Patent Publication Number: US-2022212409-A1

Title: Build material supply unit

Description:
BACKGROUND 
     The description is related to a three-dimensional (3D) printing system. A 3D printer uses additive printing processes to make 3D objects from a digital 3D object model file. More particularly, the description is related to a build material supply unit for a 3D printing system to supply build material to the 3D printing system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features of examples will be described, by way of example, in the following detailed description with reference to the accompanying drawings in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear. 
       Non-limiting examples will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  shows a cross-section of an example of a build material supply unit for a 3D printing system. 
         FIG. 2  shows a side view of a double vane of an example build material supply unit for a 3D printing system. 
         FIG. 3  shows a cross-section of a double vane of an example build material supply unit for a 3D printing system. 
         FIG. 3 a    schematically presents a controller which controls the double vane. 
         FIG. 4  shows a perspective view of an example of a build material supply unit for a 3D printing system. 
         FIG. 5  shows a side view of an example of a build material supply unit for a 3D printing system. 
         FIG. 6-11  show examples of a method for supplying build material from a tray of a build material supply unit to a spreading plane in a 3D printing system. 
     
    
    
     DETAILED DESCRIPTION 
     In some 3D printing systems, for example, a 3D object may be formed on a layer-by-layer basis where each layer is processed and combined with a subsequent layer until the 3D object is fully formed. 
     In various 3D printing systems, a 3D object being produced may be defined from a 3D object model file. Information in such a 3D object model file comprises 3D geometric information that describes the shape of the 3D model. The 3D geometric information in a 3D object model file may define solid portions of a 3D object to be printed or produced. To produce a 3D object from a 3D object model, the 3D model information may be processed to provide 2D planes or slices of the 3D model. Each 2D slice generally comprises an image and/or data that may define an area or areas of a layer of build material as being solid object areas where the build material is to be solidified during a 3D printing process. 
     In some powder-bed 3D printing systems, such as binder or fusing agent jetting systems, a 2D slice of a 3D object model may be produced by spreading a thin layer of build material over a print bed in a build unit of the 3D printing system. This layer of build material is to receive a functional agent such as a binding agent or a fusing agent. Conversely, areas of a build material layer that are not defined as object areas by a 2D slice comprise non-object areas where the build material is not to be solidified and will not receive a functional agent. The procedure of spreading build material and applying a functional agent is repeated until completion of the 3D object. In some such systems, energy, such as curing or fusing energy, may be applied to cause solidification of build material where an agent was applied. 
     Within 3D printing systems, the term “build material” is to be generally understood as a physical substance that can be used to generate an object via 3D printing. Examples of build materials for additive manufacturing include polymers, crystalline plastics, semi-crystalline plastics, polyethylene (PE), polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), amorphous plastics, Polyvinyl Alcohol Plastic (PVA), Polyamide (e.g., nylon), thermo(setting) plastics, resins, transparent powders, colored powders, metal powder, ceramics powder such as for example glass particles, and/or a combination of at least two of these or other materials wherein such combination may include different particles each of different materials or different materials in a single compound particle. Examples of blended build materials include alumide, which may include a blend of aluminum and polyamide, and plastics/ceramics blends. There exist more build materials and blends of build materials that can be managed by an apparatus of this disclosure. In some 3D printing systems, the build material is in powder form. In other 3D printing systems, the build material is in the form of paste material, solid material, slurry material or liquid material. 
     Some 3D printing systems may comprise, among others, a build platform, a build material storage, a printhead to apply the functional agent to the build material, a recoater, a control unit, a build material supply unit, and a user interface. In one example, the build material supply unit may be integrated into the build unit of the 3D printing system. In another example, a build unit may comprise one or multiple build material supply units. In one example, the 3D object may be generated on a build platform by alternatively applying a build material and a functional agent (and energy, where appropriate). In one example, the build material may be applied layer wise by the recoater moving in a spreading plane. In an example initial state, the spreading plane and the build platform may coincide, while the build platform may move downwards with increasing number of applied layers of build material. 
