BACKGROUND

[0001] Additive manufacturing machines can produce three-dimensional (3D) objects by building up layers of build material. Data from a digital 3D object model can be processed into slices that define an area or areas of each layer of build material to be formed into an object layer. An object can be formed when the areas of build material from each layer are solidified according to the 3D object model. In some 3D printing devices, for example, inkjet printheads can selectively print (i.e., deposit) liquid functional agents such as fusing agents or liquid binding agents onto layers of build material within predefined areas that are to become layers of a 3D object. The liquid agents can facilitate the solidification of the build material within the printed areas.

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 a side view of an example 3D printer in which an example platform mover and related methods can provide precision vertical movement and horizontal angular tilt correction of a build platform;

[0004] FIG. 2a shows a top view of an example build platform and components of an example build platform mover;

[0005] FIG. 2b shows a side view of an example build platform and components of an example build platform mover; [0006] FIG. 3 shows a perspective view of a build platform with an illustration of the X-axis and Y-axis running along the platform;

[0007] FIGs. 4 and 5 show flow diagrams of example methods of moving a build platform in a 3D printer.

[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, 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 kinds of energy, such as ultra-violet light, can be applied to facilitate the solidification of the printed build material.

[0010] One of the challenges with such additive manufacturing processes is forming each material layer with an accurate thickness that is consistent across the whole layer. When build layers are formed with inaccurate and/or uneven layer thickness, the strength and quality of the resulting object can be reduced. A build cycle for building an object can include hundreds or thousands of layers of build material. For each layer, a build platform or platen can undergo a vertical downward displacement on the order of 40 - 120 microns, for example. The accuracy of thickness for each layer depends in part on how accurately the build platform can be lowered following the formation of a previous layer. The consistency of a layer’s thickness across the platform can depend on whether or not the platform experiences any tilting as it is lowered between the formation of each layer. In some example additive manufacturing machines, a platform mover or “lift”, can be controlled to lower the platform in fixed increments that control the thickness of each layer of build material. In such example machines, a drive system may be expected to lower the platform in fixed increments that are within a tolerance of a few microns for each layer. [0011] However, in some example machines and processes, vertical downward displacements of the build platform can be inaccurate, and there can be significant tilt or angular deviations of the platform from a horizontal orientation in two axes. Existing machines and processes use a single drive mechanism along with a linear or rotary encoder to create and control the displacement of the build platform. The drive mechanism can include a drive motor with an encoder, and the mechanism is often located near the center of the platform. In some examples there may also be additional guide rods to constrain rotational degrees of freedom of the platform. [0012] Because the encoder is often operatively mounted to the drive motor and not the build platform itself, however, the encoder provides information about the motor but not about the platform specifically. Information about displacement of the platform is inferred from the encoded motor data, and it therefore can be inaccurate. For example, backlash in the motor gears can cause the platform displacement information to be inaccurate. In addition, a build platform incrementally lowered downward in the Z-axis for each new build layer can tilt or rotate around both the X- axis and the Y-axis as the platform is lowered. Even small angles of rotation, such as 0.10 degrees of rotation, will create significant vertical displacements or offsets from nominal toward the outer edges of the platform. As additive manufacturing machines continue to scale up in size, vertical displacements from a tilted platform become more magnified with the corresponding increase in size of the build platforms, which creates the potential for even greater variations in object layer thickness.

[0013] There are various reasons that build platforms can tilt during a build process. For example, objects comprising build material treated with liquid agents have a higher density than raw build material (e.g., powder), and a solid object formed near a corner or edge of the platform can create an uneven load that causes the platform to tilt. In some examples, components used during and after a build process can create forces that cause the platform to tilt. For instance, some processes use tubes that provide air flow to extract loose powder through the bottom of the build platform. As the platform is lowered, changing forces from the tubing can act against the platform causing it to tilt. In some examples, a counter-rotating roller can translate across the platform to spread and compress powdered build material. The force from the roller as it moves across the platform can create a see-saw tilting effect in the platform. Still more causes of platform tilt can include component wear and/or tolerances in platform drive mechanisms. [0014] Accordingly, example platform movers and methods described herein provide for precision vertical displacement of build platforms including correction of platform angular tilt within 3D printing devices and other additive manufacturing systems. During an object build process in a 3D printing device, for example, precision vertical displacements of the build platform between the formation of each build layer can move the platform downward in a uniform manner that maintains the platform in a horizontally parallel or tilt-free orientation. The precision displacements ensure that each layer of build material formed on the platform has an accurate and uniform thickness across the entire platform surface. In some examples, making a precision displacement can include detecting if the platform is tilted and correcting for any detected platform tilt by making vertical adjustments to the platform at different points across the platform. An example platform mover can include multiple, independent lifting/lowering systems to support the platform at different points across the platform, and to move the platform vertically up and down in the Z-axis dimension. Each lift system contacts or is coupled to the platform at a different platform location (i.e. , underneath the platform). Each lift system is independently controllable to adjust the vertical position of the platform at its respective platform location so that unwanted tilt or horizontal angular rotation in the platform can be corrected prior to the formation of each object layer. Example platform movers can also include a multi-axis tilt sensor to detect angular rotation of the build platform about either or both of the X-axis and Y-axis dimensions. With each vertical downward movement of the build platform, prior to forming a new build material layer, a controller can monitor the multi-axis tilt sensor and determine when to make any vertical adjustments at the lift system platform locations to correct for detected angular rotation (i.e. , tilt) of the platform.

