Patent Publication Number: US-9423756-B2

Title: Electrophotography-based additive manufacturing system with reciprocating operation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a continuation of U.S. patent application Ser. No. 13/242,669, filed Sep. 23, 2011, entitled “Electrophotography-Based Additive Manufacturing System With Reciprocating Operation”, the contents of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to additive manufacturing systems for building three-dimensional (3D) parts and support structures. In particular, the present disclosure relates to systems and processes for building 3D parts and support structures with electrophotography-based systems and/or ionography-based systems. 
     Additive manufacturing systems are used to build 3D parts from digital representations of the 3D parts (e.g., AMF and STL format files) using one or more additive manufacturing techniques. Examples of commercially available additive manufacturing techniques include extrusion-based techniques, ink jetting, selective laser sintering, powder/binder jetting, electron-beam melting, and stereolithographic processes. For each of these techniques, the digital representation of the 3D part is initially sliced into multiple horizontal layers. For each sliced layer, a tool path is then generated, which provides instructions for the particular additive manufacturing system to form the given layer. 
     For example, in an extrusion-based additive manufacturing system, a 3D part or model may be printed from a digital representation of the 3D part in a layer-by-layer manner by extruding a flowable part material. The part material is extruded through an extrusion tip carried by a print head of the system, and is deposited as a sequence of roads on a substrate in an x-y plane. The extruded part material fuses to previously deposited part material, and solidifies upon a drop in temperature. The position of the print head relative to the substrate is then incremented along a z-axis (perpendicular to the x-y plane), and the process is then repeated to form a 3D part resembling the digital representation. 
     In fabricating 3D parts by depositing layers of a part material, supporting layers or structures are typically built underneath overhanging portions or in cavities of objects under construction, which are not supported by the part material itself. A support structure may be built utilizing the same deposition techniques by which the part material is deposited. The host computer generates additional geometry acting as a support structure for the overhanging or free-space segments of the 3D part being formed. Support material is then deposited from a second nozzle pursuant to the generated geometry during the printing process. The support material adheres to the modeling material during fabrication, and is removable from the completed 3D part when the printing process is complete. 
     In two-dimensional (2D) printing, electrophotography (i.e., xerography) is a popular technology for creating 2D images on planar substrates, such as printing paper. Electrophotography systems include a conductive support drum coated with a photoconductive material layer, where latent electrostatic images are formed by charging and then image-wise exposing the photoconductive layer by an optical source. The latent electrostatic images are then moved to a developing station where toner is applied to charged areas of the photoconductive insulator to form visible images. The formed toner images are then transferred to substrates (e.g., printing paper) and affixed to the substrates with heat or pressure. 
     SUMMARY 
     An aspect of the present disclosure is directed to an additive manufacturing system for printing a 3D part using electrophotography. The system includes a rotatable photoconductor component having a surface, and a first development station and a second development station. The first development station is configured to develop a layer of a first material on the surface of the rotatable photoconductor component while the rotatable photoconductor component rotates in a first rotational direction. The second development station is configured to develop a layer of a second material on the surface of the rotatable photoconductor component while the rotatable photoconductor component rotates in a second rotational direction that is opposite of the first rotational direction. The system also includes a rotatable transfer component configured to receive the developed layers from the surface of the rotatable photoconductor component, and a platen configured to receive the developed layers from the rotatable transfer component in a layer-by-layer manner to print the 3D part from at least a portion of the received layers. The system further includes a controller configured to selectively rotate the rotatable photoconductor component in first and second rotational directions, to rotate the rotatable transfer component at a synchronized rate and in counter-rotation with the rotatable photoconductor component, and to move the platen at a synchronized rate with and in the direction of the rotation of the rotatable transfer component, and to index the platen along a z-axis between layers. 
     Another aspect of the present disclosure is directed to an additive manufacturing system for printing a 3D part using electrophotography, where the system includes a rotatable photoconductor component having a surface, and a plurality of development stations configured to develop layers of materials on the surface of the rotatable photoconductor component while the rotatable photoconductor component rotates in a first rotational direction. The system also includes a mechanism configured to interchangeably engage the plurality of development stations with the surface of the rotatable photoconductor component, and a platen configured to operably receive the developed layers in a layer-by-layer manner to print the 3D part from at least a portion of the received layers. The system further includes at least one second development station configured to develop layers of a second material on the surface of the rotatable photoconductor component while the rotatable photoconductor component rotates in the second rotational direction. The system further includes a controller configured to selectively rotate the rotatable photoconductor component in the opposing rotational directions, and to operate the mechanism to interchangeably engage the plurality of first development stations with the surface of the rotatable photoconductor component. 
     Another aspect of the present disclosure is directed to a method for printing a 3D part and support structure. The method includes rotating a rotatable photoconductor component in a first rotational direction, and developing a layer of the support structure from a first development station onto a surface of the rotatable photoconductor component while rotating in the first rotational direction. The method also includes transferring the developed layer of the support structure to a rotatable transfer component that rotates at a synchronized rate with the rotatable photoconductor component, and transferring the developed layer of the support structure from the rotatable transfer component to a platen. The method further includes rotating the rotatable photoconductor component in a second rotational direction that is opposite of the first rotational direction, and developing a layer of the 3D part from a second development station onto the surface of the rotatable photoconductor component while rotating in the second rotational direction. The method further includes transferring the developed layer of the 3D part to the rotatable transfer component, and transferring the developed layer of the 3D part from the rotatable transfer component onto the layer of the support structure previously transferred to the platen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an additive manufacturing system of the present disclosure for printing 3D parts and support structures using electrophotography. 
         FIGS. 2A-2L  are schematic illustrations of the additive manufacturing system of the present disclosure, illustrating a printing operation to print a 3D part and support structure. 
         FIGS. 3A-3L  are schematic illustrations of a portion of the additive manufacturing system of the present disclosure, illustrating an alternative printing operation to print a 3D part and support structure. 
         FIG. 4  is a schematic illustration of a first alternative additive manufacturing system of the present disclosure, which includes four development stations. 
         FIG. 5  is a schematic illustration of a second alternative and preferred additive manufacturing system of the present disclosure, which includes carousels of development stations. 
         FIG. 6  is a schematic illustration of a third alternative additive manufacturing system of the present disclosure, which includes a heated chamber. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed to an additive manufacturing system for printing 3D parts and support structures using electrophotography. As discussed below, the system prints the 3D parts and support structures in a layer-by-layer manner with the use of a photoconductor, a transfer medium, and a platen, each of which are configured to move in a reciprocating or bi-directional manner. The reciprocating movements allow the system to have smaller dimensions, while also efficiently printing with multiple materials, such as multiple part and/or support materials, and colorants. The reciprocating movements also increase printing rates and registration accuracies compared to multiple-engine systems. 
     As shown in  FIG. 1 , system  10  includes photoconductor drum  12 , transfer drum  14 , and platen  16 , each of which are configured to move in a reciprocating or bi-directional manner to print 3D part  18  and support structure  19  in a layer-by-layer manner. In the shown example, support structure  19  is the bottom-most layers of the printed stack to assist in the removal of 3D part  18  from platen  16  after the printing operation is complete. However, support structure  19  may be one or more structures printed to provide vertical support along the z-axis for overhanging regions of any of the layers of 3D part  18 . This allows 3D part  18  to be printed with a variety of geometries. 