     In example 3D printing systems that use powdered material, the powdered material may be conveyed from a powder storage unit to a build material supply unit and then to the build platform, located next to the build material supply unit, and on which a 3D object is build layer by layer. An example build material supply unit provides a predetermined dose of build material, which is an amount of powder sufficient to form a layer on the build platform. The example build material supply unit may distribute the predetermined dose of build material uniformly along the length of the build platform. A recoater may spread the supplied build material across the build platform. The build material supply unit may as well reduce powdery build material that becomes airborne during the supplying. An example build material unit is further easy to dismantle for cleaning. 
     Examples described relate to a mechanism that distributes build material from a build material inlet to a uniform, linear output for supplying to a build platform of a 3D printing system while minimizing airborne build material. In addition, the example apparatus described herein reduces the number of active parts because the mechanism has dosing and distributing functions, both of which are carried out during rotation of the mechanism. As such, the example build material supply unit is a simplified design and is compatible with a continuous feeding strategy, whereby material is continually input into the spreading plane, and the example 3D printing system can produce a dose of powder in a reduced time, increasing productivity. In addition, the simplified design reduces the number of failure modes, improving reliability and up time of the 3D printing system. Examples described herein may relate to binder jetting and other powder-bed 3D printing systems. 
       FIGS. 1-11  show a build material supply unit  10  of a 3D printing system and related methods wherein like reference numerals correspond to the same components. Now referring to  FIG. 1  which shows a cross-section of the build material supply unit  10  for a 3D printing system. The build material supply unit  10  comprises a tray  11 . The tray  11  comprises a build material supply opening  12 . The build material supply opening  12  defines a spreading plane  13  in which a recoater  16  of the 3D printing system spreads build material. The spreading plane coincides with the build platform of the 3D printing system in an initial state, joined together along a single edge. The build material supply unit  10  further comprises a double vane  14  that is rotatably mounted inside the tray  11 . The rotation axis  15  of the double vane  14  extends along the tray  11  below the spreading plane  13 . Each vane of the double vane  14  has a convex front side  17  and a rear side  18 . 
     The build material supply unit  10  also comprises a controller  9  to control the rotation of the double vane  14 . By rotating the double vane  14  into a trimming position, shown in  FIG. 6 c   , a predetermined dose of build material with the front side  17  of one vane  19  of the double vane  14  is supplied to the spreading plane  13 . In the trimming position, the front side  17  of the one vane  19  approaches the spreading plane  13  such that the predetermined dose of build material is enclosed between the front side  17  of the one vane  19  and the spreading plane  13 . In the trimming position, the recoater  16  of the 3D printing system trims excess build material on the front side of the one vane  19  to the rear side  18  of the other vane  20 . The one vane  19  of the double vane and the other vane  20  of the double vane may be aligned in the same sense of rotation along which they are offset by an angle of 180 degrees. Thus, with half a turn of the double vane, the one vane  19  takes over the functions of the other vane  20  and vice versa. Excess build material may comprise the amount of build material loaded on the front side  17  of the one vane  19  that exceeds the predetermined dose of build material. In some examples, the double vane  14  may be rotated in a single direction. This direction may determine the front side  17  and the rear side  18  of a vane as seen from the rotating direction. The excess build material trimmed to the rear side  18  of the other vane  20  will be lowered into the tray  11  upon further rotation of the double vane  14 . In some examples, the predetermined dose of build material may be in the order of a few grams of build material. In some examples, predetermined dose of build material may be one of the following: 6 grams, 8 grams, 10 grams, 12 grams, 14 grams, and 16 grams. 
     In an example, the tray  11  may comprise a circular cross-section below the spreading plane. The semi-circular of the tray  11  may be arranged to the evolvent of the rotating double vane  14  below the spreading plane  13 . The distance between the rotation axis  15  of the double vane  14  to the build material supply opening  12  may be adopted to avoid interference of the recoater  16 , which moves within the spreading plane  13 , with the double vane  14  in the trimming position shown in  FIG. 6 c    or in the supply position shown in  FIG. 6 e   ?. In an example material supply unit  10 , the distance between the rotation axis  15  of the double vane  14  to the build material supply opening  14  may be in the order of about ten millimeters. 
