Patent Publication Number: US-2017355135-A1

Title: Feedback control system for printing 3d parts

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned, co-pending U.S. Patent Application Ser. No. 62/286,490, entitled: “Large format electrophotographic 3D printer,” by C. Sreekumar et al.; and to commonly assigned, co-pending U.S. patent application Ser. No. 15/091,789, entitled: “Printing 3D parts with controlled surface finish,” by T. Tombs et al., each which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains to the field of additive manufacturing systems for printing three-dimensional parts and support structures, and more particularly to a feedback control system for accurately printing three-dimensional parts with precise dimensions and accurately replicated features. 
     BACKGROUND OF THE INVENTION 
     Additive manufacturing systems are used to build three-dimensional (3D) parts from digital representations of the 3D parts using one or more additive manufacturing techniques. Common forms of such digital representations would include the well-known AMF and STL file formats. 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 a plurality of horizontal layers. For each sliced layer, a tool path is then generated, that 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 (sometimes referred to as a 3D model) can be printed from the 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 printhead of the system, and is deposited as a sequence of layers 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 printhead 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 defining the support structure for the overhanging or free-space segments of the 3D part being formed, and in some cases, for the sidewalls of the 3D part being formed. The support material adheres to the part material during fabrication, and is removable from the completed 3D part when the printing process is complete. 
     In two-dimensional (2D) printing, electrophotography (also known as xerography) is a technology for creating 2D images on planar substrates, such as printing paper and transparent substrates. Electrophotography systems typically include a conductive support drum coated with a photoconductive material layer, where latent electrostatic images are formed by electrostatic charging, followed by image-wise exposure of 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, or alternatively to discharged 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 and/or pressure. 
     U.S. Pat. No. 9,144,940 (Martin), entitled “Method for printing 3D parts and support structures with electrophotography-based additive manufacturing,” describes an electrophotography-based additive manufacturing method that is able to make a 3D part using a support material and a part material. The support material compositionally includes a first charge control agent and a first copolymer having aromatic groups, (meth)acrylate-based ester groups, carboxylic acid groups, and anhydride groups. The part material compositionally includes a second charge control agent, and a second copolymer having acrylonitrile units, butadiene units, and aromatic units. 
     The method described by Martin includes developing a support layer of the support structure from the support material with a first electrophotography engine, and transferring the developed support layer from the first electrophotography engine to a transfer medium. The method further includes developing a part layer of the 3D part from the part material with a second electrophotography engine, and transferring the developed part layer from the second electrophotography engine to the transfer medium. The developed part and support layers are then moved to a layer transfusion assembly with the transfer medium, where they are transfused together to previously-printed layers. 
     It is difficult to make an accurate reproduction of a three-dimensional part by transferring hundreds or thousands of toner layers one at a time to a platen when using only the spatial information of the desired end product is used. Small changes or drift in the registration of the layers caused by changes in temperature, slip in conveyance elements, vibration, drive variability or other disturbances create errors in the dimensions of the printed object. Electrophotographic interactions between subsystems often cause periodic and artifacts such as spots, bands and streaks in the printed layers. Non-periodic artifacts are also difficult to avoid in electrophotography. Hence, there is a need for a method to improve the accurate reproduction of the precise dimensions and features when printing a three-dimensional object using electrophotography, and to eliminate print artifacts associated with the electrophotographic process. 
     SUMMARY OF THE INVENTION 
     The present invention represents a method for printing a three-dimensional part and with an electrophotography-based additive manufacturing system, the method including: 
     providing a part material compositionally including part material particles; 
     developing a part layer of the three-dimensional part from the part material with a first electrophotography engine; 
     transferring the developed part layer from the first electrophotography engine to a transfer medium; 
     transfusing the transferred part layer together to previously-printed layers using a layer transfusion assembly; 
     measuring a surface height profile of the transfused part layers using a surface profilometer; and 
     controlling a thickness profile of a subsequently-printed part layer responsive to the measured surface height profile. 
     This invention has the advantage that print artifacts, misregistration and other errors caused by the printing process are compensated for so that the printed 3D part is a precise replica of the digital information describing the desired object to be printed. 
     It has the further advantage that part yield is improved, thereby reducing the cost to print 3D parts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic front view of an exemplary electrophotography-based additive manufacturing system for printing 3D parts and support structures from part and support materials; 
         FIG. 2  is a schematic front view showing additional details of the electrophotography engines in the additive manufacturing system of  FIG. 1 ; 
         FIG. 3  is a schematic front view showing an alternative electrophotography engine, which includes an intermediary drum or belt; 
         FIG. 4  is a schematic front view illustrating a layer transfusion assembly for performing layer transfusion steps; 
         FIG. 5  is a flowchart showing a method for constructing a 3D part and support structure in accordance with an exemplary embodiment; and 
         FIG. 6  is a schematic front view illustrating a layer transfusion assembly for performing layer transfusion steps including a surface profilometer for measuring surface height profiles in accordance with the present invention. 
     
    
    
     It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is inclusive of combinations of the embodiments described herein. References to “a particular embodiment” and the like refer to features that are present in at least one embodiment of the invention. Separate references to “an embodiment” or “particular embodiments” or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to the “method” or “methods” and the like is not limiting. It should be noted that, unless otherwise explicitly noted or required by context, the word “or” is used in this disclosure in a non-exclusive sense. 
       FIGS. 1-4  illustrate an exemplary additive manufacturing system  10 , which uses an electrophotography-based additive manufacturing process for printing 3D parts from a part material (e.g., an ABS part material), and associated support structures from a removable support material. As shown in  FIG. 1 , additive manufacturing system  10  includes a pair of electrophotography (EP) engines  12   p  and  12   s , belt transfer assembly  14 , biasing mechanisms  16  and  18 , and layer transfusion assembly  20 . 