     While described herein as drums, one or both of photoconductor drum  12  and transfer drum  14  may alternatively be rollers, belt assemblies, or other rotatable assemblies. Furthermore, the components of system  10  are desirably retained within an enclosable housing (not shown) that prevents ambient light from being transmitted to the components of system  10  during operation. 
     System  10  also includes controller  20 , which is one or more microprocessor-based engine control systems and/or digitally-controlled raster imaging processor systems, and which is configured to operate the components of system  10  in a synchronized manner based on printing instructions received from host computer  21 . Host-computer  21  is one or more computer-based systems configured to communicate with controller  20  to provide the print instructions (and other operating information). For example, host computer  21  may transfer information to controller  20  that relates to the sliced layers of 3D part  18  and support structure  19 , thereby allowing system  10  to print 3D part  18  and support structure  19  in a layer-by-layer manner. 
     Photoconductor drum  12  includes conductive drum  22  and photoconductive surface  24 , where conductive drum  22  is an electrically-conductive drum (e.g., fabricated from copper, aluminum, tin, or the like) that is electrically grounded and configured to rotate in a reciprocating or bi-directional manner around shaft  26 . Shaft  26  is correspondingly connected to drive motor  28 , which is configured to rotate shaft  26  (and photoconductor drum  12 ) in opposing rotational directions in the direction of arrows  40  and  44  based on commands from controller  20 . 
     Photoconductive surface  24  is a thin film extending around the circumferential surface of conductive drum  22 , and is derived from one or more photoconductive materials, such as amorphous silicon, selenium, zinc oxide, organic materials, and the like. As discussed below, surface  24  is configured to receive latent-charged images of the sliced layers of 3D part  18  and support structure  19  (or negative images), and to attract charged particles of the part and support materials to the charged or discharged image areas, thereby creating the layers of 3D part  18  and support structure  19 . 
     As further shown, system  10  also includes charge inducers  30   a  and  30   b , imager  32 , development stations  34   a  and  34   b , cleaning stations  36   a  and  36   b , and discharge devices  38   a  and  38   b , each of which may be in signal communication with controller  20 . Charge inducer  30   a , imager  32 , development station  34   a , cleaning station  36   a , and discharge device  38   a  define a first image-forming assembly  39   a  for surface  24 . Correspondingly, charge inducer  30   b , imager  32 , development station  34   b , cleaning station  36   b , and discharge device  38   b  define a second image-forming assembly  39   b  for surface  24 . 
     In the shown example, first image-forming assembly  39   a  is used to form a layer of a part material  42  for printing 3D part  18  (e.g., layer  18   a , shown below in  FIGS. 2G-2K ), where a supply of part material  42  is retained by development station  34   a , and drive motor  28  and shaft  26  rotate photoconductor drum  12  in the direction of arrow  40  (hereafter “first rotational direction”). Charge inducer  30   a  is configured to generate a uniform electrostatic charge on surface  24  as surface  24  rotates in the first rotational direction past charge inducer  30   a . Suitable devices for charge inducer  30   a  include corotrons, scorotrons, charging rollers, and other electrostatic charging devices. 
     Imager  32  is a digitally-controlled, pixel-wise light exposure apparatus configured to selectively emit electromagnetic radiation toward the uniform electrostatic charge on surface  24  as surface  24  rotates in the direction of arrow  40  past imager  32 . The selective exposure of the electromagnetic radiation to surface  24  is directed by controller  20 , and causes discrete pixel-wise locations of the electrostatic charge to be removed (i.e., discharged to ground), thereby forming a latent image charge pattern on surface  24 . Suitable devices for imager  32  include scanning laser (e.g., gas or solid state lasers) light sources, light emitting diode (LED) array exposure devices, and other exposure devices conventionally used in 2D electrophotography systems. In alternative embodiments, suitable devices for imager  32  and charge devices  30   a  and  30   b  include ion-deposition systems configured to selectively directly deposit charged ions or electrons to surface  24  to form the latent image charge pattern. As such, as used herein, the term “electrophotography” includes ionography. 
     Development station  34   a  is a first electrostatic and magnetic development station that retains the supply of part material  42  (or other suitable material) in powder form, and that applies a layer of part material  42  to surface  24 . In particular, as surface  24  (containing the latent charged image) rotates in the first rotational direction from imager  32  to development station  34   a , part material  42  is attracted to the appropriately charged or discharged regions of the latent image on surface  24 , utilizing either charged area development or discharged area development (depending on the electrophotography mode being utilized). 
     Development station  34   a  may function in a similar manner to single or dual component development systems and toner cartridges used in 2D electrophotography systems. For example, development station  34   a  may include an enclosure for retaining the charged part material  42 , and one or more devices for transferring the charged part material  42  to surface  24 , such as conveyor, fur brushes, paddle wheels, rollers, and/or magnetic brushes. Suitable materials for part material  42  may vary depending on the desired part properties, such as one or more thermoplastic resins. Examples of suitable thermoplastic resins for part material  42  include polyolefins, polyester, nylon, toner materials (e.g., styrene-acrylate/acrylic materials), and combinations thereof. In dual-component arrangements, part material  42  may also include a carrier material with the thermoplastic resin(s). For example, the carrier material is magnetically permeable and appropriately coated with a material to triboelectrically charge the thermoplastic resin(s) of part material  42 . 
     The resulting developed part layer of part material  42  (e.g., layer  18   a , shown below in  FIGS. 2G-2K ) carried by surface  24  is rotated in the direction of arrow  40  to a transfer region in which the part layer is transferred from photoconductor drum  12  to transfer drum  14 , as discussed below. After the part layer is transferred from photoconductor drum  12  to transfer drum  14 , drive motor  28  and shaft  26  continue to rotate photoconductor drum  12  in the first rotational direction such that the region of surface  24  that previously held the part layer passes cleaning station  36   a . Cleaning station  36   a  is a station configured to remove any residual, non-transferred portions of part material  42 . Suitable devices for cleaning station  36   a  include blade cleaners, brush cleaners, electrostatic cleaners, vacuum-based cleaners, and combinations thereof. 
     After passing cleaning station  36   a , surface  24  continues its rotation such that the cleaned regions of surface  24  pass discharge device  38   a  to remove any residual electrostatic charge on surface  24 . Suitable devices for discharge device  38   a  include optical systems, high-voltage alternating-current corotrons and/or scorotrons, one or more rotating dielectric rollers having conductive cores with applied high-voltage alternating-current, and combinations thereof. 
     As shown, while surface  24  is rotated in the first rotational direction, controller  20  desirably deactivates or disengages charge inducer  30   b , cleaning station  36   b , and discharge device  38   b . This prevents these components from interfering with the formation, development, and transfer of the part layer. In one embodiment, controller  20  may also disengage development station  34   b . However, as shown in  FIG. 1 , development station  34   b  is disposed downstream in the first rotational direction of arrow  40  from discharge device  38   a , and upstream from charge inducer  30   a . As such, a region of surface  24  that passes development station  34   b  while rotating in the first rotational direction is uncharged, thereby preventing charged particles from development station  34   b  from being attracted to surface  24 . 