     An example double vane  14  may comprise a double bent blade. Yet another example double vane  14  may be a sheet metal with two parallel longitudinal bends. In some examples, the double vane  14  may be made of stainless-steel. In another example, the double blade  14  may be made of aluminum. In yet another example, the double vane  14  may be made of a metal other than aluminum or stain-less steel. In further examples, the double vane may be made of a material that comprises a certain sturdiness. The double vane  14  may be easily replaceable and easy to clean. The double vane  14  may be arranged to meet the requirements of a certain printing process. It represents a robust system for dosing and feeding build material to a 3D printing system. 
     The example double vane  14  may comprise two vanes. In one example, the vanes of the double vane  14  are aligned in the same sense of rotation along which they are offset by an angle of 180 degrees. In another example, the double vanes are offset by an angle different from 180 degrees. In one example, the convex front side  17  of a vane of the double vane  14  may be formed in a curved shape. In another example, the convex front side  17  of the one vane of the double vane  14  may be angularly bent. In yet another example the convex front side  17  of the double vane is rather slightly bent such with a shallow angle of curvature. In some examples, the double vane may comprise a number of vanes that is greater than two. In some other examples, the double vane may comprise an even number of vanes. 
     In some examples, the rear side  18  of the other vane of the double vane  14  may be concave. In other examples, the rear side  18  of the other vane of the double vane  14  may be flat. In yet other examples, the surface of the rear side  18  of the double vane  14  may be ripped or bent. The rear side of the vane helps reduce the amount of powder that becomes airborne by decreasing the height that the build material falls freely into the tray  11  after trimming. Without the rear side  18  of the other vane  20  the build material would freely fall down to the bottom of the tray along a distance corresponding to the diameter of the vane. The “other” vane prevents or at least strongly reduces this free fall to a slipping down around double the distance between the rotation axis and the spreading plane. The height of the rotation axis  15  of the double vane  14  with respect to the spreading plane  13  may be arranged to further avoid the development of airborne build material. 
     In some examples, the recoater  16  may be a roller. In other examples, the recoater  16  may be a counter-rotating roller. In another example, the recoater  16  may be a slider or blade. In some examples, the recoater  16  may comprise a carriage to move the recoater  16 . The recoater  16  may be arranged to move over the build material supply opening  12 . An example 3D printing system may comprise more than one recoater  16 . In another example, for a 3D printing system comprising two parallel build material supply units  10 , the recoater  16  may move across a printing platform in a first direction to deposit a first layer of powder from one build material supply unit  10  on one side of the build platform and then moves in a second, opposite, direction to deposit another layer of powder from a second build material supply unit  10  on the other side of the build platform. 
     In some examples, a 3D printing system may comprise one build material supply unit  10  located next to the build platform. In another example, a 3D printing system may comprise two or more build material supply units  10  located next to the build platform. In an example 3D printing system, the at least one build material supply unit  10  may be arranged to minimize the travel distance of the build material from the build material supply unit  10  to the build platform. 
       FIG. 2  shows a side view of an example double vane  14  of an example build material supply unit  10  for a 3D printing system. The build material supply unit  10  may comprise deflectors  21  to uniformly distribute build material which is accumulated on the rear side  18  of the vanes of the double vane  14  over the length of the tray  11 . The tray  11  extends along the rotation axis  15  of the double vane  14 . 
     In some examples, the deflectors  21  are provided on the rear side  18  of the other vane and the convex front side  17  of the one vane of the double vane  14 . The double vane  14  may be arranged such that the deflectors  21  extend from the rear side  18  of one vane  19  partly to the convex front side  17  of the other vane  20 . In some examples, the deflectors  21  are inclined with respect to the rotation axis  15  of the double vane  14  such as to distribute build material accumulated in the trimming position on the rear side  18  of one vane  19  over the length of the tray  11  upon trimming of excess build material and upon further rotation of the double vane  14 . 
