Patent Publication Number: US-11648732-B2

Title: Indexing in 3D printing

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/097,226, filed Oct. 26, 2018, which is a 371 application of PCT Application No. PCT/US2016/043718, filed Jul. 22, 2016. The contents of both U.S. application Ser. No. 16/097,226 and PCT Application No. PCT/US2016/043718 are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     Additive manufacturing processes can produce three-dimensional (3D) objects by providing a layer-by-layer accumulation and unification of material patterned from a digital model. In 3D printing, for example, digitally patterned portions of successive material layers can be joined together by fusing, binding, or solidification through processes including melting, sintering, extrusion, and irradiation. The quality, strength, and functionality of objects produced by such systems can vary depending on the type of additive manufacturing technology used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples will now be described with reference to the accompanying drawings, in which: 
         FIG.  1    shows a perspective view of an example 3D printing system suitable for implementing a multiple pass indexing method that enables more than one nozzle to print a liquid agent over a region of a build platform; 
         FIG.  2    shows a bottom view of an example of a print bar suitable to provide platform-wide printing of a liquid agent onto a layer of powder on a build platform; 
         FIG.  3    shows an example printhead die that includes eight rows of nozzles; 
         FIG.  4   a    shows an example of multiple pass 3D printing with print bar indexing; 
         FIG.  4   b    shows an example of multiple pass 3D printing with build platform indexing; 
         FIG.  5    shows an example of several layers of a 3D object to demonstrate the effect of single pass indexing; 
         FIGS.  6   a  and  6   b    show examples of different multiple pass indexing schemes that may be suitable for multiple pass 3D printing and indexing in a 3D printing system; 
         FIGS.  7  and  8    are flow diagrams showing example methods of printing a 3D object. 
     
    
    
     Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. 
     DETAILED DESCRIPTION 
     In some examples of three-dimensional (3D) printing, 3D objects can be produced in a 3D printing system (i.e., a 3D printer) by depositing and processing layers of build material, such as layers of powdered nylon, or polyamide. Each layer of build material (i.e., powder) can be deposited and processed on a build platform within a system work space. The build platform, sometimes referred to as a powder bed, can be moved vertically downward to increase the height of the work space as additional layers of powder are deposited and processed. Processing can include the selective application of a liquid agent onto layers of the powder in areas where the powder is to be fused together. For example, a liquid fusing agent can be applied to cover a cross-sectional area of a 3D object being printed, according to a 3D digital model. The fusing agent can coat the exterior surface of the powder and penetrate into a layer of powder. Processing can also include exposing the powder to a fusing energy such as visible light radiation, infrared (IR) radiation, and ultraviolet radiation. The fusing agent deposited onto the build powder can absorb the radiation and convert it into thermal energy. The thermal energy can fuse (i.e., melt and coalesce) those areas of the powder to which the fusing agent has been applied. This process can be repeated with each layer of powder deposited into the work space until each cross-sectional area is fused together to form a 3D object. 
     In some examples, 3D printing systems can implement inkjet technology to deposit the liquid fusing agent onto the layers of build powder. For example, a liquid agent dispenser can include a drop-on-demand printhead that can be scanned over the build platform to selectively deliver a fusing agent or other liquid onto a powder bed. Printheads can include, for example, thermal inkjet or piezoelectric inkjet printheads that have arrays of liquid ejection nozzles to jet the liquid agents onto the powder. In some examples, multiple printheads can be aligned end-to-end along the length of a print bar to enable a page-wide, or platform-wide, coverage of the powder bed through a single scan of the print bar over the build platform. 
     3D printers that include powder beds and liquid jetting systems with scanning printheads are susceptible to various nozzle oriented defects that can result in reduced quality in printed objects, such as reduced surface color quality and reduced part strength. For example, printhead nozzles can become blocked from airborne powder, other ambient debris, and/or dried agents. Other defects can include nozzles with drop ejection directionality differences, nozzles with drop-weight and drop-shape differences, and nozzles with differences in colorant concentration. In some examples, color concentration differences between nozzles can result from temperature variation across a single printhead die during printing, and/or die-to-die variations when multiple die are printing from a print bar, for example. In other examples, print bars with multiple aligned dies can exhibit die stitching defects that can cause irregular print patterns. 
