Powder for layerwise manufacturing of objects

Abstract

The invention describes powders for use in the production of spatial structures, i.e. molded bodies, using layer build-up methods, as well as methods for their efficient production. The powders have the special feature that they have good flow behavior, for one thing, and at the same time, have such a composition that the molded body that can be produced with the powder, using rapid prototyping, has significantly improved mechanical and/or thermal properties. According to a particularly advantageous embodiment, the powder has a first component that is present in the form of essentially spherical powder particles, which is formed by a matrix material, and at least one further component in the form of stiffening and/or reinforcing fibers, which are preferably embedded in the matrix material.

Claims

1. A method for the production of a powder comprising essentially spherical particles of an aromatic polyether ketone plastic, comprising: cooling a coarse granulate comprising a plastic matrix material to form brittle, coarse granulates; grinding the brittle, coarse granulates; and separating the ground granulate into a fraction spectrum, wherein the grinding is carried out with a pinned disk mill.

2. A method for the production of a powder comprising essentially spherical particles of an aromatic polyether ketone plastic, comprising: cooling a coarse granulate comprising a plastic matrix material to form brittle, coarse granulates; grinding the brittle, coarse granulates; separating the ground granulate into a fraction spectrum; and smoothing the ground granulate, wherein the smoothing is carried out by embedding or accumulating at least one of microparticles or nanoparticles.

3. A method for producing a powder comprising a first component in the form of essentially spherical powder particles and at least one of a stiffening fiber or a reinforcing fiber, wherein the first component comprises a matrix material, comprising: cooling a coarse granulate plastic comprising the matrix material to form brittle, coarse granulates; grinding the brittle, coarse granulates; and separating the ground granulate into a fraction spectrum.

4. The method according to claim 3, wherein the coarse granulate is a fiber-reinforced plastic matrix material.

5. The method according to claim 3, wherein the grinding is carried out with a pinned disk mill.

6. The method according to claim 3, wherein the grinding is carried out with cooling.

7. The method according to claim 3, wherein the separating is carried out with an air separator.

8. The method according to claim 3, further comprising: smoothing the ground granulate.

9. The method according to claim 8, wherein the smoothing is carried out by embedding or accumulating at least one of microparticles or nanoparticles.

Description

(1) In the following, the invention will be explained in greater detail, using exemplary embodiments:

(2) The drawing shows:

(3) FIG. 1 a fundamental diagram to show the functional principle of the layer build-up method;

(4) FIG. 2 the detail II in FIG. 1;

(5) FIG. 3 a schematic representation of a method for the production of the powder according to a first embodiment;

(6) FIG. 4 a schematic view of a powder according to a further embodiment of the invention;

(7) FIG. 5 a schematic view of a powder according to a further variant of the invention;

(8) FIG. 6 a schematic representation of a method for the production of the powder according to FIG. 5, corresponding to one embodiment;

(9) FIG. 7 a schematic representation of another method for the production of the powder according to FIG. 5;

(10) FIG. 8 a schematic view of a cut-out of a component that can be produced using the powder according to the invention; and

(11) FIG. 8A the detail VIII in FIG. 8.

(12) FIG. 1 schematically shows how a component is produced by means of layer build-up methods. It can be seen that successive powder layers 12-1, 12-2, . . . 12-n having a thickness S are being applied to a platform 10 that can be lowered into a construction space, step by step. After a layer has been applied, the particles 18 (see FIG. 2) are selectively melted in targeted areas, completely or in part, by an energy beam from an energy source 16, whereby the regions 14 indicated with cross-hatching in the figure are formed, which thereby become an integral part of the component being produced. The platform is subsequently lowered by the layer thickness S, whereupon a new powder layer having the layer thickness S is applied. The energy beam passes over a predetermined area once again, whereby the corresponding regions are melted and melded, i.e. joined with the regions in the previous layer that were melted. In this manner, a multi-layer powder block having an embedded molded body of a complex structure is gradually formed. The molded body is removed from the powder block and generally the residual powder that adheres to it or is sintered to it is cleaned away manually.

