Abstract:
The invention relates to apparatus and methods for producing three-dimensional objects and auxiliary systems used in conjunction with the aforementioned apparatus and methods. The apparatus and methods involve continuously printing radially about a circular and/or rotating build table using multiple printheads. The apparatus and methods also include optionally using multiple build tables. The auxiliary systems relate to build material supply printhead cleaning diagnostics, and monitoring operation of the apparatus.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application incorporates by reference, and claims priority to and the benefit of, U.S. Provisional Patent Application Ser. No. 60/472,922, which was filed on May 23, 2003. 

   FIELD OF THE INVENTION 
   The present invention relates to apparatus and methods for creating three-dimensional objects by printing. 
   BACKGROUND 
   Generally, 3D printing involves the use of an inkjet type printhead to deliver a liquid or colloidal binder material to layers of a powdered build material. The printing technique involves applying a layer of a powdered build material to a surface typically using a roller. After the build material is applied to the surface, the printhead delivers the liquid binder to predetermined areas of the layer of material. The binder infiltrates the material and reacts with the powder, causing the layer to solidify in the printed areas by, for example, activating an adhesive in the powder. The binder also penetrates into the underlying layers, producing interlayer bonding. After the first cross-sectional portion is formed, the previous steps are repeated, building successive cross-sectional portions until the final object is formed. See, for example, U.S. Pat. Nos. 6,375,874 and 6,416,850, the disclosures of which are incorporated herein by reference in their entireties. 
   Apparatus for carrying out 3D printing typically move the printheads over the print surface in raster fashion along orthogonal X and Y axes. In addition to the time spent printing, each printhead move requires time for acceleration, deceleration, and returning the printhead to the starting position of the next move. The inefficiencies inherent in these reciprocating motions reduce the productivity of the 3D printing process. 
   It is, therefore, an object of the present invention to provide apparatus and methods for continuously and efficiently performing 3D printing. 
   SUMMARY 
   Generally, the invention relates to apparatus and methods for producing three-dimensional objects, such as casting cores, toys, bottles, cans, architectural models, automotive parts, molecular models, models of body parts, cell phone housings, and footwear, more rapidly and efficiently than heretofore achievable. Additionally, the invention relates to systems and methods for maintaining and operating the aforementioned apparatus. In particular, if a user wants to produce large volumes of three-dimensional objects rapidly, a 3D printing apparatus in accordance with the invention can achieve a high throughput by continuously printing, using multiple printheads. 
   In one aspect, the invention relates to an apparatus for fabricating a three-dimensional object from a representation of the object stored in memory. The apparatus includes a rotary build table for receiving successive layers of a build material and an array having at least one printhead disposed above the build table. In one embodiment, the rotary table rotates continuously. 
   In another aspect, the invention relates to an apparatus for fabricating a three-dimensional object from a representation of the object stored in memory. The apparatus includes a generally circular build table for receiving successive layers of a build material and an array having at least one printhead disposed above the build table and movable relative to the build table. In one embodiment, the generally circular build table is movable in a vertical direction. In various embodiments, the printhead is movable over at least a portion of a build surface defined by the generally circular build table and the printhead can move continuously about the build table. In one embodiment, the array is configured to dispense fluid at substantially any radial location of the build table by moving the array radially to the desired location. 
   In yet another aspect, the invention relates to a method of fabricating a three-dimensional object. The method includes the steps of depositing successive layers of a build material on a rotary build table and depositing a liquid in a predetermined pattern on each successive layer of the build material to form the three-dimensional object. In various embodiments, the method includes the steps of: rotating the build table continuously, distributing the build material over at least a portion of the build table with a spreader, measuring an amount of excess build material deposited on the build table, and adjusting the amount of build material deposited on the build table based on the amount of excess build material measured. Additionally, the liquid can be deposited by an array of one or more printheads. 
   In still another aspect, the invention relates to a method of fabricating a three-dimensional object. The method includes the steps of depositing successive layers of a build material on a generally circular build table and depositing a liquid in a predetermined pattern on each successive layer of the build material to form the three-dimensional object. In various embodiments, the liquid is deposited by an array of at least one printhead and the printhead is movable over at least a portion of a build surface defined by the generally circular build table. In addition, the printhead can move continuously about the build table and the build table can move in a vertical direction. 
