JPH11131103A - Composition for powder injection molding and production of powder injection molded goods - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compsn. for powder injection molding for obtaining sintered compacts having excellent dimensional accuracy without the occurrence of warpage, deformation, etc., in parts by uniformly progressing sintering shrinkage in a powder injection molding method using powder for sintering and a process for producing powder injection molded goods by using the same. SOLUTION: The compsn. for powder injection molding is manufactured by kneading the powder for sintering and resins of which pyrolysis ends at a temp. below the sintering shrinkage initiation temp. of the powder for sintering as the respective components of an org. binder. This compsn. is molded to form injection moldings. The respective components of the org. binder are removed at the temp. below the sintering shrinkage initiation temp. of the powder for sintering by which the degreased moldings are obtd. in a degreasing stage of the injection moldings. This degreased bodies are sintered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a powder injection molding composition by kneading a sintering powder and at least one kind of thermoplastic resin as an organic binder component, and molding the same to form an injection molded article. And removing the organic binder contained in the injection-molded body to obtain a degreased body, and then sintering the degreased body to form a sintered body, a powder injection molding composition used in a powder injection molding method, and The present invention relates to a method for producing the powder injection molded article.

[0002]

2. Description of the Related Art The powder injection molding method is widely used as a technique capable of mass-producing three-dimensionally complex parts with high dimensional accuracy. In the powder injection molding method, an organic binder which is a thermoplastic resin is dispersed and mixed in a powder for sintering, molded by an injection molding method, and then the resin contained in the molded body is removed, that is, degreasing is performed. Thereafter, the compact is sintered to produce a desired sintered part.

In this powder injection molding method, a thermal decomposition method is widely used as a method for removing an organic binder from a molded article. For example, JP-A-2-57615 discloses an example in which an iron-based powder is used as a sintering powder, a thermoplastic resin and / or a wax is mainly used as an organic binder, and a plasticizer, a lubricant, and a degreasing accelerator are added. It is described in.

[0004]

However, in the conventional organic binder composition and degreasing conditions in the thermal decomposition degreasing step, the relationship between the sintering shrinkage starting temperature of the sintering powder and the decomposition ending temperature of the organic binder component is considered. I didn't. Here, the powder sintering shrinkage start temperature is a temperature at which the shrinkage of the size of the compact starts due to sintering of the sintering powder constituting the compact, which is the material, particle size and shape of the powder used. And the heating atmosphere. In the prior art, when the decomposition end temperature of the organic binder component is higher than the sintering shrinkage starting temperature of the sintering powder, when the sintering shrinkage of the molded body proceeds, the organic binder contained in the molded body is increased. There is a problem that the atmosphere in the furnace is contaminated by the decomposition gas, and as a result, the dimensional accuracy of the sintered body is reduced. Hereinafter, this problem will be described in detail with examples.

For example, as a sintering powder, an iron powder having a sintering shrinkage initiation temperature of 450 ° C. in a hydrogen atmosphere is used.
A case where an organic binder composition in which the thermal decomposition end temperature of the organic binder is 600 ° C. will be described.

In the degreasing step, in order to completely remove the organic binder, the degreasing step is performed at 600 ° C. in an inert atmosphere.
As described above, when a component requiring high dimensional accuracy is manufactured using a powder that is easily oxidized, such as iron, degreasing must be performed at a temperature lower than 600 ° C. This is because, even under an inert atmosphere, the oxygen powder slightly contained in the atmosphere oxidizes the iron powder on the surface of the molded body that is in contact with the atmosphere. Here, sintering shrinkage of the powder is inhibited on the surface where oxidation has progressed, as compared with the surface where oxidation has not progressed such as the surface in contact with the firing jig. As a result, the degreased body shrinks non-uniformly, causing deformation of the sintered body and lowering dimensional accuracy. Therefore, in the case of iron powder, to prevent oxidation of the powder,
Degreasing must be performed at least at 400 ° C. or lower, and at 400 ° C., the organic binder remaining in the degreased body without being thermally decomposed must be removed in the initial stage of the sintering process.

