Doctor blade unit for powder-based additive manufacturing systems

Abstract

The present invention relates to a doctor blade unit for an additive manufacturing system using powder as a starting material. The doctor blade unit according to the present invention achieves improved straightness of the doctor blade and improved parallelism to the construction level. The doctor blade 1 is suspended by an adjustable fastening member on a cross member 3, which is fixed to a guide bracket 6 on one side by a fixed bearing 4 and on the other side by a floating bearing 5.

Description

[Technical field]

[0001] The present invention relates to a doctor blade unit for an additive manufacturing system using powder as a starting material.Improved straightness of the doctor blade and parallelism with respect to the construction level are achieved with the method of the doctor blade unit according to the invention. [Background technology]

[0002] Apparatuses and methods for the additive manufacturing of workpieces, also known under the term Additive Manufacturing (AM), are known from the prior art. Also spoken of are generative manufacturing methods or 3D printing. Here, the raw material may be in powder, wire or liquid form. Powder methods include, for example, Selective Laser Melting (SLM), Selective Laser Sintering (SLS) or Electron Beam Melting (EBM). Here, the raw material consists of plastic or metal.

[0003] In methods using powdered raw materials, the raw material is applied in the form of a layer onto a lowerable work surface where it is melted or sintered in sections. The work surface is then lowered the thickness of the layer. Another layer is then applied to that layer, following a similar procedure to the first layer. Here, the sections to be melted or sintered are selected so that a three-dimensional workpiece is built up layer by layer.

[0004] For the quality of the components, it is essential that the individual layers have a homogeneous thickness. In particular, the layers must be of the same thickness over the entire plane and must also be parallel to the construction level, firstly on the lowerable working surface and then on each pre-treatment layer. To achieve this, a precise application of the powder is necessary, which requires, first of all, a highly accurate doctor blade and doctor blade guide. Here, in the case of prior art doctor blades, two problems arise, which become more severe the larger the doctor blade and the higher the temperature of the construction space. The first of these is the deflection of the doctor blade, which is caused by a deviation from the straightness of the doctor blade, which leads, as a rule, to a thinning of the powder layer in the middle of the path of movement, and the second problem is thermal expansion.

[0005] Initially, additive manufacturing was a rapid prototyping method that provided only design models, and the scale of the models was often small. However, advances in technology have enabled the manufacture of larger functional parts. In particular, in addition to powder materials, which were initially limited to polymeric materials, materials made of metals, glass, and ceramics can now be used. As a result, the construction space has become larger, and the size of the doctor blade has also become larger, resulting in the problems mentioned above.

[0006] Moreover, processing metals and ceramics requires significantly higher melting and sintering temperatures than plastics. Therefore, in addition to more powerful lasers, electron beam guns are used for these methods as irradiation units that can apply more energy to the powder. The temperatures generated also heat the doctor blade in order to distribute the newly applied powder to the last layer that has just solidified, forming a new layer. The thermal expansion of the doctor blade that occurs in the process causes stresses and deformations in the doctor blade between the fastening points.

[0007] In the case of polymer powders, the thermal problem is not inherently significant, since temperatures of up to about 180° C. are usually generated in the preheated powder bed, whereas in the case of metals such as titanium and titanium alloys, temperatures of up to more than 1200° C. are required in the powder bed. The greatest problems therefore arise in the case of electron beam processes with metal powders, which is the main application field of the present invention. There, for example, temperatures of about 400° C. are typically generated in the center of the doctor blade, and about 50° C. in its edge regions, which are not on the powder bed. As a result, considerable stresses are generated at different depths in the suspension system of the blade and in the blade itself due to thermal expansion, which leads to deviations from linearity.

[0008] EP 2010370 B1, for example, describes a powder coating device which is intended to allow easy replacement and in the process maintain parallelism to the construction level. Here, an exchangeable coating module is used which can be inserted into a frame-like receiver. The two blades are screwed to the edges via webs in the coating module. The blades are set to the construction level via a frame-like receiver, on which the coating module is hooked via webs. For larger dimensions, this results in the problem that the blades, which are only attached to the outer side, are deflected. In the event of thermal distortion, the double suspension has a particularly negative effect on the straightness and parallelism of the blades to the construction level, firstly of the blades of the coating module in the frame-like receiver and then of the frame-like receiver in the entire device. Summary of the Invention

[0009] In view of these problems, it is an object of the present invention to provide a doctor blade unit by means of which the drawbacks of the devices from the prior art are overcome. In particular, it is an object of the present invention to provide a device which is able to apply a powder layer which is particularly homogeneous and which is arranged parallel to the construction level, even in the case of large dimensions and high operating temperatures.