     Examples of suitable components and functional operations for additive manufacturing system  10  include those disclosed in U.S. Patent Application Publication No. 2013/0077996 (Hanson et al.), entitled “Electrophotography-based additive manufacturing system with reciprocating operation;” in U.S. Patent Application Publication No. 2013/0077997 (Hanson et al.), entitled “Electrophotography-based additive manufacturing system with transfer-medium service loop;” in U.S. Patent Application Publication No. 2013/0186549 (Comb et al.), entitled “Layer transfusion for additive manufacturing;” and in U.S. Patent Application Publication No. 2013/0186558 (Comb et al.), entitled “Layer transfusion with heat capacitor belt for additive manufacturing,” each of which is incorporated herein by reference. 
     EP engines  12   p  and  12   s  are imaging engines for respectively imaging or otherwise developing layers of the part and support materials, where the part and support materials are each preferably engineered for use with the particular architecture of EP engine  12   p  and  12   s . The part material compositionally includes part material particles, and the support compositionally includes support material particles. In an exemplary embodiment, the support material compositionally includes support material particles including a first charge control agent and a first copolymer having aromatic groups, (meth)acrylate-based ester groups, carboxylic acid groups, and anhydride groups; and the part material compositionally includes part material particles including a second charge control agent, and a second copolymer having acrylonitrile units, butadiene units, and aromatic units. As discussed below, the developed part and support layers are transferred to belt transfer assembly  14  (or some other appropriate transfer medium) with biasing mechanisms  16  and  18 , and carried to the layer transfusion assembly  20  to produce the 3D parts and associated support structures in a layer-by-layer manner. 
     In the illustrated configuration, belt transfer assembly  14  includes transfer belt  22 , which serves as the transfer medium, belt drive mechanisms  24 , belt drag mechanisms  26 , loop limit sensors  28 , idler rollers  30 , and belt cleaner  32 , which are configured to maintain tension on the transfer belt  22  while transfer belt  22  rotates in rotational direction  34 . In particular, the belt drive mechanisms  24  engage and drive the transfer belt  22 , and the belt drag mechanisms  26  function as brakes to provide a service loop design for protecting the transfer belt  22  against tension stress, based on monitored readings from the loop limit sensors  28 . 
     Additive manufacturing system  10  also includes a controller  36 , which includes one or more control circuits, microprocessor-based engine control systems, or digitally-controlled raster imaging processor systems, and which is configured to operate the components of additive manufacturing system  10  in a synchronized manner based on printing instructions received from a host computer  38 . Host computer  38  includes one or more computer-based systems configured to communicate with controller  36  to provide the print instructions (and other operating information). For example, host computer  38  can transfer information to controller  36  that relates to the individual layers of the 3D parts and support structures, thereby enabling additive manufacturing system  10  to print the 3D parts and support structures in a layer-by-layer manner. 
     The components of additive manufacturing system  10  are typically retained by one or more frame structures, such as frame  40 . Additionally, the components of additive manufacturing system  10  are preferably retained within an enclosable housing (not shown) that prevents ambient light from being transmitted to the components of additive manufacturing system  10  during operation. 
       FIG. 2  illustrates EP engines  12   p  and  12   s  in additional detail. EP engine  12   s  (i.e., the upstream EP engine relative to the rotational direction  34  of transfer belt  22 ) develops layers of support material  66   s , and EP engine  12   p  (i.e., the downstream EP engine relative to the rotational direction  34  of transfer belt  22 ) develops layers of part material  66   p . In alternative configurations, the arrangement of EP engines  12   p  and  12   s  can be reversed such that EP engine  12   p  is upstream from EP engine  12   s  relative to the rotational direction  34  of transfer belt  22 . In other alternative configuration, additive manufacturing system  10  can include one or more additional EP engines for printing layers of additional materials. 
     In the illustrated configuration, EP engines  12   p  and  12   s  utilize identical components, including photoconductor drums  42 , each having a conductive drum body  44  and a photoconductive surface  46 . Conductive drum body  44  is an electrically-conductive drum (e.g., fabricated from copper, aluminum, tin, or the like) that is electrically grounded and configured to rotate around shaft  48 . Shaft  48  is correspondingly connected to drive motor  50 , which is configured to rotate the shaft  48  (and the photoconductor drum  42 ) in rotation direction  52  at a constant rate. 
     Photoconductive surface  46  is a thin film extending around the circumferential surface of conductive drum body  44 , and is preferably derived from one or more photoconductive materials, such as amorphous silicon, selenium, zinc oxide, organic materials, and the like. As discussed below, photoconductive surface  46  is configured to receive latent-charged images of the sliced layers of a 3D part or support structure (or negative images), and to attract charged particles of the part or support material of the present disclosure to the charged (or discharged image areas), thereby creating the layers of the 3D part and support structures. 
     As further shown, EP engines  12   p  and  12   s  also include charging device  54 , imager  56 , development station  58 , cleaning station  60 , and discharge device  62 , each of which is in signal communication with controller  36 . Charging device  54 , imager  56 , development station  58 , cleaning station  60 , and discharge device  62  accordingly define an image-forming assembly for surface  46  while drive motor  50  and shaft  48  rotate photoconductor drum  42  in the rotation direction  52 . 
     In the illustrated example, the image-forming assembly for photoconductive surface  46  of EP engine  12   s  is used to form support material layers  64   s  of support material  66   s , where a supply of support material  66   s  is retained by development station  58  of EP engine  12   s , along with associated carrier particles. Similarly, the image-forming assembly for photoconductive surface  46  of EP engine  12   p  is used to form part material layers  64   p  of part material part material  66   p , where a supply of part material  66   p  is retained by development station  58  of EP engine  12   p , along with associated carrier particles. Charging device  54  is configured to provide a uniform electrostatic charge on the photoconductive surface  46  as the photoconductive surface  46  rotates in the rotation direction  52  past the charging device  54 . Suitable devices that can be used for the charging device  54  include corotrons, scorotrons, charging rollers, and other electrostatic devices. 
