DE102018220085A1 - Method of manufacturing a rolling bearing component - Google Patents

Abstract

The invention relates to a method for producing a rolling bearing component made of polyimide, comprising the following steps: - producing at least one green body using the direct molding method, polyimide powder being introduced into a mold and pressed to the green body; - subsequent sintering of the green body by supplying heat into the pressed polyimide powder The method according to the invention is characterized in that the green compact is surrounded during sintering with a pressurized fluid and / or another medium which is not stable in shape and which additionally compresses the polyimide powder in the green compact.

Description

The present invention relates to a method for producing a rolling bearing component from polyimide according to the preamble of claim 1. According to the present invention, polyimide can be used in pure form or as a component of an alloy or mixture, polyimide in particular making up the largest proportion.

In practice, polyimide has proven to be an advantageous material for rolling bearing components, for example for cages or balls of rolling bearings. Conventionally, such components are manufactured, as exemplarily in EP 2 886 583 A1 is described, either by a so-called hot compression molding process, also called hot press process, or by a direct forming process, also called direct molding process. The hot-compression molding process results in a tribologically favorable, high-strength polyimide material that is sawn, rotated and ground after the hot-compression molding process step, with up to 40% of the material removed from the press mold being lost by machining goes.

In the direct forming process, the material yield is significantly better because the starting material polyimide powder (PI powder) is immediately brought into a geometry close to the component to be manufactured. Molding is carried out at room temperature or comparatively low temperatures, for example at a temperature between 0 ° C. and 100 ° C., preferably between 15 ° C. and 40 ° C., as in detail in EP 2 886 583 A1 is specified. Pressing into the desired shape is followed by sintering of the pressed polyimide powder in a sintering furnace for several hours, preferably at a temperature of 250 ° C. to 600 ° C. The component is solidified during sintering. The sintering of the so-called green compact, which is removed from the press mold by the direct forming method, is generally carried out in an oven, with nitrogen or an inert gas as a protective gas being able to flow through the oven.

In practice, it has been found that the rolling bearing components, which were produced by the direct-forming process with subsequent sintering, do not have the desired tribological properties and strength properties, which is why the hot-compression molding process is used almost exclusively a correspondingly low material yield, which increases the cost of manufacturing the rolling bearing components.

The present invention has for its object to provide a method for producing a rolling bearing component made of polyimide, with which the desired high tribological and strength properties are achieved and at the same time the material yield is improved.

The object of the invention is achieved by a method with the steps of claim 1. Particularly advantageous and expedient embodiments of the invention are specified in the dependent claims.

• - Herstellen wenigstens eines Grünlings im Direktformverfahren, wobei Polyimidpulver in eine Pressform eingebracht und zu dem Grünling verpresst wird.
• - Nachfolgendes Versintern des Grünlings durch Zufuhr von Wärme in das verpresste Polyimidpulver.

The method according to the invention for producing a rolling bearing component from polyimide comprises the following steps:

• - Production of at least one green compact in the direct molding process, polyimide powder being introduced into a mold and pressed to the green compact.
• - Subsequent sintering of the green body by applying heat to the compressed polyimide powder.

According to the invention, the green compact is surrounded during sintering with a pressurized fluid and / or another medium which is not stable in shape and which additionally compresses the polyimide powder in the green compact.

The additional pressing of the green body during sintering leads to particularly high-quality material properties of the rolling bearing component thus produced, both from a tribological and strength point of view.

At the same time, the desired high material yield can be achieved by using the direct forming process. In particular, the polyimide powder is pressed in the direct molding process into a shape which already has a cross-sectional shape corresponding to the rolling bearing component or has a geometry close to the component to be produced, so that during the subsequent finishing of the rolling bearing component it is advantageous for machining, at least to a greater extent, possibly with the exception of Cutting (including sawing), drilling and / or grinding can be dispensed with. In particular, the rolling bearing component made of polyimide is neither milled nor turned during its manufacture. A separation of the cross-sectional shape of the green body removed from the press mold into smaller semi-finished parts with cross-sectional shapes which are comparatively closer to or more similar to the rolling bearing component to be produced can also advantageously be dispensed with. This does not rule out that the green compact is divided in the longitudinal direction before or after its sintering, for example from a rod material produced, in particular a hollow cylinder, with one corresponding to the roller bearing component Cross-sectional shape to produce several rolling bearing components. It is also conceivable to process the green compact in accordance with the geometrical requirements of the rolling bearing component, such as drilling, punching, deforming or the like.

To improve the surface quality, the green compact can be ground before or after its sintering, preferably after its sintering.

For example, a maximum of 25% or 20% or preferably a maximum of 10%, in particular only up to 5%, of the material volume is removed from the green compact until the rolling bearing component is produced in its final form.

The additional pressing of the green compact during sintering by the pressurization with fluid and / or the other medium that is not stable in shape can take place at a pressure which is lower than the pressing pressure in the previous direct molding process. In principle, however, the same pressure or even a higher pressure is also possible.

