Patent Publication Number: US-2017368816-A1

Title: Resin slot extruder for additive manufacturing system

Description:
BACKGROUND 
     The present disclosure relates to additive manufacturing systems and processes for printing or otherwise building three-dimensional (3D) parts with layer-based, additive manufacturing techniques. In particular, the present disclosure relates to systems for printing 3D parts using optical-based additive manufacturing techniques. 
     Additive manufacturing systems are used to print or otherwise build 3D parts from digital representations of the 3D parts (e.g., STL format files) using one or more additive manufacturing techniques. Examples of commercially available additive manufacturing techniques include extrusion-based techniques, jetting, selective laser sintering, powder/binder jetting, electron-beam melting, digital light processing, and stereolithographic processes. For each of these techniques, the digital representation of the 3D part is initially sliced into multiple horizontal layers. For each sliced layer, a tool path or image is then generated, which provides instructions for the particular additive manufacturing system to print the given layer. 
     For example, in an extrusion-based additive manufacturing system, a 3D part may be printed from a digital representation of the 3D part in a layer-by-layer manner by extruding a flowable part material. The part material is extruded through an extrusion tip carried by a print head of the system, and is deposited as a sequence of roads on a substrate in an x-y plane. The extruded part material fuses to previously deposited part material, and solidifies upon a drop in temperature. The position of the print head relative to the substrate is then incremented along a z-axis (perpendicular to the x-y plane), and the process is then repeated to form a 3D part resembling the digital representation. 
     In another example, in a stereolithography-based additive manufacturing system, a 3D part may be printed from a digital representation of the 3D part in a layer-by-layer manner by tracing a laser beam across a vat of photocurable resin. For each layer, the laser beam draws a cross-section for the layer on the surface of the liquid resin, which cures and solidifies the drawn pattern. After the layer is completed, the system&#39;s platform is lowered by a single layer increment. A fresh portion of the resin may then recoat the previous layer, and the laser beam may draw across the fresh resin to pattern the next layer, which joins the previous layer. This process may then be repeated for each successive layer. Afterwards, the uncured resin may be cleaned, and the resulting 3D part may undergo subsequent curing. 
     SUMMARY 
     An aspect of the present disclosure is directed to a slot extruder for use with an additive manufacturing system. The slot extruder includes a plenum configured to receive a photocurable resin, an elongated slot positioned at a bottom end of the plenum and configured to receive the photocurable resin from the plenum, and one or more resin inlet ports extending into the plenum. The slot extruder also preferably includes one or more mechanisms configured to controllably pressurize and depressurize the photocurable resin in the plenum. 
     Another aspect of the present disclosure is directed to a selectively-operated slot extruder for use with an additive manufacturing system, which includes a plenum configured to receive a photocurable resin, an elongated slot configured to receive the photocurable resin from the plenum, and a plurality of resin inlet ports extending into the plenum. The slot extruder also includes one or more heater array assemblies configured to receive commands from a controller assembly of the additive manufacturing system. 
     Another aspect of the present disclosure is directed to an additive manufacturing system, which includes a build platen, a platen gantry configured to move the platen along a first axis, a laser assembly comprising a plurality of individually-operable laser emitters, and one or more slot extruders configured to extrude one or more photocurable resin films over the platen. The system also includes one or more gantries configured to move the laser assembly and the one or more slot extruders along a scan length axis such that each of the plurality of laser emitters traverses across the platen, and such that each of the one or more slot extruders traverses across the platen. The system further includes a controller assembly configured to operate the platen gantry, the laser assembly, the one or more slot extruders, and the one or more gantries to print three-dimensional parts on the platen in a layer-by-layer manner from the photocurable resin films. 
     Another aspect of the present disclosure is directed to an additive manufacturing system that includes a build platen, a platen gantry configured to move the platen along a first axis, one or more imaging devices configured to emit laser beams, and one or more slot extruders configured to extrude one or more photocurable resin films over the platen. The system also includes one or more gantries configured to move the slot extruders along a scan length axis such that each of the one or more slot extruders traverses across the platen, and a controller assembly configured to operate the platen gantry, the one or more imaging devices, the one or more slot extruders, and the one or more gantries to print one or more three-dimensional parts on the platform in a layer-by-layer manner from the photocurable resin films. 
     DEFINITIONS 
     Unless otherwise specified, the following terms as used herein have the meanings provided below: 
     The term “additive manufacturing system” refers to a system that prints, builds, or otherwise produces 3D parts and/or support structures at least in part using an additive manufacturing technique. The additive manufacturing system may be a stand-alone unit, a sub-unit of a larger system or production line, and/or may include other non-additive manufacturing features, such as subtractive-manufacturing features, pick-and-place features, two-dimensional printing features, and the like. 
     The terms “command”, “commanding”, and the like, with reference to a controller assembly commanding a device (e.g., a laser emitter, a gantry, a motor, or the like), refers to the direct and/or indirect relaying of control signals from the controller assembly to the device such that the device operates in conformance with the relayed signals. The signals may be relayed in any suitable form, such as communication signals to a microprocessor on the device, applied electrical power to operate the device, and the like. 
     The term “polymer” includes both homopolymers and copolymers (e.g., polymers of two or more different monomers). Correspondingly, 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 copolymer”, “one or more copolymers”, and “copolymer(s)” may be used interchangeably and have the same meaning. 
     The term “(meth)acrylate” includes acrylate and methacrylate. Similarly, the term “(meth)acrylic acid” includes acrylic acid and methacrylic acid. 
     The term “providing”, such as for “providing a device”, 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. 
     Unless otherwise specified, temperatures referred to herein are based on atmospheric pressure (i.e. one atmosphere). 
     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. 
     The terms “about” and “substantially” are used herein with respect to measurable values and ranges due to expected variations known to those skilled in the art (e.g., limitations and variabilities in measurements). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of an additive manufacturing system having slot extruders of the present disclosure and a laser assembly positioned on a first side of the system. 
         FIG. 2  is a side view of the additive manufacturing system shown in  FIG. 1 . 
         FIG. 3  is a top view of the additive manufacturing system with slot extruders and laser assembly positioned on a second side of the system. 
         FIG. 4  is a side view of the additive manufacturing system shown in  FIG. 3 . 
         FIG. 5  is a top view of the additive manufacturing system, illustrating a 3D part being printed with the slot extruders and laser assembly. 
         FIG. 6  is a side view of the additive manufacturing system shown in  FIG. 5 . 
         FIG. 7  is a top view of the 3D part being printed, illustrating a resin film extrusion area. 
         FIG. 8  is a view of the 3D part being printed, illustrating a resin film extrusion area that is reduced along an axis of travel 
         FIG. 9  is a view of the 3D part being printed, illustrating a resin film extrusion area that is also reduced along a span axis. 
         FIGS. 9A and 9B  are views of the 3D part being printed, illustrating printed containment dikes for the resin film. 
         FIG. 10  is a perspective view of the slot extruder extruding and applying a resin film. 
         FIG. 11  is an expanded side view of the slot extruder extruding and applying the resin film on a working surface, where a lateral end wall of the slot extruder is omitted. 
         FIG. 12  is an englarged side view of the slot extruder illustrated in  FIG. 11 . 
         FIG. 13  is a perspective view of a portion of an embodiment of the slot extruder extruding and applying a resin film, where an end wall of the slot extruder is omitted. 
         FIG. 14  is an englarged side view of the slot extruder illustrated in  FIG. 13 . 
         FIG. 15  is a top perspective view of the slot extruder illustrated in  FIG. 13 , where a lateral end wall and a top cover of the slot extruder are omitted. 
         FIG. 16  is an enlarged perspective view of a plenum of the slot extruder illustrated in  FIG. 13 . 
         FIG. 17  is another enlarged perspective view of a slot of a plenum illustrated in  FIG. 16 . 
         FIG. 18  is a further enlarged perspective view of the slot illustrated in  FIG. 17 . 
         FIG. 19  is a top view of an alternative additive manufacturing system having multiple slot extruders, such as for multiple-material applications. 
         FIG. 20  is a perspective view of the slot extruder illustrated in  FIG. 19  extruding and applying a resin film. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed to a slot extruder for use in an additive manufacturing system, where the slot extruder is configured to apply films of one or more photocurable resins, preferably under low-shear conditions. The slot extruder may be used in combination with an imaging device of the additive manufacturing system (e.g., a laser assembly), where the imaging device is configured to selectively cross-link each extruded resin film in a predefined pattern to print a 3D part in a layer-by-layer manner As discussed below, the slot extruder can apply substantially uniform films of photocurable resins, including viscous photocurable resins, at fast rates and with large surface areas. 
     Current additive manufacturing techniques with photocurable resin films, such as stereolithography (SLA) and digital light process (DLP) 3D printers, use a variety of different techniques for applying photopolymer layers to a part under construction. These techniques typically include submerging the 3D part into a resin vat, trapping resin between the surface of the 3D part and a transparent low-surface-energy window, impelling a wave across the 3D part with a counter-rotating cylinder, applying and peeling a web carrying a resin layer, and passing a doctor blade across the resin layer on the 3D part. 
