BACKGROUND

[0001] Additive manufacturing processes can produce three-dimensional (3D) objects by providing a layer-by-layer accumulation and solidification of build material according to digital 3D object models. In some examples, printheads can selectively print (i.e. , deposit) liquid functional agents such as fusing agents or binding agents onto layers of build material within predefined areas that are to become layers of a part. The liquid agents enable solidification of build material within the printed areas. For example, in some binder jetting processes heat can be applied during printing to at least partially cure and dry each part layer where liquid binding agent has been applied. This layer-by-layer process can be repeated until an entire part (a “green” part) is printed. The fabricated green part can then undergo further post-processing, such as infiltration or sintering, to bring the part to its full mechanical strength.

BRIEF DESCRIPTION OF THE DRAWINGS [0002] Examples will now be described with reference to the accompanying drawings, in which:

[0003] FIG. 1 shows a block diagram of an example 3D printing system suitable for modifying material spreading parameters during the formation of a 3D object; [0004] FIG. 2 shows a block diagram of an example controller such as the controller shown in FIG. 1; [0005] FIG. 3 shows an example of a build process in which an object is formed in a build area of an example 3D printing system using different powder spreading parameters in different regions of the object;

[0006] FIG. 4 shows an example of a build process in which multiple objects of different sizes, and in varying build locations, are formed in a build area of an example 3D printing system using different powder spreading parameters in different regions of the objects; and,

[0007] FIGs. 5, 6 (shown as FIGs. 6A and 6B), and 7, show flow diagrams that illustrate example methods of forming layers of an object in a 3D printing device. [0008] Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.

DETAILED DESCRIPTION

[0009] In some additive manufacturing processes, including some 3D printing processes, 3D objects can be formed from layers of build material. Examples of different build materials include plastics, metals, and ceramic materials that can be used in different forms such as powders, fibers, and so on. In some example processes, layers of build material are spread over a build platform one layer at a time, and portions of each material layer are combined with portions of a subsequent layer until a 3D object is fully formed. In some examples, a liquid agent such as a fusing agent or binding agent can be printed or deposited onto portions of each material layer, and heat or other types of energy, such as ultra-violet light, can be applied to facilitate the solidification of the printed build material.

[0010] In some powder-based additive manufacturing processes, such as binder jetting 3D printing processes, layers of powder or other build material are spread over a build platform by a powder recoater, where they are processed layer- by-layer to form 3D objects. In some examples, a powder recoater is implemented as a rotating roller. In such processes, the manner in which the layers of powder (i.e. , build material) are spread over the platform can impact the quality of the finished objects. For example, the visual quality of an object or objects within a given build can be improved when the powder spreading parameters are set so that the rotating roller rotates at a faster speed and moves across the build platform at a faster speed. However, when the powder spreading parameters are set at faster speeds, the roller tends to generate powder layers that have lower density, which can result in finished objects having reduced mechanical strength. In general, objects produced with higher density layers have greater mechanical strength, and higher density layers can be generated using slower powder spreading parameters that cause the roller to rotate more slowly and translate more slowly across the build platform. As indicated above, however, slower translational and rotational roller speeds can reduce the visual quality of objects being produced.

[0011] Crazing on the bottom surfaces of objects is one visual anomaly that can occur when powder spreading parameters are set for slower translational and rotational speeds. Crazing can appear as cracks or fissures of various lengths and depths in the bottom surface of an object. Crazing can begin in the initially formed layers and can transfer through to subsequently formed layers, extending for example, through the first 10 to 20 layers of the bottom surface of an object. The layers eventually coalesce, which prevents the crazing from continuing deeper into the object. In such an example, therefore, while slower powder spreading parameters can generate higher density layers resulting in objects having improved mechanical strength, the visual quality of the bottom surfaces of such objects may suffer from crazing through the first 10 to 20 layers.

[0012] Accordingly, example methods and devices described herein, control powder spreading parameters to enable the generation of 3D objects with improved visual and mechanical properties. The methods and devices help to achieve both high visual quality and high mechanical strength by adjusting powder spreading parameters on a per object basis. That is, during the process of generating an object, powder spreading parameters can be modified such that different layers within the same object are spread differently over the build platform, resulting in layers with different properties, such as layers having different densities, for example. Adjusting the powder spreading parameters in this manner (i.e. , per object) helps produce powder layers that result in finished objects that have both high visual quality and high mechanical strength.

