculated from the above pressure drop equation.

Accordingly, when a raw material is used for forming a green compact, a moving rate U (m/s) of a punch is adjusted by pressing power ΔP (Pa) of the punch so as to be not more than a rate calculated from the following equation (1). The raw material has a viscosity μ (Pa·s) and includes a melted binder, and the green compact has a length L (m) and a thin wall portion t of a corresponding tube diameter De (m). As a result, a raw material moving as a fluid is sufficiently extruded and fills a hole of a die corresponding to a thin wall portion, whereby a satisfactory green compact is formed.


U=ΔP/(32μ×LDe2 (1)

A satisfactory green compact is obtained even when the moving rate U of the punch is smaller than that calculated from the equation (1). However, when the moving rate U of the punch is extremely small, the time required for the compacting is increased, and the productivity is thereby decreased. In view of this, the moving rate U of the punch is preferably set to be not less than 80% of the rate calculated from the equation (1) at the latest.

By increasing the pressing power ΔP of the punch, the moving rate U of the punch can be a high rate, and the time required for the compacting can be reduced.

The compacting pressure may be approximately 500 to 800 MPa in producing sintered parts having an ordinary density by a die pressing method, and the compacting pressure may be more than 1 GPa in producing sintered parts having a high density. In contrast, in compacting a raw material having flowability as in the present invention, as shown in FIG. 1, the compacting pressure of the upper punch is approximately 0.72 MPa in a case of forming a thin wall portion 0.2 mm thick and is approximately 0.94 MPa in a case of forming a thin wall portion 0.1 mm thick. The compacting can be performed at extremely low pressure compared to the pressure in a case of the die pressing method. Therefore, in compacting a raw material having flowability, a high-load pressing device having a press capacity of several tens of tons to several hundreds of tons, which may be used in the die pressing method, is not necessary, and a pressing device may be reduced in size. In compacting a raw material having flowability, a pressing device for precisely controlling a pressing power ΔP of a punch and a moving rate U of the punch, is preferably used. Therefore, a uniaxial servo pressing device for precisely controlling a stroke is preferably used.

“Improvement of Die Assembly”

As described above, a die assembly having a punch, which is precisely controlled by driving a servo device in compacting, is preferable, and a die assembly having the following structure is more preferable. In this case, in order to reduce the time of heating a raw material and the time of cooling a green compact after compacting, a cooling device that is relatively difficult to control is arranged inside the die assembly, and a high-frequency induction heating device that is easy to control is arranged outside of the cooling device. That is, since a high-frequency induction heating device is used, the surface of a hole of a die is directly heated by eddy currents generated in the vicinity of the surface, whereby the time of heating is greatly reduced. The cooling device for running a cooling medium is buried close to the surface of the hole of the die, whereby the surface of the hole of the die is rapidly cooled by running a cooling medium. According to such a structure, shortening of a cycle time of heating and cooling the die is easily controlled.

FIGS. 4A and 4B are sectional views showing an embodiment preferably used for performing the production method of the present invention, and the embodiment has a die assembly and a structure for obtaining one green compact by using the die assembly. FIG. 4A shows a condition in which a raw material is supplied, and FIG. 4B shows a condition in which compacting is finished. FIGS. 4A and 4B show a reference numeral 1 indicating a raw material to be compacted and a reference numeral 2 indicating a green compact with a cylindrical portion and a bottom after the raw material 1 is compacted. FIGS. 4A and 4B also show a reference numeral 3 indicating a fixed die (lower die) for compacting, into which the raw material 1 is supplied, and a reference numeral 4 indicating a movable die (upper die) that is provided above the fixed die 3 and compacts the raw material 1.

The fixed die 3 is provided with a cylindrical die 5 having a hole 5a at the center. The die 5 is a die part for extruding the raw material and is made of a magnetic metal that has good heat conductivity and is heated by high-frequency induction, such as iron. The die 5 is formed so as to have small mass and small volume in order to decrease the thermal capacity as much as possible. The movable die 4 is provided with an upper inner punch 6 for extruding the raw material 1 by lowering in the hole 5a of the die 5, and the movable die 4 is also provided with an upper outer punch 7 for limiting the height of the raw material 1 that is extruded.