     In the shown embodiment, the second image-forming assembly  39   b  for surface  24  is used to form a layer of a support material  46  (e.g., layer  19   a , shown below in  FIGS. 2A-2E ) for printing support structure  19 , where a supply of support material  46  is retained by development station  34   b , and drive motor  28  and shaft  26  rotate photoconductor drum  12  in the direction of arrow  44  (hereafter “second rotational direction”). Charge inducer  30   b  is configured to generate a uniform electrostatic charge on surface  24  as surface  24  rotates in the second rotational direction past charge inducer  30   b . Suitable devices for charge inducer  30   b  include those discussed above for charge inducer  30   a.    
     Imager  32  operates in the second image-forming assembly  39   b  in the same manner as described above in regards to the first image-forming assembly  39   b . In this operation, imager  32  selectively emits electromagnetic radiation toward the uniform electrostatic charge on surface  24  as surface  24  rotates in the second rotational direction past imager  32 . This selective exposure of the electromagnetic radiation to surface  24  is directed by controller  20 , and causes discrete pixel-wise locations of the electrostatic charge to be removed (i.e., discharged to ground), thereby forming a second latent image charge pattern on surface  24 . 
     Development station  34   b  is a second electrostatic and magnetic development station that retains the supply of a support material  46  (or other suitable material) in powder form, and applies a layer of support material  46  to surface  24 . In particular, as surface  24  (containing the second latent image) rotates from imager  32  to development station  34   b  in the second rotational direction, support material  44  is attracted to the appropriately charged or discharged regions of the latent image on surface  24 , utilizing either charged area development or discharged area development (depending on the electrophotography mode being utilized). 
     Development station  34   b  may function in a similar manner to single or dual component development systems and toner cartridges used in 2D electrophotography systems, as described above regarding development station  34   a . Suitable materials for support material  46  may vary depending on the desired support structure properties, such as one or more thermoplastic resins that are compatible with part material  42  and that may be separated from 3D part  18  after the printing operation is complete (e.g., different solubilities, different melting temperatures, and the like). In dual-component arrangements, support material  46  may also include a carrier material with the thermoplastic resin(s). For example, the carrier material is magnetically permeable and appropriately coated with a material to triboelectrically charge the thermoplastic resin(s) of part material  46 . In an alternative example, the carrier material may be coated with the thermoplastic resin(s) of support material  46 . 
     The resulting developed support layer of support material  46  (e.g., layer  19   a , shown below in  FIGS. 2A-2E ) is rotated with surface  24  in the second rotational direction to the transfer region in which the support layer is transferred from photoconductor drum  12  to transfer drum  14 , as discussed below. After the support layer is transferred from photoconductor drum  12  to transfer drum  14 , drive motor  28  and shaft  26  continue to rotate photoconductor drum  12  in the second rotational direction such that the region of surface  24  that previously held the support layer passes cleaning station  36   b . Cleaning station  36   b  is a station configured to remove any residual, non-transferred portions of support material  46 . Suitable devices for cleaning station  36   b  include those discussed above for cleaning station  36   a.    
     After passing cleaning station  36   b , surface  24  continues to rotate in the second rotational direction such that the cleaned regions of surface  24  pass discharge device  38   b  to remove any residual electrostatic charge on surface  24 . Suitable devices for discharge device  38   b  include those discussed above for discharge device  38   a.    
     In this operation, while surface  24  is rotated in the second rotational direction, controller  20  desirably deactivates or disengages charge inducer  30   a , cleaning station  36   a , and discharge device  38   a . This prevents these components from interfering with the formation, development, and transfer of the support layer. In one embodiment, controller  20  may also disengage development station  34   a . However, as shown in  FIG. 1 , development station  34   a  is disposed downstream in the second rotational direction of arrow  44  from discharge device  38   b , and upstream from charge inducer  30   b . As such, a region of surface  24  that passes development station  34   a  while rotating in the second rotational direction is uncharged, thereby preventing charged particles from development station  34   a  from being attracted to surface  24 . 
     Transfer drum  14  is a second electrically-conductive drum (e.g., fabricated from copper, aluminum, tin, or the like) that is electrically grounded, and includes circumferential surface  47 . The layers of part material  42  and support material  46  may be transferred from photoconductor drum  12  to surface  47  of transfer drum  14  using one or more of electrostatic forces, temperature, and pressure. For example, transfer drum  14  may be temperature-controlled, as described below, and may also be electrically biased with a potential having a magnitude and sign that electrostatically attracts the layers of part material  42  and support material  46  from photoconductor drum  12  at the transfer region. 
     Surface  47  may include a layer of one or more appropriately selected surface energy materials optimized to effectively receive the transferred layers from surface  24  and also to completely transfer the received layers of part material  42  and support material  46  to platen  16 . Examples of suitable materials for surface  47  include fluorinated polymers, such as polytetrafluoroethylenes (PTFE), fluorinated ethylene propylenes, and perfluoroalkoxy polymers. Examples of suitable commercially available fluorinated polymers include PTFE available under the trade designation “TEFLON” from E.I. du Pont de Nemours and Company, Wilmington, Del. 
     Transfer drum  14  is configured to rotate in a reciprocating or bi-directional manner around shaft  48 . Shaft  48  is correspondingly connected to drive motor  50 , which is configured to rotate shaft  48  (and transfer drum  14 ) in opposing rotational directions based on commands from controller  20 . For example, when controller  20  directs motor  28  to rotate photoconductor drum  12  in the first rotational direction of arrow  40  to form a layer of part material  42 , controller  20  also directs motor  50  to rotate transfer drum  14  in the direction of arrow  52  at a synchronized rate with, and counter to, the rotation of photoconductor drum  12  in the direction of arrow  40 . As shown, the rotational directions of arrows  40  and  52  are opposite, which prevents frictional sliding at the transfer region between photoconductor drum  12  and transfer drum  14 . 
     Likewise, when controller  20  directs motor  28  to rotate photoconductor drum  12  in the second rotational direction of arrow  44  to form a layer of support material  46 , controller  20  also directs motor  50  to rotate transfer drum  14  in the direction of arrow  54 . The rotation of transfer drum  14  in the direction of arrow  54  is also desirably performed at a synchronized rate with, and counter to, the rotation of photoconductor drum  12  in the direction of arrow  44 , where the rotational directions of arrows  44  and  54  are also opposite to prevent frictional sliding at the transfer region. 
     System  10  also includes heaters  56   a  and  56   b  and cooling units  58   a  and  58   b , disposed around the circumference of transfer drum  14 . Heaters  56   a  and  56   b  are heating elements configured to heat the transferred layers of part material  42  and support material  44 , thereby rendering the heated layers tacky. After being heated, the resulting tacky layers may be transferred to platen  16  or to previously formed layers of 3D part  18  and/or support structure  19 , as discussed below. Cooling units  58   a  and  58   b  are configured to lower the temperature of surface  47  after the given layers of part material  42  or support material  46  have been transferred to platen  16 , and prior to receiving a subsequent layer from photoconductor drum  12 . 
     Controller  20  operates heater  56   a  and cooling unit  58   a  in coordination with the formation of the layers of part material  42 , and operates heater  56   b  and cooling unit  58   b  in coordination with the formation of the layers of support material  46 . For example, while transfer drum  14  is rotated in the direction of arrow  52 , controller  20  directs heater  56   a  to operate to tackify the received layer of part material  42 . The heated layer is then transferred from surface  47  of transfer drum  14  to platen  16  (or to a previously formed layer of 3D part  18  and/or support structure  19 ). The continued rotation of transfer drum  14  in the direction of arrow  52  allows surface  47  of transfer drum  14  to pass cooling unit  58   a  to cool surface  47 . 