(13) The layer thickness is selected to be between 20 and 300 urn, depending on the area of application, whereby the majority of the powder particles 18 have a grain size D of approximately ⅓ of the layer thickness S, as can be seen in FIG. 2.

(14) Conventionally, the powder is formed by a thermoplastic, for example PA 11 or PA 12, whereby the mechanical strength of the molded bodies remains limited, due to the low modulus of elasticity in the range of 1.4 GPa, and the low tensile strength in the range of 40 to 50 MPa.

(15) The invention gives different approaches for the production of molded bodies having significantly improved mechanical properties, which will be explained in greater detail in the following:

EMBODIMENT 1

(16) The powder has a first matrix component that is present in the form of essentially spherical powder particles (18), which is formed by an aromatic polyether ketone plastic, particularly a polyaryl ether ketone (PEEK) plastic, having the repetition unit oxy-1,4-phenylene-oxy-1,4-phenylene-carbonyl-1,4-phenylene of the general formula

(17) ##STR00002##

(18) Such a material can be purchased, for example, under the trade name “PEEK,” from the company Victrex Plc. The material properties lie at a tensile strength of more than 90 MPa and a modulus of elasticity in the range of more than 3.5 GPa (according to ISO 527). In addition, this material is characterized by an extremely good temperature stability, so that the molded parts built from it can be used even in areas that are subject to great thermal stress.

(19) The production of powder particles from this material preferably takes place according to one of the following methods:

(20) 1. Spray-drying,

(21) 2. Grinding; and

(22) 3. Melt-spraying.

(23) Spray-Drying:

(24) For this purpose, as can be seen in FIG. 3, a suspension is first produced, having a matrix micropowder 22 stirred into a liquid phase, such as ethanol or an ethanol/water mixture 20. The particles of the matrix micropowder 22 have dimensions that lie significantly below the particle size DP of the powder particle 30 to be produced. In this connection, uniform mixing of the phases in the vessel must be assured.

(25) The suspension is sprayed through a nozzle, not shown in detail, whereby droplets 32 containing matrix micropowder are formed. The liquid phase 26, specifically the surface tension of this phase, guarantees an essentially spherical shape of the droplets.

(26) Subsequently, for example in a subsequent heating segment, the liquid component 26 of the droplets 32 is vaporized and/or evaporated, leaving essentially spherical agglomerates 30 behind. These agglomerates 30 form the powder particles to be used in the subsequent layer build-up method. Accordingly, the process parameters of the method are selected in such a manner that the particles are produced in the desired grain size distribution.

(27) Grinding:

(28) An alternative method consists in that the material, which can be purchased, for example, as a coarse granulate having a grain size of approximately 3 mm, is ground to produce a suitable micropowder.

(29) In this process, the coarse granulate is first cooled to a temperature that lies below the temperature at which the material becomes brittle. Cooling takes place, for example, by means of liquid nitrogen. In this state, the coarse granulate can be ground in a pinned disk mill, for example. The ground powder is finally separated, preferably in an air separator, to obtain a predetermined fraction spectrum.

(30) The method step of grinding can take place with additional cooling.

(31) In order for the ground powder to get a sufficiently smooth and preferably spherical surface, it is advantageous to subject the ground material to smoothing treatment, for example by embedding or accumulation of microparticles and/or nanoparticles, such as Aerosil.

(32) Melt-Spraying:

(33) A third method variant of the production of micropowder made of aromatic polyether ketone, particularly a polyaryl ether ketone, consists in that a melt-spraying method is used.

(34) In this process, the material is melted in a crucible that has a connection to a spray nozzle with which the material is atomized.

(35) In this process, small droplets leave the nozzle. Because of the surface tension of the material, these droplets assume an essentially spherical shape. If the droplets are subsequently moved through a cooling segment, they solidify in this spherical shape, so that powder is present in the grade desired for the layer build-up method.

(36) Preferably, hot gas is used for the atomization. The hot gas that is used for spraying, i.e. for atomization of the melted material, is produced by means of a so-called pebble heater.

(37) As a rule, a separating step follows the method step of spraying, in order to obtain powder particles in accordance with a predetermined fraction spectrum.