     The use of redundant nozzles in a printhead can help to remedy some nozzle oriented defects. However, because printheads can have many thousands of nozzles, adding nozzle redundancy can increase the cost of a printing system considerably. In addition, the use of redundant nozzles can include examining the performance of each nozzle on a printhead to detect which nozzles are defective, and then employing redundant nozzles to remedy the defective nozzles. In addition to the added costs associated with detecting defective nozzles, the additional time expended between printhead scanning cycles to examine thousands of nozzles can have a noticeable adverse impact on printing speed. 
     Accordingly, in some examples described herein, a multiple pass indexing 3D printer enables printing a 3D object by scanning a printhead, or print bar, over a build platform multiple times and in different indexed positions in order to deposit a liquid agent onto a layer of build powder. As used herein, ‘printhead’ can refer to an elongated print bar having multiple printhead die aligned generally end-to-end to provide a fixed array of printhead nozzles that can cover an entire width of a print zone, such as the width of a powder bed. Indexing the printhead between multiple passes in an orthogonal direction relative to the scanning/printing direction of the printhead enables more than one nozzle to print over a region of the build platform. Regions that may have been missed or misprinted on a first pass by defective nozzles can be covered by different nozzles on a subsequent pass. In some examples, single pass indexing can be implemented where the printhead is indexed between powder layers after a single pass per each layer. Single pass indexing can decrease the time to print each layer as indexing the printhead orthogonally and translating the printhead back to a start position can both occur while a next layer of powder is being deposited onto the build platform. 
     Multiple printhead passes at different indexed positions can remedy defects such as missing nozzles, nozzles with drop ejection directionality differences, nozzles with drop-weight and drop-shape differences, nozzles with color concentration differences, and die stitching defects. Each pass of the printhead can print a similar or different loading of agents. The process can be repeated multiple times per each layer of build powder while the printhead is in a different orthogonal offset for each pass. In some examples, instead of indexing the printhead, the build platform can be indexed. Indexing the printhead and/or build platform in this manner can reduce costs associated with using physical redundant nozzles and improve build speed and throughput by reducing down time that may otherwise be expended for detecting nozzle defects and servicing the printhead. This solution may additionally reduce costs associated with color calibrations and scanner alignments. 
     In a particular example, a method of printing a 3D object includes scanning, or moving, a print bar in a first direction over a build platform of a 3D printer to deposit a liquid agent onto a layer of build powder. After scanning in the first direction, the print bar is indexed in a second direction that is substantially orthogonal to the first direction. The print bar is then scanned back over the build platform in a third direction that is opposite of the first direction to deposit additional liquid agent onto the layer of build powder. 
     In another example, a non-transitory machine-readable storage medium stores instructions that when executed by a processor of a three-dimensional (3D) printer cause the 3D printer to apply a layer of build powder onto a build platform of a 3D printer. The printer can deposit a liquid agent onto the powder with multiple passes of a print bar over the platform. During a first pass, the print bar can be passed over the platform with the print bar and platform in a first relative position to one another. After the first pass, the print bar and platform can be indexed relative to one another to put the print bar and platform into a second relative position to one another. During a second pass, the print bar can pass over the platform with the print bar and platform in the second relative position. 
     In another example, a device for printing 3D objects includes a build platform to receive build powder. The device also includes a print bar to scan back and forth over the platform in multiple passes while selectively depositing a liquid agent onto the build powder. A motorized indexing arm is coupled to the print bar to index the print bar after each pass of the print bar over the platform. The indexing arm indexes the print bar in a direction orthogonal to the scanning direction of the print bar during each pass. 
       FIG.  1    shows a perspective view of an example three-dimensional (3D) printing system  100  suitable for implementing a multiple pass indexing method that enables more than one nozzle to print a liquid agent over a region of a build platform. The example printing system  100  includes a moveable printing platform  102 , or build platform  102  that can serve as a floor to a work space  104  in which a 3D object (not shown in  FIG.  1   ) can be printed. The work space  104  can include fixed walls  105  (illustrated as front wall  105   a , side wall  105   b , back wall  105   c , side wall  105   d ) around the build platform  102 . The fixed walls  105  and platform  102  can contain a volume of powdered build material deposited layer by layer into the work space  104  during printing of a 3D object. For purposes of this description and to help illustrate different elements and functions of the 3D printing system  100 , the front wall  105   a  of the work space  104  is shown as being transparent. During printing, a build volume within the work space  104  can include all or part of a 3D object formed by layers of powder that are processed with the application of liquid fusing agent and fusing energy (e.g., radiation). The build volume can also include non-processed powder that surrounds and supports the 3D object within the work space  104 . 
