Patent Publication Number: US-2022234299-A1

Title: Layer transfusion sequencing for selective deposition-based additive manufacturing

Description:
This application is being filed as a PCT International Patent application on May 15, 2020 in the name of Evolve Additive Solutions, Inc., a U.S. national corporation, applicant for the designation of all countries, and J. Samuel Batchelder, a U.S. Citizen, and Manish Boorugu, a U.S. Citizen, inventors for the designation of all countries, and claims priority to U.S. Provisional Patent Application No. 62/849,213 filed May 17, 2019, the contents of which are herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to systems and methods for additive manufacturing of three-dimensional (3D) parts, and more particularly, to additive manufacturing systems and processes for sequencing the characteristics of layers selectively transfused to build 3D parts and their support structures. 
     Additive manufacturing is generally a process for manufacturing a three-dimensional (3D) object by selective layer deposition utilizing a computer model of the objects. The basic operation of an additive manufacturing system consists of slicing a three-dimensional computer model into thin cross sections, translating the result into position data, and the position data to control equipment which manufacture a three-dimensional structure in a layerwise manner using one or more additive manufacturing techniques. Additive manufacturing entails many different approaches to the method of fabrication, including fused deposition modeling, ink jetting, selective laser sintering, powder/binder jetting, electron-beam melting, electrophotographic imaging, and stereolithographic processes. 
     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. In some instances, the host computer can optionally generate additional geometry acting as a 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 an electrostatographic 3D printing process, slices of the digital representation of the 3D part and its support structure are printed or developed using an electrophotographic engine. The electrostatographic engine generally operates in accordance with 2D electrophotographic printing processes, using charged powder materials that are formulated for use in building a 3D part (e.g., a polymeric toner material). The electrostatographic engine typically uses a support drum that is coated with a photoconductive material layer, where latent electrostatic images are formed by electrostatic charging following image-wise exposure of the photoconductive layer by an optical source. The latent electrostatic images are then moved to a developing station where the polymeric toner is applied to charged areas, or alternatively to discharged areas of the photoconductive insulator to form the layer of the charged powder material representing a slice of the 3D part. The developed layer is transferred to a transfer medium, from which the layer is transfused to previously printed layers with heat and pressure to build the 3D part. 
     In addition to the aforementioned commercially available additive manufacturing techniques, a novel additive manufacturing technique has emerged, where particles are first selectively deposited in an imaging process, forming a layer corresponding to a slice of the part to be made; the layers are then bonded to each other, forming a part. This is a selective deposition process, in contrast to, for example, selective sintering, where the imaging and part formation happens simultaneously. The imaging step in a selective deposition process can be done using electrophotography. 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 
     In one aspect of the present disclosure, a method for making a three-dimensional (3D) part in a layerwise manner with an electrostatographic based additive manufacturing system includes establishing a first control parameter profile, establishing a second control parameter profile, establishing a transfusion sequence usable to transfuse a plurality of layers, and transfusing n+m layers on a bonding region of previously accumulated layers of the 3D part according to the transfusion sequence, where n and m are each a positive integer. The first control parameter profile includes a first combination of temperature and pressure parameters usable to transfuse a single layer of the 3D part, the second control parameter profile includes a second combination of temperature and pressure parameters usable to transfuse a single layer of the 3D part, and the second combination of temperature and pressure parameters is different from the first combination of temperature and pressure parameters. The transfusion sequence specifies the use of each of the first control parameter profile and the second control parameter profile in a specified order. Thermal energy is added to the previously accumulated layers from the bonding region to a thermal diffusion depth within the previously accumulated layers with transfusion of each layer, and a total thickness of the n+m layers is less than the thermal diffusion depth. The transfusion step includes transfusing n layers on the bonding region of previously accumulated layers of the 3D part according to the first control parameter profile, and, after transfusing the n layers, transfusing m layers on the bonding region of previously accumulated layers of the 3D part according to the second control parameter profile. 
     In another aspect of the present disclosure, a method for making a three-dimensional (3D) part in a layerwise manner with an electrostatographic based additive manufacturing system includes establishing a first control parameter profile, establishing a second control parameter profile, establishing a transfusion sequence usable to transfuse a plurality of layers, transfusing n+m layers on a bonding region of previously accumulated layers of the 3D part according to the transfusion sequence, where n and m are each a positive integer, and transfusing another n+m layers on the bonding region of previously accumulated layers of the 3D part according to the transfusion sequence. The first control parameter profile includes a first combination of temperature and force parameters usable to control the electrostatographic based additive manufacturing system to build a single layer of the 3D part, and is configured to build layers with relatively high performance in a first category and lower performance in a second category. The second control parameter profile includes a second combination of temperature and force parameters usable to control the electrostatographic based additive manufacturing system to build a single layer of the 3D part, and is configured to build layers with relatively high performance in the second category and lower performance in the first category. The second combination of temperature and force parameters is different from the first combination of temperature and force parameters. The transfusion sequence specifies the use of each of the first control parameter profile and the second control parameter profile in a specified order, and includes instructions to transfuse n layers on the bonding region of previously accumulated layers of the 3D part according to the first control parameter profile and to transfuse m layers on the bonding region of previously accumulated layers of the 3D part according to the second control parameter profile after transfusion of the n layers. Thermal energy is added to the previously accumulated layers from the bonding region to a thermal diffusion depth within the previously accumulated layers with transfusion of each layer, and a total thickness of the n+m layers is less than the thermal diffusion depth. 
     In another aspect, a method for making a three-dimensional (3D) part in a layerwise manner with an electrostatographic based additive manufacturing system includes establishing a first control parameter profile, establishing a second control parameter profile, establishing a transfusion sequence usable to perform transfusion assembly passes with a transfusion assembly of the electrostatographic based additive manufacturing system involving n+d transfusion assembly passes, where n and d are each a positive integer, transfusing n layers on a bonding region of previously accumulated layers of the 3D part according to the first control parameter profile, and performing d non-transfusing passes with the transfusion assembly such that at least one of the d non-transfusing passes is performed using the second control parameter profile. The first control parameter profile includes a first combination of parameters (e.g., temperature and pressure) usable to transfuse a single layer of the 3D part, the second control parameter profile includes a second combination of parameters, and the second combination of parameters is different from the first combination of parameters. The transfusion sequence specifies the use of each of the first control parameter profile and the second control parameter profile in a specified order to transfuse a plurality of layers. The d non-transfusing passes do not transfuse any a new layers on the bonding region. 
     The present summary is provided only by way of example, and not limitation. Other aspects of the present invention will be appreciated in view of the entirety of the present disclosure, including the entire text, claims and accompanying figures. 
     Definitions 
     Unless otherwise specified, the following terms as used herein have the meanings provided below: 
     The term “copolymer” refers to a polymer having two or more monomer species. 
     The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the inventive scope of the present disclosure. 
     Reference to “a” chemical compound refers one or more molecules of the chemical compound, rather than being limited to a single molecule of the chemical compound. Furthermore, the one or more molecules may or may not be identical, so long as they fall under the category of the chemical compound. 
     The terms “at least one” and “one or more of” an element are used interchangeably, and have the same meaning that includes a single element and a plurality of the elements, and may also be represented by the suffix “(s)” at the end of the element. 
     Directional orientations such as “above”, “below”, “top”, “bottom”, and the like are made with reference to a direction along a printing axis of a 3D part. In the embodiments in which the printing axis is a vertical z-axis, the layer-printing direction is the upward direction along the vertical z-axis. In these embodiments, the terms “above”, “below”, “top”, “bottom”, and the like are based on the vertical z-axis. However, in embodiments in which the layers of 3D parts are printed along a different axis, the terms “above”, “below”, “top”, “bottom”, and the like are relative to the given axis. 
     The term “providing”, such as for “providing a material” and the like, when recited in the claims, is not intended to require any particular delivery or receipt of the provided item. Rather, the term “providing” is merely used to recite items that will be referred to in subsequent elements of the claim(s), for purposes of clarity and ease of readability. 
     The term “selective deposition” refers to an additive manufacturing technique where one or more layers of particles are fused to previously deposited layers utilizing heat and pressure over time where the particles fuse together to form a layer of the part and also fuse to the previously printed layer. 
     The term “electrostatography” refers to the formation and utilization of latent electrostatic charge patterns to form an image of a layer of a part, a support structure or both on a surface. Electrostatography includes, but is not limited to, electrophotography where optical energy is used to form the latent image, ionography where ions are used to form the latent image and/or electron beam imaging where electrons are used to form the latent image. 
     Unless otherwise specified, temperatures referred to herein are based on atmospheric pressure (i.e. one atmosphere). 
     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). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified schematic front view of an example electrophotography-based additive manufacturing system for building 3D parts and associated support structures in a layerwise manner, in accordance with embodiments of the present disclosure. 
         FIG. 2  is a schematic front view of example electrophotographic engines, in accordance with embodiments of the present disclosure. 
         FIG. 3  is a schematic front view of an example electrophotographic engine, which includes a transfer drum or belt, in accordance with embodiments of the present disclosure. 
         FIG. 4  is a schematic front view of an example layer transfusion assembly, in accordance with embodiments of the present disclosure. 
         FIG. 5  is a graph of temperature versus depth schematically illustrating an example cyclical temperature swing for the building of a given layer of a 3D part. 
         FIG. 6  is a cross-sectional view of a 3D part schematically illustrating a building error void growing through a series of relatively hot layer transfusions. 
         FIG. 7  is a cross-sectional view of a 3D part schematically illustrating a building error void bridged through a series of relatively cold layer transfusions. 
         FIG. 8  is a schematic cross-sectional view of a 3D part built according to an example transfusion sequence that transfuses layers in a sequence of different transfusion control parameter profiles, in accordance with embodiments of the present disclosure. 
         FIG. 9  is a flow chart of an example method of building a 3D part and associated support structures in a layerwise manner according to a transfusion sequence, in accordance with embodiments of the present disclosure. 
         FIGS. 10A, 10B, 10C and 10D  are flow charts of example methods of transfusing layers according to a transfusion sequence, in accordance with embodiments of the present disclosure. 
     