     Alternatively, while transfer drum  14  is rotated in the direction of arrow  54 , controller  20  directs heater  56   b  to operate to tackify the received layer of support material  46 . The heated layer is then transferred from transfer drum  14  to platen  16  (or to a previously formed layer of 3D part  18  and/or support structure  19 ). The continued rotation of transfer drum  14  in the direction of arrow  54  allows surface  47  of transfer drum  14  to pass cooling unit  58   b  to cool surface  47  down. 
     Platen  16  is a platform assembly of system  10  that is configured to receive the layers of part material  42  and/or support material  46  for printing 3D part  18  and/or support structure  19  in a layer-by-layer manner. Platen  16  is supported by x-axis gantry  60  and z-axis gantry  62 . X-axis gantry  60  is a first linear guide mechanism configured to move platen  16  along the horizontal x-axis in reciprocating directions, referred to by arrows  64  and  66 . The movement of platen  16  with x-axis gantry  60  is operated by drive motor  68  based on commands from controller  20 . In the shown embodiment, x-axis gantry  60  moves platen  16  in the reciprocating directions with back-and-forth motions along the x-axis. In alternative embodiments, x-axis gantry  60  (and, optionally, z-axis gantry  62 ) may move platen  16  in the reciprocating directions with loop motions and/or pivoting motions (e.g., with a gimbal design). 
     Z-axis gantry  62  is a second linear guide mechanism configured to move platen  16  and x-axis gantry  60  along the vertical z-axis to adjust the elevation of platen  16  relative to transfer drum  14 . The movement of platen  16  and x-axis gantry  60  with z-axis gantry  62  is operated by z-axis motor  70  based on commands from controller  20 . 
     System  10  also includes fixing elements  72  and  74 , which are heating devices configured to heat the formed layers of 3D part  18  and support structure  19 , as discussed below. Suitable devices for fixing elements  72  and  74  include non-contact radiant heaters, convection heating devices, contact heating devices (e.g., heated rollers and/or platens), and the like. Fixing elements  72  and  74  may also be variable heat devices, and they may also be configured as pressure plates, for fusing together the printed material. 
     The reciprocating movements of the components of system  10  may vary depending on the particular layers being developed and printed.  FIGS. 2A-2L  illustrate an example printing operation to print 3D part  18  and support structure  19  with system  10 , where multiple layers of support structure  19  are initially printed, followed by the printing of multiple layers of 3D part  18 . In comparison, as discussed below,  FIGS. 3A-3L  illustrate an example printing operation to print 3D part  18  and support structure  19  with system  10 , where the layers of part and support materials are printed in a co-planar or alternating manner, such as when the layers of 3D part  18  and support structure  19  are printed in the same increments, or as alternating successive layers. 
     When printing multiple successive layers of support material  46  without switching to part material  42  (e.g., as shown below in  FIGS. 2A-2F ), photoconductor drum  12  may continue to rotate in the second rotational direction of arrow  44  over multiple revolutions. This allows multiple development cycles to be performed for developing and transferring the successive layers of support material  46 . Transfer drum  14  correspondingly rotates in the direction of arrow  54  over multiple revolutions at a rate that is synchronized with the rotation of photoconductor drum  12 . Similarly, platen  16  moves in the directions of arrows  64  and  66  at a synchronized rate with the rotation of transfer drum  14 . 
     As used herein, the term “development cycle” refers to a cycle to develop a layer of a material with a photoconductor drum (e.g., photoconductor drum  12 ), and to transfer the developed layer from the photoconductor drum (e.g., to transfer drum  14 ), regardless of the rotational direction of the photoconductor drum. For example, a development cycle may be performed to print a layer of support material  46  by rotating photoconductor drum  12  in the second rotational direction such that a portion of surface  24  passes charge inducer  30   b , imager  32 , development station  34   b , and transfer drum  14 , thereby developing and transferring a layer of support material  46 . Optionally, the term “development cycle” may also include one or more steps for cleaning and/or discharging the surface of the photoconductor drum (e.g., surface  24  with cleaning station  36   b  and discharge device  38   b ). 
     Similarly, when printing multiple successive layers of part material  42  without switching to support material  46  (e.g., as shown below in  FIGS. 2G-2L ), photoconductor drum  12  may continue to rotate in the first rotational direction of arrow  40  over multiple revolutions. This allows multiple development cycles to be performed for developing and transferring the successive layers of part material  42 . Transfer drum  14  correspondingly rotates in the direction of arrow  52  over multiple revolutions at a rate that is synchronized with the rotation of photoconductor drum  12 . Similarly, platen  16  moves in the directions of arrows  64  and  66  at a synchronized rate with the rotation of transfer drum  14 . 
     As shown in  FIG. 2A , during the operation to print 3D part  18  and support structure  19 , controller  20  directs z-axis motor  70  and z-axis gantry  60  to position platen  16  at a suitable height along the z-axis such that a top surface of platen  16  (referred to as top surface  76 ) is disposed below and adjacent to surface  47  of transfer drum  14 . If the height of platen  16  is too low relative to transfer drum  14 , then the layer of part material  42  or support material  46  will not be able to transfer from transfer drum  14  to top surface  76 . Alternatively, if the height of platen  16  too high relative to transfer drum  14 , the transferred layers may be squeezed between transfer drum  14  and platen  16 , thereby potentially damaging 3D part  18  and support structure  19 . As such, controller  20  desirably positions platen  16  at the suitable height to effectively transfer the layer(s) of part material  42  and/or support material  46  from transfer drum  14  to top surface  76 . 
     Additional factors that may affect the transfer of the developed layers of part and support materials from transfer drum  14  to the top layer of the stack of 3D part  18  and support structure  19  include the temperature of the layer being transferred (e.g., how tacky the material is), the contact duration between the transferred layer and the top layer of the stack, the adhesive properties of the part and support materials, and the like. In one embodiment, system  10  operates with one or more feedback process control loops to monitor the pressures between transfer drum  14  and platen  16  and/or the temperatures of the layers of part material  42  and support material  46 . Examples of suitable techniques for such feedback process control loops, and a suitable transfusion technique for transferring the developer layers, are disclosed in co-filed U.S. Provisional Patent Application No. 61/538,491, and entitled “Layer Transfusion For Electrophotography-Based Additive Manufacturing”, the contents of which are incorporated by reference. This provides a variable temperature and pressure control to transfer the layers from transfer drum  14  to platen  16 . 
     As discussed above, system  10  may initially form one or more layers of support structure  19  on top surface  76  to assist in the removal of 3D part  18  from platen  16  after the printing operation is complete. When forming a layer of support material  46 , controller  20  may direct photoconductor drum  12  and transfer drum  14  to rotate in the directions of arrows  44  and  54 , respectively. At this point in the process, controller  20  also desirably disables or disengages charge inducer  30   a , cleaning station  36   a , discharge device  38   a , (and optionally development station  34   a ) to prevent them from interfering with the formation of the layer of support material  46 . 