     Each of these techniques attempt to distribute the flowable resin over the surface of the 3D part, level the resin to a uniform z-height increment, and allow the polymerizing optical energy (e.g., a laser beam) to reach the applied resin. However, these techniques develop problems when the layer surface area of the 3D part is large, and when the resin being applied has a high viscosity. Additional problematic factors can also include short times between the printing of the layers, small printed features, and multiple-material applications. 
     There are several root causes of these problems. First, attempts to quickly level a high-viscosity material against a surface can generate destructive, in-plane shear stress on the part surface. Additionally, the disengagement of a z-height defining surface, such as a web or window, can generate destructive normal stresses on the part surface. Furthermore, shearing one photopolymer on top of a different photopolymer can induce resin mixing, which typically produces an undesirable intermediate state material. 
     These deficiencies can become readily apparent when attempting to print 3D parts with a laser assembly having arrays of hundreds or thousands of laser diodes, such as discussed in Batchelder et al., Patent Application Ser. No. 62/083,553, entitled “Additive Manufacturing System With Laser Assembly” (“the &#39;553 application”) and International Application Serial No. PCT/US2015/062388, the contents of both are incorporated by reference in their entireties. The laser assemblies discussed in the &#39;553 application and the &#39;388 International application are preferably capable of printing with high-viscosity, flowable photocurable resins (e.g., from 50-5,000 centipoise), thin layers (e.g., 10-100 micrometers), high printing speeds (e.g., 30-150 inches/second), and short layer times (e.g., 0.5-10 seconds). 
     As can be appreciated, current techniques for applying the resin layers are not capable of meeting these demands By way of example, a one-inch diameter roller that counter-rolls across a one-meter wide 3D part surface at 50 inches/second, applying a 100-inches/second effective surface velocity difference over a 50-micrometer gap filled with 10 poise resin, will place about 30 pounds-force (about 130 Newtons) of in-plane shear force on the surface of the 3D part. The imparted shear force is sufficient to mechanically distort the 3D part and/or break off fine feature details. 
     One alternative to this design may include slowing down the application of the resin layers. However, this would significantly impede the overall printing operation, and substantially counteract the benefits of a high-speed laser assembly. 
     To overcome the above-mentioned deficiencies, the slot extruder of the present disclosure is capable of applying a high-viscosity, flowable photocurable resin to a surface at high speeds with minimal or otherwise reduced levels of stress applied to the surface. More particularly, the slot extruder preferably shears the photocurable resin within the extruder to define the resin film thickness, and then apply the resin film to the part surface with minimal (or otherwise substantially reduced) amounts of normal and shear forces. This allows 3D parts to be printed from photocurable resins having high viscosities (for increased part strengths and film-deformation resistance), thin layers (for good part resolutions), high printing speeds and short layer times (for reduced printing durations and increased protection). 
       FIGS. 1 and 2  illustrate an exemplary non-limiting system  10  that includes a laser assembly  12  in use with a pair of spaced apart slot extruders  14  of the present disclosure (individually referred to as slot extruders  14   a  and  14   b ). In this non-limiting example, system  10  also includes housing  16  having a working region  18  generally defining a T-shape arrangement. Housing  16  also typically and optionally has an enclosable lid or casing to enclose working region  18  during operation. 
     The following discussion describes the components of the example system  10 , as shown in  FIGS. 1 and 2 , and how they operate with slot extruders  14 . However, slot extruders  14  may alternatively be used with any suitable additive manufacturing system for printing 3D parts from photocurable resin films, such as but not limited to, stereolithography (SLA) 3D printers, digital light processing (DLP) 3D printers, and the like. 
     In the shown embodiment, system  10  also includes platform  20  and platform gantry  22  (shown in  FIG. 2 ), where platform  20  is a receiving surface for printing the 3D parts in a layer-by-layer manner. Platform gantry  22  is an assembly configured to controllably move platform  20  along (or substantially along) the vertical z-axis within, such as in incremental steps. This allows platform  20  to be lowered incrementally to print each layer of a 3D part. Platform gantry  22  may operate to move platform  20  with one or more motors (e.g., stepper motors and encoded DC motors), gears, pulleys, belts, screws, linear bearings and rails, and the like. 
     Laser assembly  12  and slot extruders  14  are mounted in working region  18 , above the height of platform  20 , with gantry  24 . Gantry  24  is configured to controllably move laser assembly  12  and slot extruders  14  back-and-forth along the x-axis (scan direction axis) within overhead region  18 . Gantry  24  may operate to move laser assembly  12  with one or more motors (e.g., stepper motors and encoded DC motors), gears, pulleys, belts, screws, linear bearings and rails, and the like. 
     In some embodiments, gantry  24  also prevents or otherwise restricts laser assembly  12  and slot extruders  14  from moving along the y-axis (swath direction axis) and/or the z-axis. However, in other embodiments, gantry  24  may controllably move laser assembly  12  and slot extruders  14  along the y-axis, such as in small randomized offsets along the y-axis to overcome potential laser failures. In yet further alternative embodiments, gantry  24  may controllably move laser assembly  12  and slot extruders  14  along the z-axis, if desired. Gantry  24  also preferably prevents laser assembly  12  and slot extruders  14  from roll, pitch, or yaw movements relative to platform  20 . 
     System  10  also includes controller assembly  26 , which is one or more computer-based systems and/or one or more control circuits configured to monitor and operate the components of system  10 , such as laser assembly  12 , slot extruders  14 , motors for platen gantry  22 , gantry  24 , and various sensors, calibration devices, display devices, and/or user input devices. For example, one or more of the control functions performed by controller assembly  26  can be implemented in hardware, software, firmware, and the like, or combinations thereof. 
     Controller assembly  26  may communicate over communication line  28  with the various components of system  10 , such as laser assembly  12 , slot extruders  14 , motors for platen gantry  22 , gantry  24 , and various sensors, calibration devices, display devices, and/or user input devices. While communication line  28  is illustrated as a single signal line, it may include one or more electrical, optical, and/or wireless signal lines and intermediate control circuits, allowing controller assembly  26  to communicate with various components of system  10 . 
     Controller assembly  26  may also include one or more computer-based systems having computer-based hardware, such as data storage devices, processors, memory modules and the like for generating, storing, and transmitting tool path and related printing instructions to system  10 . Furthermore, one or more portions of controller assembly  26  may be retained by laser assembly  12 . For instance, controller assembly  26  may include one or more intermediate controllers mounted to laser assembly  12  and slot extruders  14 , which may relay data (e.g., compressed image data and timing information) to additional control boards of laser assembly  12  (e.g., laser driver boards), and/or to additional control boards of slot extruders  14  (e.g., heater driver boards). 
     Laser assembly  12  is an exemplary, non-limiting laser array assembly for use with slot extruders  14 , and preferably includes multiple arrays of laser emitters  30 , where each laser emitter  30  that can be selectively and independently operated to emit laser beams on a voxel-by-voxel basis. The emitted laser beams from laser emitters  30  selectively cross-link the resin film in a predefined pattern to form a layer of a 3D part. Examples laser array assemblies for laser assembly  12  include those disclosed in the &#39;553 application and the &#39;388 International application. 
     As can be seen in  FIG. 1 , the arrays of laser emitters  30  are separated along the y-axis to define a swath width  32 . Swath width  32  can define the maximum y-axis limits for a build envelope in which a 3D part may be printed, referred to as build envelope  34 . As discussed below, swath width  32  can be any reasonably desired distance, and preferably does not directly affect the printing speeds for system  10 . 
     Laser emitters  30  may also be arranged in a series of y-axis rows that include a leading row  36  and a trailing row  38 , and which are offset along the x-axis. The terms “leading” and “trailing” are used for convenience, and are not intended to limit the use of laser assembly  12 . As will be discussed below, gantry  24  is configured to controllably move laser assembly  12  and slot extruders  14  in back-and-forth directions along the x-axis for printing layers. With laser assembly  12  at the left-side set position in overhead region  18 , as shown in  FIGS. 1 and 2 , the leading row  36  of laser emitters  30  is located to the left side of build envelope  34  by a distance  36   a  (shown in  FIG. 1 ), which accounts for accelerations and decelerations of laser assembly  12  and slot extruders  14 . 
     Slot extruders  14  of the present disclosure are a pair of opposing resin slot extruders to producing film of a photocurable resin for each layer of the 3D part, as discussed below. In the shown example, slot extruder  14   a  is operably mounted to laser assembly  12  and/or to gantry  24  adjacent to leading row  36  of laser emitters  30 . Correspondingly, slot extruder  14   b  is operably mounted to laser assembly  12  and/or to gantry  24  adjacent to trailing row  38  of laser emitters  30 . This arrangement allows laser assembly  12  to operate with bidirectional printing. 
     Slot extruders  14  may each receive a supply of the photocurable resin from one or more material resin supply devices  40 . Resin supply device(s)  40  may include one or more cartridges, reservoirs, and the like for retaining and feeding the photocurable resin to slot extruders  14  through feed lines  40   a,  preferably under positive pressure. For instance, the photocurable resin may be controllably pumped from resin supply device(s)  40  to slot extruders  14 . In some embodiments, resin supply device(s)  40  may be integrally connected to slot extruders  14 , and/or otherwise carried by gantry  24 . 
