JPH05194041A - Method for producting sintered part - Google Patents

Abstract

PURPOSE: To provide a process for producing a sintered compact by which the asymmetrical forming components with a complicated shape are mass-produced with an excellent strength characteristic (compact density) while economical advantages adapted to the material of the powder metallurgy in comparison with the melting metallurgy.

CONSTITUTION: A powder material is filled in a disposable press die made of a foamed plastic and whose phase is made stable, and isostatic pressing is conducted so as to obtain compact mapping a form the press die accurately.

COPYRIGHT: (C)1993,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a powder material which is compressed by cold isostatic pressing into a powder compact in a press mold which shrinks under the action of a pressure medium. The present invention relates to a method for manufacturing a sintered molded part, which is subsequently sintered.

[0002]

2. Description of the Related Art The production of metal and ceramic molded parts based on powder metallurgy is constantly improving the product quality due to its relatively high economic efficiency and versatility of processable materials compared to fusion metallurgy. It is becoming increasingly important. Two significantly different techniques for powder compaction as an important part of powder metallurgy are axial compaction in a robust or external mold and flexibility to shrink under the pressure of the compacting medium. Is used in parallel with hydrostatic compression in the mold. With the current mature axial compression technology it is also possible to economically produce geometrically complex shaped parts, for example gears, as mass-produced parts. The optional correction of the molded part after powder compaction and sintering results in a dimensional accuracy that requires minimal or no mechanical finishing such as turning or grinding. The drawback of this technique is that the pressing pressure can only be applied in one axial direction. On the one hand, this limits the geometries that can be produced according to this technique, and on the other hand it is not possible with this technique to achieve a uniform density distribution. Depending on the material and particle properties, axial compression can economically compress powder materials to residual porosities of 10 to 20%. The limit of compaction is reached when the individual powder particles can no longer move relative to each other under compressive force and are prevented from their movement. The compressed density itself determines the material density achievable during sintering and the mechanical strength of the molded part, with the respective remaining percentages of the residual porosity of the finished sintered molded part being determined by its mechanical properties. It works against you.

Due to hydrostatic compression, the powder material is conventionally packed in an elastic tube, which itself lacks the inherent shape stability, so that it is particularly large in size for ease of handling. It is placed in a metal sleeve with a wall provided with a through hole. The filled and closed tube is inserted into a hydrostatic press and compressed with a pressure medium, often water, to a hydrostatic pressure of 1000 to 4000 bar. Due to the omnidirectional pressure exerted on the powder material, the blockage of the powder particles only begins at higher green compact densities than in the case of axial compression, and the density distribution becomes very uniform. However, this method can only produce geometrically simple green compact shapes, usually cylindrical rods and disks. The reason is the high elastic modulus required for reusable rubber or plastic tubes that shrink under pressure medium. The semi-finished product obtained after sintering must therefore be mechanically finished to give the desired geometry. This is done by plastic deformation and / or cutting of the semi-finished product. The variety of products actually produced according to this method is therefore limited on the one hand to geometrically simple molded parts, and on the other hand to high material quality obtained only by powder metallurgy, high machining costs and thus overall production. Limited to complex molded parts for special applications that justify the cost.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention that the advantage of hydrostatic or omnidirectional compression at high and uniform green compact density is that axially compressed or dimensionally correct geometrically complex molded parts. To provide a method that can be combined with the advantages of manufacturing in an economical way, in particular, while utilizing the advantages of powder metallurgy that is more economical and material-friendly than fusion metallurgy itself, It is to manufacture a non-symmetrical molded part with excellent strength characteristics so as to have a target size and also as a mass-produced part. This task cannot be achieved by axial compression in a rigid outer mold and a rigid mold.

[0005]

According to the present invention, a powder material is filled in a disposable press die made of foamed plastic and having a stable shape, and compressed so as to obtain a green compact which is an exact copy of the press die. It is solved by being done.

It has been found that among the expanded plastics, expanded polystyrene is particularly excellent as an economical disposable press mold because it has a large geometric shaping capability. However, other foamed plastics such as polyurethane are also suitable as stable, disposable press molds with a controlled shrinkage shape under the action of a pressure medium.

Foamed plastics, especially foamed polystyrene, do not stick and bond with the powder to be compressed during isostatic pressing.
That is, the surface quality of the sinter-molded material given by the original press die surface is maintained even in the finished green compact or sinter-molded part. Expanded polystyrene as a stable packaging material in all the special shapes required generally results in the respective desired shape-stable contours due to the expansion of the material in the mold in wet steam. The disposable press mold according to the invention is manufactured in principle by the same method as the moldings for packaging.

The surface of expanded polystyrene is generally somewhat reticulated or textured. This mesh pattern on the surface can be transferred to the molded part. In order to avoid this, according to an advantageous embodiment of the invention, a thin cover layer of organic material is coated and sprayed onto the finished press die. As a result, at the same time, it is possible to enhance the sealing property of the mold wall against the pressure medium during hydrostatic compression. Both measures help improve the surface quality of the molded part.