     The build platform  102  is moveable within the work space  104  in an upward and downward direction as indicated by up arrow  106  and down arrow  108 , respectively. When printing of a 3D object begins, the build platform  102  can be located in an upward position toward the top of the work space  104  as a first layer of powdered build material is deposited onto the platform  102  and processed. After a first layer of powder has been processed, the platform  102  can move in a downward direction  108  as additional layers of powdered build material are deposited onto the platform  102  and processed. 
     The example 3D printing system  100  includes a supply of powdered build material  110 , or powder. The build material, alternately referred to herein as “powder”, can comprise powdered material made from various materials that are suitable for producing 3D objects. Such powdered materials can include, for example, polymers, glass, ceramics (e.g., alumina, Al 2 O 3 ), Hydroxyapatite, metals, and so on. The printing system  100  can feed powder from the supply  110  into the work space  104  using a spreader  112  to controllably form the powder into layers over the build platform  102 , and/or over other previously deposited layers of powder. A spreader  112  can include, for example, a roller, a blade, or another type of material spreading device. Although not illustrated, in some examples a carriage can be associated with the powder supply  110  and/or powder spreader  112  to convey the supply and spreader over the build platform  102  during the forming of a layer of powder onto the platform. 
     The example 3D printing system  100  also includes a liquid agent dispenser  114 . While other types of liquid dispensers are possible, the example dispenser  114  shown and described herein comprises a drop-on-demand printhead  114  that can be scanned, or moved, over the build platform  102  to selectively deliver a fusing agent or other liquid onto a powder bed. Examples of drop-on-demand printheads include thermal inkjet and piezoelectric inkjet printheads that comprise an array of liquid ejection nozzles. In some examples, the printhead  114  has a length dimension that enables it to span the full depth  116  of the build platform  102 . Thus, a printhead  114 , alternately referred to herein as a print bar  114 , can enable a page-wide or platform-wide coverage of the powder bed through a single scan of the print bar over the build platform  102 . In some examples, a 3D printing system  100  can include more than one print bar  114 . 
       FIG.  1    shows an example of the scanning motion (illustrated by direction arrow  120 ) of the print bar  114 . In some examples a carriage (not shown) can be associated with the print bar  114  to convey the print bar  114  over the build platform  102  during the application of a liquid agent onto a layer of powder on the platform  102 . In some examples the print bar  114  can be coupled to a conveyor  140  that can be controlled to scan the print bar  114  over the platform  102 , as illustrated by the dashed-line print bar representation  122 . As discussed in more detail below, the print bar  114  can be scanned back and forth over the platform  102  in different indexed positions. Although not shown in the example of  FIG.  1   , during printing a portion of a 3D object would be present within the work space  104  as the print bar  114  scans over the work space and ejects droplets  124  of a fusing agent or other liquid. 
       FIG.  2    shows a bottom view of an example of a print bar  114  suitable to provide platform-wide printing of a liquid agent onto a layer of powder on the build platform  102 . Platform-wide printing is enabled in part, by the print bar  114  having multiple printhead die  117  positioned in parallel along the length  121  of the print bar  114  in an end-to-end alignment  119 . As shown in the blow up view of  FIG.  2   , the ends of the multiple printhead die  117  can be arranged in an overlapping alignment of nozzles to help provide a seamless printing transition between the multiple die. With a continuous array of nozzles  115  spanning its length  121 , the print bar  114  can scan over the full width  118  and depth  116  of the build platform  102  as the nozzles jet droplets  124  of a fusing agent, colorant, or other liquid onto layers of powder within the work space  104 . The bottom view of the print bar  114  shown in  FIG.  2    is provided for the purpose of illustrating an example arrangement of printhead die  117  and nozzles  115  on the bottom side of the print bar, while the print bar  114  ( 122 ) in  FIG.  1    is shown from a top perspective view with the nozzles  115  facing downward to eject liquid agent droplets  124  over the build platform  102 . 
     While the example printhead die shown in  FIG.  2    include two rows of nozzles  115 , other nozzle configurations on a printhead die are possible and contemplated.  FIG.  3    shows an example printhead die  117  that includes eight rows  123  of nozzles. Such an arrangement can enable multiple liquid agents, such as different ink colors and/or different fusing agents, to be applied to a powder layer in a single pass over the build platform  102 . In some examples, each of the two pairs of adjacent rows of nozzles  115  can be associated with a different fluid slot (not shown) formed in the substrate of the die  117 . Each fluid slot can supply a different liquid agent to nozzles in an associated pair of adjacent rows of nozzles. 