[0015] In some examples, a platform mover in a 3D printer includes multiple independent lift systems to lower and raise a build platform. The platform mover can also include a multi-axis tilt sensor to detect tilt in the build platform. The platform mover can include a controller to execute an instruction to lower the platform by a target amount, and to generate drive signals to control each lift system to lower the build platform by the target amount while correcting tilt detected by the multi-axis tilt sensor.

[0016] In some examples, a method of moving a build platform in a 3D printing device can include receiving an instruction to lower a build platform a target amount, and producing drive signals to control multiple independent lift systems to lower the build platform the target amount. The method can also include measuring an angle of tilt in the build platform, and adjusting a vertical position of at least one of the lift systems to reduce the angle of tilt.

[0017] In some examples, a 3D printer platform mover, includes three independently controllable support structures each coupled to a bottom surface of a build platform at a different one of three non-collinear platform locations. The platform mover includes a separate drive mechanism rotatably coupled to each support structure, where each drive mechanism is to raise and lower its respective support structure independent of the other drive mechanisms. The platform mover also includes a controller to execute instructions to lower all three support structures equally to achieve a target displacement, and to move the support structures unequally to maintain or orient the build platform in a tilt-free, horizontal orientation. [0018] FIG. 1 shows a block diagram of a side view of an example 3D printer 100 in which an example platform mover and related methods can provide precision vertical movement and horizontal angular tilt correction of a build platform. The example 3D printer 100 comprises a thermal fusion based 3D printer capable of forming a 3D object through an additive build process that generally includes spreading layers of build material over a build platform within a build area, printing a liquid fusing agent onto areas of the build material layers, and applying fusing energy to the build material layers to fuse together the areas of printed build material, forming the 3D object. While some components of an example 3D printer 100 are shown in FIG. 1 and described herein, the example 3D printer 100 may comprise additional components and may perform additional functions not specifically illustrated or discussed herein. Thus, the 3D printer 100 is shown by way of example, and it is not intended to represent a complete 3D printing system.

[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 area 104 is enclosed within a build box 108 having walls that surround the build platform 102 to contain build material 109 on 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 view of other components, objects, and materials, inside the box 108. [0020] The build platform 102 can move in a vertical direction (i.e. , up and down according to direction arrow 111 ) along the Z-axis, as further discussed below. During a build process, a build material distributor 110 can translate back and forth along the X-axis (as indicated by direction arrow 112) forming layers of build material on the build platform 102. The build material distributor 110 can include, for example, a powder supply and a powder spreading mechanism such as a roller or blade (not specifically shown) to move across the build platform 102 to spread layers of build material.

[0021] A liquid agent dispenser 114 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 build material layer that has been spread over the build platform 102. A liquid agent dispenser 114 can include, for example, a printhead or printheads, such as thermal inkjet or piezoelectric inkjet printheads. In some examples, a printhead liquid agent dispenser 114 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 112) as it ejects liquid droplets onto a build material layer. In some examples, a printhead dispenser 114 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.

[0022] The example 3D printer 100 can also include a thermal energy source 116 such as a thermal radiation source. A thermal radiation source 116 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 116 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 116 can include a thermal radiation module comprising a thermic light lamp, such as 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.