     Photoconductor drum  12  desirably rotates in the second rotational direction at a predetermined operating rotational rate, which may depend on several factors such as the diameter of photoconductor drum  12 , the desired printing rate of system  10 , the speed of the development cycle with second image-forming assembly  39   b , the cross-sectional dimensions of the developed layers, maximum platen/part length, and the like. The operating rotational rate is also dictated by the achievable rate of movement for platen  16 . X-axis gantry  60  desirably moves platen  16  back-and-forth in the directions of arrows  64  and  66  once per printed layer. For an average printing rate of 50 layers per minute (i.e., about 1.2 seconds per layer), platen  16  moves back-and-forth between transfer drum  14  and fixing element  74  about every 1.2 seconds. 
     X-axis gantry  60  may move platen  16  back-and-forth at different rates, where platen  16  is moved at a first rate when engaged with transfer drum  14  (i.e., a rate that is synchronized with the rotational rate of transfer drum  14 ), and at a second faster rate when moving back-and-forth between transfer drum  14  and fixing element  74 . Assuming, for example, that the movement back-and-forth between transfer drum  14  and fixing element  74  accounts for about 50% of the 1.2 seconds (i.e., for about 0.6 seconds), platen  16  is then engaged with transfer drum  14  for about 0.6 seconds. This corresponds to the maximum development cycle time for developing each layer of support material  46  with photoconductor drum  12 . 
     When performing a single development cycle per revolution of photoconductor drum  12  (i.e., developing and transferring a single layer of support material  46  per revolution) at the maximum development cycle time of 0.6 seconds, the operating rotational rate for photoconductor drum  12  is about 0.6 seconds per revolution. However, when performing multiple development cycles per revolution of photoconductor drum  12  (i.e., developing and transferring multiple layers of support material  46  per revolution), the operating rotational rate for photoconductor drum  12  may be decreased. For example, when performing two development cycles per revolution of photoconductor drum  12  at the maximum development cycle time of 0.6 seconds, the operating rotational rate for photoconductor drum  12  is about 1.2 seconds per revolution; and when performing four development cycles per revolution photoconductor drum  12  at the maximum development cycle time of 0.6 seconds, the operating rotational rate for photoconductor drum  12  is about 2.4 seconds per revolution. 
     As photoconductor drum  12  rotates in the second rotational direction at the operating rotational rate, controller  20  directs charge inducer  30   b  to generate a uniform electrostatic charge on surface  24 . Controller  20  then directs imager  32  to selectively expose surface  24  to electromagnetic radiation to form a latent image charge pattern on surface  24  corresponding to the dimensions of the layer of support material  46  (or a corresponding negative image). 
     As photoconductor drum  12  continues to rotate in the second rotational direction (bypassing the deactivated or disengaged charge inducer  30   a ), charged particles of support material  46  from development station  34   b  are attracted to the appropriately charged or discharged regions of the latent image on surface  24 . This forms layer  19   a  of support material  46  on surface  24 . 
     As photoconductor drum  12  continues to rotate in the second rotational direction (bypassing the deactivated or disengaged discharge device  38   a  and cleaning station  36   a ), the charged particles of layer  19   a  are attracted to surface  47  of transfer drum  14 . As shown in  FIG. 2B , this transfers layer  19   a  from photoconductor drum  12  to transfer drum  14  while transfer drum  14  rotates in the direction of arrow  54  at a rate that is synchronized with the operating rotational rate of photoconductor drum  12 . 
     The synchronized rotational rate of transfer drum  14  depends on the operating rotational rate of photoconductor drum  12 , and on the relative diameters between photoconductor drum  12  and transfer drum  14 . For photoconductor drum  12  having an average diameter of about 6.0 inches and transfer drum  14  having an average diameter of 4.0 inches, and the operating rotational rate of photoconductor drum  12  is about 2.4 seconds per revolution, the synchronized rotational rate for transfer drum  14  is about 1.6 seconds per revolution. 
     As shown in  FIG. 2C , upon being transferred to surface  47 , layer  19   a  is then heated to a tacky state with heating element  56   b . As transfer drum  14  continues to rotate in the direction of arrow  54 , layer  19   a  reaches a second transfer region at top surface  76  of platen  16 . As shown in  FIG. 2D , controller  20  directs motor  68  to and platen  16  in the direction of arrow  66  via x-axis gantry  60 , at a rate that is synchronized with the rotational rate of transfer drum  14  in the direction of arrow  54 . This prevents frictional resistance from being generated therebetween, allowing layer  19   a  to be transferred from surface  47  and laminated onto top surface  76  of platen  16  with accurate placement. 
     As shown in  FIG. 2E , as platen  16  and the received layer  19   a  continue to move in the direction of arrow  66 , layer  19   a  is exposed to fixing element  74 , which further heats layer  19   a  to assist in securing layer  19   a  to top surface  76  of platen  16 . After layer  19   a  is printed and heated, controller  20  may then direct z-axis motor  70  to appropriately move and reposition platen  16  and layer  19   a  via z-axis gantry  60 . Controller may also direct motor  68  to move platen  16  and layer  19   a  in the direction of arrow  64  via x-axis gantry  60  to reset the placement of platen  16  and layer  19   a  for printing subsequent layers of support structure  19  on top of layer  19   a.    
     As mentioned above, the synchronized movement rate of platen  16  depends on the rotational rate of transfer drum  14 . Following the above-discussed example in which transfer drum  14  has a diameter of about 4.0 inches and rotates at a rotational rate of about 1.6 seconds per revolution, this corresponds to a movement rate for the circumference of surface  47  of transfer drum  14  of about 7.9 inches per second. Platen  16 , therefore, also moves at about 7.9 inches per second to remain synchronized with transfer drum  14 . Since platen  16  may be engaged with transfer drum  14  for about 0.6 seconds to transfer layer  19   a  from transfer drum  19  to platen  16  with the synchronized movement, a movement rate of 7.9 inches per second corresponds to a maximum layer size along the x-axis of about 4.7 inches. 
     As shown in  FIG. 2F , the above-discussed process may be repeated to print each successive layer of support structure  19  in a continuous manner by rotating photoconductor drum  12  and transfer drum  14  in the directions of arrows  44  and  54 , respectively. For each printed layer, platen  16  is also desirably moved in the direction of arrow  66  to expose the printed layers of support structure  19  to fixing element  74 , and then lowered a single increment along the z-axis. 
     When system  10  has completed the given layers of support structure  19 , and is ready to print layers of 3D part  18 , controller  20  may direct motor  68  to position platen  16  such that the top surface of support structure  19  (referred to as top surface  78 ) is set at a start location along the x-axis, as shown. This start position allows top surface  78  of support structure  19  to receive layers of part material  42  while transfer drum  14  subsequently rotates in the direction of arrow  40  and platen  16  subsequently moves in the direction of arrow  64 . 
     When switching from support material  46  to part material  42 , drive motor  28  slows photoconductor drum  12  down from the operating rotational rate in the second rotational direction to a zero rotation state. Drive motor  28  then rotates photoconductor drum  12  in the first rotational direction of arrow  40  (i.e., opposite of the second rotational direction of arrow  44 ), from the zero rotational state to the operating rotational rate. 
     As shown in  FIG. 2G , when forming a layer of part material  42  on top surface  78  of support structure  19 , controller  20  may direct photoconductor drum  12  and transfer drum  14  to rotate in the directions of arrows  40  and  52 , respectively. At this time in the process, controller  20  also desirably deactivates or disengages charge inducer  30   b , cleaning station  36   b , discharge device  38   b , (and optionally development station  34   b ) to prevent them from interfering with the formation of the layer of part material  42 . 