     In some examples, the deflectors  21  of the double vane  14  may be regularly spaced along the rotation axis  15  of the double vane  14 . In this way, the build material accumulated by the rear side  18  of the vane and guided by the deflectors  21  is evenly distributed along the length of the tray  11 . 
       FIG. 3  shows a cross-section of an example double vane  14  of an example build material supply unit  10  for a 3D printing system. An example double vane  14  may have a z-profile. The z-profile may comprise two parallel outer sections  31 . Both outer section  31  may be connected to a central double vane section  32  via an elbow  33  as shown in  FIG. 3 . In some examples, the elbow  33  may be a longitudinal bending edge. In one example, the longitudinal bending edge may be a rounded edge. In another example, the longitudinal bending edge may be a sharp edge transition between the outer double vane sections  31  and the central double vane section  32 . 
     In one example, the z-profile may be arranged such that the predetermined dose of build material may be enclosed between the spreading plane  13  and one outer double vane section  31  when the double vane  14  is rotated into the trimming position. In the trimming position, the elbow  33  approaches the spreading plane  13 . In one example, the elbow  33  may come close to the spreading plane  13  from below in the trimming position. In another example, the elbow may connect the spreading plane  13  in the trimming position. In yet another example, the elbow  33  may align with the spreading plane  13  in the trimming position. 
     In some examples, the predetermined dose of build material sets boundary conditions for configuring the bending angle of the elbow  33  and, in dependence of the angle, for the length of the outer section  31  in the trimming position. In another example, the predetermined dose of build material sets boundary conditions for configuring the length of the outer section  31 , and in dependence of the outer section, for the bending angle of the elbow  33  in the trimming position. The resulting z-shape may determine the distance between the rotation axis  15  and the spreading plane  13 . In another example, the distance between the rotation axis  15  and the spreading plane  13  may determine the z-shape of the double vane  14  under consideration the predetermined dose of build material. 
       FIG. 3 a    schematically presents the controller  9  of the build material supply unit  10  which controls rotation of the double vane  14  by a driving system  34 . The controller  9  may control rotation speed of the double vane  14 . The controller  9  may control the angular position of the double vane  14  in the trimming and the supplying position. The controller  9  may further synchronize the movement of the recoater  16  and the rotation of the double vane  14 . 
       FIG. 4  shows a perspective view of an example of a build material supply unit  10  for a 3D printing system. In one example, the double vane  14  is mounted with its rotation axis  15  below the spreading plane  13 . The distance between the rotation axis  15  of the double vane  14  to the build material supply opening  12  may be designed such that in the trimming position, the bending edge  33  of the double vane approaches the spreading plane  13  and prevents build material of the enclosed predetermined dose of build material to be moved by the recoater to the other vane  20 . The recoater  16  may be arranged to move across the build material supply opening  12 . The deflectors  21  may redirect the falling excess build material towards the lateral ends of the tray  11  such that the build material supply unit  10  supplies the predetermined dose of build material uniformly along the spreading plane  13 . 
       FIG. 5  shows a side view of an example of a build material supply unit  10  for a 3D printing system. In some examples, the build material supply unit  10  may comprise a thermal blanket  52 . The thermal blanket  52  may surround the tray  11  to control the temperature of the build material which is accumulated in the tray  11 . In some examples, the thermal blanket  52  may preheat the build material which may cause the build material to become sticker or more cohesive. In some examples, preheating the build material may cause the build material to form a semi-hard cake once supplied to the spreading plane  13 . 
