Method for producing a form segment and form segment
The invention relates to a method for producing a mould segment of a vul-
canization mould for a vehicle tyre having a profiled tread, wherein, on an in particular planar surface of a first mould segment part prefabricated by a non-generative manufacturing process, a further mould segment part is built up in layers by means of a generative manufacturing process by melting metal powder and is materially bonded to the first mould segment part,
wherein the further mould segment part is built up together with the ele-
ments forming the profile negative of the tread of the vehicle tyre, and wherein the further mould segment part is built up with an outer layer of completely melted metal powder which surrounds on the outside an inner region adjoining the first mould segment part.
The invention relates further to a mould segment of a vulcanization mould for a vehicle tyre, composed of a prefabricated first mould segment part, produced by a non-generative manufacturing process, and a further mould segment part which is built up in layers by means of a generative manufac- turing process by melting metal powder, is materially bonded to the first mould segment part and is provided with the elements forming the profile negative of the tread of the vehicle tyre, and wherein the further mould segment part has an outer layer and an inner region adjoining the first mould segment part, wherein the outer layer con- sists of completely melted metal powder and surrounds the inner region on
— the outside.
In known manner, the vulcanization of vehicle tyres takes place in heat presses, in which a green tyre is inserted into a suitable vulcanization mould and vulcanized under the action of pressure and temperature.
The mould segments, which form a mould segment ring, shape and heat the tread of the tyre and impress the tread profile into it.
Mould segments are traditionally produced from steel or aluminium alloys by casting processes with subse- quent machining or solely by machining.
It is known to produce the complete profile negative of the mould segments of a tread or also individual elements of such a profile negative, for example lamellae or ribs, by generative manufacturing processes, in particular by SLM (selective laser melting), and to fasten or arrange it/them in traditional- ly manufactured mould segment parts.
The advantage of generative manu- facturing processes is to be seen in particular in the time- and cost-efficient preparation and in the freedom in terms of the shaping of the mould seg- ments and the components thereof.
EP 2 709 792 BI discloses a method for manufacturing a moulding element, which in particular is a mould segment, for a mould for a pneumatic tyre, wherein the moulding element has a sintered part and a non-sintered part fixedly bonded to the sintered part.
The sintered part is a shell, which con- tains a core which is manufactured as one piece with the shell and has a meshed structure, which thus contains a plurality of cavities.
The cavities occupy far more than half of the volume of the core.
The shell is thereby produced together with the meshed structure layer by layer from molten metal powder.
In the formation of this meshed structure in layers with corre- sponding cavities, non-melted metal powder remains in the cavities due to the manufacturing process.
The mesh parts forming the network structure consist of solid melted material.
The core therefore contains large propor- tions of cavities filled with air and metal powder, as a result of which the thermal conductivity of the mould segment part is significantly poorer com- pared to a mould segment part consisting of solid material.
The meshed structure is said to improve the mechanical stability and reduce internal stresses, which during reworking by machining cause changes in the geome- try of the mould segment part due to distortion.
DE 10 2004 028 462 Al discloses a mould segment of a vulcanization mould of a pneumatic vehicle tyre, which mould segment has a base part having a three-dimensional grid structure, and a mould segment part adjoin- ing the three-dimensional grid structure. The three-dimensional grid struc- ture is produced directly on the base part from a sinterable powdered materi- al by a laser melting process, that is to say by a generative manufacturing process. The grid structure creates a network of cavities, through which a heating medium, in particular water vapour, is passed. The mould segment part consists of sintered steel material which is built up in layers from pow- dered material by means of a laser melting process and which has micro- channels and micropores distributed over its entire width and its entire cross section. The microchannels and micropores, in combination with the grid structure and a central vent hole, allow the vulcanization mould to be vent-
ed. Such a mould segment can be produced quickly and inexpensively. US 2016/0039160 A1 discloses a mould segment which is produced layer by layer from metal powder by a laser melting process. The laser melting is carried out in such a way that the finished mould segment has a porous vent- ing channel with a porosity of at least 0.2 running between its inner surface and its outer surface. The object underlying the invention is to provide a method which makes it possible to produce a mould segment having substantially better heat- conducting properties than the mould segment having the meshed structure according to EP 2 709 792 BI, at a high process speed and with variably adjustable heat-conducting properties, wherein non-melted metal powder within the segment is at least largely to be avoided. A further object underly- ing the invention is to provide a mould segment having these properties. With regard to the method, the stated object is achieved according to the invention in that the inner region is built up at least partially with a volumet- ric energy input which differs from the volumetric energy input during the build-up of the outer layer, in such a way that the inner region is formed with a porosity of 0.01 to 0.5. As regards the mould segment, the stated object is achieved according to the invention in that the inner region has a porosity of 0.01 to 0.5. According to the invention, a porosity is established in the melted inner re- gion.
