WO2004014586A2

WO2004014586A2 - Production of powders containing compacting auxiliary agents

Info

Publication number: WO2004014586A2
Application number: PCT/EP2003/008489
Authority: WO
Inventors: Benno Gries; Bernhard Szesny
Original assignee: Benno Gries; Bernhard Szesny
Priority date: 2002-08-02
Filing date: 2003-07-31
Publication date: 2004-02-19
Also published as: WO2004014586A8; DE10235413A1; WO2004014586A3

Abstract

The invention relates to methods for producing powders, particularly metal powders, that contain compacting auxiliary agents. In a first step, a powdery compacting auxiliary agent is mixed with a starting powder at a temperature lower than the melting point of the compacting auxiliary agent. In a second step, the mixture obtained in the first step is heated to a temperature higher than the melting temperature of the compacting auxiliary agent. According to the invention, the mixture is not subjected to movement during heating and successive cooling. The invention also relates to methods for producing sinterable metallic shaped parts from these powders containing compacting auxiliary agents.

Description

Production of powder containing press aids

In powder metallurgy, the three successive manufacturing steps required to manufacture the starting powders, manufacture compacts (shaping) and sinter the compacts (compacting), whereby the last two steps can also be carried out together in modern processes. During the shaping process, the starting powder is pressed into the desired shape and compacted in the process. Although corresponding manufacturing steps in the manufacture of brake pads, as well as engineering and service ceramics are very similar, one does not speak of "powder-ceramic" manufacturing processes. All cases, however, have in common that powdered raw materials are used to first produce compacts, which are then sintered A major advantage of this process compared to conventional casting processes, for example, is that it is possible to produce sintered parts close to the final shape with little or no effort for postprocessing

Sintering shrinkage. Unfortunately, the shrinkage is rarely isotropic in practice, since the shape is usually pressed axially and not isostatically. The pressed part is not quite homogeneous in terms of its pressing density, since internal friction occurs during the pressing by sliding the powder particles against one another and sliding the powder particles on the wall of the pressing tool.

A uniform press density is particularly critical when the sintered part is to have a defined, locally homogeneous and open porosity, or is sintered in the final contour because no rework is to be carried out, or the pressing, also called green body, is special during further handling or processing is exposed to mechanical loads. The former is important, for example, with sintered metal filter elements, tantalum sintered anodes for capacitors, but also with other sintered parts that are later to be infiltrated. In the latter applications, infiltration takes place, for example with bronze, strontium titanate or copper. Applications are in the area of WCu composites, UV lamp electrodes or plain bearings etc. In order to reduce the friction during pressing and to achieve the most uniform possible pressing density in the green body, an organic lubricant, the so-called pressing aid, is therefore usually added to the powder to be pressed. The most frequently used are pressing aids from the wax substance class, which can be decomposed without residue during the sintering process. The selection of the right wax is based on physical and chemical criteria. As a rule of thumb, a wax with a low softening range has better lubricating properties and allows lower pressures with the same press density than one with a high softening range. However, the wax has too

Effects on the strength of the compact, the so-called "green strength". This should be as high as possible, so that the pressure is not destroyed when pressed out of the mold, and can be handled by manipulation machines without being destroyed or damaged. Furthermore, it often depends assuming that the wax wets the surface of the powder particles, in which case it is preferred to use polar waxes which may have surface-active properties, for example higher fatty acids or their metal salts, so-called metal soaps or their esters or amides.

For the characterization of the pellet, in addition to the press density at constant

Press pressure also the mechanical stability of the compact, since it must not be damaged when it is ejected from the mold or during further handling. For a given powder, high press densities also mean high mechanical stability, which can be achieved by high press pressures. The mechanical stability can be quantified by various methods such as chatillon tests or abrasion methods. With these processes, a press is produced under standardized conditions and its mechanical strength or mechanical abrasion is measured. In the latter case, compacts with a standardized weight are produced at standardized pressures and placed in a cylindrical wire basket which rotates about its longitudinal axis. After a predetermined number of revolutions, the weight of the tube basket including its contents weighed, and the percentage of abrasion is determined from the gross weight (so-called "rattler test"). The higher the abrasion resistance, the lower the abrasion at a given number of revolutions.