   In various embodiments of the foregoing aspects, the apparatus includes a build material delivery system. The system includes a storage means for holding the build material and a conveying means for delivering the build material to the build table. In one embodiment, the storage means includes at least two storage chambers for holding at least two build material components separate from each other and the system further includes a blender for mixing the build material components in a predetermined ratio for delivery to the build table. In addition, the apparatus can include a spreader for distributing the build material over at least a portion of the build table. The spreader can be a counter-rotating roller, and the counter-rotating roller can be skewed with respect to a radius of the rotary build table to induce excess build material to migrate over an edge of the build table. 
   In additional embodiments, the apparatus can include a sensor disposed below an edge of the build table to detect an amount of the excess build material. An amount of build material delivered to the build table can be adjusted in response to the amount of excess build material detected. In one embodiment, the sensor can automatically monitor printhead condition, and the apparatus can automatically modify its operation in response to a signal from the sensor. In one example, printhead cleaning is initiated if print quality is inadequate. In another example, the apparatus can utilize the redundant printheads in areas where the printing coverage is inadequate. 
   In other embodiments, the array can include a plurality of printheads disposed above the build table. In one embodiment, the array is configured to dispense fluid at substantially any radial location of the rotary build table without adjustment. In another embodiment, the array prints an entire surface of the build table by continuous consecutive radial scanning motions. In addition, the array can be adjusted incrementally radially and/or can be displaced from a normal printing position for servicing. Further, the array can be displaced radially with respect to the rotary build table. The array can include redundant printheads. 
   In further embodiments, the apparatus defines an opening for removing the three-dimensional object. In one embodiment, the three-dimensional object is removed through a top opening of the build table. Additionally, the apparatus can include a sensor to monitor at least one performance characteristic of the apparatus, such as print quality, printing errors, print speed, printhead condition, build material quantity, and table position. In one embodiment, the array is movable in response to a signal from the sensor. The apparatus can also include a plurality of rotary build tables. 
   In still other embodiments, the invention can include methods and apparatus for cleaning the printheads of the apparatus. Methods of cleaning the printhead can include wiping the printhead with a roller including a cleaning fluid, drawing a vibrating member across the printhead, drawing a cleaning fluid across the printhead by capillary action through a wick, and/or combinations thereof. In addition, the methods can include optionally the step of applying a vacuum to the printhead to remove debris. The apparatus for cleaning a printhead used in a 3D printer can include a wick disposed adjacent the printhead for drawing a cleaning fluid across the printhead. 
   In another aspect, the invention relates to an apparatus for cleaning a printhead used in a 3D printer. The pressure in the interior of a printhead is typically lower than atmospheric pressure. This negative pressure is balanced by the surface tension of the meniscuses that form over the outlets of the printhead nozzles. It is desirable to flush the accumulated powder off the face of the printhead with a clean wash solution without allowing the solution to be drawn into the printhead when the meniscuses are destroyed. This goal is achieved in this apparatus by maintaining an environment outside the printhead in which the pressure is lower than the pressure inside the head. In addition, this induced pressure differential causes binder to flow out of the heads through the nozzles, flushing out any powder that may have lodged in the nozzle passageways. The apparatus includes a base, a cam track disposed within the base, a cap carrier slidably engaged with the cam track, and a sealing cap defining a cavity and disposed on the carrier. The cap being transportable into engagement with the face of the printhead by the carrier. In various embodiments the apparatus includes a cleaning fluid source in communication with the cap for cleaning the printhead face and a vacuum source in communication with the cap for removing used wash fluid and debris. 
   In further embodiments, the apparatus can also include a spring coupled to the carrier and the base to bias the carrier into a receiving position for receiving the printhead. In one embodiment, the carrier includes a stop disposed on a distal end of the carrier for engaging the printhead as the printhead enters the apparatus. The printhead slides the carrier rearward along the cam track after engaging the stop and until the printhead face and cap sealably engage. In a further embodiment, the apparatus includes a latch pawl coupled to the base for engaging with the carrier to prevent forward movement of the carrier and a squeegee disposed on a proximal end of the carrier. The squeegee is positioned to engage the printhead face as the printhead exits the apparatus. 