     As photoconductor drum  12  rotates in the first rotational direction at the operating rotational rate, controller  20  directs charge inducer  30   a  to generate a uniform electrostatic charge on surface  24 . Controller  20  then directs imager  32  to selectively expose surface  24  to electromagnetic radiation to form a latent image charge pattern on surface  24  corresponding to the dimensions of the layer of part material  42  (or a corresponding negative image). 
     As photoconductor drum  12  continues to rotate in the first rotational direction (bypassing the deactivated or disengaged charge inducer  30   b ), charged particles of part material  42  from development station  34   a  are attracted to the appropriately charged or discharged regions of the latent image on surface  24 . This forms layer  18   a  of part material  42  on surface  24 . 
     As photoconductor drum  12  continues to rotate in the first rotational direction (bypassing the deactivated or disengaged discharge device  38   b  and cleaning station  36   b ), the charged particles of layer  18   a  are attracted to surface  47  of transfer drum  14 . As shown in  FIG. 2H , this transfers layer  18   a  from photoconductor drum  12  to transfer drum  14  while transfer drum  14  rotates in the direction of arrow  52  at a rate that is synchronized with the operating rotational rate of photoconductor drum  12 . The synchronized rotational rate of transfer drum  14  in the direction of arrow  52  also depends on the operating rotational rate of photoconductor drum  12 , and on the relative diameters between photoconductor drum  12  and transfer drum  14 , as discussed above. 
     As shown in  FIG. 2I , upon being transferred to surface  47 , layer  18   a  is then heated to a tacky state with heating element  56   a . As transfer drum  14  continues to rotate in the direction of arrow  52 , layer  18   a  reaches a second transfer region at top surface  78  of support structure  19 . As shown in  FIG. 2J , controller  20  directs motor  68  to move platen  16  in the direction of arrow  64  via x-axis gantry  60 , at a rate that is synchronized with the rotational rate of transfer drum  14  in the direction of arrow  52 , as discussed above. This prevents frictional resistance from being generated therebetween, allowing layer  18   a  to be transferred from surface  47  and laminated onto top surface  78  of support structure  19  with accurate placement. 
     As shown in  FIG. 2K , as platen  16  and the received layer  18   a  continue to move in the direction of arrow  64 , layer  18   a  and support structure  19  are exposed to fixing element  72 , which further heats layer  18   a  and support structure  19  to assist in securing layer  18   a  to top surface  78  of support structure  19 . In some embodiments, fixing element  72  moves vertically to contact the top of layer  18   a , applying pressure as well as heat to fuse together the printed material. Controller  20  may then direct z-axis motor  70  to appropriately move and reposition platen  16 , layer  18   a , and support structure  19  via z-axis gantry  60 , and then direct motor  68  to move platen  16 , layer  18   a , and support structure  19  in the direction of arrow  66  via x-axis gantry  60  to reset the placement platen  16  and layer  18   a  for printing subsequent layers of 3D part  18  on top of layer  18   a.    
     As shown in  FIG. 2L , the above-discussed process may then be repeated to print each successive layer of 3D part  18  in a continuous manner by rotating photoconductor drum  12  and transfer drum  14  in the directions of arrows  40  and  52 , respectively. For each printed layer, platen  16  is also desirably moved in the direction of arrow  64  to expose the printed layers of 3D part  18  and support structure  19  to fixing element  72 , and then lowered a single increment along the z-axis. 
     In situations in which additional layers of support structure  19  and/or 3D part  18  are to be printed over the shown layers of 3D part  18 , the above-discussed steps may be repeated in a reciprocating manner until 3D part  18  and support structure  19  are completed. When system  10  has completed printing of 3D part  18  and support structure  19 , the printed stack of 3D part  18  and support structure  19  may be removed from system  10 , and support structure  19  may be removed from 3D part  18 . 3D part  18  may then undergo one or more post-printing operations. 
     In the examples illustrated in  FIGS. 2A-2L , 3D part  18  and support structure  19  are printed in distinct blocks of layers, where system  10  prints the layers of support structure  19  (e.g., the steps shown in  FIGS. 2A-2F ) and then prints the layers of 3D part  18  (e.g., the steps shown in  FIGS. 2G-2L ). In these examples, photoconductor drum  12  continued to rotate in the same rotational direction over multiple revolutions for each material. However, in many situations, the layers of 3D part  18  and support structure  19  are printed in a co-planar manner, where a given layer of 3D part  18  is printed in the same increment as a given layer of support structure  19 . In these operations, photoconductor drum  12  may be operated in the reciprocating or bi-directional manner after each development cycle. In other words, after rotating in the second rotational direction of arrow  44  to develop and transfer a single layer of support material  46 , photoconductor drum  12  may reverse the rotation to the first rotational direction of arrow  40  to develop and transfer a single layer of part material  42 . This reciprocating motion may then be repeated to form alternating layers of the support and part materials. 
     As shown in  FIGS. 3A-3L , the reciprocating or bi-directional motion of system  10  is particularly suitable and efficient for printing multiple materials (e.g., part and support materials) in a co-planar or alternating manner, such as when the layers of 3D part  18  and support structure  19  are printed in the same increments, or as alternating adjacent layers. As shown in  FIG. 3A , surface  24  of photoconductor drum  12  includes a first region  24   a  (illustrated by a single dashed line) and a second region  24   b  (illustrated by a pair of dashed lines), which are diametrically opposed across photoconductor drum  12 . 
     In the shown example, first region  24   a  is positioned between charge inducer  30   a  and development station  34   b , and second region  24   b  is positioned between discharge device  38   b  and development station  34   a . To form a layer of 3D part  18 , controller  20  rotates photoconductor drum  12  in the first rotational direction of arrow  40  at the operating rotational rate. As shown in  FIG. 3B , this positions first region  24   a  at charge inducer  30   a  to generate a uniform electrostatic charge on surface  24  at first region  24   a.    
     As shown in  FIG. 3C , the continued rotation of photoconductor drum  12  in the direction of arrow  40  positions first region  24   a  at imager  32 . Controller  20  directs imager  32  to selectively expose surface  24  at first region  24   a  to electromagnetic radiation to form a latent image charge pattern on first region  24   a  corresponding to the dimensions of the layer of support material  46  (or a corresponding negative image). 
     As shown in  FIG. 3D , as photoconductor drum  12  continues to rotate in the direction of arrow  40  (bypassing the deactivated or disengaged charge inducer  30   b ), charged particles of part material  42  from development station  34   a  are attracted to the appropriately charged or discharged regions of the latent image on surface  24  at first region  24   a . This forms layer  18   b  of part material  42  at first region  24   a.    
     As shown in  FIG. 3E , as photoconductor drum  12  continues to rotate in the direction of arrow  40  (bypassing the deactivated or disengaged discharge device  38   b  and cleaning station  36   b ), the charged particles of layer  18   b  are attracted to surface  47  of transfer drum  14 , as discussed above. 
     As shown in  FIG. 3F , after layer  18   b  is transferred from photoconductor drum  12  to transfer drum  14 , photoconductor drum  12  continues to rotate in the direction of arrow  40  such that first region  24   a  passes cleaning station  36   a  and discharge device  38   a . This cleans and removes any residual electrostatic charge from surface  24  at first region  24   a . As further shown, this also positions second region  24   b  of surface  24  between charge inducer  30   b  and development station  34   a . In other words, the diametrically-opposed locations of first region  24   a  and second region  24   b  position second region  24   b  upstream from charge inducer  30   b  for developing a layer of support structure  19 . Photoconductor drum  12  may also continue to rotate in the direction of arrow  40  up to about 180 degrees further to ensure that the entirety of surface  24  is cleaned. 