[0023] An example build platform mover 118 (shown in FIG. 1 within dashed line 118) can comprise a number of components. FIGs. 2a and 2b show a top view and side view, respectively, of an example build platform 102 and the components of an example build platform mover 118. The build platform in FIG. 2a is shown as transparent in order to provide a view of some of the components of the build platform mover 118 that are in contact with the underside of the platform 102. Referring generally to FIGs. 1 , 2a, and 2b, the components of an example build platform mover 118 can include multiple independent lift systems to lower and raise the build platform 102. Each independent lift system can include a support structure 120 and an independent drive mechanism 122 associated with a respective support structure 120. A controller 124 can control the drive mechanisms to move the support structures 120 in order to move and level the build platform 102. An example support structure 120 can include a leadscrew 120 to be driven up and down by a drive mechanism 122. An example drive mechanism 122 can include a drive nut with a motor (not separately illustrated) controlled by controller 124 to rotate the drive nut and move the support structure up and down. While three support structures 120 are shown in the examples in FIGs. 1 , 2a, and 2b, each with a respective independent drive mechanism 122, in other examples a different number of multiple support structures may be suitable. Examples of different numbers of support structures 120 that may be suitable include two support structures and four support structures.

[0024] Controller 124 generally represents processing and memory resources, programming, electronic circuitry, and other components for controlling various functions of the example 3D printer 100, including spreading build material layers onto the build platform 102, selectively delivering liquid agents onto areas of build material layers, applying thermal energy to build material layers, controlling components of the build platform mover 118 to move the platform 102 vertically and to correct angular rotation or tilt in the platform, and so on. In some examples, the controller 124 can execute programming instructions to lower the build platform 102 by a fixed, target amount, and convert the instructions into drive signals to control each of the independent drive mechanisms 122. In some examples, the controller 124 can execute programming instructions to monitor a linear encoder and determine from the encoder if the build platform 102 has been lowered a target amount. In some examples, the controller 124 can execute programming instructions to monitor a multi axis tilt sensor and determine from the sensor if the build platform 102 has any angular tilt or rotation about the X-axis or Y-axis, and further to send drive signals to one or multiple of the independent drive mechanisms 122 to correct any such angular tilt in the build platform 102.

[0025] The three independently driven support structures 120 shown in the examples of FIGs. 1 , 2a, and 2b, are coupled to the bottom surface of the build platform 102 at three non-collinear platform locations, A, B, and C. The platform 102 therefore corresponds with a plane determined at the non-collinear platform locations, A, B, and C, and the independently driven support structures 120 can move the platform locations A, B, and C, independently in equal or non-equal vertical distances (i.e. , in the Z-axis). When the platform is in a horizontal orientation (i.e., not tilted), for example, an equal or uniform amount of displacement by each of the three support structures 120 at platform locations A, B, and C, moves the platform vertically along the Z-axis and maintains the horizontal orientation of the platform. However, with additional reference to FIG. 3, if the platform becomes tilted with an angular rotation around the X-axis or Y-axis, the support structures 120 can be controlled to vertically move the three platform locations A, B, and C, in a manner that removes the tilt and restores the platform to a horizontal orientation. As shown in FIG. 3, an angular tilt or rotation of the platform 102 around the X-axis and Y-axis can be referred to, respectively, as theta-X and theta-Y. FIG. 3 shows a perspective view of the build platform 102 with a representation of the X-axis and Y-axis running along the platform 102. While the example build platform mover 118 itself is not specifically shown in FIG. 3, the three non-collinear platform locations, A, B, and C, where support structures 120 contact the bottom surface of the build platform 102 are shown with dashed lines.

[0026] Referring now generally to FIGs. 1 , 2a, 2b, and 3, the example build platform mover 118 additionally comprises sensors to detect and measure the vertical displacement of the platform 102 as well as any theta-X and theta-Y angular rotation or tilt of the platform around the X-axis and Y-axis. In some examples, the build platform 102 can be encoded with a linear encoder 126 mounted to the platform 102 near its center point to detect an amount of vertical displacement of the platform in the Z-axis. The controller 124 can read the linear encoder signal and determine from the encoder position if the vertical displacement of the platform 102 matches a target amount of displacement. Examples of a linear encoder 126 can include an optical encoder, magnetic encoder, inductive encoder, and capacitive encoder. In some examples, the build platform mover 118 can include a multi-axis tilt sensor 128 mounted to the build platform 102. In some examples, a multi-axis tilt sensor 128 can measure angular tilt (i.e. , theta-X and theta-Y) in the platform with respect to the direction of gravity. The controller 124 can read the tilt sensor 128 signal and determine if the measured theta-X and theta-Y angles are below a given threshold value. The controller 124 can produce drive signals to control the independent drive mechanisms 122 to correct for angular tilt by reducing theta-X and theta-Y angles to within the threshold value.