    
    
     While the above-identified figures set forth one or more embodiments of the present invention, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the invention. The figures may not be drawn to scale, and applications and embodiments of the present invention may include features, steps and/or components not specifically shown in the drawings. 
     DETAILED DESCRIPTION 
     Embodiments of the disclosure are described more fully hereinafter with reference to the accompanying drawings. Elements that are identified using the same or similar reference characters refer to the same or similar elements. The various embodiments of the disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. 
     Specific details are given in the following description to provide a thorough understanding of the embodiments. However, it is understood by those of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, circuits, systems, networks, processes, frames, supports, connectors, motors, processors, and other components may not be shown, or shown in block diagram form in order to not obscure the embodiments in unnecessary detail. 
     As will further be appreciated by one of skill in the art, the present disclosure may be embodied as methods, systems, devices, and/or computer program products, for example. Accordingly, the present disclosure may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. The computer program or software aspect of the present disclosure may comprise computer readable instructions or code stored in a computer readable medium or memory. Execution of the program instructions by one or more processors (e.g., central processing unit), such as one or more processors of a controller, results in the one or more processors performing one or more functions or method steps described herein. Any suitable patent subject matter eligible computer-readable media or memory may be utilized including, for example, hard disks, CD-ROMs, optical storage devices, or magnetic storage devices. Such computer-readable media or memory constitute more than mere transitory waves or signals. 
     The computer-readable medium or memory mentioned herein, may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a random axis memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, and a portable compact disc read-only memory (CD-ROM). Note that the computer-usable or computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory. 
     As mentioned above, during an electrostatographic 3D part additive manufacturing or printing operation, electrostatographic engines develop each layer of a 3D part out of charged powder materials (e.g., polymeric toners) using the electrostatographic process. A completed layer of the 3D part typically includes a part portion formed of part material by one electrostatographic engine that is transferred to a suitable transfer medium, such as a transfer belt or drum, and/or a support structure portion formed of support material by a different electrostatographic engine that is applied to the transfer medium in registration with the corresponding part portion. Alternatively, the part portion may be developed and transferred to the transfer medium in registration with a previously printed support structure portion on the transfer medium. Further, a plurality of layers can be imaged in a reverse order of printing and stacked one on top of the other on the transfer medium to form a stack of a selected thickness. In these ways, a layer can be composed of several superimposed or separate portions of part and/or support material. 
     The transfer medium delivers the developed layers or the stack of layers to a transfusion assembly where a transfusion process is performed to form a 3D structure in a layer-by-layer manner, a stack-by-stack manner or a combination of individual layers and stacks of layers to form the 3D part and corresponding support structure. During the transfusion process, heat and pressure fuse the developed layers or stacks of layers to build surfaces of the 3D structure. After the 3D structure is built, the support structures can then be dissolved or disintegrated in an aqueous solution or dispersion to reveal the completed 3D part. 
     Embodiments of the present disclosure provide a system and method for adjusting thermal profiles of layers, parts being built, and transfusion assembly components. Thermal flux devices, such as heater devices and cooler devices, can be used to selectively and controllably add thermal energy and remove thermal energy at various times and locations during a transfusion cycle for a given layer. The present disclosure allows for relatively efficient use of thermal resources, by reducing or eliminating unnecessary thermal energy accumulation in a part being built while at the same time promoting relatively quick and efficient layer transfusion cycle times. Other features, aspects and benefits of the present disclosure will be recognized in view of the entirety of the present disclosure, including the accompanying figures. 
     While the present disclosure can be utilized with any electrostatography-based additive manufacturing system, the present disclosure will be described in association in an electrophotography-based (EP) additive manufacturing system. However, the present disclosure is not limited to an EP based additive manufacturing system and can be utilized with any electrostatography-based additive manufacturing system. 
       FIG. 1  is a simplified diagram of an example electrophotography-based additive manufacturing system  10  for building 3D parts and associated support structures in a layer-by-layer manner, in accordance with embodiments of the present disclosure. While illustrated as building 3D parts and associated support structures in a layer-by-layer manner, the system  10  can also be used to form stacks of layers and transfuse the stacks to form the 3D parts and associated support structures. 
     As shown in  FIG. 1 , system  10  includes one or more electrophotographic (EP) engines, generally referred to as  12 , such as EP engines  12   a - d , a transfer assembly  14 , at least one biasing mechanism  16 , and a transfusion assembly  20 . Examples of suitable components and functional operations for system  10  include those disclosed in Hanson et al., U.S. Pat. Nos. 8,879,957 and 8,488,994, and in Comb et al., U.S. Patent Application Publication Nos. 2013/0186549 and 2013/0186558. 
     The EP engines  12  are imaging engines for respectively imaging or otherwise developing completed layers of the 3D part, which are generally referred to as  22 , of the charged powder part and support materials. The charged powder part and support materials are each preferably engineered for use with the particular architecture of the EP engines  12 . In some embodiments, at least one of the EP engines  12  of the system  10 , such as EP engines  12   a  and  12   c , develops layers of the support material to form the support structure portions  22   s  of a layer  22 , and at least one of the EP engines  12 , such as EP engines  12   b  and  12   d , develops layers of the part material to form the part portions  22   p  of the layer  22 . The EP engines  12  transfer the formed part portions  22   p  and the support structure portions  22   s  to a transfer medium  24 . In some embodiments, the transfer medium  24  is in the form of a transfer belt, as shown in  FIG. 1 . The transfer medium  24  may take on other suitable forms in place of, or in addition to, the transfer belt, such as a transfer drum or sheet. Accordingly, embodiments of the present disclosure are not limited to the use of transfer mediums  24  in the form of the transfer belt. 
     In some embodiments, the system  10  includes at least one pair of the EP engines  12 , such as EP engines  12   a  and  12   b , which cooperate to form completed layers  22 . In some embodiments, additional pairs of the EP engines  12 , such as EP engines  12   c  and  12   d , may cooperate to form other layers  22 . 
     In some embodiments, each of the EP engines  12  that is configured to form the support structure portion  22   s  of a given layer  22  is positioned upstream from a corresponding EP engine  12  that is configured to form the part portion  22   p  of the layer  22  relative to a feed or in-track direction  32  of the transfer belt  24 . Thus, for example, EP engines  12   a  and  12   c  that are each configured to form the support structure portions  22   s  are positioned upstream from their corresponding EP engines  12   b  and  12   d  that are configured to form the part portions  22   p  relative to the feed direction  32  of the transfer belt  24 , as shown in  FIG. 1 . In alternative embodiments, this arrangement of the EP engines  12  may be reversed such that the EP engines that form the part portions  22   p  may be located upstream from the corresponding EP engines  12  that are configured to form the support structure portions  22   s  relative to the feed direction  32  of the transfer belt  24 . Thus, for example, the EP engine  12   b  may be positioned upstream from the EP engine  12   a , and the EP engine  12   d  may be positioned upstream of the EP engine  12   c  relative to the feed direction  32  of the transfer belt  24 . In further embodiments, different groupings of EP engines  12  are possible (e.g., trios, quartets, etc.), such as where more than two materials are utilized, or where different materials have significantly different characteristics and more EP engines  12  are dedicated to some material or materials than others. 
     As discussed below, the developed layers  22  are transferred to a transfer medium  24  of the transfer assembly  14 , which delivers the layers  22  to the transfusion assembly  20 . The transfusion assembly  20  operates to build a 3D structure  26 , which includes the 3D part  26   p , support structures  26   s  and/or other features, in a layer-by-layer manner by transfusing the layers  22  together on a build platform  28 . 
     In some embodiments, the transfer medium  24  includes a belt, as shown in  FIG. 1 . Examples of suitable transfer belts for the transfer medium  24  include those disclosed in Comb et al. (U.S. Patent Application Publication Nos. 2013/0186549 and 2013/0186558). In some embodiments, the belt  24  includes front surface  24   a  and rear surface  24   b , where front surface  24   a  faces the EP engines  12 , and the rear surface  24   b  is in contact with the biasing mechanisms  16 . 
     In some embodiments, the transfer assembly  14  includes one or more drive mechanisms that include, for example, a motor  30  and a drive roller  33 , or other suitable drive mechanism, and operate to drive the transfer medium or belt  24  in a feed direction  32 . In some embodiments, the transfer assembly  14  includes idler rollers  34  that provide support for the belt  24 . The exemplary transfer assembly  14  illustrated in  FIG. 1  is highly simplified and may take on other configurations. Additionally, the transfer assembly  14  may include additional components that are not shown in order to simplify the illustration, such as, for example, components for maintaining a desired tension in the belt  24 , a belt cleaner for removing debris from the surface  24   a  that receives the layers  22 , and other components. 
     System  10  also includes a controller  36 , which represents one or more processors that are configured to execute instructions, which may be stored locally in memory of the system  10  or in memory that is remote to the system  10 , to control components of the system  10  to perform one or more functions described herein. In some embodiments, the processors of the controller  36  are components of one or more computer-based systems. In some embodiments, the controller  36  includes one or more control circuits, microprocessor-based engine control systems, one or more programmable hardware components, such as a field programmable gate array (FPGA), and/or digitally-controlled raster imaging processor systems that are used to control components of the system  10  to perform one or more functions described herein. In some embodiments, the controller  36  controls components of the system  10  in a synchronized manner based on printing instructions received from a host computer  38  or from another location, for example. 