     In some alternative embodiments, system  10  may operate with a single slot extruder  14  operably mounted to laser assembly  12  and/or to gantry  24  at the location of slot extruder  14   a  or slot extruder  14   b.  In other alternative embodiments, one or more slot extruders  14  may be mounted to an additional gantry (or gantries), where controller assembly  26  can command the additional gantry  24  to move the slot extruder(s)  14  independently of the movement of laser assembly  12 . For example, controller assembly  26  may command a slot extruder  14  to pass over platform  20  and apply a resin film for a 3D part layer, and then move laser assembly  12  after the slot extruder  14  has completed its pass. This may be beneficial in situations where the resin film requires a small time period for the resin polymers to reach a relaxed state before the cross-linking. 
     In another example, the slot extruder(s)  14  may move along the y-axis by a separate gantry (not shown), substantially perpendicular to the movement directions of laser assembly  12  along the x-axis. This can be beneficial in system architectures where the x-axis dimensions and the y-axis dimensions of build envelope  34  are significantly different. 
     During the printing operation, controller assembly  26  may command platform gantry  22  to move platform  20  to a predetermined height along the z-axis. Controller assembly  26  may then command gantry  24  to move laser assembly  12  and slot extruders  14  along the x-axis in the direction of arrow  42  to traverse across platform  20 . 
     While moving along the x-axis, controller assembly  26  may also command slot extruder  14   a  to extrude and apply a film of the photocurable resin onto platform  20  (or the top working surface of a partially-printed 3D part). As briefly discussed above, slot extruder  14   a  is preferably capable of extruding the flowable resin at high speeds with minimal or otherwise substantially reduced levels of normal forces or sheer forces applied to the surface. This allows the resin film to be applied with a substantially uniform z-height at the fast speeds. 
     While continuing to move along the x-axis, laser emitters  30  are then selectively and independently operated to emit laser beams on a voxel-by-voxel basis within build envelope  34 . This selectively cross-links the resin film in a predefined voxel pattern to form a layer of the 3D part. 
     As shown in  FIGS. 3 and 4 , this process may continue until laser assembly  12  reaches the opposing, right-side, set position of overhead region  18 . With laser assembly  12  at this right-side set position, the trailing row  38  of laser emitters  30  are located on the right side of build envelope  34  by a distance  38   a  (shown in  FIG. 3 ), which also accounts for accelerations and decelerations of laser assembly  12  and slot extruders  14 . 
     Accordingly, the x-axis limits for build envelope  34  correspond to scan length  44 , which is preferably the distance over which laser assembly  12  and each slot extruder  14  can pass across at a substantially steady rate of movement. As such, laser assembly  12  preferably moves in the direction of arrow  42  to traverse distance  36   a  (shown in  FIG. 1 ) for acceleration, scan length  44  for printing at a substantially steady rate of movement, and distance  38   a  (shown in  FIG. 3 ) for deceleration. In other words, the cross-sectional area of build envelope  34  in which a 3D part may be printed can be defined by swath width  32  and scan length  44 . 
     Once laser assembly  12  reaches the right-side set position, controller assembly  26  may command platform gantry  22  to move platform  20  downward by a single layer increment along the z-axis. Controller assembly  26  may then command gantry  24  to move laser assembly  12  and slot extruders  14  back along the x-axis in the direction of arrow  46  to traverse across platform  20 . 
     While moving along the x-axis, controller assembly  26  may command slot extruder  14   b  to extrude and apply a film of the photocurable resin onto the surface of the previously-formed layer. Laser emitters  30  may again be selectively and independently operated to emit laser beams on a voxel-by-voxel basis within build envelope  34 . This selectively cross-links the resin film in a predefined voxel pattern to form the next layer of the 3D part, which bonds to the previously-formed layer. 
     As shown in  FIGS. 5 and 6 , this back-and-forth process may be repeated for each layer until the 3D part (referred to as 3D part  48 ) is completed. Afterwards, 3D part  48  may be removed from system  10  and undergo one or more post-processing steps, such as resin removal and/or post-curing steps. 
     In an alternative control scheme, the trailing slot extruder  14  may be commanded to extrude and apply a film of the photocurable resin onto the surface of the previously-formed layer (rather than the leading slot extruder  14 , as described above). In this alternative scheme, controller assembly  26  may initially command gantry  24  to move laser assembly  12  and slot extruders  14  the x-axis in the direction of arrow  42  to traverse across platform  20  (as shown in  FIGS. 1 and 2 ). During this initial step, the leading slot extruder  14   a  does not extrude any photocurable resin and laser assembly  12  does not emit any laser beams. 
     Instead, controller assembly  26  may command the trailing slot extruder  14   b  to extrude and apply a film of the photocurable resin onto platform  20  (or the top working surface of a partially-printed 3D part). Then, during the next pass along the x-axis in the direction of arrow  46  (as shown in  FIGS. 3 and 4 ), controller assembly  26  may command laser emitters  30  to be selectively and independently emit laser beams on a voxel-by-voxel basis within build envelope  34 . This selectively cross-links the previously-applied resin film in a predefined voxel pattern to form the next layer of the 3D part. 
     In this step, the leading slot extruder  14   b  does not extrude any photocurable resin. Rather, the trailing slot extruder  14   a  may be commanded to extrude and apply a film of the photocurable resin onto the surface of the previous cross-linked layer. This process may then be repeated such that the resin film for the next layer is applied during the same pass that is used to cross-link the current layer. 
     Furthermore, as shown in  FIG. 7 , slot extruders  14  are preferably capable of extruding and applying each resin film, referred to as resin film  50 , with widths (along the y-axis) and scan lengths (along the x-axis) that are outside of the perimeter of build envelope  34 . This arrangement allows the edges of each resin film  50 , which can exhibit non-uniform undulations, thickness anomalies, and other geometrical variations to occur outside of build envelope  34 . 
     For instance, slot extruders  14  may begin extruding and applying each resin film  50  prior to reaching build envelope  34  along the x-axis, and stop extruding and applying the resin film  50  after exiting build envelope  34  along the x-axis. This can provide x-axis edge regions  52   a  and  52   b  for each resin film  50 . Correspondingly, the spans of slot extruders  14  along the y-axis preferably extend past the opposing edges of build envelope  34  along the y-axis, such that the lateral span of each resin film  50  can extend beyond swath width  32 . This can provide y-axis edge regions  54   a  and  54   b  for each film  50 . The dimensions of x-axis edge regions  52   a  and  52   b,  and of y-axis edge regions  54   a  and  54   b,  preferably ensure that any non-uniform undulations, thickness anomalies, and other geometrical variations that can occur at the edges of resin film  50  are located outside of build envelope  34 . 
     The example shown in  FIG. 7  is suitable for 3D parts  48  have footprint sizes similar to the dimensions of build envelope  34 , and/or for applications where each resin film  50  is applied with the same cross-sectional dimensions regardless of the footprint sizes of the 3D parts  48 . However, in many cases, the cross-sectional dimensions of 3D part  48  (or multiple 3D parts  48 ) can be smaller than the dimensions of build envelope  34 . As such, in some embodiments, controller assembly  26  may command slot extruders  14  to extrude and apply resin films  50  with smaller cross-sectional dimensions, but that still have edge regions corresponding to edge regions  52   a,    52   b,    54   a,  and  54   b  that are outside of the layer perimeters of 3D parts  48 . 
     This is illustrated in  FIG. 8 , where four 3D parts  48  are being printed simultaneously. In this case, 3D parts  48  have a combined footprint perimeter  56 , which is smaller than the dimensions of build envelope  34 . As such, controller assembly  26  may command slot extruders  14  to extrude and apply resin films  50  with the smaller cross-sectional dimensions along the x-axis, as shown, such that edge regions  52   a,    52   b,    54   a,  and  54   b  each remain outside of footprint perimeter  56 . 
     Additionally, in some embodiments, as discussed below, slot extruders  14  may be selectively operated along their y-axis spans, allowing each resin film  50  to have smaller spans along the y-axis. This is illustrated in  FIG. 9 , where controller assembly  26  may command slot extruders  14  to extrude and apply resin films  50  with the smaller cross-sectional dimensions along the x-axis and/or the y-axis, as shown. In this case edge regions  52   a,    52   b ,  54   a,  and  54   b  may each remain outside of footprint perimeter  56 . As can be appreciated, this dynamic sizing of resin films  50  as shown in  FIGS. 8 and 9  can change with the dimensions of each layer (for sets of layers) for 3D parts  48 , thereby reducing the amount of the photocurable resin that is consumed per pass. 
     As can be appreciated, because the resin is a flowable material, the edges of resin film  50  will typically require lateral containment. In some embodiments, platform  20  may lower into a tank (e.g., as shown in  FIG. 1 ) having lateral walls that can contain the flowable resin of film  50 . However, in some embodiments, system  10  may incorporate a dike-forming technique for containing the resin. In these embodiments, laser assembly  12  can cure a perimeter portion of each resin film  50  to print a containment dike  57  at any suitable location between footprint perimeter  56  and the perimeter edge of resin film  50 . 
     For example, laser assembly  12  can print the geometry of containment dike  57  at or adjacent to build envelope  34  (as shown in  FIG. 9A ), adjacent to and outside of footprint perimeter  56  (as shown in  FIG. 9B ), any locations in-between, and the like. Additionally, containment dike  56  can have any suitable geometry, where square geometries and rectangular geometries are typical. This dike-forming technique is particularly suitable for use with laser assembly  12  (and/or other bitslice-based imagers) as it does not detrimentally affect the printing speeds of laser assembly  12 . It also allows reduced volumes of the resin to be extruded, thereby reducing material consumptions. 
       FIGS. 10-12  illustrates an exemplary embodiment for slot extruder  14 , which may apply to both slot extruders  14   a  and  14   b.  As illustrated in  FIGS. 10 and 11 , slot extruder  14  may include upstream sidewall  58 , downstream sidewall  60 , and top cover  62 , all of which are secured together to define an interior plenum  64  for retaining a supply of photocurable resin  66 . As illustrated in  FIG. 10 , slot extruder  14  also includes opposing end walls  68  that cap interior plenum  64 . Upstream sidewall  58 , downstream sidewall  60 , top cover  62 , and opposing end walls  68  may each be fabricated from one or more rigid materials, such as one or more polymeric and/or metallic materials. 