In this respect, the invention differs significantly from that according to EP 2 709 792 BI, since the meshed structure is formed solely of solid melted material with non-melted powder in the interspaces.
In the invention, the production of porosity in the inner region "allows" a high process speed and influencing of the heat-conducting properties, in particular in certain regions of the segment.
Unlike in a meshed structure as is known from EP 2 709 792 BI, in the case of a porosity that is produced deliberately, the geo- — metric characteristics and form are not defined exactly, even though the po- sition can be controlled by the production process.
Therefore, according to the invention, the heat-conducting properties can purposively be varied and set locally within the segment.
Any incompletely melted powder material which is present in the interior of the component as a result of the intended porosity remains in small self-contained cavities that form at random and thus differs from the defined permeability of a mesh or network.
Moreover, it has been shown that a large part of the metal powder that is not directly melted becomes sintered to the "walls" of the pores and thus enters into a materially bonded bond with the surrounding material.
According to a preferred embodiment of the invention, the inner region is built up at least partially with a lower volumetric energy input than the outer layer.
This measure allows the porosity to be set particularly well in the de- sired range of 0.01 to 0.5 in the inner region.
In this method variant, the pro- duction of points of higher porosity can be achieved with an increase in the process speed, the lower volumetric energy input which is to be applied and which brings about porosity is achieved in a simple manner, for example by increasing the manipulation speed of the laser beam of the device.
Internal stresses in the interior of the component are also reduced by the locally low- er energy inputs.
5 In an alternative embodiment, the inner region is built up at least partially with a higher volumetric energy input than the outer layer.
If there is chosen, for example, a print strategy in which the inner region is not melted in every layer but, for example, only after every fourth coating operation, a larger amount of powder must be melted and remelted.
The large amount of ener-
gy, which is physically capped, to be inputted for that purpose produces a large melt bath, the solidification of which results in inclusions of gases.
Added to this is a molten bath dynamics which promotes the formation of inclusions on solidification of the melt.
The power, for example of the laser, and thus the theoretical energy input can be chosen to be greater than the maximum energy to be absorbed by the powder surface.
Thus, a large energy input is in principle also suitable for producing porosity in the desired range.
In a further particularly preferred embodiment which is simple to produce, the inner region is built up with pores distributed with stochastic homogenei-
— ty.
In order largely to avoid loose metal powder within the inner region, it is preferred and advantageous to build up the inner region with pores, the larg- est diameter of which is < 1.0 mm.
In a further advantageous embodiment variant of the method, the inner re- gion is built up with subregions which differ from one another in terms of their porosity.
Regions of different porosity advantageously also have dif- ferent heat-conducting properties.
The heat-conducting properties of the inner region can be influenced particu- larly well by the degree of porosity.