Next, a sintered body is obtained by sintering the degreased molded body, that is, the degreased body. Here, the organic binder remaining in the degreased body is thermally decomposed in an initial stage of the sintering process,
It is removed by gasification. Here, especially when a large amount of the compact is charged into the sintering furnace, a large amount of the decomposition gas due to the residual binder is generated from around 400 ° C. And 4
From around 50 ° C., the sintering shrinkage of the iron powder proceeds in such a state that the atmosphere in the sintering furnace is contaminated with the gas. At this time,
The surface of the degreased body that was in contact with the sintering atmosphere was contaminated in the sintering furnace by the decomposition gas of the organic binder. Therefore, the shrinkage ratio becomes relatively small. As a result, there is a relative difference in shrinkage between the upper surface and the lower surface of the sintered body, and the sintered body is deformed. In particular, in the case of a thin-shaped part, this effect appears remarkably, and causes a warp deformation defect of the sintered body.

[0008] As described above, in particular, when parts are sintered in a large amount, the deformation defect generated due to the pyrolysis gas of the residual organic binder has been a serious problem of lowering the yield.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a sintered body which solves the above problems, prevents defective deformation of parts, has excellent dimensional accuracy, and a method for manufacturing the same.

[0010]

In order to achieve the above object, the composition for powder injection molding of the present invention adopts the following constitution.

That is, a powder injection molding composition is prepared by kneading a powder for sintering and at least one kind of thermoplastic resin as an organic binder component, and molding this into an injection molded body. Degreased to obtain a degreased body, and sintering the degreased body to produce a sintered body, the composition for powder injection molding in the powder injection molding method, wherein each component of the organic binder, It is characterized in that the resin is thermally decomposed below the sintering shrinkage starting temperature of the binder powder.

In order to achieve the above object, the method for producing a powder injection molded article of the present invention employs the following method.

That is, a resin that is thermally decomposed at or below the sintering shrinkage initiation temperature of the sintering powder as a component of the sintering powder and the organic binder is kneaded to prepare a powder injection molding composition. Is molded into an injection molded body, and in the degreasing step of the injection molded body, each component of the organic binder is removed at a temperature equal to or lower than a sintering shrinkage start temperature of the sintering powder to obtain a degreased body, and the degreased body is fired. It is characterized by tying.

According to the present invention, a powder injection molding composition is produced by kneading a sintering powder and at least one kind of thermoplastic resin as an organic binder, and molding this into an injection molded article. A method for producing a composition for powder injection molding and a method for producing the powder injection molded article, wherein the degreased body obtained by degreased the injection molded body is sintered to produce a sintered body, wherein each component of the organic binder is: The resin is characterized by being thermally decomposed below the sintering start temperature of the sintering powder. Thereby, when the sintering shrinkage of the molded body proceeds, no decomposition gas is generated from the residual organic binder, and the sintering can proceed without polluting the atmosphere in the sintering furnace.
Sintering shrinkage proceeds uniformly, and a sintered body having excellent dimensional accuracy without deformation can be obtained.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In the following, a composition for powder injection molding and a method for producing a powder injection molded product in the best mode for carrying out the present invention will be described.

First, in the present invention, any sintering powder of metals and ceramics generally used for powder injection molding can be used. In addition, two or more kinds of powders having different particle diameters, shapes, and materials can be mixed and used. Here, when the temperature of the powder compact is increased in a furnace, the dimensional shrinkage of the compact starts at a certain temperature with sintering of the powder. The temperature at which the dimensional shrinkage starts is called the sintering shrinkage start temperature. Here, the sintering shrinkage starting temperature of each powder can be known from the dimensional change of the compact taken out after heating the compact to an arbitrary temperature in a predetermined atmosphere.
The temperature at which dimensional shrinkage due to sintering of the molded body starts is the sintering shrinkage starting temperature of the powder.

The factors governing the sintering shrinkage initiation temperature include the material of the powder used, the amount of oxygen in the powder, the shape and particle size, the heating atmosphere, and the like. Generally, in the case of powders of the same material, the smaller the particle size and the higher the packing density, the lower the powder sintering shrinkage onset temperature. The present invention is particularly effective when a powder having a small particle diameter and a low sintering shrinkage initiation temperature is used.