     In some examples, the build material supply unit  10  may comprise a build material inlet  53 . The build material inlet  53  may be coupled to a build material feeder  51 , wherein the build material feeder  51  may feed build material through the build material inlet  53  into the tray  11 . The build material feeder  51  may feed build material from a build material storage of the 3D printing system through the build material inlet  53  into the tray  11 . In one example, the build material inlet  53  may be located at the bottom of the tray  11  opposite the build material supply opening  12 . In some examples, the build material inlet  53  is located centrally along the rotation axis  15  of the double vane  14 . The amount of build material fed by the build material feeder  51  in one cycle may be synchronized to correspond to the predetermined dose of build material supplied to the spreading plane  13  in one cycle. 
     In one example, the build material supply unit  10  may comprise a plurality of material inlets  53 . In some examples, the deflectors  21  of the double vane  14  may be arranged to the configurations of the build material inlet  53  in the tray  11  such that the deflectors  21  distribute build material uniformly along the tray  11 . In one example the tray  11  may comprise two build material inlets  53  which divide the tray  11  into two sections. The deflectors  21  of the double vane  14  corresponding to the first section may be inclined towards the build material inlet  53  of the first section whereas the deflectors  21  of the double vane  14  corresponding to the second section may be inclined to the build material inlet of the second section respectively. 
     The build material may be fed through the build material inlet  53  into the tray  11  by a feeder  51 . In another example, the feeder  51  may comprise a pneumatic conveyance system. In another example, the feeder  51  may comprise an auger. In yet another example, the feeder  51  may comprise an Archimedes screw. 
     Before the build material unit  10  may be operated in steady state, it may undergo an initialization. During initialization, the double vane  14  may rotate some full turns to uniformly distribute build material within the tray  11  while the recoater  16  is on hold. In steady state, the build material supply unit  10  may supply the predetermined dose of build material for building one layer of a 3D object uniformly along the spreading plane  13 . The build material feeder  51  may be synchronized to feed the predetermined dose of build material into the tray  11  such that the build material level in the tray  11  is maintained at steady state. 
     In some examples, the configuration of the deflectors  21  may determine the speed at which build material is distributed along the tray  11 . In one example, the length of the deflectors  21  may determine the distance the build material travels along the tray  11  during rotation. In another example, the size of the deflectors  21 , the angle the deflectors  21  are tilted with regards to the rotation axis as well as the distance between deflectors  21  may determine the amount of powder that is redirected towards the longitudinal ends of the tray  11  during rotation. In some examples, the deflectors may be arranged to meet the build material characteristics. 
     In some examples, the sense of orientation of the deflectors  21  is modified to the build material inlet  53 . In an example, where build material is supplied over the length of the tray  11 , the deflectors may not be inclined but still arranged on the double vane to comb through the build material to avoid conglomerating of build material. In another example, where build material is supplied over the length of the tray  11 , the double vane  14  may not comprise deflectors  21 . In an example of a central build material inlet, the deflectors may be oriented symmetrically with respect to the build material inlet. In some examples, stickier powder may require enhanced height of deflectors for distribution to counteract fins that may otherwise occur when sticky powder hits deflectors. 
       FIG. 5  further shows an example predetermined dose of build material  54  being supplied at the build material supply opening  11  of the build material supply unit  10 . The predetermined dose of build material  54  will be spread into the spreading plane  13  by the recoater  16  which moves over the build material supply unit  10 . 
     In some examples, the build material supply unit  10  may further comprise a collecting unit  55 . The collecting unit  55  may be positioned at least at one longitudinal end of the tray  11  and arranged to collect excess build material. In some example, the collecting unit  55  may be a removable storage. In some examples, the build material collected in the collecting unit  55  may be processed for reuse. The build material unit  10  may be arranged to minimize build material that is collected in the collecting unit  55 . 
     An example build material supply unit  10  may also comprise a controller  9 . The controller  9  may be programmed to measure the amount of build material loaded on the front side  17  of one vane  19 . The amount of build material loaded on the front side  17  of one vane  19  may be measure for example by measuring the torque applied to the double vane  14 . In some examples, the torque to be applied to the double vane  14  may be measured using pulse-width modulation. In some examples, this measurement indicates the level of build material in the tray  11 . In another example, the level of build material in the tray  11  may be measured using laser measurement. The amount of build material supplied by the build material feeder  51  may be controlled closed-loop as a response of the measured level of build material in the tray  11 . 
