Patent Publication Number: US-2022227040-A1

Title: Selective deposition-based additive manufacturing device and method of printing 3d parts with semi-crystalline materials

Description:
This application is being filed as a PCT International Patent application on May 29, 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, inventor for the designation of all countries, and Eric Carl Stelter, a U.S. Citizen, inventor and applicant for the designation of all countries, and claims priority to U.S. Provisional Patent Application No. 62/855,566, filed May 31, 2019, the contents of which are herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to additive manufacturing systems and methods for producing three-dimensional (3D) parts with semi-crystalline materials. In particular, the present disclosure relates to selective deposition-based additive manufacturing systems for producing 3D parts with semi-crystalline polymers, and methods of producing 3D parts with feedstock materials having a semi-crystalline polymeric matrix using the selective deposition-based additive manufacturing systems. 
     Additive manufacturing is generally a process for manufacturing a three-dimensional (3D) object in additive manner 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. The host computer generates 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, the present disclosure is directed to a selective deposition-based additive manufacturing system capable of building a three-dimensional (3D) part utilizing a semi-crystalline polymeric material. The system includes at least one electrostatographic engine configured to develop one or more layers of particles of semi-crystalline polymeric material corresponding to one or more slices of a 3D model of a 3D part. The system includes a transfer medium configured to receive the one or more layers of particles of the semi-crystalline polymeric material on a front side from the at least one electrostatographic engine and to move the one or more layers away from the electrostatographic engine and a platen configured to carry the 3D part or support being printed. The system includes a gantry coupled to the platen and configured to move the platen into registration with the one or more layers, and a heater configured to heat a top surface of the 3D part being printed to a transfuse temperature. The system includes a layer transfer assembly having a roller contacting a back side of the transfer medium and a driver configured to cause the layer to transfer from the front side of the transfer medium to the heated top surface of the part. The system includes a cooler configured to cool the melted semi-crystalline polymeric material at a maximum rate of at least 20° C. per second such that the semi-crystalline material is in a super-cooled state wherein the semi-crystalline polymeric material does not completely initially crystallize. 
     Another aspect includes a method of printing a 3D part in a layer-wise manner with a semi-crystalline polymeric material utilizing a selective deposition-based additive manufacturing system. The method includes developing a first partial layer of semi-crystalline polymeric material using at least one electrostatotographic engine and conveying the first layer from the at least one electrostatographic engine to a transfusion assembly with a transfer medium. The method includes moving a platen carrying a 3D part being printed with a gantry to a location upstream from the transfusion assembly and heating a top layer of the 3D part to a transfusion temperature. The method includes registering the layer with the 3D part and moving the 3D part and the layer through a transfusion assemble to thermally transfer the layer to the top surface of the 3D part while applying a pressure to a back surface of the transfer medium with a roller. The method includes cooling the transfused layer at a rate of least 20° C. per second such that the semi-crystalline material is super-cooled and repeating the above steps until the 3D part is printed. 
     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, and includes terpolymers (i.e., copolymers having three monomer species). 
     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. For example, “at least one polyamide”, “one or more polyamides”, and “polyamide(s)” may be used interchangeably and have the same meaning. 
     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 scope of the present disclosure. 
     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. 
     The term “semi-crystalline” refers to polymers that form crystalline structures upon cooling after being melted. Non-limiting examples of semi-crystalline materials include polyolefins such as polyethylene and polypropylene; polyesters, polyamides, such as, but not limited to, nylons, polysulfones such as polyethersulfone (PES) and ketones, such as, but not limited to, polyetheretherketone (PEEK), and polyetherketoneketone (PEKK). 
     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). 
     All references cited herein are incorporated by reference in their entireties. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph of specific volume versus temperature for a typical amorphous polymer and a typical semi-crystalline polymer. 
         FIG. 2  is a simplified diagram of an example electrophotography-based additive manufacturing system for printing 3D parts and associated support structures, in accordance with embodiments of the present disclosure. 
         FIG. 3  is a schematic front view of a pair of example electrophotography (EP) engines of the system for developing layers of the part and support materials. 