     Top cover  62  may include a plurality of inlet ports  70  for receiving a plurality of supplies of resin  66  from feed lines  40   a.  While illustrated with six inlet ports  70  extending through top cover  62 , slot extruder  14  may alternatively include any suitable number of inlet ports  70  (e.g., one or more inlet ports  70 ), which may extend through top cover  62  and/or any other suitable location of slot extruder  14 . 
     In some preferred embodiments, feed lines  40   a  and/or inlet ports  70  may include controllable pumps and/or valves  70   a,  which can communicate with controller assembly  26  over communication line(s)  28 . This preferably allows controller assembly  26  to independently control each pump/valve  70   a  to independently regulate the flows of resin  66  through the separate inlet ports  70 . Furthermore, in some additional embodiments, feed lines  40   a  are sealed (e.g., hermetically sealed) to inlet ports  70  to prevent resin  66  and retained pressurized air from leaking out of plenum  64 . 
     Slot extruder  14  may also include a gas inlet line  72  for introducing pressurized air (or other gases, such as an inert gas) into an overhead region  74  of plenum  64 , above the fill level of resin  66 . As discussed below, the pressurized air can assist in reducing pressure drops along the y-axis span of plenum  64 , and to provide a positive internal pressure within plenum  64  for extruding resin  66  from slot extruder  14  as resin film  50 . 
     In the shown example, upstream sidewall  58  and downstream sidewall  60  converge at the bottom side to define slot  76  at the base of plenum  64 . Slot  76  preferably extends linearly along the y-axis. However, in some embodiments, slot  76  may have a small non-linear bowing or curvature, if desired. The span distance for slot  76  (and plenum  64 ) is preferably long enough to extrude and apply resin film  50  with a y-axis span that is capable of extending beyond the y-axis perimeter of build envelope  34 , such as by edge regions  54   a  and  54   b  (e.g., as shown above in  FIGS. 7 and 8 ). 
     Examples of suitable span distances  78  for slot  76  and plenum  64  include those greater than about 1 foot, greater than about 2 feet, and/or greater than about 3 feet. In some embodiments, span distance  78  ranges from about 1 foot to about 4 feet, from about 2 feet to about 4 feet, and/or from about 3 feet to about 4 feet. In other embodiments, span distance  78  may be larger, such as from about 4 feet to about 20 feet, from about 6 feet to about 15 feet, and/or from about 8 feet to about 12 feet. 
     As discussed below, resin  66  typically has a substantially uniform and known viscosity suitable for laminar-flow extrusion. At the known viscosity and a constant applied pressure (from air inlet line  72 ), resin  66  is forced from plenum  64 , through slot  76  where it is subjected to shear forces, and extruded onto a working surface  80 . Working surface  80  (shown in  FIG. 11 ) may be the top surface of platform  20  and/or the top working surface of 3D part  48 ). 
     As slot extruder  14  is moved along the x-axis in the direction of arrow  82  (corresponding to either arrow  42  or arrow  46 ), the extruded resin  66  is applied onto working surface  78  with minimal lateral and normal forces to produce resin film  50  having a substantially uniform thickness (outside of edge regions  52   a,    52   b,    54   a,  and  54   b ). Slot extruder  14  can optionally operate in combination with vacuum slot  84  and air knife unit  86 , each of which may extend along the y-axis by span distance  78 . Vacuum slot  84  preferably draws a weak vacuum upstream of slot  76 , and air knife unit  86  preferably applies a weak air knife downstream of slot  76 . This combination eliminates or otherwise reduces air bubbles from becoming trapped under the applied resin film  50  as slot extruder  14  moves along the x-axis. 
     Furthermore, in some embodiments, the pressurized air from air knife unit  86  optionally may be operated in conjunction with a refrigeration unit (not shown) or other suitable heat exchange unit for chilling the air. This can assist in cooling the resin films  50  prior to and/or after curing. Furthermore, in embodiments that incorporate multiple slot extruders  14  (e.g., as shown in  FIGS. 1-6 ), each slot extruder  14  may include its own air knife unit  86  for blowing the chilled air on the printed 3D part  48  during each bidirectional pass over platform  20 . 
     Moreover, in further embodiments, each air knife unit  86  may also function as a series of air gauges, which can measure back pressure of the emitted air as it blows across each resin film  50 . The measured back pressures can accordingly provide a non-contact technique for measuring the gap between the slot of air knife unit  86  and the surface of resin film  50 . These measurement signals can be transmitted over communication line(s)  28  to controller assembly  26 , which can then use the measurement signals to detect the height of each resin film  50 . 
     Based on these detected heights, controller assembly  26  can then adjust the extrusion profiles of slot extruders  14  to correct for height deviations. This can be particularly beneficial for preventing layer height errors from accumulating over multiple passes. In additional embodiments, system  10  and controller assembly  12  may also correct for voxel height errors using one or more imaging sensors, such as disclosed in Comb et al., U.S. Patent Publication No. 2015/0266242, entitled “Additive Manufacturing With Virtual Planarization Control”, the disclosure of which is incorporated by reference in its entirety. 
     As shown in  FIG. 12 , the bottom of slot  76  (and slot extruder  14 ) may pass over working surface  80  by an average gap distance  88  along the z-axis, which may be any suitable distance that allows resin film  50  to be extruded and applied to working surface  80 , preferably at high speeds, with minimal or otherwise reduced levels of shear or normal forces applied to working surface  80 . While extruding, resin  66  is sheared while flowing through slot  76  to partially define the thickness for resin film  50  (referred to as film thickness  90 ). This is achieved by the dimensions of slot  76 , referred to as slot width  92  along the x-axis and slot height  94  along the z-axis. In preferred embodiments, slot width  92  and slot height  94  each have less than 10% variation along the y-axis (i.e., along span distance  78 ), and more preferably are each substantially uniform along the y-axis. 
     The shearing of resin  66  in slot  76  allows the resulting resin film  50  to be applied onto working surface  80  with minimal (or otherwise reduced) amounts of normal and lateral forces. This allows 3D part  48  to be printed from resin  66  have a high viscosity, thin layers, high printing speeds, and short layer times, without the risk of damaging or distorting the previously-printed layers. 
     As resin  66  exits slot  76 , its polymeric chains typically undergo a relaxation phase that generate a die swell effect that increases the thickness of the extruded resin  66  beyond slot width  92 . As such, in some embodiments, slot width  92  is preferably smaller than the desired film thickness  90  for resin film  50  to account for the die swell effect. Examples of suitable average dimensions for slot width  92  range from about 10 micrometers to about 200 micrometers, from about 20 micrometers to about 100 micrometers, and/or from about 20 micrometers to about 50 micrometers. With reference to the desired film thickness  90 , the average dimensions for slot width  92  range from about 80% to about 100%, from about 90% to about 100%, and/or from about 95% to about 100% of the average film thickness  90 . 
     In some embodiments, slot  76  is positioned close to working surface  80 , such as from about one to about two times the average film thickness  90 . In these embodiments, the presence of working surface  80  can create a small additional flow resistance that can increase monotonically with gap distance  88 . This flow resistance can usefully provide flow leveling feedback that tends to fill in depressions and under-apply a layer thickness to elevated portions of the 3D part  48  being printed. 
     Back pressure leveling of the extruded film allows for compensation for required changes in flow rate for different regions of the part being printed. When the back pressure between part being printed and the pressure in the plenum  66  is increased, the driving force (caused by a difference in pressure) is decreased and results in a decrease in flow rate. When the back pressure between the part being printed and the pressure in the plenum  66  is decreased, the driving force is increased and results in an increase in flow rate. Therefore, the pressure within the plenum  66  can be manipulated to control the flow rate of the photopolymer resin as the film of resin is extruded to accommodate for, by way of example, a change in a level of the photocurable resin within the plenum  66 . In some embodiments, the back pressure from the flow of photocurable resin interacting with the part surface is at least about 5% of the total pressure drop from the pressure in the plenum  66  measured at a top surface of the photopolymer resin pool within the plenum  66  to ambient pressure which is typically atmospheric pressure. 
     Since the bottom of slot extruder  14  that is closest to working surface  80  is essentially a knife edge, the draw down of the extruded resin  66  is typically less than the film thickness  90 . On the other hand, as mentioned above, die swell can cause the natural film thickness  90  to be greater than slot width  92 . However, some residual in-plane tension can allow the extruded resin  66  to bridge voids in the 3D part  48  being printed. 
     In some embodiments, gap distance  88  is about 100% to about 120% of the film thickness  90 , from about 100% to about 110% of the film thickness  90 , from about 100% to about 105% of the film thickness  90 , and/or is substantially the same as film thickness  90 . It has been found that viscous drag of the extruded resin  66  against the trailing edge of slot  76  causes the extruded resin  66  to accumulate in front of slot  76  (referred to as “push”), which generates the additional pressure required to make film thickness  90  substantially the same as gap distance  88 . 
     Moreover, this additional pressure can serve to better expel air, for example, from between the resin film  50  and the underlying working surface  80 . Another advantage of this embodiment is that there is more feedback pressure from working surface  80  to doctor or level the resin film  50  in the presence of part surface undulations. Thus, as can be appreciated from the above discussion, gap distance  88  for slot  76  does not solely dictate the film thickness  90  for resin film  50 . 
     The amount of pressure required to extrude resin  66  through slot  76  can generally follow a Poiseuille laminar flow profile, such as described in Equation 1: 
     
