Patent Publication Number: US-2021170483-A1

Title: System and methods for printing defect detection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 62/945,359 filed Dec. 19, 2019, the contents of which are herein incorporated by reference in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     Various aspects of the present disclosure relate generally to systems and methods for fabricating components by additive manufacturing, and specifically, to systems and methods for promoting uniformity in a powder bed of a binder-jet three-dimensional printer. 
     BACKGROUND OF THE DISCLOSURE 
     Powder bed three-dimensional fabrication is an additive manufacturing technique based on binding particles of a powder to form a three-dimensional object within the powder bed. Binder jetting is one type of powder bed three-dimensional fabrication. Binder jetting includes delivering powder, e.g., metal powder, to a print bed, spreading the powder into a layer, and depositing a binder material, e.g., a binder fluid, on top of the powder to bind the powder together. The binder material is deposited in a pre-determined pattern (e.g., in a cross-sectional shape of the three-dimensional object) to successive layers of powder in a powder bed such that the powder particles bind to one another where the binder material is located to form a three-dimensional green part. In the context of binder jet printing of three-dimensional metal objects, a three-dimensional green part may be formed by printing as described above, and may then be processed further into a finished three-dimensional metal part. For example, excess, unbound metal powder may be removed from the powder bed. Then, the three-dimensional green part may be heated in a furnace to remove the binder material and/or sintered to form the final, three-dimensional part. 
     When binder-jetting the three-dimensional green part, defects may occur when powder particles or binder are distributed unevenly on the powder bed. For example, defects such as unintended mounds or blisters, divots, and/or smears may occur when powder particles are distributed in an unintended manner, are overly cohesive, and/or are disturbed by forces experienced during printing. Additionally, when the powder particles are overly mobile, powder may be ejected during the printing process, by, e.g., the deposition of binder on the powder bed. The ejected powder may deposit on the print head, where the powder can potentially block or clog orifices needed for depositing binder. 
     The systems and methods of the present disclosure may address one or more of the deficiencies described above and/or may address other aspects of the prior art. 
     SUMMARY 
     Examples of the present disclosure relate to, among other things, systems and methods for defect detection during additive manufacturing. Each of the examples disclosed herein may include one or more of the features described in connection with any of the other disclosed examples. 
     In one aspect, an additive manufacturing method may include depositing a first amount of metal powder onto a powder bed of a printing system, spreading the first amount of metal powder across the powder bed to form a first layer, and depositing a first amount of binder material on the first layer. The additive manufacturing method may also include exposing the first layer to a first lighting condition, imaging the first layer under the first lighting condition to generate a first image, analyzing the first image, and determining whether to adjust at least one printing parameter based on the analyzing. 
     In another aspect, an additive manufacturing apparatus may include a powder bed for receiving metal powder, a spreader configured to traverse the powder bed to spread the metal powder to form a layer, a first light source configured to illuminate the powder bed, and a second light source configured to illuminate the powder bed, wherein the first light source is positioned so as to form a first angle relative to the powder bed, wherein the second light source is positioned so as to form a second angle relative to the powder bed, and wherein the first angle is less than the second angle. The additive manufacturing apparatus may also include an imaging device configured to image the powder bed, and a vapor deposition device configured to deposit vapor onto the powder bed. The additive manufacturing apparatus may include a controller configured to control the light source, the imaging device, the spreader, and the vapor deposition device so that: the spreader traverses the powder bed to spread a layer of powder on the powder bed, the first light source illuminates the layer, the imaging device images the layer illuminated by the first light source, the second light source illuminates the layer, the imaging device images the layer illuminated by the second light source, and the controller adjusts at least one printing parameter of the at least one additive manufacturing apparatus. 
     In another aspect, an additive manufacturing method may include depositing a first amount of powder onto a powder bed, spreading the first amount of powder across the powder bed to form a first layer of powder, applying a vapor to the first layer, and depositing a binder on the first layer. The additive manufacturing method may also include imaging the first layer under a first lighting condition to generate a first image, imaging the first layer under a second lighting condition to generate a second image, and determining whether to adjust at least one printing parameter based on the first image and the second image. 
     Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the features, as claimed. As used herein, the terms “comprises,” “comprising,” “including,” “having,” or other variations thereof, are intended to cover a non-exclusive inclusion such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements, but may include other elements not expressly listed or inherent to such a process, method, article, or apparatus. Additionally, the term “exemplary” is used herein in the sense of “example,” rather than “ideal.” It should be noted that all numeric values disclosed or claimed herein (including all disclosed values, limits, and ranges) may have a variation of +/−10% (unless a different variation is specified) from the disclosed numeric value. In this disclosure, unless stated otherwise, relative terms, such as, for example, “about,” “substantially,” and “approximately” are used to indicate a possible variation of +/−10% in the stated value. Moreover, in the claims, values, limits, and/or ranges of various claimed elements and/or features means the stated value, limit, and/or range +/−10%. The terms “object,” “part,” and “component,” as used herein, are intended to encompass any object fabricated through the additive manufacturing techniques described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various exemplary embodiments and together with the description, serve to explain the principles of the disclosed embodiments. There are many aspects and embodiments described herein. Those of ordinary skill in the art will readily recognize that the features of a particular aspect or embodiment may be used in conjunction with the features of any or all of the other aspects or embodiments described in this disclosure. 
         FIG. 1A  is an exemplary block diagram of an additive manufacturing system, 
         FIG. 1B  illustrates an exemplary binder jet fabrication subsystem, and  FIG. 1C  illustrates another exemplary binder jet fabrication subsystem; 
         FIGS. 2A and 2B  are schematic side views showing an exemplary powder bed of a binder jet fabrication subsystem with an exemplary defect and exemplary components of a defect detection system; 
         FIGS. 3A and 3B  are top views of the powder bed of  FIGS. 2A and 2B  as imaged by the defect detection system; 
         FIGS. 4A and 4B  are schematic side views showing an exemplary powder bed of a binder jet fabrication subsystem with another exemplary defect and exemplary components of a defect detection system; 
         FIGS. 5A and 5B  are top views of the powder bed of  FIGS. 4A and 4B  as imaged by the defect detection system; 
         FIGS. 6A and 6B  are schematic side views showing an exemplary powder bed of a binder jet fabrication subsystem with another exemplary defect and exemplary components of a defect detection system; 
         FIGS. 7A and 7B  are top views of the powder bed of  FIGS. 6A and 6B  as imaged by the exemplary defect detection system; 
         FIGS. 8A and 8B  are schematic side views showing an exemplary powder bed of a binder jet fabrication subsystem with another exemplary defect and exemplary components of a defect detection system; 
         FIGS. 9A and 9B  are top views of the powder bed of  FIGS. 8A and 8B  as imaged by the exemplary defect detection system; 
         FIGS. 10A and 10B  are top views of an exemplary portion of a defect-free powder bed as imaged with the exemplary defect detection system; 
         FIG. 11  is a flow chart illustrating an exemplary defect detection and/or correction method; 
         FIG. 12  is a flow chart illustrating an exemplary defect detection and/or correction method; and 
         FIG. 13  is a flow chart illustrating an exemplary defect detection and/or correction method. 
         FIGS. 14A-B  is a schematic side view showing an exemplary powder bed of a binder jet fabrication subsystem with an exemplary defect and exemplary components of a defect detection system. 
         FIGS. 15A-B  are top views of the powder bed of  FIGS. 14A and 14B  as imaged by the exemplary defect detection system. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure include systems and methods to facilitate and improve the efficacy and efficiency of additive manufacturing, in particular, to mitigate issues in low-speed or high-speed printing. Reference now will be made in detail to examples of the present disclosure described above and illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       FIG. 1A  illustrates an exemplary system  100  for forming a printed object, according to an embodiment of the present disclosure. System  100  may include a printer, for example, a binder jet fabrication subsystem  102 , and one or more treatment site(s), for example, a de-powdering subsystem  104 , a curing subsystem  105 , and a sintering furnace subsystem  106 . Binder jet fabrication subsystem  102  may be used to form an object from a build material, for example, by delivering successive layers of build material and binder material to a build plate. As shown in  FIG. 1A , a build box subsystem  108  may be movable and may be selectively positioned in one or more of binder jet fabrication subsystem  102 , de-powdering subsystem  104 , curing subsystem  105 , and sintering furnace subsystem  106 . For example, build box subsystem  108  may be coupled or couplable to a movable assembly. Alternatively, a conveyor (not shown) may help transport the object between portions of system  100 . 
     The build material may be a bulk metallic, polymeric, and/or ceramic powder delivered and spread in successive layers. The binder material may be, for example, a polymeric liquid that may be deposited onto and may be absorbed into layers of the build material. One or more of binder jet fabrication subsystem  102 , de-powdering subsystem  104 , curing subsystem  105 , and sintering furnace subsystem  106  may include a debinding station to treat the printed object to remove a binder material from the build material. Furnace subsystem  106  may heat and/or sinter the build material of the printed object. System  100  may also include a user interface  110 , which may be operatively coupled to one or more components, for example, to binder jet fabrication subsystem  102 , de-powdering subsystem  104 , curing subsystem  105 , and sintering furnace subsystem  106 , etc. In some embodiments, user interface  110  may be a remote device (e.g., a computer, a tablet, a smartphone, a laptop, etc.). User interface  110  may be wired or wirelessly connected to one or more of binder jet fabrication subsystem  102 , de-powdering subsystem  104 , and sintering furnace subsystem  106 . System  100  may also include a control subsystem  116 , which may be included in user interface  110 , or may be a separate element. 
     Binder jet fabrication subsystem  102 , de-powdering subsystem  104 , curing subsystem  105 , sintering furnace subsystem  106 , user interface  110 , and/or control subsystem  116  may each be connected to the other components of system  100  directly or via a network  112 . Network  112  may include the Internet and may provide communication through one or more computers, servers, and/or handheld mobile devices, including the various components of system  100 . For example, network  112  may provide a data transfer connection between the various components, permitting transfer of data including, e.g., geometries, the printing material, one or more support and/or support interface details, binder materials, heating and/or sintering times and temperatures, etc., for a part or a part to be printed. 
