Patent Publication Number: US-9904223-B2

Title: Layer transfusion with transfixing for additive manufacturing

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority to U.S. Provisional Application No. 61/538,491, filed on Sep. 23, 2011, and entitled “LAYER TRANSFUSION FOR ELECTROPHOTOGRAPHY-BASED ADDITIVE MANUFACTURING”. 
     Reference is hereby made to co-filed U.S. patent application Ser. No. 13/624,495, entitled “LAYER TRANSFUSION FOR ADDITIVE MANUFACTURING”. 
     Reference is also hereby made to co-filed U.S. patent application Ser. No. 13/624,513, entitled “LAYER TRANSFUSION WITH PART HEATING FOR ADDITIVE MANUFACTURING”. 
     Reference is also hereby made to co-filed U.S. patent application Ser. No. 13/624,519, entitled “LAYER TRANSFUSION WITH ROTATABLE BELT FOR ADDITIVE MANUFACTURING”. 
    
    
     BACKGROUND 
     The present disclosure relates to additive manufacturing systems for building three-dimensional (3D) parts and support structures. In particular, the present disclosure relates to additive manufacturing systems and processes for building 3D parts and support structures using an imaging process, such as electrophotography. 
     Additive manufacturing systems are used to build 3D parts from digital representations of the 3D parts (e.g., STL format files) using one or more additive manufacturing techniques. Examples of commercially available additive manufacturing techniques include extrusion-based techniques, ink jetting, selective laser sintering, powder/binder jetting, electron-beam melting, and stereolithographic processes. For each of these techniques, the digital representation of the 3D part is initially sliced into multiple horizontal layers. For each sliced layer, a tool path is then generated, which provides instructions for the particular additive manufacturing system to form the given layer. 
     For example, in an extrusion-based additive manufacturing system, a 3D part or model may be printed from a digital representation of the 3D part in a layer-by-layer manner by extruding a flowable part material. The part material is extruded through an extrusion tip carried by a print head of the system, and is deposited as a sequence of roads on a substrate in an x-y plane. The extruded part material fuses to previously deposited part material, and solidifies upon a drop in temperature. The position of the print head relative to the substrate is then incremented along a z-axis (perpendicular to the x-y plane), and the process is then repeated to form a 3D part resembling the digital representation. 
     In fabricating 3D parts by depositing layers of a part material, supporting layers or structures are typically built underneath overhanging portions or in cavities of objects under construction, which are not supported by the part material itself. A support structure may be built utilizing the same deposition techniques by which the part material is deposited. The host computer generates additional geometry acting as a support structure for the overhanging or free-space segments of the 3D part being formed. Support material is then deposited from a second nozzle pursuant to the generated geometry during the printing process. The support material adheres to the modeling material during fabrication, and is removable from the completed 3D part when the printing process is complete. 
     In two-dimensional (2D) printing, electrophotography (i.e., xerography) is a popular technology for creating 2D images on planar substrates, such as printing paper. Electrophotography systems include a conductive support drum coated with a photoconductive material layer, where latent electrostatic images are formed by charging and then image-wise exposing the photoconductive layer by an optical source. The latent electrostatic images are then moved to a developing station where toner is applied to charged areas of the photoconductive insulator to form visible images. The formed toner images are then transferred to substrates (e.g., printing paper) and affixed to the substrates with heat or pressure. 
     SUMMARY 
     An aspect of the present disclosure is directed to an additive manufacturing system for printing a thermoplastic part. The additive manufacturing system includes an imaging engine configured to develop imaged layers of a thermoplastic-based powder, a movable build platform, and a transfer medium having a transfer surface configured to sequentially receive and convey the imaged layers from the imaging engine to the build platform and an opposing contact surface. The system also includes a heater configured to heat the imaged layers on the transfer medium to a fusion temperature, and a layer transfuse element configured to be heated to a transfer temperature and further configured to transfuse a heated imaged layer conveyed by the transfer medium onto previously fused layers of a thermoplastic part being printed by engaging the contact surface, and to disengage from the contact surface without releasing the transfer medium from the transfused layer. The system further includes a cooling unit configured to actively cool the transfused layer while it remains on the transfer medium. 
     Another aspect of the present disclosure is directed to an additive manufacturing system for printing a thermoplastic part, which includes an imaging engine configured to develop imaged layers of a thermoplastic-based powder, a movable build platform, and a rotatable belt having a transfer surface configured to sequentially receive and convey the imaged layers from the imaging engine to the build platform and an opposing contact surface. The system also includes a heater configured to heat the imaged layers on the rotatable belt to a fusion temperature, and a fusion roller configured to be heated to a transfer temperature and further configured to transfuse a heated imaged layer conveyed by the rotatable belt onto previously fused layers of a thermoplastic part being printed on the build platform by engaging the contact surface so as to press the heated imaged layer on to the previously fused layers, and to disengage from the contact surface without releasing the rotatable belt from the transfused layer. The system further includes a cooling unit configured to actively cool the transfused layer while it remains on the rotatable belt. 
     Another aspect of the present disclosure is directed to a method for printing a three-dimensional part with an additive manufacturing system. The method includes imaging a layer of the 3D part from a thermoplastic-based powder, transferring the imaged layer to a transfer medium, and heating the imaged layer to a fusion temperature while the imaged layer is retained on the transfer medium. The method also includes transfusing the heated layer to a surface of the 3D part being printed on a build platform while the imaged layer remains retained on the transfer medium, actively cooling the transfused layer to below the fusion temperature, and delaminating the transfused layer from the transfer medium after cooling the transfused layer below the fusion temperature to thereby maintain dimensional integrity of the transfused layer. 
     In some embodiments, the additive manufacturing system is configured to print or otherwise produce the layers at a rate that is faster than a passive thermal diffusion rate of the thermoplastic or 3D part. 
     DEFINITIONS 
     Unless otherwise specified, the following terms as used herein have the meanings provided below: 
     The terms “transfusion”, “transfuse”, “transfusing”, and the like refer to the adhesion of layers with the use of heat and pressure, where polymer molecules of the layers at least partially interdiffuse. 
     The term “transfusion pressure” refers to a pressure applied during a transfusion step, such as when transfusing layers of a 3D part together. 
     The term “deformation temperature” of a 3D part refers to a temperature at which the 3D part softens enough such that a subsequently-applied transfusion pressure, such as during a subsequent transfusion step, overcomes the structural integrity of the 3D part, thereby deforming the 3D part. 
     Unless otherwise specified, temperatures referred to herein are based on atmospheric pressure (i.e. one atmosphere). 
     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 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 schematic illustration of an electrophotography-based additive manufacturing system of the present disclosure having a layer transfusion assembly with a press plate. 
         FIG. 2  is a top view of a build platform receiving a heated layer, illustrating an air knife arrangement. 
         FIG. 3  is a graphical illustration of number of printed layers for a 3D part versus an average part temperature, showing plot lines for a 3D part printed without active cooling and for a 3D part printed with active cooling. 
         FIGS. 4A-4D  are expanded views of the layer transfusion assembly, illustrating a layer transfer technique of the present disclosure. 
         FIGS. 5A-5E  are expanded views of an alternative layer transfusion assembly of the electrophotography-based additive manufacturing system, having a nip roller, and which illustrate the layer transfer technique. 
         FIGS. 6A-6F  are expanded views of another alternative layer transfusion assembly of the electrophotography-based additive manufacturing system, having separate transfusion and release rollers, and which illustrate the layer transfer technique. 
         FIG. 7  is an expanded view of another alternative layer transfusion assembly of the electrophotography-based additive manufacturing system, having an enclosable chamber. 
         FIG. 8  is a sectional side of a rotatable transfer belt. 
         FIG. 9A  is a top view of a first embodiment of the rotatable transfer belt, having receiving regions and encoder markings. 
         FIG. 9B  is a top view of a second embodiment of the rotatable transfer belt, having holes for engagement in a tractor-feed manner. 
         FIG. 9C  is a bottom view of a third embodiment of the rotatable transfer belt, having rear ribs for engagement in a timing-belt manner. 
         FIG. 10  is a flow diagram of a first embodied method for the layer transfer technique of the present disclosure, having a combined transfusion and transfixing step, and an active cooling step. 