[0013] In some examples, in the start-up region of an object, powder spreading parameters can be set so that build powder for the initial object layers can be spread across the build platform by a counter rotating roller that rotates and translates at relatively high speeds. For example, the counter rotating roller may translate across the build platform at 10 inches per second while rotating in a counter-translational direction at 10 rotations per second. While these higher speeds are provided by way of example, other high translational and rotational speeds, as well as rotational directions, can be determined with the general intent of minimizing shear between the powder recoating roller and the printed surface of the underlying powder layer. The minimized shear helps to avoid visual defects on the bottom surfaces of the object, such as crazing. An example of the range of layers over which higher speed powder spreading parameters may be applicable, is from approximately 10 layers below the first layer of the downward facing object surface to approximately 20 layers above the first layer of the downward facing object surface.

[0014] For the remaining layers of an object, it may be appropriate to use slower powder spreading parameters to achieve powder layers having higher density. For example, after spreading layers in the start-up region of an object (e.g., the first 10 to 20 layers) at higher speeds, the speed of the counter rotating roller can be modified (i.e. , slowed down) to translate across the build platform at 2 to 4 inches per second while rotating in a counter-translational direction at 3 to 6 rotations per second.

[0015] In a particular example, a method of forming layers of an object in a 3D printing device can include forming build material layers over a build platform of a 3D device, processing each layer according to an object model to form an object, and during the forming of layers, modifying material spreading parameters of the printing device so that some layers within the object are spread using different material spreading parameters than other layers within the object.

[0016] In another example, a powder-layer based additive manufacturing device includes a build platform on which to generate a 3D object, and a roller spreader to spread build material layers of the 3D object over the platform. The device also includes a controller to modify operating parameters of the roller spreader during spreading of the build material layers so that some build material layers of the 3D object are spread over the platform by the roller spreader operating at different speeds.

[0017] In another example, a method of forming layers of an object in a three- dimensional (3D) printing device includes forming powder layers over a build platform using a roller device in a build area of a 3D printing device, and printing liquid functional agent onto areas of the powder layers to form multiple objects based on a 3D object model. During the forming of the powder layers, material spreading parameters of the roller device can be modified so that some layers within the objects are spread using different material spreading parameters than other layers within the objects.

[0018] FIG. 1 shows a block diagram of an example 3D printing system 100 suitable for modifying material spreading parameters during the formation of a 3D object. The example 3D printing system 100 may be variously referred to herein as 3D printer 100, 3D printing device 100, printing system 100, and the like. In the example system 100, during the formation of a 3D object or objects within a particular build or print job, powder layers can be spread over the build platform using different powder spreading parameters for different layers of the object or objects. The example 3D printing system 100 generally comprises a binder jetting 3D printing system 100 that enables the formation of a 3D object (sometimes referred to as a “green object” or “green part”) in a layer-by-layer build process using a metal powder build material and a binder liquid, as discussed in more detail herein below. However, aspects of the example 3D printing system 100 described and illustrated herein are not limited to such a binder jetting 3D printing system, as various aspects may be similarly applicable to other systems, including other powder bed-based additive manufacturing systems in which layers of powder and other build material are to be spread over a build platform and processed with liquid functional agents to facilitate the solidification of the build material. Furthermore, the 3D printing system 100 depicted in FIG. 1 is shown by way of example, and it is not intended to represent a complete 3D printing system. Thus, it is understood that such an example 3D printing system 100 may comprise additional components and may perform additional functions not specifically illustrated or discussed herein.

[0019] As shown in FIG. 1 , an example 3D printer 100 includes a moveable build platform 102 to serve as the floor to a work space or build area 104 in which a 3D object or objects 106 can be formed. The build platform 102 is movable in a vertical direction (i.e. , up and down) along the Z-axis by a lift mechanism 103. The build area 104 is enclosed within a build box 108 having walls that surround the build platform 102 to contain build material 109 spread over the platform 102 during a build process. In the side view shown in FIG. 1 , the front wall of the build box 108 is not shown in order to provide a better view of other components, objects, and materials, inside the box 108.