     In some embodiments, the controller  36  communicates over suitable wired or wireless communication links with the components of the system  10 . In some embodiments, the controller  36  communicates over a suitable wired or wireless communication link with external devices, such as the host computer  38  or other computers and servers, such as over a network connection (e.g., local area network (LAN) connection), for example. 
     In some embodiments, the host computer  38  includes one or more computer-based systems that are configured to communicate with the controller  36  to provide the print instructions (and other operating information). For example, the host computer  38  may transfer information to the controller  36  that relates to the sliced layers of the 3D parts and support structures, thereby allowing the system  10  to print the layers  22  and form the 3D part including any support structures in a layer-by-layer manner. 
     The components of system  10  may be retained by one or more frame structures. Additionally, the components of system  10  may be retained within an enclosable housing (not shown) that prevents components of the system  10  from being exposed to ambient light during operation, and which helps control the temperature and humidity of the air circulating through the system. 
       FIG. 2  is a schematic front view of the EP engines  12   a  and  12   b  of the system  10 , in accordance with exemplary embodiments of the present disclosure. In the shown embodiment, the EP engines  12   a  and  12   b  may include the same components, such as a photoconductor drum  42  having a conductive body  44  and a photoconductive surface  46 . The conductive body  44  is an electrically-conductive body (e.g., fabricated from copper, aluminum, tin, or the like), that can be electrically grounded (or biased to a particular voltage other than ground) and configured to rotate around a shaft  48 . The shaft  48  is correspondingly connected to a drive motor  50 , which is configured to rotate the shaft  48  (and the photoconductor drum  42 ) in the direction of arrow  52  at a substantially constant rate. While embodiments of the EP engines  12  are discussed and illustrated as utilizing a photoconductor drum  42 , a belt having a conductive material, or other suitable bodies, may also be utilized in place of the photoconductor drum  42  and the conductive body  44 . 
     The photoconductive surface  46  is a thin film extending around the circumferential surface of the conductive body  44  (shown as a drum but can alternatively be a belt or other suitable body), 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, the 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 to the charged or discharged image areas, thereby creating the layers  22  of the 3D part  26   p , or support structure  26   s.    
     As further shown, each of the exemplary EP engines  12   a  and  12   b  also includes a charge inducer  54 , an imager  56 , a development station  58 , a cleaning station  60 , and a discharge device  62 , each of which may be in signal communication with the controller  36 . The charge inducer  54 , the imager  56 , the development station  58 , the cleaning station  60 , and the discharge device  62  accordingly define an image-forming assembly for the surface  46 , while the drive motor  50  and the shaft  48  rotate the photoconductor drum  42  in the direction  52 . 
     The EP engines  12  use the charged particle material (e.g., a relatively poor electrical conductor such as polymeric or thermoplastic toner, a ceramic or magnetic material, etc.), generally referred to herein as  66 , to develop or form the layers  22 . In some embodiments, the image-forming assembly for the surface  46  of the EP engine  12   a  is used to form support structure portions  22   s  of the support material  66   s , where a supply of the support material  66   s  may be retained by the development station  58  (of the EP engine  12   a ) along with carrier particles. Similarly, the image-forming assembly for the surface  46  of the EP engine  12   b  is used to form part portions  22   p  of the part material  66   p , where a supply of the part material  66   p  may be retained by the development station  58  (of the EP engine  12   b ) along with carrier particles. 
     The charge inducer  54  is configured to generate a uniform electrostatic charge on the surface  46  as the surface  46  rotates in the direction  52  past the charge inducer  54 . Suitable devices for the charge inducer  54  include corotrons, scorotrons, charging rollers, and other electrostatic charging devices. 
     The imager  56  is a digitally-controlled, pixel-wise light exposure apparatus configured to selectively emit electromagnetic radiation toward the uniform electrostatic charge on the surface  46  as the surface  46  rotates in the direction  52  past the imager  56 . The selective exposure of the electromagnetic radiation to the surface  46  is directed by the controller  36 , and causes discrete pixel-wise locations of the electrostatic charge to be removed (i.e., discharged), thereby forming latent image charge patterns on the surface  46 . 
     Suitable devices for the imager  56  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 the charge inducer  54  and the imager  56  include ion-deposition systems configured to selectively directly deposit charged ions or electrons to the surface  46  to form the latent image charge pattern. In accordance with this embodiment, the charge inducer  54  may be eliminated. In some embodiments, the electromagnetic radiation emitted by the imager  56  has an intensity that controls the amount of charge in the latent image charge pattern that is formed on the surface  46 . As such, as used herein, the term “electrophotography” can broadly be considered as “electrostatography,” or a process that produces a charge pattern on a surface. Alternatives also include such things as ionography. 
     Each development station  58  is an electrostatic and magnetic development station or cartridge that retains the supply of the part material  66   p  or the support material  66   s , along with carrier particles. The development stations  58  may 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  may include an enclosure for retaining the part material  66   p  or the support material  66   s , and carrier particles. When agitated, the carrier particles generate triboelectric charges to attract the powders of the part material  66   p  or the support material  66   s , which charges the attracted powders to a desired sign and magnitude, as discussed below. 
     Each development station  58  may also include one or more devices for transferring the charged particles of the support material  66   p  or  66   s  to the surface  46 , such as conveyors, fur brushes, paddle wheels, rollers, and/or magnetic brushes. For instance, as the surface  46  (containing the latent charged image) rotates from the imager  56  to the development station  58  in the direction  52 , the charged part material  66   p  or the support material  66   s  is attracted to the appropriately charged regions of the latent image on the surface  46 , utilizing either charged area development or discharged area development (depending on the electrophotography mode being utilized). This creates successive layers  22   p  or  22   s  on the surface  46  as the photoconductor drum  42  continues to rotate in the direction  52 , where the successive layers  22   p  or  22   s  correspond to the successive sliced layers of the digital representation of the 3D part or support structure. 
     In some embodiments, the thickness of the layers  22   p  or  22   s  on the surface  46  depends on the charge of the latent image charge pattern on the surface. Thus, the thickness of the layers  22   p  or  22   s  may be controlled through the control of the magnitude of the charge in the pattern on the surface using the controller  36 . For example, the controller  36  may control the thickness of the layers  22   p  or  22   s  by controlling the charge inducer  54 , by controlling the intensity of the electromagnetic radiation emitted by the imager  56 , or by controlling the duration of exposure of the surface  46  to the electromagnetic radiation emitted by the imager  56 , for example. 
     The successive layers  22   p  or  22   s  are then rotated with the surface  46  in the direction  52  to a transfer region in which layers  22   p  or  22   s  are successively transferred from the photoconductor drum  42  to the belt  24  or another transfer medium, as discussed below. While illustrated as a direct engagement between the photoconductor drum  42  and the belt  24 , in some preferred embodiments, the EP engines  12   a  and  12   b  may also include intermediary transfer drums and/or belts, as discussed further below. 
     After a given layer  22   p  or  22   s  is transferred from the photoconductor drum  42  to the belt  24  (or an intermediary transfer drum or belt), the drive motor  50  and the shaft  48  continue to rotate the photoconductor drum  42  in the direction  52  such that the region of the surface  46  that previously held the layer  22   p  or  22   s  passes the cleaning station  60 . The cleaning station  60  is a station configured to remove any residual, non-transferred portions of part or support material  66   p  or  66   s . Suitable devices for the cleaning station  60  include blade cleaners, brush cleaners, electrostatic cleaners, vacuum-based cleaners, and combinations thereof. 
     After passing the cleaning station  60 , the surface  46  continues to rotate in the direction  52  such that the cleaned regions of the surface  46  pass the discharge device  62  to remove any residual electrostatic charge on the surface  46 , prior to starting the next cycle. Suitable devices for the discharge device  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 biasing mechanisms  16  are configured to induce electrical potentials through the belt  24  to electrostatically attract the layers  22   s  and  22   p  from the EP engines  12   a  and  12   b  to the belt  24 . Because the layers  22   s  and  22   p  are each only a single layer increment in thickness at this point in the process, electrostatic attraction is suitable for transferring the layers  22   s  and  22   p  from the EP engines  12   a  and  12   b  to the belt  24 . In some embodiments, the thickness of the layers  22   p  or  22   s  on the surface  24   a  of the belt  24  depends on the electrical potential induced through the belt by the corresponding biasing mechanism  16 . Thus, the thickness of the layers  22   p  or  22   s  may be controlled by the controller  36  through the control of the magnitude of the electrical potential induced through the belt by the biasing mechanisms  16 . 
     The controller  36  preferably controls the rotation of the photoconductor drums  42  of the EP engines  12   a  and  12   b  at the same rotational rates that are synchronized with the line speed of the belt  24  and/or with any intermediary transfer drums or belts. This allows the system  10  to develop and transfer the layers  22   s  and  22   p  in coordination with each other from separate developer images. In particular, as shown, each part of the layer  22   p  may be transferred to the belt  24  with proper registration with each support layer  22   s  to produce a combined part and support material layer, which is generally designated as layer  22 . As can be appreciated, some of the layers  22  transferred to the 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 embodiment, the part portions  22   p  and the support structure portions  22   s  may optionally be developed and transferred along the belt  24  separately, such as with alternating layers  22   s  and  22   p . These successive, alternating layers  22   s  and  22   p  may then be transferred to the layer transfusion assembly  20 , where they may be transfused separately to print or build the structure  26  that includes the 3D part  26   p , the support structure  26   f , and/or other structures. 