         FIG. 4  is a schematic front view of an example electrophotography engine, which includes an intermediary drum or belt. 
         FIG. 5  is a schematic front view of an example transfusion assembly of the system for performing layer transfusion steps with the developed layers. 
     
    
    
     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 
     The present disclosure is directed to selective deposition-based additive manufacturing systems for printing 3D parts with semi-crystalline materials or material feedstocks that have a semi-crystalline polymeric matrix, and methods of printing 3D parts with semi-crystalline materials or material feedstocks that have a semi-crystalline polymeric matrix using such systems. During a selective deposition 3D part additive manufacturing or printing operation, an electrostatography engine can develop each layer of the 3D part (and any associated support material) from a polymeric toners or powder-based materials using an electrostatographic process. The developed layers are then transferred to a transfer medium (e.g., a flexible, continuous belt), which conveys the layers to a transfusion assembly where those layers are transfused (e.g., transferred and fused using heat and pressure) to build a 3D part and support structures in a layer-by-layer manner. 
     Typically, amorphous polymeric materials have been used to print 3D parts because the physical properties of amorphous polymeric materials, specifically their densities, gradually change with temperature. In contrast, semi-crystalline polymeric materials generally have more abrupt changes in density near a melting temperature is reached, which causes the semi-crystalline material to mechanically distort in an additive manufacturing process. As the semi-crystalline material cools, crystalline structures are rapidly formed which causes rapid shrinkage. When attempting to print 3D parts, the shrinkage causes intra-layer curling and de-lamination between layers. 
     The glass transition temperature, T g , is usually defined as a second order phase transition where the specific heat undergoes a step increase from lower to higher temperature. Generally, it is accompanied by a step change in the slope of the specific volume, as shown in FIG. 1 . The melt temperature, T m , is a first order phase transition where heat is released as the temperature is increased through T m . There are generally specific volume changes in semicrystalline polymers near T m , as shown in  FIG. 1 . 
     Semi-crystalline materials behave significantly differently than amorphous polymeric materials. Semi-crystalline materials typically have an amorphous domain and a crystalline domain. The amorphous domain behaves as mentioned above, and softens and flows after the glass transition temperature is reached, while the crystalline domain has abrupt changes in density. 
     As illustrated in  FIG. 1 , the slope of the line of the semi-crystalline domain is high proximate the melting temperature (T m ), which is indicative of a high thermal expansion coefficient. Parts made by additive manufacturing systems in this temperature range are therefore prone to curl. 
     Electrostatography-based additive manufacturing systems produce layers of material as substantially unheated sheets. The layers are moved from an imaging engine to a transfusion assembly where the part being printed can be preheated, the layer transfused to the prior printed layer and the heat can be removed in a sufficiently short amount of time. The heat can be rapidly removed from a semicrystalline part being printed, supercooling the transferred layer to an amorphous-like material in the short period of time, which prevents intra-layer curling and de-lamination between the layers such that the 3D part can be printed with semi-crystalline based materials. Once the part is printed, the entire part is allowed to crystallize in a controlled manner, similar to that of an injection molded part. 
     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. 2  is a simplified diagram of an example electrophotography-based additive manufacturing system  10  for printing 3D parts and associated support structures, in accordance with embodiments of the present disclosure. As shown in  FIG. 1 , system  10  includes one or more EP engines, generally referred to as  12 , such as EP engines  12   p  and  12   s , a transfer assembly  14 , biasing mechanisms  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 Publication Nos. 2013/0186549 and 2013/0186558. 
     The EP engines  12   p  and  12   s  are imaging engines for respectively imaging or otherwise developing layers, generally referred to as  22 , of the powder-based part and support materials, where the part and support materials are each preferably engineered for use with the particular architecture of the EP engine  12   p  or  12   s . 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 the 3D part  26 , which may include support structures and 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. 3 . 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 example transfer assembly  14  illustrated in  FIG. 3  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. 
