Patent Description:
Embodiments herein relate to methods and systems for forming three-dimensional printed parts, in particular printed parts with an improved surface finish.

Additive manufacturing systems are used to build 3D parts from digital representations of the parts 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 digitally 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.

One particularly desirable additive manufacturing method is selective toner electrophotographic process (STEP) additive manufacturing, which allows for rapid, high quality production of 3D parts. STEP manufacturing is performed by applying layers of thermoplastic material that are 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). The layers are placed down in an X-Y plane, with successive layers positioned on top of one another in a Z-axis perpendicular to the X-Y plane.

A support structure is sometimes built utilizing the same deposition techniques by which the part material is deposited. The supporting layers or structures are often built underneath overhanging portions or in cavities of parts under construction that are not supported by the part material itself. The part material adheres to the support material during fabrication and the support material is subsequently removable from the completed 3D part when the printing process is complete. In typical STEP processes layers of the part material and support material are deposited next to each other in a common X-Y plane. These layers of part and support material are each built on top of one another (layers of part material built on top of other layers of part material; and layers of support material built on to top of other layers of support material) along the Z-axis to create a composite part that contains both part material and support material.

The US patent application <CIT> teaches a method for printing an article using a selective toner electrophotographic process ("STEP") includes successively depositing multiple layers of part material and support material, the layers deposited substantially parallel to a first plane; wherein: a) the multiple layers of part material and support material extend in a perpendicular to the first plane; and b) at least some of the layers of part material and support material are separated from each other to form a gap between the layers of part material and layers of support material.

Although STEM deposition can produce very high quality parts, it is still desirable to form even better parts. For example, in some implementations it is still desirable to have better surface properties, such as improved surface finishes, and in particular smoother surface features. The desire for improved surface finishes, such as smoother surfaces, is particularly true for surfaces outside of the X-Y planes, such as surfaces that have a significant Z-axis component, because these surfaces are sometimes the most difficult on which to obtain a smooth surface finish. These surfaces with a significant Z-axis component are often formed in junction with a support material, and therefore surfaces with improved finish are desired where support material is used to form the part.

The present application is directed to a method for printing an article using a selective toner electrophotographic process (STEP). The method includes forming a gap (also referenced as a trench or canyon) between adjacent layers of part material and support material, and then applying pressure and heat to transfer some of the part material and support material into the gap. As the part material and support material flow into the gap they come together to form an enhanced surface that is smoother than would otherwise typically be obtained. Part of this enhancement is a result of depositing partial layers of material, referred to herein as enhancement layers, adjacent to the gap. These enhancement layers increase the amount of material (both part and support) adjacent to the gap.

As pressure is applied from the z-axis (or another axis perpendicular to the layers) these enhancement layers function to transfer that pressure down through layers beneath them. The material forming these lower layers (as well as the enhancement layers themselves) are thus under increased pressure, which results in various embodiments a horizontal (x and y direction) flow of material, along with some downward flow of material, into the gap adjacent to the enhancement layers. Once in the gap the material flows upward in the gap. Note that in other implementations the layers are not formed in the same orientation as described above, but the same principals of flow of material into a gap so as to improve surface finish can be observed.

Thus, pressure from the top of the composite, such as applied by a transfuse roller, during formation results in an increased pressure build up in areas having enhancement layers. The increased pressure causes horizontal undertow flow of material (typically both part material and support material) into the gap. The gap gradually fills with material from the bottom, and new flows of material into the gap have an at least partially-upward flow direction in the (z-axis). This upward flow of the part material and support material causes a smoothing of the interface between the part and support materials, thereby forming a smoother finished part.

Thus, in an example embodiment, the enhanced walls of the material along the gap effectively functional like a piston that moves down when rolled by a transfuse roller. Part and support material just outside of the gap is first pressed down. As the tops of the gap sidewalls are pressed down, the material beneath the tops of the trench sidewalls is forced to move sideways into the gap in an undertow. "Undertow" refers to a primarily horizontal flow under the surface as material, also with some downward flow. As material moves out from under the opposing gap walls the part and support material flow into the gap and upward to converge upon one another in the gap. It will be appreciated that in some embodiments the orientation of the layers and gap varies from that described in this example, but similar flow properties and surface improvements are observed. This convergence can occur at the centerline of the gap in some embodiments, such as if the dimensions of the enchantment layers are the same, and the viscosities of the part and support materials are the same. Upon convergence the part and support material moves in the only direction available, which is vertically up the gap because lower portions of the gap are already filled. Generally when the gap is almost filled (the top of the gap is just below the z-axis elevation of the tops of the sidewalls) the flow stops, as the downward pressure over the trench balances the higher downward pressure over the trench sidewalls less the pressure drop from the undertow flow times the viscous flow resistance. It will be appreciated that as described herein the gap is a space between the layers of deposited build material and support material. Multiple layers of build and support material stacked onto one another can form a trench between the layers (the trench essentially multiple gap layers stacked on top of one another). Upon application of transfusion pressure the gap is at least partially (and generally mostly or completely) filled with part and support material flowing into it. Thus the gap (or trench) is filled with material as the layers are deposited and transfusion (described below) occurs.

Thus, in certain embodiments the present application is directed to a method of successively depositing multiple layers of part material and support material, the layers deposited substantially parallel to an X-Y plane (or another plane, referred to herein as a "first plane"). At least some of the layers of part material and support material are offset from each other in the X-Y plane (or other plane) to form a gap or trench between the part material and support material. The multiple layers of part material and support material extend in a Z-direction perpendicular to the X-Y plane, or another direction perpendicular to first plane). Heat and pressure are applied to the top surface of the aggregated layers of part material and support material such that a portion of the part material and support material flows into and at least partially fills the gap between the part material and support material and make contact with one another. The contact area forms an interface that, when the support is removed, results in a part surface that has improved surface properties, including reduced roughness. Typically during this flow into the gap at least a portion of the part material and support material flows upward in a Z direction normal to the X-Y. In some cases the gap is not vertical, but rather slanted or inclined (or has another orientation), in which case the part and support material will flow into that gap, but it may not be normal to the X-Y plane, but rather include a component that is normal to the X-Y plane. The result of this upward (or other direction flow in the case of non-vertical gaps or trenches) flow is that each layer of build material and support material, including material from the edge enhancement layers, is spread vertically over a Z-axis dimension greater than their thickness prior to application of heat and pressure.