         FIG. 11  is a flow diagram of a second embodied method for the layer transfer technique, having separate transfusion and transfixing steps, and an optional active cooling step. 
         FIG. 12  is a flow diagram of a third embodied method for the layer transfer technique, having a part surface heating step, separate transfusion and transfixing steps, and an active cooling step. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed to a layer transfer technique for printing 3D parts and support structures in a layer-by-layer manner, where each layer is printed from a part or support material in a thermally-controlled manner. The layer transfer technique is performed with an imaging system, such as an electrophotography-based additive manufacturing system. For example, each layer may be developed using electrophotography and carried from an electrophotography (EP) engine by a transfer medium (e.g., a rotatable belt or drum). The layer is then transferred to a build platform to print the 3D part (or support structure) in a layer-by-layer manner, where the successive layers are transfused together to produce the 3D part (or support structure). 
     In comparison to 2D printing, in which developed toner particles can be electrostatically transferred to printing paper by placing an electrical potential through the printing paper, the multiple printed layers in a 3D environment effectively prevents the electrostatic transfer of part and support materials after a given number of layers are printed (e.g., about 15 layers). Instead, in the present disclosure, a layer retained by the transfer medium is heated to at least a fusion temperature of the layer material. The heated layer is then pressed against a previously-printed layer (or to a build platform) to transfuse the layers together (i.e., a transfusion step). This allows numerous layers of 3D parts and support structures to be built vertically, beyond what is otherwise achievable via electrostatic transfers. 
     As discussed below, the fusion temperature is a temperature that sufficiently melts the layer material to a fusable state such that polymer molecules of the layer material at least partially interdiffuse during the subsequent transfusion step to promote interlayer or interfacial entanglement. The fusion temperature is high enough to promote the layer transfusion, but it can be too hot for the transfused layer to cleanly release or otherwise delaminate from the transfer medium. This can potentially result in portions of the transfused layer remaining adhered to the transfer medium, or smear upon release from the transfer medium, which negatively impacts feature detail, dimensional accuracy, and porosity of a printed 3D part. 
     Accordingly, in some embodiments, the layer transfer technique may also include a “transfixing step”, in which the transfer medium and/or the transfused layer is cooled prior to releasing the transfused layer from the transfer medium. While not wishing to be bound by theory, it is believed that this transfixing step cools down the interface between the transfer medium and the transfused layer, thus increasing the adhesive force of the interdiffused polymers in adjacent layers relative to the adhesive force of the transfused layer to the surface of the transfer medium, thereby maintaining the transfused layer adhered to the 3D part in a fixed spatial position. This allows the transfused layer to cleanly release from the transfer medium and remain adhered to the 3D part. 
     Furthermore, because the imaging system is capable of printing the layers at speeds that are much faster than the rate at which heat diffuses through the variable thermal resistance of the 3D parts, heat accumulation in the 3D parts has been observed. As such, as the height of a given 3D part grows, heat dissipation from passive thermal diffusion becomes insufficient to cool the heated layers. The faster the layer speed, the faster the heat accumulation in the bulk of the 3D part. As successive layers are continuously printed, this heat accumulation may exceed a deformation temperature of the 3D part, causing the bulk of the 3D part to soften enough reduces its structural integrity. Such a soft part may deform under a subsequently-applied transfusion pressure during a subsequent transfusion step. 
     In some embodiments, heat accumulation can be reduced by slowing down the printing process. As can be appreciated, this can substantially increase the time required to print 3D parts, thereby reducing throughput. Instead, to overcome this issue while maintaining fast printing rates, the layer transfer technique may include an “active cooling step” to prevent the 3D part from accumulating additional heat, thereby maintaining the 3D part at a lower “average part temperature” that is lower than the deformation temperature of the 3D part. 
     In particular, after each layer of the 3D part is transfused, the heat added to the 3D part from the transfused layer is substantially removed prior to the transfusion of the next layer. This holds the 3D part at an average part temperature that is desirably balanced to promote interlayer adhesion and reduce the effects of curling, while also being low enough to prevent the 3D part from softening too much (i.e., below its deformation temperature). 
     As shown in  FIG. 1 , system  10  is an example additive manufacturing system for printing 3D parts and support structures using electrophotography, which incorporates the layer transfer technique of the present disclosure. System  10  includes EP engine  12 , transfer belt  14 , rollers  16 , build platform  18 , and press plate  20  for printing 3D parts (e.g., 3D part  22 ) and any associated support structures (not shown). Examples of suitable components and functional operations for system  10  include those disclosed in U.S. patent application Ser. Nos. 13/242,669 and 13/242,841. 
     In alternative embodiments, system  10  may include different imaging engines for imaging the layers. As discussed below, the layer transfer technique focuses on the transfer of layers from belt  14  (or other transfer medium) to build platform  18 , and on belt  14 , rather than on the particular imaging engine. However, the layer transfer technique is particularly suitable for use with electrophotography-based additive manufacturing systems (e.g., system  10 ), where the multiple printed layers in a 3D environment effectively prevents the electrostatic transfer of part and support materials after a given number of layers are printed, as discussed above. 
     System  10  also includes controller  24 , which is one or more control circuits, microprocessor-based engine control systems, and/or digitally-controlled raster imaging processor systems, and which is configured to operate the components of system  10  in a synchronized manner based on printing instructions received from host computer  26 . Host computer  26  is one or more computer-based systems configured to communicate with controller  24  to provide the print instructions (and other operating information). For example, host computer  26  may transfer information to controller  24  that relates to the sliced layers of 3D part  22  (and any support structures), thereby allowing system  10  to print 3D part  22  in a layer-by-layer manner. 
     As discussed in the U.S. patent application Ser. Nos. 13/242,669 and 13/242,841, EP engine  12  is configured to develop successive layers  28  of a thermoplastic-based powder using electrophotography. As used herein, the term “electrophotography” includes ionography. The thermoplastic-based powder includes one or more thermoplastic materials (e.g., an acrylonitrile-butadiene-styrene (ABS) copolymer), and may also include one or more additional components for development with EP engine  12  and triboelectric attraction to belt  14 . 
     The developed layers  28  of the thermoplastic-based powder are then rotated to a first transfer region in which layers  28  are transferred from EP engine  12  to belt  14 . Belt  14  is an example transfer medium for transferring or otherwise conveying the developed layers  28  from EP engine  12  to build platform  18  with the assistance of press plate  20 . In the shown embodiment, belt  14  includes front or transfer surface  14   a  and rear or contact surface  14   b , where front surface  14   a  faces EP engine  12 . As discussed below, in some embodiments, belt  14  may be a multiple-layer belt with a low-surface-energy film that defines front surface  14   a , and which is disposed over a base portion that defines rear surface  14   b.    
     System  10  may also include biasing mechanism  29 , which is configured to induce an electrical potential through belt  14  to electrostatically attract layers  28  of the thermoplastic-based powder from EP engine  12  to belt  14 . Because layers  28  are each only a single layer increment in thickness at this point in the process, electrostatic attraction is suitable for transferring layers  28  from EP engine  12  to belt  14 . However, as mentioned above, the multiple printed layers  28  for 3D part  22  effectively prevents electrostatic transfer of layers  28  from belt  14  to build platform  18  after a given number of layers  28  are printed. 
     Rollers  16  are a series of drive and/or idler rollers or pulleys that are configured to maintain tension on belt  14  while belt  14  rotates in the rotational direction of arrows  30 . This allows belt  14  to maintain a substantially planar orientation when engaging build platform  18  and press plate  20 . System  10  may also include service loops (not shown), such as those disclosed in U.S. patent application Ser. No. 13/242,841. 
     System  10  also includes heater  32 , located upstream from build platform  18  and press plate  20 , based on the rotational direction of belt  14 . Heater  32  is one or more heating devices configured to heat layers  28  to at least a fusion temperature of the thermoplastic-based powder prior to reaching press plate  20 . Examples of suitable devices for heater  32  include non-contact radiant heaters (e.g., infrared heaters or microwave heaters), convection heating devices (e.g., heated air blowers), contact heating devices (e.g., heated rollers and/or platens), combinations thereof, and the like, where non-contact radiant heaters are preferred. Each layer  28  desirably passes by (or through) heater  32  for a sufficient residence time to heat the layer  28  to the intended fusion temperature. 