     Moreover, network  112  may be connected to a cloud-based application (not shown in  FIG. 1 ), which may also provide a data transfer connection between the various components and cloud-based application in order to provide a data transfer connection, as discussed above. The cloud-based application may be accessed by a user in a web browser, and may include various instructions, applications, algorithms, methods of operation, preferences, historical data, etc., for forming the part or object to be printed based on the various user-input details. Alternatively or additionally, the various instructions, applications, algorithms, methods of operation, preferences, historical data, etc., may be stored locally on a local server (not shown) or in a storage and/or processing device within or operably coupled to one or more of binder jet fabrication subsystem  102 , de-powdering subsystem  104 , curing subsystem  105 , sintering furnace subsystem  106 , user interface  110 , and/or control subsystem  116 . In this aspect, binder jet fabrication subsystem  102 , de-powdering subsystem  104 , curing subsystem  105 , sintering furnace subsystem  106 , user interface  110 , and/or control subsystem  116  may be disconnected from the Internet and/or other networks, which may increase security protections for the components of system  100 . In either aspect, an additional controller (not shown) may be associated with one or more of binder jet fabrication subsystem  102 , de-powdering subsystem  104 , and sintering furnace subsystem  106 , etc., and may be configured to receive instructions to form the printed object and to instruct one or more components of system  100  to form the printed object. 
       FIG. 1B  illustrates an exemplary binder jet fabrication subsystem  102  operating in conjunction with build box subsystem  108 . Binder jet fabrication subsystem  102  may include a powder supply  120 , a spreader  122  (e.g., a roller) configured to be movable across a powder print bed  124  of build box subsystem  108 , a print head  126  movable across powder bed  124 , and a controller  128  in electrical communication (e.g., wireless, wired, Bluetooth, etc.) with print head  126 . Powder bed  124  may comprise powder particles, for example, micro-particles of a metal, micro-particles of two or more metals, or a composite of one or more metals and other materials. 
     Spreader  122  may be movable across powder bed  124  to deposit and spread a layer of powder, from powder supply  120 , across powder bed  124 . Print head  126  may comprise a discharge orifice  130  and, in certain implementations, may be actuated to dispense a binder material  132  (e.g., through delivery of an electric current to a piezoelectric element in mechanical communication with binder material  132 ) through discharge orifice  130  to the layer of powder spread across powder bed  124 . In some embodiments, the binder material  132  may be one or more fluids configured to bind together powder particles. 
       FIG. 1B  also shows a vapor deposition device  150  that is configured to deposit vapor (e.g., steam)  152  on the powder bed  124  prior to deposition of binder material  132 , as discussed further below. One or more light sources and one or more image capturing devices may be included as components of binder jet fabrication subsystem  102 . In some embodiments, controller  128  may be configured to control (e.g., output control signals to) a (relatively) low-angle light source  180  and/or a (relatively) high-angle light source  190 . For example, low-angle light source  180  may form an angle in the range of about 1 degree to about 10 degrees with respect to powder bed  124 . In at least some embodiments, low-angle light source  180  may form an angle of about 2 degrees to about 5 degrees with respect to powder bed  124 . High-angle light source  190  may form an angle of about 70 degrees to about 110 degrees with respect to powder bed  124  (i.e., from 90 degrees vertical to about 20 degrees from vertical in either direction). In at least some embodiments, high-angle light source  190  may form an angle of approximately 90 degrees with respect to powder bed  124 . The particular angle may be selected at least in part based on the type(s) of defects that low-angle light source  180  and/or high angle light source  190  are being employed to identify. The low angle light source and high angle light source may be any illumination source that provides sufficient illumination level to distinguish the defects. In one embodiment the light source may have a brightness of 10-10000 and preferably 100-1000 lumens. In another embodiment the light source may consist of a collimated (that is substantially parallel) light to avoid an angular component. In another embodiment, the light source may comprise one or more light diffusers to create a diffuse illumination condition. 
     The high angle light source may be any illumination source that provides a sufficient illumination level to distinguish the defects. The light source must be significantly brighter than the ambient light in the room, such that the ambient lighting in the room has a minimal impact on the lighting conditions of the print bed. The high angle light is placed in such a manner to allow distribution of light that is approximately even across the entire print surface. This can be accomplished by adding a diffuser to the high angle light to evenly distribute the light over the print surface. To control the effects of the lighting on different powder materials, the image capture device is configured with specific exposure and gain values for different materials that maximize the contrast of areas of the bed that contain no binder to areas of the bed that contain binder. In an embodiment, the low angle light source may comprise a laser. A raster scanner may be employed or a diffusing element may be used to illuminate the print bed with the laser light. A raster scanner may comprise a laser configured to project a beam of light configured to produce a substantially round shape, which may be scanned (for example using a galvanometer scanner to rapidly move the beam location across the bed in two axes). Alternatively, a raster scanner may be configured to produce a beam of lightwith an elongated shape (that is, a line source), which may be scanned in a single axis along the print bed to illuminate defects. A diffusing element may comprise an optical element, such as a lens, a diffraction grating, or other optical element, intended to reshape a beam into a suitable shape for providing illumination, such as a one line or multiple lines, or an area with uniform or substantially uniform illumination. Other beam shapes and rastering strategies may be employed, with the objective of providing a uniform illumination to the print bed. In some embodiments, a laser light source may allow for higher illumination intensity, or improved signal to noise for image capturing, compared to a non-laser light source. 
     The low angle light source is intended to hit the powder bed at a low angle of incidence, which will highlight any imperfections on the ideally flat, smooth surface of the print bed. In one embodiment a collimated light is employed, in which all light travels out of the reflector in parallel. In another embodiment a light source with a narrow beam angle can also be used. In another embodiment, a source further away from the area of inspection is employed to accomplish a similar effect. For this lighting source, an even lighting condition across the bed is preferred, but not required as long as the image is not overexposed in any area as to create areas of the image that are primarily white on a print bed. Along with the angle and positioning of the light, the exposure and gain values can be modified on the image capture device to prevent this. The exposure and gain may also need to be modified based on the material that is being printed on. 
     Controller  128  may output command signals to, and receive data (e.g., imaging data) from, an image capturing device  174 . In an exemplary configuration, low-angle light source  180 , high-angle light source  190 , and image capturing device  174  may be provided at predetermined fixed locations within binder jet fabrication subsystem  102 . However, image capturing device  174  may be connected to a mechanism that provides locomotion to image capturing device  174 , which may facilitate acquisition of images from different positions and/or minimize or eliminate obstructions. If desired, a plurality of low-angle light sources  180 , a plurality of high-angle light sources  190 , and/or a plurality of image capturing devices  174  may be provided at one or more different locations of binder jet fabrication subsystem  102 . Image capturing device  174  may be, for example, any suitable light-detecting device, such as a camera capable of detecting visible light and/or a charge-coupled device capable of detecting ultraviolet light, infrared light, etc., or a combination thereof. When low-angle light source  180  and high-angle light source  190  are configured to produce visible light, light sources  180 ,  190  may produce light having different colors (i.e., different wavelengths of light within the visible spectrum), such as red and blue light. When different colors of light are used, image capturing device  174  may be configured to distinguish between different colors of light (e.g., a color charge-coupled device (CCD)). In an exemplary configuration, high-angle light source  190  may produce red light, while low-angle light source  180  may produce blue light. By capturing a color image with image capturing device  174 , the defect detection system may be configured to analyze blue, red, combined red and blue, and darkened (shadow-containing) regions at the same time. This may, in at least some embodiments, eliminate the need to apply low-angle light and high-angle light at different times. 
     Moreover, a plurality of low-angle light sources  180  and/or a plurality of high-angle light sources  190  may be arranged at different angles and/or orientations. For example, a first low-angle light source  180  may be located so as to direct light along a translation direction of spreader  122  (e.g., X-direction, see  FIGS. 2A-10B ) with a second low-angle light source  180  located so as to direct light along a horizontal direction orthogonal to the translation direction of spreader  122  (e.g., Y-direction, see  FIGS. 2A-10B ). Such light may facilitate the identification of defects that tend to form aligned with, or orthogonal to (e.g., smears, divots), the translation direction of spreader  122 . If multiple low-angle or high-angle light sources  180 ,  190  are included, the multiple low-angle or high-angle light sources  180 ,  190  may produce the same color light or different colored light. For example, in the embodiment described above, the horizontal and orthogonal low-angle light sources  180  may be the same color light (i.e., wavelength) or different color light (i.e., wavelength). Further, although red and blue light are provided as examples herein, it is understood that any suitable color (wavelength) light may be included. 
     In some embodiments, light sources  180  and/or  190  may be configured to produce light outside of the visible spectrum (e.g., ultraviolet light or infrared light), while in others, they may be configured to produce light within the visible spectrum. In some aspects, combinations of light sources  180  and/or  190  may be configured to produce different types of light, for example, light sources  180  and light sources  190  may be configured to produce different types of light, and/or different light sources  180  may produce different types of light, and/or different light sources  190  may produce different types of light. 
     In operation, controller  128  may actuate print head  126  to deliver binder material  132  from print head  126  to each layer of the powder in a pre-determined, two-dimensional pattern, as print head  126  moves across powder bed  124 . In embodiments, the movement of print head  126 , and the actuation of print head  126  to deliver binder material  132 , may be coordinated with movement of spreader  122  across powder bed  124 . For example, spreader  122  may spread a layer of the powder across powder bed  124 , and print head  126  may deliver the binder in a pre-determined, two-dimensional pattern, to the layer of the powder spread across powder bed  124 , to form a layer of one or more three-dimensional objects or parts  134 . These steps may be repeated (e.g., with the pre-determined two-dimensional pattern for each respective layer) in sequence, with the build plate incrementing down each layer to form subsequent layers until, ultimately, the one or more parts  134  are formed in powder bed  124 . 
     Vapor deposition device  150  may move with (and in advance of) print head  126 , to deposit vapor (i.e., to condense vapor), such as water vapor, on powder bed  124 . For example, vapor deposition device  150  may distribute vapor (e.g., steam) to predetermined locations of powder bed  124 , causing condensation of the vapor into the powder. Vapor deposition device  150  may move together with print head  126 , and may, if desired, be included as part of print head  126 . Alternatively, vapor deposition device  150  may move independently of print head  126 . 
     During operation and motion of spreader  122 , print head  126 , and vapor deposition device  150 , each of these components may move with respect to powder bed  124 . Thus, for a period of time between the deposition of each layer of powder, image capturing device  174  may have a substantially or completely unobscured view of powder bed  124 . At the same time, one or both of light sources  180  and  190  may be configured to direct light toward powder bed  124 , without significant light blockage from spreader  122 , print head  126 , or vapor deposition device  150 . Image capturing device  174  may be configured to provide controller  128  with image information corresponding to an entirety of powder bed  124  or one or more areas of powder bed  124 . In some embodiments, image capturing device  174  may image a first portion of powder bed  124  at a first timing, and a different second (or subsequent) portion of powder bed  124  at a second timing. Such imaging may be useful when different portions of powder bed  124  are obscured by one of the moving components at different times. Details of a defect detection system, which may include light sources  180  and  190  and image capturing device  174 , is described in further detail below. 