[0020] An example 3D printer 100 can include a build material supply 110 to provide powder 109 or other build material 109. During a build process, a rotating recoater roller 112 translates over the build platform 102 along the X-axis (e.g., as indicated by direction arrow 114) and moves powder 109 from the supply 110, spreading it in a layer over the build platform 102 on top of a previous powder layer. With each new layer, a powder delivery platform 116 driven by lift mechanism 117 in the supply 110 can push more powder upward as indicated by direction arrow 118, making the powder available to roller 112. After each new powder layer is spread over the build platform 102 and processed with liquid agent and heat, as discussed herein below for example, the build platform 102 can move downward as indicated by direction arrow 120, making room in the build area 104 for a next layer.

[0021] A liquid agent dispenser 122 can deliver a liquid functional agent such as a binder liquid or a liquid fusing agent and/or detailing agent in a selective manner onto areas of a powder layer that has been spread over the build platform 102. Areas of powder layers that are to be printed can be determined in accordance with a digital 3D object model that includes geometric information describing the shape of the object to be printed. Such a 3D object model can be processed into 2D slices, where each 2D slice defines the portions of a powder layer that are to be printed on by the liquid agent dispenser 122 in order to form a layer of the 3D part. [0022] A liquid agent dispenser 122 can include, for example, a printhead or printheads, such as thermal inkjet or piezoelectric inkjet printheads. In some examples, a printhead liquid agent dispenser 122 can comprise a platform-wide array of liquid ejectors (i.e. , nozzles, not shown) that spans across the full Y-axis dimension of the build platform 102. A platform-wide liquid agent dispenser can move bi directionally along the X-axis (as indicated by direction arrow 124) as it ejects liquid droplets onto a build material layer. In some examples, a printhead dispenser 122 can comprise a scanning type printhead that spans across a limited portion or swath of the build platform 102 in the Y-axis dimension as it moves bi-directionally in the X- axis while ejecting liquid droplets onto a build material layer. Upon completing each swath, a scanning type printhead can move in the Y-axis direction in preparation for printing liquid droplets onto another swath of the build material layer.

[0023] The example 3D printer 100 can also include a thermal energy source 126 such as a thermal radiation source. A thermal radiation source 126 can apply radiation (R) from above the build area 104 to heat build material layers on the build platform 102. In some examples, a thermal radiation source 126 can comprise a platform-wide scanning energy source that scans across the build platform 102 bi directionally in the X-axis, while covering the full width of the build platform 102 in the Y-axis. In some examples, a thermal radiation source 126 can include a thermal radiation module comprising a thermic light lamp, such as a quartz-tungsten infrared halogen lamp. Other thermal energy sources can include, for example, resistive heating elements (not shown) disposed within walls of the build box 108 or the build platform 102.

[0024] Referring still to FIG. 1 , an example 3D printing system 100 additionally includes an example controller 128. The example controller 128 can control various components and operations of the 3D printing system 100 to facilitate the printing of 3D objects as generally described herein, such as controllably spreading powder onto the build platform 102, selectively applying/printing liquid functional agent onto portions of the powder, and exposing the powder to radiation R. In some examples, the controller 128 can modify powder spreading parameters to control the roller 112 to rotate and translate at different speeds when spreading different layers of an object or objects. More generally, the example controller 128 controls components of the 3D printing system 100 to perform operations such as discussed below with regard to the flow diagrams of FIGs. 5, 6 (shown as FIGs. 6A and 6B), and 7.