In an embodiment, a method for printing an article using a selective toner electrophotographic process is described, the method including successively depositing multiple layers of part material and support material, the layers deposited substantially parallel to an X-Y plane; wherein: a) the multiple layers of part material and support material extend in a Z-direction perpendicular to the X-Y plane; and b) at least some of the layers of part material and support material are offset from each other in the X-Y plane to form a gap between the layers of part material and layers of support material; application of heat and pressure to the part material and support material such that a portion of the part material and support material flows into and at least partially fills the gap between the part material and support material; and at least a portion of the part material and support material flows upward with a component in a Z-direction normal to the X-Y plane.

According to the invention, the method further includes deposit of an edge enhancement layer between at least some of the multiple layers of part material and support material; the edge enhancement layers includes a layer of part material or a layer of support material selectively printed adjacent to the gap.

In an embodiment, the printed part material or support material of the edge enhancement layers has a volume substantially equal to the volume of the gap.

In an embodiment, an edge enhancement layer is deposited every second, third, fourth, fifth, sixth, seventh, eighth, ninth, or tenth layer.

In an embodiment, the edge enhancement layer has an average width of <NUM> to <NUM> pixels.

In an embodiment, the average width of the gap between the part regions and support regions is from <NUM> to <NUM> pixels.

In an embodiment, the gap is from <NUM> to <NUM> pixels in width and the average width of the edge enhancement layer is from <NUM> to <NUM> pixels in width.

In an embodiment, the average width of the gap between the part material and support material is from <NUM> to <NUM> pixels.

In an embodiment, the part region forms a first perimeter defining a first side of the gap and the support region forms a second perimeter defining a second side of the gap.

In an embodiment, the method further includes reheating and recooling the build surface so as to cause the gap to diminish and the part region surface to become progressively smoother.

In an embodiment, the surface roughness of vertical part surfaces is less than <NUM>.

Unless otherwise specified, the following terms as used herein have the meanings provided below:
The term "copolymer" refers to a polymer having two or more monomer species.

The terms "preferred" and "preferably" refer to embodiments of the invention that may afford certain benefits, under certain circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the inventive scope of the present disclosure.

Reference to "a" chemical compound refers one or more molecules of the chemical compound, rather than being limited to a single molecule of the chemical compound. Furthermore, the one or more molecules may or may not be identical, so long as they fall under the category of the chemical compound.

The terms "at least one" and "one or more of" an element are used interchangeably, and have the same meaning that includes a single element and a plurality of the elements, and may also be represented by the suffix "(s)" at the end of the element.

Directional orientations such as "above", "below", "top", "bottom", and the like are made with reference to a direction along a printing axis of a 3D part. In the embodiments in which the printing axis is a vertical z-axis, the layer-printing direction is the upward direction along the vertical z-axis. In these embodiments, the terms "above", "below", "top", "bottom", and the like are based on the vertical z-axis. However, in embodiments in which the layers of 3D parts are printed along a different axis, the terms "above", "below", "top", "bottom", and the like are relative to the given axis.

The 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).

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 terms "resilient material" and "flowable material" describe distinct materials used in the printing of a 3D part and support. The resilient material has a higher viscosity and/or storage modulus relative to the flowable material.

Unless otherwise specified, pressures referred to herein are based on atmospheric pressure (i.e. one atmosphere).

Embodiments of the present disclosure relate to a selective deposition-based additive manufacturing system, such as an electrostatography-based additive manufacturing system, to print 3D parts and/or support structures with high resolution and smooth surfaces. During a printing operation, electrostatographic engines develop or otherwise image each layer of the part and support materials using the electrostatographic process. The developed layers are then transferred to a layer transfusion assembly where they are transfused (e.g., using heat and/or pressure over time) to print one or more 3D parts and support structures in a layer-by-layer manner.

<FIG> show example components of STEP manufacturing systems, while FIGS. <NUM> to <NUM> show further aspects of methods and techniques for producing 3D printed parts with improved surface properties. <FIG> is a simplified diagram of an exemplary electrophotography-based additive manufacturing system <NUM> configured to perform a selective deposition process to printing 3D parts and associated support structures, in accordance with embodiments of the present disclosure. As shown in <FIG>, system <NUM> includes one or more EP engines, generally referred to as <NUM>, such as EP engines 12p and <NUM>, a transfer assembly <NUM>, biasing mechanisms <NUM>, and a transfusion assembly <NUM>. Examples of suitable components and functional operations for system <NUM> include those disclosed in<CIT>and<CIT>, and in <CIT> and <CIT>.

The EP engines 12p and <NUM> are imaging engines for respectively imaging or otherwise developing layers, generally referred to as <NUM>, 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 12p or <NUM>. As discussed below, the developed layers <NUM> are transferred to a transfer medium (such as belt <NUM>) of the transfer assembly <NUM>, which delivers the layers <NUM> to the transfusion assembly <NUM>. The transfusion assembly <NUM> operates to build the 3D part <NUM>, which may include support structures and other features, in a layer-by-layer manner by transfusing the layers <NUM> together on a build platform <NUM>.

In some embodiments, the transfer medium includes a belt <NUM>, as shown in <FIG>. Examples of suitable transfer belts for the transfer medium or belt <NUM> include those disclosed in Comb et al. Patent Application Publication Nos. <NUM>/<NUM> and <NUM>/<NUM>. In some embodiments, the belt <NUM> includes front surface 24a and rear surface 24b, where front surface 24a faces the EP engines <NUM>, and the rear surface 24b is in contact with the biasing mechanisms <NUM>.