A high green strength is desirable from the point of view of production technology. However, there are limits to the press pressure due to the materials available for the press tool and the maximum achievable press force. Technically, the pressing force used and thus the achievable density of the compact, the so-called pressing density, is therefore limited. An improved green strength can, however, be achieved by carrying out the shaping in such a way that the press density within the compact is as uniform as possible. A non-uniform press density manifests itself, among other things, by high abrasion or low green strength, since poorly compressed areas have low strength. A uniform press density should therefore lead to higher abrasion resistance of the green body with the same press pressure.

In addition, a high press density for sintered parts to be infiltrated later can prevent the control of the porosity after sintering. The control of the content of infiltration agent in the infiltrated component is done by controlling the necessarily open porosity after sintering, which in turn can only be controlled indirectly via the porosity of the press body and thus the press density. A low pressing density, which is necessary from the point of view of infiltration, can lead to such low green strengths that the compacts can no longer be handled without damage. An object of the present invention is to solve this conflict of objectives between green density and green strength.

The introduction of pressing aids into the starting powder happens industrially in different ways. Mixtures of different starting powders are often used so that powders have to be mixed with other powders before use or the powders are not fine enough and have to be ground. This often happens when wet, with the pressing aid dissolved in the grinding fluid. Spray drying is often carried out after the grinding process. This method is more suitable for ceramic powder. in the

In the case of ductile metal powders, deformation takes place to form thin flakes, which in turn are only comminuted after a very long grinding time. In the case of brittle or hard starting powders, comminution takes place. This process therefore changes the particle size distribution and particle shape the powder defined grain size distribution or

Obtained grain shape in Grünköφer or sintered piece, so this method is unsuitable. In addition, handling liquids, often of an organic nature, is necessary, which requires special precautionary measures. Powders are obtained which are wetted with the pressing aid. The pressing aid also has the task here of making granules which may have formed during spray drying mechanically stable.

Another method is the mixing of powdery pressing aids with the starting powders in a free-fall mixer, the powder being hardly mechanically loaded and therefore the particle size distribution or agglomerate size distribution is not changed significantly. In the simplest case, the free-fall mixer is an only partially filled, rotating container. This process can also be carried out in a mixer equipped with circulating tools, e.g. a paddle mixer from Lödige. In this case, mixing of the metal powder with the powdered wax is achieved, but none

Wetting. This method is the most commonly used method, although it produces a relatively inhomogeneous wax distribution. Due to the poor wax distribution, this process requires a higher concentration of wax in order to reduce the internal friction during pressing and the wall friction with the press mold. However, higher wax concentrations are disadvantageous because they result in lower ones Press densities, based on the powder content, are obtained in the green body, and the thermal removal during the sintering process takes a very long time.

EP 0781 180 B1 describes a process for mixing metal powders based on iron with powdered polyethers, the mixing being carried out without the addition of solvents. The key point is the use of the special pressing aid, which means that the green strength can be increased compared to the use of conventional pressing aids, for example a commercially available wax. The powdered polyether is added in an amount of up to 10% by weight.

EP 0 968 068 B1 also discloses a special pressing aid for producing sinterable molded parts from metal powders. The pressing aid contains polyethylene glycol with a molecular weight of 3000 to 6500 g mol and can, for example, be mixed with the starting powder in a drum mixer. In one embodiment, the drum mixer is heated to a temperature which is above the softening temperature of the pressing aid provided for the subsequent pressing process.

Another method used is so-called hot mixing. Here it is

Powder mixed with the pressing aid in an intensive mixer until it reaches a temperature above the melting point of the pressing aid due to the energy input. When mixing hot, a drum mixer with circulating tools can be used, which is heated externally by means of a heating jacket. Very good wetting is achieved by the pressing aid. However, the powder is mechanically stressed by this high energy input, so that agglomerated powders in particular are deagglomerated. Furthermore, the capillary action of the molten pressing aid and the movement of the mixed material result in partial compaction and granulation of the mixed material. Powders produced in this way have a comparatively high bulk density. It is characteristic of the hot mixing that the powder bed for The melting aid of the pressing aid is in motion. If a mixture of two powders is to be used, the problem often arises during hot mixing that the pressing aid preferably wets a component when the melting point is reached, and granulate particles are formed which contain this powder component. This segregation is promoted by the mechanical movement of the mix.