The inner region, or at least a subregion of the inner region, is preferably built up with a porosity < 0.5, in particular
< 0.3, and preferably with a porosity > 0.03, preferably > 0.07 and particu- larly preferably > 0.1. As far as the thickness of the outer layer is concerned, it is preferred if the outer layer is built up in a layer thickness of 0.1 mm to 3.0 mm at least on the side and end faces of the mould segment. The outer layer also covers the elements of the profile negative, so that they are built up at least to some extent from solid material. In a preferred vari- ant, it can be provided that the elements of the profile negative are built up at least to some extent in such a way that they each comprise a subregion of the inner region and have an outer layer with a layer thickness of 0.1 mm to
3.0 mm. The outer layer is preferably further built up with a layer thickness of 0.1 mm to 3.0 mm on the inner surface having the elements of the profile nega- tive, and in the regions between the elements of the profile negative. It is advantageous if the first mould segment part is prefabricated in such a way that it has the form of a rectangular plate or the form of a near-net- shape segment spine or the final contour of a segment spine, wherein the further mould segment part is applied to the surface situated opposite the segment spine, wherein, when it is in the form of a rectangular plate or when it has the near-net shape of the segment spine, the first mould segment part is reworked in order to obtain the final contour of the segment spine. As a result, a first mould segment part having the final contour of the segment spine can be produced temporally and spatially independently and separably from the generative build-up of the further mould segment part. In a specific embodiment, the surface, situated opposite the segment spine, of the first mould segment part has a planar design, and the further mould segment part is applied to the planar surface. A planar surface is necessary according to the current state of the art for the generative manufacturing process of the powder-bed-based laser melting process. This process is known, for example, by the name "selective laser melting". It is advantageous if the volume of the mould segment part produced by means of a generative process is approximately equal to or greater than the volume of the first mould segment part in the final contour. The mould segment is preferably reworked by machining and/or is subjected to a heat and surface treatment process. Material and surface properties can thereby be adjusted and minimal mould tolerances maintained. There is suitable as the generative process in particular at least one of the processes from the following group: laser melting process, sintering process, build-up welding process, and other melting processes. Further features, advantages and details of the invention will be described in detail with reference to the schematic drawing, which shows an exemplary embodiment, in which:
Fig. 1 is a sectional view of part of a tyre vulcanization mould in the closed state,
Fig. 2is a view of a mould segment,
Fig. 3 shows a segment ring composed of mould segments, and
Fig. 4 is a radial section (cross section) of a mould segment which has been — produced.
Fig. 1 shows, schematically, the fundamental components of a conventional vulcanization mould for a pneumatic vehicle tyre for passenger cars.
The vulcanization mould is situated within a heat press, the components of which are not shown and which conventionally has a press upper part and a press lower part and the corresponding mechanisms for positioning the tyre to be vulcanized, for operating the components of the vulcanization mould, for introducing the heating media and for removing the finished vulcanized tyre.
The vulcanization mould shown schematically in Fig. 1 is a multipart con-
— tainer mould having a lower heating plate 1, a lower sidewall shell 2, an up- per heating plate 3, an upper sidewall shell 4, a lower bead ring 5 and an upper bead ring 15. The components of the vulcanization mould that are moved in the axial direction (double arrow P1) for opening and closing in- clude the upper heating plate 3 with the upper sidewall shell 4 arranged
— thereon.
The vulcanization mould further has a segment ring 6, which is conventionally composed of 7 to 14 segment shoes 7 which extend annularly and contain the shaping mould segments 8 in the same number.
In a special mould construction, the number of mould segments can also be up to 120.
The segment shoes 7 are moved apart in the radial direction (double arrow
P2in Fig. 1) on opening and closing of the vulcanization mould, enclose the tyre to be vulcanized on closing and, after vulcanization, release the finished vulcanized tyre.
On the inner side of the segment shoe 7 shown in Fig. 1 there can be seen one of the mould segments 8, which shapes the profiled tread of the tyre.
On the upper heating plate 3 there is arranged a closing ring 9 which has a bevelled inner surface which cooperates with bevelled outer surfaces of the segment shoes 7 of the segment ring 6 in such a way that, on closing of the vulcanization mould, the segment shoes 7 are moved together in the radial direction to form the closed segment ring 6. The lower heating plate 1, the upper heating plate 3 and the closing ring 9 contain heat-
ing chambers 10, 11, 12a, 12b and 12c, into which at least one heating me- dium, for example saturated steam (water vapour), is introduced for vulcani- zation of the tyre.
In this way, the green tyre (not shown) is heated from out-
side by way of the segment shoes 7, the sidewall shells 2, 4 and the bead rings 5, 15. This heating is usually referred to as external heating. So-called internal heating is further provided, which has a conventional bladder (not shown), which is arranged in known manner and is filled at least with a heat- ing medium under pressure in order to centre the green tyre in the vulcaniza- tion mould from the inside, wherein the bladder is brought into a torus shape matching the tyre.