     As shown in  FIG. 3G , to develop a layer of support structure  19 , controller  20  reverses the rotation of photoconductor drum  12  to rotate photoconductor drum  12  in the second rotational direction of arrow  44 . When switching from part material  42  to support material  46 , drive motor  28  slows photoconductor drum  12  down from the operating rotational rate in the first rotational direction to a zero rotation state. Drive motor  28  then rotates photoconductor drum  12  in the second rotational direction (i.e., opposite of the first rotational direction), from the zero rotational state to the operating rotational rate. As shown in  FIG. 3H , this positions second region  24   b  at charge inducer  30   b  to generate a uniform electrostatic charge on surface  24  at second region  24   b.    
     As shown in  FIG. 3I , the continued rotation of photoconductor drum  12  in the direction of arrow  44  then positions second region  24   b  at imager  32 . Controller  20  directs imager  32  to selectively expose surface  24  at second region  24   b  to electromagnetic radiation to form a latent image charge pattern on surface  24  at second region  24   b  corresponding to the dimensions of the layer of support material  46  (or a corresponding negative image). 
     As shown in  FIG. 3J , as photoconductor drum  12  continues to rotate in the direction of arrow  44  (bypassing the deactivated or disengaged charge inducer  30   a ), charged particles of support material  46  from development station  34   b  are attracted to the appropriately charged or discharged regions of the latent image on surface  24  at second region  24   b . This forms layer  19   b  of support material  46  on surface  24  at second region  24   b.    
     As shown in  FIG. 3K , as photoconductor drum  12  continues to rotate in the direction of arrow  44  (bypassing the deactivated or disengaged discharge device  38   a  and cleaning station  36   a ), the charged particles of layer  19   b  are attracted to surface  47  of transfer drum  14 , as discussed above. 
     As shown in  FIG. 3L , after layer  19   b  is transferred from photoconductor drum  12  to transfer drum  14 , photoconductor drum  12  continues to rotate in the direction of arrow  44  such that second region  24   b  passes cleaning station  36   b  and discharge device  38   b . This cleans and removes any residual electrostatic charge from surface  24  at second region  24   b . As further shown, this also repositions first region  24   a  of surface  24  between charge inducer  30   a  and development station  34   b  (i.e., upstream from charge inducer  30   a  for developing the next layer of 3D part  18 , as discussed above). Photoconductor drum  12  may also continue to rotate in the direction of arrow  44  up to about 180 degrees further to ensure that the entirety of surface  24  is cleaned. 
     The processes discussed above in  FIGS. 2A-2L  and  FIGS. 3A-3L  may be used interchangeably to form the various layers of 3D part  18  and support structure  19 . For example, the steps shown in  FIGS. 2A-2F  may be repeated (with photoconductor drum  12  being continuously rotated in the second rotational direction of arrow  44  over multiple revolutions) to form multiple layers of support structure  19 , the steps shown in  FIGS. 2G-2L  may be repeated (with photoconductor drum  12  being continuously rotated in the first rotational direction of arrow  40  over multiple revolutions) to form multiple layers of 3D part  18 , the steps shown in  FIGS. 3A-3L  may be repeated (in either order) to form single layer increments with part and support materials and/or alternating layers of part and support materials, and combinations and variations thereof. 
     In some embodiments, a layer portion of support material  46  and a layer portion of part material  42  may both be developed and transferred to transfer drum  14  prior to being laminated on platen  16 . For example, system  10  may develop a layer portion of support material  46  and transfer the layer portion to transfer drum  14 . Prior to transferring the layer portion of support material  46  from transfer drum  14  to platen  16 , system  10  may develop a layer portion of part material  42  and transfer the layer portion to transfer drum  14  to coincide with the previously formed layer portion of support material  46 . System  10  may then transfer the resulting layer from transfer drum  14  to platen  16  in a single lamination step and then further heat the resulting laminated layer with either fixing element  72  or fixing element  74 . 
     While illustrated in  FIGS. 2A-2L  and  FIGS. 3A-3L  as operating in separate and distinct steps, the separate components of system  10  desirably operate in a continuous manner to reduce overall printing times. For example, as discussed above, system  10  may perform multiple development cycles during each revolution of photoconductor drum  12  and transfer drum  14 . Accordingly, system  10  is suitable for printing 3D parts and support structures from part and support materials at high rates and with good part resolutions. In some embodiments, system  10  may print layers of 3D part  18  and support structure  19  at a rate of at least about 40 layers per minutes (e.g., about 50 layers per minute) with accurate registrations, layer thicknesses ranging from about 5 micrometers to about 125 micrometers, and layer dimensions along the y-axis of at least 51 centimeters (about 11 inches). 
     The resolutions of the 3D parts and support structures may also be varied based on the printing rate. For example, 3D part  18  may be printed at a “high quality” resolution, in which system  10  operates at a slower rate, but prints with lower layer thicknesses. Alternatively, 3D part  18  may be printed at a “draft quality” resolution, in which system  10  operates a faster rate, but prints with greater layer thicknesses. Furthermore, 3D part  18  may be printed in “gray scale”, in which a lower density of part material  42  is developed onto surface  24 . Numerous resolutions and speeds therebetween may also be incorporated. 
     Furthermore, while illustrated with part material  42  and support material  46 , system  10  may alternatively operate with a variety of different materials, such as two part materials with different compositions and/or different colors. Moreover, as shown in  FIGS. 4 and 5 , the additive manufacturing systems of the present disclosure may include additional development stations to build 3D parts and/or support structures with a variety of different compositions and colors. 
     For example,  FIG. 4  illustrates system  110 , which is similar to system  10  (shown in  FIGS. 1, 2A-2L, and 3A-3L ) for printing 3D part  118  and support structure  119  in a layer-by-layer manner using electrophotography, where reference numbers are increased by “100” from those of system  10 . As shown in  FIG. 4 , system  110  includes development stations  134   a  and  134   b , which correspond to development stations  34   a  and  34   b  of system  10 . However, system  110  also includes development stations  134   c  and  134   d , where development station  134   c  is located adjacent to development station  134   a , and development station  134   d  is located adjacent to development station  134   b . Suitable devices for development stations  134   c  and  134   d  includes those discussed above for development stations  34   a  and  34   b.    
     In this embodiment, development stations  134   c  and  134   d  allow system  110  to print 3D part  118  and/or support structure  119  with additional materials (e.g., additional colors). For example, development stations  134   c  and  134   d  may include colorants  180  and  182 , which may diffuse into the developed layers of part material  142  and/or support material  146 . As such, the layers of 3D part  118  and/or support structure  119  may be printed with different colors based on colorants  180  and  182 . Alternatively, development stations  134   a ,  134   c , and  134   d  may each include part materials  142 ,  180 , and  182  having different compositions or colors, where system  110  may selectively form layers of 3D part  118  with one or more of part materials  142 ,  180 , and  182 . 