     The EP engine  12   s  develops layers of powder-based support material, and the EP engine  12   p  develops layers of powder-based part/build material. In some embodiments, the EP engine  12   s  is positioned upstream from the EP engine  12   p  relative to the feed direction  32 , as shown in  FIG. 2 . In alternative embodiments, the arrangement of the EP engines  12   p  and  12   s  may be reversed such that the EP engine  12   p  is upstream from the EP engine  12   s  relative to the feed direction  32 . In further alternative embodiments, system  10  may include three or more EP engines  12  for printing layers of additional materials. 
     System  10  also includes 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 controller  36  includes one or more control circuits, microprocessor-based engine control systems, and/or digitally-controlled raster imaging processor systems, and is configured to operate the components of system  10  in a synchronized manner based on printing instructions received from a host computer  38  or a remote location. In some embodiments, the host computer  38  includes one or more computer-based systems that are configured to communicate with 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 3D parts  26  and support structures in a layer-by-layer manner. 
     The components of system  10  may be retained by one or more frame structures (not shown for simplicity). Additionally, the components of system  10  may be retained within an enclosable housing (not shown for simplicity) that prevents components of the system  10  from being exposed to ambient light during operation. 
       FIG. 3  is a schematic front view of the EP engines  12   s  and  12   p  of the system  10 , in accordance with example embodiments of the present disclosure. In the illustrated embodiment, the EP engines  12   p  and  12   s  may include the same components, such as a photoconductor drum  42  having a conductive drum body  44  and a photoconductive surface  46 . The conductive drum body  44  is an electrically-conductive drum (e.g., fabricated from copper, aluminum, tin, or the like) that is 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 constant rate. 
     The photoconductive surface  46  is a thin film extending around the circumferential surface of the conductive drum body  44 , and is preferably derived from one or more photoconductive materials, such as amorphous silicon, selenium, zinc oxide, organic materials, and the like. As discussed below, 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 of the 3D part or support structure. 
     As further shown, each of the example EP engines  12   p  and  12   s  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 . 
     Each of the EP engines  12  uses the powder-based material (e.g., polymeric or thermoplastic toner), generally referred to herein by reference character  66 , to develop or form the layers  22 . In some embodiments, the image-forming assembly for the surface  46  of the EP engine  12   s  is used to form support layers  22   s  of powder-based 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   s ) along with carrier particles. Similarly, the image-forming assembly for the surface  46  of the EP engine  12   p  is used to form part layers  22   p  of powder-based 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   p ), generally 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. 
     Each 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  the past 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 to ground), 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 device 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. As such, as used herein, the term “electrophotography” includes ionography. 
     Each development station  58  is an electrostatic and magnetic development station or cartridge that retains the supply of 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 part or 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  as the photoconductor drum  12  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. 
     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 other 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   p  and  12   s  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   p  and  22   s  from the EP engines  12   p  and  12   s  to the belt  24 . Because the layers  22   p  and  22   s  are each only a single layer increment in thickness at this point in the process, electrostatic attraction is suitable for transferring the layers  22   p  and  22   s  from the EP engines  12   p  and  12   s  to the belt  24 . 
     The controller  36  preferably rotates the photoconductor drums  36  of the EP engines  12   p  and  12   s  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   p  and  22   s  in coordination with each other from separate developer images. In particular, as shown, each part 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 layers  22   p  and the support layers  22   s  may optionally be developed and transferred along the belt  24  separately, such as with alternating layers  22   p  and  22   s . These successive, alternating layers  22   p  and  22   s  may then be transferred to layer transfusion assembly  20 , where they may be transfused separately to print or build the 3D part  26  and support structure. 
     In a further alternative embodiment, one or both of the EP engines  12   p  and  12   s  may also include one or more intermediary transfer drums and/or belts between the photoconductor drum  42  and the belt or transfer medium  24 . For example, as shown in  FIG. 4 , the EP engine  12   p  may also include an intermediary 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 intermediary 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   s  may include the same arrangement of an intermediary 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   p  and  12   s  can be beneficial for thermally isolating the photoconductor drum  42  from the belt  24 , if desired. 
