JPH04333501A - Production of metal sintered body - Google Patents

Abstract

PURPOSE:To produce a metal sintered body by forming a fine metal powder along with a thermoplastic binder, rapidly heating and degreasing the formed body in a short time to remove the binder and sintering the formed body. CONSTITUTION:A formed body 8 is embedded in the powder 15 of a backup material placed in a vacuum box 9, the opening of the box 9 is covered with a flexible sheet 16, the box is evacuated, and the formed body 8 is hydrostatically backed up by the powder 15, rapidly heated at the rate of about 100-300 deg.C/hr and degreased while preferably lowering the vacuum as the degreasing proceeds. Consequently, the degreasing time is reduced from 60-100hr in the conventional process to 4-6hr.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method of manufacturing a metal sintered body, and more particularly to a so-called metal powder injection method in which metal powder is injection molded together with a thermoplastic binder, the binder is removed by degreasing, and then sintered. The present invention relates to a degreasing method in a method of manufacturing a metal sintered body based on a molding method.

0002

[Prior Art] In the conventional manufacturing of metal sintered bodies, metal powder filled in a mold is first press-molded to form a green compact.
Next, this green compact is placed in a sintering furnace, heated at a predetermined temperature, and sintered to form a sintered body, which is the so-called powder metallurgy method.

However, in the case of powder metallurgy, since the metal powder is molded using a press, the shapes of products that can be molded are restricted, and only products with extremely simple shapes can be formed.

For this reason, in recent years, a so-called metal powder injection molding method has been developed, which is a method for producing a sintered body using injection molding as a metal powder molding technique.

[0005] According to the metal powder injection molding method, metal powder is molded by injection molding, so unlike press molding in powder metallurgy, products of any complex shape can be easily and accurately molded. Now it can be molded.

In this manufacturing method, a thermoplastic binder is mixed with fine metal powder having a particle size of about 10 μm or less, and a so-called green body, which is an injection molded product, is first formed by injection molding.

[0007] As a thermoplastic binder, ethylene
Thermoplastic synthetic resins such as vinyl acetate copolymer, poly(meth)acrylate, polyethylene, polypropylene, polystyrene, and polyamide, plasticizers such as dibutyl phthalate, waxes such as paraffin wax, etc. An appropriate mixture is used.

Next, in order to remove the binder in the green body before sintering, the green body is placed in a heating furnace and heated to about 300°C, which is the highest melting point temperature of the binder component. Degreasing is performed to remove the binder in the body by melting, decomposition, evaporation, etc. to form a so-called brown body, which is a binder-free body from which the binder has been removed.

Finally, the brown body from which the binder has been removed is heated and sintered at a predetermined sintering temperature in a sintering furnace to form a sintered body.

Conventional degreasing, when the product shape is simple,
As shown in FIG. 3, the green body 1 is placed on a porous ceramic plate 3 such as an alumina filter placed on a tray 2 for absorbing the molten binder.

If the product shape is complex, as shown in FIG. 4, the green body 4 is placed on a box-shaped tray 5 with a porous ceramic plate 6 underneath to improve the absorption of the molten binder. At the same time, the surrounding area is filled with refractory powder 7 such as alumina and degreased.

0012

[Problems to be Solved by the Invention] However, since the binder thermally expands during degreasing, deformation, cracking, blistering, etc. are likely to occur in the brown body after degreasing.

[0013] In order to prevent such defects from occurring, heating for degreasing is performed at a slow rate of about 3 to 5°C per hour so that the binder components with the lowest melting point are gradually removed. It was done at speed.

If degreasing at such a slow heating rate does not prevent defects due to complex product shapes, the green body may be replaced with alumina as shown in FIG. It has also been done to degrease the binder by embedding it in powder of a refractory material such as powder and backing it up, and then absorbing the binder into the powder of the backup material to promote degreasing.

However, even if the green body is embedded in refractory powder and degreased, a large amount of gas is generated due to thermal decomposition of the binder as degreasing progresses, causing the layer of refractory powder to collapse. Since a gap is created between the green body and the green body, sufficient backup cannot be achieved, and defects also occur.

[0016] As described above, conventionally, when degreasing is heated at 3°C/hour, it takes a very long time of about 100 hours to heat up to 300°C, and furthermore, it has not been possible to completely prevent the occurrence of defects.

The object of the present invention is to eliminate these drawbacks of the prior art and to provide a degreasing method that has a very fast heating rate, short degreasing time, and almost no defects.

[0018]

[Means for Solving the Problems] That is, the present invention includes a molding process of molding a molding composition comprising fine metal powder and a binder to form a green body, and a molding process of heating the green body to release the binder. A method for manufacturing a metal sintered body comprising a degreasing step of removing fat to form a brown body, and a sintering step of sintering the brown body to form a sintered body. The process consists of embedding the decompression box in backup material powder in the box, reducing the pressure by vacuum evacuation after covering the opening of the decompression box with a flexible sheet, and heating the green body to a predetermined degreasing temperature. - The body is degreased while being hydrostatically backed up with backup material powder under reduced pressure, and more preferably, the degree of vacuum in the vacuum box during the degreasing operation is adjusted to reduce the amount of binder from the green body as the degreasing progresses. This method of manufacturing a metal sintered body is characterized in that the back-up force is adjusted to be gradually lowered and weakened depending on the degree of removal.