     In a further alternative embodiment, one or both of the EP engines  12   a  and  12   b  may also include one or more transfer drums and/or belts between the photoconductor drum  42  and the belt or transfer medium  24 . For example, as shown in  FIG. 3 , the EP engine  12   b  may also include an intermediate transfer drum  42   a  that rotates in the direction  52   a  that opposes the direction  52 , in which drum  42  is rotated, under the rotational power of motor  50   a . The intermediate transfer drum  42   a  engages with the photoconductor drum  42  to receive the developed layers  22   p  from the photoconductor drum  42 , and then carries the received developed layers  22   p  and transfers them to the belt  24 . 
     The EP engine  12   a  may include the same arrangement of an intermediate transfer drum  42   a  for carrying the developed layers  22   s  from the photoconductor drum  42  to the belt  24 . The use of such intermediary transfer drums or belts for the EP engines  12   a  and  12   b  can be beneficial for thermally isolating the photoconductor drum  42  from the belt  24 , if desired. 
       FIG. 4  illustrates an embodiment of the layer transfusion assembly  20 . As shown, the transfusion assembly  20  includes the build platform  28 , a nip roller  70 , pre-transfusion heaters  72  and  74 , an optional post-transfusion heater  76 , and air jets  78  (or other cooling units). The build platform  28  is a platform assembly or platen of system  10  that is configured to receive the heated combined layers  22  (or separate layers  22   p  and  22   s ) for printing the part  26 , which includes a 3D part  26   p  formed of the part layers  22   p , and support structure  26   s  formed of the support layers  22   s , in a layer-by-layer manner. In some embodiments, the build platform  28  may include removable film substrates (not shown) for receiving the printed layers  22 , where the removable film substrates may be restrained against build platform using any suitable technique (e.g., vacuum drawing). 
     The build platform  28  is supported by a gantry  84  or other suitable mechanism, which can be configured to move the build platform  28  along the z-axis and the x-axis (and, optionally, also the y-axis), as illustrated schematically in  FIG. 1  (the y-axis being into and out of the page in  FIG. 1 , with the z-, x- and y-axes being mutually orthogonal, following the right-hand rule). The gantry  84  may produce cyclical movement patterns relative to the nip roller  70  and other components, as illustrated by broken line  86  in  FIG. 4 . The particular movement pattern of the gantry  84  can follow essentially any desired path suitable for a given application. The gantry  84  may be operated by a motor  88  based on commands from the controller  36 , where the motor  88  may be an electrical motor, a hydraulic system, a pneumatic system, or the like. In one embodiment, the gantry  84  can included an integrated mechanism that precisely controls movement of the build platform  28  in the z- and x-axis directions (and optionally the y-axis direction). In alternate embodiments, the gantry  84  can include multiple, operatively-coupled mechanisms that each control movement of the build platform  28  in one or more directions, for instance, with a first mechanism that produces movement along both the z-axis and the x-axis and a second mechanism that produces movement along only the y-axis. The use of multiple mechanisms can allow the gantry  84  to have different movement resolution along different axes. Moreover, the use of multiple mechanisms can allow an additional mechanism to be added to an existing mechanism operable along fewer than three axes. 
     In the illustrated embodiment, the build platform  28  can be heatable with heating element  90  (e.g., an electric heater). The heating element  90  is configured to heat and maintain the build platform  28  at an elevated temperature that is greater than room temperature (25° C.), such as at a desired average part temperature of 3D part  26   p  and/or support structure  26   s , as discussed in Comb et al., U.S. Patent Application Publication Nos. 2013/0186549 and 2013/0186558. This allows the build platform  28  to assist in maintaining 3D part  26   p  and/or support structure  26   s  at this average part temperature. 
     The nip roller  70  is an example heatable/coolable element or heatable/coolable layer transfusion element, which is configured to rotate around a fixed axis with the movement of the belt  24 . In particular, the nip roller  70  may roll against the rear surface  22   s  in the direction of arrow  92  while the belt  24  rotates in the feed direction  32 . In the shown embodiment, the nip roller  70  is heatable with a heating element  94  (e.g., an electric heater). The heating element  94  is configured to heat and maintain nip roller  70  at an elevated temperature that is greater than room temperature (25° C.), such as at a desired transfer temperature for the layers  22 . In further embodiments, a cooling element such as an air knife can be used to cool the nip roller  70 . 
     The pre-transfusion heater  72  includes one or more heating devices (e.g., an infrared heater and/or a heated air jet) that are configured to heat the layers  22  on the belt  24  to a selected temperature of the layer  22 , such as up to a fusion temperature of the part material  66   p  and the support material  66   s , prior to reaching nip roller  70 . Each layer  22  desirably passes by (or through) the heater  72  for a sufficient residence time to heat the layer  22  to the intended transfer temperature. The pre-transfusion heater  74  may function in the same manner as the heater  72 , and heats the top surfaces of the 3D part  26   p  and support structure  26   s  on the build platform  28  to an elevated temperature, and in one embodiment to supply heat to the layer upon contact. 
     Optional post-transfusion heater  76  is located downstream from nip roller  70  and upstream from air jets  78 , and is configured to heat the transfused layers  22  to an elevated temperature. 
     As mentioned above, in some embodiments, prior to building the part  26  on the build platform  28 , the build platform  28  and the nip roller  70  may be heated to their selected temperatures. For example, the build platform  28  may be heated to the average part temperature of 3D part  26   p  and support structure  26   s . In comparison, the nip roller  70  may be heated to a desired transfer temperature for the layers  22 , which may be cooler than a peak transfuse temperature because it may be desirable for a nip entrance to be hot (for fusing build material particles together) and a nip exit to be cooler (for reduced adhesion to the belt  24 ). 
     As further shown in  FIG. 4 , during operation, the gantry  84  may move the build platform  28  (with 3D part  26   p  and support structure  26   s ) in a reciprocating pattern  86 . In particular, the gantry  84  may move the build platform  28  along the x-axis below, along, or through the heater  74 . The heater  74  heats the top surfaces of 3D part  26   p  and support structure  26   s  to an elevated temperature, such as the transfer temperatures of the part and support materials. As discussed in Comb et al., U.S. Patent Application Publication Nos. 2013/0186549 and 2013/0186558, the heaters  72  and  74  may heat the layers  22  and the top surfaces of 3D part  26   p  and support structure  26   s  to about the same temperatures to provide a consistent transfusion interface temperature. Alternatively, the heaters  72  and  74  may heat layers  22  and the top surfaces of 3D part  26   p  and support structure  26   s  to different temperatures to attain a desired transfusion interface temperature. 
     The continued rotation of the belt  24  and the movement of the build platform  28  align the heated layer  22  with the heated top surfaces of 3D part  26   p  and support structure  26   s  with proper registration along the x-axis. The gantry  84  may continue to move the build platform  28  along the x-axis, at a rate that is synchronized with the rotational rate of the belt  24  in the feed direction  32  (i.e., the same directions and speed). This causes the rear surface  24   b  of the belt  24  to rotate around the nip roller  70  to nip the belt  24  and the heated layer  22  against the top surfaces of 3D part  26   p  and support structure  26   s . This presses the heated layer  22  between the heated top surfaces of 3D part  26   p  and support structure  26   s  at the location of the nip roller  70 , which at least partially transfuses the heated layer  22  to the top layers of 3D part  26   p  and support structure  26   s  that form a build surface  96 . 
     As the transfused layer  22  passes the nip of the nip roller  70 , the belt  24  wraps around the nip roller  70  to separate and disengage from the build platform  28 . This assists in releasing the transfused layer  22  from the belt  24 , allowing the transfused layer  22  to remain adhered to 3D part  26   p  and support structure  26   s . Maintaining the transfusion interface temperature at a transfer temperature that is higher than its glass transition temperature, but lower than its fusion temperature, allows the heated layer  22  to be hot enough to adhere to the 3D part  26   p  and support structure  26   s , while also being cool enough to readily release from the belt  24 . 
     After release, the gantry  84  continues to move the build platform  28  along the x-axis to the post-transfusion heater  76 . At optional post-transfusion heater  76 , the top-most layers of 3D part  26   p  and the support structure  26   s  (including the transfused layer  22 ) may then be heated to at least the fusion temperature of the thermoplastic-based powder in a post-fuse or heat-setting step. This optionally heats the material of the transfused layer  22  to a highly fusible state such that polymer molecules of the transfused layer  22  quickly interdiffuse to achieve a high level of interfacial entanglement with 3D part  26   p.    
     Additionally, as the gantry  84  continues to move the build platform  28  along the x-axis past the post-transfusion heater  76  to the air jets  78 , the air jets  78  blow cooling air towards the top layers of 3D part  26   p  and support structure  26   s . This actively cools the transfused layer  22  down to or below the average part temperature, as discussed in Comb et al., U.S. Patent Application Publication Nos. 2013/0186549 and 2013/0186558. 
     To assist in keeping the 3D part  26   p  and support structure  26   s  at the average part temperature, in some preferred embodiments, the heater  74  and/or the heater  76  may operate to heat only the top-most layers of 3D part  26   p  and support structure  26   s . For example, in embodiments in which heaters  72 ,  74 , and  76  are configured to emit infrared radiation, the 3D part  26   p  and support structure  26   s  may include heat absorbers and/or other colorants configured to restrict penetration of the infrared wavelengths to within the top-most layers. Moreover, infrared radiation penetration into the part  26   p  and support structure  26   s  may be substantially less than a thermal diffusion depth (D TD ) related to thermal diffusivity within the accumulated layers and the length of time a heated surface is permitted to diffuse heat into the accumulated layers, as explained further below. Alternatively, the heaters  72 ,  74 , and  76  may be configured to blow heated air across the top surfaces of 3D part  26   p  and support structure  26   s . In either case, limiting the thermal penetration into 3D part  26   p  and support structure  26   s  allows the top-most layers to be sufficiently transfused, while also reducing the amount of cooling required to keep 3D part  26   p  and support structure  26   s  at the average part temperature. 
     The gantry  84  may then actuate the build platform  28  downward, and move the build platform  28  back along the x-axis to a starting position along the x-axis, following the reciprocating rectangular pattern  86 . The build platform  28  desirably reaches the starting position for proper registration with the next layer  22 . In some embodiments, the gantry  84  may also actuate the build platform  28  and 3D part  26   p /support structure  26   s  upward for proper registration with the next layer  22 . The same process may then be repeated for each remaining layer  22  of 3D part  26   p  and support structure  26   s.    