   In still another aspect, the invention relates to a method of cleaning a printhead used in a 3D printer. The method includes the step of receiving the printhead within an apparatus that includes a base, a cam track disposed within the base, a cap carrier slidably engaged with the cam track, and a sealing cap defining a cavity and disposed on the carrier. Additional steps include engaging the face of the printhead with the cap, drawing a vacuum on the cavity, and introducing a cleaning fluid into the cavity and into contact with the printhead face. In one embodiment, the method includes the step of removing the cleaning fluid from the cavity. The method can further include disengaging the cap from the printing surface and wiping the printing surface with a squeegee as the printhead is withdrawn from the apparatus. 
   In another aspect, the invention relates to an apparatus for cleaning or reconditioning a printhead. The apparatus includes a nozzle array for spraying a washing solution towards a face of a printhead and a wicking member disposed in proximity to the printhead face for removing excess washing solution from the printhead face. 
   In various embodiments, the nozzle array includes one or more individual nozzles. The wicking member and the printhead are capable of relative movement. A fluid source can also be included in the apparatus for providing washing solution to the nozzle array under pressure. In another embodiment, the wicking member includes at least one of a permeable material and an impermeable material. 
   The nozzle array can be positioned to spray the washing solution at an angle with respect to the printhead face. In another embodiment, the wicking member is disposed in close proximity to the printhead face, without contacting print nozzles located on the printhead face. The spacing between the wicking member and the print nozzles can be automatically maintained. In one embodiment, the spacing is maintained by causing a portion of the wicking member to bear on the printhead face in a location removed from the print nozzles. The apparatus can also include a basin for collecting washing solution and debris. 
   In another aspect, the invention relates to a method of cleaning or reconditioning a printhead. The method includes the steps of positioning a face of the printhead relative to at least one nozzle and operating the at least one nozzle to spray washing solution towards the printhead face. Excess washing solution is then removed from the printhead face by passing a wicking member in close proximity to the printhead face, without contacting the printhead face. 
   In one embodiment, the step of operating the at least one nozzle includes spraying the washing solution at an angle to the printhead face. In another embodiment, the method can include the step of operating the printhead to expel washing solution ingested by the printhead during cleaning. The method can include automatically maintaining a space between the wicking member and print nozzles located on the printhead face by, for example, causing a portion of the wicking member to bear on the printhead face in a location removed from the print nozzles. 
   These and other objects, along with advantages and features of the present invention herein disclosed, will become apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, like reference characters generally refer to the same parts throughout the different views. In addition, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
       FIG. 1  is a schematic top perspective view of one embodiment of an apparatus for 3D printing in accordance with the invention; 
       FIG. 2  is an enlarged schematic side perspective view of the apparatus of  FIG. 1 ; 
       FIG. 3  is an enlarged schematic perspective view of a portion of the apparatus of  FIG. 1 ; 
       FIG. 4  is a schematic top view of the apparatus of  FIG. 1  illustrating the spreader apparatus; 
       FIG. 5A  is a schematic partial cross-sectional view of the apparatus of  FIG. 1  taken at line  5 A- 5 A in  FIG. 4 ; 
       FIG. 5B  is an enlarged schematic perspective view of an overflow sensor in accordance with the invention; 
       FIG. 6A  is a schematic perspective view of one embodiment of a system for 3D printing including a 3D printing apparatus and a build material delivery system in accordance with the invention; 
       FIG. 6B  is a schematic perspective view of an alternative embodiment of a system for 3D printing including a 3D printing apparatus and a build material delivery system in accordance with the invention; 
       FIG. 7A  is a schematic perspective view of one embodiment of an apparatus for 3D printing in accordance with the invention with a build drum partially cut-away; 
       FIG. 7B  is a schematic perspective view of the apparatus of  FIG. 7A  with a portion of the build material removed from the build drum; 
       FIG. 8A  is an enlarged schematic perspective of one embodiment of a printbar assembly including a print diagnostic station in accordance with the invention; 
       FIG. 8B  is a schematic representation of the diagnostic station of  FIG. 8A ; 
       FIGS. 9A-9J  are schematic representations of one embodiment of an apparatus and method for cleaning a printhead in accordance with the invention; 
       FIG. 10  is a schematic representation of one step of the method of cleaning a printhead depicted in  FIGS. 9A-9J ; 
       FIG. 11  is a schematic perspective view of an alternative embodiment of a printhead cleaning station in accordance with the invention; 
       FIGS. 12A-12C  are schematic side and perspective views of a printhead being cleaned at the cleaning station of  FIG. 11 ; 
       FIGS. 13A-13D  are schematic perspective views of another alternative embodiment of a printhead cleaning station in accordance with the invention; 
       FIGS. 14A-14D  are schematic representations of one embodiment of a radial printing process in accordance with the invention; and 
       FIGS. 15A and 15B  are schematic top views of an alternative embodiment of an apparatus for 3D printing in accordance with the invention. 