       
         
           
             
               
                 
                   Pressure 
                   = 
                   
                     
                       12 
                        
                       
                           
                       
                        
                       Q 
                        
                       
                           
                       
                        
                       μ 
                        
                       
                           
                       
                        
                       
                         H 
                         Slot 
                       
                     
                     
                       
                         W 
                         Slot 
                         3 
                       
                        
                       
                           
                       
                        
                       
                         D 
                         Span 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     where Q is the volumetric extrusion flow rate (volume/second), μ is the dynamic viscosity of resin  66 , H Slot  is the slot height  94 , W Slot  is the slot width  92  (cubed in Equation 1), and D Span  is the span distance  78  for slot  76 . 
     For example, pushing the flowable resin  66  through slot  76  in a low-pressure situation (e.g., 10 inches/second flow rate, 0.1 Poise viscosity, 4-mil slot height, and 4-mil slot width) requires a pressure of about 0.04 pounds/square-inch (psi) applied evenly along span distance  78 . Alternatively, a high-pressure situation (e.g., 150 inches/second flow rate, 10 Poise viscosity, 1-mil slot height, and 1-mil slot width) requires a pressure of about 260 psi applied evenly along span distance  78 . 
     As can be appreciated, these variables can be modified to achieve a variety of different pressures between these low and high-pressure situations. For instance, in some embodiments, slot extruder  14  may have a slot width  92  and a slot height  94  each ranging from about 25 micrometers to about 50 micrometers, and be commanded to extrude the flowable resin  66  at a flow rate ranging about 50 inches/second to about 150 inches/second, where resin  66  may have a dynamic viscosity ranging from about 50 centipoise to about 5,000 centipoise. 
     Examples of suitable dimensions for slot height  94  range from about 50% to about 200% of slot width  92 , from about 75% to about 150% of slot width  92 , and/or from about 90% to about 110% of slot width  92 . In some embodiments, slot height  94  is substantially the same length as slot width  92 . 
     The length of slot  76  along the y-axis (e.g., span distance  78 ) is substantially greater than slot width  92 . The length of slot  76  is preferably more than 5 times larger than slot width  92 , more preferably more than 10 times larger than slot width  92 , even more preferably more than 15 times larger than slot width  92 . In some embodiments, the length of slot  76  is preferably more than 20 times larger than slot width  92 , and in further embodiments, more than 30 times larger than slot width  92 . For example, a length of slot  76  of one meter and a slot width  92  of 50 micrometers provides a factor of 20, and a length of slot  76  of one meter and a slot width  92  of 25 micrometers provides a factor of 40. 
     To extrude and apply resin film  50  having a substantially uniform thickness  90 , the pressure and level of resin  66  in plenum  64  should be as uniform as possible. Otherwise, pressure variations along the length of plenum  64  can vary the volumetric extrusion rates of resin  66  though slot  76  at different points along span distance  78 , which can result in thickness variations in resin film  50 . These thickness variations may accordingly cause 3D part  48  to accumulate surface undulations, such as hills and valleys, over successive layers that can reduce part quality. 
     However, due to its long dimensions, plenum  64  can exhibit pressure drops in the flowing resin  66  along its length. In some embodiments, slot extruder  14  may include the gas inlet line  72  (shown in  FIGS. 10 and 11 ) for introducing pressurized air (or other gases, such as an inert gas) into overhead region  74  of plenum  64 , above the fill level of resin  66 . The pressurized air can provide a constant positive pressure buffer along the length of plenum  64  to assist in reducing pressure fluctations along its length. This is in addition to providing the positive internal pressure for extruding resin  66  from slot  76  when commanded by controller assembly  26 . 
     In addition to the pressurized air, slot extruder  14  may also include multiple inlet ports  70  and pumps/valves  70   a  (best shown above in  FIG. 10 ), as discussed above. This allows controller assembly  26  to independently regulate the flows of resin  66  through the separate inlet ports  70 , such as in response to measured local fill levels. For example, as discussed below, in some embodiments, slot extruder  14  may include multiple capacitance sensors distributed along the length of plenum  64  to measure local fill levels of resin  66  in plenum  64 . 
       FIGS. 13-18  illustrate a preferred architecture for slot extruder  14 , which can incorporate capacitance sensing and resin heating. As illustrated in  FIGS. 13-15 , upstream sidewall  58  and downstream sidewall  60  respectively include indentations  96  and  98  each extending along the y-axis. Indentations  96  and  98  themselves respectively retain plates  100  and  102  that also extend along the y-axis, and may be fabricated from one or more ceramic materials. Accordingly, plates  100  and  102  are the portions of slot extruder  14  that primarily define the width and length of slot  76 , rather than upstream sidewall  58  and downstream sidewall  60 . 
     As further illustrated, plenum  64  is separated along its length by a plurality of spaced apart reinforcing supports  104  that stiffen slot extruder  14 , while also allowing resin  66  to flow along the length of plenum  64  to maintain a substantially uniform pressure and fill level. Upstream sidewall  58  and downstream sidewall  60  may also respectively include coolant ports  106  and  108 . This allows coolant fluids to flow from a heat exchange unit (not shown) and through upstream sidewall  58  and downstream sidewall  60 , as discussed below. 
     Each side of slot extruder  14  may also includes a heater array assembly  110  extending along the length of plenum  64 . Each heater array assembly  110  may includes a series of adjacent flex circuits  112  that can propagate out of plenum  64  to communicate with heater drivers  114  and capacitance level sensors  116  (depicted in  FIG. 14 ), each which may communicate with controller assembly  26  over communication line(s)  28 . 
     In some embodiments, slot extruder  14  may also include one or more ultrasonic transducers (not shown) at slot  76  to further assist in extruding resin  66 . For instance, each heater array assembly  110  may optionally include one or more ultrasonic transducer(s) at or near slot  76 . In these embodiments, flex circuits  112  that can propagate out of plenum  64  can also communicate with transducer drivers  117  (depicted in  FIG. 14 ), which may also communicate with controller assembly  26  over communication line(s)  28 . 
     As best illustrated in  FIGS. 15-17 , flex circuits  112  may be secured to sidewalls  58  and  60 , as well as to plates  100  and  102 , and connect multiple adjacent heater elements  118  (best shown in  FIGS. 17 and 18 ) to heater drivers  114  and capacitance level sensors  116 . As shown in  FIG. 18 , each heater element  118  can include a lithographed-serpentine geometry that is positioned down and through slot  76 . 
     During operation, controller assembly  26  and/or heater drivers  114  can selectively apply electrical power to heater elements  118 , which heats the powered heater elements  118 . This heat may then transfer to the resin  66  in plenum  64 , particularly proximate region  66  adjacent to heater elements  118 . 
     Even in embodiments in which resin  66  does not behave as a thermoplastic material, its viscosity will generally decrease when heated. As such, this arrangement provides a local electronically-addressable ability to cause flow to stop along the length of slot  76 . During an extrusion run, heater elements  118  may heat up resin  66  adjacent to slot  76 , and the air pressure within plenum  64  may be increased (via air line  72 ) to drive resin  66  through slot  76 . While passing through slot  76 , resin  66  is heated and sheared to a desired viscosity for extrusion as resin film  50 . After the extrusion run is completed, the applied pressure may be reduced and heater elements  118  may be turned off. This allows the coolant fluid flowing through coolant conduits  106  and  108  to locally cool the resin  66  residing in plenum  64  to a more viscous state, thereby controllably preventing resin  66  from flowing through slot  76 . 
