DE102020127325A1 - Press tool, press device, method for pressing and spherical ceramic green body - Google Patents

Abstract

The invention relates to a multi-part pressing tool (10) for producing a spherical ceramic green body (1), the pressing tool (10) being designed to include an upper punch (2), a lower punch (3) counteracting the upper punch and a press die (4), wherein the upper punch (2) has a first concave surface (2a) in the form of a first shell of a first spherical section and the lower punch (3) has a second concave surface (3a) in the form of a second shell of a second spherical section. The press die (4) is designed in several parts, all parts (41, 42) of the multi-part press die (4) together forming a third surface (4a) in the form of a shell of a spherical segment. The concave first surface (2a), the concave second surface (3a) and the third surface (4a) are dimensioned in such a way that together they form a spherical pressing chamber (5) for forming the green compact (1).

Description

The invention relates to a multi-part pressing tool for producing a spherical ceramic green compact, the pressing tool being designed to include an upper punch, a lower punch which counteracts the upper punch and a press die. The invention further relates to a press device and method for uniaxial or biaxial pressing. Finally, the invention relates to a spherical ceramic green body produced by such a method.

There is currently an increasing demand in the field of ceramic roller bearings for a wide variety of applications, which will increase significantly in the coming years. The focus here is particularly on hybrid bearings, in which only the rolling elements are made of ceramic and the bearing rings are made of metal. The limiting factor for widespread use of such hybrid bearings, especially in applications for the automotive industry, is currently the cost of such ceramic rolling elements. These costs are mainly due to a very complex process route for the production of the sintered blanks and the finish machining of them.

A sintered blank that is as homogeneous as possible, which is characterized above all by uniform properties and a high degree of roundness, is decisive for optimal finishing of spherical ceramic rolling elements. These properties are significantly influenced by the choice of shaping process used to form the ceramic green body.

A two-stage molding process is usually chosen. In the first step, a powder preparation or a granulate comprising ceramic powder and binder is pressed into a green compact by means of uniaxial or biaxial pressing.

In uniaxial/biaxial pressing, the powder preparation or granules are filled into a one-piece press die. This is usually cylindrical and has no direct shaping function. The desired spherical geometry of the green body is formed only via the upper and lower punches. For this purpose, a half-shell is introduced into each of the stamps.

In order to take account of the corresponding process fluctuations and to avoid severe tool wear, the two punches are not brought into contact with one another during shaping. This leaves a small gap between the punches. As a result, the formed green compact is not completely spherical. A cylindrical portion remains at the level of the equator of the sphere. This is called Saturn's ring.

However, this green body still has inhomogeneities in terms of its density, which after debinding of the green body in a subsequent sintering step lead to non-uniform shrinkage behavior and increased out-of-roundness of the sintered blank or component. For this reason, in a second shaping step, the already pre-compacted green body is recompacted on all sides by cold isostatic pressing. As a result, the aforementioned differences in density can be evened out and almost evened out.

However, the outer geometry of the blank is significantly influenced by the first shaping step. In the subsequent second shaping step of cold isostatic pressing, the inner microstructure of the blank and thus its shrinkage behavior are only made more uniform, while major changes in shape no longer take place.

The Saturn ring formed in the first shaping step proves to be very disadvantageous in the later finishing of the sintered blank, since it impedes the rotational movement of the ball in a ball mill and thus makes processing more difficult. For this reason, the Saturn ring must first be removed in a kind of pre-machining ("prelapping") using a large amount of abrasive, usually in the form of a diamond suspension, before the final spherical geometry can be produced in subsequent fine machining.

It is therefore an object of the invention to develop the outer geometry of a spherical ceramic green compact formed by means of uniaxial or biaxial pressing without such a Saturn ring. A suitable pressing tool, a suitable press device and suitable pressing methods should be specified for this purpose.

The task is for a multi-part pressing tool for the production of a spherical ceramic green compact, wherein the pressing tool is designed to include an upper punch, a lower punch that counteracts the upper punch and a press die, wherein the upper punch has a first concave surface in the form of a first shell of a first spherical segment and wherein the lower punch has a second concave surface in the form of a second shell of a second spherical segment, solved in that the press die is designed in several parts, with all parts of the multi-part press die together forming a form the third surface in the form of a shell of a layer of spheres, wherein the concave first surface, the concave second surface and the third surface are dimensioned such that together form a spherical press chamber for forming the green body.

As a result, no ring of Saturn is formed in the area of the equator of the green compact, which would have to be removed in a time-consuming manner after debinding and sintering of the green compact.

It has proven useful if the press die comprises an even number of parts, with two parts being arranged opposite one another and formed in a mirror-inverted manner. In particular, the press die is designed in two or four parts. Opposite parts of the press die for forming the press chamber can preferably be moved towards one another.

In a preferred embodiment of the pressing tool, either only the upper punch (uniaxial pressing) or the upper punch and the lower punch (biaxial pressing) are designed to be movable.