     Although the exemplary embodiment depicted in  FIG. 1B  depicts a single part  134  being printed, it should be understood that the powder print bed  124  may include more than one part  134  in embodiments in which more than one part  134  is printed at once. Further, the powder bed  124  may be delineated into two or more subsections, stacked vertically, with one or more objects disposed within each subsection. 
     An exemplary binder jet fabrication subsystem  102  may comprise a powder supply actuator mechanism  136  that elevates powder supply  120  as spreader  122  spreads the powder across powder bed  124 . Similarly, build box subsystem  108  may comprise a build box actuator mechanism  138  that lowers powder bed  124  incrementally as each layer of powder is distributed across powder bed  124 . The distance the powder bed is lowered in each increment defines the layer thickness of the printing process. 
     In another exemplary embodiment, layers of powder may be applied to powder bed  124  by a hopper followed by a spreading device, such as a compaction roller. The hopper may move across powder bed  124 , depositing powder along the way. The spreading device may be configured to follow the hopper, spreading the deposited powder to form a layer of powder. 
     For example,  FIG. 1C  illustrates another binder jet fabrication subsystem  102 ′ operating in conjunction with a build box subsystem  108 ′. In this aspect, binder jet fabrication subsystem  102 ′ may include a powder supply  120 ′ in a metering apparatus, for example, a hopper  121 . Binder jet subsystem  102 ′ may include a defect detection system, including light sources  180  and  190  and image capturing device  174 , which may function in a similar manner as a defect detection system of binder jet subsystem  102 . Binder jet subsystem  102 ′ may also include one or more spreaders  122 ′ (e.g., one or more rollers) configured to be movable across powder bed  124 ′ of build box subsystem  108 ′, a print head  126 ′ movable across powder bed  124 ′, and a controller  128 ′ in electrical communication (e.g., wireless, wired, Bluetooth, etc.) with one or more of hopper  121 , spreaders  122 ′, and print head  126 ′. Powder bed  124 ′ may comprise powder particles, for example, micro-particles of a metal, micro-particles of two or more metals, or a composite of one or more metals and other materials. 
     Hopper  121  may be any suitable metering apparatus configured to meter and/or deliver powder from powder supply  120 ′ onto a top surface  123  of powder bed  124 ′. Hopper  121  may be movable across powder bed  124 ′ to deliver powder from powder supply  120 ′ onto top surface  123 . The delivered powder may form a pile  125  of powder on top surface  123 . 
     The one or more spreaders  122 ′ may be movable across powder bed  124 ′ downstream of hopper  121  to spread powder, e.g., from pile  125 , across powder bed  124 . The one or more spreaders  122 ′ may also compact the powder on top surface  123 . In either aspect, the one or more spreaders  122 ′ may form a layer  127  of powder. The aforementioned powder delivery and spreading steps may be successively performed in order to form a plurality of layers  129  of powder. Additionally, although two spreaders  122 ′ are shown in  FIG. 1C , binder jet fabrication subsystem  102 ′ may include one, three, four, etc. spreaders  122 ′. 
     Print head  126 ′ may comprise a discharge orifice  130 ′ and, in certain implementations, may be actuated to dispense a binder material  132 ′ (e.g., through delivery of an electric current to a piezoelectric element in mechanical communication with binder material  132 ′) through discharge orifice  130 ′ to the layer of powder spread across powder bed  124 ′. In some embodiments, the binder material  132 ′ may be one or more fluids configured to bind together powder particles. 
     In operation, controller  128 ′ may actuate print head  126 ′ to deliver binder material  132 ′ from print head  126 ′ to each layer  127  of the powder in a pre-determined two-dimensional pattern, as print head  126 ′ moves across powder bed  124 ′. As shown in  FIG. 1C , controller  128 ′ may be in communication with hopper  121  and/or the one or more spreaders  122 ′ as well, for example, to actuate the movement of hopper  121  and the one or more spreaders  122 ′ across powder bed  124 ′. Additionally, controller  128 ′ may control the metering and/or delivery of powder by hopper  121  from powder supply  120 ′ to top surface  123  of powder bed  124 ′. In embodiments, the movement of print head  126 ′, and the actuation of print head  126 ′ to deliver binder material  132 ′, may be coordinated with movement of hopper  121  and the one or more spreaders  122 ′ across powder bed  124 ′. For example, hopper  121  may deliver powder to bed  124 ′, and spreader  122 ′ may spread a layer of the powder across powder bed  124 ′. Then, print head  126 ′ may deliver the binder in a pre-determined, two-dimensional pattern, to the layer of the powder spread across powder bed  124 ′, to form a layer of one or more parts  134 ′. These steps may be repeated (e.g., with the pre-determined two-dimensional pattern for each respective layer) in sequence to form subsequent layers until, ultimately, the one or more parts  134 ′ are formed in powder bed  124 ′. 
     Although the exemplary embodiment depicted in  FIG. 1C  depicts a single part  134 ′ being printed, it should be understood that the powder bed  124 ′ may include more than one part  134 ′ in embodiments in which more than one part  134 ′ is printed at once. Further, the powder print bed  124 ′ may be delineated into two or more subsections  127 , stacked vertically, with one or more objects disposed within each subsection. 
     As in  FIG. 1B , build box subsystem  108  may comprise a build box actuator mechanism  138  that lowers powder bed  124  incrementally as each layer  127  of powder is distributed across powder bed  124 . Accordingly, hopper  121 , the one or more spreaders  122 , and print head  126  may traverse build box subsystem  108  at a pre-determined height, and build box actuator mechanism  138  may lower powder bed  124  to form part  134 . 
     Although not shown, binder jet fabrication subsystems  102  may include a coupling interface that may facilitate the coupling and/or uncoupling of the build box subsystems  108 ,  108 ′ with the binder jet fabrication subsystems  102 ,  102 ′, respectively. The coupling interface may comprise one or more of (i) a mechanical aspect that provides for physical engagement, and/or (ii) an electrical aspect that supports electrical communication between the build box subsystem  108 ,  108 ′ to the binder jet fabrication subsystem  102 ,  102 ′. 
     It should be appreciated that the techniques herein may reference aspects of  FIGS. 1A-1C  (e.g., the binder jet fabrication subsystem  102 , the powder bed  124 , the print head  126 , etc.). Any reference to systems or components in  FIGS. 1A and 1B  may also be applicable to corresponding components in  FIG. 1C  (e.g., reference numerals with corresponding prime (&#39;) designations). For example, references and descriptions to the binder jet fabrication subsystem  102  may also be applicable to the binder jet fabrication subsystem  102 ′, references and descriptions to the powder bed  124  may also be applicable to the powder bed  124 ′, and so on. 
     Binder jetting thin layers of powders (in particular, fine powders) at low or high speed may introduce defects, such as blisters (e.g., regions of trapped gas, trapped fluid, or powder and fluid rearrangement). Blisters may tend to be caused by surface tension effects, such as beading, which may cause one or more defects to protrude above the surface of the printed layer after deposition of binder). Binder jetting may also introduce defects such as regions of missing binder (e.g. due to one or more blocked binder discharge orifices), divots (e.g., regions where the step of spreading powder does not leave a smooth and uninterrupted powder surface, but instead leaves a depression, hole, or other region of “missing” powder), beading (e.g., surface-tension driven balling of binder or binder and powder on the powder bed), and/or smearing (e.g., regions where, during the step of spreading a new layer of powder, movement occurs in one or more underlying layers, causing distortion of the part), among others. The deposition of vapor on the powder bed, e.g., by vapor deposition device  150 , may, in some embodiments, facilitate the spread of binder material  132  and reduce the occurrence of defects, such as beading of binder fluid and powder due to the above-described surface tension effects. However, when an excessive amount of vapor, or an excessive amount of binder fluid, or a combination of binder fluid and vapor, is deposited by vapor deposition device  150 , binder material  132  may form blisters. Such blisters may be produced by air that becomes trapped beneath the binder and/or condensed vapor in the one or more layers of powder. Further, the deposition of an excessive amount of vapor may cause unwanted spreading of binder material  132  or “bleeding.” When an inadequate amount of vapor is deposited, powder may lack cohesion and may become undesirably mobile. Such powder may be easily ejected (projected upward) when binder material  132  is deposited thereon, and may block or clog one or more discharge orifices  130 . These blocked discharge orifices  130  may become unable to deposit binder, leading to one or more areas of powder bed  124  with missing binder, also referred to as a jets-out condition. Mobile powder may also experience surface tension effects during the deposition of binder resulting in rearrangement (beading) of the powder in the layer being printed, leading to a rough surface on the resulting printed parts; or leading to poor binder penetration into the powder bed resulting in parts without the desired strength; or leading to the printed layer protruding vertically above the next layer of powder, resulting in pushing of the printed part (e.g. smearing). 
     Additionally, spreading successive layers of powders at high speeds may introduce issues associated with non-uniform powder distribution and/or the inability of layers of powder to withstand the forces that may occur during printing (e.g., forces imparted by spreader  122 ). Insufficient metering of powder may produce areas that lack powder due to uneven distribution of the powder. Spreading of powder that is excessively cohesive may result in a transient “particle jamming” behavior inside the powder pile, where the powder particles temporarily transition from a free-flowing powder to a jammed or tightly packed body. When this jamming occurs, the powder jammed powder may not distribute into an even and uniform layer of powder, leaving a gap, hole, or divot in the deposited powder layer. If areas with an insufficient supply of powder become aligned (vertically), a hole or divot may become increasingly larger and/or deeper. Such divots may be present in a printed area (e.g., an area in which binder was deposited which forms a part), or a non-printed area (e.g., an area of powder which is not intended to form a part and one in which binder was not deposited). Divots, when present, may result in undesirable characteristics in the printed parts  134 . For example, divots may create or cause voids or empty spaces within the part  134 , which may adversely affect mechanical properties, such as ultimate strength, yield strength, toughness, or decreased elongation during mechanical tensile testing. Divots may also tend to introduce flaws on the surface of the part, including regions where desired features are not present or are incorrectly shaped, regions having increased surface roughness, or other undesirable surface characteristics. 
     Smearing may occur when a previously-printed layer shifts, cracks, or breaks apart. For example, smearing may occur if an excessive quantity of powder is spread, or when previously printed layers are unable to withstand the force of spreader  122 . Smearing may also cause undesirable surface roughness on an exterior surface of part  134 , particularly on downward-facing surfaces or overhangs. In some cases, smearing may introduce distortion of printed features, causing a part to fall outside of a specified dimensional tolerance. 
     One or more aspects of this disclosure may address one or more of the above issues. 