     As defined above, the fusion temperature is a temperature that sufficiently melts the thermoplastic-based powder to a fusable state. Thus, the fusion temperature will vary depending on the particular layer material used. For example, for an ABS copolymer material, the fusion temperature may range from about 180° C. to about 220° C. depending on the particular copolymer composition. Heating the thermoplastic-based powder does not necessarily require every component of the thermoplastic-based powder to melt. Rather, the overall thermoplastic-based powder needs to reach a fusable state for subsequent transfusion. This typically refers to the one or more thermoplastic materials of the thermoplastic-based powder being sufficiently melted to the fusable state. 
     Build platform  18 , press plate  20 , and heater  32  may collectively be referred to as layer transfusion assembly  33 . Layer transfusion assembly  33  is configured to transfuse the heated layers  28  from the belt  14  to the previously-transfused layers of 3D part  22  (or onto build platform  18 ) in a layer-by-layer manner. 
     Build platform  18  is a platform assembly or platen of system  10  that is configured to receive the heated layers  28  for printing 3D part  22  in a layer-by-layer manner. Build platform  18  is supported by z-axis gantry  34 , which is a linear guide mechanism configured to incrementally lower build platform  18  along the vertical z-axis relative to belt  14  and press plate  20  after each pressing step. The movement of build platform  18  with z-axis gantry  34  is operated by z-axis motor  36  based on commands from controller  24 , where z-axis motor  36  may be an electrical motor, a hydraulic system, a pneumatic system, or the like. 
     In the shown embodiment, build platform  18  is heatable with heating element  38  (e.g., an electric heater). Heating element  38  is configured to heat and maintain build platform  18  at an elevated temperature that is greater than room temperature (25° C.), such as at the desired average part temperature of 3D part  22 . This allows build platform  18  to assist in maintaining 3D part  22  at this average part temperature. 
     As mentioned above, the average part temperature for 3D part  22  is desirably high enough to promote interlayer adhesion and to reduce the effects of curling, while also being low enough to prevent 3D part  22  from softening too much (i.e., below its deformation temperature). Suitable average part temperatures for 3D part  22  range from greater than the average solidification temperature of the thermoplastic material(s) of the thermoplastic-based powder to about the glass transition temperature of the thermoplastic material(s). More desirably, the average part temperature is maintained at about the creep relaxation temperature of the thermoplastic material(s) of the thermoplastic-based powder, or within about 10° C. above or below thereof. Examples of suitable techniques for determining the creep relaxation temperatures of materials are disclosed in Batchelder et al., U.S. Pat. No. 5,866,058. 
     For example, when printing layers  28  of an ABS copolymer-based powder, the average part temperature for 3D part  22  may be about 100° C. This average part temperature allows 3D part  22  to maintain its structural integrity when pressed between build platform  18  and press plate  20  during subsequent transfusion steps. Furthermore, when the top-most layer of 3D part  22  maintained at this temperature and receives a heated layer  28  at a fusion temperature of about 200° C., the transfusion interface temperature for transfusing the layers together starts at about 150° C. This temperature is suitable for the polymer molecules of the layers  28  to at least partially interdiffuse to promote interfacial entanglement. 
     Press plate  20  is an example heateable element or heatable layer transfusion element, which is configured to press belt  14  and a given heated layer  28  downward against 3D part  22  and build platform  18  during each transfusion step. Press plate  20  may be actuated with a servo mechanism (not shown) configured to move press plate  20  along the vertical z-axis during each transfusion step. 
     The particular pressure applied during each transfusion step is desirably high enough to adhere the heated layer  28  to the previously-transfused layer  28  (or to build platform  18 ), allowing the polymer molecules to at least partially interdiffuse. However, the transfusion pressure is also desirably balanced to prevent press plate  20  from compressing 3D part  22  too much, thereby allowing 3D part  22  to maintain its dimensional integrity. 
     In the shown embodiment, press plate  20  is heatable with heating element  40  (e.g., an electric heater). Heating element  40  is configured to heat and maintain press plate  20  at an elevated temperature that is greater than room temperature (i.e., 25° C.). However, in comparison to the elevated temperature of build platform  18 , heating element  40  may heat and maintain press plate  20  at a temperature that is less than the desired average part temperature of 3D part  22 . For example, in situations where the intended average part temperature for 3D part  22  is 100° C., heating element  40  may heat and maintain press plate  20  at about 70° C. 
     The lower temperature for press plate  20  will cool belt  14  from the side of rear surface  14   b  (i.e., a transfixing step). As discussed above, if the transfused layer  28  remains too hot, it may remain adhered to front surface  14   a  of belt  14 , rather than cleaning releasing from belt  14 . As such, cooling belt  14  with the contact from press plate  20  allows the interface between front surface  14   a  of belt  14  and the transfused layer  28  to cool down enough to cleanly release the transfused layer  28  from belt  14 . 
     On the other hand, due to the large contact surface area between belt  14  and press plate  20 , if press plate  20  is maintained at a temperature that is too low (e.g., 25° C.), the contact duration between press plate  20  and belt  14  during the transfusion step may cool the transfused layer  28  down too much, thereby lowering the transfusion interface temperature, which can reduce the interlayer adhesion. As such, in some embodiments, heating element  40  desirably maintains press plate  20  at a temperature that balances these competing thermal effects to facilitate both transfusion and transfixing in a single, combined step. 
     System  10  may also include one or more air knives  42  or other cooling units, where air knife  42  is an example cooling unit configured to blow localized cooling air to the top layers of 3D part  22 . As best shown in  FIG. 2 , air knife  42  is located adjacent to the lateral side of build platform  18  to direct the cooling air laterally relative to the direction of movement of belt  14 . This allows air knife  42  to extend along the entire length of 3D part  22 , providing good air flow over the top layers of 3D part  22 , including the fused layer  28 . In an alternative embodiment, system  10  may also include a second air knife  42  (not shown) located at the opposing lateral side of build platform  18 . In embodiments having air knives  42  or other cooling units, press plate  20  may be heated to the fusion temperature to promote interdiffusion in the transfusion step. Then, upon release of press plate  20 , a separate transfix step may be performed by cooling the transfused layer before release of the layer from the belt  14 . 
     As mentioned above, when system  10  prints layers  28  at high speeds, the printed layers  28  do not have sufficient time to cool down to the desired average part temperature before successive layers  28  are printed. As such, as the height of 3D part  22  grows, heat dissipation from passive thermal diffusion becomes insufficient to cool the heated layers. 
     This is depicted in  FIG. 3 , which is a graphical illustration of the number of layers printed versus the average part temperature for a 3D part printed without the active cooling (represented by line  44 ) and for 3D part  22  printed with the active cooling (represented by line  46 ). As shown by line  44 , without the active cooling, the heat added by each layer at its fusion temperature will accumulate in the 3D part, causing the average part temperature to increase until the deformation temperature of the 3D part is reached, as illustrated by threshold line  48 . At threshold line  48 , the temperature in the bulk of the 3D part is high enough such that the part material substantially softens. When the 3D part reaches this point, the transfusion pressure applied by press plate  20  during subsequent transfusion steps may overcome the structural integrity of the 3D part, thereby deforming the 3D part. 
     Air knife  42 , however, actively cools each layer after the transfusion step to prevent the additional heat from accumulating. As shown by line  46 , the active cooling substantially removes the heat provided by each layer  28 , thereby providing substantially zero heat accumulation after each printed layer  28 . As such, 3D part  22  may be substantially maintained at an average part temperature that is below its deformation temperature during the entire printing operation. 
     In some embodiments, it can be beneficial for the average part temperature to be high enough such that the bulk of 3D part  22  exhibits a small amount of softening. It has been found that when the bulk of 3D part  22  exhibits a small amount of softening that still maintains its overall structural integrity, subsequent transfusion steps with press plate  20  may mildly compress the bulk of 3D part  22 , thereby increasing the part density. The increased part density correspondingly reduces brittleness and porosity of the resulting 3D part  22 , and increases its z-axis strength. These properties are beneficial for a variety of applications. 