EMBODIMENT 2

(38) As shown schematically in FIG. 4, powder having a first component present in the form of essentially spherical powder particles 118, which is formed by a matrix material, and at least one further component in the form of stiffening and/or reinforcing fibers 140. The matrix component can be formed by a metal or by a thermoplastic plastic was used.

(39) The following experimental example was carried out:

(40) PA12 powder having a grain size distribution with d50 at about 50 μm was mixed with 10 vol.-% carbon fibers of two different types, having an average fiber length L50 of about 70 μm and a fiber thickness of 7 μm. It was possible to process the powder obtained in this way on commercially available rapid prototyping machines, to produce defect-free molded bodies.

(41) It was possible to significantly improve the mechanical properties of the sample body produced on the basis of this powder/fiber mixture, according to the layer build-up method, as compared with a component that did not contain any fibers. Specifically, it was possible to increase the modulus of elasticity to more than 3.8 GPa and the tensile strength to approximately 70 MPa.

(42) These experimental results were compared with results obtained with components that were obtained by means of injection-molding of PA12 mixed with fibers, whereby the fibers that were added to the injection-molding mass were present in the same volume concentration and the same size distribution. The measured results show that the mechanical properties of the components obtained according to the layer build-up method are not inferior, in any regard, to those of the injection-molded parts. In fact, it was actually possible to increase the modulus of elasticity in the case of the sintered body.

(43) Although the proportion of fibers in the micropowder can be varied, depending on the average grain size and its distribution, it generally cannot be raised above 25% without causing problems. In order to nevertheless be able to achieve improved material properties, the third embodiment of the invention is offered.

EMBODIMENT 3

(44) According to the third embodiment, which is illustrated schematically in FIG. 3, a powder is created that contains significantly higher proportions of fiber, namely above 30 vol.-%, but nevertheless has such a composition that it can be used in a layer build-up method, because of its good flow capacity.

(45) The particular feature is that the fibers 240 are embedded in essentially spherical powder molded bodies 218, which form the matrix material of the component to be produced, preferably in such a manner that they are essentially completely surrounded by the matrix material, as shown in FIG. 5.

(46) For the production of such a powder, the methods described above, i.e. spray-drying, grinding, and melt-spraying, can be used with slight modifications:

(47) Spray-Drying:

(48) This method is shown schematically in FIG. 6. It differs from the method described above, on the basis of FIG. 3, only in that not only matrix micropowder 322 but also stiffening or reinforcing fibers 340 are stirred into the liquid phase, such as an ethanol or an ethanol/water mixture 320. The particles of the matrix micropowder 322 have dimensions that lie significantly below the particle size DP of the powder particle 330 to be produced. The fiber lengths are also selected in such a manner that their average length is not greater than the average grain size of the powder particles to be achieved. In this connection, again, uniform mixing of the phases in the vessel must be assured.

(49) When spraying the suspension through a nozzle, not shown in detail, droplets 332 that contain matrix micropowder and fiber(s) form. The liquid phase 326, specifically the surface tension of this phase, guarantees an essentially spherical shape of the droplets.

(50) If, subsequently, the volatile component 326 of the droplets 332 is vaporized and/or evaporated, again, essentially spherical agglomerates 330 remain behind. These agglomerates 330 form the powder particles to be used in the subsequent layer build-up method. Accordingly, the process parameters are selected in such a manner that the particles are produced in the desired grain size distribution.

(51) Good results can be achieved with the spray-drying if micropowders having an average grain size d50 between 3 and 10 μm, preferably 6 μm, are used.

(52) If fibers are stirred in, they should preferably be used at an average length L50 from 20 to 150 μm, preferably 40 to 70 μm.

(53) In the case of a metallic matrix material, the lengths of the fibers should generally be selected to be shorter. An advantageous range for the average fiber length L50 lies between 10 and 100 μm, preferably between 10 and 80 μm.

(54) It is advantageous to adjust the process parameters in such a manner that essentially spherical microdroplets having an average diameter D50 of 10 to 70 μm are formed.

(55) The vaporization and/or evaporation step is advantageously carried out while the droplets are being moved through a heating segment.