     Imager  56  is a digitally-controlled, pixel-wise light exposure apparatus configured to selectively emit electromagnetic radiation toward the uniform electrostatic charge on the photoconductive surface  46  as the photoconductive surface  46  rotates in the rotation direction  52  past the imager  56 . The selective exposure of the electromagnetic radiation on the photoconductive surface  46  is controlled by the controller  36 , and causes discrete pixel-wise locations of the electrostatic charge to be removed (i.e., discharged to ground), thereby forming latent image charge patterns on the photoconductive surface  46 . The imager  56  in the EP engine  12   p  is controlled to provide a latent image charge pattern in accordance with a specified pattern for a particular part material layer  64   p , and the imager  56  in the EP engine  12   s  is controlled to provide a latent image charge pattern in accordance with a specified pattern for a corresponding support material layer  64   s.    
     Suitable devices for imager  56  include scanning laser light sources (e.g., gas or solid state lasers), light emitting diode (LED) array exposure devices, and other exposure device conventionally used in 2D electrophotography systems. In alternative embodiments, suitable devices for charging device  54  and imager  56  include ion-deposition systems configured to selectively deposit charged ions or electrons directly to the photoconductive surface  46  to form the latent image charge pattern. As such, as used herein, the term “electrophotography” includes “ionography.” 
     Each development station  58  is an electrostatic and magnetic development station or cartridge that retains the supply of part material  66   p  or support material  66   s , preferably in powder form, along with associated carrier particles. The development stations  58  typically function in a similar manner to single or dual component development systems and toner cartridges used in 2D electrophotography systems. For example, each development station  58  can include an enclosure for retaining the part material  66   p  or support material  66   s  and carrier particles. When agitated, the carrier particles generate triboelectric charges to attract the part material particles of the part material  66   p  or the support material particles of the support material  66   s , which charges the attracted particles to a desired sign and magnitude, as discussed below. 
     Each development station  58  typically include one or more devices for transferring the charged part material  66   p  or support material  66   s  to the photoconductive surface  46 , such as conveyors, fur brushes, paddle wheels, rollers or magnetic brushes. For instance, as the photoconductive surface  46  (having the latent image charge pattern) rotates past the development station  58  in the rotation direction  52 , the particles of charged part material  66   p  or support material  66   s  are attracted to the appropriately charged regions of the latent image on the photoconductive surface  46 , utilizing either charged area development or discharged area development (depending on the electrophotography mode being utilized). This creates successive part material layers  64   p  and support material layers  64   s  as the photoconductor drum  42  continues to rotate in the rotation direction  52 , where the successive part material layers  64   p  and support material layers  64   s  correspond to the successive sliced layers of the digital representation of the 3D part and support structures. 
     The successive part material layers  64   p  and support material layers  64   s  are then rotated with photoconductive surfaces  46  in the rotation direction  52  to a transfer region in which the part material layers  64   p  and support material layers  64   s  are successively transferred from the photoconductor drums  42  to the transfer belt  22 , as discussed below. While illustrated as a direct engagement between photoconductor drum  42  and transfer belt  22 , in some preferred embodiments, EP engines  12   p  and  12   s  may also include intermediary transfer drums or belts, as discussed further below. The EP engines  12   p  and  12   s  are configured so that the part material layers  64   p  are transferred onto the transfer belt in registration with the corresponding support material layers  64   s  to provide combined layers  64 . 
     After a given part material layer  64   p  or support material layer  64   s  is transferred from the photoconductor drum  42  to the transfer belt  22  (or an intermediary transfer drum or belt), drive motor  50  and shaft  48  continue to rotate the photoconductor drum  42  in the rotation direction  52  such that the region of the photoconductive surface  46  that previously held the developed layer passes the cleaning station  60 . The cleaning station  60  is configured to remove any residual, non-transferred portions of part material  66   p  or support material  66   s  from the photoconductive surface  46 . Suitable types of cleaning devices for use in the cleaning station  60  include blade cleaners, brush cleaners, electrostatic cleaners, vacuum-based cleaners, and combinations thereof. 
     After passing the cleaning station  60 , the photoconductive surface  46  continues to rotate in the rotation direction  52  such that the cleaned regions of the photoconductive surface  46  pass by the discharge device  62  to remove any residual electrostatic charge on photoconductive surface  46  prior to starting the next cycle. Suitable types of discharge devices  62  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. 
     The transfer belt  22  is a transfer medium for transporting the developed part material layers  64   p  and support material layers  64   s  from photoconductor drum  42  (or an intermediary transfer drum or belt) to the layer transfusion assembly  20  ( FIG. 1 ). Examples of suitable types of transfer belts  22  include those disclosed in Comb et al. in the aforementioned U.S. Patent Application Publication No. 2013/0186549 and U.S. Patent Application Publication No. 2013/0186558 by Comb et al. The transfer belt  22  includes a front surface  22   a  and a rear surface  22   b , where the front surface  22   a  faces the photoconductive surfaces  46  of photoconductor drums  42  and the rear surface  22   b  is in contact with biasing mechanisms  16  and  18 . 
     Biasing mechanisms  16  and  18  are configured to induce electrical potentials through transfer belt  22  to electrostatically attract the part material layers  64   p  and support material layers  64   s  from EP engines  12   p  and  12   s , respectively, to the transfer belt  22 . Because the part material layers  64   p  and support material layers  64   s  each represent only a single layer increment in thickness at this point in the process, electrostatic attraction is suitable for transferring the part material layers  64   p  and support material layers  64   s  from the EP engines  12   p  and  12   s  to the transfer belt  22 . 
     Preferably, the controller  36  rotates the photoconductor drums  42  of EP engines  12   p  and  12   s  at the same rotational rates, such that the tangential velocity of the photoconductive surfaces  46  are synchronized with the line speed of the transfer belt  22  (as well as with any intermediary transfer drums or belts). This allows the additive manufacturing system  10  to develop and transfer the part material layers  64   p  and support material layers  64   s  in coordination with each other from separate developed images. In particular, as shown, each part material layer  64   p  is transferred to transfer belt  22  in proper registration with each support material layer  64   s  to produce the combined layer  64 . As discussed below, this allows the part material layers  64   p  and support material layers  64   s  to be transfused together. To enable this, the part material  66   p  and support material  66   s  preferably have thermal properties and melt rheologies that are the same or substantially similar. Within the context of the present invention, “substantially similar thermal properties and melt rheologies” should be interpreted to be within 20% of regularly measured properties such as glass transition temperature, melting point and melt viscosity. As can be appreciated, some combined layers  64  transported to layer transfusion assembly  20  may only include support material  66   s  or may only include part material  66   p , depending on the particular support structure and 3D part geometries and layer slicing. 