     While illustrated with air knife  42 , system  10  may alternatively include a variety of different cooling units configured to actively cool each layer after the transfusion step to prevent the additional heat from accumulating, such as refrigeration units, liquid-cooling units, and the like. Furthermore, one or more air knives  42  (or other cooling units) may be located at other locations around build platform  18  and press plate  20  to direct the cooling air towards the top layers of 3D part  22 . Optionally, system  10  may also include additional heaters (e.g., heaters  270  and  272 , shown in  FIGS. 6A-6F ) to heat the top layer(s) or surface of 3D part  22  to the fusion temperature prior to each subsequent transfusion step. 
       FIG. 4A-4D  illustrate an example process for printing a layer  28  with system  10  using the layer transfer technique of the present disclosure. At the start of the printing operation, build platform  18  and press plate  20  may be heated to their desired temperatures. For example, build platform  18  may be heated to the desired average part temperature for 3D part  22 , and press plate  20  may be heated to a temperature that is lower than the desired average part temperature for 3D part  22 . 
     The printing operation initially involves developing a layer  28  with EP engine  12  (shown in  FIG. 1 ) and transferring the developed layer to heater  32  via belt  14 . As shown in  FIG. 4A , as the developed layer  28  passes by (or through) heater  32 , heater  32  heats the layer  28  and the associated region of belt  14  to at least the fusion temperature of the thermoplastic-based powder. 
     As shown in  FIG. 4B , the continued rotation of belt  14  in the direction of arrow  30  aligns the heated layer  28  above build platform  18  with proper registration location along the x-axis. Press plate  20  may then actuate downward, as illustrated by arrow  50 , to press the heated layer  28  onto the previously-printed layer of 3D part  22 . As shown in  FIG. 4C , because layer  28  is heated to at least the fusion temperature of the part material, the pressed layer  28  transfuses to the top surface of 3D part  22 . 
     Examples of suitable durations for pressing during the transfusion step range from about 0.1 seconds to about 1.0 second, with particularly suitable durations ranging from about 0.1 seconds to about 0.5 seconds. In some embodiments, the pressing duration is a fixed value for each layer  28 . In alternative embodiments, the pressing duration may be varied based on the dimensions and geometry of 3D part  22 . For example, the pressing duration may be reduced for layers  28  having smaller cross-sectional areas and/or fine-feature details, as discussed below. 
     The temperature of press plate  20 , being lower than the desired average part temperature, and substantially lower than the fusion temperature, begins to draw heat from the heated region of belt  14 . This assists in releasing the transfused layer  28  by cooling belt  14  from rear side  14   b , as discussed above. 
     After the transfusion/transfixing step is completed, press plate  20  may then be retracted upward, as illustrated by arrow  52 , to release the pressure applied to belt  14  and the transfused layer  28 . In the embodiment shown in  FIG. 4D , this releases the transfused layer  28  from belt  14 , allowing the transfused layer  28  to remain transfused to 3D part  22 . Additionally, z-axis gantry  34  may lower build platform  18  downward, such as by a single layer increment, as illustrated by arrow  54 . 
     In an alternate embodiment, press plate  20  may be heated to the fusion temperature to assist in the transfusion of layer  28 . In this embodiment, layer transfusion assembly  33  is desirably configured such that retracting press plate  20  upward does not immediately release the transfused layer  28  from belt  14 . Rather, belt  14  may be maintained in a relatively constant position upon the retraction of press plate  20  by positioning build platform  18  in a higher position relative to that shown in  FIG. 4D  during the transfusion step. A separate transfixing step may then be utilized. 
     In this transfixing step, air knife  42  may be activated to cool the transfused layer  28  before releasing it from belt  14 . After a duration sufficient to cool the layer material down below the fusion temperature, which maintains the transfused layer  28  in a fixed spatial position and adhered to 3D part  22 , build platform  18  is then lowered to release transfused layer  28  from belt  14 . 
     Air knife  42  may also be activated to blow cooling air onto the transfused layer  28  after delamination from belt  14 . This actively cools the transfused layer  28  down to the desired average part temperature. Examples of suitable durations for this active cooling step range from about 1.0 second to about 2.0 seconds, which may correspond with the transfer and alignment of the next layer  28 . 
     As can be appreciated, the transfer of layer  28  from belt  14  to build platform  18  requires a pause during the pressing step. Otherwise, the movement of belt  14  in the rotational direction of arrows  30  during the transfusion step may cause a mis-registration of the pressed layer  28 , potentially resulting in lower part quality. These pauses during each transfusion step may be accommodated with the use of service loops, such as those disclosed in U.S. patent application Ser. No. 13/242,841. As further shown in  FIGS. 4B-4D , as a present layer  28  is being transfused, the next layer  28   a  may be positioned at heater  32 . This allows heater  32  to sufficiently heat the next layer  28   a  during the required pause while the present layer  28  is transfused to 3D part  22 . After the layer  28  is transfused and cooled, the same process may then be repeated for layer  28   a , and each subsequent layer for 3D part  22 . 
       FIGS. 5A-5E  illustrate layer transfusion assembly  133 , which is an alternative to layer transfusion assembly  33  of system  10  (shown in  FIGS. 1, 2, and 4A-4D ), and where the reference numbers of the respective components are increased by “ 100 ” from system  10 . As shown in  FIG. 5A , layer transfusion assembly  133  includes nip roller  120  in lieu of press plate  20 , where nip roller  120  is another example heateable element or heatable layer transfusion element, and is configured to rotate around a fixed axis with the movement of belt  114 . In particular, nip roller  120  may roll against rear surface  114   b  in the direction of arrow  156  while belt  114  rotates in the direction of arrows  130 . In some embodiments, nip roller  120  may function as a drive roller for belt  114 . 
     As further shown, air jets  142  (or other suitable cooling units) are used in lieu of air knife  42 , and are located downstream from the interface between belt  114  and nip roller  120 . Air jets are configured to blow cooling air towards the top layers of 3D part  122  to actively cool the layers after each transfusion step, as discussed below. 
     In this embodiment, build platform  118  is supported by gantry  134 , which is a guide mechanism configured to move build platform  118  along the z-axis and the x-axis to produce a reciprocating rectangular pattern, where the primary motion is back-and-forth along the x-axis. Gantry  134  may be operated by motor  136  based on commands from controller  124 , where motor  136  may be an electrical motor, a hydraulic system, a pneumatic system, or the like. Accordingly, the pressure that is applied during each transfusion step is performed by build platform  118 . 
     Prior to printing 3D part  122 , build platform  118  and nip roller  120  may be heated to their desired temperatures, as discussed above for build platform  18  and press plate  20 . In comparison to press plate  20 , heating element  140  may heat nip roller  120  to a higher temperature (e.g., to the average part temperature) since there is a relatively short time for the heat to locally flow from nip roller  120  through belt  114 . 
     During the printing operation, heater  132  heats a developed layer  128  and the associated region of belt  114  to at least the fusion temperature of the thermoplastic-based powder. Belt  114  may then move the heated layer  132  to a predetermined registration location along the x-axis, as shown. Gantry  134  may then actuate build platform  118  upward to engage belt  114 , which presses the top layer of 3D part  122  against the heated layer  124 , as illustrated by arrow  158 . Alternatively, nip roller  120  may be actuated downward to meet the top layer or surface of 3D part  122 . 
     As shown in  FIG. 5B , this presses the heated layer  128  between the top layer of 3D part  122  and belt  114  at the location of nip roller  120 . While build platform  118  remains engaged with belt  114 , gantry  134  may then move build platform  118  (and 3D part  122 ) along the x-axis in the direction of arrow  160 , at a rate that is synchronized with the rotational rate of belt  114  in the direction of arrow  130  (i.e., the same directions and speed). This presses belt  114  and the heated layer  128  between the top layer of 3D part  122  and nip roller  120 . 
     As shown in  FIG. 5C , this causes rear surface  114   b  of belt  114  to roll across nip roller  120  to nip belt  114  and the heated layer  128  against the top layer of 3D part  122 . Because layer  128  is heated to the fusion temperature of the part material and 3D part  122  is maintained at the average part temperature, the pressed layer  128  transfuses to the top layer of 3D part  122  in a similar manner to that discussed above for 3D part  22  and layer  28 . 