Next, the organic binder used in the present invention is made of at least one kind of thermoplastic resin. Here, all the components constituting the organic binder in the present invention are resins that are thermally decomposed and removed at or below the sintering shrinkage starting temperature of the sintering powder contained in the powder injection molding composition. And

For example, as a powder for sintering, an average particle size of about 1
When a carbonyl iron powder of μm is used, its sintering shrinkage onset temperature is about 450 ° C. For example, when the above powder is used, paraffin wax, stearic acid, dibutyl phthalate and the like can be used for imparting fluidity to the composition for powder injection molding. It is also possible to use a combination of two or more of these resins. These components are removed by heating at 400 ° C. or lower in an inert atmosphere. Next, polybutyl methacrylate, poly-α-methylstyrene, or the like is used as a polymer component that imparts strength to the powder injection molded article. These polymer components are also resins whose decomposition is completed at about 400 ° C. in an inert atmosphere. Note that two or more of these resins can be used in combination.

First, the sintering powder and the organic binder are kneaded using a pressure kneader or the like to obtain a powder injection molding composition. After kneading, the mixture is granulated using a pelletizer or the like, if necessary, to obtain a composition for injection molding.

Next, the injection molding composition is molded by an injection molding machine to obtain an injection molded article having a desired shape.

Next, degreasing is performed to remove the organic binder contained in the compact. The degreasing atmosphere is selected according to the powder used, such as air, an inert atmosphere, and a reducing atmosphere. When a material that is easily oxidized such as iron is used as the sintering powder, the degreasing step is preferably performed in an inert atmosphere such as nitrogen or argon. In the present invention, degreasing may be performed under an arbitrary reduced pressure atmosphere. Further, in the present invention, a part of the organic binder component may be removed by solvent extraction or the like. By using these techniques, in the present invention, all the organic binder components can be removed below the sintering shrinkage onset temperature of the powder.

Next, the obtained degreased body is sintered at a predetermined atmosphere and temperature to obtain a sintered body. In the present invention,
Since all the organic binder components are decomposed and removed at a temperature lower than the temperature at which the sintering shrinkage of the powder starts, no decomposition gas of the residual organic binder is generated at the stage where the sintering shrinkage of the powder proceeds. Therefore, the atmosphere in the sintering furnace is not contaminated by the decomposition gas, the sintering shrinkage of the degreased body proceeds uniformly, and a sintered part having excellent dimensional accuracy without deformation can be produced.

[0024]

The present invention will be described in more detail with reference to the following examples.

(Example 1) Spherical carbonyl iron powder having an average particle diameter of 1 µm was used as a sintering powder, and poly-α-methylstyrene was 65% by weight as an organic binder.
Paraffin wax 30% by weight, stearic acid 5% by weight
And a mixture of 150 parts
The mixture was kneaded at ℃ to obtain a composition for powder injection molding. The mixing ratio of the powder and the organic binder was 9.2 parts by weight of the organic binder with respect to 100 parts by weight of the iron powder. Here, the thermal decomposition end temperature of the organic binder composition in this embodiment is about 400 ° C.

The composition for injection molding obtained in this manner is molded using an injection molding machine, and has a thickness of 1.5 mm and a diameter of 2 mm.
A 0 mm flat watch component was obtained. Here, in the present embodiment, the target value of the thickness of the sintered body in consideration of the shrinkage rate is:
1230 ± 50 μm.

Next, 16 compacts were arranged on a 12 cm square alumina plate, and 42 plates, that is, 672 in total, were degreased and sintered using a batch furnace. .

First, the molded body was subjected to thermal decomposition degreasing in a nitrogen atmosphere. First, the temperature is raised from room temperature to 150 ° C. at a rate of 2.5 ° C./min.
° C / min, followed by 0.6 ° C / min up to 350 ° C, followed by 4 ° C.
The temperature was raised to 00 ° C. at a rate of 0.7 ° C./min and held at 400 ° C. for 1 hour. The organic binder removal rate of the molded body after the completion of degreasing, that is, the degreasing rate was 97%.