     In an alternative and generally less-preferred configuration, part material layers  64   p  and support material layers  64   s  may optionally be developed and transferred along transfer belt  22  separately, such as with alternating part material layers  64   p  and support material layers  64   s . These successive, alternating layers  64   p  and  64   s  may then be transported to layer transfusion assembly  20 , where they may be transfused separately to print the 3D part and support structure. 
     In some configurations, one or both of EP engines  12   p  and  12   s  can also include one or more intermediary transfer drums or belts between the photoconductor drum  42  and the transfer belt  22 . For example,  FIG. 3  illustrates an alternate configuration for an EP engine  12   p  that also includes an intermediary drum  42   a . The intermediary drum  42   a  rotates in a rotation direction  52   a  opposite to the rotation direction  52 , under the rotational power of drive motor  50   a . Intermediary drum  42   a  engages with photoconductor drum  42  to receive the developed part material layers  64   p  from the photoconductor drum  42 , and then carries the received part material layers  64   p  and transfers them to the transfer belt  22 . 
     In some configurations, the EP engine  12   s  ( FIG. 2 ) can use a same arrangement using an intermediary drum  42   a  for carrying the developed support material layers  64   s  from the photoconductor drum  42  to the transfer belt  22 . The use of such intermediary transfer drums or belts for EP engines  12   p  and  12   s  can be beneficial for thermally isolating the photoconductor drum  42  from the transfer belt  22 , if desired. 
       FIG. 4  illustrates an exemplary configuration for the layer transfusion assembly  20 . In the illustrated embodiment, the layer transfusion assembly uses a heating process to fuse the combined layer  64  to the previously printed layers of the 3D part  80  and support structure  82 . In other embodiments, the layer transfusion assembly  20  can use other types of transfusion processes to perform the fusing operation. For example, a solvent process can be used to soften the part material  66   p  and the support material  66   s  so that they can be fused to the previously printed layers of the 3D part  80  and support structure  82  by pressing them together. 
     As shown, the layer transfusion assembly  20  includes build platform  68 , nip roller  70 , heaters  72  and  74 , post-fuse heater  76 , and air jets  78  (or other cooling units). Build platform  68  is a platform assembly or platen that is configured to receive the heated combined layers  64  (or separate part material layers  64   p  and support material layers  64   s ) for printing a 3D part  80  and support structure  82 , in a layer-by-layer manner. In some configurations, the build platform  68  may include removable film substrates (not shown) for receiving the combined layers  64 , where the removable film substrates may be restrained against build platform using any suitable technique (e.g., vacuum drawing, removable adhesive, mechanical fastener, and the like). 
     The build platform  68  is supported by gantry  84 , which is a gantry mechanism configured to move build platform  68  along the z-axis and the x-axis in a reciprocating rectangular motion pattern  86 , where the primary motion is back-and-forth along the x-axis. Gantry  84  may be operated by a motor  88  based on commands from the controller  36 , where the motor  88  can be an electrical motor, a hydraulic system, a pneumatic system, or the like. 
     In the illustrated configuration, the build platform  68  is heatable with heating element  90  (e.g., an electric heater). Heating element  90  is configured to heat and maintain the build platform  68  at an elevated temperature that is greater than room temperature (e.g., about 25° C.), such as at a desired average part temperature of 3D part  80  and support structure  82 , as discussed by Comb et al. in the aforementioned U.S. Patent Application Publication No. 2013/0186549 and U.S. Patent Application Publication No. 2013/0186558. This allows build platform  68  to assist in maintaining the 3D part  80  and support structure  82  at the desired average part temperature. 
     Nip roller  70  is a heatable element or a heatable layer transfusion element, which is configured to rotate around a fixed axis with the movement of transfer belt  22 . In particular, nip roller  70  may roll against the rear surface  22   b  in rotation direction  92  while the transfer belt  22  rotates in the rotation direction  34 . In the illustrated configuration, nip roller  70  is heatable with heating element  94  (e.g., an electric heater). Heating element  94  is configured to heat and maintain nip roller  70  at an elevated temperature that is greater than the room temperature (e.g., 25° C.), such as at a desired transfer temperature for combined layers  64 . 
     Heater  72  includes one or more heating device (e.g., an infrared heater or a heated air jet) configured to heat the combined layers  64  to a temperature near an intended transfer temperature of the part material  66   p  and support material  66   s , such as at least a fusion temperature of the part material  66   p  and support material  66   s , preferably prior to reaching nip roller  70 . Each combined layer  64  preferably passes by (or through) heater  72  for a sufficient residence time to heat the combined layer  64  to the intended transfer temperature. Heater  74  may function in the same manner as heater  72 , and heats the top surfaces of 3D part  80  and support structure  82  to an elevated temperature, such as at the same transfer temperature as the heated combined layers  64  (or other suitable elevated temperature). 
     As mentioned above, the support material  66   s  used to print support structure  82  preferably has thermal properties (e.g., glass transition temperature) and a melt rheology that are similar to or substantially the same as the thermal properties and the melt rheology of the part material  66   p  used to print 3D part  80 . This enables the part material  66   p  of the part material layer  64   p  and the support material  66   s  of the support material layer  64   s  to be heated together with heater  74  to substantially the same transfer temperature, and also enables the part material  66   p  and support material  66   s  at the top surfaces of 3D part  80  and support structure  82  to be heated together with heater  74  to substantially the same temperature. Thus, the part material layers  64   p  and the support material layers  64   s  can be transfused together to the top surfaces of 3D part  80  and support structure  82  in a single transfusion step as combined layer  64 . This single transfusion step for transfusing the combined layer  64  is typically impractical without sufficiently matching the thermal properties and the melt rheologies of the part material  66   p  and support material  66   s.    