It is also known to dissolve the pressing aid in a suitable organic solvent, to wet the starting powder with this solution, and then to dry it. Drying must take place while the powder is moving, otherwise local accumulations of the pressing aid would form. The expenditure on equipment is high. In addition, you must work with organic liquids that have a high risk potential. Grain size distribution or agglomerate size distribution remain essentially unchanged. In principle, this process can also be carried out using water as a solvent, but then only water-soluble pressing aids can be used which, although they have good stability to the granules, generally have no lubricating effect. Examples of such pressing aids are methyl cellulose, polyacrylic acid or polyethylene glycols. Water also has the disadvantage of reacting with most metal powders and undesirable oxide layers on the

To form metal powder particles which reduce the sinterability or lead to porosity after sintering.

Regardless of the method of incorporating the pressing aid, there is extensive literature and patents that vary the chemical composition of the pressing aid and thereby attempt to improve the green strength and other properties of the compact by interfering with the chemical or physical interaction between the pressing aid and metal powder and mixtures or alloys of different pressing aids can be used (e.g. WO 01/40416 AI). However, the pressing aid is introduced using one of the above methods. It was an object of the invention to provide a method for mixing a powder with a pressing aid which is as simple as possible and can be carried out without great expenditure on apparatus and which does not have the disadvantages mentioned above. In particular, complete wetting of the

Guaranteed powder with the pressing aid, and the powder in terms of grain size distribution, grain shape and agglomeration state are not changed significantly. Flowable products should be produced without organic solvents or water, and no segregation and / or granulation of powder components should be caused when mixing powder mixtures with the pressing aid.

The surprisingly simple solution now consists in briefly bringing the mixtures of starting powder and pressing aid produced at temperatures below the melting point of the pressing aid to a temperature above the

To heat the melting point of the pressing aid in a stationary powder bed, so that an infiltration of the self-contained powder bed by the molten pressing aid takes place.

The invention therefore relates to a process for producing powders mixed with pressing aids, in a first step pulverulent pressing aids being mixed with a starting powder below the melting temperature of the pressing aid, and in a second step the mixture obtained from the first step to a temperature of at least the melting temperature of the pressing aid is heated, the mixture not moving during heating and subsequent cooling.

In view of the use of pressing aids, in particular waxes, in powder metallurgy, which has been carried out for decades, and the need for improvement that still exists, it is all the more astonishing that this process has so far not become known. The method according to the invention can be carried out using the simplest of means. The mixing of powdered pressing aid and starting powder is preferably carried out in a free-fall mixer. Known mixers, such as drum mixers or a partially filled, rotated container, for example a drum or a double cone mixer, can be used as free-fall mixers, for example. A drum or compulsory mixer equipped with circulating tools can also be used. Regardless of the type of mixer used, it is essential that the melting point of the pressing aid is not reached during the mixing process.

The mixture obtained can be heated, for example, in a drying cabinet, a belt dryer or other suitable apparatus. It is essential that the mixture obtained is static during heating and cooling, i.e. is not moved in itself and that the mixing on the one hand and heating and cooling on the other hand are carried out in different units.

The process according to the invention is furthermore distinguished by the fact that neither organic solvents nor complex drying apparatuses, such as, for example, spray drying, are necessary.

According to the invention, the temperature during the heating corresponds at least to the melting temperature of the pressing aid. The temperature is preferably 5 to 100 ° C. above the melting temperature of the pressing aid, particularly preferably 10 to 50 ° C.

Depending on the chosen pressing aid, heating to 50 to 250 ° C, preferably to 60 to 180 ° C, may be necessary.

The heat treatment is preferably carried out over a period of 0.1 to 10 h, particularly preferably from 0.5 to 2.2 h. A characteristic of pressing aids, in particular waxes, is that they have a very low viscosity just above their melting point and can therefore completely wet the starting powder by capillary forces. The powders produced in this way then accumulate in a bonded state, but rub on a sieve again to form smaller agglomerates without being ground, as a result of which good flowability is achieved.