       FIGS. 2A, 2B, 4A, 4B, 6A, 6B, 8A, and 8B  are schematic side views corresponding to a powder bed, e.g., of exemplary binder jet fabrication subsystem  102  and/or binder jet fabrication subsystem  102 ′ and exemplary defects that may occur in powder bed  124  during binder jetting. In  FIGS. 2A, 2B, 4A, 4B, 6A, 6B, 8A, and 8B , a horizontal direction X may correspond to an exemplary translation direction of spreader  122 ,  122 ′. A vertical direction Z orthogonal to the X-direction may correspond to a stacking direction along which layers of powder are formed. As discussed above and as shown in  FIGS. 1B and 1C , binder jet fabrication subsystem  102 ,  102 ′ may include at least one source of light, such as a source of low-angle light  180  or high-angle light  190 , as well as one or more image capturing devices  174 . In one aspect, at least one source of low-angle light  180  and at least one source of high-angle light  190  may be located within binder jet fabrication subsystem  102 ,  102 ′. In one aspect, the at least one source of low-angle light  180  and at least one source of high-angle light  190  may also be substantially isolated from exposure to the inside of the print environment (e.g. from powder) through the use of separating feature (e.g. transparent windows). These sources of light  180 ,  190  may each be operable to direct illumination in a direction toward powder bed  124 , in which a three-dimensional object or part  134  may be manufactured in a layer-by-layer manner. Controller  128 ,  128 ′ of the defect detection system, and other components of binder jet fabrication subsystem  102 ,  102 ′ are omitted from  FIGS. 2A, 2B, 4A, 4B, 6A, 6B, 8A, and 8B  for clarity. Additionally, low-angle light source  180  is depicted in different positions in  FIGS. 2B, 4B, 6B, and 8B  as compared to FIGSs.  2 A,  4 A,  6 A, and  8 A, for the purposes of clarity. However, it is to be understood that low-angle light source  180  may be provided in any suitable location in the binder jet printing subsystem  102 ,  102 ′. 
     As can be seen in  FIGS. 2A and 2B , low-angle light source  180  or high-angle light source  190  may be activated to generate and direct light toward powder bed  124  at a particular time. For example, as shown in  FIG. 2A , low-angle light source  180  may be activated (e.g., by controller  128 ,  128 ′) to generate and direct low-angle light  182  in a direction toward powder bed  124  (and one or more parts  134 ). When one or more blisters  210  are present on powder bed  124 , the low-angle light  182  may create a blister shadow  214  that extends in a direction away from low-angle light source  180 . Additionally, reflected low-angle light  182  (not shown in  FIG. 2A ) may have a sufficiently low angle with respect to powder bed  124  so as to create a blister bright spot  212  on the mound formed by blister  210 , due at least in part to the potentially non-uniform shape that blister  210  may have. As can be seen in  FIG. 2A , a region  200  of powder bed  124  substantially free of defects due to powder spreading or vapor deposition may not create a shadow or bright spot. As can be seen in  FIG. 2B , when high-angle light source  190  is activated and generates high-angle light  192 , the position of this light source  190  with respect to blister  210  may not result in the creation of a blister shadow  214 , although, high-angle light source  190  may be used to detect the presence of blisters  210  having certain sizes and/or shapes. Thus, low-angle light  182  may be particularly suitable for creating blister shadows  214  when one or more blisters  210  are present. 
       FIGS. 3A and 3B  are exemplary top views of powder bed  124  from the perspective of image capturing device  174  when low-angle light  182  or high-angle light  192  is respectively directed onto powder bed  124  in the manner shown in  FIGS. 2A and 2B , respectively. In  FIGS. 3A and 3B  (and  FIGS. 5A, 5B, 7A, 7B, 9A, 9B, 10A, and 10B ), the horizontal direction X may correspond to the translation direction of spreader  122 ,  122 ′. The Y-direction may be a horizontal direction orthogonal to the X-direction, as described above.  FIG. 3A  shows a top view of a portion of powder bed  124  on which a plurality (e.g., three) exemplary parts  134  are being formed, e.g., by binder jet fabrication subsystem  102  or  102 ′.  FIGS. 3A and 3B  show the same parts  134  under different lighting conditions.  FIGS. 3A and 3B  may correspond to a portion of a field of view of image capturing device  174  or the entire field of view of image capturing device  174 . 
     As can be seen in  FIG. 3A , a plurality of parts  134  may be formed within areas of powder bed  124  on which binder fluid or binder material  132  has been deposited. A remainder of powder bed  124  may include binder-free or unbound powder  140 . In one aspect, unbound powder  140  may have a different, e.g., darker, appearance in an image captured under low-angle light  182  (may reflect less light) as compared to the binder-containing parts  134  formed by bound powder  142  when low-angle light  182  is directed toward powder bed  124 . 
       FIG. 3A  illustrates a number of blisters  210  present on powder bed  124 . When low-angle light  182  encounters a blister  210 , the blister  210  may create a blister bright spot  212  that corresponds to a mound of the blister  210 . In one aspect, the blister bright spot  212  may be brighter than an area surrounding blister  210  and/or brighter than an average brightness of a top layer of powder bed  124 . Additionally, each blister  210  may cast a blister shadow  214  that extends away from low-angle light source  180  ( FIG. 2A ). Thus, each blister  210  may present a blister bright spot  212  and an adjacent (contiguous) blister shadow  214 , which can be detected using image capturing device  174 , as is shown in  FIG. 3A . Blisters  210  may be identified or differentiated from other types of defects based on the circularity of e.g. blister bright spot  212 , which compares the perimeters of the defect to a circle of the same area. For example, a circularity of approximately 1 may be indicative of a blister  210 , while a lower circularity may indicate the presence of another defect. Additionally or alternatively, blisters  210  may be detected or differentiated from other defects based on a measure of the aspect ratio of the of defect—e.g., a defect that is significantly longer in the y direction than in x (or vice versa) is unlikely to be a blister. 
     To accomplish this, first the image captured using the image capturing device  174  and low angle light  182  is transformed using a homography matrix to align key points in the image to the actual image sent to the print electronics  126 . The resulting image is then broken down into regions of interest (ROI) that can then be processed sequentially individually for detect, either sequentially, or alternatively in parallel. 
     To detect blistering, each ROI is then scanned using a blob detection algorithm. This will provide areas on interest (AOI) to more closely inspect. Each AOI is inspected for circularity, inertia, and convexity. An object that has a circularity and inertia of approximately  1 , will be flagged as a potential blister. If a significant amount of blistering is found in a region, corrective action may be taken as outlined in the present disclosure. 
       FIG. 3B  may correspond to a portion or entirety of an image captured by image capturing device  174  when high-angle light source  190  generates high-angle light  192  in the manner shown in  FIG. 2B . As can be seen in  FIG. 3B , each of the parts  134  may reflect less of the high-angle light  192  toward image capturing device  174  so as to form relatively dark portions of powder (e.g., due to the presence of binder, or due to different surface texture imparted by the binder deposition process). Regions of powder bed  124  including unbound powder  140  may reflect more of the high-angle light  192 , so as to form relatively light portions of powder bed  124 . In one aspect, blisters  210 , blister bright spots  212 , and/or blister shadows  214  may be difficult to observe or detect when high-angle light  192  is directed toward powder bed  124 . 
       FIGS. 4A and 4B  illustrate exemplary lighting conditions under low-angle and high-angle light, respectively, for detection of one or more missing binder (jets-out condition) defects  220  in powder bed  124 .  FIG. 4A  illustrates low-angle light  182  from low-angle light source  180  directed toward powder bed  124 . In  FIGS. 4A and 4B , part  134  includes regions of bound powder  142 , as well as regions of missing binder  220  (which may be formed by unbound powder  140  located on portions of part  134 ). These regions may reflect the low-angle light  182  toward image capturing device  174 . 
     When low-angle light  182  is directed toward regions of missing binder (jets-out condition)  220  and regions of bound powder  142 , the reflected low-angle light  184  may tend to be directed in a substantially uniform manner toward image capturing device  174 , as represented in  FIG. 4A . Observation or detection of missing binder  220  may be difficult when light is reflected in this manner. However, when high-angle light  192  is directed toward regions of missing binder  220  and bound powder  142  of part  134 , as shown in  FIG. 4B , light may tend to be more readily absorbed by bound powder  142  as compared to unbound powder  140 , creating lighter and darker areas of powder. Thus, regions of missing binder  220  may be identified or detected by the greater amount of high-angle light  194  reflected by the regions of missing binder  220 , which may appear as lighter regions in images captured by image capturing device  174 . As understood,  FIGS. 4A and 4B  illustrate regions of bound powder  142  and missing binder  220  for ease of illustration. However, a cross-sectional view taken in a plane extending parallel to the X-direction (e.g., the view of  FIGS. 4A and 4B ), may include only missing binder  220  or only bound powder  142 . 
       FIGS. 5A and 5B  are exemplary top views from the perspective of image capturing device  174 , according to  FIGS. 4A and 4B , respectively.  FIGS. 5A and 5B  may correspond to a portion of the field of view of image capturing device  174 . As shown in  FIG. 5A  and described above, when low-angle light  182  is directed toward parts  134  on powder bed  124 , regions of missing binder (jets-out condition)  220  within part  134  may be imperceptible or difficult to detect. However, when high-angle light  192  is directed toward the same powder bed  124  and parts  134 , the missing binder  220  (which amounts to the presence of unbound powder) within one or more parts  134  may be readily discerned or detected as lighter regions within parts  134 . This may occur, at least in part, due to a larger amount of high-angle light  192  being reflected by the unbound powder ( FIG. 4B ). In one aspect, the regions of unbound powder  140  forming missing binder  220  may be formed in substantially linearly-extending streaks or lines. These lines may extend in a direction approximately aligned with a direction of motion of print head  126 . For example, unbound powder  140  and the corresponding lighter areas of part  134  may be formed due to one or more blocked discharge orifices  130 ,  130 ′, which may prevent binder material  132  from being deposited in the regions of missing binder  220 . 
     To accomplish this, first the image captured using the image capturing device  174  and high angle light  190  is transformed using a homography matrix to align key points in the image to the actual image sent to the print electronics  126 . This resulting image now has the same scale as the image printed and each pixel of the image represents one jet on the printhead electronics. This can be used to identify the specific printhead, or nozzle that ejects each drop. 
     The resulting image is then thresholded at a material specific value to differentiate areas where there is bound powder on the print bed to areas where there is only unbound powder. The thresholded image is then compared directly to the actual image on a pixel-wise basis along each row. If there are a significant number of pixels that have unbound powder in a row compared to the actual image, then that row is said to have jets out. If multiple rows of sequential jets out are detected, corrective actions may be taken. 