Fig. 2 shows a mould segment 8 in a schematic oblique view, Fig. 3 shows, likewise schematically, an oblique view of a segment ring 6 composed of a plurality of mould segments 8. Each mould segment 8 has an outer side which is remote from the cavity 20 (Fig. 1) of the vulcanization mould and which is usually and hereinbelow referred to as the segment spine 17, with which the mould segment 8 is arranged in a segment shoe 7 of the vulcaniza- tion mould. Each mould segment 8 further has a mould side 14 which faces the cavity 20 of the vulcanization mould and which has the profile negative of the tread in the form of ribs, lamellae and the like, wherein these elements are not shown. Ribs in known manner form grooves in the tread, lamellae in known manner form sipes.
Fig. 4 shows a cross section through a mould segment 8. The mould segment 8 consists of two regions, a mould segment part 16 having the segment spine 17, and a mould segment part 18 having the mould side 14. The mould seg- ment part 16 is a prefabricated component, in particular consisting of steel, which already has the final contour of the segment spine 17. The surface 16a, situated opposite the segment spine 17, of the mould segment part 16 preferably has a planar design. The mould segment part 18 is built up on this surface 16a of the mould segment part 16 in a materially bonded manner by means of a generative manufacturing process using and with the melting of metal powder, preferably by means of a laser. In this process, which is known as the SLM (selective laser melting) process, metal powder is applied layer by layer and melted layer by layer, whereby the mould segment part 18 is fixedly bonded to the mould segment part 16 by means of melted and then solidified metal powder and is built up in an additive manner. The mould side 14 with the elements located thereon, such as ribs and lamellae, any wear indicators and the symbols provided, is preferably also built up. The required venting channels or vent holes are preferably subsequently drilled through the finished mould segment 8 in the conventional manner and in an automated manner. The mould segment part 18 is built up in such a way that it has an outer lay- er 19 of solid material, that is to say of completely melted metal powder, and a porous inner region 20. A completely melted, solid material is understood as being a material which has a porosity (ratio of cavity volume to total vol- ume) < 0.01, wherein this "type" of porosity consists of unintentional de- fects, that is to say gas inclusions in the material structure. The inner region 20, on the other hand, has a porosity of 0.01 to 0.5, wherein the inner region can have subregions or partial volumes which differ from one another in terms of their porosity. The porosity of the inner region 20 or of at least one of the subregions provided is in particular > 0.03, preferably > 0.07, particu- larly preferably > 0.1. The pore size of the pores, their largest diameter, is 20 — not more than 1.0 mm. The inner region, also in subregions with different porosity which are optionally provided, has pores arranged with stochastic homogeneity, that is to say pores which are in a uniform random allocation. The outer layer 19 is formed with a constant or a varying layer thickness of
0.1 to 3.0 mm at least on the side and end faces of the mould segment part
18. The mould elements forming the profile negative, such as ribs and lamel- lae, are built up in a possible embodiment from completely melted metal powder, along the inner surface 14a of the mould side 14 — in the regions between the ribs and lamellae (not shown) protruding on the mould side 14 — the layer thickness is preferably likewise 0.1 mm to 3.0 mm. In an alterna- tive embodiment, at least some, preferably all, the elements forming the pro- file negative are formed with an outer layer and an inner region, wherein thin, for example 0.5 mm thick, lamellae can also be composed of solid ma- terial. List of reference signs
1.................lower heating plate
2................lower sidewall shell
3.................upper heating plate
4.................upper sidewall shell
5.................lower bead ring
6.................segment ring
7.................segment shoe
8................ mould segment
9.................closing ring 10,11........... heating chamber 12a, 12b, 12¢... heating chamber
14................mould side
14a...............inner surface
15................upper bead ring
16................mould segment part
16a...............surface
17................segment spine
18................ mould segment part
19................outer layer
20................inner region
Pi................double arrow in the axial direction
P2................double arrow in the radial direction