It is therefore preferable to sieve off the powder with press aids after heating and cooling. This can be done in a conventional

Screening machine, for example a tumble screening machine. The size of the sieve openings depends essentially on the particle diameter of the powder mixed with pressing aids. For example, screen openings of 300 to 2000 μm have proven to be suitable. Here, the product in cake form is grated.

In the process according to the invention, the powdery pressing aid is preferably used in an amount of 0.2 to 10% by weight, based on the total amount of pressing aid and starting powder, particularly preferably in an amount of 0.3 to 6% by weight, in particular preferably from 0.5 to 4% by weight.

In principle, any pressing aids can be used as powdered pressing aids, as long as they are available in solid, powdery form. Examples of pressing aids from the group of the fatty acids or their metal salts, the fatty acid bisamides, the fatty acid monoamides, the fatty acid esters, the polyethylenes or their oxidic derivatives, and the waxes are mentioned, the use of waxes being preferred. Zinc stearate, paraffin waxes and montan ester waxes are particularly preferably used.

There are preferably powdered, so-called micronized pressing aids with a particle diameter of a maximum of 5 to a maximum of 500 microns, ie from -5 to -500 μm, particularly preferably from a maximum of 10 to a maximum of 100 μm. These are preferably commercially available powders which have been sprayed from the melt, so that the grain size of the pressing aid corresponds to the droplet size when spraying. However, pressing aids ground cryogenically in impact mills can also be used.

A large number of different starting powders can be used in the process according to the invention. In principle, all ceramic or metallic powders or also composite powders, mixtures and alloys can be used.

However, metal powders or a mixture of metal powders or a mixture of metal powders with a carbide or oxide powder are preferably used. Metal powders of tungsten, molybdenum, niobium and tantalum, but also of iron, cobalt, copper and nickel may be mentioned as examples of metal powder. The metal powders can be in pure form. But it is also possible that they are in

Alloys are present or contain further additives, such as graphite, oxides or carbides, in particular tungsten carbide.

Examples of suitable alloy powders are tungsten-nickel alloys and tungsten carbide-cobalt alloys. Also to be mentioned are single and multi-phase alloy powders, alloy powders such as those e.g. can be used for tool steels, bearing and filter materials. It is also possible to use a mixture of alloy powders.

In a particular embodiment, an oxidic powder is used as the starting powder

Powder, a powder of a carbide or a mixture of carbide with metal powders is used. A mixture of tungsten carbide with a metal powder, preferably a cobalt metal powder, may be mentioned as an example.

The method according to the invention can be varied in addition to the production of

Powders can also be used to produce flowable granules, for example by heating the mixture produced in a free-fall mixer in motion in an aggregate suitable for building up granules. This can be, for example, a continuous rotary tube dryer or a heatable pelletizing plate. However, static heating of the powder is preferred, ie the powder is preferably not moved in itself during the heating. Cooling below the melting point of the pressing aid is also preferably static.

The mixture of powders with powdered pressing aid is very particularly preferably heated as a static, i.e. at rest, powder filling on a tray in a drying cabinet or on the belt of a

Belt dryer or in a glow box in an industrial furnace, e.g. in a walking beam furnace or push-through furnace.

The metal powders mixed with pressing aids produced in the process according to the invention are particularly suitable for producing sinterable metallic

Moldings.

The invention therefore furthermore relates to a method for producing sinterable metallic moldings from metal powders mixed with pressing aids, powdered pressing aids containing a metal powder in a

Mixed temperature below the melting temperature of the pressing aid, the mixture obtained as a self-contained powder bed heated to a temperature of at least the melting temperature of the pressing aid, cooled in a self-standing manner and finally sieved, and the metal powder thus prepared containing press aids is poured into a mold, below Compressed pressure and finally ejected from the mold as a pressed molded part.

Particular embodiments of the method according to the invention are explained in more detail by the following examples, which examples are not to be understood as restricting the general inventive concept. Examples

The mean grain diameters given in the examples were determined using Fisher Sub Sieve Sizer (FSSS, ASTM B 330), the knock volume and knock density according to ASTM B 527 and the flowability using Hall Flow measurements

Using a 2/10 "funnel (ASTM B 417) determined.