[0010] This object is achieved by a method doctor blade unit according to claim 1. Advantageous design variations are the subject matter of the dependent claims.

[0011] A doctor blade unit according to the present invention for an additive manufacturing system using a powder as a starting material includes a doctor blade and a suspension system that is mounted so as to be horizontally movable in the additive manufacturing system.

[0012] When referring to a doctor blade in this application, this term includes both the actual blade and the composite unit formed with them, for example, thus in the case of a double blade, including the two blades and the means connecting them.

[0013] The suspension system includes a cross member attached to a locating bearing on one side and a floating bearing on the other side, the cross member being connected to a guide bracket via the locating bearing and the floating bearing, respectively.

[0014] The doctor blade is fastened in a suspended manner below the cross member via a number of fastening means along its length, which fastening means allow for an adjustable spacing between the doctor blade and the cross member in order to position the doctor blade parallel with respect to the construction level of the additive manufacturing system. For this purpose, depending on the length of the doctor blade, a number of fastening means are provided which keep the spacing between them short enough to prevent deflection and allow precise adjustment of the straightness of the suspended doctor blade. And through the adjustable fastening means, parallelism with respect to the construction level can be established.

[0015] An important aspect is the planarity of the applied layer. Here, the powder grain size of the metal powder used sets a limit to the maximum achievable position. With the doctor blade system according to the invention, a planarity of the powder layer is aimed for which corresponds to a deviation of less than half the average powder grain size (diameter) d50. Typical metal powders for EBM have an average grain size (diameter) d50 of about 50 μm. In special cases, powders with smaller grain sizes are also used. The accuracy aimed for in normal system operation is therefore a deviation of maximum 25 μm, preferably maximum 10 μm.

[0016] The doctor blade may consist of a single blade, double blade or multiple blades. The doctor blade is placed under a solid block arranged so as to be symmetrically centered in the middle, through which the fastening means are connected to the cross member. The connection to the solid block is made in cutouts arranged in the area of ​​the vertical fastening means by horizontal bolts and spacer elements inserted between the individual blades. The blades are made of flexible thin metal sheets, machined with comb-like teeth to flexibly adapt to irregularities. The pre-wiper is screwed parallel to the blade on the side of the solid block. Its lower end is usually spaced 1 mm, generally about 0.5 mm, from the lower end of the blade. The pre-wiper serves the purpose of minimizing the pressure of the powder on the blade and of initially distributing and smoothing the powder. The whole block is covered with a roof-like structure, which can be flat, curved or angular in shape.

[0017] The guide bracket is preferably suitable for torque-rigid guiding in the additive manufacturing system, meaning that the guide bracket is connected to the additive manufacturing system in such a way that a torque can be transmitted from the guide bracket to the additive manufacturing system, so that only translational movements in a given direction above the construction level are possible, other spatial directions and rotations are not possible.

[0018] The horizontally movable mounting of the suspension system in the additive manufacturing system can take place, for example, via rails or guide rods. They can be present singularly or multiplely, in particular double, on each side. They do not have to be arranged horizontally and do not have to function as supports, but rather can preferably be oriented vertically. The latter is particularly advantageous in the case of embodiments in which the rails and rail receivers of the guide bracket are in the form of a dovetail connection and a double rail, in order to form a torque-rigid connection of the guide bracket to the building space of the additive manufacturing system.

[0019] The cross member of the doctor blade unit can be configured as a hollow body, which contributes to a low weight and at the same time ensures high rigidity. Here, the hollow body can be open, i.e. cylindrical, at the end side or closed. The cross member preferably has a rectangular or elliptical cross section, which increases the resistance to torque during installation.

[0020] The locating bearing preferably surrounds the cross member on at least two sides and prevents movement of the cross member relative to the locating bearing in all three spatial directions.

[0021] A centering pin can be mounted on the inner circumference of the locating bearing, which engages with a positive lock with a corresponding hole in the cross member. In this way, a reliable positioning of the cross member on the guide bracket is achieved. Furthermore, the centering pin helps the fastening of the cross member and prevents any movement of the cross member in a direction perpendicular to the direction of movement of the cross member above the construction level, especially in a plane parallel to the construction level.

[0022] The floating bearing may surround the cross member at its periphery with a sliding element in such a way that the cross member is allowed to move relative to the floating bearing along its longitudinal axis while being prevented from moving in two spatial directions perpendicular thereto, through which a compensating movement takes place when the doctor blade and the cross member expand during a temperature rise, so that no stresses are generated in the doctor blade which would lead to deformation.