Next, the compact was sintered in a sintering furnace having an inner volume of 80 L in an atmosphere of hydrogen flow of 1 L / min. The sintering shrinkage start temperature of the iron powder used in the present example in a hydrogen atmosphere was investigated. As a result, it was found that the shrinkage significantly progressed from 450 ° C. That is, the sintering shrinkage start temperature of the iron powder used in this example is 450 ° C. The temperature rise rate in the sintering step was 10 ° C./min,
After holding at 0 ° C. for 2 hours, it was further held at 1250 ° C. for 2 hours to obtain a sintered body.

Next, in order to examine the dimensional accuracy of the obtained sintered body, the thickness of the portion shown in FIG. 1 was measured. The dimensional measurement was performed on a total of 168 sintered bodies, 42 pieces of 4 pieces per alumina plate. As a result, defects such as warpage did not occur at all, and sintered bodies all within the target value dimensional tolerance were obtained. Table 1 shows the dimensions of the sintered body.

Example 2 Spherical carbonyl iron powder having an average particle size of 3.6 μm was used as a sintering powder, and polybutyl methacrylate 7 was used as an organic binder.
A mixture consisting of 0% by weight, 20% by weight of paraffin wax, and 10% by weight of dibutyl phthalate was added and kneaded at 160 ° C. using a pressure kneader to obtain a composition for powder injection molding. The mixing ratio of the powder and the organic binder was 8.5 parts by weight of the organic binder with respect to 100 parts by weight of the iron powder. Here, the thermal decomposition end temperature of the organic binder composition in this embodiment is about 400 ° C.

The composition for injection molding thus obtained was molded using an injection molding machine to obtain a molded product similar to that of Example 1. Here, in this embodiment, the target value of the thickness of the sintered body in consideration of the shrinkage is 1250 ± 50 μm.

Next, the molded body was treated in the same manner as in Example 1,
Each of 72 pieces was subjected to degreasing and sintering using a batch type furnace.

First, the molded body was subjected to thermal decomposition and degreasing in a nitrogen atmosphere in the same manner as in Example 1. First, the temperature is raised from room temperature to 150 ° C. at a rate of 2.5 ° C./min.
0.4 ° C / min up to 0 ° C, then 0.6 ° C up to 350 ° C.
The temperature was raised at a rate of 0.7 ° C./minute from the temperature of 400 ° C./minute to 400 ° C., and the temperature was maintained at 400 ° C. for 1 hour. Here, the removal rate of the organic binder, that is, the degreasing rate was 98%.

Next, the compact was sintered in the same sintering furnace as in Example 1 under an atmosphere of hydrogen flow of 1 L / min. The sintering shrinkage onset temperature of the carbonyl iron powder used in this example in a hydrogen atmosphere was investigated.
It was found to be 00 ° C. The temperature was raised at a rate of 10 ° C./min in the sintering step. The temperature was maintained at 400 ° C. for 2 hours, and then the temperature was further maintained at 1250 ° C. for 2 hours to obtain a sintered body.

Next, in order to examine the dimensional accuracy of the obtained sintered body, the thickness of the sintered body was measured in the same manner as in Example 1. As a result, defects such as warpage did not occur at all, and sintered bodies all within the target value dimensional tolerance were obtained. Table 1 shows the dimensions of the sintered body.
Shown in

Example 3 Example 1 was used as a sintering powder.
The same carbonyl iron powder as above was used, and as an organic binder, a mixture of 68% by weight of paraffin wax, 10% by weight of urethane wax, 20% by weight of polybutyl methacrylate, and 2% by weight of stearic acid was added, and a planetary mixer was used. And kneaded at 150 ° C. to obtain a composition for powder injection molding. The mixing ratio of the powder and the organic binder was 8.8 parts by weight of the organic binder with respect to 100 parts by weight of the iron powder. Here, the thermal decomposition end temperature of the organic binder in this embodiment is about 400 ° C.

The composition for injection molding thus obtained was molded using an injection molding machine to obtain a molded product similar to that of Example 1. Here, in this embodiment, the target value of the thickness of the sintered body in consideration of the shrinkage is 1245 ± 50 μm.

Next, the 672 compacts were degreased and sintered under the following conditions. First, the molded body was kept in a heptane gas phase at 85 ° C. for 5 hours to extract and remove paraffin wax, urethane wax and stearic acid. Thereafter, in order to remove the remaining polybutyl methacrylate, the molded body was subjected to 400 ° C./min.
The temperature was raised to 400 ° C., the temperature was maintained at 400 ° C. for 1 hour, and thermal decomposition degreasing was performed. Here, the removal rate of the organic binder, that is, the degreasing rate was 98%.