To determine the abrasion resistance, the powders were processed by uniaxial pressing at a pressing pressure of 6.7 t / cm ² and placed in a cylindrical wire basket, which rotates about its longitudinal axis. After a predetermined number of revolutions, the weight of the wire basket and its contents were weighed, and the percentage of abrasion was determined from the gross weight (so-called “rattler test”). The higher the abrasion resistance, the lower the abrasion at a given number of revolutions. The abrasion test was carried out a commercially available device from Minerva Kiki Co. Ltd., Japan with the

Name Rattler Tester used. The speed of rotation was 87 revolutions per minute.

Example 1 (comparative example)

Tungsten metal powder microns with an average grain diameter according to FSSS of 12, a knock volume of 11.4 ^cm3 / 100 g and a melt flow rate of 2.95 s (Hall Flow, 2/10 "funnels) was without pressing aids by uniaxial pressing with a floating form and fixed lower punch to With a press pressure of 6.7 t / cm ² , the dimensions and weight of the

Presslings a pressing density of 13.92 g / cm ³ . An abrasion test of pressings ("rattler test") showed 4.23% abrasion after 150 revolutions. Although these values suggest a direct processability to pressed and sintered parts, this unwaxed powder cannot be used industrially without pressing aids due to the high internal friction. Example 2 (comparative example)

The tungsten powder from Example 1 was mixed with 0.8% by weight of powdered zinc stearate for 2 hours in a stainless steel barrel. The flowability was then determined to be 4.3 s. The knock volume was 10.6 cm ³ / 100g. The press density was 14.52 g / cm ³ , the abrasion test, however, gave 100% abrasion after 150 revolutions. As the density improves, the strength of the compact deteriorates dramatically, making it difficult to handle the compact after it is ejected. In addition, there is poorer flowability of the powder obtained.

Example 3 (example according to the invention)

The tungsten metal powder from Example 2 mixed with 0.8% by weight of powdered zinc stearate was statically heated for 2 hours at 170 ° C. as a powder bed on a tray in a drying cabinet, and then statically cooled. The cake obtained in this way was lightly ground on the 500 μm sieve fabric of a tumbler sieve machine (Allgaier type). The tungsten metal powder thus obtained had the following properties:

Tap volume 12.2 cm ³ / 100g, Hall Flow 2.6 s / 50 g, press density 12.2 g / cm ³ , abrasion

14.05% after 150 revolutions. Very good flowability and good handling properties are achieved despite the comparatively low press density because of the good green strength. The latter is a prerequisite for a controlled porosity after sintering, which facilitates later infiltration of the sintered compact by, for example, copper or strontium titanate.

Examples 4 to 6 (Comparative Examples)

A mixture of 3.8% by weight of nickel metal powder, 1.2% by weight of carbonyl iron powder and 95% by weight of tungsten metal powder (H.C. Starck GmbH, type HC

800) in a free-fall mixer at room temperature (Example 4). Mixtures of the same type were produced with 1.5% by weight of powdered paraffin wax (melting point 57 ° C.) or with 1.5% by weight of a powdery montan ester wax (melting point 80 ° C), the proportions by weight of the other components being reduced proportionately (Examples 5 and 6). Pellets were produced from all powders as described in Example 1. The following table shows the values obtained with these powders:

It can clearly be seen that the wax content lowers the flowability and the pressed density of the compact in example 5 increases in spite of the lower specific weight of the wax compared to example 4, since the wax reduces the internal friction and the wall friction during pressing. While the paraffin wax does not contribute to better abrasion resistance, the montan ester wax seems to improve it due to its polar character. When using montan ester wax, only a comparatively low pressing density is achieved, since this wax obviously cannot act as a pressing aid due to its comparatively high melting point, since the temperature increase during pressing does not suffice to reach the melting point of the wax.