FIG. 4A shows a reference numeral 9 indicating a cooling device buried close to a forming surface (wall surface of the hole) 5b of the die 5, and the cooling device has tubular portions, inside of which a cooling medium, such as cold water, runs. In order to reduce temperature variations on the forming surface 5b during cooling, a distance A of the tubular cooling device 9 to the forming surface 5b, and a pitch B between the tubular portions of the tubular cooling device 9 in a longitudinal direction, are approximately the same, as shown in FIG. 4B. A cooling medium (cold water) runs in the tubular portions of the cooling device 9 during cooling, and the cold water is drained by blowing pressurized air in the tubular portions so that the tubular portions are empty at the discharge of the green compact. This empty state of the tubular portions is maintained during heating. The discharge of the green compact is described later.

FIG. 4A shows a reference numeral 8 indicating a heating device provided around the cooling device. The heating device 8 has a high-frequency induction heating coil 8a buried within an insulator 8b, and the heating device 8 is wound around the cooling device 9. High-frequency induction heating has a large heating capability and is easily controlled, whereby the heating device 8 is arranged outside of the cooling device 9. A reference numeral 10 indicates a lower inner punch which receives the raw material 1 to be compacted, and the lower inner punch 10 and the upper inner punch 6 provide pressing power to the raw material 1 in the axial direction. The lower inner punch 10 pushes up the green compact 2 that is cooled and is solidified after compacting, and the lower inner punch 10 moves in the direction of an up-arrow, whereby the green compact 2 is discharged from the hole 5a.

FIGS. 4A and 4B show a reference numeral 11 indicating a heater for maintaining the entirety of the fixed die 3 at 120° C. and a reference numeral 12 indicating a heater for maintaining the entirety of the movable die 4 at 80° C. Accordingly, the die 5 and the fixed die 3 are maintained at 120° C., and the movable die 4 is maintained at 80° C. during forming. A reference numeral 14 indicates a thermal insulating plate arranged at the outer circumference of the die 5. The thermal insulating plate 14 thermally separates the die 5 from the fixed die 3, whereby the thermal capacity of the entirety of the lower die is decreased as much as possible. Since the fixed die 3 is continuously maintained at 120° C., the die 5 is maintained at similar temperature by air transmission.

The structure of the die assembly is described above, and the die assembly has a simple structure. Therefore, the die assembly is low cost, and maintenance thereof is easily performed.

FIG. 5 is a block diagram showing a structure of a control system of a die assembly. FIG. 5 shows a reference numeral 20 indicating a die assembly controlling structure including driving structures of the fixed die 3 and the movable die 4. FIG. 5 also shows a reference numeral 21 indicating a heating controller for controlling the heater 11 for maintaining the temperature of the fixed die 3, the heater 12 for maintaining the temperature of the movable die 4, and the high-frequency induction heating coil 8a. The heating controller 21 drives the high-frequency induction heating coil 8a at a frequency of approximately 25 kHz. FIG. 5 shows a reference numeral 22 indicating a cooling medium and air controller for supplying a cooling medium or air in the tubular portion of the cooling device 9. FIG. 5 also shows a reference numeral 23 indicating a die assembly controller for controlling operation of the die assembly controlling structure 20, and shows a reference numeral 24 indicating a control device for controlling each controller.

An operation of the die assembly having the above structure is described with reference to FIGS. 4A, 4B, and 6. A raw material 1 is supplied to the hole 5a of the die 5 (FIG. 6: Raw material supply steps T0 to T2), and simultaneously heating of the die 5 of the fixed die 3 is started (FIG. 6: Steps T1 to T2), as shown in FIG. 4A. Since the heating is performed by high-frequency induction, eddy currents are generated in the vicinity of the surface of the die 5 made of a magnetic material provided inside of the heating coil 8a, and the entirety of the die 5, including the surface thereof, is directly heated by the eddy currents. Therefore, the forming surface 5b of the die 5, that is, the surface contacting the raw material 1, is directly heated, whereby heat is transmitted to the raw material 1 at high efficiency.