     After the transfusion operation is completed, the resulting 3D part  26   p  and support structure  26   s  may be removed from system  10  and undergo one or more post-printing operations. For example, support structure  26   s  may be sacrificially removed from 3D part  26   p  using an aqueous-based solution, such as an aqueous alkali solution. Under this technique, support structure  26   s  may at least partially dissolve in the solution, separating it from 3D part  26   p  in a hands-free manner. 
     In comparison, part materials are chemically resistant to aqueous alkali solutions. This allows the use of an aqueous alkali solution to be employed for removing the sacrificial support structure  26   s  without degrading the shape or quality of 3D part  26   p . Examples of suitable systems and techniques for removing support structure  26   s  in this manner include those disclosed in Swanson et al., U.S. Pat. No. 8,459,280; Hopkins et al., U.S. Pat. No. 8,246,888; and Dunn et al., U.S. Patent Application Publication No. 2011/0186081; each of which are incorporated by reference to the extent that they do not conflict with the present disclosure. 
     Furthermore, after support structure  26   s  is removed, 3D part  26   p  may undergo one or more additional post-printing processes, such as surface treatment processes like sand blasting and ball rolling. Examples of suitable surface treatment processes include those disclosed in Priedeman et al., U.S. Pat. No. 8,123,999; and in Zinniel, U.S. Pat. No. 8,765,045. 
     In alternate embodiments, transfusion assembly  20  can utilize an optional planishing roller or pressing element that serves to compact a layer to reduce voids, potentially to heat a layer to remove at least some moisture and solvents, and/or to create films that support tensile loading. Planishing of a given layer  22  can occur before that layer  22  is transfused with accumulated layers of the part  26 . 
     When transfusing layers  22  to build the 3D part  26 , the accumulated layers of the part  26  undergo a cyclical temperature swing associated with the transfusion of each new layer  22 .  FIG. 5  is a graph of temperature versus depth schematically illustrating an example cyclical temperature swing for the building of a given layer of a 3D part over τ layer , the cycle time for transfusion of a given layer  22 . As shown in  FIG. 5 , the vertical z-axis represents depth from the build surface  96  into accumulated layers of the 3D part  26 , and the horizontal axis represents temperature. The stippled region  100  plotted on the graph of  FIG. 5  depicts the cyclical temperature fluctuations at different depths within the accumulated layers of the 3D part  26 , from a relative minimum temperature T min  (at a left-side boundary in  FIG. 5 ) to a relative maximum temperature T max  (at a right-side boundary in  FIG. 5 ). A core temperature T core  of the previously accumulated layers of the part  26  is indicated, and represents the bulk temperature of a substantial portion of the previously accumulated layers of the part  26  away from the build surface  96 . 
     Generally, in at least one embodiment of a build process, a layer  22  to be transfused onto the accumulated layers of the 3D part  26  at the build surface  96  are heated while being conveyed to a heatable transfusion nip element  70  (e.g., nip roller or another suitable pressing element), such as by the heater  72 . Concurrently, the accumulated layers of the 3D part  26  are heated while the layer  22  is being conveyed, such as by the heater  74 , a build platform heater, etc. The heatable transfusion nip element  70  can also be heated while the layer  22  is being conveyed. Therefore, the temperature in the accumulated layers of the 3D part  26  generally increases over a first part of the cycle time τ layer , with the first part of the cycle time τ layer  occurring at least through the time the layer  22  reaches the heatable transfusion nip element  70 . Where a post-transfusion heater  76  is used, the first part of the cycle time τ layer  can continue after the accumulated layers of the 3D part  26  with the newly-transfused layer  22  passes the heatable transfusion nip element  70  and until the accumulated layers of the 3D part  26  pass the post-transfusion heater  76 . 
     After the passing the heatable transfusion nip element  70  (and any post-transfusion heater  76 ), the accumulated layers of the 3D part  26  with the newly-transfused layer  22  can be cooled, such as with the air jets  78  that blow cooling air at the build surface  96  where the new layer  22  was just transfused. Therefore, the temperature in the accumulated layers of the 3D part  26  generally decreases over a last part of the cycle time τ layer , with the last part of the cycle time τ layer  occurring after the time the layer  22  passes the heatable transfusion nip element  70  and any post-transfusion heater  76 . 
     As just described, the temperature within the region  100  shown in  FIG. 5  at shallow depths generally increases then decreases during the cycle time τ layer . However, it should be noted that in further embodiments more complex temperature fluctuations are possible, such as though the use of staged heating and cooling cycles before and/or after transfusion. Moreover, the cycle time τ layer  and other transfusion-related properties (such as the degree of heating and cooling) can vary layer-to-layer in some embodiments. 
     The core temperature T core  of the accumulated layers of the 3D part  26  can remain substantially constant at significant depths, which is depicted in  FIG. 5  by a narrow, peaked shape of the region  100  that is nearly linear at the greatest z-axis depths. While the core temperature T core  can vary across multiple layer transfusion operations, depending on heating and cooling operations performed by the system  10 , in some embodiments the core temperature T core  can be maintained at a substantially constant bulk temperature across multiple layer transfusions, or even across most or all layer transfusions. In some embodiments, the core temperature T core  can be maintained with a desired depth-dependent variation, such as to substantially linearly decline toward the greatest z-axis depths. 
     A thermal diffusion depth D TD =√{square root over (τ layer  κ/π)}, where τ layer  is the cycle time for the surface temperature to vary sinusoidally about an average temperature for a given layer (e.g., between heating and cooling of the accumulated layers of the 3D part  26 ) and κ is the thermal diffusivity for the part and/or support build material (e.g., a polymeric toner). An arbitrary temperature profile for a layer can be expressed as a superposition of such layer times, and the resulting thermal depth becomes a weighted average of these times. For example, a system with a 4.2 second layer time spends 0.6 seconds heating the layer and 1.8 seconds cooling the layer, for an effective τ layer  of about 2.4 seconds. The thermal diffusion depth D TD  represents a characteristic distance that thermal energy diffuses from the build surface  96  into the accumulated layers of the 3D part  26  during the cycle time τ layer . In one example embodiment, the build material thermal diffusivity  κ  is 180 mil 2 /s and the cycle time τ layer  is 2.4 seconds, such that the thermal diffusion depth D TD  is 298 μm (11.7 mil). If layer thicknesses (Γ) are 20 μm (0.8 mil), then the thermal diffusion depth D TD  in this example is the equivalent of the thickness of about fifteen layers. The thermal diffusion depth D TD  can be in a range of approximately 125 to 750 μm in some embodiments. 
     In some electrostatography-based additive manufacturing processes, fusion of the layers might be termed a sedimentary process, where the number of transfusion pressures experienced by the part being built removes substantially all voids from the part. However, in some alternative processes, the part is porous (with lower density) at the most recently transferred layer, becoming fully fused some ten to twenty layers into the part. 
     Once pressed into a melt, toner particles quickly come up to temperature. By way of example, the thermal diffusivity of ABS filament and soluble support material, sold by Stratasys, Inc. located in Eden Prairie, Minn., are roughly κ=160 mil 2 /s at fusion temperatures. Example toner particles used in EP build processes are about s=0.82 mils=21 microns in diameter. The thermal diffusion time associated with heat diffusing a distance of one particle diameter is 0.82/160=4.2 milliseconds. 
     Once melted, the new toner particles of a layer  22  are flattened into a film as they flow and are cast against the transfer medium  24 , leaving a consolidated part surface with significantly less, if any, pores. In this flattening process, lower temperature (“cold”) toner particles of a new layer  22  are pressed by the pressing component  70  (e.g. in the form of a nip roller) and the transfer medium  24  into one or more previously accumulated layers of the part  26  at a higher temperature. Below the bonding region of the part  26 , at least a bulk region of the part located away from the build surface  96  can remain at a lower temperature. Another advantage of pressing a layer of toner into a melt is that gas that evolves from heating the toner particles (such as water vapor) is more inclined to reside at the layer-belt interface than in trapped voids in the part. 
     Thus, in some embodiments, instead of using a sedimentary process where gradual consolidation of a layer of part material occurs as an increasing number of subsequent layers are applied, a layer of part material can instead be fully consolidated as it is applied by pressing the layer into a rapidly heated part surface, and then subsequently cooled. Using disclosed methods, the materials rapidly inter-diffuse across the interface, then the combination is cooled. In this process, less heat is required, because the thermal diffusion depth D TD  is shallower. However, the presently-disclosed embodiments can still be used with a sedimentary process. 
     Different combinations of control parameters governing the system  10  produce different performance characteristics in built 3D parts  26 . In at least some instances, combinations of control parameters result in performance tradeoffs, each having both positive and negative aspects. For instance, if a layer  22  is transfused relatively “hot” (that is, with a relatively hot build surface  96 ), the strength of the layer in the resultant part  26  can be very good; but the air cooling impingement velocity from the air jets  78  has to be relatively high to remove the extra heat from the “hot” transfusion, which tends to cause relatively fragile structures like shelves or bridges to fail. Bridging occurs with a given layer is transfused over a gap, hole, opening or the like in the bonding region of previously accumulated layers. Shelving occurs when a given layer overhangs the bonding region of the previously accumulated layers by more than a threshold amount (e.g., by more than the thickness of a layer). If relatively mild or low impingement air cooling on the build surface  96  from the air jets  78  is used instead, the new layer  22  can better shelve or bridge over random printing defects; but the transfuse temperature needs to be reduced to keep from overheating the core of the part  26 , which tends to reduce the strength of the resultant fused part  26 . If a relatively high force or pressure from the transfuse nip element  70  is used, the toner particles come into intimate contact, allowing the polymer chains to reptate and create a relatively strong part; but such high transfuse nip element pressure can generate creep and flow in the core of the part  26 , tending to cause the geometric accuracy of the resultant part  26  to deteriorate. If instead a relatively low transfuse nip element pressure is used, the geometric features of the resultant part  26  generally remain in their as-printed positions in space; but the lower pressure generates a relatively weaker part  26 . 