In some embodiments, the transfer assembly <NUM> includes one or more drive mechanisms that include, for example, a motor <NUM> and a drive roller <NUM>, or other suitable drive mechanism, and operate to drive the transfer medium or belt <NUM> in a feed direction <NUM>. In some embodiments, the transfer assembly <NUM> includes idler rollers <NUM> that provide support for the belt <NUM>. The example transfer assembly <NUM> illustrated in <FIG> is highly simplified and may take on other configurations. Additionally, the transfer assembly <NUM> 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 <NUM>, a belt cleaner for removing debris from the surface 24a that receives the layers <NUM>, and other components.

The EP engine <NUM> develops layer or image portions <NUM> of powder-based support material, and the EP engine 12p develops layer or image portions 22p of powder-based part/build material. In some embodiments, the EP engine <NUM> is positioned upstream from the EP engine 12p relative to the feed direction <NUM>, as shown in <FIG>. In alternative embodiments, the arrangement of the EP engines 12p and <NUM> may be reversed such that the EP engine 12p is upstream from the EP engine <NUM> relative to the feed direction <NUM>. In further alternative embodiments, system <NUM> may include three or more EP engines <NUM> for printing layers of additional materials, as indicated in <FIG>.

Example system <NUM> also includes controller <NUM>, which represents one or more processors that are configured to execute instructions, which may be stored locally in memory of the system <NUM> or in memory that is remote to the system <NUM>, to control components of the system <NUM> to perform one or more functions described herein. In some embodiments, the controller <NUM> 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 <NUM> in a synchronized manner based on printing instructions received from a host computer <NUM> or a remote location.

In some embodiments, the host computer <NUM> includes one or more computer-based systems that are configured to communicate with controller <NUM> to provide the print instructions (and other operating information). For example, the host computer <NUM> may transfer information to the controller <NUM> that relates to the sliced layers of the 3D parts and support structures, thereby allowing the system <NUM> to print the 3D parts <NUM> and support structures in a layer-by-layer manner. The controller <NUM> may also use signals from one or more sensors to assist in properly registering the printing of the part or image portion 22p and/or the support structure or image portion <NUM> with a previously printed corresponding support structure portion <NUM> or part portion 22p on the belt <NUM> to form the individual layers <NUM>.

The components of system <NUM> may be retained by one or more frame structures (not shown for simplicity). Additionally, the components of system <NUM> may be retained within an enclosable housing (not shown for simplicity) that prevents components of the system <NUM> from being exposed to ambient light during operation.

<FIG> is a schematic front view of the EP engines 12p and <NUM> of the system <NUM>, in accordance with example embodiments of the present disclosure. In the illustrated embodiment, the EP engines 12p and <NUM> may include the same components, such as a photoconductor drum <NUM> having a conductive drum body <NUM> and a photoconductive surface <NUM>. The conductive drum body <NUM> is an electrically-conductive drum (e.g., fabricated from copper, aluminum, tin, or the like) that is electrically grounded and configured to rotate around a shaft <NUM>. The shaft <NUM> is correspondingly connected to a drive motor <NUM>, which is configured to rotate the shaft <NUM> (and the photoconductor drum <NUM>) in the direction of arrow <NUM> at a constant rate.

The photoconductive surface <NUM> can be a thin film extending around the circumferential surface of the conductive drum body <NUM>, 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 <NUM> 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 12p and <NUM> also includes a charge inducer <NUM>, an imager <NUM>, a development station <NUM>, a cleaning station <NUM>, and a discharge device <NUM>, each of which may be in signal communication with the controller <NUM>. The charge inducer <NUM>, the imager <NUM>, the development station <NUM>, the cleaning station <NUM>, and the discharge device <NUM> accordingly define an image-forming assembly for the surface <NUM> while the drive motor <NUM> and the shaft <NUM> rotate the photoconductor drum <NUM> in the direction <NUM>.

Each of the EP engines <NUM> uses the powder-based material (e.g., polymeric or thermoplastic toner), generally referred to herein by reference character <NUM>, to develop or form the layers <NUM>. In some embodiments, the image-forming assembly for the surface <NUM> of the EP engine <NUM> is used to form support layers <NUM> (e.g., image portions) of powder-based support material <NUM>, where a supply of the support material <NUM> may be retained by the development station <NUM> (of the EP engine <NUM>) along with carrier particles. Similarly, the image-forming assembly for the surface <NUM> of the EP engine 12p is used to form part layers 22p (e.g., image portion) of powder-based part material 66p, where a supply of the part material 66p may be retained by the development station <NUM> (of the EP engine 12p) along with carrier particles. Additional EP engines <NUM> may be included that utilize other support or part materials <NUM>.

The charge inducer <NUM> is configured to generate a uniform electrostatic charge on the surface <NUM> as the surface <NUM> rotates in the direction <NUM> past the charge inducer <NUM>. Suitable devices for the charge inducer <NUM> include corotrons, scorotrons, charging rollers, and other electrostatic charging devices.

Each imager <NUM> is a digitally-controlled, pixel-wise light exposure apparatus configured to selectively emit electromagnetic radiation toward the uniform electrostatic charge on the surface <NUM> as the surface <NUM> rotates in the direction <NUM> the past imager <NUM>. The selective exposure of the electromagnetic radiation to the surface <NUM> is directed by the controller <NUM>, 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 <NUM>.

Suitable devices for the imager <NUM> 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 <NUM> and the imager <NUM> include ion-deposition systems configured to selectively directly deposit charged ions or electrons to the surface <NUM> to form the latent image charge pattern.

Each development station <NUM> is an electrostatic and magnetic development station or cartridge that retains the supply of the part material 66p or the support material <NUM>, along with carrier particles. The development stations <NUM> 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 <NUM> may include an enclosure for retaining the part material 66p or the support material <NUM> and carrier particles. When agitated, the carrier particles generate triboelectric charges to attract the powders of the part material 66p or the support material <NUM>, which charges the attracted powders to a desired sign and magnitude, as discussed below.