     During operation, while rotating photoconductor drum  122  in the first rotational direction of arrow  140 , controller  120  may transfer part material  142  or colorant  180  by selectively engaging either development station  134   a  or development station  134   c  with surface  124 , thereby attracting the respective materials from the engaged development station. Alternatively, while rotating photoconductor drum  122  in the second rotational direction of arrow  144 , controller  120  may apply either support material  146  or part material  182  by selectively engaging either development station  134   b  or development station  134   d  with surface  124 , thereby attracting the respective material from the engaged development station. Accordingly, system  110  increases the number of materials that may be used to form 3D parts and support structures. 
       FIG. 5  illustrates system  210 , which is similar to system  10  (shown in  FIGS. 1, 2A-2L, and 3A-3L ) and system  110  (shown in  FIG. 4 ) for printing 3D part  218  and support structure  219  in a layer-by-layer manner using electrophotography, where reference numbers are increased by “200” from those of system  10 . As shown in  FIG. 5 , system  210  is a preferred embodiment of the present disclosure that includes carousels  284  and  286 , which are independently rotatable carousels that include development stations  284   a - 284   e  and  286   a - 286   e , receptively. 
     Development stations  284   a - 284   e  and  286   a - 286   e  may each function in a similar manner to development stations  34   a  and  34   b  of system  10 , and to development stations  134   a - 134   d  of system  110 . However, in this embodiment, development stations  284   a - 284   e  and  286   a - 286   e  allow system  210  to print 3D part  218  and/or support structure  219  with even more materials (e.g., additional colors). 
     For example, development station  284   a  may include a part material, development station  286   a  may include a support material, and development stations  284   b - 284   e  and  286   a - 286   e  may include different colorants that may diffuse into the developed layers of the part material and/or support material. In this embodiment, the colorants retained in development stations  284   b - 284   e  and  286   a - 286   e  may apply colors to the part and/or support materials using one or more color creation techniques, such as selective spot colors, subtractive color creation using cyan, yellow, magenta, and black materials, and/or additive color creation (e.g., partitive color creation) using cyan, yellow, magenta, red, green, blue, and black materials. 
     During operation, controller  220  may rotate carousels  284  and  286  independently of each other in a unidirectional or bi-directional manner, and independent from the rotations of photoconductor drum  212  and transfer drum  214 , to align the desired development stations of development stations  284   a - 284   e  and  286   a - 286   e  with photoconductor drum  212 . As such, system  212  may operate in the same manner as discussed above for system  10  to print 3D parts and support structures (e.g., 3D part  218  and support structure  219 ), where the materials from development stations  284   a - 284   e  and  286   a - 286   e  may be selectively developed and transferred to platen  216  to provide a variety of different functional and/or aesthetic qualities to the printed 3D parts and support structures. 
     The reciprocating or bi-directional rotations of photoconductor drum  222  and transfer drum  214  (and the reciprocating motions of platen  216 ) further allow controller  220  to selectively develop and transfer materials from either carousel  284  or carousel  286  based on the rotational directions of photoconductor drum  212  and transfer drum  214 . While illustrated with two carousels of development stations (i.e., carousels  284  and  286 ), system  210  may alternatively include three or more carousels of development stations. Furthermore, while illustrated with five development stations per carousel, the carousels of system  210  may alternatively include different numbers of development stations based on particular material requirements for printing 3D parts and support structures. Moreover, while depicted with carousels (i.e., carousels  284  and  286 ), system  210  may alternatively incorporate different mechanisms for selectively and interchangeably engaging the multiple development stations with photoconductor drum  222 . 
       FIG. 6  illustrates system  310 , which is similar to system  10  for printing 3D part  318  and support structure  319  in a layer-by-layer manner using electrophotography, where reference numbers are increased by “300” from those of system  10 . While illustrated with a pair of development stations  334   a  and  334   b , system  310  may alternatively include additional development stations, such as those of system  110  (shown in  FIG. 4 ) and/or system  210  (shown in  FIG. 5 ). 
     As shown in  FIG. 6 , system  310  may operate in the same manner as system  10 , and also includes heated chamber  388 . Heated chamber  388  extends around platen  316 , and defines an enclosable environment for printing 3D part  318  and support structure  319 . In the shown example, heated chamber  388  partially encloses z-axis gantry  362  and the bottom portion of transfer drum  314 , allowing z-axis gantry  362  and transfer drum  314  to extend through the walls of heated chamber  388 . For example, the ceiling portion of heated chamber  388  may include a slot opening (not shown) configured to receive the bottom portion of transfer drum  314  while transfer drum  314  rotates, while also desirably minimizing the dissipation of heat from heated chamber  388 . 
     Heated chamber  388  is configured to be heated to, and maintained at, one or more temperatures that are in a window between the solidification temperature and the creep relaxation temperature of part material  342  and/or support material  346 . This reduces the risk of mechanically distorting (e.g., curling) 3D part  318  and support structure  319 , where the creep relaxation temperature of part material  342  is proportional to the glass transition temperature of part material  342 . Examples of suitable techniques for determining the creep relaxation temperatures of the part and support materials are disclosed in Batchelder et al., U.S. Pat. No. 5,866,058. 
     The use of heated chamber  388  may require the use of cooling units  390  and  392 . Cooling units  390  and  392  are gas jet (e.g., air jet) units configured to blow localized air to the top layers of the stack of 3D part  318  and support structure  319  when platen  316  moves in the directions of arrows  364  and  366 . In an alternative embodiment, cooling units  390  and  392  may be integrated in fixing elements  372  and  374 . The use of fixing units  372  and  374 , and cooling units  390  and  392  further increase the capability of system  310  to provide variable temperature and pressure control to transfer the layers from transfer drum  314  to platen  316 . 
     Because system  310  is capable of printing layers of part material  342  and support material  346  at high speeds, the tackified materials for the printed layers do not have sufficient time to cool below their creep relaxation temperatures before successive layers are printed. As such, heat from the tackified materials can build up in the printed layers, preventing them from cooling down to sufficient temperatures to vertically support the successive layers. Cooling units  390  and  392  direct gas (e.g., air) to the top printed layer of the stack to cool the top layer down (e.g., to about the creep relaxation temperature of the part or support material). This allows the cooled layer to have sufficient strength to vertically support successively printed layers, while also reducing the risk of curling effects. 
     As mentioned above, proper transfer of the developed layers of part and support materials from transfer drum  314  to the top layer of the stack of 3D part  318  and support structure  319  is dependent on multiple factors, such as the pressure between transfer drum  314  and the top layer of the stack, the temperature of the layer being transferred (e.g., how tacky the material is), the contact duration between the transferred layer and the top layer of the stack, the adhesive properties of the part and support materials, and the like. 
     In one embodiment, system  310  operates with one or more feedback process control loops to monitor and adjust the height of platen  316  based on measured pressures between transfer drum  314  and platen  316 . In an additional embodiment, system  310  also operates with one or more feedback process control loops to monitor and adjust the temperatures of heaters  356   a  and  356   b , of heated chamber  388 , and of cooling units  390  and  392  based on measured temperatures of the layers of part and support materials. Examples of suitable techniques for such feedback process control loops are disclosed in co-filed U.S. Provisional Patent Application No. 61/538,491, and entitled “Layer Transfusion For Electrophotography-Based Additive Manufacturing”. 
     The terms “about” and “substantially” are used herein with respect to measurable values and ranges due to expected variations known to those skilled in the art (e.g., limitations and variabilities in measurements). All temperatures referred to herein are based on atmospheric pressure (i.e. one atmosphere). 
     Although the present disclosure has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the disclosure.