       FIGS. 6 and 7  illustrate some examples of the tradeoffs just discussed.  FIG. 6  is a cross-sectional view of a 3D part  26 ′ schematically illustrating a building error void  104  growing through a series of all relatively hot layer transfusions into large void  104 ′. With relatively hot layer transfusions, relatively high cooling air flows from post-transfusion air jets  78  is usually required. Such high cooling air flows can act like an air knife that can cause shelf and/or bridge structures to fail through cooling air shear effects. Unintuitively, such void enlargement effects in relatively high-temperature built parts  26 ′ may be more prone to failure, even though relatively hot transfusions generally produce mechanically stronger inter-layer bonds and resultant parts, because destructive levels of air cooling are often required to maintain suitable part temperatures. Failures of shelves and/or bridges or the presence of voids that have grown to significant sizes may lead to rejection of the resultant part  26 ′. Even if the part  26 ′ with the large void  104 ′ is acceptable, the presence of the large void  104 ′ can cause at least a local reduction of part strength. 
       FIG. 7  is a cross-sectional view of a 3D part  26 ″ schematically illustrating a building error void  104 ″ contained or “healed” by a bridge  106  through a series of all relatively cold layer transfusions. With relatively cold layer transfusions, cooling air flows from post-transfusion air jets  78  can be relatively mild, which can avoid air knife effects that can cause shelf and/or bridge structures to fail through cooling air shear effects. Therefore, while the relatively low-temperature built part  26 ″ may generally produce, on average, weaker inter-layer bonds than with relatively hot layer transfusions, the overall impact of the building defect (void)  104 ″ on the part  26 ″ is constrained and healed by the ability to maintain the structural integrity of the bridge  106  and the subsequent transfusion of additional layers. 
     When the parts  26 ′ and  26 ″ shown in  FIGS. 6 and 7  are made using entirely “hot” or “cold” layer transfusions, the types of tradeoffs discussed above tend to persist. However, by building a 3D part according to a sequence of control parameter profiles that differ, alternate or otherwise vary across multiple layer transfusions, the positive aspects of each control parameter profile can be realized. In this way coarse performance tradeoffs can be at least partially transcended, with the sequencing providing a blended set of performance benefits that can promote positive performance characteristics and reduce the risk of negative effects of particular control parameter profiles (such as avoiding a risk of building error voids growing unacceptably large in a series of relatively “hot” layer transfusions). For example, a first control parameter profile can be configured to build layers with relatively high performance in a first category (e.g., 3D part fracture strength or reliable transfer of a layer from a transfer medium to the part build surface) and lower performance in a second category (e.g., quality of bridge or shelf formation), while a second control parameter profile can be configured to build layers with relatively high performance in the second category and lower performance in the first category. The number and type of controllable parameters of the profiles and transfusion sequence (e.g., temperature and pressure or force parameters) will vary depending upon the particular configuration of a given electrophotography-based additive manufacturing system  10 , such as based on the type, number and location of heating, cooling and pressure-application elements of the system  10 . Moreover, the individual parameters of a given control parameter profile can be constant values, a range or window of proscribed values, a time-dependent polynomial function, a slew rate limitation, etc., and the given control parameter profile can include combinations of different types of parameters (e.g., with some parameters being constants and others being a range of values). Control parameter profiles can specify targets (open-loop control parameters for relevant components of system  10 ) or actual (sensed) values, or combinations of targets and actual values. The use of actual values in control parameter profiles generally requires that the system  10  provide suitable sensors in order to provide sensed data used to implement actual value control as a function of closed-loop sensor feedback. The transfusion sequence and control parameter profiles can be implemented by the controller  36 , and can be implemented via computer or processor-readable instructions. 
     For example, in some embodiments, a cycle of two different control parameter profiles can be used to establish the transfusion sequence. However, sequences of more than two control parameter profiles can also be used that may have a regular pattern (e.g., each control parameter profile is used in turn as a cycle with the cycle then beginning again to build a desired number of layers) or a more complex irregular pattern (e.g., two different profiles alternate with a third different profile used every certain number of layers, such as a third profile used every seventh layer in the sequence), as desired. In some embodiments, the transfusion sequence can include a combination of any suitable number of different control parameter profiles that builds a plurality of layers of a 3D part with those layers built according to the transfusion sequence having a collective thickness that is less than, or significantly less than, the thermal diffusion depth D TD , so that temperature and pressure applied to the build surface  96  can be expected to fuse within the accumulated layers of the 3D part regardless of the specific control parameters of a given control parameter profile (e.g., even if a given layer is transfused relatively “cold”). Further, in some embodiments, multiple transfusion sequences can be established and then a given transfusion sequence selected for use with a desired number of layer transfusions and a different transfusion sequence selected for use with a desired number of further layer transfusions. Such selection of different transfusion sequences can allow for sequences tailored to particular characteristics of portions of builds, such as a relatively simple transfusion sequence used for layers made mostly or entirely of support material and a more complex transfusion sequence used for layers made of a significant amount of part material in which build error tolerances are tighter. In some still further embodiments, a transfusion sequence can include mode switching in which one or more dummy transfusion passes in which a new layer is not transfused onto a part build surface but other typical transfusion heating, cooling and pressure application steps are performed. It should be noted that for some applications, transfusion sequences can be begun but not fully completed, such as where a part is fully built in the middle of a transfusion sequence. 
       FIG. 8  is a schematic cross-sectional view of a 3D part  126  built according to an example transfusion sequence  129  that builds a plurality of layers  22  in a sequence of different transfusion control parameter profiles. A first control parameter profile P 1  includes a first combination of temperature and pressure (or force) parameters usable to transfuse a single layer  22  of the 3D part  126 , and a second control parameter profile P 2  includes a second combination of temperature and pressure parameters usable to transfuse a single layer  22  of the 3D part  126 . The control parameter profiles P 1  and P 2  each specify controllable settings for the operation of the system  10 . The temperature parameters for each profile P 1  and P 2  can include pre-transfusion image heater temperature (e.g., for heater  72  and/or  74 ), heatable transfusing nip element temperature (e.g., for nip roller  70 ), accumulated part layer heater temperature (e.g., for heater  90 ), accumulated part layer heater angle of incidence (e.g., providing selective heating of certain portions of the accumulated part layers based on heater orientation), post-nip cooling air flow rate (e.g., from jets  78 ). In some embodiments, a range of heating transfusing nip element temperature can be approximately 60 to 180° C., or approximately 110 to 140° C., with an outer surface of the nip element having a higher transient temperature (e.g., approximately 160 to 280° C.) that then cools by conduction with an interior of the nip element. The pressure or force parameters for each control parameter profile P 1  and P 2  can include heatable transfusing nip element pressure (e.g., for nip roller  70  as applied at the build surface  96 ). The second combination of temperature and pressure parameters of the second control parameter profile P 2  is different from the first combination of temperature and pressure parameters of the first control parameter profile P 1 , though the difference can be just a single different temperature of pressure parameter or a different of many temperature and/or pressure parameters. Additionally, each control parameter profile P 1  and P 2  can include control parameters specifying layer thickness, layer material(s), and/or other characteristics, though in some embodiments layer thickness can be essentially constant for all control parameter profiles P 1  and P 2 . For example, layer thicknesses can be varied in a range of 5 to 22 μm in some embodiments, or 10 to 18 μm in still other embodiments. With respect to layer material(s), the control parameter profile P 1  or P 2  can constrain one or more layers to just one material or can exclude that material. 
     The transfusion sequence  129  specifies an ordered sequence of using the control parameter profiles P 1  and P 2  to build the layers  22  of the 3D part  126 . As shown schematically in the example embodiment of  FIG. 8 , n layers  22  are built according to the first control parameter profile P 1 , followed by m layers  22  built according to the second control parameter profile P 2 , followed by a repeating sequence of n+m layers according to the respective first and second control parameter profiles P 1  and P 2 . The values of n and m are each positive integers. In some embodiments n=m, though in other embodiments n and m can be different. In some embodiments either or both n and/or m can be greater than one, such as in an example embodiment in which the second control parameter profile P 2  transfuses layers relatively cold and m&gt;1 such that multiple layers  22  of the 3D part  126  are built relatively cold in direct sequence to provide a relatively good chance for small build defects such as voids to be bridged or shelved before n&gt;1 layers are built relatively hot according to the first control parameter profile P 1  with relatively intense heat and post-transfusion cooling air flow. In some example transfusion sequences, a point of the pattern is to periodically introduce a layer  22  that has been sintered into a contiguous film prior to transfusion, which can bridge or shelve over defects in previously accumulated part layers, and thereby increase the chances that the resultant part will complete successfully. The sintering can be achieved done by heating the layer  22  up a lot before transfusing. Single-layer bridge layers may be fragile, so post-transfusion cooling air rate should be decreased to not blow them away; though in many situations it is better to transfer part and support materials in more of a powder form, so that solvents and trapped gas can better get out of the newly fused layer (and avoid blistering). As another example, the first control parameter profile P 1  includes a relatively high heatable transfusing nip element pressure or force, and the second control parameter profile P 2  includes a relatively low heatable transfusing nip element pressure or force, which allows all accumulated layers of the 3D part  126  to experience relatively high pressure when within the thermal diffusion depth D TD  while reducing a total impulse average on the build surface  96  over time. 
     The following tables provide example embodiments of control parameter profiles and associated transfusion sequences. These example tables are disclosed merely by way of example. Numerous other profiles and sequences are possible. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Heatable 
                   