     Examples of liquid agents suitable for ejection from nozzles in a print bar  114  can include water-based dispersions comprising a radiation absorbing agent. The radiation absorbing agent can comprise, for example, an infrared (IR) radiation absorber, a near infrared radiation absorber, an ultraviolet radiation absorber, or a visible light absorber. In some examples, a fusing agent can be an ink-type formulation as the radiation absorbing agent. In some examples, a fusing agent can be an ink or other liquid that absorbs energy in the IR spectrum but reflects energy in the visible light spectrum. Dye based and pigment based colored inks are examples of inks that include visible light absorbing agent. 
     As shown in  FIG.  1   , the example 3D printing system  100  also includes a fusing energy source such as radiation source  126 . The radiation source  126  can be implemented in a variety of ways including, for example, as a curing lamp or as light emitting diodes (LEDs) to emit IR, near-IR, UV, or visible light, or as lasers with specific wavelengths. The radiation source  126  can depend in part on the type of fusing agent and/or powder being used in the printing process. In different examples, the radiation source  126  can be attached to a carriage (not shown) to be scanned across the work space  104 . The radiation source  126  can apply radiation R to layers of powder in the work space  104  to facilitate the heating and fusing of the powder. In some examples, a fusing agent  124  can be selectively applied by print bar  114  to a layer of powder to enhance the absorption of the radiation R and help convert the absorbed radiation into thermal energy. In areas where fusing agent has been applied to the powder, the absorbed radiation can heat the powder sufficiently to cause fusing of the powder. 
     Referring still to  FIG.  1   , the example 3D printing system  100  additionally includes an example controller  128 . The controller  128  can control various operations of the printing system  100  to facilitate the printing of 3D objects as generally described above, such as spreading powder into the work space  104 , selectively applying fusing agent  124  to portions of the powder, and exposing the powder to radiation R. In addition, as described in more detail below, the controller  128  can control the 3D printing system  100  to perform multiple passes of the print bar  114  over the build platform  102  in different indexed positions to deposit a liquid agent onto the powder. Indexing the print bar  114  and/or build platform  102  to different positions between each print bar pass enables more than one nozzle to print over a region of the build platform  102  and helps to provide coverage of regions that may have been missed or misprinted by a defective nozzle. 
     As shown in  FIG.  1   , 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 various 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, magnetic tape, 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), 3MF formatted data, and other data and/or instructions executable by a processor  130  of the 3D printing system  100 . 
     An example of executable instructions to be stored in memory  132  include instructions associated with a build module  134  and an indexing module  136 , while examples of stored data can include object data  138 . In general, modules  134  and  136  include programming instructions executable by processor  130  to cause the 3D printing system  100  to perform operations related to printing 3D objects within a work space  104 , including performing indexing a print bar  114  and/or build platform  102  between multiple print bar  114  passes over the platform  102 . Such operations can include, for example, the operations of methods  700  and  800 , described below with respect to  FIGS.  7  and  8   , respectively. 
     In some examples, controller  128  can receive object data  138  from a host system such as a computer. Object data  138  can represent, for example, object files defining 3D object models to be produced on the 3D printing system  100 . Executing instructions from the build module  134 , the processor  130  can generate print data for each cross-sectional slice of a 3D object model from the object data  138 . The print data can define, for example, each cross-sectional slice of a 3D object model, the liquid agents to be used to cover the build powder within each cross-sectional slice, and how fusing energy is to be applied to fuse each layer of powder. The processor  130  can use the print data to control components of the printing system  100  to process each layer of powder. Thus, the object data can be used to generate commands and/or command parameters for controlling the distribution of build powder from a supply  110  onto the build platform  102  by a spreader  112 , the application of fusing agents by a print bar  114  onto layers of the powder, the application of radiation by a radiation source  126  to the layers of powder, and so on. 