Preferably the second concave surface forms a mirror image of the first concave surface. However, the first and second concave surfaces may differ in extent.

A press device for uniaxially or biaxially pressing a spherical ceramic green compact has proven itself, with the press device comprising a multi-part pressing tool according to the invention.

The use of the multi-part pressing tool for uniaxial/biaxial pressing of the spherical green compacts makes it possible to relocate part of the shaping contour to the multi-part press die. At the time of spherical shaping, the press chamber and thus the press die are closed, so that the formed spherical green body does not have a raised Saturn ring at the equator due to the small dividing seam of the press die, but only a visible narrow ring-shaped mark. Such markings are also present on the dividing seams of the pressing tool at the transition from upper punch to press die and from lower punch to press die on the green body. However, these do not lead to the necessary time-consuming removal of material projecting beyond the spherical surface of the green compact after debinding and sintering of the green compact.

• Bereitstellen einer erfindungsgemäßen Presseneinrichtung,
• Öffnen der Presskammer des mehrteiligen Presswerkzeuges mittels des Oberstempels,
• Einfüllen einer Pulverzubereitung oder eines Granulats enthaltend mindestens ein keramisches Pulver und ein Bindemittel zur Ausbildung des kugelförmigen keramischen Grünlings oder
• Einbringen eines Vorformlings aus einer Pulverzubereitung oder einem Granulat enthaltend mindestens ein keramisches Pulver und ein Bindemittel, und Schließen der Presskammer mittels des Oberstempels unter Ausbildung eines Pressdruckes in der Presskammer und Ausbildung des kugelförmigen keramischen Grünlings.

A method for uniaxially pressing a spherical ceramic green compact according to the present invention comprises the following steps:

• providing a press device according to the invention,
• Opening the pressing chamber of the multi-part pressing tool using the upper punch,
• Filling in a powder preparation or a granulate containing at least one ceramic powder and a binder to form the spherical ceramic green body or
• Introduction of a preform from a powder preparation or a granulate containing at least one ceramic powder and a binder, and closing the pressing chamber by means of the upper punch, forming a pressing pressure in the pressing chamber and forming the spherical ceramic green body.

• Bereitstellen einer erfindungsgemäßen Presseneinrichtung,
• Öffnen der Presskammer des mehrteiligen Presswerkzeuges mittels des Oberstempels und des Unterstempels,
• Einfüllen einer Pulverzubereitung oder eines Granulats enthaltend mindestens ein keramisches Pulver und ein Bindemittel zur Ausbildung des kugelförmigen keramischen Grünlings oder
• Einbringen eines Vorformlings aus einer Pulverzubereitung oder einem Granulat enthaltend mindestens ein keramisches Pulver und ein Bindemittel, und Schließen der Presskammer mittels des Oberstempels und des Unterstempels unter Ausbildung eines Pressdruckes auf den Vorformling in der Presskammer und Ausbildung des kugelförmigen keramischen Grünlings.

A second method of biaxially pressing a spherical ceramic green compact according to the present invention comprises the following steps:

• providing a press device according to the invention,
• Opening of the pressing chamber of the multi-part pressing tool by means of the upper punch and the lower punch,
• Filling in a powder preparation or a granulate containing at least one ceramic powder and a binder to form the spherical ceramic green body or
• Introduction of a preform from a powder preparation or a granulate containing at least one ceramic powder and a binder, and closing the pressing chamber by means of the upper punch and the lower punch, forming a pressing pressure on the preform in the pressing chamber and forming the spherical ceramic green body.

“Opening the press chamber” is understood to mean that at least the upper punch is moved out so far that powder, granulate or preform can be filled into the press chamber via an opening.

A preform can be formed, for example, by an extrusion process or a pressing process.

The formed spherical ceramic green body has at least three ring-shaped marks if the die is made in two parts and is divided horizontally. Unless the press If the die has more than two parts and, for example, has not only one horizontal but also two or more vertical dividing seams, then the green body still has at least two linear markings connecting the ring-shaped markings in the area of its surface. The markings are therefore arranged in the area of dividing seams, which are present in the press chamber at the transition between the first surface and the third surface, between the second surface and the third surface and between the adjoining parts of the multi-part press die in the area of the third surface.

After debinding and sintering of the green compact, the time required for finishing the ceramic ball to a defined diameter is significantly reduced.

A ceramic powder in the form of silicon nitride (Si ₃ N ₄ ) and/or aluminum oxide (Al ₂ O ₃ ) and/or zirconium oxide (ZrO ₂ ) and/or ZTA (zirconium oxide-reinforced aluminum oxide) is preferably used to form the ceramic green compact. ) and/or ATZ (Alumina Reinforced Zirconia) and/or Silicon Carbide (SiC).

• 1 einen Längsschnitt durch eine Presseneinrichtung umfassend ein Presswerkzeug, und
• 2 einen kugelförmigen keramischen Grünling.

the 1 and 2 are intended to exemplify the present invention. So shows:

• 1 a longitudinal section through a press device comprising a pressing tool, and
• 2 a spherical ceramic green compact.