Examples 7 to 8 (examples according to the invention)

Waxed mixtures, which were obtained according to Examples 5 and 6, were statically heated in a drying cabinet as a powder bed on a tray tray for 1 h to temperatures above the melting point of the wax used (105 ° C.), again cooled statically, and the cake obtained was sieved on a 500 μm sieve. The powder obtained in Example 7 contained wax wax with a melting point of 57 ° C, the powder obtained in Example 8 Montanesterwachs (melting point 80 ° C). Pellets were produced from the powders obtained as described in Example 1. The following values for the powder and pellet properties were obtained:

The comparison of Examples 5 and 7 shows that it is possible with the method according to the invention to produce compacts with a uniform and reproducible very high level

Green strength, i.e. with low abrasion, although the compact density is lower than that of compacts made from metal powder mixed only with wax. The powder according to Example 7 had no acceptable flow properties, while that according to Example 8 had the same flowability as the unwaxed mixture in Example 4, had a high compression density with very little scattering, and had the required high green strength or abrasion resistance. The interaction of the pressing aid with the metallic wall of the press mold, which is desirable per se, naturally leads to a stronger interaction with the wall of the metallic funnel used in the case of the wax-infiltrated powders because of the better wax distribution

Determination of the flowability according to ASTM B 417 so that the flowability is reduced. Example 8 is an example of a powder according to the invention which contains a wax which is optimized with regard to the flowability. Examples 9 and 10 (9 = according to the invention)

93.5 parts by weight of tungsten carbide with an average grain diameter according to FSSS of 5 μm were mixed with 6.5 parts by weight of cobalt metal powder (CoMP TVC, H.C. Starck GmbH) with an average grain diameter according to FSSS of 0.9 μm in a mixing container (Turbula system). 98 parts by weight of the powder mixture obtained in this way were then mixed uniformly with 2 parts by weight of powdered paraffin wax (melting point 57 to 59 ° C.), and then statically heated to 100 ° C. in a drying cabinet for 1 h in a tray. After static cooling, the cake obtained was sieved through a sieve with a sieve opening of 1 mm (Example 9)

For comparison, all three components were mixed in a compulsory mixer until the temperature of the mix had risen to 80 ° C (hot mixing). After static cooling, the mix was comminuted over a sieve with a sieve opening of 1 mm (example 10).

The following values were determined to characterize the compression and flow properties:

It can be seen that the principle according to the invention is also effective in powder mixtures. With slightly poorer flowability, lower values for the tap density are achieved in comparison with the powder produced according to Example 10, which is an indicator of a rather loose structure of the powder pile. This is from This is an advantage when WCCo mixtures are to be processed into porous sintered agglomerates, such as those used in thermal spraying, or when shaping by pressing a uniform press density and thus a uniform shrinkage is required in order to be able to work close to the final contour and grinding work on the sintered Reduce fitting.

Examples 11 and 12 (Example 12 according to the invention)

WC powder with an average grain diameter in accordance with FSSS of 5 μm and cobalt metal powder (CoMP IVC, HC Starck GmbH) with an average grain diameter in accordance with FSSS of 0.9 μm was, according to WO 00/42230 AI, into a 200 kg mixture with the composition 94% by weight. % WC and 6% by weight cobalt intensely mixed. Here, the powder components were mixed intensively in a mixer using both circulating tools and a rotor / stator system, the mixture being heated up considerably by the energy input.

3 kg were removed from the cooled mixture and the remaining 197 kg were hot mixed with 4 kg of powdered paraffin wax (melting point 57 to 59 ° C.) in the above mixer, i.e. intensively mixed under high energy input until the mix has risen to a temperature above the melting point of the wax, and then intensively mixed for a further 20 min. After the static cooling, the mixture was ground through a sieve with sieve openings of 1 mm, and then sieved over 315 μm (Example 11). Two fractions were obtained, the first fraction having particles with a grain size smaller than 315 μm (-315 μm) and the second fraction having particles with a grain size larger than 315 μm (+ 315 μm). The 2nd fraction therefore remained on the sieve tray. 38% by weight of the

2nd fraction (+ 315 μm) and 62% by weight of the 1st fraction (- 315 μm) obtained. The wax and cobalt contents of both fractions differed significantly:

You can clearly see that cobalt and wax accumulate in the larger agglomerates (segregation). These are obviously built up during hot mixing by the high shear forces, again generated by the high-speed mixing tools (rotor / stator system). If such a hard metal mixture is pressed and sintered, a sintered structure with local cobalt enrichments is obtained, which has a negative effect on the wear and strength behavior of such a hard metal material. In addition, such a mixture cannot be dispersed, which would be necessary for spray drying, if appropriate, to produce flowable granules, since the larger agglomerates sediment out.