Each development station <NUM> may also include one or more devices for transferring the charged part or the support material 66p or <NUM> to the surface <NUM>, such as conveyors, fur brushes, paddle wheels, rollers, and/or magnetic brushes. For instance, as the surface <NUM> (containing the latent charged image) rotates from the imager <NUM> to the development station <NUM> in the direction <NUM>, the charged part material 66p or the support material <NUM> is attracted to the appropriately charged regions of the latent image on the surface <NUM>, utilizing either charged area development or discharged area development (depending on the electrophotography mode being utilized). This creates successive layers 22p or <NUM> as the photoconductor drum continues to rotate in the direction <NUM>, where the successive layers 22p or <NUM> correspond to the successive sliced layers of the digital representation of the 3D part or support structure.

The successive layers 22p or <NUM> are then rotated with the surface <NUM> in the direction <NUM> to a transfer region in which layers 22p or <NUM> are successively transferred from the photoconductor drum <NUM> to the belt <NUM> or other transfer medium, as discussed below. While illustrated as a direct engagement between the photoconductor drum <NUM> and the belt <NUM>, in some preferred embodiments, the EP engines 12p and <NUM> may also include intermediary transfer drums and/or belts, as discussed further below.

After a given layer 22p or <NUM> is transferred from the photoconductor drum <NUM> to the belt <NUM> (or an intermediary transfer drum or belt), the drive motor <NUM> and the shaft <NUM> continue to rotate the photoconductor drum <NUM> in the direction <NUM> such that the region of the surface <NUM> that previously held the layer 22p or <NUM> passes the cleaning station <NUM>. The cleaning station <NUM> is a station configured to remove any residual, non-transferred portions of part or support material 66p or <NUM>. Suitable devices for the cleaning station <NUM> include blade cleaners, brush cleaners, electrostatic cleaners, vacuum-based cleaners, and combinations thereof.

After passing the cleaning station <NUM>, the surface <NUM> continues to rotate in the direction <NUM> such that the cleaned regions of the surface <NUM> pass the discharge device <NUM> to remove any residual electrostatic charge on the surface <NUM>, prior to starting the next cycle. Suitable devices for the discharge device <NUM> 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 <NUM> are configured to induce electrical potentials through the belt <NUM> to electrostatically attract the layers 22p and <NUM> from the EP engines 12p and <NUM> to the belt <NUM>. Because the layers 22p and <NUM> are each only a single layer increment in thickness at this point in the process, electrostatic attraction is suitable for transferring the layers 22p and <NUM> from the EP engines 12p and <NUM> to the belt <NUM>.

The controller <NUM> preferably rotates the photoconductor drums of the EP engines 12p and <NUM> at the same rotational rates that are synchronized with the line speed of the belt <NUM> and/or with any intermediary transfer drums or belts. This allows the system <NUM> to develop and transfer the layers 22p and <NUM> in coordination with each other from separate developer images. In particular, as shown, each part layer 22p may be transferred to the belt <NUM> with proper registration with each support layer <NUM> to produce a combined part and support material layer or combined image layer, which is generally designated as layer <NUM>. As can be appreciated, some of the layers <NUM> transferred to the layer transfusion assembly <NUM> may only include support material <NUM> or may only include part material 66p, depending on the particular support structure and 3D part geometries and layer slicing.

In an alternative embodiment, the part layers 22p and the support layers <NUM> may optionally be developed and transferred along the belt <NUM> separately, such as with alternating layers 22p and <NUM>. These successive, alternating layers 22p and <NUM> may then be transferred to layer transfusion assembly <NUM>, where they may be transfused separately to form the layer <NUM> and print or build the 3D part <NUM> and support structure.

In a further alternative embodiment, one or both of the EP engines 12p and <NUM> may also include one or more intermediary transfer drums and/or belts between the photoconductor drum <NUM> and the belt or transfer medium or belt <NUM>. For example, as shown in <FIG>, the EP engine 12p may also include an intermediary drum 42a that rotates in the direction 52a that opposes the direction <NUM>, in which drum <NUM> is rotated, under the rotational power of motor 50a. The intermediary drum 42a engages with the photoconductor drum <NUM> to receive the developed layers 22p from the photoconductor drum <NUM>, and then carries the received developed layers 22p and transfers them to the belt <NUM>.

The EP engine <NUM> may include the same arrangement of an intermediary drum 42a for carrying the developed layers <NUM> from the photoconductor drum <NUM> to the belt <NUM>. The use of such intermediary transfer drums or belts for the EP engines 12p and <NUM> can be beneficial for thermally isolating the photoconductor drum <NUM> from the belt <NUM>, if desired.

<FIG> illustrates an embodiment of the layer transfusion assembly <NUM>. As shown, the exemplary transfusion assembly <NUM> includes the build platform <NUM>, a nip roller <NUM>, and pre-transfusion heaters <NUM> and <NUM>. In some embodiments, the transfusion assembly includes, an optional post-transfusion heater <NUM>, and/or a cooler (e.g., air jets <NUM> or other cooling units), as shown in <FIG> and <FIG>. The build platform <NUM> is a platform assembly or platen of system <NUM> that is configured to receive the heated combined layers <NUM> (or separate layers 22p and <NUM>) for printing the part <NUM>, which includes a 3D part 26p formed of the part layers 22p, and support structure <NUM> formed of the support layers <NUM>, in a layer-by-layer manner. In some embodiments, the build platform <NUM> may include removable film substrates (not shown) for receiving the printed layers <NUM>, where the removable film substrates may be restrained against build platform using any suitable technique (e.g., vacuum drawing).

The build platform <NUM> is supported by a gantry <NUM> or other suitable mechanism, which can be configured to move the build platform <NUM> along the z-axis and the x-axis (and, optionally, also the y-axis), as illustrated schematically in <FIG> (the y-axis being into and out of the page in <FIG>, with the z-, x- and y-axes being mutually orthogonal, following the righthand rule). The layers are put down generally parallel to an x-y plane, and the layers stack on top of one another along the z-axis. The gantry <NUM> may produce cyclical movement patterns relative to the nip roller <NUM> and other components, as illustrated by broken line <NUM> in <FIG>. The particular movement pattern of the gantry <NUM> can follow essentially any desired path suitable for a given application. The gantry <NUM> may be operated by a motor <NUM> based on commands from the controller <NUM>, where the motor <NUM> may be an electrical motor, a hydraulic system, a pneumatic system, or the like. In one embodiment, the gantry <NUM> can included an integrated mechanism that precisely controls movement of the build platform <NUM> in the z- and x-axis directions (and optionally the y-axis direction). In alternate embodiments, the gantry <NUM> can include multiple, operatively-coupled mechanisms that each control movement of the build platform <NUM> 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 <NUM> 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 <NUM> can be heatable with heating element <NUM> (e.g., an electric heater). The heating element <NUM> is configured to heat and maintain the build platform <NUM> at an elevated temperature that is greater than room temperature (<NUM>), such as at a desired average part temperature of 3D part 26p and/or support structure <NUM>, as discussed in <CIT>and <CIT>. This allows the build platform <NUM> to assist in maintaining 3D part 26p and/or support structure <NUM> at this average part temperature.