     Post-fuse heater  76  is located downstream from nip roller  70  and upstream from air jets  78 , and is configured to heat the transfused layers to an elevated temperature to perform a post-fuse or heat-setting operation. Again, the similar thermal properties and melt rheologies of the part and support materials enable the post-fuse heater  76  to post-heat the top surfaces of 3D part  80  and support structure  82  together in a single post-fuse step. 
     Prior to printing 3D part  80  and support structure  82 , build platform  68  and nip roller  70  may be heated to their desired temperatures. For example, build platform  68  may be heated to the average part temperature of 3D part  80  and support structure  82  (due to the similar melt rheologies of the part and support materials). In comparison, nip roller  70  may be heated to a desired transfer temperature for combined layers  64  (also due to the similar thermal properties and melt rheologies of the part and support materials). 
     During the printing operation, transfer belt  22  carries a combined layer  64  past heater  72 , which may heat the combined layer  64  and the associated region of transfer belt  22  to the transfer temperature. Suitable transfer temperatures for the part and support materials include temperatures that exceed the glass transition temperatures of the part material  66   p  and the support material  66   s , which are preferably similar or substantially the same, and where the part material  66   p  and support material  66   s  of combined layer  64  are softened but not melted (e.g., to a temperature ranging from about 140° C. to about 180° C. for an ABS part material). 
     As further shown in the exemplary configuration of  FIG. 4 , during operation, gantry  84  moves the build platform  68  (with 3D part  80  and support structure  82 ) in a reciprocating rectangular motion pattern  86 . In particular, the gantry  84  moves build platform  68  along the x-axis below, along, or through heater  74 . Heater  74  heats the top surfaces of the 3D part  80  and support structure  82  to an elevated temperature, such as the transfer temperatures of the part and support materials. As discussed by Comb et al. in the aforementioned U.S. Patent Application Publication No. 2013/0186549 and U.S. Patent Application Publication No. 2013/0186558, heaters  72  and  74  can heat the combined layers  64  and the top surfaces of the 3D part  80  and support structure  82  to about the same temperatures to provide a consistent transfusion interface temperature. Alternatively, heaters  72  and  74  can heat the combined layers  64  and the top surfaces of the 3D part  80  and support structure  82  to different temperatures to attain a desired transfusion interface temperature. 
     The continued rotation of transfer belt  22  and the movement of build platform  68  align the heated combined layer  64  with the heated top surfaces of the 3D part  80  and support structure  82  with proper registration along the x-axis. The gantry  84  continues to move the build platform  68  along the x-axis at a rate that is synchronized with the tangential velocity of the transfer belt  22  (i.e., the same directions and speed). This causes rear surface  22   b  of the transfer belt  22  to rotate around nip roller  70  and brings the heated combined layer  64  into contact with the top surfaces of 3D part  80  and support structure  82 . This presses the heated combined layer  64  between the front surface  22   a  of the transfer belt  22  and the heated top surfaces of 3D part  80  and support structure  82  at the location of nip roller  70 , which at least partially transfuses the heated combined layer  64  to the top layers of 3D part  80  and support structure  82 . 
     As the transfused combined layer  64  passes the nip of nip roller  70 , the transfer belt  22  wraps around nip roller  70  to separate and disengage the transfer belt from the build platform  68 . This assists in releasing the transfused combined layer  64  from the transfer belt  22 , enabling the transfused combined layer  64  to remain adhered to the 3D part  80  and the support structure  82 , thereby adding a new layer to the 3D part and the support structure  82 . Maintaining the transfusion interface temperature at a transfer temperature that is higher than the glass transition temperatures of the part and support materials, but lower than their fusion temperatures, enables the heated combined layer  64  to be hot enough to adhere to 3D part  80  and support structure  82 , while also being cool enough to readily release from transfer belt  22 . Additionally, as discussed earlier, the similar thermal properties and melt rheologies of the part and support materials allow them to be transfused in the same step. 
     After release, the gantry  84  continues to move the build platform  68  along the x-axis to the post-fuse heater  76 . At the post-fuse heater  76 , the top-most layers of 3D part  80  and support structure  82  (including the transfused combined layer  64 ) are preferably heated to at least the fusion temperature of the part and support materials in a post-fuse or heat-setting step. This melts the part and support materials of the transfused layer  64  to a highly fusible state such that polymer molecules of the transfused layer  64  quickly inter-diffuse to achieve a high level of interfacial entanglement with the 3D part  80  and the support structure  82 . 
     The gantry  84  continues to move the build platform  68  along the x-axis past post-fuse heater  76  to air jets  78 , the air jets  78  blow cooling air towards the top layers of 3D part  80  and support structure  82 . This actively cools the transfused layer  64  down to the average part temperature, as discussed by Comb et al. in the aforementioned U.S. Patent Application Publication No. 2013/0186549 and U.S. Patent Application Publication No. 2013/0186558. 
     To assist in keeping 3D part  80  and support structure  82  at the desired average part temperature, in some arrangements, one or both of the heater  74  and post-heater  76  can be configured to operate to heat only the top-most layers of 3D part  80  and support structure  82 . For example, in embodiments in which heaters  72 ,  74 , and  76  are configured to emit infrared radiation, 3D part  80  and support structure  82  can include heat absorbers or other colorants configured to restrict penetration of the infrared wavelengths to within only the top-most layers. Alternatively, heaters  72 ,  74 , and  76  can be configured to blow heated air across the top surfaces of 3D part  80  and support structure  82 . In either case, limiting the thermal penetration into 3D part  80  and support structure  82  allows the top-most layers to be sufficiently transfused, while also reducing the amount of cooling required to keep 3D part  80  and support structure  82  at the desired average part temperature. 