   

   DETAILED DESCRIPTION 
   Embodiments of the present invention are described below. It is, however, expressly noted that the present invention is not limited to these embodiments, but rather the intention is that variations, modifications, and equivalents that are apparent to the person skilled in the art are also included. 
     FIGS. 1-3  depict an apparatus  10  for 3D printing. The apparatus  10  produces three-dimensional objects by depositing alternating layers of build material and binder on a build surface or in a container to print multiple layers that ultimately form the three-dimensional object. The apparatus  10  includes a rotary build table, in this case a build drum  12 , a structural frame  14 , a base  16 , at least one printbar assembly  18 , a powdered build material dispenser assembly  20 , and a spreader assembly  22 . In the embodiment shown, the apparatus  10  includes two printbar assemblies  18 A,  18 B. The apparatus  10  further includes a component-mounting surface  26  attached to the frame  14 . In one embodiment, the component mounting surface  26  may be movable to provide access to the build drum  12 . The various assemblies  18 ,  20 ,  22  are typically mounted to the component mounting surface  26  and/or the frame  14 . It is generally advantageous, for maintenance purposes, for the assemblies  18 ,  20 ,  22  to be stationary and the build drum  12  to rotate. For example, with redundant stationary printbar assemblies  18 , a user can change out one printbar assembly  18  while the other printbar assembly  18  continues to operate. In addition, the apparatus  10  can include essentially any number of printbar assemblies  18  mounted in a variety of configurations for accomplishing printhead redundancy, increasing print speeds, and/or printing multiple colors. 
   The build drum  12  shown is generally cylindrical in shape and is mounted about a center shaft  28  attached to the base  16  and the frame  14 . A bottom surface  17  of the build drum  12  may be substantially perpendicular to a sidewall  19  of the build drum  12 , or the bottom surface  17  can be angled. For example, the bottom surface  17  may be conical, such that the surface tilts toward a center point of the build drum  12 . The tilt may be from about 1 degree to about 15 degrees or more. In such an arrangement, the dispenser, the spreader, and the printbars should be slanted to correspond to the angle of tilt. 
   In a particular embodiment, the build drum  12  is mounted on a rotary actuator  29  that rotates the build drum  12  about the center shaft  28 . The rotary actuator  29  could be hydraulically, pneumatically, or electrically driven. The rotary actuator  29  can include gears and belts for driving the build drum  12 . In addition, the rotary actuator  29  may include one or more encoders  46 , or similar devices, that cooperate with a controller to monitor and adjust the speed and/or position of the build drum  12 . The encoders  46  can also be used to control the firing of the printheads  48 , such that the printheads  48  print accurately and repeatedly, regardless of variations in the rotational speed of the build drum  12 . 