                   
                 Heatable 
                   
               
               
                   
                 Pre- 
                 Transfusing 
                   
                 Post-nip 
                 Transfusing 
                   
               
               
                   
                 transfusion 
                 Nip 
                 Accumulated 
                 Cooling 
                 Nip 
                   
               
               
                 No. of 
                 Heater 
                 Element 
                 Part Heater 
                 Air Flow 
                 Element 
                 Layer 
               
               
                 Layers 
                 Temp. 
                 Temp. 
                 Temp. 
                 Rate 
                 Pressure 
                 Thickness 
               
               
                   
               
             
            
               
                 n ≥ 1 
                 High 
                 High 
                 High 
                 High 
                 Moderate 
                 Constant 
               
               
                 m &gt; 1 
                 Low 
                 Low 
                 Low 
                 Moderate 
                 Moderate 
                 Constant 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Heatable 
                   
                   
                 Heatable 
                   
               
               
                   
                 Pre- 
                 Transfusing 
                   
                 Post-nip 
                 Transfusing 
                   
               
               
                   
                 transfusion 
                 Nip 
                 Accumulated 
                 Cooling 
                 Nip 
                   
               
               
                 No. of 
                 Heater 
                 Element 
                 Part Heater 
                 Air Flow 
                 Element 
                 Layer 
               
               
                 Layers 
                 Temp. 
                 Temp. 
                 Temp. 
                 Rate 
                 Pressure 
                 Thickness 
               
               
                   
               
             
            
               
                 n ≥ 1 
                 Moderate 
                 Moderate 
                 Moderate 
                 Moderate 
                 High 
                 Constant 
               
               
                 m &gt; 1 
                 Moderate 
                 Moderate 
                 Moderate 
                 Moderate 
                 Low 
                 Constant 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Heatable 
                   
                   
                 Heatable 
                   
               
               
                   
                 Pre- 
                 Transfusing 
                   
                 Post-nip 
                 Transfusing 
                   
               
               
                   
                 transfusion 
                 Nip 
                 Accumulated 
                 Cooling 
                 Nip 
                   
               
               
                 No. of 
                 Heater 
                 Element 
                 Part Heater 
                 Air Flow 
                 Element 
                 Layer 
               
               
                 Layers 
                 Temp. 
                 Temp. 
                 Temp. 
                 Rate 
                 Pressure 
                 Thickness 
               
               
                   
               
             
            
               
                 n ≥ 1 
                 High 
                 High 
                 High 
                 High 
                 Moderate 
                 Thin 
               
               
                 m ≥ 1 
                 Low 
                 Low 
                 Low 
                 Low to 
                 Moderate 
                 Thin 
               
               
                   
                   
                   
                   
                 Moderate 
                   
                   
               
               
                 q = 1 
                 Moderate 
                 Moderate 
                 Moderate 
                 Moderate 
                 High 
                 Thick 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Target Pre- 
                 Target  
                 Post-nip  
               
               
                   
                 No. of  
                 Transfusion  
                 Nip  
                 Cooling Air 
               
               
                   
                 Layers 
                 Layer Temp. 
                 Temp. 
                 Flow Rate 
               
               
                   
                   
               
             
            
               
                   
                 n = 5 
                 130° C. 
                 210° C. 
                 60% 
               
               
                   