     The EP engines  12   p  and  12   s  have an associated maximum printable area. For example, the EP engines in the NexPress SX3900 have a maximum printing width in the cross-track direction (i.e., the y-direction) of about 340 mm, and a maximum printing length in the in-track direction (i.e., the x-direction) of about 904 mm. When building a 3D part  80  and support structure  82  having a footprint that is smaller than the maximum printable area of the EP engines  12   p  and  12   s , the gantry  84  next actuates the build platform  68  downward, and moves the build platform  68  back along the x-direction following the reciprocating rectangular motion pattern  86  to an appropriate starting position in the x-direction in proper registration for transfusing the next combined layer  64 . In some embodiments, the gantry  84  may also actuate the build platform  68  with the 3D part  80  and support structure  82  upward to bring it into proper registration in the z-direction for transfusing the next combined layer  64 . (Generally the upward movement will be smaller than the downward movement to account for the thickness of the previously printed layer.) The same process is then repeated for each layer of 3D part  80  and support structure  82 . 
     In prior art arrangements, the size of the 3D parts  80  that could be fabricated was limited by the maximum printable area of the EP engines  12   p  and  12   s . It would be very costly to develop specially designed EP engines  12   p  and  12   s  having maximum printable areas that are larger than those used in typical printing systems. Commonly assigned, co-pending U.S. Patent Application No. 62/286,490, entitled “Large format electrophotographic 3D printer,” which is incorporated herein by reference, describes methods for using EP engines to produce large parts by printing into a plurality of tile regions on a large build platform. 
       FIG. 5  shows a flow chart summarizing a method for constructing a 3D part and support structure  270  from a support material  66   s  and a part material  66   p  using an additive manufacturing system  10 , such as that shown in  FIG. 1 , in accordance with the present invention. The part to be constructed is specified using part and support structure shape data  205 , which is a digital representation specifying the desired shape of the 3D part and support structure  270 . Common forms of such digital representations would include the well-known AMF and STL file formats. 
     The 3D part and support structure  270  is formed in a layer-by-layer manner using a layer formation process  200 . A develop support structure layer step  220  is used to develop a support material layer  64   s  ( FIG. 2 ) of the support structure  82  ( FIG. 4 ) from the support material  66   s  ( FIG. 2 ) using a first EP engine  12   s  ( FIG. 2 ). The developed support material layer  64   s  is transferred from the first EP engine  12   s  to a transfer belt  22  ( FIG. 2 ), or some other appropriate transfer medium, using a transfer support structure layer to transfer medium step  230 . Similarly, a develop part structure layer step  225  is used to develop a part material layer  64   p  ( FIG. 2 ) of the 3D part  80  ( FIG. 4 ) from the part material  66   p  ( FIG. 2 ) corresponding to the content to be constructed using a second EP engine  12   p  ( FIG. 2 ). The developed part material layer  64   p  is then transferred from the second EP engine  12   p  to the transfer belt  22  using a transfer part structure layer to transfer medium step  235 . As discussed earlier, the developed part material layer  64   p  is preferably transferred to the transfer belt  22  in registration with the developed support material layer  64   s  to form a combined layer  64  ( FIG. 2 ). 
     A move transfer medium to layer transfusion assembly step  240  is then used to move the transfer medium (e.g., transfer belt  22 ) bearing the developed part material layer  64   p  and developed support material layer  64   s  to a layer transfusion assembly  20  ( FIG. 4 ). The transfer belt  22  is aligned with an appropriate starting position of the build platform  68  of the layer transfusion assembly  20 . A transfuse part and support structure layer to previous layers step  245  is then used to transfuse the developed part material layer  64   p  and developed support material layer  64   s , adding a layer to the 3D part  80  and support structure  82 , providing a transfused part and support layer  250 . 
     In some embodiments, a tiling method can be used to form a 3D part and support structure  270  having a larger footprint than the EP engines  12   s ,  12   p  can provide. For example, the method described in commonly assigned, co-pending U.S. Patent Application Ser. No. 62/286,490, entitled: “Large format electrophotographic 3D printer,” by C. Sreekumar et al., which is incorporated herein by reference, can be used to form large format 3D parts. 
     A measure surface height profile step  255  is then used to measure a surface height profile  260  of the transfused layers of the 3D part  80  and support structure  82 . In an exemplary embodiment, the measure surface height profile step  255  measures the surface height profile  260  using a surface profilometer  100  as shown in  FIG. 6 . Within the context of the present disclosure, a surface height profile can include one or more surface height measurements that characterize the height of the surface of the transfused layers of the 3D part  80  and support structure  82 . In some embodiments, the surface height profile can be a point measurement of the surface height at a particular position. In other embodiments, the surface height profile can include a plurality of surface height measurements taken at a set of different positions, which can vary in one or both of the x- and y-directions. 
     Any type of surface profilometer  100  known in the art can be used in accordance with the present invention. In the illustrated configuration, the surface profilometer is positioned to measure the surface profile after the current layer has been transfused onto the build platform  68 , but before the build platform returns to the start position. However, it will be obvious to one skilled in the art that the surface profilometer  100  can be positioned in a variety of locations. 
     Surface profilometers  100  can generally be categorized as either contact or non-contact devices. For contact devices, a physical probe  102  contacts the surface of the transfused layers of the 3D part  80  and support structure  82 , and the surface height is determined by providing an electrical signal representing the displacement of the probe  102 . There are many ways that surface profilometer  100  can measure the displacement of the probe  102 , including the use of capacitive, inductive or resistive elements. In an exemplary embodiment, the surface profilometer  100  uses a linear variable differential transformer (LVDT), such as those available from Measurement Specialties of Hampton, Va. Further information about the operation of such devices can be found in the Application Note “The LVDT: construction and principle of operation” available from the Measurement Specialties web site. An LVDT is an absolute displacement transducer that converts a linear displacement into a proportional electrical signal containing phase (for direction) and amplitude information (for distance). The LVDT operation does not require electrical contact between the moving part (probe  102 ) and the transformer (which is a type of inductive element), but rather relies on electromagnetic coupling. 
     For non-contact devices, the probe  102  is not a mechanical element that physically touches the surface of the transfused layers of the 3D part  80  and support structure  82 , but rather senses the surface position by directing a beam of some sort (e.g., electromagnetic or acoustic) onto the surface and detecting a reflected beam. In such cases, the beam serves as the probe  102 . 