   The build drum  12  receives build material from the build material dispenser assembly  20  that is located adjacent to the build drum  12 . In particular, the build material dispenser assembly  20  is mounted above the build drum  12  and dispenses build material onto the build drum  12  as it rotates. Typically, the build material dispenser assembly  20  deposits a predetermined amount of material onto the build drum  12  in the form of a line substantially along a radius of the build drum  12 . Alternatively, the build material dispenser assembly  20  could include nozzles for spraying the material onto the build drum  12 . In addition, the build material dispenser assembly  20  could include a volumetric adjuster, for manually or automatically adjusting the amount of material being deposited. The build material dispenser assembly  20  is supported on the component-mounting surface  26 . In one embodiment, the build material dispenser assembly  20  may be supplied by a larger dispenser assembly located remotely from the apparatus  10  (see  FIGS. 6A and 6B ). Further, the build material dispenser assembly  20  may include an agitator to maintain the build material in a loose powder form. 
   Located adjacent the build material dispenser assembly  20  is the spreader assembly  22 . The spreader assembly  22  spreads the build material uniformly across the build drum  12  as it rotates. The spreader assembly  22  is shown in greater detail in  FIG. 3 . The spreader assembly  22  includes a counter-rotating spreader roll  52  that spreads the build material radially across the build drum  12 , thereby forming a build surface  24 . The spreader assembly  22  also includes a roll scraper  54  that removes build material that may become stuck to the roll  52 . The spreader assembly  22  is also mounted on the component-mounting surface  26 . 
   The operation of the build drum  12  varies in different embodiments to accommodate the multiple layers of build material. For example, in one embodiment, the build drum  12  moves downwardly relative to the assemblies  18 ,  20 ,  22  mounted on the component mounting surface  26 . In a particular embodiment, at least a portion of the center shaft  28  and the build drum  12  are threaded and the build drum  12  threadedly engages the center shaft  28 . As the build drum  12  rotates, it moves down the center shaft  28 . In another embodiment, as shown in  FIGS. 2 and 5A , the build drum  12  includes a bottom surface  17  that moves downwardly relative to the build drum  12  to continuously receive layers of build material. The bottom surface  17  is moved vertically by one or more linear actuators  191 . The linear actuators could be hydraulically, pneumatically, or electrically driven. In yet another embodiment, the assemblies  18 ,  20 ,  22  move upwardly relative to the build drum  12  and the build surface  24 . 
   It is advantageous for a user to be able to remove finished parts without stopping the printing process, therefore, the build drum  12  may include structure for facilitating removal of completed parts. In one example, the build drum  12  includes an opening in its bottom or side surface that allows for removal of the parts from the bottom and/or side, while the apparatus  10  continues to print above. In this example, the apparatus  10  may print a bottom plate covering essentially the entire build surface  24  before printing any parts. The bottom plate(s) would separate the layers of printed parts to prevent the inadvertent removal of build material or unfinished parts. Alternatively, the user could stop the printing process and remove the parts manually from the top, bottom, or side (see  FIGS. 7A and 7B ). 
   As shown in  FIG. 4 , the spreader assembly  22  is disposed slightly non-radially, with respect to the build drum  12 . The build material dispenser assembly  20  deposits a substantially radial line of material in front of the spreader assembly  22  as the build drum  12  rotates (arrow  44 ). The apparatus  10  can be configured to operate with the drum  12  rotating in a counter-clockwise direction when viewed from the top as illustrated or clockwise in a mirror image of the configuration shown. The non-radial spreader assembly  22  spreads the material, forcing the excess material to migrate towards a center opening  56  in the build drum  12 . The excess material falls into an overflow tray  68  (see  FIGS. 1-2 ) located beneath the build drum  12 . In one embodiment, the apparatus  10  is configured to reclaim the excess material for later use. In the embodiment shown, the apparatus  10  includes an overflow sensor  58 . The sensor  58  monitors the amount of excess material falling through the center opening  56 . The sensor  58  sends a signal to the apparatus controller indicative of the amount of excess material measured. The apparatus  10  can, in response to the signal, adjust the amount of material dispensed by the build material dispenser assembly  20 . 
   The sensor  58  is shown in greater detail in  FIGS. 5A and 5B .  FIG. 5A  depicts the general location of the sensor  58  on the apparatus  10 . The sensor  58  is disposed within the center opening  56  and is mounted to the non-rotating center shaft  28 .  FIG. 5B  is an enlarged view of the sensor  58 . The sensor  58  includes a shaft  66  for mounting the sensor  58  to the center shaft  28 . At a distal end of the shaft  66  is a paddlewheel assembly including a magnetic sensor  60  and a series of magnets  62  located on individual legs  64  of the paddlewheel  65 . As excess material falls, it impinges on the legs  64 , causing the paddle wheel  65  to rotate. The speed and/or period of rotation can be used to ascertain the amount of excess material being deposited, which can be adjusted accordingly. Alternatively, other types of sensors or more than one sensor can be used. 