     The indexing module  136  includes further executable instructions to enable a processor  130  to control the 3D printing system  100  to perform multiple passes of the print bar  114  over the build platform  102  in different indexed positions to deposit a liquid agent onto a single or same layer of powder. More specifically, indexing module instructions can execute to control a conveyor  140  to scan the print bar  114  back and forth across the platform  102  to apply a liquid agent onto a powder layer in multiple passes. The instructions can further control a motorized print bar indexing arm  142  to index the print bar in directions that are substantially orthogonal to the scanning direction of the print bar  114 . The instructions can also control print data to adjust the distribution of liquid agent among individual nozzles and/or rows of nozzles. For example, because indexing the print bar  114  and/or build platform  102  to different positions between print bar passes enables more than one nozzle to print over a region of the build platform  102 , the print data that controls the color, type, and/or loading of liquid agent to be deposited over that region can be shifted accordingly to different nozzles and/or rows of nozzles to correspond with the direction of indexing. For example, indexing the print bar  114  in a +Y direction (e.g., see description of  FIG.  4   a   ) may result in print data being shifted to nozzles or rows of nozzles that are in the −Y direction on the print bar  114 . Indexing of the print bar  114  can occur at different times, such as before or after a first pass, before or after a second pass, and so on. In some examples, instead of indexing the print bar  114 , the indexing module instructions can control indexing of the build platform  102 . Thus, the indexing module instructions can control a motorized platform indexing arm  144  to index the build platform  102  in directions that are substantially orthogonal to the scanning direction of the print bar  114 . 
       FIG.  4   a    shows an example of multiple pass 3D printing with print bar indexing.  FIG.  4   a    shows several top down views of an example build platform  102  during multiple passes of a print bar  114  over the platform  102  to deposit a liquid agent onto a powder layer of a 3D object  146 . The print bar  114  in  FIG.  4   a    is shown as transparent in order to illustrate the arrangement of printhead die  117  aligned on the bottom side of the print bar  114 . In view (a) of  FIG.  4   a   , the print bar  114  begins on the right side of the platform  102  in a first Y-coordinate indexed position with respect to the illustrated XY coordinate plane  148 . In a first pass, the print bar  114  is scanned over the build platform  102  in a first direction  150  from the right side to the left side of the platform  102  (i.e., in a −X direction) to print a liquid agent onto a layer of a 3D object  146 . As shown in view (a) of  FIG.  4   a   , in some examples a print defect  152  can occur due to one or multiple defective nozzles on the print bar  114 . The print defect  152  is illustrated as a white line to indicate a region of the build platform  102  or powder layer that was not printed on with liquid agent. 
     As shown in view (b) of  FIG.  4   a   , when the print bar  114  completes its first pass over the build platform  102 , the printing system  100  indexes the print bar  114  in the +Y direction in preparation for a second pass over the platform  102 . In some examples, the system can index the print bar  114  in −Y direction. In any case, the indexing direction  154  of the print bar  114  comprises a second direction  154  of movement that is substantially orthogonal to the scanning or printing direction of the print bar  114 . Indexing the print bar  114  in the second direction  154  moves the print bar  114  to a second Y-coordinate indexed position (i.e., in XY coordinate plane  148 ) in preparation for a second pass over the platform  102 . 
     In some examples, the indexing offset amount, or distance, of print bar movement in the Y direction can be on the order of one half the length of a printhead die  117 , to one full length of a printhead die  117 . Other indexing distances are also possible. In general, the index offset distance can be a minimum distance that moves nozzles far enough on the 3D object  146  that print defects from defective nozzles on a first pass can be remedied on a subsequent pass, and so defects from the subsequent pass will be less apparent. As described below with respect to  FIG.  6   , different indexing schemes that define varying indexing directions and distances, as well as varying multiple print bar pass patterns, can be implemented and controlled through executable instructions from indexing module  136 . 
     As shown in view (c) of  FIG.  4   a   , after the print bar  114  is indexed in the (+) Y direction to a second indexed position, the print bar  114  is then scanned in a third direction  156  over the build platform  102  to deposit a liquid agent onto the same layer of the 3D object in a second pass. The third direction  156  is opposite the first direction  150 . As shown by the object  146  in view (c), the print defect  152  from defective nozzles on the first pass has been resolved by working nozzles that have applied liquid agent to the region of the build platform  102  that was not printed on in the first pass. In some examples, the print bar  114  can then be indexed in the −Y direction back to the first Y-coordinate position (i.e., the starting position) in preparation to print onto another layer of powder added to the 3D object  146 . Alternatively, the process can be repeated multiple times on the same layer of the 3D object  146 , with the print bar  114  being indexed to a different orthogonal offset per pass. 