Next, the degreased body was sintered under the same conditions as in Example 1 to obtain a sintered body. The dimensions of the sintered body were measured in the same manner as in Example 1. As a result, no deformation such as warpage occurred at all, and sintered bodies all within the target value dimensional tolerance were obtained. Table 1 shows the dimensions of the sintered body.

Comparative Example 1 Example 1 was used as a sintering powder.
Using the same carbonyl iron powder as above, a mixture composed of 46% by weight of ethylene-vinyl acetate copolymer, 9% by weight of polyethylene, 35% by weight of paraffin wax, and 10% by weight of dibutyl phthalate was added as an organic binder. And kneaded at 150 ° C. to obtain a composition for powder injection molding. The mixing ratio between the powder and the organic binder was such that 100 parts by weight of the iron powder and 9 parts of the organic binder were mixed.
2 parts by weight. Here, the thermal decomposition end temperature of the organic binder composition in this comparative example is about 600 ° C.

The obtained composition for injection molding was molded using an injection molding machine in the same manner as in Example 1 to obtain a molded article. next,
In the same manner as in Example 1, 672 pieces of the compact were subjected to degreasing and sintering using a batch furnace.

First, the molded body was subjected to thermal decomposition degreasing under the same degreasing conditions as in Example 1. Here, the degreasing rate is 7
8%. Next, the degreased body was sintered in the same sintering furnace as in Example 1 under the same conditions, and the thickness of the obtained sintered body was measured. As a result, the sintering shrinkage of the surface that was in contact with the furnace atmosphere during sintering was inhibited, and as shown in FIG. 2, all the sintered bodies were warped downward. The warp deformation increased the apparent thickness of the sintered body and was out of the dimensional tolerance. Table 1 shows the dimensions of the sintered body.

Comparative Example 2 The compact obtained in Comparative Example 1 was degreased using the same degreasing furnace as in Example 1. First, the temperature is raised from room temperature to 150 ° C. at a rate of 2.5 ° C./min.
0.6 ° C./min up to 0 ° C., followed by 0.7 ° C. up to 600 ° C.
The temperature was raised at a rate of ° C / min, and the temperature was maintained at 600 ° C for 1 hour. Here, the degreasing rate was 96%. However, in the molded body after the completion of degreasing, discoloration of the molded body due to oxidation of the iron powder was observed on the upper surface in contact with the degreasing atmosphere.

Next, the obtained compact was sintered under the same conditions as in Example 1 to obtain a sintered body. Further, the dimensions of the obtained sintered body were measured in the same manner as in Example 1. As a result, all the sintered bodies were warped as shown in FIG. 2 and were out of dimensional tolerance. Table 1 shows the dimensions of the sintered body.

Comparative Example 3 Example 2 was used as a sintering powder.
Using the same carbonyl iron powder as above, a mixture consisting of 24% by weight of polyethylene, 24% by weight of polypropylene, 28% by weight of polymethyl methacrylate, 15% by weight of stearic acid, and 9% by weight of dibutyl phthalate was added as an organic binder. Kneading was performed at 170 ° C. using a pressure kneader to obtain a composition for powder injection molding. The mixing ratio of the powder and the organic binder was 8.5 parts by weight of the organic binder with respect to 100 parts by weight of the iron powder. Here, the thermal decomposition end temperature of the organic binder composition in this comparative example is about 600.
° C.

The obtained composition for injection molding was molded using an injection molding machine in the same manner as in Example 2 to obtain a molded product. next,
The molded body was degreased and sintered under the same preparation conditions as in Example 2.

First, the compact was heated under the same conditions as in Example 2 to degrease it. Here, the degreasing rate is 75
%Met. Next, the degreased body was sintered in the same sintering furnace as in Example 2 under the same conditions.

Next, the dimensions of the obtained sintered body were measured in the same manner as in Example 2. As a result, all the sintered bodies were warped as shown in FIG. 2 and were out of dimensional tolerance.
Table 1 shows the dimensions of the sintered body.