     Because heater elements  118  can be selectively operated, in some cases, slot extruder  14  can extrude resin  66  form less than the entire length of slot  76 , such as discussed above for the smaller resin films  50  (e.g., shown above in  FIG. 9 ). This selectively extrusion for resin film  50  can be beneficial for a variety of applications. 
     Additionally, heater array assemblies  110  may also communicate with capacitive level sensors  116  to indicate the local level of resin  66  along the length of plenum  64 . In particular, the capacitance of resin  66  at various locations along the length of plenum  64  can be monitored to identify variations in fill levels. If these variations are detected, controller assembly  26  can direct feed lines  40   a  and/or pumps/valves  70   a  to operate to introduce additional amounts of resin  66  to plenum  64  at the identified lower-filled locations. This can assist in maintaining substantially uniform fill levels for resin  66 , which can accordingly increase the uniformity of pressure along the length of slot  76  (i.e., to prevent local pressure drops). 
     The capacitance level sensing can also be used to provide feedback on the flow rate during an extrusion run. In this case, the extrusion flow rate through slot  76  can be linked to the drive pressure in plenum  64  by measuring the flow, such as by capacitively monitoring the change in the resin level inside plenum  64  during while extruding. In other embodiments, film thickness  90  of resin film  50  may also be monitored with trailing optical, capacitive, and/or acoustic sensors (not shown). These external sensors can also be used in a feedback loop to control the drive pressure in plenum  64 . 
       FIG. 19  illustrates an alternative embodiment, which includes multiple slot extruders  14  on each end of laser assembly  12 , such as for multiple-material applications. This embodiment is particularly beneficial in combination the embodiment discussed above having the selective extrusion along the length of slot  76  (shown above in  FIGS. 13-18 ). In this case, slot extruders  14   a  and  14   b  may each include a first resin  66 , slot extruders  14   c  and  14   f  may include a second resin  66 , and slot extruders  14   d  and  14   e  may include a third resin  66 . The first, second, and third resins  66  may be the same materials or different materials, as desired. 
     The selective extrusion of each slot extruder  14  allows the different resins  66  to be selectively extruded onto platform  20  or working surface  80  of 3D part  48 . This allows multiple photocurable resins to be used, such as resins with different functional properties (e.g., different strengths, flexibilities, and the like) as well as resins with different colors. 
     In some embodiments, the sets of slot extruders  14  (e.g., slot extruders  14   e,    14   f , and  14   b ) may extrude their resins  66  during the same pass, effectively functioning as co-extruders. In alternative embodiments, each slot extruder  14  may be configured with multiple plenums  64  and slots  76  to function as a co-extruder. These co-extruder embodiments can provide a variety of different multi-film combinations for tailoring the physical and/or aesthetic properties of the 3D parts. 
       FIG. 20  illustrates another alternative embodiment, in which each slot extruder  14  may operate with a multiple-zone architecture. In this embodiment, plenum  64  may be separated into multiple zones  120 , where each zone  120  may be aligned with one of the inlet ports  70  for controllably and independently receiving resin  66  from the associated feed line  40   a.  For example, reinforcing supports  104  can optionally extend down to slot  76  to separate plenum  64  into the multiple zones  120 , where the span length of each zone  120  along the y-axis is preferably selected such that pressure drops associated with flow from the inlet ports  70  to any portion of slot  76  is substantially the same (e.g., within about 5% pressure variation). 
     As shown, air inlet line  72  (shown above in  FIG. 10 ) may optionally be omitted. Instead of extruding based on gas pressure throughout plenum  64 , each zone  120  is preferably filled with resin  66  up to inlet port  70  and feed line  7 a (i.e., overhead region  74 , shown above in  FIG. 14 , is omitted). As such, the pressure within each zone  120  can be controlled by the flow of resin  66  through inlet port  70 , which can be independently regulated or metered with flow regulators  70   a.  The regulation or metering by flow regulators  70   a  can accordingly be made in response to pressure signals from one or more pressure sensors  122  located in each zone  120 . 
     For example, each pressure sensor  122  can monitor the pressure of resin  66  in its associated zone  120 , and transmit signals over communication lines  28  to controller assembly  26  (and/or to local control boards of controller assembly  26 ) corresponding to the monitored pressures. Based on the received pressure signals from each pressure sensor  122 , controller assembly  26  can adjust the flow of the pressurized resin  66  into each zone  120  to preferably maintain a substantially uniform pressure across each zone  120 . 
     In some preferred embodiments, flow regulators  70   a  operate with fluid pumps. In this case, controller assembly  26  may independently command each flow regulator  70   a  to selectively adjust the feed rate of resin  66  into is associated zone  120  to maintain a predefined pressure. The use of pump-based flow regulators  70   a  can be beneficial for pressurizing and depressurizing each zone  120 , where depressurization can be obtained by reversing the flow of resin  66  with the pumps  70   a,  which can draw back the resin  66 . 
     Resin  66  may be extruded through slot  76  by increasing the flow of resin  66  into each zone  120  with each associated flow regulator  70   a.  Because resin  66  is relatively incompressible compared to air or other gases, the direct pressurization and depressurization of resin  66  can increase the response time for extruding resin  66  through slot  76  and for halting the extrusion (e.g., by draw backs with pumps  70   a ). Additionally, this embodiment precludes the need to vent the pressurized air after each extrusion run. 
     Slot extruder  14  can otherwise operate in the same manner as discussed above for the previous embodiments. Accordingly, slot extruders  14  of the present disclosure can operate with a variety of different pressurization techniques for maintaining substantially uniform pressures across plenum  64 , and for extruding resin  66  through slot  76 . 
     With respect to the resin  66 , system  10 , laser system  12 , and slot extruders  14  may print 3D parts (e.g., 3D part  48 ) from a variety of different photocurable resins, such as those used in stereolithography-based systems. For example, resin  66  may include one or more monomers and oligomers capable of polymerizing and cross-linking to change the flowable resin  66  of resin film  50  into a hardened, solid layer of 3D part  48 . As discussed below, in some further embodiments, resin  66  may also include one or more (pre-polymerized) polymers having available cross-linking groups. 
     For ease of discussion, the monomers, oligomers, and polymers are collectively referred to as the reactant compounds. It is understood that reference to a monomer unit, such as “methyl methacrylate” may collectively refer to the methyl methacrylate monomer, an oligomer that is polymerized at least in part from the methyl methacrylate monomer, and a polymer that is polymerized at least in part from the methyl methacrylate monomer. 
     Examples of suitable reactant compounds for resin  66  include one or more (meth)acrylic compounds, epoxy-functional compounds, and combinations thereof. The particular reactant compounds used may vary depending on the photoinitiation architecture. For example, reactant compounds that cross-link with ethylenically-unsaturated groups (e.g., (meth)acrylate compounds) can polymerize and/or cross-link with the use of free radical photoinitiators. In comparison, epoxy-functional compounds can typically polymerize and/or cross-link with the use of cationic photoinitiators. 