     One type of non-contact device that can be used for the profilometer  100  is an optical device. In such devices, a radiation-emitting element (e.g., a laser) is used to direct a beam of radiation onto the surface of the transfused layers of the 3D part  80  and support structure  82 . A detector is then used to sense the surface height based on detecting radiation reflected from the surface. In some optical profilometer devices, the beam of radiation is directed onto the surface at an angle relative to the surface normal. The detector captures an image of the surface and determines a spatial position of the reflected beam. The surface height can then be determined based on the well-known parallax effect. In other optical sensing devices, the surface height can be determined using interference effects or other optical sensing methods. In some embodiments, the LJ-G series High-accuracy 2D Laser Displacement Sensor available from the Keyence Corporation of Itasca, Ill. can be used to provide 2D surface height profiles. 
     Another type of non-contact device that can be used for the profilometer is an ultrasonic sensing device. Ultrasonic sensing devices typically direct an ultrasonic beam in the form of a short burst of ultrasonic sound waves toward the surface. When the sound is reflected, it returns to the sensor as an echo. The distance between the ultrasonic sensor and the target is calculated from the signal&#39;s return time and the propagation velocity of the measurement medium. In some embodiments, the M30M1 Series Ultrasonic Sensors available from the Balluff, Inc. of Florence, Ky. can be used to provide surface height profiles. 
     In some configurations, the surface profilometer  100  can use a 2D surface height sensor that directly provides a 2D surface height profile of the surface of the transfused layers of the 3D part  80  and support structure  82 . In other configurations, the surface profilometer  100  can use a 1D surface height sensor that provides a 1D surface height profile or one or more point surface height sensors that determine a surface height profile at a single point. In such cases, 2D surface height profiles can be determined by measuring surface heights as the build platform  68  moves the surface of the transfused layers of the 3D part  80  and support structure  82  past the surface profilometer  100 , or as the surface profilometer  100  is moved past the surface of the transfused layers of the 3D part  80  and support structure  82 . In one exemplary configuration, a plurality of point surface height sensors are positioned at different locations across the width (i.e., the y-direction) of the build platform  68 , and 1D surface height profiles are determined at each location as the build platform  68  moves the surface of the transfused layers of the 3D part  80  and support structure  82  past the point sensors in the x-direction. This effectively provides a 2D surface height profile. In this case, the resolution of the surface height profile in the cross-track y-direction will be defined by the number of point surface height sensors, and therefore may be lower than the resolution in the in-track x-direction, which will be defined by the measurement sampling frequency. 
     Increasing the number of surface height measurements in one or both of the x- and y-directions improves the resolution of the surface height profile  260 , enabling more accurate corrections, but it also increases the amount of time needed for the measurements. If the time to make the measure the surface height profile  260  becomes excessive, it may be desirable to use two different build platforms  68  so that a second part can be printed and transfused while the surface height profile  260  for the first part is being measured. 
     In some configurations, the surface profilometer  100  may be arranged to only provide surface height profile information for the most important portions of the 3D part  80  and support structure  82  (e.g., near edges of the 3D part  80 ), or to provide higher resolution surface height profile information in the most important portion. For example, the locations of one or more point sensors can be repositioned on a layer-by-layer basis based on the part geometry specified by the part and support structure shape data  205  ( FIG. 5 ) in order to focus on the most important portions of each layer. 
     Returning to a discussion of  FIG. 5 , a control thickness for additional layers step  265  is next used to adjust one or more settings of the additive manufacturing system  10  ( FIG. 1 ) in order to control the thickness profile of subsequently-printed part and support structure layers formed using the layer formation process  200  responsive to the measured surface height profile  260 . In an exemplary embodiment, a control system  104  ( FIG. 6 ) is used to analyze the measured surface height profile  260  and adjust one or more settings associated with the EP engines  12   p ,  12   s  ( FIG. 2 ) to control the thickness profile of the subsequently-printed part and support structure layers. In some cases, the control system  104  can be the controller  36  ( FIG. 1 ), or can be a component of the controller  36 . In other cases, the control system  104  can be an independent data processing system. 
     The layer formation process  200  is repeated for each of the layers that make up the 3D part and support structure  270 , with the surface height profile  260  being measured for each iteration and used to control the thickness of subsequent layers. After repeating the layer formation process  200  for all of the layers, the resulting 3D part and support structure  270  is removed from the additive manufacturing system  10  and post-printing operations can be used to remove the support structure  82 , leaving the final 3D part  80 . 
     In some configurations, the surface height profile  260  is analyzed to determine an overall height error by comparing an overall height of the transfused layers relative to an expected nominal height. The overall height of the transfused layers can be determined by measuring the surface height at an appropriate location, or more preferably can be determined by averaging the surface height determined at a plurality of locations across the surface of the transfused layers of the 3D part  80  and support structure  82 . The overall layer thickness of subsequently printed layers can then be controlled in accordance with the determined overall height error. For example, if it is determined that the overall surface height is slightly lower (or higher) than the expected nominal height, then the layer thickness of one or more of the subsequently printed layers can be increased (or decreased) in order to compensate for the height error. Consider the case where the nominal layer thickness is 40 μm. After printing 10 layers, the expected surface height of the top layer would be 10×40 μm=400 μm. However, if analyzing the measured surface height profile  260  shows that the average surface height is 398 μm, then the EP engines  12   p ,  12   s  can be controlled to increase the layer thickness of one or more of the subsequent layers until the average surface height matches the expected surface height. For small surface height errors, it may be possible to fully correct the error in a single layer. For layer surface height errors it may take several layers before the error is fully corrected. 
     The layer thickness of the subsequently printed layers can be controlled in a variety of manners. In an exemplary configuration, the layer thickness can be controlled by adjusting appropriate parameters for one or more components of the EP engines  12   p ,  12   s  ( FIG. 2 ). For example, the charge provided by the charging device  54  ( FIG. 2 ) can be adjusted by controlling a charging voltage, the exposure provided by the imager  56  ( FIG. 2 ) can be adjusted by controlling a light intensity provided by the light sources, or the amount of toner deposited by the development station  58  ( FIG. 2 ) can be adjusted by controlling a bias voltage. 