Comparative Example 4 The compact obtained in Comparative Example 3 was degreased using the same degreasing furnace as in Example 2. First, the temperature is raised from room temperature to 150 ° C. at a rate of 2.5 ° C./min.
0.6 ° C./min up to 0 ° C., followed by 0.7 ° C. up to 600 ° C.
The temperature was raised at a rate of ° C / min, and the temperature was maintained at 600 ° C for 1 hour to obtain a degreased body. Here, the degreasing rate was 98%. However, in the molded body after the completion of degreasing, discoloration of the molded body due to oxidation of the iron powder was observed on the surface in contact with the atmosphere.

Next, the obtained degreased body was sintered under the same conditions as in Example 2 to obtain a sintered body. Further, in the same manner as in Example 2, the dimensions of the obtained sintered body were measured. as a result,
All the sintered bodies were warped as shown in FIG. 2 and were out of dimensional tolerance. Table 1 shows the dimensions of the sintered body.

Comparative Example 5 Example 1 was used as a sintering powder.
The same carbonyl iron powder as above was used, and as an organic binder, 68% by weight of paraffin wax, 10% by weight of urethane wax, 20% by weight of polyethylene, and 2% by weight of stearic acid
A mixture consisting of 100% by weight was added and kneaded at 150 ° C. using a planetary mixer to obtain a composition for powder injection molding. The mixing ratio of the powder and the organic binder was 8.8 parts by weight of the organic binder with respect to 100 parts by weight of the iron powder. Here, the thermal decomposition end temperature of the organic binder composition in this comparative example is about 550 ° C.

Next, the 672 compacts were degreased and sintered under the following conditions. First, the compact was held in a heptane gas phase at 85 ° C. for 5 hours to extract and remove paraffin wax, urethane wax and stearic acid. Thereafter, in order to remove the remaining polyethylene, the molded body was heated to 400 ° C. at a rate of 4 ° C./min in a nitrogen atmosphere, held at 400 ° C. for 1 hour, and subjected to thermal decomposition degreasing. The degreasing rate of the obtained degreased body is 84%
Met.

Next, after sintering the degreased body under the same conditions as in Example 1, the dimensions of the sintered body were measured. As a result, all the sintered bodies were warped as shown in FIG. 2 and were out of dimensional tolerance. Table 1 shows the dimensions of the sintered body.

(Comparative Example 6) The compact obtained in Comparative Example 5 was degreased and sintered with the same number of charges as in Comparative Example 5. First, the molded body was heated at 85 ° C in a heptane gas phase at 5 ° C.
After holding for a while, paraffin wax, urethane wax and stearic acid were extracted and removed. Thereafter, in order to remove the remaining polyethylene, the molded body was heated to 550 ° C. at a rate of 4 ° C./min in a nitrogen atmosphere, held at 550 ° C. for 1 hour, and subjected to thermal decomposition degreasing. The degreasing rate of the obtained degreased body was 94%. However, in the molded body after the completion of degreasing, discoloration of the molded body due to oxidation of the iron powder was observed on the surface in contact with the atmosphere.

Next, the degreased body was sintered under the same conditions as in Example 1 to obtain a sintered body. The dimensions of the sintered body were measured in the same manner as in Example 1. As a result, all the sintered bodies were warped as shown in FIG. 2 and were out of dimensional tolerance. Table 1 shows the dimensions of the sintered body.

[0057]

[Table 1]

In Table 1, in Examples 1 to 3 of the present invention, the thicknesses of the sintered bodies were all within the target dimensional tolerance, and the dimensional accuracy was superior to Comparative Examples 1 to 6. Is evident.

First, in Example 1, 97% of the organic binder can be removed at a degreasing temperature lower than the sintering temperature of the powder. Therefore, at a temperature at which the sintering shrinkage of the powder progresses, the decomposed gas of the residual binder is removed from the degreased body. Does not occur. Therefore, the sintering shrinkage of the degreased body proceeds uniformly, and a sintered body having a dimensional tolerance can be obtained.