     As further shown, as the transfused layer  128  passes the nip of nip roller  120 , belt  114  wraps around nip roller  120  to separate and disengage from build platform  118 . This assists in releasing the transfused layer  128  from belt  114 , allowing the transfused layer  128  to remain adhered to 3D part  122 . Additionally, air jets  142  blow cooling air towards the top layers of 3D part  122  as build platform  118  moves along the x-axis past nip roller  120  to transfix the transfused layer  128 . The transfused layer  128  may be cooled down to the average part temperature by continuing to blow cooling air against the 3D part  122  in between transfusion steps, thus overlapping the transfixing and active cooling steps. 
     When build platform  118  moves 3D part  122  past nip roller  120 , gantry  134  may actuate build platform  118  (and 3D part  122 ) downward, as illustrated by arrow  162 . As shown in  FIG. 5D , gantry  134  may then move build platform  118  (and 3D part  122 ) along the x-axis in the direction of arrow  164 , back to a starting position along the x-axis. As shown in  FIG. 5E , build platform  118  desirably reaches the starting position as the next heated layer  128  is positioned above 3D part  122 . The same process may then be repeated, where gantry  134  actuates build platform  118  upward to press belt  114  and the next heated layer  128  between the top layer of 3D part  122  and roller  120 , as illustrated by arrow  166 . In this step, however, the height of the top surface of 3D part  122  is offset downward, such as by a single layer increment, compared to the previous pressing step. 
     As can be appreciated, moving build platform  118  (and 3D part  122 ) in the reciprocating rectangular pattern allows the transfusion steps to be performed while belt  114  continuously rotates. In particular, moving build platform  118  at a rate that is synchronized with the rotational rate of belt  114 , along with the use of nip roller  120 , which rotates against rear surface  114   b  of belt  114 , allows the transfusion and transfixing steps to be performed rapidly (e.g., within about 0.1 to about 0.5 seconds). This allows the active cooling steps to range from about 1.0 second to about 2.0 seconds, which may correspond with the transfer and alignment of the next heated layer  28 . While the reciprocating rectangular pattern is described as a rectangular pattern with sharp axial corners (defined by arrows  160 ,  162 ,  164 , and  166 ), gantry  134  may move build platform  118  (and 3D part  122 ) in a reciprocating rectangular pattern having rounded or oval-defining corners, so long as build platform  118  moves linearly along the x-axis during the transfusion steps. 
       FIGS. 6A-6F  illustrate layer transfusion assembly  233 , which is an alternative to layer transfusion assembly  133  (shown in  FIGS. 5A-5E ), and where the reference numbers of the respective components are increased by “200” from system  10  (shown in  FIGS. 1, 2, and 4A-4D ). Layer transfusion assembly  233  may function in a similar manner to layer transfusion assembly  133 , where build platform  218  may move in a reciprocating rectangular pattern. 
     However, as shown in  FIG. 6A , layer transfusion assembly  233  includes fusion roller  220  and release roller  268  in lieu of a single nip roller  120 , where fusion roller  220  and release roller  268  are each configured to rotate around an axis with the movement of belt  214 . The use of separate rollers (i.e., fusion roller  220  and release roller  268 ) separates the functions of each roller, allowing them to be optimized for their particular purposes. For example, fusion roller  220  may be heated to the fusion temperature of the thermoplastic-based powder, and release roller  268  may be maintained at a substantially lower temperature to assist in delaminating the transfused layers  228  from belt  214 . 
     As further shown, system  210  also includes heaters  270  and  272  and air jets  274 . The separation of fusion roller  220  and release roller  268  creates separate transfusion and release steps, and allows a transfixing step to be performed therebetween via cooling by air jets  274 . By postponing the release step, fusion roller  220  may be heated to the fusion temperature best suited for the transfusion step, rather than a compromise temperature that facilitates both transfusion and release. This increases the interlayer adhesion between the transfused layers  228 . 
     Prior to printing 3D part  222 , build platform  218  and fusion roller  220  may be heated to their desired temperatures. For example, build platform  218  may be heated to the average part temperature and fusion roller  220  may be heated to the fusion temperature of the thermoplastic-based powder. During the printing operation, belt  214  carries a developed layer  228  past heater  232 , which heats the developed layer  228  and the associated region of belt  214  to at least the fusion temperature of the thermoplastic-based powder. 
     Additionally, platen gantry  234  moves build platform  218  along the x-axis in the direction of arrow  276  below, along, or through heater  270 . Heater  270  may function in the same manner as heaters  32  and  232 , and heats the top surface of 3D part  222  to an elevated temperature, such as at the fusion temperature of the layer material. 
     As shown in  FIG. 6B , the continued rotation of belt  214  and the movement of build platform  218  align the heated layer  228  with the heated top surface of 3D part  222  with proper registration along the x-axis. Furthermore, the heated layer  228  and the heated top surface of 3D part  222  each pass heater  272 , which may be configured to heat and/or maintain both the heated layer  228  and the heated top surface of 3D part  222  at the fusion temperature of the layer material. This prevents the heated layer  228  from cooling down prior to reaching fusion roller  220 , and brings the temperature of the heated top surface of 3D part  222  to or near the fusion temperature before the next transfusion step is performed. In alternative embodiments, one or more of heaters  232 ,  270 , and  272  may be provided a single heater configured to direct heat in multiple directions (e.g., towards both the layer  228  and the top surface of 3D part  222 ). 
     Gantry  234  may continue to move build platform  218  (and 3D part  222 ) along the x-axis in the direction of arrow  276 , at a rate that is synchronized with the rotational rate of belt  214  in the direction of arrow  230  (i.e., the same directions and speed). This causes rear surface  214   b  of belt  214  to rotate around fusion roller  220  to nip belt  214  and the heated layer  228  against the top surface of 3D part  222 . This engages build platform  218  and belt  214 , and presses the heated layer  228  between the heated top surface of 3D part  222  and belt  214  at the location of fusion roller  220 . 
     Because layer  228  and the heated top layer of 3D part  222  are each heated to the fusion temperature of the layer material, the pressed heated layer  228  transfuses to the heated top surface of 3D part  222  with a high level of interlayer adhesion. By separating fusion roller  220  and release roller  268 , with a cooling step therebetween via air jets  274 , layer transfusion assembly  233  allows the layers to be heated to an optimal transfusion interface temperature, and to be cooled to a temperature that fixes the layers before release. For example, the transfusion interface temperature for transfusing the layers together may be at about the fusion temperature of the layer material (e.g., about 200° C.). This substantially increases the extent to which the polymer molecules of the transfused layers interdiffuse to promote interfacial entanglement, while also maintaining dimensional accuracy of 3D part  222 . 
     As shown in  FIG. 6C , after passing fusion roller  220 , and while build platform  218  remains engaged with belt  214 , belt  214 , build platform  218 , and 3D part  222  pass air jets  274 . Air jets  274  may function in the same manner as air jets  142  and  242  for cooling belt  214  the side of rear surface  214   b . In alternative embodiments, air jets  274  may be a variety of different cooling units, such as refrigeration units, liquid-cooling units, and the like. 
     As discussed above, if the transfused layer  228  remains too hot, portions of it may remain adhered to front surface  214   a  of belt  214 , rather than cleaning releasing from belt  214 . As such, cooling belt  214  with air jets  274  allows the interface between front surface  214   a  of belt  214  and the transfused layer  228  to cool so that the transfused layer  228  will remain adhered to 3D part  222  and cleanly release from belt  214 . This also partially assists in the active cooling of 3D part  222  to maintain 3D part  222  at the average part temperature below its deformation temperature. 
     As further shown in  FIG. 6D , as the transfused layer  228  passes the nip of release roller  268 , belt  214  rotates around release roller  268  to separate and disengage from build platform  218 . This assists in releasing the transfused layer  228  from belt  214 , in an “assisted delamination” step, allowing the transfused layer  228  to remain adhered to 3D part  222 . Additionally, air jets  242  blow cooling air towards the top layers of 3D part  222  as build platform  218  moves along the x-axis past release roller  268 . This actively cools the transfused layer  228  down to the average part temperature, as discussed above. 