The nip roller <NUM> 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 <NUM>. In particular, the nip roller <NUM> may roll against the rear surface <NUM> in the direction of arrow <NUM> while the belt <NUM> rotates in the feed direction <NUM>. In the shown embodiment, the nip roller <NUM> is heatable with a heating element <NUM> (e.g., an electric heater). The heating element <NUM> is configured to heat and maintain nip roller <NUM> at an elevated temperature that is greater than room temperature (<NUM>), such as at a desired transfer temperature for the layers <NUM>.

The pre-transfusion heater <NUM> includes one or more heating devices (e.g., an infrared heater and/or a heated air jet) that are configured to heat the layers <NUM> on the belt <NUM> to a selected temperature of the layer <NUM>, such as up to a fusion temperature of the part material 66p and the support material <NUM>, prior to reaching nip roller <NUM>. Each layer <NUM> desirably passes by (or through) the heater <NUM> for a sufficient residence time to heat the layer <NUM> to the intended transfer temperature. The pre-transfusion heater <NUM> may function in the same manner as the heater <NUM>, and heats the top surfaces of the 3D part 26p and support structure <NUM> on the build platform <NUM> to an elevated temperature, and in one embodiment to supply heat to the layer upon contact.

The part and support materials 66p and <NUM> of the layers 22p and <NUM> may be heated together with the heater <NUM> to substantially the same temperature, and the part and support materials 66p and <NUM> at the top surfaces of the 3D part 26p and support structure <NUM> may be heated together with heater <NUM> to substantially the same temperature. This allows the part layers 22p and the support layers <NUM> to be transfused together to the top surfaces of the 3D part 26p and the support structure <NUM> in a single transfusion step as the combined layer <NUM>. As discussed below, a gap can be placed between the support layers <NUM> and part layers 22p, and under heat and pressure part and support material are pressed together in a manner such as to produce an improved interface with reduced surface roughness.

An optional post-transfusion heater <NUM> may be provided downstream from nip roller <NUM> and upstream from air jets <NUM>, and configured to heat the transfused layers <NUM> to an elevated temperature in a single post-fuse step.

As mentioned above, in some embodiments, prior to building the part <NUM> on the build platform <NUM>, the build platform <NUM> and the nip roller <NUM> may be heated to their selected temperatures. For example, the build platform <NUM> may be heated to the average part temperature (e.g., bulk temperature) of 3D part 26p and support structure <NUM>. In comparison, the nip roller <NUM> may be heated to a desired transfer temperature or nip entrance temperature for the layers <NUM>.

As further shown in <FIG>, during operation, the gantry <NUM> may move the build platform <NUM> (with 3D part 26p and support structure <NUM>) in a reciprocating pattern <NUM>. In particular, the gantry <NUM> may move the build platform <NUM> along the x-axis below, along, or through the heater <NUM>. The heater <NUM> heats the top surfaces of 3D part 26p and support structure <NUM> to an elevated temperature, such as the transfer temperatures of the part and support materials. As discussed in <CIT> and <CIT>, the heaters <NUM> and <NUM> may heat the layers <NUM> and the top surfaces of 3D part 26p and support structure <NUM> to about the same temperatures to provide a consistent transfusion interface temperature. Alternatively, the heaters <NUM> and <NUM> may heat layers <NUM> and the top surfaces of 3D part 26p and support structure <NUM> to different temperatures to attain a desired transfusion interface temperature.

The continued rotation of the belt <NUM> and the movement of the build platform <NUM> align or register the heated layer <NUM> (e.g., combined image layer) with the heated top surfaces of 3D part 26p and support structure <NUM> with proper registration along the x-axis. The gantry <NUM> may continue to move the build platform <NUM> along the x-axis, at a rate that is synchronized with the rotational rate of the belt <NUM> in the feed direction <NUM> (i.e., the same directions and speed). This causes the rear surface 24b of the belt <NUM> to rotate around the nip roller <NUM> to nip the belt <NUM> and the heated layer <NUM> against the top surfaces of 3D part 26p and support structure <NUM>. This presses the heated layer <NUM> between the heated top surfaces of 3D part 26p and support structure <NUM> at the location of the nip roller <NUM>, which at least partially transfuses the heated layer <NUM> to the top layers of 3D part 26p and support structure <NUM>.

As the transfused layer <NUM> passes the nip of the nip roller <NUM>, the belt <NUM> wraps around the nip roller <NUM> to separate and disengage from the build platform <NUM>. This assists in releasing the transfused layer <NUM> from the belt <NUM>, allowing the transfused layer <NUM> to remain adhered to 3D part 26p and support structure <NUM>. 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 <NUM> to be hot enough to adhere to the 3D part 26p and support structure <NUM>, while also being cool enough to readily release from the belt <NUM>. Additionally, the close melt rheologies of the part and support materials allow them to be transfused in the same step. The temperature and pressures can be selected, as is discussed below, to promote flow of part material and support material into a gap between the two materials. Often the rheologies are preferably close, they can be transfused with glass transition temperatures that are significantly different from one another in some constructions. This flow into the gap, typically accompanied by an upward movement of the part and support material, results in a smoother interface between the part and support, plus a smoother surface for the part after removal of the support.