     An example build material supply unit  10  may comprise a driving system  34  to rotate the double vane  14 . In one example, the driving system  34  may comprise a motor and the controller  9 . The controller  9  may further comprise an encoder to control the rotation angle of the double vane  14 . The driving system  34  may also comprise a gearbox to increase torque applied to the rotating axis  15 . The driving system  34  may also comprise a coupling that enables dislodging the double vane  14  from the tray  11  for maintenance and cleaning. In some examples, the build material supply unit  10  may further comprise a locking system arranged at the opposite side of the driving system to preload the double vane against the driving system axial datum. 
     In an example of the build material supply unit  10  the controller  9  may actuate the driving system  34 . In an example, the controller  9  may also measure the level of build material in the tray  11 . 
       FIG. 6  shows how the controller  9  controls example positions of the double vane  14  and the recoater  16  during one cycle. In some examples, one cycle may correspond to a half turn of the double vane  14 .  FIG. 6 a    shows a state of the build material supply unit  10  after initialization. During initialization, the controller  9  may rotate the double vane  14  a predetermined number of full turns to uniformly distribute the build material within the tray  11  while halting the recoater  16 . The predetermined number of full turns may depend on the arrangement of the deflectors  21  on the double vane  14 . In an example, the deflectors  21  may be spaced apart and inclined with respect to the rotation axis  15  such that the build material reaches the next adjacent deflector in outwardly longitudinal direction during a half turn. The number of full turns during initialization may correspond to at least one quarter of the total number of deflectors  21 . 
     As the build material has been distributed uniformly along the longitudinal axis of the tray  11  the double vane is ready for operation. The controller  9  controls the build material feeder  51  to supply the predetermined dose of build material through the build material inlet to the tray  11  from a build material storage in each cycle. Upon rotation, the double vane  14  may be controlled by the controller  9  to load a portion of build material on the convex front side  17  of one vane  19 , while the recoater  16  may be controlled by the controller  9  to move from the build platform towards the tray  11  as shown in  FIG. 6 b   . In one example position of the double vane  14 , the controller  9  may further rotate the double vane may until one elbow  33  of the vane aligns with the spreading plane  13  as shown  FIG. 6 c   . In this position, the vane encloses the predetermined dose of build material to be fed to the spreading plane  13  between its convex front side  17 , the spreading plane  13  and the inner side section of the tray  11  below the spreading plane  13 . In this trimming position, the controller  9  may halt double vane  14  until excess build material has been trimmed and the controller  9  may have moved recoater  16  across the tray. 
     The controller  9  may continue to move the recoater towards a direction away from the build platform thereby trimming excess build material. The recoater  16  relocates excess build material such that it is accumulated on the rear side  18  of the other vane as shown in  FIG. 6 d   . The rear side of the other vane  20  catches the excess build material. Free fall of the excess build material into the tray  11  and related airborne build material is thus avoided. 
       FIG. 6 e    shows the controller  9  to further rotate the double vane  14  until the outer section of the one vane  19  aligns with the spreading plane  13  and the predetermined dose of build material is lifted to the spreading plane  13 . The controller then moves the recoater  16  in a opposite direction towards the build platform and which spreads the predetermined dose of build material into the spreading plane  13  as shown in  FIG. 6 f   . Upon further rotating the double vane  14  by the controller  9 , excess build material is lowered into the tray  11  by the other vane  20 . In some examples, the predetermined dose of build material may be slightly modified in that the controller  9  halts the double vane  14  at different angles in the trimming and supplying position. 
     The controller  9  may synchronize the movement of the recoater  16  and the rotation of the double vane  14  may such that the recoater  16  does not interfere with the double vane  14 . Also, the height of the double vane  14  may be arranged such that outer sections  31  and the elbows  33  do not interfere with the recoater  16 . 