   Referring back to  FIGS. 1-3 , two printbar assemblies  18 A,  18 B are shown disposed about the apparatus  10 . Each printbar assembly  18  includes a printhead carrier  42 , for carrying at least one printhead  48 , a service station  34 , a printhead diagnostics station  38 , a printbar motor  36 , a printbar cable guide  32 , and a printbar slide  30 . One of the two assemblies  18 A,  18 B can be redundant to the other. Alternatively, many more printbar assemblies  18  could be included on the apparatus  10 . The printbar cable guide  32  guides and secures the electrical connections to the printheads  48 . The printbar slide  30  is attached to the component-mounting surface  26  and supports the printhead carrier  42 , the service station  34 , the printhead diagnostics station  38 , and the printbar motor  36 . The print bar motor  36  can be a servo type motor, used to radially move the printbar assembly  18  relative to the build drum  12  along the slide  30 . It is generally advantageous to use a positioning system capable of accurate and repeatable control, because this directly influences the accuracy of the objects being produced. The printhead carrier  42  is radially movable to position the printheads  48  for printing and for performing service on the printheads  48 . 
   The printhead carrier  42  can be moved along a radius of the build drum  12  to correct for deficiencies in print quality. For example, the printhead carrier  42  supports a printhead array  40 , which may include any number of printheads  48 , for example a single printhead  48  or eight rows of six printheads  48 . The printhead array  40  may include redundant printheads  48 , which compensate for the deficiencies in print quality. The printheads  48  can be commercially available inkjet type printheads or custom manufactured printheads to suit a particular application. The printheads  48  include multiple jets, for example 512 jets, each jet for depositing a drop of binder onto the build surface  24 . 
   The printheads  48  can be moved incrementally back and forth along the radius in a “shingling” fashion to compensate for irregularities in printing, for example, if some jets are not working, misfire, or are out of alignment. Shingling allows the apparatus  10  to produce stronger parts, because printing errors are averaged out. For example, shingling reduces the affect of jets that are not printing properly by offsetting the jets by a small amount such that any line of unprinted build material caused by a missing jet is in a different location on each print layer. Shingling can be carried out in various ways, for example, in response to an error message or the apparatus  10  can be programmed to continuously shingle by moving the printheads  48  in and out along the radius a random distance between the printing of each layer. Alternatively, the apparatus  10  can be programmed to run a printing routine, where the printheads  48  are moved a set distance for a specific number of print layers and then reset to a starting position. For example, the printheads  48  can be moved out along the radius 1/16″ for each print layer until the printheads  48  have been moved a total of ¼″. Then, the printheads  48  can be moved back in along the radius to their starting position or be moved back incrementally. Therefore, the apparatus  10  is printing over the same areas with different printheads  48  to average out any errors. 
     FIGS. 14A-14D  depict generally a radial scanning print process, where a printhead array moves continuously in and out along a radius of a build drum, as the build drum rotates continuously. In such a process, the printhead array scans an entire build surface of the 3D printer.  FIG. 14A  is a schematic isometric view of a 3D printer  200  in accordance with the invention. The 3D printer  200  is similar to the 3D printer  10  previously described with respect to  FIGS. 1-3 . The 3D printer  200  includes a build drum  212  and two printbar assemblies  201 A,  201 B. Each printbar assembly  201 A,  201 B includes a printhead array  202 .  FIG. 14B  is a schematic top view of the 3D printer  200  of  FIG. 14A . The printbar assemblies  201 A,  201 B include printhead carriers  203  that move in and out, generally along a radius of the build drum  212 , as shown by arrow  204 . As shown in  FIG. 14B , the build drum  212  includes a build surface  224  and rotates counter-clockwise, as shown by arrow  244 . Generally, the build drum  212  moves relatively slowly, while the printhead carriers  203  move more rapidly. 