     When build platform  218  moves 3D part  222  past release roller  268 , gantry  234  may actuate build platform  218  (and 3D part  222 ) downward, as illustrated by arrow  278 . For example, build platform  218  may be incrementally offset downward by a single layer increment. As shown in  FIG. 6E , gantry  234  may then move build platform  218  (and 3D part  222 ) along the x-axis in the direction of arrow  280 , back to a starting position along the x-axis. 
     As shown in  FIG. 6F , build platform  218  desirably reaches the starting position for proper registration with the next layer  228 . The same process may then be repeated for each remaining layer  228  of 3D part  222 . Layer transfusion assembly  233  provides the benefits of transfusing the layers together at the fusion temperature to increase interlayer adhesion, while also sufficiently cooling down the interface between front surface  214   a  of belt  214  and the transfused layers  228  to transfix the layers  228  in place before release from belt  214 , and further promotes a clean release by assisting the delamination from belt  214 . 
     In addition, for each printed layer  228 , the combination of air jets  242  and  274  (or other cooling units) substantially removes the heat that is added from heating elements  232 ,  270 ,  272 , and from the heated fusion roller  220 , prior to printing the next layer  228 . This active cooling substantially removes the heat provided by each layer  228 , thereby providing substantially zero heat accumulation after each printed layer  228 . As such, 3D part  222  may be substantially maintained at an average part temperature that is below its deformation temperature during the entire printing operation. Further, the top layer surface temperature of the printed 3D part  222  may be brought back up to the fusion temperature after delamination using heater  270  and/or  272  of layer transfusion assembly  233  for optimal transfusion of the next layer  228 . 
     System  10  with layer transfusion assemblies  33 ,  133 , and  233  is suitable for printing 3D parts (and any support structures) from thermoplastic-based powders at high rates and with good part resolutions. In some embodiments, system  10  may print layers of a 3D part at a rate of at least about 40 layers per minutes (e.g., about 50 layers per minute) with accurate registrations, layer thicknesses ranging from about 5 micrometers to about 125 micrometers, and layer dimensions along the y-axis up to at least about 51 centimeters (about 11 inches). For example, system  10  may print a 3D part at a rate of about three inches in height along the vertical z-axis per hour. 
     The resolutions of the 3D parts may also be varied based on the printing rate. For example, each 3D part may be printed at a “high quality” resolution, in which system  10  operates at a slower rate, but prints with lower layer thicknesses. Alternatively, a 3D part may be printed at a “draft quality” resolution, in which system  10  operates a faster rate, but prints with greater layer thicknesses. Furthermore, a 3D part may be printed in “gray scale”, in which a lower density of the part material is developed. Numerous resolutions and speeds therebetween may also be incorporated. In each of these situations, the controller may adjust the applied pressures, temperatures, and/or contact durations during the transfusion steps to account for the different printing rates. 
     System  10  is illustrated as being configured to print 3D parts (e.g., 3D parts  22 ,  122 , and  222 ) from a single thermoplastic-based powder. However, the additive manufacturing systems of the present disclosure may also be configured to print 3D parts and/or support structures from multiple part materials and/or support materials derived from thermoplastic-based powders (e.g., multiple compositions and/or colors). Examples of suitable multiple-material systems include those disclosed in U.S. patent application Ser. Nos. 13/242,669 and 13/242,841. 
     In some embodiments, controller  24  may monitor the applied pressure, the temperature of the layers, and the contact durations during the transfusion steps to maximize or otherwise increase the effectiveness in transferring the layers from the front surface of the transfer belt to the build platform. In an open-loop embodiment, one or more of the applied pressure, temperature, and contact durations may be fixed parameters for a given part material and overall printing rate. 
     Alternatively, in a closed-loop embodiment, controller  24  may adjust one or more of these parameters in response to the monitored signals using the one or more process control loops. For example, the controller may adjust the pressure applied by press plate  20  or build platforms  118  and  218  in response to changes in the monitored pressure and/or changes in the monitored temperature of the layers. Moreover, controller  24  may adjust the contract durations during the transfusion steps to compensate for changes in the temperatures of layers and/or fluctuations in the monitored applied pressures. 
     As shown in  FIG. 7 , in some embodiments, system  10  may also include a chamber  284 , which can extend around layer transfusion assembly  33 , and may define an enclosable environment for printing 3D part  22 . While illustrated in use with layer transfusion assembly  33 , chamber  284  is equally suitable for use with layer transfusion assemblies  133  and  233 . Chamber  284  is a temperature-controllable chamber, which provides greater control over the active cooling step. For example, chamber  284  may be maintained at the average part temperature of 3D part  22 . 
     In these embodiments, chamber  284  may partially enclose z-axis gantry  34  and belt  14 , allowing z-axis gantry  34  and belt  14  to extend through the walls of chamber  284 . In alternative embodiments, heater  32  may be located outside and upstream of chamber  284 . In further alternative embodiments, chamber  284  may be located below press plate  20 , allowing build platform  18  to be lowered down into chamber  284 . These embodiments further assist in maintaining 3D part  22  at an average part temperature that is below its deformation temperature. 
     As further shown in  FIG. 7 , layer transfusion assembly  33 ,  133 , or  233  may also include pressure sensors (e.g., pressure sensor  286 ) and/or capacitive sensors (e.g., capacitive sensor  288 ), each of which is configured to communicate with controller  24  over one or more communication lines (not shown). Pressure sensor  286  is one or more sensor assemblies configured to measure the transfusion pressure applied between build platform  18  and press plate  20  (or between build platforms  118 / 218  and rollers  120 / 220 ), allowing controller  24  to monitor the applied transfusion pressure and adjust the height of build platform  18  and/or press plate  20  using one or more process control loops. Examples of suitable sensor assemblies for pressure sensor  286  include one or more strain gauges retained on build platform  18  and/or press plate  20 . 
     Capacitive sensor  288  is one or more capacitive sensor assemblies configured to measure the electrical resistance between build platform  18  and press plate  20  (or between build platforms  118 / 218  and rollers  120 / 220 ). For example, during a transfusion step, capacitive sensor  288  may induce an electrical current from platen  18  to press plate  20  (or vice versa), and measure the intensity of the resulting electrical current through the printed layers  28  of 3D part  22  and belt  14 . Since the thickness of belt  14  is constant, the resulting electrical current will reduce as the 3D part  22  grows through the printing of successive layers  28 . 
     Thus, capacitive sensor  288  is suitable for monitoring the height of 3D part  22  and the number of layers  28  transferred to build platform  18 . This allows controller  24  to accurately predict the applied pressure during a subsequent pressing step rather than merely relying on the calculated height of a single layer increment. This accurate prediction allows build platform  18  to be quickly raised to an intended height, rather than relying solely on feedback signals from pressure sensor  286 . 
     Build platforms  18 ,  118 , and  218 , press plate  20 , and rollers  120  and  220  may each also include one or more temperature sensors (not shown) configured to respectively measure the temperatures of the build platforms and press plate/rollers, allowing controller  24  to hold them at the above-discussed temperatures. In a further alternative embodiment, system  10  may include temperature sensors (not shown) configured to measure the temperatures of the 3D part layers. For example, system  10  may include an ultrasonic transducer for measuring the temperature of the layer  28  retained by belt  14  and/or the temperatures of the previously transfused layers  28  of 3D part  22  using acoustic thermometry. 
     In some embodiments, controller  24  and/or host computer  26  may receive operational mode selections for operating system  10  in different modes. For example, a user may select operational modes such as high quality printing, draft quality printing, and gray scale, as discussed above. Alternatively, system  10  may receive the operational mode selections as default or system generated modes (e.g., a default of a high quality printing). These received operational mode selections may alternatively (or additionally) be set based on the geometry of the 3D part, such as if the 3D part has a small cross-sectional area and/or fine-feature details, as discussed above. 
     Upon receipt of these operational mode selections, controller  24  and/or host computer  26  may set transfusion parameters for performing the transfusion steps, based on the received operational mode selections. For example, the transfusion pressure, temperature(s), and or duration for each transfusion step may be set or adjusted based on the received operational mode select. This provides greater control over the transfusion steps when operating system  10  to improve printing accuracies and/or printing rates. 