     In some examples, rather than indexing the print bar  114 , the printing system  100  may instead index the build platform  102  in the positive (+) or negative (−) Y directions.  FIG.  4   b    shows an example of multiple pass 3D printing with build platform indexing.  FIG.  4   b    shows several top down views of an example build platform  102  during multiple passes of a print bar  114  over the platform  102  to deposit a liquid agent onto a powder layer of a 3D object  146 . The print bar  114  in  FIG.  4   b    is shown as transparent in order to illustrate the arrangement of printhead die  117  aligned on the bottom side of the print bar  114 . In view (a) of  FIG.  4   b   , the print bar  114  begins on the right side of the platform  102  with the build platform  102  in a first Y-coordinate indexed position with respect to the illustrated XY coordinate plane  148 . 
     In a first pass, the print bar  114  is scanned over the build platform  102  in a first direction  158  from the right side to the left side of the platform  102  (i.e., in a −X direction) to print a liquid agent onto a layer of a 3D object  146 . As shown in view (a) of  FIG.  4   b   , in some examples a print defect  152  can occur due to one or multiple defective nozzles on the print bar  114 . The print defect  152  is illustrated as a white line to indicate a region of the build platform  102  or powder layer that was not printed on with liquid agent. 
     As shown in view (b) of  FIG.  4   b   , when the print bar  114  completes its first pass over the build platform  102 , the printing system  100  indexes the build platform  102  in the negative −Y direction in preparation for a second pass of the print bar  114  over the platform  102 . In some examples, the system can index the platform  102  in a positive +Y direction. The platform indexing direction  160  is substantially orthogonal to the scanning or printing direction of the print bar  114 . Indexing the build platform  102  moves the platform  102  to a second Y-coordinate indexed position in preparation for a second pass of the print bar  114  over the platform  102 . 
     The index offset used when indexing the build platform  102  can be similar to the index offset used when indexing the print bar  114 . As noted above, in some examples the index offset can be on the order of one half the length of a printhead die  117 , to one full length of a printhead die  117 , with other offset values being possible. Indexing the build platform  102  shifts the 3D object  146  with respect to nozzles on the print bar  114  so that different nozzles will be positioned to cover regions where there may be print defects caused by defective nozzles on the first pass. 
     As shown in view (c) of  FIG.  4   b   , after the build platform  102  is indexed in the −Y direction to a second indexed position, the print bar  114  is then scanned back over the platform  102  in a direction  162  opposite the first scan direction  158  to deposit a liquid agent onto the same layer of the 3D object in a second pass. As shown by the object  146  in view (c), the print defect  152  from defective nozzles on the first pass has been resolved by working nozzles that have applied liquid agent to the region of the build platform  102  that was not printed on in the first pass. In some examples, the build platform  102  can then be indexed in the +Y direction back to the first Y-coordinate position (i.e., the starting position) in preparation to print onto another layer of powder added to the 3D object  146 . Alternatively, the process can be repeated multiple times on the same layer of the 3D object  146 , with the platform  102  being indexed to a different orthogonal offset per pass. 
     As mentioned above, in some examples single pass indexing can be implemented where the printhead is indexed between powder layers after a single pass per each layer.  FIG.  5    shows an example of several layers of a 3D object to demonstrate the effect of single pass indexing. In part (a) of  FIG.  5   , each layer n, n+1, and n+2, has been printed with a single pass of the print bar  114 , but without indexing in between layers. Each of three layers n, n+1, and n+2, in  FIG.  5   , shows a missing region  170  (illustrated as regions  170   a ,  170   b ,  170   c ) where a defective nozzle or nozzles missed printing onto the layer. It is apparent in part (a) of  FIG.  5   , how the missing regions  170  are lined up due to no indexing. In part (b) of  FIG.  5   , the print bar  114  has been indexed by a distance  172  in between each layer. The missing regions  170  in part (b) of  FIG.  5    are separated horizontally by the indexing distance  172 . While the missing region  170   c  in layer n+2 appears to be empty, the missing region  170   b  is darkened, which indicates it has been partially or fully fused by the fusing of above layer n+2 and by heat conducting from layer n. Furthermore, the missing region  170   a  in layer n has been partially or fully fused by the fusing of layer n+1 directly above. Objects printed with single pass printing will have greater strength if indexing between the layers is implemented. This is because regions that do not get printed on due to a nozzle defect can be partially or fully fused due to energy applied to the next layer or heat conducting from the previous layer. 