[0023] The doctor blade preferably has a length of 0.1 to 5 m, more preferably 0.2 to 4.5 m, or 0.25 to 4 m, or 0.3 to 3 m, or 0.3 to 2.5 m. The advantages of the doctor blade unit according to the present invention are greater with a longer doctor blade.

[0024] The parallelism deviation ΔP is defined as the difference between the maximum and minimum spacing between the doctor blade and the construction level and is preferably 1-50 μm, more preferably 2-45 μm or 3-40 μm or 4-35 μm or 5-30 μm or 10-25 μm. According to the definition, the parallelism deviation ΔP detects both the inclination of the doctor blade and the deviation from straightness. It is a measure of the quality of the adjustment of the doctor blade. The doctor blade unit according to the invention therefore realizes an excellent uniformity of the layer thickness and, as a result, an improvement of the component properties can be achieved. Thus, for example, smaller layer thicknesses can still be produced reliably, which results in an improved surface quality and resolution.

[0025] The suspension system preferably has 3 to 30, preferably 3 to 25 or 3 to 20 or 3 to 15 or 3 to 10 or 3 to 5 fastening means per meter of doctor blade length. This number of fastening means has proven to be sufficient, since even in the case of very long doctor blades and high operating temperatures it is possible to reliably set the straightness of the doctor blade.

[0026] The fastening means is preferably a threaded bolt, which must have a sufficiently long thread so that it is possible to provide the required adjustment travel, and which can be adjusted simply by means of a nut. This is done during the installation of the doctor blade. If necessary, the individual bolts can be readjusted to re-establish straightness and parallelism.

[0027] In the case of the doctor blade unit, in one preferred design variant, during operation of the additive manufacturing system, the change of the distance of the doctor blade from the cross member is possible by electromechanical and / or hydraulic actuators acting on the fastening means. In this automated variant of the doctor blade unit, deviations from straightness and parallelism are measured and monitored by optical and / or mechanical sensors. This information is used to activate actuators, e.g. actuating motors or hydraulic lines with actuators, which accordingly correct the length of the fastening means in order to re-establish straightness and parallelism. With the help of this active system, a further increase in the uniformity of the layer thickness is again achieved, which results in a further improvement of the component properties. In particular, this has the advantage that uninterrupted operation is possible, since the additive manufacturing system does not have to be stopped for readjustments. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a side cross-sectional view of a doctor blade unit. [Diagram 2] FIG. 2 is a perspective view of the doctor blade unit as viewed from the locating bearing side. [Diagram 3] FIG. 3 is a perspective view of the doctor blade unit as viewed from the floating bearing side. [Figure 4] FIG. 4 is a perspective cross-sectional view of the doctor blade unit with a section through the fastening means. [Diagram 5] FIG. 5 is an enlarged detail of FIG. 4 showing the fastening means and the lower area of ​​the doctor blade. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The drawings depict only preferred embodiments of the invention as an example, and are not to be construed as limiting.

[0030] FIG. 1 is a cross-sectional side view of a doctor blade unit according to the invention, which is again shown in perspective in FIG. 2 and FIG. 3. The design variant of the doctor blade unit shown comprises a doctor blade (1) with a double blade. The suspension system (2) comprises a cross member (3), which is mounted on the left side in a locating bearing (4) and on the right side in a floating bearing (5), which in turn are connected to a guide bracket (6). In this example, the doctor blade (1) is fixed to the cross member (3) by seven fastening means (7) in the form of bolts with an end thread and associated nuts. The length of the doctor blade (1) is 2 m. The doctor blade suspension system therefore has 3.5 fastening means per meter of doctor blade length. The parallelism deviation ΔP thus achieved was 10 μm.

[0031] The cross member (3) supports its own weight and the mechanical load of the doctor blade (1). Its deflections can be compensated for via fastening means (7), so that the doctor blade (1) suspended below it can be arranged linearly and parallel to the construction level. As can be seen in figures 2 and 3, the cross member (3) is constructed in this example as a tubular hollow body with a rectangular cross section and rounded edges. The guide bracket (6) comprises two rail receivers (8) which can be engaged in the manner of a dovetail joint with rails with a substantially trapezoidal cross section in each case for torque-rigid guidance. These rails are attached to the lateral working surfaces of the construction container. In this way, a distortion-free movement of the doctor blade unit over the construction level for levelling the applied powder is ensured.