     System  10  may then image a layer of the 3D part from a thermoplastic-based powder (e.g., develop a layer with EP engine  12 ), transfer the imaged layer to a transfer medium, heat the imaged layer while the imaged layer is retained on the transfer medium, and transfuse the heated layer to a surface of the three-dimensional part based on the set transfusion parameters. 
     In some embodiments, the set transfusion parameters allow the transfusion pressure, temperature(s), and or duration for each transfusion steps to vary between different transfusion steps. For example, if a first portion of a 3D part contains a simple block geometry and a second portion of the 3D part contains a fine-feature geometry, controller  24  and/or host computer  26  may set the transfusion parameters such that the layers used to form the simple block geometry are transfused differently (e.g., higher transfusion pressure) from those used to form the fine-feature geometry (e.g., lower transfusion pressure). 
       FIGS. 8 and 9A-9C  illustrate a suitable embodiment for belt  14  (shown in  FIGS. 1, 2, and 4A-4D ), and is equally suitable for belt  114  (shown in  FIGS. 5A-5E ) and belt  214  (shown in  FIGS. 6A-6F ). In the embodiment shown in  FIG. 8 , belt  14  is a multiple-layer belt that includes layer or film  290  (defining front surface  14   a ) and base portion  292  (defining rear surface  14   b ). 
     Film  290  and base portion  292  are desirably derived from materials that are suitable for transferring the layers  28  of part (or support) materials from EP engine  12  to build platform  18 , that are thermally stable at the fusion temperatures of the part and support materials, and that are robust for continued operation at high rotational speeds while being repeatedly heated and cooled during the heating and active cooling steps. 
     Film  290  is derived from one or more low-surface energy materials, thereby allowing the received layers  28  to effectively release from front surface  14   a  to build platform  18 . Examples of suitable materials for film  290  include one or more fluorinated polymers, such as polytetrafluoroethylenes (PTFE), fluorinated ethylene propylenes, and perfluoroalkoxy polymers. Examples of suitable commercially available fluorinated polymers include PTFE available under the trade designation “TEFLON” from E.I. du Pont de Nemours and Company, Wilmington, Del. 
     Base portion  292  is derived from one or more materials that promote good electrostatic attraction for the thermoplastic-based powders to front surface  14   a  via triboelectric charges. Examples of suitable materials for base portion  292  include one or more polyimide materials, such as those commercially available under the trade designation “KAPTON” from E.I. du Pont de Nemours and Company, Wilmington, Del., which may be doped with one or more conductive materials to promote the triboelectric charges. In some embodiments, belt  14  may also include one or more additional layers between film  290  and base portion  292 , such as one or more tie layers. 
       FIGS. 9A-9C  illustrate alternative embodiments for belt  14  for engaging with various drive rollers of system  10 . As shown in  FIG. 9A , front surface  14   a  of belt  14  may include receiving region  294  and edge regions  296  on opposing lateral sides of receiving region  294 . Receiving region  294  is the region of front surface  14   a  on which layers  28  are retained for transfer between EP engine  12  and build platform  18 . Edges regions  296  are the regions at which one or more drive mechanisms may engage drive belt  14 . 
     For example, one or more rollers (e.g., rollers  16 , nip roller  120 , fusion roller  220 , release roller  268 , and/or any service-loop roller) may engage front surface  14   a  and/or rear surface  14   b  at edge regions  296  to ensure the rollers to not interfere with the developed layers  28 . In some embodiments, pairs of opposing rollers (not shown) may simultaneously engage front surface  14   a  and rear surface  14   b  at edge regions  296  to nip and drive belt  14  in the direction of arrow  30 . 
     Registration along the x-axis may be maintained with the use of encoder markings  298 . Encoder markings  298  may be pre-marked on front surface  14   a  and/or rear surface  14   b  at preset increments along the x-axis, or may be printed with the developed layers  28  to identify relative locations of the developed layers  28  along the x-axis. System  10  may also include one or more optical readers (e.g., optical reader  299 ) to locate encoder markings  298  as belt  14  rotates in the direction of arrow  30 . 
     Alternatively, as shown in  FIG. 9B , belt  14  may include an array of holes  300  or other openings that extend through film  290  and base portion  292  adjacent to the lateral edges of belt  14 . Holes  300  are configured to engage with reciprocating gear teeth (not shown) of one or more rollers (e.g., rollers  16 , nip roller  120 , fusion roller  220 , release roller  268 , and/or any service-loop roller) to drive belt  14  in a tractor-feed manner. In this embodiment, registration along the x-axis may also be maintained with the use of encoder markings  298 , if desired. Alternatively, holes  300  may themselves function as encoder markings in the same manner. System  10  may also include one or more optical readers (e.g., optical reader  299 ) to locate encoder markings  298  and/or holes  300  as belt  14  rotates in the direction of arrow  30 . 
       FIG. 9C  shows yet another alternative embodiment in which belt  14  includes rear ribs  302  that extend laterally along rear surface  14   b . Ribs  302  are configured to engage with reciprocating gear teeth (not shown) of one or more rollers (e.g., rollers  16 , nip roller  120 , fusion roller  220 , release roller  268 , and/or any service-loop roller) to drive belt  14  in a timing-belt manner. In this embodiment, registration along the x-axis may also be maintained with the use of encoder markings corresponding to encoder markings  298 , if desired. Alternatively, ribs  300  may themselves function as encoder markings in the same manner. System  10  may also include one or more optical readers (e.g., optical reader  299 ) to locate the encoder markings and/or holes ribs as belt  14  rotates in the direction of arrow  30 . 
       FIGS. 9A-9C  illustrate example engagement mechanisms for belt  14 , allowing belt  14  to engage with one or more drive mechanisms of system  10 . However, belt  14  may alternatively include different engagement mechanisms as particular designs may require. 
       FIGS. 10-12  are flow diagrams of embodied methods for the layer transfer technique of the present disclosure, which may be performed with system  10 .  FIG. 10  illustrates method  310 , which may be performed with system  10  having layer transfusion assembly  33  (shown in  FIGS. 1, 2, and 4A-4D ) and/or layer transfusion assembly  133  (shown in  FIGS. 5A-5E ). As shown, method  310  includes step  312 - 324 , and initially involves developing or otherwise imaging a layer (step  312 ), such as with EP engine  12 . The imaged layer may then be transferred on a transfer medium (e.g., belts  14  and  114 ) from a first location at EP engine  12  to a second location at the layer transfusion assembly (e.g., layer transfusion assemblies  33  and  133 ) (step  314 ). 
     Prior to reaching the second location at the layer transfusion assembly, the layer is heated to at least a fusion temperature of the thermoplastic-based powder (e.g., at heaters  32  and  132 ) (step  316 ). Upon reaching the layer transfusion assembly, the heated layer is then transfused and transfixed in a combined step (step  318 ). 
     For example, for layer transfusion assembly  33 , press plate  20  may engage build platform  18  to transfuse the heated layer  28  to the top surface of 3D part  22 . Because press plate  20  may be heated to a temperature that is lower than the fusion temperature, the contact between press plate  20  and rear surface  14   b  of belt  14  cools down the interface between belt  14  and the transfused layer  28 , increasing the adhesive force of the interdiffused polymers in the transfused layer  28  and 3D part  22  relative to the adhesive force of the transfused layer  28  to surface  14   a  of belt  14 , thereby maintaining the transfused layer adhered to the 3D part in a fixed spatial position. 
     The transfused and transfixed layer may then be released from the transfer medium (step  320 ), such as by retracting press plate  20  and/or build platform  18 , or by the separation of belt  114  from build platform  118  by belt  114  winding around nip roller  120 . The transfixing step discussed above allows the transfused layer to cleanly release from the transfer medium and remain adhered to the 3D part. 
     The 3D part may then be actively cooled (e.g., with air knives  42  and air jets  142 ) (step  322 ). As discussed above, because the imaging system (e.g., system  10 ) is capable of printing the layers at speeds that are much faster than the rate at which heat diffuses through the variable thermal resistance of the 3D parts, heat can accumulate in the 3D parts, which, if not accounted for, can exceed a deformation temperature of the 3D part, causing the bulk of the 3D part to soften enough reduces its structural integrity. Such a soft part may deform under a subsequently-applied transfusion pressure during a subsequent transfusion step. 