     Alternatively, if there is a pixel that is shown to have bound powder in the captured image but the printed image shows there should be no binder present, bleeding is indicated. Small amounts of bleeding are expected as binder migrates in the print bed, but if a ROI contains large amounts of bleeding, as calculated by comparing the number of pixels containing bleeding vs the area printed, corrective action can be taken.  FIGS. 6A and 6B  illustrate exemplary lighting conditions for detection of one or more cavity or divot defects  230  in powder bed  124 .  FIG. 6A  illustrates low-angle light  182  from low-angle light source  180  directed toward a part  134  containing a divot  230 . The low-angle light  182  may have a sufficiently low angle such that at least a portion of divot  230  casts a shadow  234  and remains relatively dark in comparison to the powder immediately surrounding divot  230 . In one aspect, divot  230  may form a divot bright spot  232  at an end of divot  230  farthest from low-angle light source  180 . In one aspect, divot bright spot  232  may be brighter than an average brightness of a top layer of powder bed  124 . Divot shadow  234  may be caused by blocked low-angle light  182 , and may be closer to the low-angle light source  180  than a corresponding divot shadow  234 . In contrast, a blister  210  may form a blister bright spot  212  closer to low-angle light source  180  as compared to a corresponding blister shadow  214 . In one aspect, divot shadow  234  may be formed in a majority of divot  230 , and may extend outward from an end portion of divot  230  closest to low-angle light source  180 . In contrast to divot  230 , regions  200  may be substantially free of shadows and bright spots. As shown in  FIG. 6B , high-angle light  192  directed toward a part  134  containing one or more divots  230  may produce shadows and bright spots that are less pronounced as compared to the divot bright spots  232  and shadows  234  when low-angle light source  180  is employed to illuminate part  134 . Thus, while it may be possible to discern or detect divots  230  with the use of high-angle light  192 , low-angle light  182  may facilitate more accurate identification of one or more divot defects  230 , as can be seen in  FIGS. 7A and 7B . One or more regions  200  may be substantially free of shadows or bright spots. 
     To accomplish this, first the image captured using the image capturing device  174  and low angle light  182  is transformed using a homography matrix to align key points in the image to the actual image sent to the print electronics  126 . The resulting image is then broken down into regions of interest (ROI) that can then be processed sequentially individually for detect, either sequentially, or alternatively in parallel. 
     To detect divots, each ROI is then scanned using a blob detection algorithm. This will provide areas on interest (AOI) to more closely inspect. Each AOI that is darker than the surrounding powder bed is inspected for circularity, inertia, and convexity. Divots, in general, have lower circularity and inertia (&lt;. 8 ). In addition, there will be a bright spot adjacent to the defect. If that spot is further from the side light  182 , it indicates that the defect is below the print surface and is likely a divot. If the bright spot is between the defect and the side light  182 , it indicates a defect that occurs above the print surface, such as smearing. If a significant amount of blistering is found in a region, corrective action may be taken. 
       FIGS. 7A and 7B  are exemplary top views of powder bed  124  from the perspective of image capturing device  174 , according to  FIGS. 6A and 6B , respectively.  FIGS. 7A and 7B  may illustrate an entirety or a portion of the field of view of image capturing device  174 . In one aspect, this field of view may cover an entirety or substantially an entirety of powder bed  124 , or this field of view may cover only a portion of powder bed  124 . 
     In the powder bed  124  shown in  FIG. 7A , a plurality of divots  230  may form one or more defect-containing regions  250 . While divots  230  may form at any location within powder bed  124 , divots  230  may tend to form within a part  134 . Each of these divots  230  may form a divot bright spot  232  and a divot shadow  234 , which are most clearly identified with the use of low-angle light source  180 , as shown in  FIG. 7A . However, as can be seen in  FIG. 7B , each divot  230  may also be observed or detected by dark portions or shadows present in powder bed  124 , although these may be more subtle to detect. In at least some embodiments, one or more divots  230  may be identified based on a ratio of the length to width of a potential divot (e.g., a ratio of the length to width of shadow  234  with or without bright spot  232 ). For example, a divot may be identified based on a ratio (e.g., aspect ratio) of about 1:1 of bright to dark regions. Divots  230  may be identified based on the contrast or sharpness of the potential defect. For example, a divot  230  may be characterized by softer or smoother transitions between dark regions (e.g., divot shadows  234 ) and light regions (e.g., divot bright spots  232 ). By contrast, a smearing defect may have a sharper dividing line (e.g., higher contrast, higher sharpness) between light and dark regions. 
       FIGS. 8A and 8B  illustrate exemplary lighting conditions under low-angle and high-angle light, respectively, for detection of one or more cracking or smearing defects in powder bed  124 .  FIG. 8A  illustrates low-angle light  182  from low-angle light source  180  directed toward a part  134  containing a smear  240 . Each smear  240  may include a raised portion  242  that extends above a level of powder region  200  (which may be substantially free of defects due to powder spreading or binder deposition), and an adjacent recessed portion  244  that extends below the level of the deposited powder region  200 . During the application of low-angle light  182 , each raised portion  242  may reflect an increased amount of light so as to form a smear bright spot  246  and a darkened portion or smear shadow  248  farther from low-angle light source  180 . In one aspect, smear bright spots  246  may be brighter than an average brightness of a top layer of powder bed  124 . Each recessed portion  244  may also form a smear shadow  248 . As can be seen in  FIG. 8B , smears  240  may also form dark portions or shadows  248  in powder bed  124  upon application of high-angle light  192 . Smear shadows  248  may be formed when smears  240  create sufficiently deep cracks. Walls of these cracks may be angled so as to reflect light away from image capturing device  174  or further into the crack, effectively trapping the light within the crack, resulting in a shadow  248 . These smear shadows  248 , formed by the application of high-angle light  192 , may have an appearance similar to a series of cracks and may be thinner as compared to smear shadows  248  produced by low-angle light  182 . One or more regions  200  may be substantially free of shadows or bright spots. Smears  240  may also be identified based on the contrast or sharpness of the potential defect. For example, a smear  240  may have a sharper dividing line (e.g., a high-contrast, high-sharpness line) between dark regions (e.g., smear shadows  248 ) and light regions (e.g., smart bright spots  246 ). Thus, smears  240  may be differentiated from divots  230 , which may have relatively softer or smoother transitions between dark and light regions, as described above. 
     To accomplish this, first the image captured using the image capturing device  174  and low angle light  182  is transformed using a homography matrix to align key points in the image to the actual image sent to the print electronics  126 . The resulting image is thing broken down into regions of interest (ROI) that can then be processed sequentially individually for detect, either sequentially, or alternatively in parallel. 
     To detect smearing, each ROI is then scanned using a blob detection algorithm. This will provide areas on interest (AOI) to more closely inspect. Each AOI that is darker than the surrounding powder bed is inspected for circularity, inertia, and convexity. Smearing, in general, have low circularity and inertia (&lt;0.5). In addition, there will be a bright spot adjacent to the defect. If that spot is between the defect and the side light  182 , it indicates that the defect is above the print surface. If this defect is in an area in the print bed where we expect bound powder, the defect is classified as smearing and corrective action outlined &lt;somewhere else in the document&gt;can be performed. 
       FIGS. 9A and 9B  are exemplary top views from the perspective of image capturing device  174  according to the lighting conditions of  FIGS. 8A and 8B , respectively. Like  FIGS. 7A and 7B ,  FIGS. 9A and 9B  may correspond to an entirety or substantially an entirety of the field of view of image capturing device  174 , or a portion of the field of view. 
     In the exemplary powder bed  124  shown in  FIG. 9A , a plurality of smears  240  may form one or more smear-containing regions  260 . While smears  240  may form at any location within powder bed  124 , smears  240  may tend to form within parts  134 . When smear bright spots  246  are formed by the application of low-angle light  182 , each smear bright spot  246  may be located adjacent to one or more smear shadows  248 . Thus, the presence of smearing may be observed or detected based on the presence of a series of bright portions or bright spots  246  and a series of dark portions or shadows  248 . In at least some embodiments, one or more smears  240  may be identified based on a ratio of the length to width of a potential smear (e.g., a ratio of the length to width of shadow  248  with or without bright spot  246 , or a ratio of the length to the width of bright spot  246 ). For example, a smear may be identified (e.g., by controller  128 ,  128 ′) based on a ratio (e.g., aspect ratio) of about  10 : 1 . The aspect ratio of smear shadows  248  may depend, at least in part, on the angle of the low-angle light. Additionally, the presence of multiple smears  240  in close proximity to one another may cause one smear shadow  248  to interrupt the smear bright spot  246  or smear shadow  248  of an adjacent smear  240 . When this occurs, the aspect ratio of the shadow to the bright spot may not conform to an expected aspect ratio for a smear  240 . Such smears  240  may instead be identified, for example, based the distance between adjacent bright spots  246  and shadows  248 . This distance may be measured along a direction towards the low-angle light source  182 . 
       FIGS. 14A-B  illustrate exemplary lighting conditions under low-angle light source  1401  and high-angle light source  1402 , respectively, for detection of one or more beading defects  1403  in powder bed  1404 .  FIG. 14A  illustrates low-angle light  1405  from low-angle light source  1401  directed toward a part  1406  containing a beading defect  1403  in a region of the deposited binder  1408 . Each beading defect  1403  may include a portion containing a rough or textured region. During the application of low-angle light  1405 , each textured portion of the beading defect  1403  may reflect a decreased amount of light (i.e. a darker region  1406 ) relative to a part without a beading defect, or relative to a region of unbound powder  1407 . The darker region may be substantially uniform in intensity. During the application of high-angle light, a region containing a beading defect  1403  may reflect substantially the same amount of light as a region of a part not containing a beading defect. As can be seen in  14 B, when high-angle light source  1402  is activated and generates high-angle light  1409 , the position of this light source  1402  with respect to beading  1403  may not result in the creation of a dark region in reflected light  1410 , although, high-angle light source  1403  may be used to detect the presence of beading  1403  having certain sizes and/or shapes. Thus, low-angle light  1409  may be particularly suitable for detecting beading dark regions when beading defects are present. 
       FIGS. 15A-B  are exemplary top views from the perspective of image capturing device  1411  according to the lighting conditions of  FIGS. 14A and 14B , respectively. Like  FIGS. 7A and 7B ,  FIGS. 15A and 15B  may correspond to an entirety or substantially an entirety of the field of view of image capturing device, or a portion of the field of view. Each image is of the same print bed in which a series of parts having cubic cross sections approximately 15 mm wide are being fabricated. Certain of the parts have beading defects  1501  whereas others do not. 