According to another measure, the rough surface modification of the expanded polystyrene press die can be substantially prevented by heating the molding die for a short time during the foaming process.

The foamed plastic material itself has a relatively high shape stability. Sufficient shape stability for the press die is optimized by the press die with a small wall thickness and the ribs and reinforcements provided on the wall, while adapting to the volume of molded parts and powder packing method in each case. it can. Regions of different wall thickness do not affect the quality and dimensional accuracy of the sintered molded part. Since expanded polystyrene consists of only 3% by volume polystyrene and 97% by volume air or gas, initially also mold walls of very different thickness are compressed to a virtually uniform wall thickness after the isostatic pressing step.

Depending on the size and shape of the sintered molding material,
The disposable press mold according to the invention is made up of two or more parts, for example in the form of two half-shells. In the press mold,
Inserting a solid core, which allows the production of sinter-molded material with a hollow interior, or which is an integral part of the sinter-molded part, for example as an axis in geometrically complex molded parts can do.

According to one embodiment of the method according to the invention, the press mold is removed from the green compact after isostatic pressing and before sintering. However, green compacts are generally sintered with a disposable press mold. This eliminates the die-cutting process and protects the green body from surface damage until it obtains an inherent stability due to sintering. The sintering temperatures of the materials in question are invariably above the temperatures at which foamed plastics such as expanded polystyrene decompose and / or burn without residue. Therefore, the disposable press mold can be processed simultaneously during sintering without any problems.

All materials currently processed by powder metallurgy into sinter-molded parts can also be used in the disposable press mold according to the invention. Ceramic powders can be processed as well as metal powders such as alloy steel, bronze or cemented carbide powders.

The method according to the invention makes it possible, by far and unexpectedly, to significantly expand the variety of molded parts which can be produced in a qualitatively and cost-competitively manner by the cold isostatic method. Hydrostatic compression of a press mold filled with expanded plastic, especially expanded polystyrene, allows the production of geometrically complex molded parts with high dimensional accuracy (shape close to the final product) under normal compression conditions. Becomes The fact that the thickness of the press die has practically no influence on the achievement of geometries close to the final product and thus does not at all require the production of a press die with a wall thickness as uniform as possible is unexpected. This advantage has great practical implications.

On account of the expanded use of expanded plastics, in particular expanded polystyrene, as parts of complex shape, for example as molded packaging material or as molds for the so-called full-molding process, a complex disposable press mold according to the invention. Mature technology is available for cheap manufacturing of. Due to the above-mentioned shape stability of the press mold according to the present invention, a disadvantageous but unavoidable enlargement of the green compact end, so-called "elephant's foot phenomenon" occurs when the hydrostatic molding part is compressed in an elastic tube. Absent.

Since foamed plastics, in particular expanded polystyrene, are water-repellent, the pressure medium normally used for isostatic pressing, ie water, is particularly suitable. Water does not stain the press walls during compression.

Due to the large volume fraction of air in expanded polystyrene, it is proposed to optionally add a gas absorber to the expanded polystyrene press mold for hydrostatic compression for safety reasons. In all cases where the expanded polystyrene press mold has a large volume and the press does not have a device for releasing gas from the chamber during the compression process, the filled press mold is placed in a compression tube normally used with a gas absorber. It is advantageous to put it in and insert the device into the compression chamber for compression after closing. An open porous incompressible iron skeleton, which can take up the generated gas volume and strongly compress it there, is particularly suitable as a gas absorber.

[0018]

According to the method according to the invention, ceramic mass-produced parts, which are used more and more widely in industrial products, can be processed in the same way as complex containers made of steel or molded parts made of cemented carbide. It can be manufactured. The method according to the invention is equivalent, even if not superior in quality, to lost wax castings and die castings, even in the case of complex molded parts with undercuts and through holes. However, in addition to this, the method according to the invention is more economical than the known sinter and fusion metallurgical methods involving a machining step.

Claims (8)

[Claims] 1. A powder material is obtained by cold isostatic pressing.
In a method of manufacturing a sintered molded part, in which a powder compact is compressed into a compact in a press mold that contracts under the action of a pressure medium, and the compact is subsequently sintered, the powder material foams. A method for producing a sinter-molded part, which comprises filling a disposable press die made of plastic and having a stable shape, and compressing the die so as to obtain a green compact that accurately replicates the press die. 2. The manufacturing method according to claim 1, wherein a press die made of expanded polystyrene is used. 3. The manufacturing method according to claim 1, wherein a press die having an organic cover layer is used. 4. The manufacturing method according to claim 1, wherein a press die having different ribs and having ribs and reinforcing portions incorporated in the wall is used. 5. One of the claims 1 to 4, characterized in that the green compact of the molded part is pre- or finish-sintered with the press die, whereby the press die chemically decomposes and / or burns without residue. Manufacturing method described in. 6. The method according to claim 1, wherein the ceramic powder material is compressed. 7. The method according to claim 1, wherein the alloy steel powder is compressed. 8. The filled press mold together with the gas absorber is placed in a compression tube and the compression tube is closed,
8. The method according to claim 1, wherein the compression is performed as it is.
The manufacturing method described in one of 1.