     To overcome this issue while maintaining fast printing rates, the 3D part may be actively cooled between each transfusion step  318  to maintain the 3D part at an average part temperature that is lower than the deformation temperature of the 3D part. Steps  312 - 324  may then be repeated for each layer of the 3D part until the printing operation is completed (as indicated by arrow  324 ). By heating each layer to at least the fusion temperature of the thermoplastic-based powder, followed by transfusing/transfixing, and active cooling allows system  10  to print 3D parts with good part quality and strengths (e.g., z-strengths). 
       FIG. 11  illustrates method  326 , which is similar to method  310  (shown in  FIG. 10 ), and may be performed with system  10  having layer transfusion assembly  233  (shown in  FIGS. 6A-6F ) (and with layer transfusion assembly  33  in the embodiment in which belt  14  remains in contact with the transfused layer  28  after press plate  20  retracts). Method  326  includes steps  328 - 342 , where steps  328 ,  330 ,  332 ,  338 ,  340 , and  342  may be performed in the same manner as the respective steps of method  310 . 
     However, instead of a combined transfusion and transfixing step  318  of method  310 , method  326  includes a transfusion step  334  and transfixing step  335 , which are separate. for example, layer transfusion assembly  233  includes a heated fusion roller  220  (for transfusion step  334 ) and release roller  268  (for release step  338 ), which are separated by air jets  274  (for a cooling or transfixing step  336 ). This allows the layers to be heated to an optimal transfusion interface temperature at the heating step  332 , and during the transfusion step  334 , and then to be cooled to a temperature that fixes the layers (at transfixing step  336 ) before release at release step  338 . This substantially increases the extent to which the polymer molecules of the transfused layers interdiffuse to promote interfacial entanglement, while also maintaining dimensional accuracy of the 3D part. 
     Moreover, the release of the transfused layer from the transfer medium may be assisted during the release step  338 . For example, release roller  268  may assist in releasing transfused layer  228  from belt  214  by increasing the angle of separation between belt  214  and build platform  218 , which increases the ease at which transfused layer  228  delaminates from belt  214 . 
     As further shown in  FIG. 11 , the active cooling step  340  may be an optional step of method  326  (as illustrated with the broken lines  344 ). For example, system  10  may instead operated at a lower printing speed to allow heat to diffuse from the 3D part. However, as discussed above, the active cooling step  340  is desirable for maintaining the structural integrity of the 3D part while printing at high speeds. 
       FIG. 12  illustrates method  346 , which is similar to method  310  (shown in  FIG. 10 ) and method  326  (shown in  FIG. 11 ), and may be performed with system  10  having layer transfusion assembly  233  (shown in  FIGS. 6A-6F ). Method  346  may also be performed with layer transfusion assembly  33  in the embodiment that includes one or more heaters corresponding to heaters  270  and  272 . Method  346  includes steps  348 - 364 , where steps  348 ,  350 ,  352 ,  356 ,  358 ,  360 ,  362 , and  364  may be performed in the same manner as the respective steps of method  326 . 
     However, method  346  also includes step  354 , in which the top surface or layer(s) of the 3D part is also pre-heated prior to the transfusion step  356 . For example, with layer transfusion assembly  233 , heaters  270  and  272  may heat the top surface or layer(s) of 3D part  222  to at least the fusion temperature of the thermoplastic-based powder. Because layer  228  and the heated top surface/layer of 3D part  222  are each heated to the fusion temperature of the layer material, the pressed heated layer  228  transfuses to the heated top surface/layer of 3D part  222  with a high level of interlayer adhesion (during transfusion step  358 ). 
     Furthermore, by separating fusion roller  220  and release roller  268 , with a cooling or transfixing step  358  therebetween via air jets  274 , layer transfusion assembly  233  allows the layers to be heated to an optimal transfusion interface temperature, and to be cooled to a temperature that fixes the layers before release. This substantially increases the extent to which the polymer molecules of the transfused layers interdiffuse to promote interfacial entanglement, while also maintaining dimensional accuracy of the 3D part. 
     EXAMPLES 
     The present disclosure is more particularly described in the following examples that are intended as illustrations only, since numerous modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. 
     Example 1 
     For the printing operations of Example 1, 3D parts were printed with an additive manufacturing system corresponding to the system shown in  FIGS. 1, 2, and 4A-4D , having a moveable press plate and a vertically-actuatable build platform, each of which were heated. The 3D parts were printed from an ABS part material, where each layer was developed with an electrophotography engine and transferred to a rotatable transfer belt looped around a service loop design. The developed layer was carried by the transfer belt to an infrared heater to heat the layer to a fusion temperature of ABS part material. 
     The heated layer was then transferred to and aligned over the build platform. The press plate was actuated downward to transfuse the heated layer against a previously-transfused layer of the 3D part (or to the build platform for the initial layer). The press plate was then retracted, which cleanly delaminated the layer from the transfer belt, allowing the layer to remain transfused to the 3D part. 
     Cooling air from an air knife was also directed to the top layers of the 3D part. This removed the additional heat from the transfused layer to provide an average part temperature of that maintained the structural integrity of the 3D part, where the 3D part did not slump or melt due to heat accumulation. This process was repeated for each layer of the 3D part. The air knife and the press plate successfully maintained the average part temperature below the deformation temperature of the 3D part during the entire printing operation. The resulting 3D parts exhibited good part resolution, dense fill, and good dimensional integrity. 
     Example 2 
     For the printing operations of Example 2, 3D parts were printed with an additive manufacturing system corresponding to the system shown in  FIGS. 5A-5E , having a nip roller and a moveable build platform, each of which were heated. The 3D parts were printed from an ABS part material, where each layer was developed with an electrophotography engine and transferred to a rotatable transfer belt looped around a service loop design. In these Examples, the service loop was used to protect the transfer belt against tension stress. The developed layer was carried by the transfer belt to an infrared heater to heat the layer to a fusion temperature of the ABS part material. 
     The heated layer was then transferred to and aligned over the build platform. The build platform was actuated upward to transfused the heated layer against a previously-transfused layer of the 3D part (or to the build platform for the initial layer). In particular, actuating the build platform upward pressed the heated layer and transfer belt between the build platform (or against a previously-transfused layer of the 3D part) and the nip roller. The build platform was then moved at a synchronized rate with the transfer belt, and then released at a downstream location. This cleanly delaminated the layer from the transfer belt, allowing the layer to remain transfused to the 3D part. 
     Cooling air from air lets was also directed to the top layers of the 3D part. This removed the additional heat from the transfused layer to provide an average part temperature of that maintained the structural integrity of the 3D part, where the 3D part did not slump or melt due to heat accumulation. The build platform was then moved back to its starting position, and the process was repeated for each layer of the 3D part. In these Examples, the air jets also successfully maintained the average part temperature below the deformation temperature of the 3D part during the entire printing operation. The resulting 3D parts also exhibited good part resolution, dense fill, and good dimensional integrity. 
     Comparative Examples A and B 
     For the printing operations of Comparative Examples A and B, 3D parts were printed with the same additive manufacturing systems respectively used for Examples 1 and 2. However, for Comparative Examples A and B, the air knife or jet cooling was omitted. Otherwise, the processes were performed in the same manner as discussed above for Examples 1 and 2. 
     For the printing operations of Comparative Examples A and B, prior to the completion of each printed 3D part, the printed layers began to compress and flatten. As discussed above, this is believed to be due to heat accumulating in the printed layers, which was unable to diffuse sufficiently between each printed layer. The accumulated heat softened the bulk of the 3D part, causing it to compress during subsequent transfusion steps. This resulted in deformed 3D parts. 
     As such, the layer transfer technique of the present disclosure including the active cooling is beneficial for printing 3D parts at high rates using electrophotography. The active cooling was successfully implemented to remove the added heat from each fused layer prior to the transfusion of the next layer. This allowed the 3D parts printed with the systems in Examples 1 and 2 to be maintained at average part temperatures below their deformation temperatures, but high enough to promote good interlayer adhesion and reduced curl. 
     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.