1 shows a longitudinal section through a press device 11 comprising a multi-part pressing tool 10 for the production of a spherical ceramic green compact 1 (cf 2 ), wherein the pressing tool 10 has an upper punch 2 and a lower punch 3 counteracting the upper punch 2 as well as a press die 4 . The upper punch 2 has a first concave surface 2a in the form of a first shell of a first spherical section. The lower punch 3 has a second concave surface 3a in the form of a second jacket of a second spherical section. The press die 4 is multi-part, here in two parts, formed, the parts 41, 42 of the multi-part press die 4 together forming a third surface 4a in the form of a shell of a spherical layer. The concave first surface 2a, the concave second surface 3a and the third surface 4a are dimensioned such that together they form a spherical press chamber 5 for forming the green compact 1. The two-part press die 4 here is designed to be divided horizontally.

The in the pressing tool 10 according to 1 formed spherical ceramic green compact 1 has according to 2 on its surface 9 three annular markings 8a. If the press die 4 has more than two parts 41, 42 and, for example, has not only one horizontal but also two or more vertical dividing seams, then the green body 1 also has at least two linear markings 8b connecting the ring-shaped markings 8a in the area of its surface 9 present. The markings 8a, 8b are accordingly arranged in the area of dividing seams 12, 12a, 12b (cf 1 ) present in the press chamber 5 at the transition between the first surface 2a and the third surface 4a, between the second surface 3a and the third surface 4a and between the parts 41, 42 of the multi-part press die 4 adjoining one another in the region of the third surface 4a are.

Reference List

1

greenling

2

upper stamp

2a

first surface

3

bottom stamp

3a

second surface

4

press die

4a

third surface

41, 42

part of the press die

5

press chamber

8a, 8b

mark

9

surface of the green body

10

pressing tool

11

press equipment

12, 12a, 12b

dividing seam

Claims (10)

Multi-part pressing tool (10) for producing a spherical ceramic green compact (1), the pressing tool (10) being designed to include an upper punch (2), a lower punch (3) counteracting the upper punch and a press die (4), the upper punch (2nd ) has a first concave surface (2a) in the form of a first shell of a first spherical segment and wherein the lower punch (3) has a second concave surface (3a) in the form of a second shell of a second spherical segment, characterized in that the press die (4) is constructed in several parts, all parts (41, 42) of the multi-part press die (4) together having a third surface (4a) in the form of a jacket form a layer of spheres, the concave first surface (2a), the concave second surface (3a) and the third surface (4a) being dimensioned in such a way that together they form a spherical press chamber (5) for forming the green compact (1). Multi-part pressing tool (10). claim 1 , characterized in that the press die (4) comprises an even number of parts (41, 42), with two parts (41, 42) being arranged opposite one another and being shaped as a mirror image. Multi-part pressing tool (10). claim 1 or claim 2 , characterized in that the press die (4) is formed in two or four parts. Multi-part pressing tool (10) according to one of Claims 1 until 3 , characterized in that the opposite parts (41, 42) of the press die (4) to form the press chamber (5) are movable towards each other. Multi-part pressing tool (10) according to one of Claims 1 until 4 , characterized in that only the upper punch (2) or the upper punch (2) and the lower punch (3) are designed to be movable. Multi-part pressing tool (10) according to one of Claims 1 until 5 , characterized in that the second concave surface (3a) forms a mirror image of the first concave surface (2a). Press device (11) for uniaxially or biaxially pressing a spherical ceramic green compact (1), the press device (11) having a multi-part pressing tool (10) according to one of Claims 1 until 6 includes. A method for uniaxially pressing a spherical ceramic green compact (1), comprising the steps of: providing a pressing device (11) according to claim 7 , opening the pressing chamber (5) of the multi-part pressing tool (10) by means of the upper punch (2), filling in a powder preparation or granules containing at least one ceramic powder and a binder to form the spherical ceramic green compact (1) or introducing a preform from a powder preparation or a granulate containing at least one ceramic powder and a binder, and closing the pressing chamber (5) by means of the upper punch (2) with formation of a pressing pressure in the pressing chamber (5) and formation of the spherical ceramic green compact (1). A method for biaxially pressing a spherical ceramic green body (1), comprising the steps of: providing a press device (11) according to claim 7 , opening the press chamber (5) of the multi-part press tool (10) by means of the upper punch (2) and the lower punch (3), filling in a powder preparation or granules containing at least one ceramic powder and a binder to form the spherical ceramic green compact (1) or Insertion of a preform made from a powder preparation or granulate containing at least one ceramic powder and a binder, and closing the press chamber (5) by means of the upper punch (2) and the lower punch (3), forming a pressing pressure on the preform in the press chamber (5) and forming the spherical ceramic green body (1). Spherical ceramic green compact (1), produced by a method according to claim 9 or claim 10 , which has at least three ring-shaped markings (8a) and optionally also at least two linear markings (8b) connecting the ring-shaped markings (8a) in the area of its surface (9).

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