[0025] FIG. 2 shows a block diagram of an example controller 128. As shown in FIG. 2, an example controller 128 can include a processor (CPU) 130 and a memory 132. The controller 128 may additionally include other electronics (not shown) for communicating with and controlling components of the 3D printing system 100. Such other electronics can include, for example, discrete electronic components and/or an ASIC (application specific integrated circuit). Memory 132 can include both volatile (i.e. , RAM) and nonvolatile memory components (e.g., ROM, hard disk, optical disc, CD-ROM, flash memory, etc.). The components of memory 132 comprise non- transitory, machine-readable (e.g., computer/processor-readable) media that can provide for the storage of machine-readable coded program instructions, data structures, program instruction modules, JDF (job definition format), plain text or binary data in various 3D file formats such as STL, VRML, OBJ, FBX, COLLADA, 3MF, and other data and/or instructions executable by a processor 130 of the 3D printing system 100. Examples of data that can be received and/or generated by the system 100 and stored in memory 132, include 3D object model data 134 and powder spreading parameters (PSPs) 136. In some examples, 3D object model data 134 can include powder spreading parameters such as PSPs 136 to be applied while spreading different powder layers and/or portions of powder layers. Thus, powder layers can be formed and printed in accordance with a digital 3D object model that includes geometric information describing the shape of the object to be printed as well as powder spreading parameters that determine how different powder layers are to be spread during formation of a 3D object. An example of executable instructions that can be stored in memory 132 include instructions associated with a PSP instruction module 138.

[0026] FIG. 3 shows an example of a build process in which an object 140 is formed in a build area 104 of an example 3D printing system 100 using different powder spreading parameters 136 in different regions of the object 140. That is, different powder spreading parameters have been used to spread powder layers in the start-up region 142 of object 140 and the remainder region 144 of object 140. In general, as noted above, powder spreading parameters can be set within the start-up region 142 so that the recoating roller rotates and translates at higher speeds in order to minimize shear between the powder recoating roller and the printed surface of the underlying powder layer, and they can then be modified or reset to operate within the remainder region 144 so that the recoating roller rotates and translates at slower speeds in order to achieve powder layers having higher density. As shown in FIG. 3, in the start-up region 142 of the object 140, the powder spreading parameters 136 comprise a first roller translational speed, T1 , and a first roller counter-rotational speed, R1 . An example of powder spreading parameters 136 that may be used in the start-up region 142 include a roller translational speed T1 of 10 inches per second and a roller counter-rotational speed of 10 rotations per second. The start-up region 142 of an object 140 generally includes the initial layers of the object that are the first, or earlier layers, to be spread by the roller 112. The number of layers in a start-up region 142 can vary, but in some examples may range from approximately 10 to 20 layers. In addition, in some examples as noted above, the higher speed powder spreading parameters may also be applied for a number of layers prior to reaching the start-up region 142. Accordingly, as shown in FIG. 3, an example of the range of layers over which higher speed powder spreading parameters may be applicable can include the start-up region 142 of the object 140, as well as a pre-region 143 of layers formed prior to reaching the start-up region 142 of the object 140. An example of the layers comprising the pre-region 143 can be, approximately, the 10 layers below or prior to the first layer of the start-up region 142 (i.e. , the first layer of the downward facing surface of the object 140). [0027] As further shown in FIG. 3, within the remainder region 144 of the object 140, the powder spreading parameters 136 have been modified to a second roller translational speed, T2, and a second roller counter-rotational speed, R2. An example of powder spreading parameters 136 that may be used in the remainder region 142 of an object 140 include a roller translational speed T2 of 2 to 4 inches per second and a roller counter-rotational speed R2 of 3 to 6 rotations per second. In some examples, modifying the material spreading parameters can include modifying the parameters gradually over a number of layers. Thus, parameter modification may begin within the start-up region 142, a number of layers prior to reaching the first layer of the remainder region 144. The remainder region 144 of an object 140 generally includes the remaining layers of an object to be spread over the build platform 102 after the initial layers in the start-up region 142 have been spread over the platform and printed. In such example build processes, the controller 128 can execute instructions from a PSP module 138, for example, to determine which powder spreading parameters 136 to use in the start-up region 142, and which powder spreading parameters 136 to use in the remainder region 144 of an object.