[0019]

[Operation] The present invention is constructed as described above, and during degreasing,
The green body is embedded in the vacuum material powder under reduced pressure in a vacuum box, and the opening of the vacuum box is covered with a flexible sheet, so that when pressure is applied from this opening surface, the vacuum The material layer is constantly in close contact with the entire circumference of the green body and is backed up by hydrostatic pressure, so even if thermal decomposition gas from the binder is generated, there will be no gap between the backed up material layer and the green body. Since no voids are formed, defects such as cracks will not occur even if heated rapidly.

Furthermore, during degreasing, the amount of binder removed from the green body increases as the degreasing progresses.
As the strength of the green body weakens, the degree of decompression is lowered to weaken the back-up force of the back-up material.
Since the green body is backed up with an appropriate backing force depending on its strength, the occurrence of defects can be further prevented.

[0021]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a vertical cross-sectional view showing an embodiment of the present invention. Reference numeral 9 denotes a vacuum box with an open top, and a heat-resistant filter 11 made of ceramic, stainless steel, etc. is attached to the top of the vacuum box 1. A vacuum exhaust pipe 10 attached to the vacuum exhaust pipe 10 is connected to the vacuum exhaust pipe 10, and a vacuum connecting pipe 12 that connects to a vacuum pump (not shown) is connected to the vacuum connecting pipe 12. An intake pipe 13 is connected thereto.

Reference numeral 15 denotes a back-up material powder, which is a refractory powder such as alumina, filled in the vacuum box 9, and the back-up material powder 15 is sucked into the vacuum exhaust pipe 10 during evacuation. This is prevented by a filter 11, and the degree of pressure reduction in the pressure reduction box 9 can be adjusted by adjusting the amount of intake air from the intake pipe 13 with a control valve 14. There is.

Degreasing is performed by embedding the green body 8 in the backup material powder 15 and then using a vacuum box 9 whose upper opening is covered with a heat-resistant flexible sheet 16 such as metal foil.
is placed in a heating furnace, a vacuum connection pipe 12 is connected to the vacuum exhaust pipe 10 that comes out from a piping hole provided in the furnace, and the inside of the decompression box 9 is heated while being depressurized by the operation of a vacuum pump. .

During this degreasing work, when the vacuum pump is operated and the pressure inside the decompression box 9 is reduced, the flexible sheet 16 covering the opening is adsorbed to the surface of the layer of the backup material 15, and the backup material 15 is Pressure is applied based on the pressure difference with the atmosphere, making it solid.

Therefore, regardless of the shape of the green body 8 embedded in the backup material 15, the green body 8 is in a state where the entire surface is uniformly pressed by the backup material 15 and is subjected to hydrostatic pressure backup. Become.

When heating is performed in this state, the heat heats the layer of the back-up material 15 and then is transmitted to the surface of the green body 8, and the binder in the green body 8 is softened and melted from the surface in order, and the back-up material 15 is heated. The binder is sucked out into the binder 15 by the decompression action, and communicating holes, which are holes formed by the binder being sucked out, are formed in the green body 8 at an accelerated rate.

At this time, since the backup material 15 is properly backed up, all of the binder is sucked out through the communicating holes, and the binder expands and is sucked out from various directions. There is no chance of the green body cracking.

[0029] Note that the heating at this time was not done at a very slow rate of 3 to 5°C/hour as in the past, but at a rate of 100 to 30°C/hour.
The temperature is raised to about 300°C, which is the highest melting point among the components constituting the binder, at an extremely fast rate of 0°C/hour.

Furthermore, as the degreasing progresses and the binder is sucked out, the strength of the green body 8 becomes weaker, so it is necessary to weaken the back-up force by the back-up material 15 according to the progress of the degreasing. This is done by sucking atmospheric air through the intake pipe 13 by adjusting the control valve 14 based on the degreasing time and temperature, thereby lowering the degree of vacuum in the vacuum box 9.

The degree of vacuum in the vacuum box 9 during degreasing varies depending on the shape of the green body, etc., but in general, the degree of vacuum in the vacuum box 9 is adjusted to 10 torr until the degreasing temperature reaches 100°C.
The degree of pressure reduction is preferably about 50 to 100 torr at 100 to 200°C, and about 200 to 300 torr at 200 to 300°C.

Next, as shown in Fig. 2, a relatively complex part with a thickness of about 9 mm and a length of about 35 mm, which was conventionally prone to cracking and other defects during degreasing, was degreased under various conditions. An example will be explained while comparing it with a conventional example.

First, the following three types of compositions were used for injection molding. However, the composition is shown in parts by weight.