Fluid is understood to mean any liquid or gas or even a viscous substance. Gas-liquid mixtures are also suitable, for example.

By combining this pressure with the comparatively high temperature during sintering, the temperature being generally higher than in the previous direct molding process, for example a multiple, in particular at least two or three times, when the temperature is given in degrees Celsius, the desired high-quality results Material properties achieved.

The heat can be supplied to the compressed polyimide powder during sintering, preferably by conduction in the fluid and / or the medium. An alternative would be heat radiation, which, however, can in principle also be provided at the same time as the heat conduction. As a rule, convection will also take place between the green compact and the fluid or the medium and in particular between a heated container surrounding the fluid or the medium and the fluid or the medium.

Sintering can take place, for example, in a container filled with a liquid fluid, the at least one green body being inserted into the fluid. In this case, the liquid level of the fluid can be pressurized, for example in that the container has two mold halves, which together enclose a space receiving the fluid, and are moved towards one another. Correspondingly, the two mold halves can also be moved towards one another if another medium that is not dimensionally stable is used instead of a fluid.

According to a preferred embodiment of the invention, the green compact is subjected to a vacuum before the pressurized sintering. This is possible, for example, by evacuating a container in which the at least one green body is positioned before being filled with the pressurized fluid or medium.

By applying a vacuum to the green body prior to sintering, undesired gases, in particular its pores, can be removed. When the pressurized fluid is subsequently applied, this fluid can then penetrate into the previously evacuated pores and improve an advantageously provided impregnation of the green body with the pressurized fluid.

In principle, it is also possible to impregnate the green compact with a different fluid, in particular a different liquid, than the liquid which is subsequently used for the pressurization, before the pressurized sintering. In this case, the green body is preferably first subjected to a vacuum, then impregnated with the desired impregnating fluid and then surrounded with another pressurized fluid and / or correspondingly another medium and sintered at the same time.

Highly resistant oils, for example silicone oil or phenylmethylpolysiloxanes or a liquid metal, can be used as the pressurized fluid.

According to one embodiment of the invention, several green compacts are sintered simultaneously through the environment with the same pressurized fluid or medium.

According to one embodiment of the invention, the at least one green compact is produced with a hollow core, for example by shaping a hollow cylinder or a ring.

The roller bearing component is in particular a roller bearing cage, preferably a roller bearing cage of a roller bearing, in particular a ball bearing, in a medical instrument, for example a dental handpiece.

The roller bearing cage can be used, for example, in a dental technology warehouse with a speed value nx dm ≥ 500,000 mm / min, where n is the speed in revolutions per minute and dm describes the average bearing diameter in millimeters.

The invention will be described below by way of example using an exemplary embodiment and the figures.

• 1 die Herstellung eines Wälzlagerbauteils mit dem Hot-Compression-Molding-Verfahren gemäß dem Stand der Technik;
• 2 die Herstellung eines Wälzlagerbauteils mit dem Direct-Forming-Verfahren gemäß dem Stand der Technik;
• 3 die Herstellung eines Wälzlagerbauteils aus Polyimid gemäß einem erfindungsgemäßen Verfahren.

Show it:

• 1 the production of a rolling bearing component with the hot compression molding method according to the prior art;
• 2nd the production of a rolling bearing component using the direct forming method according to the prior art;
• 3rd the production of a rolling bearing component made of polyimide according to a method according to the invention.

In the 1 the so-called hot compression molding process, also called hot press process, is shown schematically. As you can see, a semi-finished product is initially made, usually in a solid plate shape, for example made of polyimide, in a press mold 1 made while heating and pressurizing at the same time. Polyimide powder, for example, is used as the raw material that is pressurized and heated and thereby sintered 2nd used that previously in the mold 1 , comprising at least two mold halves 3rd , 4th was introduced. The semi-finished product is made from the press mold 1 removed and then it is first sawn into a large number of smaller blocks, with approx. 15% of the material of the semi-finished product being lost during machining. The smaller blocks are then machined by turning, with another 20% of the material being lost by cutting. In the final grinding, another 5% of the material is lost, so that a total of only 60% of the raw material originally used is used.

In the alternative method direct forming, also called direct forming method, as in the 2nd is shown, a material yield of 95% of the raw material originally used can be achieved, but the material quality is inferior, in particular the strength and the tribological properties are less favorable. Direct molding again uses polyimide powder 2nd into a mold 1 with two mold halves 3rd , 4th introduced and shaped under pressure, but at low temperature. Then the so-called green body 5 , as the semi-finished product is also called, from the mold 1 removed and sintered in a sintering furnace. A final grinding can then be carried out in order to achieve the desired final shape of the rolling bearing component. The material yield is, for example, 95%.