[0028] FIG. 4 shows an example of a build process in which multiple objects of different sizes, and in varying build locations, are formed in a build area 104 of an example 3D printing system 100 using different powder spreading parameters 136 in different regions of the objects. As shown in FIG. 4, the start-up regions 146 and 148 of objects 150 and 152, respectively, begin at the same level and powder layer within the build area 104. Therefore, to achieve the same visual and mechanical strength properties at the surfaces of the objects 150 and 152, the same powder spreading parameters 136 can be applied to the initial start-up layers of both objects 150 and 152 within the respective start-up regions 146 and 148. For example, the powder spreading parameters 136 can comprise a first roller translational speed, T1 (e.g., 10 inches per second), and a first roller counter-rotational speed, R1 (e.g., 10 rotations per second). As noted above with regard to FIG. 3, the initial (e.g., higher speed) powder spreading parameters may also be applied for a number of layers within a pre region 143 prior to reaching the start-up regions 146 and 148. Accordingly, as shown in FIG. 4, an example of the range of layers over which initial (e.g., higher speed) powder spreading parameters may be applicable can include a pre-region 143 in addition to the start-up regions 146 and 148 of objects 150 and 152, respectively. As the layers within start-up regions 146 and 148 are completed, and the powder layers within the remainder regions 154 and 156 of objects 150 and 152 are to begin being spread over the build platform, the powder spreading parameters 136 can be modified to a second roller translational speed, T2 (e.g., 2 to 4 inches per second), and a second roller counter-rotational speed, R2 (e.g., 3 to 6 rotations per second).

[0029] The powder spreading parameters can remain the same throughout spreading of the remaining layers of objects 150 and 152. This is because these parameters are suitable for spreading powder layers for both the remainder regions 154 and 156 of objects 150 and 152. Furthermore, the parameters can remain the same after the remainder region 154 of object 150 is completed. Flowever, the powder spreading parameters can be modified again when layers for the pre-region 143 and start-up region 158 of object 160 are to be spread over the platform. Therefore, as shown in FIG. 4, the powder spreading parameters can be modified back to the first roller translational speed, T 1 and the first roller counter-rotational speed, R1 . It is also noted that other parameters are possible and contemplated herein. For example, the powder spreading parameters might be modified to provide a third roller translational speed, T3, and the third roller counter-rotational speed, R3.

[0030] Referring still to FIG. 4, while the powder spreading parameters can be modified back to roller speeds T1 and R1 to accommodate the spreading of powder layers for the pre-region 143 and start-up region 158 of object 160, it should be apparent that these same powder layers are also forming layers of the remainder region 156 of object 152, and that the modified parameters T1 and R1 are not the same parameters previously being used to spread powder layers within this remainder region 156. Accordingly, in some examples the powder spreading parameters can also be modified part way through the spreading of a powder layer in order to accommodate different objects that may be at different locations within the build area 104 and/or at different levels of completion within the build process.

[0031] Thus, referring again to FIG. 4, because each of the powder layers being spread to form the pre-region 143 and start-up region 158 of object 160 will also form part of the remainder region 156 of object 152, the powder spreading parameters can be modified within the layers and on a repeating basis. Such modifications can include, for example, first modifying the parameters so that the roller operates at speeds T 1 and R1 for the first part of a layer that makes up part of the pre-region 143 or start-up region 158 of object 160, and then modifying the parameters for the second part of the layer that makes up the remainder region 156 of object 152. Such a powder spreading parameter modification can happen part way through a layer at a transitional layer location 161 , for example. These parameter modifications at a transitional layer location 161 can be repeated until the powder layers forming the pre region 143 and start-up region 158 of object 160 have all been completed.

[0032] After the powder layers forming the start-up region 158 of object 160 have all been completed, the powder spreading parameters can be modified to remain, for example, at a second roller translational speed, T2 (e.g., 2 to 4 inches per second), and a second roller counter-rotational speed, R2 (e.g., 3 to 6 rotations per second), in order to spread the remaining powder layers for the remainder region 162 of object 160, and the remainder region 156 of object 152.