A. SUS304 100, EVA resin 3.70, acrylic resin 2.76, paraffin wax 2
．． 76, DBP 1.27 B. SUS304 100, acrylic resin 7.0
, paraffin wax 1.0, stearic acid 1.0
C. Fe-8%Ni 100, EVA 3.0, acrylic 2.5, paraffin wax 2.5, D
BP 1.0

The above composition was injection molded under the following conditions to obtain a green body. Nozzle temperature 120-150℃ Injection pressure
650kg/cmsqr

To degrease the green body, the body was embedded in alumina powder having an average particle size of 2 μm filled in a vacuum box as shown in FIG. 1, and heated under the following three conditions. stomach . Raise the temperature at a rate of 100°C/hour for 30
Hold at 0°C for 3 hours. B . Raise the temperature at a rate of 200°C/hour for 30
Hold at 0°C for 3 hours. C. Raise the temperature at a rate of 300°C/hour for 30
Hold at 0°C for 3 hours.

The degree of vacuum in the vacuum box was adjusted as follows. Gradually lower the temperature to 100°C, 10 torr, 200°C, and 100 torr. Gradually lower the temperature to 300°C and 300 torr.

[0036] Compositions A to C 3 under the reduced pressure conditions as described above.
According to the results of examining nine types of Examples in which each type was degreased under three types of heating conditions (A to C), defects such as deformation, cracking, and blistering did not occur in any of the Examples. In addition, the binder removal rate measured after degreasing was 94.
．． The carbon content was extremely high, ranging from 7 to 98.7%, indicating that degreasing was performed extremely efficiently, and a good sintered body with a predetermined carbon content maintained through subsequent sintering was obtained.

On the other hand, three types of green bodies injection molded with the compositions A to C were simply embedded in alumina powder and heated under the heating conditions A to C as in the example. All of the nine types of conventional examples that were degreased had some kind of defect such as cracking, deformation, surface peeling, or blistering, and collapsed. For this reason, it was not possible to measure the binder removal rate after degreasing, and it was also impossible to form a sintered body by sintering.

[0038]

[Effects of the Invention] The present invention has the structure and operation as described above, and the green body is degreased while being well backed up by the back-up material powder under hydrostatic pressure, and can be rapidly heated during degreasing. Therefore, the degreasing time, which was conventionally about 60 to 100 hours, is significantly shortened to about 4 to 6 hours.

Next, since degreasing is carried out under reduced pressure,
For example, the boiling point of paraffin wax is 350~ at atmospheric pressure.
When the pressure is reduced to 10 torr even though the temperature is 450°C, the binder evaporates very easily at 180-250°C, and the degreasing temperature is as low as 300°C compared to the conventional degreasing temperature of 350-400°C. Since it can be degreased quickly, oxidation of the metal components that form the sintered body can be prevented.

[0040] Also, the degree of decompression can be adjusted to reduce grease during degreasing.
Since the hydrostatic back-up force by the back-up material of the body can be adjusted, the back-up force can be adjusted according to the strength of the green body. Therefore, it is possible to back up the injection molded green body according to the strength of the injection molded green body based on the type and amount of binder, and even if the amount of binder decreases and the strength decreases as degreasing progresses. Since the back-up can be performed according to the amount of pressure, the back-up force will not be too strong and cause the green body to crack.

Furthermore, since the removal rate of the binder is extremely high, the binder remaining during sintering may carbonize, causing variations in the amount of carbon contained, or lowering the melting point of the alloy to bring it to a molten state. There is no possibility that the sintered body will be deformed or become unable to be shaped.

As described above, the present invention allows degreasing to be performed in an extremely short period of time and a good product to be obtained, regardless of the type or amount of the metal component to be sintered or the binder. The present invention provides a method for manufacturing a metal sintered body based on a metal powder injection molding method, which allows a product of any shape to be easily obtained as a metal sintered body.

[Brief explanation of drawings]

[Fig. 1] A vertical cross-sectional view showing the degreasing method of the present invention,

[Figure 2] Product diagram used in the example of the present invention,

[Figure 3]
A longitudinal cross-sectional view showing a first conventional degreasing method,

[Figure 4]
FIG. 7 is a vertical cross-sectional view showing a second conventional degreasing method.

[Explanation of symbols]

8.　Green body 9.　　Decompression box 10. Vacuum exhaust pipe 11. Filter 12. Vacuum connection tube 13. Intake pipe 14. Intake volume control valve 15. Backup material powder 16.　Flexible sheet

Claims (2)

[Claims] 1. A molding step of molding a molding composition comprising fine metal powder and a binder to form a green body;
A method for manufacturing a metal sintered body, which comprises a degreasing step of heating the green body to remove the binder to form a brown body, and a sintering step of sintering the brown body to form a sintered body. The degreasing process includes embedding the green body in backup material powder in a vacuum box, covering the opening of the vacuum box with a flexible sheet and reducing the pressure by vacuum evacuation, and removing the green body from a predetermined area. 1. A method for producing a metal sintered body, which comprises a step of heating to a degreasing temperature, and is characterized in that the green body is hydrostatically backed up with backing up material powder under reduced pressure while being degreased. 2. The degree of vacuum in the vacuum box during degreasing work is adjusted to gradually lower and weaken the back-up force in accordance with the degree of removal of binder from the green body as degreasing progresses. The method for producing a metal sintered body according to claim 1.