In a preferred method according to the invention, as shown schematically in the 3rd is shown, first a green body 5 manufactured in the direct molding process by introducing polyimide powder 2nd into a mold 1 who have favourited two mold halves 3rd , 4th having. The mold halves 3rd , 4th form a mold cavity 10th from which, at least in its cross section, corresponds at least approximately to the cross-sectional shape of the rolling bearing component to be produced. Since in the present embodiment a hollow cylindrical green compact 5 is to be made is the mold cavity 10th correspondingly hollow cylindrical, at least one of the two mold halves 3rd , 4th , here the mold half 3rd , has a stamp through the hollow core 9 of the green body 5 or the green body 5 molding polyimide powder 2nd to be led.

Direct molding can take place, for example, at temperatures of less than 400 ° C., in particular less than 300 ° C. or 200 ° C., in particular less than 100 ° C.

Then the at least one green body 5 in a container 7 positioned. In the exemplary embodiment shown there are two green compacts 5 in the container 7 positioned. In the container 7 can at least one green body 5 in particular, are initially subjected to a vacuum.

The container 7 is further with a fluid 6 and / or also filled with another medium which is not stable in shape and which contains the at least one green compact 5 surrounds on all sides. Then the liquid level 8th if it is the fluid 6 is a liquid, or generally the fluid 6 or pressurizes the medium so that this pressure on the at least one green compact 5 is transmitted. Because of the fact that the fluid 6 or the medium the green body 5 Surrounded on all sides, the pressure acts on all surfaces of the green compact 5 . At the same time, heat is supplied, for example also via the fluid 6 or medium so that the green body 5 is sintered in the pressurized state. For example, the fluid 6 or the green body 5 heated to a higher temperature than the temperature at which the direct molding was previously carried out. In particular, it is heated to more than 200 ° C, 300 ° C or 400 ° C.

The combination of pressure and temperature leads to the suitable material properties of the green compact 5 .

Then the green body 5 from the container 7 are removed and processed into the finished rolling bearing component. For example, the surface of the green compact can be used for this 5 be sanded. In particular, however, machining can be carried out on a larger scale Scope can be dispensed with, so that, for example, at least 90% or at least 95% of the material of the sintered green body 5 is still present in the finished roller bearing component.

The invention was previously illustrated using a method in which the green compact 5 and so that the rolling bearing component was made of polyimide, which is, for example, pure polyimide, that is to say polyimide with impurities that cannot be avoided. However, a material is preferably used in which an additive has been added to the polyimide, for example graphite. It is also possible to add several additives. Thus, in the present case, polyimide is understood to mean any material comprising polyimide, but preferably in a composition in which polyimide constitutes the largest constituent or main constituent and in particular more than 50% by weight.

Reference symbol list

1

Mold

2nd

Polyimide powder

3.4

Mold half

5

Green body

6

Fluid

7

container

8th

Liquid level

9

core

10th

Mold cavity

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• EP 2886583 A1 [0002, 0003]

Claims (11)

A method for producing a rolling bearing component made of polyimide, comprising the following steps: 1.1 producing at least one green body (5) in the direct molding process, polyimide powder (2) being introduced into a mold (1) and being pressed into the green body (5); 1.2 subsequent sintering of the green compact (5) by supplying heat into the compressed polyimide powder (2); characterized in that 1.3 the green compact (5) is surrounded during the sintering with a pressurized fluid (6) and / or another non-rigid medium which additionally compresses the polyimide powder (2) in the green compact (5). Procedure according to Claim 1 , characterized in that during the sintering, the supply of heat into the compressed polyimide powder (2) takes place by heat conduction in the fluid (6) and / or the medium. Method according to one of the Claims 1 or 2nd , characterized in that the sintering takes place in a container (7) filled with the fluid in the form of a liquid, the at least one green body (5) being inserted into the fluid (6) and a liquid level (8) of the fluid (6) is pressurized. Method according to one of the Claims 1 to 3rd , characterized in that the green compact (5) is subjected to a vacuum before the pressurized sintering. Procedure according to Claim 4 , characterized in that the container (7) is evacuated before filling with a liquid fluid (6) and / or the medium, the at least one green compact (5) being previously positioned in the container (7). Method according to one of the Claims 4 or 5 , characterized in that the green compact (5) is impregnated with an impregnation medium after the application of the vacuum and before the sintering, or is impregnated with the pressurized fluid (6) after the application with the vacuum. Method according to one of the Claims 1 to 6 , characterized in that a high-temperature oil, in particular silicone oil or phenylmethylpolysiloxane, is used as the fluid (6). Method according to one of the Claims 1 to 7 , characterized in that a liquid metal is used as the fluid (6). Method according to one of the Claims 1 to 8th , characterized in that several green compacts (5) are sintered simultaneously by the environment with the same pressurized fluid (6) and / or medium. Method according to one of the Claims 1 to 9 , characterized in that the at least one green compact (5) is produced with a hollow core (9), in particular by forming a hollow cylinder or ring. Method according to one of the Claims 1 to 10th , characterized in that a rolling bearing cage is produced as the rolling bearing component, in particular a medical instrument or dental handpiece.

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