     In the powder bed shown in  FIG. 15A , a plurality of beading defects  1501  may form one or more defect-containing regions. Each of these beading defects  1501  may form a dark region, which is most clearly identified with the use of low-angle light source  1401 , as shown in  FIG. 14A . Beading defects  1501  may be identified based on the contrast or sharpness of the potential defect. For example, a beading defect  1501  may be characterized by a uniform brightness within a beading defect region. By contrast, a smearing defect may have a sharper dividing line (e.g., higher contrast, higher sharpness) between light and dark regions. 
     To accomplish this, first the image captured using the image capturing device  174  and low angle light  182  is transformed using a homography matrix to align key points in the image to the actual image sent to the print electronics  126 . This resulting image now has the same scale as the image printed and each pixel of the image represents one jet on the printhead electronics. The resulting image is them broken down into ROI based on image size. 
     Thresholding is then applied locally to each ROI of the image. The image is then compared to the original image. If bound powder is visible in both the tresholded image and the actual image printed, there is beading in the area. If significant beading is present, the process parameters defined below can be applied. 
     Although  FIGS. 3A, 3B, 5A, 5B, 7A, 7B, 9A, 9B, 10A, 10B, 15A-15B , show a single type of defect at a time for the purpose of explanation, an image captured by image capturing device  174  may show multiple defect types in a single image. These defects may be present in any combination and in any location of powder bed  124 . The defect detection system described herein may facilitate the identification of each defect when multiple defect types are present. 
       FIGS. 10A and 10B  are exemplary top views from the perspective of image capturing device  174  showing an exemplary powder bed  124  in which a plurality of parts  134  are being manufactured within binder jet fabrication subsystem  102 ,  102 ′.  FIGS. 10A and 10B  may correspond to powder bed  124  and parts  134 , respectively, produced by adjusting one or more printing parameters, as described below. By adjusting one or more printing parameters in response to the initial detection of any one or more of the printing defects described herein, an entirety of powder bed  124  may be formed with powder regions  200  that are substantially free of defects due to powder spreading or binder deposition or that contain relatively fewer defects. Similarly, each of the parts  134  may be substantially free of defects or may have relatively fewer defects. Additionally, the boundaries between regions of bound powder  142  and unbound powder  140  may be clear and readily detected and observed. 
       FIG. 11  is a flowchart of an exemplary method or process  1100  for defect detection and/or correction (or mitigation). Method  1100  may be repeatedly performed during additive manufacturing with, e.g., binder jet fabrication subsystem  102 ,  102 ′. In an exemplary implementation, method  1100 , as well as methods  1200  and/or  1300  described below, may be performed one or more times during the deposition of a plurality of layers of powder in subsystem  102 ,  102 ′. 
     Method  1100  may include a step  1102  during which a layer of powder is deposited on a powder print bed. This powder may include at least one of a metal, a ceramic, or a polymer. In some aspects, this powder includes metal. This deposition of powder may be performed by distributing and spreading a layer of powder via spreader  122 , as described above. At least partially concurrently and/or subsequently, during a step  1104 , binder material  132  and/or vapor  152  may be selectively applied, e.g., by print head  126 ,  126 ′, to one or more portions of the metal powder that was deposited in step  1102 . 
     Following the completion of one or both of steps  1102  and  1104 , and/or during at least a portion of steps  1102  and  1104 , light may be directed toward powder bed  124 , including both unbound powder  140  and bound powder  142 , in step  1106 . Step  1106  may include application of low-angle light  182 , high-angle light  192 , or both (at the same or different times), in the manner described above. During the application of low-angle light  182  and/or high-angle light  192 , image capturing device  174  may capture an image of a portion or an entirety of powder bed  124 . In one aspect, a plurality of images are captured by image capturing device  174  during step  1106 , including at least one image during the application of low-angle light  182  and at least one image during the application of high-angle light  192 . Thus, step  1106  may be performed once or repeated two or more times to acquire image(s) under one or both lighting conditions. In one aspect, step  1106  may be performed by continuously capturing (e.g., capturing real-time or delayed video) images of at least a portion of powder bed  124 . Exemplary images captured by image capturing device  174  during step  1106  may look, for example, like any of  FIGS. 3A, 3B, 5A, 5B, 7A, 7B, 9A, 9B, 10A , or  10 B. 
     Step  1108  may include analyzing one or more of the image(s) captured during step  1106 . This analysis may include, for example, determining the presence of any of the features associated with one or more potential defects, as described above. For example, step  1108  may include analyzing images to determine whether powder bed  124  contains blisters  210 , missing binder  220 , divots  230 , beading  1501 , and/or smears  240 . This determination may be performed by, e.g., performing a manual observation and analysis of one or more images captured in step  1106 . However, this analysis may be conducted automatically by controller  128 ,  128 ′, or by a separately located processor, for example, with image-analysis software loaded thereon, instead of, or in addition to, manual analysis. Any suitable image-analysis software, such as software for shape recognition, may be employed for image analysis via a controller. 
     Each image may be analyzed in isolation, or may be compared to another image. For example, a pair of images under different lighting conditions, including an image captured during the application of low-angle light  182  and an image captured during the application of high-angle light  192 , may be compared to each other. By performing such a comparison, it may be possible to more accurately determine whether a darkened or brightened portion of the image captured with, low-angle light  182 , for example, is formed by a defect, as opposed to bound powder  142  or unbound powder  140 . Step  1108  may include comparing one or more of the captured images with a design file (e.g., computer-aided design file, three-dimensional model, images, or other data useful for controlling the operation of print heads, such as print head  126 ,  126 ′, or any other suitable file) representative of the part(s)  134 . For example, analysis of such a design file e.g., for one or more particular layers corresponding to the layer deposited in step  1102 , may facilitate the identification of a bright spot or shadow that is produced by a defect. Step  1108  may also include comparing an image of a previous (underlying) layer of powder to an image of the current (or overlying) layer of powder. In one aspect, this comparison may allow the defect detection system to assist with or perform a determination of whether defect(s) have been resolved, severity of defects have been increased or decreased, whether new defect(s) have been created, etc. Moreover, images of a plurality of underlying layers in different lighting conditions may be compared to overlying layer(s) in different lighting conditions. For example, two images captured under low-angle light may be compared, and/or two images captured under high-angle light may be compared to each other. 
     Step  1108  may include analyzing one or more images for blisters  210  by identifying the presence of a bright spot  212  adjacent to a shadow  214 , or the presence of a shadow  214  alone. In particular, step  1108  may include identifying the presence of a bright spot closer to low-angle light source  180  and a shadow  214  farther from low-angle light source  180 , and/or by determining a circularity of a potential defect. In one aspect, a circularity value of between  0 . 5  and  1 . 0  may be indicative of the presence of a blister  210 . Additionally or alternatively, a blister  210  may be identified based on a size of the potential defect. For example, a size, or diameter, in a range of about  0 .1 mm, to about 1.0 mm, may indicate the presence of a blister  210 . Such an image may be analyzed in isolation, may be compared to a corresponding image taken during application of high-angle light  192 , or may be compared to an image taken of an underlying layer. Step  1108  may include analyzing each image (whether taken under low-angle or high-angle light) individually for the presence of one or more defects. In one aspect, image(s) taken under the application of low-angle light may be analyzed for the presence of defects that are readily discernible under low-angle light (e.g., blistering, divots, smears, beading), while image(s) taken under the application of high-angle light may be analyzed for the presence of defects that are readily discernible under high-angle light (e.g., missing binder, divots, smears). Additionally, a plurality of images may be compared to each other during step  1108 . For at least some defects, such as blistering and missing binder, the presence of bright spots and dark spots in an image captured under low-angle light, and the absence of bright spots and dark spots in the corresponding region(s) of an image captured under high-angle light, may be indicative of a defect. For example, when bright spots and dark spots are in an image captured under low-angle light but absent from an image captured under high-angle light, this may be indicative of blistering. 
     Step  1108  may include analyzing one or more images for the presence of missing binder  220 . In one aspect, this analysis may be performed to identify missing binder  220  in one or more images captured during step  1106  when high-angle light  192  was directed toward powder bed  124 . This may include comparing lighter regions of the powder bed (regions that reflect more light) indicative of unbound powder  140  to a data file including information corresponding to the part  134  or the intended location of part  134  on the powder bed. Missing binder  240  may be detected when these lighter portions are present in an area where part  134  should be present. Thus, as illustrated in  FIG. 5B  for example, the presence of one or more portions of unbound powder  140  in a region of part  134  may be identified during step  1108 . 
     Step  1108  may also include analyzing one or more images for the presence of divots  230 . This may be performed by analyzing the image(s) for the presence of a bright spot adjacent to a shadow, as exemplified by divot bright spots  232  and divot shadows  234  shown in  FIG. 7A . As described above, a divot  230  may be identified by the presence of a bright spot  232  closer to low-angle light source  180  as compared to shadow  234 . Thus, the analysis for divots  230  may be performed with an image captured during the application of low-angle light  182 . However, the analysis for divots  230  may be performed with one or more images captured during the application of high-angle light  192 , by identifying one or more dark portions or shadows present in powder bed  124 , as illustrated in  FIG. 7B . 
     Step  1108  may include analyzing one or more images for the presence of smears  240  on powder bed  124 . Smears  240  may be detected by, e.g., analyzing one or more images captured during the application of low-angle light  182  for the presence of a bright spot  246  adjacent to a shadow  248  or between a pair of shadows  248 , and/or a series of bright spots  246  and shadows  248 . Smears  240  may also be detected by determining whether an aspect ratio exceeds a predetermined threshold (e.g., 2:1, 5:1, or 10:1), and/or by evaluating contrast, sharpness, or both, of the potential defect. Step  1108  may also include analyzing one or more images captured with high-angle light  192  for the presence of a dark line or crack formed by recessed portion  244 , as shown in  FIG. 9B . 
     Step  1108  may include analyzing one or more images for the presence of beading  1501  on the powder bed. Beading may be detected by, e.g., analyzing one or more images captured during the application of low-angle light for the presence of dark (I.e. lower reflected brightness) regions partially or completely contained within a region where binder deposition is expected. 
     Step  1108  may also include analyzing the image for other or additional defects. For example, step  1108  may include analyzing one or more images for the presence of bleeding defects, which may occur when binder material  132  spreads beyond the desired area for part  134 . Detection of bleeding defects can be performed by measuring a dimension of a captured image with one or both of high angle lighting and low angle lighting, and comparing to the expected dimension of a printed image based on image data sent to the printheads. 
     Step  1110  may be performed based on the analysis performed in step  1108 , and may include performing a determination of whether any of the above-described defects are present in powder bed  124 . If no defects are detected during step  1108 , the determination in step  1110  may be negative (no), and method  1100  may proceed from step  1110  to step  1102 , in which an additional or subsequent layer of metal powder may be deposited. However, if one or more defects are detected during step  1108 , the determination in step  1110  may be affirmative (yes), and process  1100  may proceed to step  1112 , in which one or more printing parameters of binder jet fabrication subsystem  102 ,  102 ′ may be adjusted to mitigate and/or correct a detected defect. If desired, step  1112  may be omitted from process  1100 . 