     In some cases, overall height errors can be determined independently for the part material layer  64   p  and the support material layer  64   s . The layer thicknesses of the subsequently-printed part material layers  64   p  and the support material layers  64   s  can then be independently adjusted by controlling parameters in the respective EP engines  12   p ,  12   s , until the overall heights of the part material layer  64   p  and the support material layer  64   s  match each other, or until they both match the expected surface height. 
     It can be important to maintain the surface height of the transfused part material layers  64   p  and support material layers  64   s  at the same surface height. In some embodiments, the surface height profile  260  is analyzed to determine a height difference between the transfused part material layers  64   p  and support material layers  64   s . The thickness profile for one or both of the subsequently-printed part material layers  64   p  and the support material layers  64   s  can then be adjusted to reduce the height difference. 
     In some cases, the surface height profile  260  is analyzed to determine a localized height errors in localized surface regions. In this way, non-uniformities in the surface height profile can be detected and corrected. For example, it might be determined that the surface height along the left edge is low, while the surface height along the right edge is high. In the limit, localized height errors can be determined for each image pixel. The localized thickness profile of one or more subsequently printed layers can then be controlled in accordance with the determined localized height errors. Certain control parameters are more appropriate for making localized thickness adjustments than others. For example, the exposure provided by the imager  56  ( FIG. 2 ) can be adjusted by controlling the light intensity on a pixel-by-pixel basis to provide localized thickness adjustments as a function of position within the part material layer  64   p  and the support material layer  64   s.    
     In some cases, the surface height profile  260  is analyzed to detect the presence of a printing artifact. Examples of such printing artifacts would include streak artifacts (i.e., “lines” extending in the x-direction), banding artifacts (i.e., “lines” extending in the y-direction), spot artifacts, and registration artifacts. For example, a streak artifact may be detected where the surface height is lower along the length of the streak artifact than in the surrounding portions of the surface. In this case, the localized thickness profile of one or more subsequently printed layers can then be controlled to compensate for the detected printing artifact by increasing the thickness of the corresponding positions in one or more subsequent layers. 
     In addition to controlling the parameters of the EP engines  12   p ,  12   s  discussed above, other aspects of the additive manufacturing system  10  can also be controlled responsive to the measured surface height profile  260 . For example, the surface height profile  260  can be analyzed to detect registration errors between the actual x-y positions of the transfused part material layers  64   p  and support material layers  64   s  and their expected positions. In some configurations, the measured registration errors can be fed back into a registration control system associated with the EP engines  12   p ,  12   s  in order to reduce the registration errors for subsequently-printed part material layers  64   p  and support material layers  64   s . In other configurations, the position of the build platform  68  can be controlled during the transfusing process to adjust the x-y position of the transfused layers. 
     For cases where a tiling scheme is used to produce large format parts, tiling artifacts can be caused at the tile boundaries. Such tiling artifacts can be detected by analyzing the surface height profile  260 . The start position of the build platform  68  can then be adjusted for the tiles in subsequent layers in order to correct for the tiling artifacts. 
     In some configurations, the actual overall height of the transfused layers can be determined after each layer is printed. The thickness profile of the subsequently-printed part material layers  64   p  and support material layers  64   s  can be controlled by using the part and support structure shape data  205  to provide the appropriate shape data for the subsequent layers based on the shape of the part and support structure at the actual overall height (as opposed to the expected overall height). In this way, even if the layer thicknesses are off from the aim layer thickness by a small amount, the geometry of the final printed 3D part  80  can be preserved more accurately. This approach may result in printing the 3D part  80  using fewer or more layers than would be expected based on the nominal layer thickness. 
     In extreme cases where the detected printing artifacts are too large to be corrected by simply controlling the thickness of subsequently-printed layers, a number of different steps can be taken. In some cases, an operation can be applied to remove one or more of the previously printed layers (for example, by using a surface planing operation). The removed layers can then be reprinted. In other cases, the printing of the 3D part and support structure  270  can be terminated and the 3D part  80  can be discarded. 
     In the illustrated embodiments, the print material layers  64   p  and the support material layers  64   s  are printed using EP engines  12   p  and  12   s . In other embodiments, other types of printing technologies such as ink jet printing can be used to form the print material layers  64   p  and the support material layers  64   s.    
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
     
         
           10  additive manufacturing system 
           12   p  electrophotography (EP) engine 
           12   s  electrophotography (EP) engine 
           14  belt transfer assembly 
           16  biasing mechanism 
           18  biasing mechanism 
           20  layer transfusion assembly 
           22  transfer belt 
           22   a  front surface 
           22   b  rear surface 
           24  belt drive mechanism 
           26  belt drag mechanism 
           28  loop limit sensor 
           30  idler roller 
           32  belt cleaner 
           34  rotational direction 
           36  controller 
           38  host computer 
           40  frame 
           42  photoconductor drum 
           42   a  intermediary drum 
           44  conductive drum body 
           46  photoconductive surface 
           48  shaft 
           50  drive motor 
           50   a  drive motor 
           52  rotation direction 
           52   a  rotation direction 
           54  charging device 
           56  imager 
           58  development station 
           60  cleaning station 
           62  discharge device 
           64  combined layer 
           64   p  part material layer 
           64   s  support material layer 
           66   p  part material 
           66   s  support material 
           68  build platform 
           70  nip roller 
           72  heater 
           74  heater 
           76  post-fuse heater 
           78  air jets 
           80  3D part 
           82  support structure 
           84  gantry 
           86  motion pattern 
           88  motor 
           90  heating element 
           92  rotation direction 
           94  heating element 
           100  surface profilometer 
           102  probe 
           104  control system 
           200  layer formation process 
           205  part and support structure shape data 
           220  develop support structure layer step 
           225  develop part structure layer step 
           230  transfer support structure layer to transfer medium step 
           235  transfer part structure layer to transfer medium step 
           240  move transfer medium to layer transfusion assembly step 
           245  transfuse part and support structure layer to previous layers step 
           250  transfused part and support layer 
           255  measure surface height profile step 
           260  surface height profile 
           265  control thickness for additional layers step 
           270  3D part and support structure