[0032] As can be seen in Fig. 3, the floating bearing (5) is here formed by four sliding elements (9) surrounding the cross member (3), thus allowing expansion in the longitudinal axis of the cross member (3). The cross member (3) is rigidly held on the locating bearing side, but can expand in the direction of the floating bearing side and slide through the floating bearing (5), so that no binding force is applied to the cross member (3).

[0033] In figures 4 and 5, an enlarged cross-sectional view of the area around the fastening means (7) is depicted. In the illustrated example, the bolts of the fastening means (7) pass through guide sleeves inside the cross member (3). The bent area specifies the maximum working travel for the adjustment of the blades (10). The two blades (10) are fixed under a solid central block (11) so that they are symmetrically centered in the middle. The fixing is carried out via horizontal bolts in corresponding cutouts in the solid central block (11), the blades (10) are fixed via spacer elements so that they are separated from each other. On either side of them, two pre-wipers (12) are attached to the outer lateral sides of the central block (11).

[0034] For active control of the doctor blade unit (not shown), a laser sensor system can be attached, for example, to the guide bracket (6), which monitors the straightness and parallelism of the doctor blade (1). The information thus obtained can be used for readjustment, for example via an electric actuator or a piezoelectric element driving a nut. [Explanation of symbols]

[0035] 1 Doctor Blade 2. Suspension system 3 Cross member 4 Locating Bearing 5. Floating bearing 6 Guide bracket 7 Fastening means 8 Rail Support 9 Sliding Elements 10 Blades 11 Central Block 12 Pre-wiper

Claims

1. A doctor blade unit for an additive manufacturing system using a powder as a starting material, comprising a doctor blade (1) and a suspension system (2) mounted to the additive manufacturing system so as to be horizontally movable, The suspension system (2) comprises a cross member (3) attached on one side to a locating bearing (4) and on the other side to a floating bearing (5); The cross member (3) is connected to a guide bracket (6) via the locating bearing (4) and the floating bearing (5), The doctor blade (1) is fastened in a suspended manner below the cross member (3) via a number of fastening means (7) along its length, the fastening means (7) allowing for adjustment of the spacing between the doctor blade (1) and the cross member (3) in order to position the doctor blade (1) parallel with respect to the construction level of the additive manufacturing system. A doctor blade unit comprising:

2. 2. The doctor blade unit according to claim 1, characterized in that the doctor blade (1) comprises a single blade, a double blade or a multi-blade.

3. 3. Doctor blade unit according to claim 1 or 2, characterized in that the guide bracket (6) is suitable for torque-rigid guiding in the additive manufacturing system.

4. 4. Doctor blade unit according to claim 1, characterized in that the cross member (3) is configured as a hollow body.

5. A doctor blade unit according to any one of the preceding claims, characterized in that the cross member (3) has a rectangular or elliptical cross section.

6. 6. A doctor blade unit according to claim 1, characterized in that the locating bearing (4) surrounds the cross member (3) on its periphery on at least two sides and prevents movement of the cross member (3) relative to the locating bearing (4) in all three spatial directions.

7. 7. A doctor blade unit according to claim 1, characterized in that a centering pin is mounted on the inner circumference of the locating bearing (4), said centering pin engaging with a corresponding hole in the cross member (3) with a positive lock.

8. 8. A doctor blade unit according to claim 1, characterized in that the floating bearing (5) surrounds the cross member (3) on its outer periphery with a sliding element in such a way that movement of the cross member (3) relative to the floating bearing (5) along its longitudinal axis is possible while movement in two spatial directions perpendicular thereto is prevented.

9. Doctor blade unit according to any of the preceding claims, characterized in that the doctor blade (1) has a length of 0.1 to 5 m, preferably 0.3 to 3 m.

10. 10. Doctor blade unit according to claim 1, characterized in that the parallelism deviation ΔP, defined as the difference between the maximum and minimum distances between the doctor blade (1) and the construction level, is between 1 and 50 μm, preferably between 10 and 25 μm.

11. 11. Doctor blade unit according to any one of claims 1 to 10, characterized in that the suspension system (2) comprises 3 to 30, preferably 3 to 10, fastening means (7) per meter of doctor blade length.

12. 12. A doctor blade unit according to claim 1, characterized in that during operation of the additive manufacturing system, the spacing of the doctor blade (1) from the cross member (3) can be varied by electromechanical and / or hydraulic actuators acting on the fastening means (7).

13. Doctor blade unit according to any one of the preceding claims, characterized in that the fastening means (7) are threaded bolts.

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