     In one aspect, step  1112  may include adjusting one or more of: dispensing and spreading of powder material, application of binder material  132 , or application of vapor  152 . Any suitable adjustments or corrections, including the examples described below (see steps  1216 ,  1218 ,  1310 ,  1312 ,  1316 , and/or  1318 ), may be performed during step  1112 . For example, in response to detection of one or more blisters  210 , a saturation of selective portions of powder bed  124  may be decreased. This may be performed, for example, by reducing an amount, drop size, or drop resolution (deposited drops per inch) of one or both of binder material  132  and vapor  152 . 
     In response to detection of missing binder  220 , print head  126 ,  126 ′ may be moved to a maintenance station that includes equipment for removing material from one or more discharge orifices  130 ,  130 ′. If desired, print head  126 ,  126 ′ may be replaced by a manual or automated process in response to the detection of missing binder  220 . For example, if one or more ejection orifices are identified as being obstructed, clogged, or otherwise not ejecting binder, and these ejection orifices do not recover to their desired function after one or more maintenance cycles (printhead cleaning, purging of binder agent, or other cleaning actions), the print head may be replaced. Additionally or alternatively, in response to detection of missing binder  220 , an amount of deposited vapor  152  may be increased. In one embodiment, an amount of vapor deposited may be increased by approximately 5%. In one embodiment, an increased amount of deposited vapor may be achieved by increasing a pressure of a boiler vessel used for creation of vapor. In another embodiment, vapor deposition amount may be increased. In a printer which prints bi-directionally (that is, spreads powder and binder in both the positive and negative directions of travel), an amount of vapor deposited may be increased in only one direction, or may be increased in both directions. This increase may, for example, be performed for an overall area, or within a local area. Additionally or alternatively, in response to detection of missing binder  220 , an amount of deposited binder  220  may be increased, e.g., by depositing more binder from one or more discharge orifices in the region of the location of a blocked discharge orifice. In one embodiment, an amount of deposited binder may be increased by approximately 5%. In one embodiment, an increase in binder deposition may be accomplished by increasing a greyscale of an image (that is, increasing the number of pixels which are printed in comparison to the number of pixels which are unprinted). In another embodiment, an increase in binder deposition may be achieved by increasing the ejected droplet size or mass by modifying an electrical signal (waveform) sent to a printhead. In another embodiment, an increase in binder deposition may be achieved by increasing the printing resolution (number of pixels per inch) in a printing direction, e.g., by changing a resolution from 1200 pixels per inch to 1300 pixels per inch, which printing droplets of the same size and at the same greyscale. Additionally or alternatively, in response to detection of missing binder  220 , a flow of air to an air knife may be increased (e.g., to facilitate removal of excessively mobile powder, or to facilitate removal of undesirable aerosolized binder from the region of the printhead). 
     Additionally or alternatively, step  1112  may include initiating an alert (e.g., via a displayed warning or audio cue) in response to the detection of a defect. For example, step  1112  may include displaying an indication that a blister, missing binder, divot, smear and/or any other defect was detected on powder bed  124 . This indication may include information regarding the type and location of the defect (e.g., on a part or spaced away from any part), the number of defects, the density of defects, the size of defects, the severity of defects, etc. Thus, the indication may allow an operator to determine whether remedial action is necessary, or whether binder jetting may continue. Step  1112  may include creating a record, e.g., an electronic record, of one or more detected defects. This record may be used, for example, once manufacturing is complete, to provide information about a part, e.g., to assist in determining whether the part is acceptable (pass) or should be rejected (fail), or should be subjected to further inspection prior to a final determination, based on manufacturing tolerances. 
       FIG. 12  is a flowchart illustrating an exemplary process  1200  for defect detection and/or correction. Process  1200  may include steps  1202  and  1204 , which may be performed in the same manner as steps  1102  and  1104  described above. In a step  1206  that may follow step  1204 , light may be directed under a first lighting condition. The first lighting condition may correspond to light generation by only low-angle light source  180 , or only by high-angle light source  190 . During the direction of light from one of light sources  180 ,  190 , step  1208  may including capture one or more first images under this first lighting condition image with capturing device  174 . Accordingly, step  1208  may be performed simultaneously with step  1206 , or step  1208  may occur during performance of step  1206 . 
     Once image capturing under the first lighting condition is complete, light may be directed toward powder bed  124 , including metal powder and binder, in step  1210 . This lighting may create a second lighting condition that is different from the first lighting condition. For example, if low-angle light  182  is used to create the first lighting condition in step  1206 , high-angle light  192  may be used to create the second lighting condition in step  1210 , or vice-versa. In step  1212 , image capturing device  174  may capture one or more second images during the second lighting condition. As discussed above, step  1212  may be performed simultaneously with step  1210 , or step  1212  may occur during performance of step  1210 . 
     In a step  1214 , the images captured in steps  1208  and  1212  may be analyzed for the presence of one or blisters  210  (or bleeding) and/or missing binder  220 . The image analysis may be performed by comparing a plurality of images taken under the first and second lighting conditions, comparing one or more images to data contained in a design file, and/or comparing an image of a layer to an image of an underlying layer. In at least some powder beds  124 , it may be unlikely for a region of powder bed  124  to experience both blistering  210  and missing binder  220 . Rather, a region of powder bed  124  may experience either blistering  210  or missing binder  220 . This is because missing binder  220  may be associated with inadequate deposition of vapor  152  (e.g., insufficient vapor  152  may tend to form overly-mobile powder, which may block one or more discharge orifices  130 , leading to inadequate deposition of binder  220  in subsequent layers), while blistering may be associated with excessive deposition of vapor  152  or binder  220  (e.g., due to air becoming trapped underneath deposited binder  220 ). Thus, step  1214  presents different paths, to steps  1216  and  1218  respectively, for correcting missing binder defects and blistering defects. However, if both missing binder and blistering occur, both steps  1216  and  1218  may be performed, for example, in different regions of the powder bed, as described below. Alternatively, if no defects, or too few defects are detected, then no changes may be made to vapor and/or binder deposition (e.g., neither step  1216  nor  1218  may be performed). In such instance, the method may be repeated again for a subsequent layer. 
     When missing binder  220  is observed or detected in step  1214 , a setting for vapor deposition may be changed. For example, an amount of vapor  152  deposited by vapor deposition device  150  may be increased in step  1216 . In one aspect, a flow rate or quantity (mass) of deposited vapor, e.g., steam, may be set to a larger value from a previous, e.g., default, value. For example, a flow rate of water to a steamer device of vapor deposition device  150  may be set to an increased value in step  1216 . In some aspects, the amount of increase in the flow rate of vapor may be approximately 5% to approximately 10%. Additionally or alternatively, a duration of a deposition of vapor  152  and/or a pressure at which vapor  152  is expelled may be increased (set to increased value(s)) in step  1216 . Step  1216  may include increasing an amount of binder material  132  that is deposited (e.g., by increasing a deposition rate of binder material  132 , either globally or locally by depositing more binder from one or more discharge orifices in the region of the location of a blocked discharge orifice, or by depositing binder from more discharge orifices, or by increasing a resolution of printing in a print direction, e.g. by printing more dots per inch). 
     When blistering (and/or bleeding) is observed or detected in step  1214 , an amount of vapor  152  deposited by vapor deposition device  150  may be decreased in step  1218 . For example, a flow rate of water to the steamer device may be set to a reduced value in step  1216 . This decreased amount of deposited vapor  152  may be achieved by reducing the flow rate or mass of deposited vapor  152 , a duration of vapor  152  deposition and/or a pressure at which vapor  152  is expelled from vapor deposition device  150 . In some aspects, the amount of decrease in the flow rate of vapor may be approximately 5% to approximately 10%. Additionally or alternatively, step  1218  may include decreasing an amount of binder material  132  that is deposited (e.g., by decreasing a deposition rate of binder  132 , either globally or locally by depositing less binder from one or more discharge orifices in the region of the location of a blocked discharge orifice, or by depositing binder from fewer discharge orifices, or by decreasing a resolution of printing in a print direction, e.g. by printing fewer dots per inch). 
     Negative feature printing can be used to ameliorate or eliminate blistering. This process involves intentionally creating a plurality of negatively printed features within at least some of the series of cross-sectional shapes, wherein an amount of binder fluid to be deposited in the negatively printed features is less than an amount of binder fluid to be deposited in a remainder of the cross-sectional shape. The amount of binder fluid to be deposited in the negatively printed features and a size of the negatively printed features may be configured to allow gas to escape from the powder bed. Said differently, in portions of part layer where a continuous cross-section is desired formations of unbound powder are intentionally created. These formations are, however, bounded by jetted powder, preferably on all sides within the layer. The negative feature pattern therefore will be captive within the green part. When the part is sintered, the negatively printed regions are densified along with the remainder of the part, creating the desired cross-section. 
     In another embodiment, when blistering is observed or detected in step  1214 , a negative feature pattern may be added or adjusted while maintaining a constant average amount of binder, e.g. if a circular negative feature pattern is in use, a diameter of a negative feature in the pattern may be increased, allowing for air to escape and preventing formation of trapped air causing blisters. 
     In one aspect, vapor deposition device  150  may have a plurality of orifices for outputting vapor. The analysis of the first and/or second image may result in the identification of blistering  210  or missing binder  220  in one or more regions of powder bed  124 . These regions may tend to extend in one or more lines along the direction of the movement direction of discharge orifice  130  (via print head  126 ) and vapor deposition device  150  ( FIGS. 1B, 1C ). For example, when print head  126  and vapor deposition device  150  move in a horizontal (left to right) direction of  FIGS. 3B and 5B , blisters  210  or missing binder  220  may tend to form in lines along this path. Thus, one or more corrective actions (e.g., steps  1216 ,  1218 ) may be taken with respect to a vapor deposition device  150  associated with the location of the observed blister  210  or missing binder  220  defect. One of these corrective actions may be taken for a first region, while the other action may be taken for a second region. In some aspects, the corrective actions may be implemented across the entire powder bed  124 . These corrective actions may be performed, for example, when depositing vapor in step  1204  on one or more subsequent layers of metal powder. In a printer which prints bi-directionally (that is, spreads powder and binder in both the positive and negative directions of travel), an amount of vapor deposited may be increased in only one direction, or may be increased in both directions. For beading defects, the vapor deposition may be the adjusted similarly to the methods discussed below regarding missing binder. Alternatively or in addition, the vapor deposition device  150  may be subjected to an inspection and/or cleaning maintenance cycle. 