[0027] While the multi-axis tilt sensor 128 is shown and discussed as a single, gravitational based device, other examples of a multi-axis tilt sensor 128 are possible and contemplated herein. For example, a multi-axis tilt sensor 128 can comprise multiple displacement sensors such as linear encoders, where one encoder is mounted to each of the multiple support structures 120. The controller 124 can read each linear encoder signal and determine the vertical position of each of the platform locations, A, B, and C. Based on the vertical position of each of the platform locations, A, B, and C, the controller can determine theta-X and theta-Y angles of the platform 102, and whether the theta-X and theta-Y angles are below a given threshold value as discussed above. In another example, a multi-axis tilt sensor 128 can comprise multiple distance sensors mounted at fixed positions over different points of the build platform 102. An example of a distance sensor includes a laser sensor that can measure the distance from its fixed location to the top surface of a build material layer on the build platform 102. The controller 124 can read the measured distance of each distance sensor from the top surface of the build material layer and determine theta- X and theta-Y angles of the platform 102, and whether the theta-X and theta-Y angles are below a given threshold value as discussed above.

[0028] FIGs. 4 and 5 show flow diagrams of example methods 400 and 500 of moving a build platform in a 3D printer. Method 500 comprises extensions of method 400 and incorporates additional details of method 400. Methods 400 and 500 are associated with examples discussed above with regard to FIGs. 1 , 2a, 2b, and 3, and details of the operations shown in methods 400 and 500 can be found in the related discussion of such examples. The operations of methods 400 and 500 may be embodied as programming instructions stored on a non-transitory, machine-readable (e.g., computer/processor-readable) medium, such as a memory component of controller 124 shown in FIG. 1. In some examples, implementing the operations of methods 400 and 500 can be achieved by a processor of controller 124 reading and executing programming instructions stored in a memory. In some examples, implementing the operations of methods 400 and 500 can be achieved using an ASIC and/or other electronic circuitry components of controller 124 alone or in combination with programming instructions executable by a processor of controller 124.

[0029] The methods 400 and 500 may include more than one implementation, and different implementations of methods 400 and 500 may not employ every operation presented in the respective flow diagrams of FIGs. 4, and 5. Therefore, while the operations of methods 400 and 500 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 500 might be achieved through the performance of a number of initial operations, without performing other subsequent operations, while another implementation of method 500 might be achieved through the performance of all of the operations. [0030] Referring now to the flow diagram of FIG. 4, an example method 400 of moving a build platform in a 3D printer begins at block 402 with receiving an instruction to lower a build platform a target amount. The method continues with producing drive signals to control multiple independent lift systems to lower the build platform the target amount (block 404). The method also includes measuring an angle of tilt in the build platform (block 406) and adjusting a vertical position of at least one lift system to reduce the angle of tilt (block 408).

[0031] Referring now to the flow diagram of FIG. 5, another example method 500 of moving a build platform in a 3D printer is shown. Method 500 comprises extensions of method 400 and incorporates additional details of method 400. Accordingly, method 500 begins at block 502 with receiving an instruction to lower a build platform a target amount, and continues with producing drive signals to control multiple independent lift systems to lower the build platform the target amount (block 504), measuring an angle of tilt in the build platform (block 506), and adjusting a vertical position of at least one lift system to reduce the angle of tilt (block 508). [0032] After reducing the angle of tilt in the platform, the method 500 can continue with measuring a vertical position of the build platform (block 510), determining from the vertical position that the build platform is not lowered the target amount (block 512), and producing additional drive signals to control the multiple independent lift systems to lower the build platform the target amount (block 514). In some examples, measuring a vertical position of the build platform includes reading a linear encoder mounted to the build platform (block 516). In some examples, measuring an angle of tilt includes measuring theta-X and theta-Y angles of tilt in the build platform, and reducing the angle of tilt includes comparing the theta-X and theta- Y angles to a threshold angle and adjusting a vertical position of at least one lift system to reduce the theta-X and theta-Y angles below the threshold angle (block 518). In some examples, measuring an angle of tilt includes measuring the distances from multiple fixed vertical positions above the build platform to a top surface of a layer of build material, and from the measured distances, then determining the theta-X and theta-Y angles of tilt in the build platform (block 520). The method also includes forming a layer of build material on the build platform (block 522), and controlling the multiple independent lift systems to lower the build platform again by the target amount (block 524).