After release, the gantry <NUM> continues to move the build platform <NUM> along the x-axis to the post-transfusion heater <NUM>. At optional post-transfusion heater <NUM>, the top-most layers of 3D part 26p and the support structure <NUM> (including the transfused layer <NUM>) may then be heated to at least the fusion temperature of the thermoplastic-based powder in a post-fuse or heat-setting step. This optionally heats the material of the transfused layer <NUM> to a highly fusable state such that polymer molecules of the transfused layer <NUM> quickly interdiffuse (also referred to as reptate) to achieve a high level of interfacial entanglement with 3D part 26p and support structure <NUM>.

Additionally, as the gantry <NUM> continues to move the build platform <NUM> along the x-axis past the post-transfusion heater <NUM> to the air jets <NUM>, the air jets <NUM> blow cooling air towards the top layers of 3D part 26p and support structure <NUM>. This actively cools the transfused layer <NUM> down to the average part temperature, as discussed in <CIT>and<CIT>.

To assist in keeping the 3D part 26p and support structure <NUM> at the average part temperature, in some embodiments, the heater <NUM> and/or the heater <NUM> may operate to heat only the top-most layers of 3D part 26p and support structure <NUM>. For example, in embodiments in which heaters <NUM>, <NUM>, and <NUM> are configured to emit infrared radiation, the 3D part 26p and support structure <NUM> 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 <NUM>, <NUM>, and <NUM> may be configured to blow heated air across the top surfaces of 3D part 26p and support structure <NUM>. In either case, limiting the thermal penetration into 3D part 26p and support structure <NUM> allows the top-most layers to be sufficiently transfused, while also reducing the amount of cooling required to keep 3D part 26p and support structure <NUM> at the average part temperature. However generally sufficient thermal penetration is desired to promote flow of part material and support material into gaps positioned at the interface between the part and support material.

The gantry <NUM> may then actuate the build platform <NUM> downward, and move the build platform <NUM> back along the x-axis to a starting position along the x-axis, following the reciprocating rectangular pattern <NUM>. The build platform <NUM> desirably reaches the starting position for proper registration with the next layer <NUM>. In some embodiments, the gantry <NUM> may also actuate the build platform <NUM> and 3D part 26p/support structure <NUM> upward for proper registration with the next layer <NUM>. The same process may then be repeated for each remaining layer <NUM> of 3D part 26p and support structure <NUM>.

After the transfusion operation is completed, the resulting 3D part 26p and support structure <NUM> may be removed from system <NUM> and undergo one or more post-printing operations. For example, support structure <NUM> may be sacrificially removed from 3D part 26p using an aqueous-based solution, such as an aqueous alkali solution. Under this technique, support structure <NUM> may at least partially dissolve in the solution, separating it from 3D part 26p 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 <NUM> without degrading the shape or quality of 3D part 26p. Examples of suitable systems and techniques for removing support structure <NUM> in this manner include those disclosed in <CIT>; <CIT>; and<CIT>.

Furthermore, after support structure <NUM> is removed, 3D part 26p may undergo one or more additional post-printing processes, such as surface treatment processes. Examples of suitable surface treatment processes include those disclosed in <CIT>; and in <CIT>.

Referring now to <FIG>, a schematic view of an idealized multi-layer composite <NUM> containing layers of part material <NUM> and support material <NUM> are shown. The part material <NUM> and support material <NUM> are deposited as layers onto build substrate <NUM>. The part material layers <NUM> and support material layers <NUM> are built up to form the overall composite <NUM>. It will be appreciated that the part material layers <NUM> and support material layers <NUM> are not shown to scale, and in practice the layers <NUM>, <NUM> are very thin, typically on the order of <NUM> to <NUM> microns. In the embodiment shown the part material <NUM> and support material <NUM> meet together at an interface <NUM>.

<FIG> is a schematic view of part material <NUM> of <FIG>, showing the part material <NUM> after removal of the support material <NUM> from <FIG>. Here the part material <NUM> remains behind, with an exposed surface <NUM> of the part material <NUM> depicted with a smooth and uniform surface. Layers <NUM> of part material <NUM> are still shown, although it will be appreciated that in typical embodiments those layers are difficult to discern without close inspection. It will be appreciated that in practice the exposed surface <NUM> will often have some surface texture or roughness that is not desired because the layers do not generally get deposited quite as precisely as desired.

<FIG> show an additional example of a common challenge with regard to forming a multi-layer composite <NUM> in which layers of part material <NUM> and support material <NUM> are deposited as part layers <NUM> and support material layers <NUM>, in this case overlap of layers. The result is small overlapping areas <NUM> between some of the part material layers <NUM> and support material layers <NUM>. Although the part material layers <NUM> and support material layers <NUM> are often deposited with relatively high precision, it is still possible for these overlapping areas <NUM> to form due to variations in registration of the layers as they are deposited. The result, as shown in <FIG>, after removal of the support material <NUM>, can be a part material <NUM> with an exposed surface <NUM> that is not as smooth as desired. As noted above, these figures are schematic representations of the surface irregularities, and the actual finished part will show its imperfections in regard to roughness of the surface. This roughness is often more visible to the eye or to optical roughness measurement techniques than it is to a stylus roughness measuring instrument because the imperfections are formed by such small layers and those layers are somewhat flexible, and those not as easily measured as rough.

<FIG> show how the interface between the part material <NUM> and support material <NUM> can be improved by depositing the part and support material in layers that are separated from one another by a short distance that forms a gap (also referred to as a trench), and how the application of heat and pressure to the top of the composite part and support layers (as the layers are built up) results in a flow of part and support material into the gap in a manner that results in a smooth interface forming between the parts.