The 3 kg of the WC-Co mixture removed in the cold state were mixed according to the invention in a free-fall mixer with the above-mentioned paraffin wax, the same wax content being set as in the preceding

Example and no significant temperature increase occurred (Example 12, according to the invention). The material to be mixed was then poured onto a metallic drying tray as a powder bed and then heated statically in a drying cabinet at 80 ° C. for 1 hour. After the static cooling, the resulting cake was crushed through a 1 mm sieve and sieved over 315 μm, leaving no sieve residue. The segregation effect observed in example 11 with the build-up of agglomerates therefore did not occur since the wax infiltration took place in a stationary powder bed. The powder obtained according to the invention was also easy to disperse in liquid, since there was no coarse fraction which tended to sediment out. A dispersion obtainable in this way could be fed, for example, to a spray drying step in order to produce flowable granules without coarser parts being sedimented out during the production of the spray-dried dispersion of WCCo powder in liquid.

Claims

Patentansprttche

1. A process for the production of powders mixed with pressing aids, characterized in that in a first step powdery pressing aids with a starting powder at a temperature below the

Melting temperature of the pressing aid is mixed, and in a second step the mixture obtained from the first step is heated as a powder bed to a temperature of at least the melting temperature of the pressing aid, the powder bed during the second step and with subsequent cooling to temperatures below the melting point of the Pressing aid is not moved in itself.

2. The method according to claim 1, characterized in that the powder mixed with pressing aids is sieved after heating and cooling.

3. The method according to claim 1 or 2, characterized in that the powdery pressing aid is used in an amount of 0.2 to 10 wt .-%, based on the total amount of pressing aid and starting powder.

Method according to one of claims 1 to 3, characterized in that fatty acids or their esters or their amides or their metal salts, paraffin waxes, montan ester waxes, polyethylene or their oxidized derivatives or fatty alcohols are used as the powdered pressing aid.

Method according to one of claims 1 to 4, characterized in that the powdery pressing aid has a particle diameter of -5 to -500 Dm.

6. The method according to any one of claims 1 to 5, characterized in that a metal powder or a mixture of metal powders is used as the starting powder.

7. The method according to any one of claims 1 to 5, characterized in that an alloy powder or a mixture of alloy powders is used as the starting powder.

8. The method according to any one of claims 1 to 5, characterized in that an oxide powder, a powder of a carbide or a as the starting powder

Mixture of carbide with metal powders is used.

9. A process for the production of sinterable metallic moldings from metal powders mixed with pressing aids, characterized in that powdered pressing aids are mixed with a metal powder at a temperature below the melting temperature of the pressing aid, the mixture obtained as a self-contained powder bed to a temperature of at least the melting temperature of the Press auxiliary heated, cooled to temperatures below the melting point of the auxiliary and then sieved, and the so produced with

Pressing aids containing metal powder are poured into a mold, compressed under pressure, and finally ejected from the mold as a molded part.

10. The method according to claim 1), characterized in that the heating of the self-contained powder bed takes place in a continuously operable unit containing a conveyor belt.

11. The method according to claim 1), characterized in that the heating of the mixture as a powder bed on a Trägerφlatte with lateral

Boundary walls is carried out. Feid No. VIII (v) DECLARATION: HARMFUL REVELATIONS OR EXCEPTIONS FROM THE

NEUHE1TSSCHÄDL1CHKEJT

The declaration must conform to the wording prescribed in section 215; see comments on fields VIII, VIII (i) to (v) (general) and in particular the comments on field No. VIII (v). If this field is not used, this sheet should not be attached to the application.

Declaration regarding innocuous revelations or exceptions to novelty (rules 4.17 point v and 51 to 1 paragraph a point v): regarding this international application, Dr. Benno Gries and Dr. Bernhard Szesny that the subject requested in this international application was disclosed as follows:

EP 0589 088 Bl 12., 08.1998 EP 0781 180 Bl 28.11.2001 US 5,498,276

US 2002/0127129 AI 12.09.2002 EP 0968 068 B1 05.01.2000 WO 01/40416 AI 07.06.2001

□ This explanation is continued on the following sheet, "continuation sheet for field no. VIII (v)"

Form PCT / RO / 101 (Declaration Sheet (v)) (March 2001; reprinted July 2003) See comments on this application form