       FIG. 13  is a flowchart illustrating an exemplary process  1300  for defect detection and/or correction (or mitigation). Steps  1302  and  1304  of process  1300  may be performed in the same or similar manner as steps  1102 ,  1104 ,  1202 ,  1204 , as described above. During a step  1306 , light may be directed toward powder bed  124  in a manner similar to step  1106  to create a first lighting condition or a second lighting condition, as described above. Step  1306  may include capturing an image during the first or second lighting condition. In one aspect, image capturing may be repeated to ensure that at least one image is captured at each lighting condition. 
     Step  1308  may include analyzing the captured image(s) for divot defects  230  and/or smear defects  240  (e.g., by analyzing the captured image(s) for the presence of shadows and/or bright spots, as described above), and determining whether divots  230  and/or smears  240  are present in powder bed  124 . This may be performed in any suitable manner, for example, as described above with respect to step  1108 . 
     Step  1308  may include analyzing the captured image(s) for smear defects  240  by determining the presence of one or more smear bright spots  242  or one or more smear shadows  248 . For example, an image under the first lighting condition may be analyzed for the presence of smear shadows  248 , alone (e.g., for images taken under high-angle light) or in combination with smear bright spots  246  (e.g., for images taken under low-angle light). Additionally or alternatively, an image under the second lighting condition may be analyzed for the presence of cracks or darkened portions, as shown in  FIG. 9B . Step  1308  may also include comparing a plurality of images taken under the first and second lighting conditions, comparing one or more images to data contained in a design file, and/or comparing an image of a layer to an image of an underlying layer, as described above with reference to method  1100 . 
     Step  1308  may include analyzing an image for the presence of one or more divot bright spots  232  and one or more divot shadows  234 . In one aspect, an image captured under the first lighting condition (e.g., low-angle light  182 ) may be analyzed for the presence of a divot shadow  234  closer to a low-angle light source  180  as compared to a divot bright spot  232 . If desired, an image captured under the second lighting condition (e.g., high-angle light  192 ) may be analyzed for the presence of one or more darkened portions indicative of divot shadows  234  (see  FIG. 7B ). 
     When one or more smears  240  are detected in step  1308 , a step  1310  may be performed to correct (or mitigate) the occurrence of smear defects  240 . In an embodiment, Step  1310  may include decreasing a flow rate of powder (powder metering) from hopper  121  ( FIG. 1C ), for example decreasing a metered amount of powder by an amount between 5% and 10%. In a bed-to-bed binder jet printing system as described in  FIG. 1   b,  a decreased powder metering amount may be achieved by decreasing an the amount of powder delivered from the powder supply. In a binder jet printing system as described in  FIG. 1   c,  a decreased powder metering amount may be achieved by decreasing the dispensing rate from the metering hopper. In an embodiment, the dispensing rate from a metering hopper may be decreased by decreasing the rotation rate of one or more dispensing rollers; in another embodiment, the dispensing rate from a metering hopper may be decreased by decreasing a vibration frequency or amplitude. Based on the location of the smear defects  240  detected in step  1308 , a metering rate may be decreased at every location along the powder bed in the direction of travel. In an embodiment, based on the location of smear defects  240  detected in step  1308 , a metering rate may also be decreased in only a portion of the bed, for example in a region or regions of the bed where smearing is detected. For example, if smear defects  240  are detected on parts at the trailing edge of the powder bed (i.e. the last part of the powder bed traversed by the metering hopper), the metering rate may be decreased when the metering hopper is traversing the trailing edge of the powder bed, or in a region a distance, for example, 20 mm to 50 mm, ahead of the region where the smearing defects are detected. In another embodiment, step  1310  may include decreasing a speed at which powder is spread (e.g., a speed of translation of one or more spreaders  122 ,  122 ′), and/or increasing a roller speed (e.g., a speed of rotation of one or more spreaders  122 ,  122 ′), for example increasing a roller speed by between 5% and 10%. Thus, step  1310  may cause a decrease in the deposition rate of powder. Additionally, step  1310  may include changing (e.g., increasing) a vapor (e.g., steam) deposition rate. In an embodiment, a vapor deposition rate may be increased by between 5% and 10% by increasing a flow rate or a pressure. These changes may be set or stored in a memory of controller  128  and enacted during a subsequent performance of step  1302 . Thus, it may be possible to identify and correct a smearing defect during binder-jetting, thereby preventing a failure in the production of part  134 . Step  1310  may further include changing a size of a raft (i.e. sacrificial printed component) manufactured below a bottom layer of part  134 . For example, step  1310  may include increasing a thickness of such a raft, for example increasing a raft thickness by 1 mm; and/or decreasing an offset (distance) between the part  134  and the raft. By adjusting a raft in this manner, it may be possible to improve the ability of the raft to provide an anchor to part  134  during the binder deposition process, improving the ability of the part  134  to resist forces imparted by, e.g., spreader  122 ,  122 ′. In some aspects, when producing a raft by binder jetting, it may be possible to detect the occurrence of smearing within the raft. Thus, by increasing a thickness of the raft and/or decreasing an offset between the part  134  and the raft, smearing may be prevented or mitigated in part  134 . In some aspects, detection of smearing within a raft may be used as feedback to adjust a raft thickness when forming a subsequent part. Thus, occurrence of smearing may be reduced or eliminated when producing the raft or when performing a subsequent binder-jetting operation. In an embodiment, a raft may be printed continuously until no smearing defects are detected, at which point a separation layer may be printed followed by printing a part. In this manner a part with no smearing defect can be produced. 
     When one or more divots  230  are detected in step  1308 , a step  1312  may be performed to correct (or mitigate) the occurrence of divots  230 . Step  1312  may include adjusting one or more settings related to the deposition and spreading of powder. In one embodiment, step  1312  may include for example, increasing a metered amount of powder by an amount between 5% and 10%. In a bed to bed binder jet printing system as described in  FIG. 1   b,  an increased powder metering amount may be achieved by increasing an upward increment amount of the Feed Bed. In a binder jet printing system as described in  FIG. 1   c,  an increased powder metering amount may be achieved by increasing the dispensing rate from the metering hopper. In an embodiment, the dispensing rate from a metering hopper may be increased by increasing the rotation rate of one or more dispensing rollers; in another embodiment, the dispensing rate from a metering hopper may be increased by increasing a vibration frequency or amplitude. Based on the location of the divots  230  detected in step  1308 , a metering rate may be increased at every location along the powder bed in the direction of travel. In an embodiment, based on the location of divots  230  detected in step  1308 , a metering rate may also be increased in only a portion of the bed, for example in a region or regions of the bed where divots are detected. For example, if divots  230  are detected on parts at the trailing edge of the powder bed (i.e. the last part of the powder bed traversed by the metering hopper), the metering rate may be increased when the metering hopper is traversing the trailing edge of the powder bed, or in a region a distance, for example, 20 mm to 50 mm, ahead of the region where the divots are detected. In another embodiment, step  1312  may include decreasing a speed at which powder is spread (e.g., a speed of translation of one or more spreaders  122 ,  122 ′), and/or increasing a roller speed (e.g., a speed of rotation of one or more spreaders  122 ,  122 ′), for example increasing a roller speed by between 10% and 20%. The ratio of the speed of translation to the speed of rotation of spreaders  122 ,  122 ′ may be modified to increase a rate at which powder is deposited, for example increasing a number or rotations per distance of roller translation by between 10% and 20%. These changes may be set (changed) and stored in a memory of controller  128  and employed during a subsequent performance of step  1302 . Thus, it may be possible to identify and correct a divot defect during binder-jetting, thereby preventing a failure in the production of part  134 . In another aspect, step  1312  may include changing an environmental condition in the printing environment. For example, a relative humidity (or dewpoint, or other equivalent measurement of moisture contained in a printing environment) in a printing environment may be increased by between 2% and 10% decrease in relative humidity, by means of a humidifier (e.g. a nebulizing type humidifier, or a vapor-type humidifier, or any other suitable device for adding moisture to a gas in order to increase the relative humidity); or by means of decreasing a flow rate of a process gas (e.g. nitrogen gas, argon, compressed dry air). In another embodiment, relative humidity may be decreased, for example by between 2% and 10% relative humidity, by means of increasing a flow rate of a process gas (e.g. nitrogen gas, argon, compressed dry air), or by decreasing a flow rate of a humidifier. In another aspect, step  1312  may include changing powder processing for subsequent operations of binder jet fabrication subsystem  102 ,  102 ′, or subsequent layers of powder deposited in the same operation. For example, powder may be treated to provide the powder with an increased flowability in the same or subsequent manufacturing operation, for example by increasing a duration of a heating step, or by increasing a temperature of a heating step. By employing any or all of these actions, the occurrence of divots may be reduced or eliminated. 
     While steps  1310  and  1312  are illustrated as alternative paths in process  1300 , when both divots  230  and smears  240  are detected during step  1308 , two or more corrective actions of steps  1310  and  1312  may be performed. For example, powder spreading speed may be decreased and/or a raft may be modified to address both smearing and divot defects. Or, different corrective measures may be taken in different regions of the powder bed depending on where the respective type of defect is occurring. 
     Step  1314  may include analyzing the captured image(s) for the presence of blisters  210  or missing binder  220  in the same manner described above with respect to steps  1108  or  1214 . When missing binder  220  is detected, step  1316  may be performed in the same manner as step  1216 , as described above. Similarly, when one or more blisters  210  are detected in step  1308 , step  1318  may be performed in the same manner described above with respect to step  1218 . Moreover, when both missing binder and blistering are detected (e.g., in different regions of powder bed  124 ), both steps  1316  and  1318  may be performed. For example, steps  1316  and  1318  may be performed at different regions of binder jet fabrication subsystem  102 ,  102 ′. Thus, blistering and missing binder defects may be mitigated and corrected during a manufacturing process, which may prevent a failed production of one or more parts  134 . If no defects are detected in steps  1308  and  1314 , the process may return to step  1302  without making adjustments or corrections to address a defect in powder bed  124 . While step  1314  for determining the presence of blistering or missing binder is depicted as being performed after step  1308  in  FIG. 13 , step  1314  may be performed simultaneously with step  1308 , or these steps may be performed in any sequence. 
     While some aspects, including methods  1100 ,  1200 , and  1300 , may describe use of a metal powder, it is understood that the powder may include one or more polymer or ceramic materials, instead of, or in addition to, metal. 
     Other embodiments of the present disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the embodiments disclosed herein. While certain features of the present disclosure are discussed within the context of exemplary systems, devices, and methods, the disclosure is not so limited and includes alternatives and variations of the examples herein according to the general principles disclosed. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the present disclosure being indicated by the following claims.