     In embodiments that incorporate free radical polymerization and cross-linking, resin  66  may include one or more (meth)acrylic compounds, such as one or more (meth)acrylate monomers, oligomers, and polymers, and/or one or more (meth)acrylic acid monomers, oligomers, polymers. Examples of suitable (meth)acrylate monomers include methyl (meth)acrylate, ethyl (meth)acrylate, ethanediol di(meth)acrylate, trimethylolethane tri(meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, propanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, butanediol di(meth)acrylate, trimethylolbutane tri(meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate, hexanediol di(meth)acrylate, cyclohexanediol di(meth)acrylate, trimethylolhexane tri(meth)acrylate, benzyl (meth)acrylate, isobornyl (meth)acrylate, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, tetramethylol methane tetra(meth)acrylate, dipropylene glycol di(meth)acrylate, trimethylol hexane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, urethane (meth)acrylate, and mixtures thereof. 
     In embodiments that incorporate acid functionalities, examples of suitable (meth)acrylic acid monomers include ethylenically-unsaturated carboxylic acid monomers, such as acrylic acid, methacrylic acid, alpha-chloroacrylic acid, alpha-cyanoacrylic acid, crotonic acid, alpha-phenylacrylic acid, beta-acryloxypropionic acid, fumaric acid, maleic acid, sorbic acid, alpha-chlorosorbic acid, angelic acid, cinnamic acid, p-chlorocinnamic acid, beta-stearylacrylic acid, citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, tricarboxyethylene, 2-methyl maleic acid, itaconic acid, 2-methyl itaconic acid, methyleneglutaric acid, and mixtures thereof. 
     In some embodiments, the reactant compounds may also include one or more ethylenically-unsaturated monomers to impart additional properties to 3D part  48 , resin film  50 , and/or resin  66 . For instance, the monomers may include one or more ethylenically-unsaturated, hydroxyl-functional monomers, one or more ethylenically-unsaturated aromatic monomers, and the like. Examples of suitable ethylenically-unsaturated hydroxyl-functional monomers include (meth)acrylate monomers and vinyl monomers having one or more hydroxyl-functional groups. 
     The ethylenically-unsaturated aromatic monomers preferably include aromatic groups and ethylenically-unsaturated groups, such as aromatic vinyl monomers. Examples of suitable ethylenically-unsaturated aromatic monomers include styrene, methyl styrene, halostyrene, diallylphthalate, divinylbenzene, alpha-methylstyrene, vinyl toluene, vinyl naphthalene, and mixtures thereof. For ease of reference, the ethylenically-unsaturated aromatic monomers refer to non-acrylic-type monomers (e.g., vinyl monomers). 
     In embodiments that incorporate cationic polymerization and cross-linking, resin  66  may include one or more epoxy-functional monomers. Examples of suitable epoxy-functional monomers include aliphatic, cycloaliphatic, aromatic or heterocyclic monomers having, one average, at least about 1.0 polymerizable oxriane group per molecule, more preferably at least about 1.5 polymerizable oxriane groups per molecule, and even more preferably at least about 2.0 polymerizable oxriane group per molecule. The average number of oxriane groups per molecule is determined by dividing the total number of oxriane groups in the epoxy-functional monomers by the total number of epoxy-functional molecules present. 
     Examples of suitable epoxy-functional monomers include alkyl glycidyl ethers such as butyl glycidyl ether, cresyl glycidyl ether, p-terbutylphenyl glycidyl ether, polyfunctional glycidyl ethers such as diglycidyl ether of 1,4-butanedol, diglycidyl ether of neopentyl glycol, diglycidyl ether of cyclohexanedimethanol, trimethylol ethane triglycidyl ether, trimethylol propane triglycidyl ether, polyglycidyl ether of an aliphatic polyol, polyglycol diepoxide, other glycidyl ethers of polyhydric phenols obtained by reacting a polyhydric phenol with an excess of chlorohydrin, such as epichlorohydrin), and mixtures thereof. 
     Additional examples of epoxy-functional monomers include alkylene oxides (e.g., propylene oxide, styrene oxide, and butadiene oxide); and glycidyl esters (e.g., ethyl glycidate), those which contain cyclohexene oxide groups (e.g., epoxy cyclohexanecarboxylates), octadecylene oxide, epichlorohydrin, styrene oxide, vinyl cyclohexene oxide, glycidol, glycidylmethacrylate, diglycidyl ether of Bisphendl A, vinylcyclohexene dioxide, bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate, bis(2,3-epoxycyclopentyl)ether, aliphatic epoxy modified from polypropylene glycol, dipentene dioxide, epoxidized polybutadiene, silicone resin containing epoxy functionality, 1 bis(3,4-epoxycyclohexyl)adipate, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-metadioxane, vinylcyclohexene monoxide 1,2-epoxyhexadecane, and mixtures thereof. In further embodiments, the photocurable resin may include one or more epoxy-(meth)acrylate monomers, such as glycidyl (meth)acrylate. 
     In some preferred embodiments, resin  66  may also include one or more fillers, which can be beneficial for many purposes, such as increasing the viscosity (and/or modifying the rheology) of resin  66 , increase the strength and chemical resistance of the printed 3D parts (e.g., 3D part  48 ), reducing layer shrinking and curl effects, and the like. In embodiments that include fillers, preferred concentrations of the fillers in resin  66  may include more than about 1% by weight, more than about 5% by weight, and/or more than about 10% by weight, based on an entire weight of resin  66 . Preferred concentrations of the fillers in resin  66  may also include less than about 70% by weight, less than about 60% by weight, less than about 50% by weight, and/or less than about 40% by weight, based on the entire weight of resin  66 . 
     As can be appreciated, due to the thin layers that can be produced by slot extruder  14  and laser assembly  12 , the filler particles preferably have maximum particle diameters (or other sizes, such as fiber lengths) that are smaller than the layer thickness  90  produced by slot extruder  14  (shown in  FIG. 12 ) and/or than the intended voxel dimensions. Furthermore, the maximum particle diameters or dimensions for the filler particles are preferably less than about 90%, more preferably less than about 80%, and even more preferably less than about 70% of the layer thickness  90 . In some embodiments, the maximum particle diameters or dimensions for the filler particles are less than about 60% and/or less than about 50% of the layer thickness  90 . 
     The filler particles can have a unimodial or polymodial (e.g., bimodal) particle size distribution. Furthermore, the particle size distribution(s) can be selected to increase packing densities of the filler particles by selecting smaller filler particle that can fill in interstitial voids between larger filler particles (along with the above-discussed monomers, oligomers, and/or polymers). 
     Examples of suitable fillers include quartz, nitrides (e.g., silicon nitride), feldspar, kaolin, talc, titania, calcium carbonate, magnesium carbonate, glass spheres, graphite, carbon black, carbon fiber, glass fiber, wollastonite, mica, alumina, silicon carbide, zirconium tungstate, and silica particles (e.g., silicas available under the trade designations “AEROSIL” from Degussa Corp., Akron, Ohio; and “CAB-O-SIL” from Cabot Corp., Tuscola, Ill.). One interesting characteristic of slot extruder  14  is that fiber-based fillers in the applied resin film  50  tend to orient randomly in the x-y plane of the layer, potentially increasing intralayer strengths and/or other properties for 3D parts. 
     In some preferred embodiments, the surfaces of the filler particles can also be treated with one or more coupling agents in order to enhance the bond between the filler particles and the reactant compounds of resin  66 . For instance, the surfaces of the filler particles can be treated to couple functional groups (e.g., ethylenically-unsaturated groups and/or epoxy-functional groups), which can cross-link with the reactant compounds of resin  66 . Examples of suitable coupling agents include gamma-methacryloxypropyltrimethoxysil ane, gamma-mercaptopropyltriethoxysilane, gamma-aminopropyltrimethoxysilane, and the like. 
     As mentioned above, in addition to monomers, the (meth)acrylic compounds may also include one or more oligomers and/or polymers pre-polymerized from the above-discussed monomers. This can be beneficial for increasing the viscosity of resin  66  for use with slot extruders  14 . Additionally, the longer pre-polymerized chains do not shrink during the photocuring process in system  10 , effectively allowing them to function as fillers for resin  66 . 
     For instance, in some embodiments, resin  66  may include one or more copolymers pre-polymerized (i.e., prior to use in system  10 ) from monomers that include one or more (meth)acrylates, one or more (meth)acrylic acids, one or more ethylenically-unsaturated hydroxyl-functional monomers, and/or one or more ethylenically-unsaturated aromatic monomers. Preferably, these copolymers retain unreacted and available functional groups that can cross-link during the photocuring process in system  10 . For example, the copolymers may be pre-polymerized with a free-radical polymerization reaction, and retain unreacted and available epoxy-functional groups (e.g., from glycidyl (meth)acrylate monomers). These epoxy-functional groups can then function as curing groups for cross-linking during the photocuring process in system  10  (i.e., a cationic cross-linking). 