More specifically, <FIG> is a schematic view of a multi-layer composite containing layers of part material <NUM> and support material <NUM> showing a gap <NUM> between the layers. This gap is formed intentionally to create a volume into which part material <NUM> and support material <NUM> can flow during the production process under pressure and at elevated temperatures. In the depicted embodiment each of the part material <NUM> and support material <NUM> include build material edge enhancement layers <NUM> and support material enhancement layers <NUM>. These edge enhancement layers <NUM> and <NUM> are layers of part and support material that are configured to promote flow into the gap <NUM> between the part material <NUM> and support material <NUM>. These edge enhancement layers <NUM>, <NUM> are generally partial layers of build material <NUM> and support material <NUM> positioned between larger layers <NUM>, <NUM>. The part material layers <NUM>, <NUM> of the part material <NUM> define a part material surface <NUM> opening into the gap <NUM>; while the support material layers <NUM>, <NUM> of support material <NUM> define a support material surface <NUM> opening into the other side of gap <NUM>. Each of part material surface <NUM> and support material surface <NUM> will typically have some irregularities due to registration, such as the irregularities. It will be noted that <FIG> is a superposition of as-printed layers, and not would result from actually transfusing these layers together. <FIG>, shows a simplified, schematic result after transfusion (the enhancement layers have disappeared, and the gap has filled up). Note that <FIG> shows a simplified flow into the gap, wherein typically there will be some upward flow of the material in the gap region during transfusion.

During the transfusion process, as heat and pressure is applied to the top surfaces <NUM> of the composite <NUM>, and combined with heat within the layers from their earlier deposition (and optionally the addition of heat), there is a flow of part material and support material into the gap <NUM> (as noted above, this flow generally has at least a component that is normal to the plane formed by the deposited layers. That normal flow component is not shown in this figure). As the layers of build material and support material are built up and pressure is applied from above, the additional material from the edge enhancement layers promotes a horizontal undertow flow resulting in movement horizontal (with some downward) movement of material into the gap <NUM>. The material flowing into the gap <NUM> is generally both from the edge enhancement layers <NUM>, <NUM> themselves, but also from the regular layers <NUM>, <NUM>. Thus, the edge enhancement layers promote flow of part and support material in layers <NUM>, <NUM> adjacent to the gap <NUM>. As the part and support material flows into the gap <NUM> there is generally an upward flow of material within the gap because lower portions of the gap will have already been filled with material from prior edge enhancement flows. Thus, for example, it is typically observed that the part material <NUM> will not just flow into the gap <NUM>, but will flow inward and upward along the gap. A similar flow trait of inward and upward is generally observed by the support material <NUM>. It is necessary to provide for proper amounts of flow to promote this upward flow within the gap <NUM>. Generally this can be accomplished such that the overall volume of the edge enhancement layers corresponds approximately to the overall volume of the gap between the two materials. Note that in many implementations the volume of the part edge enhancement layers <NUM> will be the same as the volume of the support edge enhancement layers <NUM>; although in some implementations there can be larger edge enhancement layers formed form either the part or support material. In such implementations the flow will be such that the contact line between the two materials is typically offset from the center toward the material having smaller edge enhancement layers.

As the edge support layers <NUM> and <NUM> converge through inward and upward movement, the irregularities in the surfaces <NUM> and <NUM> are smoothened over at a convergence position <NUM>, shown in <FIG>. Note that the upward movement of the material within the gap <NUM> is not shown in <FIG>, although such movement typically occurs (see <FIG>, below, for a simplified flow representation). <FIG> in turn is a schematic view of layers of part material from <FIG> after removal of support material <NUM>, leaving behind part material <NUM> having surface <NUM> that is typically significantly smoother than would be observed with surface <NUM> prior to the reflow of the edge enhancement layers. Again, the direction of flow is not shown in <FIG>. Also, although surface <NUM> shows some surface variation, that variation is less than would be generally encountered without the present process.

Now in reference to <FIG>, the flow properties, including the upward flow of material within the gap, is represented in additional detail. <FIG> is a schematic view of a multi-layer composite containing layers of part material <NUM> and support material <NUM> showing a gap <NUM> between the layers that is formed intentionally to create a volume into which part material <NUM> and support material <NUM> can flow during the production process under pressure and at elevated temperatures. In the depicted embodiment each of the part material <NUM> and support material <NUM> include build material edge enhancement layers <NUM> and support material enhancement layers <NUM>. These edge enhancement layers <NUM> and <NUM> are layers of part and support material that are configured to flow into the gap <NUM> between the part and support material <NUM>, <NUM>, meeting approximately at the depicted centerline in the example embodiment. These edge enhancement layers <NUM>, <NUM> are generally partial layers positioned between larger layers <NUM>, <NUM>. The layers <NUM>, <NUM> of the part material <NUM> define a surface <NUM> opening into the gap <NUM>; while the layers <NUM>, <NUM> of part material <NUM> define a surface <NUM> opening into the other side of gap <NUM>. It will be appreciated that the layers are shown schematically, and in practice they are deposited on top of one another without a space between layers. Thus, referring to <FIG>, the appearance of a space <NUM> is only representing differences in layers. In fact, there is no such space in the actual deposited layers since they are pressed together during the transfusion process.

The number and dimensions of the edge enhancement layers will often vary depending upon the application. Generally, it is desired that the edge enhancement layers will have a total volume that will occupy the gap <NUM> upon application of pressure and heat. In other words, the edge enhancement layers <NUM> and <NUM> should, in aggregate, have sufficient volume that upon displacement of material into the gap <NUM> they will full fill the gap <NUM>. Note, the edge enhancement layers themselves do not entirely fill the gap <NUM>, but rather they promote flow into the gap by creating a higher pressure region at the edge of the gap <NUM>, and this higher pressure results in flow of build and support material into the gap <NUM>. Some of that material that flows into the gap will have been deposited as an edge enhancement layer, but generally there is greater flow of material from the other layers because there are more of those other layers. In some constructions the edge enhancement layers <NUM>, <NUM> will be spaced every few regular part layers <NUM> and support layers <NUM>; such as every <NUM> layers. However, in other implementations the edge enhancement layers <NUM>, <NUM> will be more frequent or less frequent, such as every <NUM>, every <NUM>, every <NUM>, every <NUM>, every <NUM>, every <NUM> or every <NUM> layers.