[0033] FIGs. 5, 6 (shown as FIGs. 6A and 6B), and 7, are flow diagrams showing example methods 500, 600 and 700, of forming layers of an object in a 3D printing device. Method 600 comprises extensions of method 500 and incorporates additional details of method 500. Methods 500, 600 and 700 are associated with examples discussed above with regard to FIGs. 1 - 4, and details of the operations shown in methods 500, 600 and 700 can be found in the related discussion of such examples. The operations of methods 500, 600 and 700 may be embodied as programming instructions stored on a non-transitory, machine-readable (e.g., computer/processor-readable) medium, such as memory/storage 132 shown in FIG. 2. In some examples, implementing the operations of methods 500, 600 and 700 can be achieved by a controller, such as a controller 128 of FIG. 2, reading and executing the programming instructions stored in a memory 132. In some examples, implementing the operations of methods 500, 600 and 700 can be achieved using an ASIC and/or other hardware components alone or in combination with programming instructions executable by a controller 128.

[0034] The methods 500, 600 and 700 may include more than one implementation, and different implementations of methods 500, 600 and 700 may not employ every operation presented in the respective flow diagrams of FIGs. 5, 6, and 7. Therefore, while the operations of methods 500, 600 and 700 are presented in a particular order within their respective flow diagrams, the order of their presentations is not intended to be a limitation as to the order in which the operations may actually be implemented, or as to whether all of the operations may be implemented. For example, one implementation of method 600 might be achieved through the performance of a number of initial operations, without performing other subsequent operations, while another implementation of method 600 might be achieved through the performance of all of the operations.

[0035] Referring now to the flow diagram of FIG. 5, an example method 500 of forming layers of an object in a 3D printing device begins at block 502 with forming build material layers over a build platform of a 3D printing device. The method continues with processing each layer according to an object model to form an object (block 504). During the forming of layers, material spreading parameters of the 3D printing device can be modified so that some layers within the object are spread using different material spreading parameters than other layers within the object (block 506). [0036] Referring now to the flow diagram of FIG. 6 (FIG. 6A), another example method 600 of forming layers of an object in a 3D printing device is shown. Method 600 comprises extensions of method 500 and incorporates additional details of method 500. Accordingly, method 600 begins at block 602 with forming build material layers over a build platform of a 3D printing device, and continues with processing each layer according to an object model to form an object (block 604). During the forming of layers, material spreading parameters of the 3D printing device are modified so that some layers within the object are spread using different material spreading parameters than other layers within the object (block 606). In some examples, modifying material spreading parameters includes modifying operating parameters of a roller spreading device (block 608). In some examples, modifying material spreading parameters includes modifying material spreading parameters between different layers of the object (block 610). In some examples, modifying material spreading parameters includes modifying material spreading parameters within a single layer of the object (block 612). In some examples, modifying material spreading parameters includes modifying the material spreading parameters gradually over a number of layers (block 614). The method 600 then continues from FIG. 6A to FIG. 6B.

[0037] As shown in the flow diagram of FIG. 6B, in some examples, modifying material spreading parameters can include spreading an initial number of layers with a spreading roller moving at first translational and rotational speeds, and spreading a remainder number of layers with the spreading roller moving at second translational and rotational speeds (block 616). In some examples, the second translational and rotational speeds are reduced speeds from the first translational and rotational speeds (block 618). In some examples, the first translational and rotational speeds are on the order of 10 inches per second and 10 rotations per second, respectively, and the second translational and rotational speeds are on the order of 3 inches per second and 4 rotations per second, respectively (block 620). In some examples, processing each layer includes printing a liquid functional agent onto each layer according to the object model, and applying energy to the layer.

[0038] Referring now to the flow diagram of FIG. 7, another example method 700 of forming layers of an object in a 3D printing device is shown. As shown at block 702, the method 700 can include forming powder layers over a build platform using a roller device in a build area of a 3D printing device. The method can continue with printing liquid functional agent onto areas of the powder layers to form multiple objects based on a 3D object model (block 704). During the forming of the powder layers, material spreading parameters of the roller device can be modified so that some layers within the objects are spread using different material spreading parameters than other layers within the objects (block 706). In some examples, modifying material spreading parameters includes modifying material spreading parameters within a single powder layer to provide first powder spreading parameters to spread a first portion of the single layer and second powder spreading parameters to spread a second portion of the single layer (block 708). In some examples, modifying material spreading parameters within a single powder layer comprises modifying material spreading parameters so that the first portion of the single layer forms part of a first object having a first property, and the second portion of the single layer forms part of a second object having a second property (block 710).