At that time, a cooling medium (cold water) is drained from the tubular portion of the cooling device 9 by blowing air by the cooling medium and air controller 22, and the tubular portion is empty. Accordingly, the thermal capacity of the die 5 is small, and efficiency of the thermal transmission is high, as described above, whereby the time required for raising the temperature by 30° C., which is from 120° C. to 150° C., is greatly reduced (FIG. 6: Heating steps T1 to T4). According to the embodiment, the temperature was raised from 120° C. to 150° C. in 2 to 3 seconds.

When the raw material 1 is heated to 150° C. at the time T4 in FIG. 6, the movable die 4 is lowered in the direction of a down-arrow and reaches a bottom dead center as shown in FIG. 4B. As a result, the raw material 1 is extruded by the upper inner punch 6, and the height of the green compact 2 is set by the upper outer punch 7 (FIG. 6: Steps T4 to T5).

In the compacting timing of the above T4 to T5, simultaneously, heating by the heating device 8 is stopped, and cooling operation of the cooling device 9 is started instead. That is, cold water at 4° C. is supplied as a cooling medium into the tubular portion of the cooling device 9. Since the tubular portion is buried close to the forming surface 5b of the die 5, the inside of the die 5 is first cooled, and the entirety of the die 5 is then cooled (FIG. 6: Cooling steps T5 to T7). In the cooling steps, the die 5 is cooled by 30° C. from 150° C. to 120° C. The die 5 has a small thermal capacity and is cooled from the inside thereof, whereby the green compact 2 is cooled at high efficiency, and the temperature is lowered in a short time. According to the embodiment, when a cold water at 4° C. was used as a cooling medium, the temperature was lowered by 30° C. in approximately 3 seconds.

After the green compact 2 is solidified by cooling, the movable die 4 is raised and is returned to the top dead center as shown in FIG. 4A, and the lower inner punch 10 is moved in the direction of the up-arrow (FIG. 6: Steps T8 to T9). Then, the green compact 2 is pushed upward from the hole 5a (FIG. 6: Steps T9 to T10) and is discharged to the outside of the die assembly by a device (not shown in the figure) (FIG. 6: Step T10). These timings of T8 to T10 form a step of ejecting a green compact.

Substantially simultaneously with the timings T8 to T10 of the step for ejecting the green compact, the cooling medium in the cooling device 9 is drained. Specifically, air is blown into the tubular portion of the cooling device 9 by the cooling medium and air controller 22, whereby the cold water is drained, and the tubular portion is emptied. After the draining, a subsequent raw material 1 is provided to the die 5 for a next heating step. By the draining, the cold water, which is heated by the maintaining heat of the die assembly, is not boiled in the tubular portion of the cooling device 9 in the next heating step.

The above timings T0 to T10 form one forming cycle, and the forming cycle time can be reduced by performing the heating steps and the cooling steps at high rates. The die assembly is moved only in vertical direction, and the operation of the die assembly is simple, whereby the operation of forming and maintenance are easily performed, and the die assembly is easy to use.

In order to obtain a green compact having a cylindrical portion 0.2 mm thick and a bottom, steps of adjusting a raw material, supplying the raw material, and compacting, were performed. In this case, the lowering rate of the upper punch was set to be a high rate of 5 mm/second so as to reduce the time required for compacting according to the above-described equation (1), and the above die assembly structure was used so as to reduce the times required for the heating steps and the cooling steps. As a result, as shown in FIG. 7, one forming cycle including steps of supplying a raw material, heating the raw material, compacting, cooling a green compact, and ejecting, was performed at 10 seconds/cycle, which is similar to that in a typical die pressing method.

A binder was removed from this green compact having a cylindrical portion and a bottom, which had compacted by the above-described cycle, and this green compact was sintered. FIG. 8 shows a cross sectional photograph of this sintered part. As shown in the photograph in FIG. 8, a satisfactory sintered part having a thickness of 0.2 mm was obtained.