During the transfusion process, as pressure is applied to the top surfaces <NUM>, and combined with heat within the layers from their earlier deposition (and optionally the addition of heat), there is a flow of part material and support material into the gap <NUM>. As the layers are built up and pressure is applied, the areas where an edge enhancement layer are present have flow into the gap <NUM>, this flow into the gap (primarily horizontal but also downward) includes material from layers beneath the edge enhancement layer, effectively producing an undertow of horizontal and downward movement of material into the gap <NUM>. The material that moves into the gap then moves inward and upward upon reaching the gap until the part and support material converges. Thus, it is typically observed that the edge enhancement layer <NUM> and material from adjacent layers formed of part material <NUM> will not just flow into the gap <NUM>, but will flow upward into the gap. A similar flow trait of inward and upward is generally observed by the support material <NUM>.

The result of that upward flow is depicted by layer outlines in <FIG>, shown as if the layers could be distinguished after transfuse. As the edge support layers <NUM> and <NUM> converge through inward and upward movement the irregularities in the surfaces <NUM> and <NUM> are smoothened over at convergence position <NUM>, shown in <FIG> in turn is a schematic view of layers of part material from <FIG> after removal support material <NUM>, leaving behind part material <NUM> having surface <NUM> that is typically significantly smoother than would be observed with surface <NUM> prior to the reflow resulting from the presence of the edge enhancement layers.

The part and support material flows toward a convergence line <NUM> in <FIG>; although it will be appreciated that the centerline refers to an interface where the layers come together, and in practice that interface can be closer to one side or the other - so closer to either the part material <NUM> or support material <NUM>. The part and support material can have different viscosities and flow properties, in which case the point of interface is not necessarily the center between them.

<FIG> is a schematic view of a composite part <NUM>, showing generalized material flow properties. Showing edge enhancement layers <NUM>, <NUM> along with part material <NUM> and support <NUM>. The application of heat and pressure onto edge enhancement layers <NUM>, <NUM> results in areas beneath them in part material <NUM> and support material <NUM> being subject to increased pressure, resulting in flow of material into the gap, with part of the gap filled with part material inflow <NUM> and part of the gap filled with support material inflow <NUM>. The arrowed lines show general directions of material flow, with the predominant flow directions being horizontal and (and downward) undertow within the original areas occupied by support and part material; and subsequent horizontal and upward flow upon entry into the gap area, converging at a convergence line <NUM>. It will be understood that this is a schematic diagram, thus the relative size, thickness and proportions of the layers, including the edge enhancement layers <NUM>, <NUM> and the height of the material filling the gap is not intended to be representative of all real-life implementations.

<FIG> is a schematic view of a multi-layer composite containing layers of part material <NUM> and support material <NUM> showing a gap <NUM> between the layers that is formed intentionally to create a volume into which part material <NUM> and support material <NUM> can flow during the production process under pressure and at elevated temperatures. In addition, the bottom layers <NUM> of part material <NUM> and support material <NUM> are printed within the gap <NUM> to further promote upward flow of the upper level layers as they flow into the gap <NUM>. In the depicted embodiment each of the part material <NUM> and support material <NUM> include build material edge enhancement layers <NUM> and support material enhancement layers <NUM>. These edge enhancement layers <NUM> and <NUM> are layers of part and support material that are configured to promote flow of part and support material into the gap <NUM> between the part and support material <NUM>, <NUM>. As discussed above, these edge enhancement layers <NUM>, <NUM> are generally partial layers position between larger layers <NUM>, <NUM>. The layers <NUM>, <NUM> of the part material <NUM> define a surface <NUM> opening into the gap <NUM>; while the layers <NUM>, <NUM> of support material <NUM> define a surface <NUM> opening into the other side of gap <NUM>. The deposition of the bottom layers <NUM> into the gap area <NUM> provide for lower portions of the gap <NUM> to be filled, thereby necessitating the upward flow of material upon entering the gap.

<FIG> is an electron micrograph of a part formed of build material having improved surface finish from edge enhancement in accordance with the present disclosure. As can be seen from <FIG>, the surface has a somewhat vertically-smeared microtexture resulting from the upward flow of the build material. This smeared surface removes the texture present from the different layers of part material as they are deposited, resulting in a significantly smoother surface than otherwise obtained. Removal of this smeared surface can reveal the underlying layers of deposited part material. Thus, one means of assessing whether the present process was used is to examine for a surface having layers demonstrating smoothing as shown in <FIG>. Another means is to inspect the part volume adjacent to vertical walls for the presence of trapped voids or contaminants that show residual distortion from the vertical shear process. It will be the gaps need not be vertical. For example, in a typical embodiment an example layer n can have some overlap along the gaps of layers n-<NUM> and n+<NUM> (layers below and above), but they do not always have the same X-Y position, and they do not always need to be oriented in the X or the Y direction.

<FIG> is an electron micrograph of a cross section of portion of composite formed of a part material and support material, in particular showing the gap area where the pat and support material converged. <FIG> shows evidence of upward flow of the part material indicated by super-imposed arrows. The upward flow direction depicts the upward flows <NUM> of schematic <FIG>. These upward flows are not always readily visible in the finished part, since they are obscured by the fact that they are formed of a single material (either part material or support material) and are obscured under the surface of the finished part. However, <FIG> does show the manner in which the flow of material occurs, and in combination with <FIG> shows how the surface is smoothened as a result of this flow.

Claim 1:
A method for printing an article using a selective toner electrophotographic process, the method comprising:
successively depositing multiple layers of part material (<NUM>) and support material (<NUM>), the layers deposited substantially parallel to a first plane; wherein:
a) the multiple layers (<NUM>, <NUM>) of part material and support material extend in a direction perpendicular to the first plane; and
b) at least some of the layers of part material and support material are separated from each other in the first plane to form a gap (<NUM>) between part material and support material within a layer;
the method being characterized in that it further comprises the following steps:
deposit of an edge enhancement layer (<NUM>, <NUM>) between at least some of the multiple layers (<NUM>, <NUM>) of part material (<NUM>) and support material (<NUM>); the edge enhancement layer comprising a layer of part material (<NUM>) or a layer of support material (<NUM>)
selectively printed adjacent to the gap of a previous layer;
application of heat and pressure to the part material and support material such that a portion of the part material and support material flows into and at least partially fills the gap (<NUM>) between the part material and support material.