     As mentioned above, executable instructions from an indexing module  136  can control the implementation by the printing device  100  of a variety of different indexing schemes. Different indexing schemes can define varying indexing directions and indexing offsets of the print bar  114  and/or the build platform  102 , as well as the number and direction of multiple passes to be made over the platform  102  by the print bar  114 .  FIGS.  6   a  and  6   b    show some examples of different multiple pass indexing schemes that may be suitable for multiple pass 3D printing and indexing in a 3D printing system  100 . The schemes will be described in terms of indexing the print bar  114  rather than indexing the build platform  102 . However, the described schemes may apply in the same or similar manner when indexing the platform  102  as they do when indexing the print bar  114 . While several schemes are illustrated and described, it should be apparent that many other indexing schemes are possible and contemplated herein. 
     Referring to  FIGS.  6   a  and  6   b   , several examples of multiple pass indexing schemes (a)-(g) are illustrated using direction arrows to indicate the XY movements within an XY coordinate plane  148  of a print bar  114  over a build platform  102  in a 3D printing system  100 . Different Y-coordinate index positions of the print bar  114 , such as a first index position and a second index position, are illustrated using circled numbers. For example, a circle with a number one indicates a first index position, and so on. Each scheme indicates multiple print bar passes and indexes that are to be made while printing or depositing liquid agent onto a single layer of powder of a 3D object. The print bar  114 , build platform  102 , and printing system  100  are not shown in  FIGS.  6   a    and  6   b.    
     Referring to  FIG.  6   a   , in an example multiple pass indexing scheme (a), a first powder layer of a 3D object can be printed by starting the print bar in a first indexed position (circle number  1 ), and scanning in a −X direction over the build platform in a first pass. A first index in the +Y direction can then move the print bar into a second indexed position (circle number  2 ), followed by scanning the print bar in a +X direction back over the build platform. A second index in the −Y direction can then return the print bar back to the first indexed position. In an alternate scheme (b), the first index direction and the second index direction can be reversed. These schemes can be repeated for subsequent powder layers of the 3D object. 
     In an example multiple pass indexing scheme (c), a first powder layer of a 3D object can be printed by starting the print bar in a first indexed position (circle number  1 ), and scanning in a −X direction over the build platform in a first pass. A first index in the +Y direction can then move the print bar into a second indexed position (circle number  2 ), followed by scanning the print bar in a +X direction back over the build platform. A next powder layer can then be applied to the 3D object, and the multiple pass indexing scheme (c) can continue in alternate ways. For example, for the next powder layer the print bar can begin in the second indexed position where it left off from the first powder layer. The first pass over the next powder layer would then be in the −X direction, followed by indexing in the −Y direction back to the first indexed position and a second pass in the +X direction. Alternatively, for the next powder layer, a return index in the −Y direction can return the print bar back to the first indexed position prior to the first pass in the −X direction. 
     In an example multiple pass indexing scheme (d), a first powder layer of a 3D object can be printed by starting the print bar in a first indexed position (circle number  1 ), and scanning in a −X direction over the build platform in a first pass. A first index in the −Y direction can then move the print bar into a second indexed position (circle number  2 ), followed by a second pass in the +X direction. A second index again in the −Y direction can then move the print bar into a third indexed position (circle number  3 ), followed by a third pass in the −X direction. A return pass in the +X direction can then be made to return the print bar to the starting side of the build platform. A next powder layer can then be applied to the 3D object, and the multiple pass indexing scheme (d) can continue in alternate ways. For example, for the next powder layer the print bar can begin in the third indexed position where it left off from the first powder layer. The first pass over the next powder layer would then be in the −X direction, followed by indexing in the +Y direction back to the second indexed position and a second pass in the +X direction, and so on. Alternatively, for the next powder layer a return index in the +Y direction can return the print bar back to the first indexed position prior to the first scan in the −X direction. While one example scheme is described for the next powder layer, other schemes for the next powder layer and subsequent powder layers are possible and are contemplated herein. 
     Referring now to  FIG.  6   b   , in an example multiple pass indexing scheme (e), four passes are made over a first powder layer (n) on a build platform while indexing the print bar in a −Y direction following each pass. After the four passes are complete, a next powder layer (n+1) is applied to the platform and 4 passes are again made over the platform. The first pass over the n+1 powder layer begins in position  4 , where the last pass over the first powder layer n ended. The four passes and the indexing over the n+1 layer proceed in just the opposite directions as the four passes and indexing over the first layer n. As noted above, other example schemes are possible and contemplated. For example, prior to beginning the first pass over the n+1 powder layer, the print bar might be indexed in the +Y direction back to position  1 . Schemes (f) and (g) of  FIG.  6   b   , illustrate examples of single pass indexing as described above with regard to  FIG.  5   . 
