Patent Application
20090291012
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.
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×L)×De2 (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.
An embodiment of the present invention is described with reference to the figures hereinafter.
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
As shown in
As shown in
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
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
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×L)×De2 (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
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.
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.
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.
An operation of the die assembly having the above structure is described with reference to
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 (
When the raw material 1 is heated to 150° C. at the time T4 in
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 (
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
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
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-132905 | May 2008 | JP | national |