       FIG. 5  illustrates a schematic view of the transfer 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. 2  (the y-axis being into and out of the page in  FIG. 2 , 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. 5 . 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  is 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 element or heatable 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 . 
     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 temperature near an intended transfer temperature of the layer  22 , such as at least 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, such as at the same transfer temperature as the heated layers  22  (or other suitable elevated temperature). 
     As mentioned above, the support material  66   s  of the present disclosure used to form the support layers  22   s  and the support structure  26   s , preferably has a melt rheology that is similar to or substantially the same as the melt rheology of the part material  66   p  of the present disclosure used to form the part layers  22   p  and the 3D part  26   p . This allows the part and support materials  66   p  and  66   s  of the layers  22   p  and  22   s  to be heated together with the heater  72  to generally the same transfer temperature, and also allows the part and support materials  66   p  and  66   s  at the top surfaces of the 3D part  26   p  and support structure  26   s  to be heated together with heater  74  to generally the same temperature. Thus, the part layers  22   p  and the support layers  22   s  may be transfused together to the top surfaces of the 3D part  26   p  and the support structure  26   s  in a single transfusion step as the combined layer  22 . 
     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. Again, the close melt rheologies of the part and support materials  66   p  and  66   s  allow the post-transfusion heater  76  to post-heat the top surfaces of 3D part  26   p  and support structure  26   s  together in a single post-fuse step. 
     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 desired 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  (due to the close melt rheologies of the part and support materials). In comparison, the nip roller  70  may be heated to a desired transfer temperature for the layers  22  (also due to the close melt rheologies of the part and support materials). 
     During the printing or transferring operation, the belt  24  carries a layer  22  past the heater  72 , which may heat the layer  22  and the associated region of the belt  24  to the transfer temperature. Suitable transfer temperatures for the part and support materials  66   p  and  66   s  of the present disclosure include temperatures that exceed the glass transition temperature of the part and support materials  66   p  and  66   s , where the layer  22  is softened but not significantly above Tm. 
     As further shown in  FIG. 5 , 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 . 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.    
     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 an optional post-transfusion heater  76 . At 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. 
     Additionally, as the gantry  84  continues to move the build platform  28  along the x-axis past the post-transfusion heater  76  to the cooler  78 , the cooler  78  removes heat from the top layers of 3D part  26   p  and support structure  26   s . This actively cools the transfused layer  22  down to 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. 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. 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. 
       FIGS. 2-5  disclose an EP additive manufacturing system that can be utilized to print 3D parts with amorphous polymeric materials that utilizes the single nip roller  70  with a typical transfusion pressure ranging between 20 psi and 80 psi because amorphous polymeric materials have a relatively high storage modulus proximate the glass transition temperature. For example, Acrylonitrile Butadiene Styrene (ABS) has a storage modulus of about 145 psi proximate the glass transition temperature which allows for a transfusion pressure between the nip roller  70  and the part being printed to be about 80 psi. 
     In contrast to amorphous polymeric materials, semi-crystalline part materials remain solid until melted. The storage modulus above T g  but T m  of typical engineering grade semi-crystalline material is about 7,250 psi. 
     The present application discloses a transfusion assembly that transfuses the layer in at a pressure compatible with semi-crystalline materials. After the transfer of the layer to the partially printed 3D part, the layer and the part are rapidly heated to bond the layer to top surface of the partially printed 3D part. The partially printed 3D part is then rapidly cooled to a temperature to minimize the formation of crystalline structures. The transfusion assembly of the present disclosure cools the semi-crystalline polymer quickly enough to prevent complete crystallization, and therefore cause the semi-crystalline material to have similar volumetric properties to that of an amorphous polymer, where rapid shrinking is prevented as the temperature is reduced. However, the rate of cooling can be controlled to obtain a desired crystallinity within the material, based upon the physical properties of the semi-crystalline material. 
     After the part has been printed and the support material is optionally removed, the part can be subjected to additional processing, such as a post build heating process. The post build process, such as heating, allows the entire part to crystallize in a controlled manner, similar to that of an injection molded part. 
     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.