(56) Grinding:

(57) An alternative method, which is shown schematically in FIG. 7, consists in that a material containing fibers, for example carbon fibers 440, which material is present, for example, as a coarse granulate 450 having a grain size or edge length of about 3 mm, is ground to produce a suitable micropowder.

(58) In this process, the coarse granulate 450 is again cooled to a temperature that lies below the temperature at which the material becomes brittle. Cooling takes place, for example, by means of liquid nitrogen. In this state, the coarse granulate can be ground, for example, in a pinned disk mill, indicated as 460. The ground powder is finally separated in a separator 480, preferably in an air separator, in accordance with a predetermined fraction spectrum that is to be achieved. The powder particles to be used are indicated as 430.

(59) In this connection, the method step of grinding can again take place with additional cooling. Also, an optional smoothing process, by means of embedding or accumulation of microparticles and/or nanoparticles, such as Aerosil, can follow.

(60) Melt-Spraying:

(61) The third method embodiment described above, namely so-called melt-spraying, can also be used for the production of powder according to FIG. 5.

(62) In contrast to the method described above, the fiber component is stirred into the melted melt of the matrix material.

(63) The embodiments described above allow the processing of both thermoplastic plastic materials and metallic materials.

(64) Different materials can also be mixed.

(65) If the matrix material is formed by a thermoplastic plastic material, the fibers are selected from the group of carbon and/or glass fibers.

(66) The average grain size of the spherical powder particles is fundamentally not supposed to be restricted. Good results with commercially available machines can certainly be achieved if the average grain size d50 of the spherical powder particles lies in the range of 20 to 150, preferably 40 to 70 μm. The flow capacity of such a powder can be further increased by homogenization of the size distribution.

(67) If the matrix material is formed by a metallic material, the fibers are preferably selected from the group of ceramic fibers and boron fibers. In the case of such a powder, the average grain size d50 of the spherical powder particles generally lies at a low value, for example in the range of 10 and 100, preferably 10 to 80 μm.

(68) From the description, it becomes evident that using the powder according to the invention, by using layer build-up methods (powder-based generative rapid prototyping method), such as according to SLS (selective laser sintering) or laser melting technology, it is possible to produce spatial structures, i.e. molded bodies, whose mechanical and/or thermal properties were previously unthinkable.

(69) Thus, the modulus of elasticity of PEEK, if it is reinforced with 10, 20, or 30 vol.-% carbon fibers, which are introduced into the powder particles or mixed with them, according to one of the methods described, can be increased to 7, 13.5, and 22.2 GPa, respectively, while it was possible to raise the tensile strength to 136, 177, and 226 MPa, respectively.

(70) If PA12 is used as the matrix material, an improvement of the mechanical properties occurs as follows, with a fiber proportion of 10, 20, and 30 vol.-%: modulus of elasticity 3.4, 6.6, and 13.9 GPa, respectively; tensile strength 66, 105, and 128 MPa, respectively.

(71) In this way, it is possible, for the first time, as indicated schematically in FIGS. 8, 8A, to use the layer build-up method for the production of hollow molded bodies 570, having a complex shape, for example multiple curvatures, with interior reinforcements, preferably three-dimensional framework-like reinforcements 572, in practical manner, making it possible to produce components that are not only extremely light, but also can withstand great thermal and mechanical stress.

(72) Of course, deviations from the embodiments described above are possible, without leaving the basic idea of the invention. Thus, subsequent treatment steps of the individual powder production methods can also be used for different methods. The smoothing process to be carried out by means of microbodies can, of course, also be used for the two methods described as alternatives.

(73) The invention therefore creates new powders for use in the production of spatial structures, i.e. molded bodies, using layer build-up methods, as well as methods for their efficient production. The powders have the special feature that they have good flow behavior, for one thing and, at the same time, have such a composition that the molded body that can be produced with the powder, using rapid prototyping, has significantly improved mechanical and/or thermal properties. According to a particularly advantageous embodiment, the powder has a first component that is present in the form of essentially spherical powder particles, which is formed by a matrix material, and at least one further component in the form of stiffening and/or reinforcing fibers, which are preferably embedded in the matrix material.