       FIG. 7  shows a method  70  performed by the controller  9  to control rotation of the double vane  14  and movement of the recoater  16  for supplying build material from the tray  11  of the build material supply unit  10  to the spreading plane  13  in a 3D printing system. In block  71 , the controller  9  rotates the double vane  14  in a trimming position. Upon rotation by the controller  9 , the double vane  14  lifts build material with the front side  17  of one vane  19  from the tray  11  into the trimming position where a predetermined dose of build material is enclosed between the front side  17  of the one vane  19  and the spreading plane  13 . 
     In block  72  the controller  9  controls the recoater  16  to trim excess build material loaded on the front side  17  of the one vane  19 . The controller  9  moves the recoater  16  for this in the spreading plane  13  from the front side  17  of the one vane  19  to the rear side  18  of the other vane  20 . The method  70  further comprises accumulating excess build material that has been trimmed by the recoater  16  on the rear side  18  of the other vane  20  of the double vane  14  in block  73 . The method  70  comprises the controller  9  to further rotate the double vane  14  thereby lowering the build material which has been accumulated on the rear side  18  of the other vane  20  into the tray  11  in block  74 . 
     In one example, the controller  9  controls the rotation speed of the double vane which corresponds to the lowering speed of lowering accumulated excess build material on the rear side  18  of the other vane  20  into the tray  11  to prevent free fall of excess build material into the tray  11 . The rear side  18  of the other vane  20  gently lowers excess build material into the tray which prevents build material becoming airborne within the build material supply unit  10 . In some examples, the development of airborne build material may cause malfunctioning of parts of the 3D printing system. In other examples, the development of airborne build material may reduce the life-time of parts of the 3D printing system. 
       FIG. 8  shows a further example method  80  performed by the controller  9  to control rotation of the double vane  14  and movement of the recoater  16  for supplying build material from the tray  11  of the build material supply unit  10  to the spreading plane  13  in a 3D printing system. The controller  9  may rotate the double vane  14  such that excess build material accumulated on the rear side  18  of the other vane  20  is distributed over the length of the tray  11  by means of deflectors  21  provided on the double vane  14  in block  81 . In one example, blocks  74  and  81  are processed by the same rotation of the double vane  14 . The controller  9  may further lower the other vane  20  of the double vane  14  by such that the excess build material is deflected by the deflectors and put into the tray. The deflectors may describe the movement of the excess build material. 
       FIG. 9  shows an example method  90  performed by the controller  9  to control rotation of the double vane  14  and movement of the recoater  16  for supplying build material in a 3D printing system. In some examples, the method  90  may comprise collecting excess build material at the longitudinal end of the tray in block  91 . In some examples, excess build material may be processed for reuse. 
       FIG. 10  also shows an example method  100  performed by the controller  9  to control rotation of the double vane  14  and movement of the recoater  16  for supplying build material in a 3D printing system. The controller  9  may rotate the double vane  14  a predetermined number of rotations to uniformly distribute the build material over the length of the tray  11 . The predetermined number of rotations may result from the arrangement of the deflectors  21  on the double vane  14 . The double vanes  21  may be spaced apart and inclined with respect to the rotation axis  15  such that build material reaches the next adjacent deflector in outwardly longitudinal direction during a half turn. The number of full turns during initialization may correspond to at least one quarter of the total number of deflectors  21 . 
       FIG. 11  shows a further example method  110  performed by the controller  9  to control rotation of the double vane  14  and movement of the recoater  16  for supplying build material in a 3D printing system. In this method, the controller  9  may measure the amount of build material loaded on the front side  17  of one vane  19  and may adapt the feeding amount of build material a feeder feeds through a build material inlet into the tray  11  in response to the measured amount of build material in block  111 . 
     While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited by the scope of the following claims and their equivalents. 
     It should be noted that the above-mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims. Features described in relation to one example may be combined with features of another example.