                 m = 1 
                 180° C. 
                 190° C. 
                 30% 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 9  is a flow chart of a method of building a 3D part  26  or  126  (and associated support structures) in a layerwise manner according to a transfusion sequence. According to the illustrated method, x different control parameter profiles P 1  to Px are established, where x is an integer greater than or equal to two (steps  200 - 1  to  200 - x ). Then a transfusion sequence is established utilizing at least two of the control parameter profiles P 1  to Px. Then build events are performed by an electrophotography-based additive manufacturing system  10  to transfuse a plurality of layers  22  according to the transfusion sequence (step  204 ). 
       FIGS. 10A to 10D  are flow charts of example methods of transfusing layers  22  according to a transfusion sequence. Either of the illustrated methods of  FIG. 10A or 10B  can be used in conjunction with the method illustrated in  FIG. 9 , for example. It should be noted that still further embodiments of layer transfusions according to a transfusion sequence are also contemplated, such as layer transfusions according to further embodiments of transfusion sequences (for example, with more than three different control parameter profiles and/or with more complex sequencing of the control parameter profiles). 
     In the method illustrated in  FIG. 10A , n layers  22  are transfused on a bonding region or build surface  96  of previously accumulated layers of a 3D part  26  or  126  according to a first control parameter profile P 1 , where n is a positive integer (step  300 ). If n&gt;1, the n layers  22  can be transfused according to the first control parameter profile P 1  in a consecutive manner at step  300 , such that no other control parameter profiles are used to transfuse layers in between any of the n layers  22 . Next, m layers  22  are transfused on a bonding region or build surface  96  of previously accumulated layers of a 3D part  26  or  126  according to a second control parameter profile P 2 , different from the profile P 1 , where m is a positive integer (step  302 ). If m&gt;1, the m layers  22  can be transfused according to the second control parameter profile P 2  in a consecutive manner at step  302 , such that no other control parameter profiles are used to transfuse layers in between any of the m layers  22 . Thermal energy is added to the previously accumulated layers of the 3D part  26  or  126  from the build surface  96  to a thermal diffusion depth D TD  within the previously accumulated layers with the transfusion of each layer  22  in steps  300  and  302 . Together, steps  300  and  302  involve transfusing n+m layers on previously accumulated layers of the 3D part  26  or  126  according to a transfusion sequence  304 . A combined thickness of the n+m layers  22  can be less than the thermal diffusion depth D TD . For example, in some embodiments the total thickness of the n+m layers can satisfy the relationship Γ*(n+m)*2&lt;D TD , where Γ is layer thickness. 
     When the transfusion sequence  304  is complete, a query is made to assess whether any further layers are desired to be transfused (step  306 ). If there are further layer transfusions desired, the method can return to step  300  and the transfusion sequence  304  can be repeated. In alternate embodiments, a different transfusion sequence (not shown in  FIG. 10A ) can be used after step  306  to transfuse additional layers  22 . If no further layer transfusions are desired, the method can conclude. 
     In the method illustrated in  FIG. 10B , n layers  22  are transfused on a bonding region or build surface  96  of previously accumulated layers of a 3D part  26  or  126  according to a first control parameter profile P 1 , where n is a positive integer (step  300 ). If n&gt;1, the n layers  22  can be transfused according to the first control parameter profile P 1  in a consecutive manner at step  300 , such that no other control parameter profiles are used to transfuse layers in between any of the n layers  22 . Next, m layers  22  are transfused on a bonding region or build surface  96  of previously accumulated layers of a 3D part  26  or  126  according to a second control parameter profile P 2 , different from the profile P 1 , where m is a positive integer (step  302 ). If m&gt;1, the m layers  22  can be transfused according to the second control parameter profile P 2  in a consecutive manner at step  302 , such that no other control parameter profiles are used to transfuse layers in between any of the m layers  22 . Next, q layers  22  are transfused on a bonding region or build surface  96  of previously accumulated layers of a 3D part  26  or  126  according to a third control parameter profile P 3 , different from both the profiles P 1  and P 2 , where q is a positive integer (step  303 ). If q&gt;1, the q layers  22  can be transfused according to the third control parameter profile P 3  in a consecutive manner at step  302 , such that no other control parameter profiles are used to transfuse layers in between any of the q layers  22 . Thermal energy is added to the previously accumulated layers of the 3D part  26  or  126  from the build surface  96  to a thermal diffusion depth D TD  within the previously accumulated layers with the transfusion of each layer  22  in steps  300 ,  302 , and  303 . Together, steps  300 ,  302 , and  303  involve transfusing n+m+q layers on previously accumulated layers of the 3D part  26  or  126  according to a transfusion sequence  304 ′. A combined thickness of the n+m+q layers  22  can be less than the thermal diffusion depth D TD . 
     When the transfusion sequence  304 ′ is complete, a query is made to assess whether any further layers are desired to be transfused (step  306 ). If there are further layer transfusions desired, the method can return to step  300  and the transfusion sequence  304 ′ can be repeated. In alternate embodiments, a different transfusion sequence (for example, the transfusion sequence  304  shown in  FIG. 10A ) can be used after step  306  to transfuse additional layers  22  following the transfusion sequence  304 ′. If no further layer transfusions are desired, the method can conclude. 
     In the method illustrated in  FIG. 10C , n+d transfusion assembly passes are made according to a first control parameter profile P 1 , where n and d are each a positive integer (step  299 ). As part of the n+d transfusion passes, n layers  22  are transfused on a bonding region or build surface  96  of previously accumulated layers of a 3D part  26  or  126  according to the first control parameter profile P 1  (step  300 ). As part of steps  299  and  300 , d transfusion assembly passes occur without a new layer  22  being transfusion on the bonding region or build surface  96 . Non-transfusing passes of the transfusion assembly  20  can involve most or all of the temperature and pressure steps performed during a typical transfusion event but no new layer is transfused, making these non-transfusion passes akin to dummy passes that do not add to the previously accumulated layers of the part  26  or  126  but can allow the system  10  time to adjust one or more transfusion parameters before transfusing the next layer  22 , to help consolidate porous materials, generate relatively glossy top surfaces on the part  26  or  126 , etc. If n&gt;1, the n layers  22  can be transfused according to the first control parameter profile P 1  in a consecutive manner at step  300 , such that no other control parameter profiles are used to transfuse layers in between any of the n layers  22 . The d non-transfusing transfusion assembly passes can occur after transfusion of the n layers, in some embodiments. Next, m layers  22  are transfused on a bonding region or build surface  96  of previously accumulated layers of a 3D part  26  or  126  according to a second control parameter profile P 2 , different from the profile P 1 , where m is a positive integer (step  302 ). If m&gt;1, the m layers  22  can be transfused according to the second control parameter profile P 2  in a consecutive manner at step  302 , such that no other control parameter profiles are used to transfuse layers in between any of the m layers  22 . Thermal energy is added to the previously accumulated layers of the 3D part  26  or  126  from the build surface  96  to a thermal diffusion depth D TD  within the previously accumulated layers with the transfusion of each layer  22  in steps  300  and  302 . Together, steps  300  and  302  involve transfusing n+m layers on previously accumulated layers of the 3D part  26  or  126  according to a transfusion sequence  304 ″. A combined thickness of the n+m layers  22  can be less than the thermal diffusion depth D TD . 
     When the transfusion sequence  304 ″ is complete, a query is made to assess whether any further layers are desired to be transfused (step  306 ). If there are further layer transfusions desired, the method can return to step  299  and the transfusion sequence  304 ″ can be repeated. In alternate embodiments, a different transfusion sequence (for example, the transfusion sequence  304 ′ shown in  FIG. 10B ) can be used after step  306  to transfuse additional layers  22  following the transfusion sequence  304 ″. If no further layer transfusions are desired, the method can conclude. 
     It generally takes more than one layer cycle time to switch between control parameter profiles. For example, the system  10  may ramp a transfuse nip element force at about 45 newtons ( 101   bf ) per layer. The temperature in the top 1-2 mm of the accumulated layers of the part  26  or  126  at or near the bonding region  96  may take 10-100 transfusion passes (or layer transfusions) to stabilize. If a slower-changing parameter is varying as part of the transfusion sequence, then a transitional state can be provided with (dummy) non-transfusion passes to allow the process parameter of the system  10  to catch up with a target parameter. Therefore, mode-switching can be used, to spend a layer time or more going-through-the-motions of a transfusion event or transfusion pass, but not actually transferring any layer from the transfer medium  24  to the bonding region  96 . More specifically, this can allow the transfusion assembly  20  passes that actually transfuse layers  22  to be performed at relatively low bulk temperature and relatively high transfer pressure, and then have transfusion assembly passes where the part build surface is only lightly pressed, no image is transferred, and the average temperature of the top 1-2 mm of the accumulated part layers is high. This helps make a part that is both accurate and strong, with a trade-off of some reduction in throughput of the system  10 . 
     In the method illustrated in  FIG. 10D , n layers  22  are transfused on a bonding region or build surface  96  of previously accumulated layers of a 3D part  26  or  126  according to the first control parameter profile P 1 , where n is a positive integer (step  300 ). If n&gt;1, the n layers  22  can be transfused according to the first control parameter profile P 1  in a consecutive manner at step  300 , such that no other control parameter profiles are used to transfuse layers in between any of the n layers  22 . Next, d 1  non-transfusing passes of the transfusion assembly  20  are performed according to a second control parameter profile P 2 , different from the profile P 1 , where d 1  is a positive integer (step  302 - 1 ), followed by x additional non-transfusing passes d x  of the transfusion assembly  20  performed according to respective control parameter profiles Px. Thermal energy is added to the previously accumulated layers of the 3D part  26  or  126  from the build surface  96  to a thermal diffusion depth D TD  within the previously accumulated layers with the transfusion of each layer  22  in steps  300  and  302 - 1  to  302 - x . Then a decision is made as to whether any additional non-transfusing passes are desired (step  305 ), and if so the method can return to step  302 - 1 . the Together, steps  300 ,  302 - 1  to  302 - x  and any repetitions pursuant to step  305  involve transfusing n layers on previously accumulated layers of the 3D part  26  or  126  according to a transfusion sequence  304 ′″. A combined thickness of then layers  22  can be less than the thermal diffusion depth Dm. 
     When the transfusion sequence  304 ′″ is complete, a query is made to assess whether any further layers are desired to be transfused (step  306 ). If there are further layer transfusions desired, the method can return to step  300  and the transfusion sequence  304 ′″ can be repeated. In alternate embodiments, a different transfusion sequence (for example, the transfusion sequence  304 ′ shown in  FIG. 10B ) can be used after step  306  to transfuse additional layers  22  following the transfusion sequence  304 ′″. If no further layer transfusions are desired, the method can conclude. 
     Table 5 provide an additional example embodiment of control parameter profiles and associated transfusion sequences. This example table is disclosed merely by way of example. Numerous other profiles and sequences are possible. In one embodiment according to Table 5, n=d, and n can be six. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                 Target Ave. 
                   
                 Heatable 
               
               
                   
                   
                 Temp. of Top  
                   
                 Transfusing  
               
               
                   
                   
                 1-2 mm of 
                 Target  
                 Nip 
               
               
                 No. of  
                 Layer(s) 
                 Accumulated 
                 Nip 
                 Element  
               
               
                 Passes 
                 Transfused?. 
                 Part 
                 Temp. 
                 Force 
               
               
                   
               
             
            
               
                 n 
                 Yes 
                 115° C. 
                 210° C. 
                 356 newtons 
               
               
                 d 
                 No 
                 115° C. 
                 210° C. 
                  67 newtons 
               
               
                 d 
                 No 
                 150° C. 
                 250° C. 
                  67 newtons 
               
               
                 d 
                 No 
                 115° C. 
                 180° C. 
                  67 newtons 
               
               
                 d 
                 No 
                 115° C. 
                 210° C. 
                 356 newtons 
               
               
                   
               
            
           
         
       
     
     The methods discussed above can be implemented on software and/or firmware, and executed with the system  10  via one or more processors, in some embodiments. 
     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.