       FIGS.  7  and  8    are flow diagrams showing example methods  700  and  800 , of printing a three-dimensional (3D) object. Methods  700  and  800  are associated with examples discussed above with regard to  FIGS.  1 - 6   , and details of the operations shown in methods  700  and  800  can be found in the related discussion of such examples. The operations of methods  700  and  800  may be embodied as programming instructions stored on a non-transitory, machine-readable (e.g., computer/processor-readable) medium, such as memory  132  shown in  FIG.  1   . In some examples, implementing the operations of methods  700  and  800  can be achieved by a processor, such as a processor  130  of  FIG.  1   , reading and executing the programming instructions stored in a memory  132 . In some examples, implementing the operations of methods  700  and  800  can be achieved using an ASIC and/or other hardware components alone or in combination with programming instructions executable by a processor  130 . 
     The methods  700  and  800  may include more than one implementation, and different implementations of methods  700  and  800  may not employ every operation presented in the respective flow diagrams of  FIGS.  7  and  8   . Therefore, while the operations of methods  700  and  800  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  800  might be achieved through the performance of a number of initial operations, without performing one or more subsequent operations, while another implementation of method  800  might be achieved through the performance of all of the operations. 
     Referring now to the flow diagram of  FIG.  7   , an example method  700  of printing a three-dimensional (3D) object begins at block  702  with scanning a print bar in a first direction over a build platform of a 3D printer to deposit a liquid agent onto a layer of build powder. In some examples, the method can include first depositing the layer of build powder onto the build platform. As shown in block  704 , the method can include indexing the print bar in a second direction substantially orthogonal to the first direction. In some examples, the indexing can include indexing the build platform (i.e., instead or, or in addition to indexing the print bar) in a direction substantially orthogonal to the first direction, as shown at block  706 . In some examples, indexing the print bar can include moving the print bar from a first indexed position to a second indexed position, as shown at block  708 . As shown at block  710 , in some examples indexing the print bar can include moving the print bar a distance of a partial length of one printhead die on the print bar, or a distance of a full length of one printhead die on the print bar. Moving the print bar a partial length of one printhead die can include moving the print bar a distance of half of a printhead die, as shown at block  712 . Is some examples, the method can include applying fusing energy to the build powder on the platform after liquid agent has been applied to the powder. The method  700  can continue as shown at block  714 , with scanning the print bar back over the build platform in a third direction opposite the first direction to deposit additional liquid agent onto the layer of build powder. 
     As shown at block  716 , with the print bar still in the second indexed position, the print bar can be scanned in the first direction over the build platform to deposit a liquid agent onto a next layer of build powder. In some examples, the method can include depositing the next layer of build powder onto the build platform. As shown at blocks  718  and  720 , respectively, the method can then include indexing the print bar in a fourth direction opposite the second direction to move the print bar back to the first indexed position, and scanning the print bar back over the build platform in the third direction to deposit additional liquid agent onto the next layer of build powder. 
     Referring now to the flow diagram of  FIG.  8   , an example method  800  of printing a three-dimensional (3D) object begins at block  802  with applying a layer of build powder onto a build platform of a 3D printer. The method  800  continues at block  804  with depositing liquid agent onto the build powder with multiple passes of the print bar over the platform. During a first pass, the print bar can pass over the platform with the print bar and platform in a first relative position to one another, as shown at block  806 . After the first pass, the print bar and platform can be indexed relative to one another to put the print bar and platform into a second relative position to one another, as shown at block  808 . In some examples, indexing the print bar and platform relative to one another can include indexing the print bar while leaving the platform in a current position, as shown at block  810 . In some examples, indexing the print bar and platform relative to one another can include indexing the platform while leaving the print bar in a current position, as shown at block  812 . 
     As shown at block  814 , the method can continue with passing the print bar over the platform with the print bar and platform in the second relative position during a second pass. A next layer of build powder can then be applied onto the build platform, as shown at block  816 . With the print bar and platform still in the second relative position, the print bar can be passed over the platform to deposit a liquid agent onto the next layer of build powder, as shown at block  818 . As shown at block  820  and  822 , respectively, the method  800  can also include indexing the print bar and platform relative to one another to put the print bar and platform back into the first relative position to one another, and with the print bar and platform in the first relative position, passing the print bar over the platform to deposit additional liquid agent onto the next layer of build powder.