On the other hand, in the case of Comparative Example 1, when the powder was degreased at a temperature of 400 ° C. or less, which is equal to or lower than the sintering shrinkage starting temperature of the powder, the degrease rate was 78%, which was lower than that in Example 1, and the degreased body contained much residual binder. ing. This residual binder is used in the sintering process.
When the sintering shrinkage of the degreased body proceeds, it is thermally decomposed and released as a gas. As in the present comparative example, particularly when the amount of the formed body charged into the sintering furnace is large, the amount of decomposition gas generated by the residual binder increases, and the atmosphere in the sintering furnace is contaminated. When the sintering of the degreased body proceeds while the atmosphere in the sintering furnace is contaminated, the sintering shrinkage of the degreased body surface in contact with the atmosphere is hindered. On the other hand, no such inhibition is observed on the surface opposite to the sintering jig. As a result, there is a relative difference in the sintering shrinkage ratio between the front surface and the back surface of the degreased body, and the sintered body is warped and out of dimensional tolerance. Such a phenomenon is remarkably observed in the case of a thin flat plate-shaped component as in this comparative example.

Next, in Comparative Example 2, a molded article using the same binder composition as in Comparative Example 1 was degreased to 600 ° C. By setting the degreasing temperature to 600 ° C., the degreasing rate is improved to 96%. However, since the degreasing temperature is high, the iron powder is oxidized on the surface of the degreased body. At this time, on the surface where oxidation has progressed, shrinkage is inhibited when subsequent sintering shrinkage proceeds. As a result, the sintered body is warped and out of dimensional tolerance.

Similar phenomena occur in the relationship between Example 2 and Comparative Examples 3 and 4, and in the relationship between Example 3 and Comparative Examples 5 and 6.

As described above, by using the composition for powder injection molding and the method for producing a powder injection molded part according to the present invention, the deformation of the part caused in the sintering process is prevented, and the powder injection molding excellent in dimensional accuracy is achieved. Molded parts can be manufactured and provided.

[0064]

According to the present invention, a resin which is thermally decomposed at or below the sintering shrinkage starting temperature of the sintering powder is kneaded as a component of the sintering powder and the organic binder to obtain a composition for powder injection molding. Prepared, molded this to an injection molded body, in the degreasing step of the injection molded body, removing each component of the organic binder at or below the sintering shrinkage start temperature of the sintering powder to obtain a degreased body, By sintering the degreased body, sintering shrinkage of the powder proceeds uniformly, and a powder injection molded part having no deformation and excellent dimensional accuracy can be obtained.

In particular, in a thin-shaped part, the difference in the sintering shrinkage ratio between the surface in contact with the sintering jig and the surface in contact with the sintering atmosphere tends to appear as warpage deformation. In addition, when a large amount of the molded body is degreased and sintered, a large amount of a decomposition gas of the residual organic binder contained in the degreased body is generated, and the warpage is increased accordingly. Therefore, the present invention is particularly effective when a large number of thin-shaped components are degreased and sintered.

[Brief description of the drawings]

FIG. 1 is a sectional view of a sintered body obtained by sintering a molded body according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a conventional sintered body obtained by sintering a molded body.

[Explanation of symbols]

 1 Thickness of sintered body

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Kotaro Ishiyama 840 Takeno, Shimotomi, Tokorozawa-shi, Saitama Prefecture Citizen Watch Co., Ltd.

Claims (2)

[Claims] 1. A powder injection molding composition is prepared by kneading a sintering powder and at least one kind of thermoplastic resin as an organic binder component, and molding this to form an injection molded body. Degreased to obtain a degreased body, and sintering the degreased body to produce a sintered body, the composition for powder injection molding in the powder injection molding method, wherein each component of the organic binder, A composition for powder injection molding, which is a resin whose thermal decomposition ends at a temperature equal to or lower than a sintering shrinkage initiation temperature of a binder powder. 2. A composition for powder injection molding is prepared by kneading a resin which is thermally decomposed at a temperature equal to or lower than a sintering shrinkage onset temperature of the sintering powder as each component of the sintering powder and the organic binder,
This is molded into an injection molded body, and in the degreasing step of the injection molded body, each component of the organic binder is removed at or below the sintering shrinkage start temperature of the sintering powder to obtain a degreased body. A method for producing a powder injection molded article, characterized by sintering.