     As discussed for the fillers, due to the thin layers that can be produced by slot extruder  14  and laser assembly  12 , the pre-polymerized copolymers preferably have chain lengths that are smaller than the layer thickness  90  produced by slot extruder  14  (shown in  FIG. 12 ) and/or than the intended voxel dimensions. Furthermore, the maximum chain lengths or dimensions for the pre-polymerized copolymers are preferably less than about 90%, more preferably less than about 80%, and even more preferably less than about 70% of the layer thickness  90 . In some embodiments, the maximum chain lengths or dimensions for the pre-polymerized copolymers are less than about 60% and/or less than about 50% of the layer thickness  90 . 
     Resin  66  also preferably includes one or more radiation-activated photoinitiators to initiate the polymerization and/or cross-linking of the reactant compounds (and optionally, the surface-treated filler particles). The photoinitiator is preferably selected to undergo a photoreaction upon absorption of light with an appropriate light wavelength, such as wavelengths in the ultraviolet, visible, and/or infrared spectrums (e.g., from the laser beams of laser emitters  30 ). In some embodiments, the photoinitiator is selected to undergo a photoreaction on absorption of ultraviolet-wavelength light. In other embodiments, the photoinitiator is selected to undergo a photoreaction upon absorption of infrared-wavelength light. In yet other embodiments, the photoinitiator is selected to undergo a photoreaction upon absorption of electron beam radiation. 
     The photoinitiator is also preferably selected based on the particular functional groups of the reactant compounds, such as free-radically-active functional groups (e.g., ethylenically-unsaturated groups) and cationically-active functional groups (e.g., epoxy-functional groups). For free-radically-active functional groups, suitable photoinitiators include radical photoinitiators, optionally combined with one or more photosensitizers and/or accelerators. Upon absportion of the laser beam radiation (from laser emitters  30 ), the radical photoinitiators decompose into free radicals. The free radicals accordingly initiate an addition polymerization and/or cross-linking at a local level (e.g., within the voxel illuminated by the laser beam). 
     Examples of suitable radical photoinitiators include hydroxy ketones, amino ketones, hydroxy ketone/benzophenones, carbonyl compounds (e.g., benzoin, a-methyl benzoin, anthraquinone, chloroanthraquinone, and acetophenone), sulfur compounds (e.g., diphenyl sulfide, diphenyl disulfide, and dithio-carbamate), polycyclic aromatic compounds (e.g., α-hloromethyl naphthalene and anthracene), and the like. Other suitable photoinitiators for polymerizing free radically photopolymerizable compositions include the class of phosphine oxides, such as bis(2,4,6-trimethylbenzoyl)phenyl phosphine oxide, bis(2,6-dimethoxybenzoyl)-(2,4,4-trimethylpentyl) phosphine oxide (CGI 403, Ciba Specialty Chemicals), a mixture of bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide and 2-hydroxy-2-methyl-1-phenylpropan-1-one, a mixture of bis(2,4,6-trimethylbenzoyl)phenyl phosphine oxide and 2-hydroxy-2-methyl-1-phenylpropane-1-one, ethyl 2,4,6-trimethylbenzylphenyl phosphinate, and the like. 
     For cationically-active functional groups, suitable photoinitiators include cationic photoinitiators. Upon absportion of the laser beam radiation (from laser emitters  30 ), the cationic photoinitiators react and produce acid compounds (e.g., Lewis acids and Bronsted acids). These acids then initiate a condensation polymerization and/or cross-linking at a local level (e.g., within the voxel illuminated by the laser beam). 
     Suitable cationic photoinitiators include onium salts (e.g., iodonium salts), diazonium salts, organometallic complexes, boron trifluoride/tetrahydrofuran, dimethylbenzyl esters, and the like. Examples of suitable onium salts include iodonium salts (e.g., diaryliodonium and triaryliodonium salts), sulfonium salts (e.g., triarylsulfonium salts), pyridium salts, phosphonium salts, quinolinium salts, and the like. Additional examples of suitable cationic photoinitiators include thioxanthone derivatives, benzolphosphine oxides, bis-acylphosphine oxides, hydroxaryl ketones, hydrophilic initiators (e.g., quarternary ammonium salts, sulphonates, and thiosulphates). 
     Furthermore, many photoinitiators are capable of initiating both free radical and cationic reactions. Additional examples of suitable photoinitiators include those available under the trade designation “IRGACURE” from BASF Schweiz AG, Ludwigshafen, Germany (formerly Ciba Specialty Chemicals). Examples of suitable concentrations of the photoinitiators in resin  66  may range from about 0.1% by weight to about 10% by weight, and more preferably from about 0.5% by weight to about 5% by weight, based on the entire weight of resin  66 . 
     Resin  66  may also include one or more additional additives that are preferably soluble or dispersible in the reactant compounds, and that preferably do not interfere with the curing processes. Examples of suitable additional additives include colorants (e.g., pigments and dyes), polymer stabilizers (e.g., antioxidants, light sensitizers, ultraviolet absorbers, and antiozonants), heat stabilizers (e.g., organosulfur compounds), biodegradable additives, and combinations thereof. In embodiments that incorporate additional additives, the additional additives may collectively constitute from about 0.1% by weight to about 20% by weight of resin  66 , more preferably from about 0.2% by weight to about 10% by weight, and even more preferably from about 0.5% by weight to about 5% by weight, based on the entire weight of resin  66 . 
     Examples of suitable light sensitizers include those capable of absorbing light having wavelengths ranging from about 300 to about 1000 nanometers, such as ketones, coumarin dyes (e.g., ketocoumarins), xanthene dyes, acridine dyes, thiazole dyes, thiazine dyes, oxazine dyes, azine dyes, aminoketone dyes, porphyrins, aromatic polycyclic hydrocarbons, p-substituted aminostyryl ketone compounds, aminotriaryl methanes, merocyanines, squarylium dyes, and pyridinium dyes. Ketones (e.g., monoketones or alpha-diketones), ketocoumarins, aminoarylketones, and p-substituted aminostyryl ketone compounds are preferred sensitizers. 
     Resin  66  may be formulated to have any suitable dynamic (shear) viscosity. In some preferred embodiments, resin  66  may be formulated to have dynamic viscosities that are significantly higher than those typically used for stereolithography applications. the high viscosities can be beneficial for many purposes, such as for increased part strengths (e.g., due to the incorporation of higher molecular weight polymer chains and/or filler particles), deformation resistance of resin film  50 , and the like. 
     Examples of suitable dynamic viscosities for resin  66  (in an uncured state) include those less than about 7,500 centipoise, more preferably less than about 6,000 centipoise, and in some embodiments less than about 5,000 centipoise. In some embodiments, additional examples of suitable dynamic viscosities for resin  66  (in an uncured state) include those greater than about 25 centipoise, and more preferably greater than about 50 centipoise. 
     In some particular embodiments, resin  66  may have a dynamic viscosity (in an uncured state) ranging from about 50 centipoise to about 500 centipoise. In other particular embodiments, resin  66  may have a dynamic viscosity (in an uncured state) ranging from about 500 centipoise to about 1,000 centipoise. In further particular embodiments, resin  66  may have a dynamic viscosity (in an uncured state) ranging from about 1,000 centipoise to about 2,500 centipoise. In further particular embodiments, resin  66  may have a dynamic viscosity (in an uncured state) ranging from about 2,500 centipoise to about 5,000 centipoise. 
     During the curing process with system  10  (or any other suitable additive manufacturing system), the absorbed radiation initiates the free radical and/or cationic reactions at the illuminated voxels of resin  66  in resin film  50 . This polymerizes and/or cross-links the reactant compounds (and optionally, the surface-treated filler particles), which transitions resin  66  from the uncured state in which resin  66  is flowable and extrudable, to the cured state in which the cured resin film  50  becomes a hardened, solid layer of 3D part  48  that bonds to the previously produced layers. The resulting hardened, solid layer is then non-flowable and preferably hydrophobic (e.g., a contact angle greater than about 90 degrees) for use in the printed 3D part (e.g., 3D part  48 ). 
     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.