     FIGS. 14C and 14D  are enlarged schematic top views of the 3D printer  200  of  FIG. 14A . As shown in  FIG. 14C , the printhead array  202  includes six printheads  248  staggered along a length of the printhead carrier  203 ; however, the array  202  could be made up of essentially any number or arrangement of printheads  248 . The six staggered printheads  248  define the printing swath width  206 . In one embodiment, each printhead  248  prints a ½″ swath, resulting in a swath width  206  of about 3″. The width  206  is obtained with all of the jets printing; however, different swath widths and shapes can be achieved by controlling the number and arrangement of jets that actually fire. As the printhead carrier  203  moves the printhead array  202  radially in and out, the printheads  248  print on the in stroke, as shown by arrow  205 . 
     FIG. 14D  depicts the specific details of the print swaths. Generally, the swaths print canted to a radius of the build drum  212 , because the build drum  212  is rotating as the printheads  248  are printing along the radius. The printhead travel path  207  includes a print stroke  208  and a return stroke  209  (the lines shown represent the centerline of the printhead array  202 ). The return stroke  209  occurs as the printhead carrier  203  moves radially outward, and the print stroke  208  occurs as the printhead carrier  203  moves radially inward. When printing, not all of the jets are firing along the entire print stroke  208 , resulting in a used printable area  213  and an unused printable area  211 . This is done to compensate for the fact that the printed swaths would otherwise overlap as the build drum  212  rotates. As shown, the printed segments  210  abut one another, thereby forming a fully printed area, as shown. The used printable area  213  of the swath is widest at a point furthest from the center of the build drum  212 . 
   It should be noted that the various 3D printers disclosed herein print based on polar coordinates (i.e., r, θ), as opposed to linear printers, which print based on rectangular coordinates (i.e., x, y). The disclosed 3D printers include logic for converting rectangular coordinates to polar coordinates for printing on a radial build surface. The converting logic typically resides in the controller that controls the operation of the 3D printer. 
   In addition, because the printheads are printing along a radius, not all of the jets of the printhead print every time. In particular, the jets located closest to the center of the print arrays tend to print less, thereby resulting in a longer duty life. Correspondingly, the printheads located on the outsides of the print arrays tend to fail first. 
   In one embodiment, the apparatus  10  can include one or more sensors to measure the print quality or other characteristics of the apparatus  10 , such as print speed, printhead condition (e.g., an empty or dirty printhead), misfiring jets, build material quantity, and/or build drum position. In a particular embodiment, a sensor can monitor the print quality by determining if the printheads  48  are printing properly and, if not, can send a signal to the apparatus controller to shift the printheads  48  to compensate for printheads  48  that are not printing properly. For example, the controller could move the printheads  48  radially a very small amount for shingling purposes. In one embodiment, a sensor can be used to determine whether all, or at least a minimum number, of jets are firing and, if not, signal the user to replace a printhead  48 . Additionally, sensors can be used to monitor and control other functions, such as running diagnostic tests, performing cleaning of the printheads  48 , refilling the build material dispenser assembly  20 , cleaning the spreader assembly  22 , and performing any other desired function of the apparatus  10 . 
   The printbar assembly  18  can also be moved for diagnostic or service purposes. Moving the printhead array  40  radially from the build drum  12  provides the user with access to the printheads  48  for maintenance purposes, such as cleaning or replacement. Printhead cleaning is described in detail with respect to  FIGS. 9A-9J ,  10 ,  11 ,  12 A- 12 C, and  13 A- 13 D. The printhead array  40  can also be moved radially outwardly to run a diagnostic routine of the printhead array  40  (see  FIGS. 8A and 8B ). In an alternative embodiment, the printbar assembly  18  can be raised from the build drum  12  for service purposes. 
   The size and exact configuration of the apparatus  10  can vary to suit a particular application. For example, the apparatus  10  could be sized to fit on a tabletop to produce relatively small three-dimensional objects, or the apparatus  10  could have a substantial footprint for producing relatively large three-dimensional objects. In a particular embodiment, the build drum  12  has an outside diameter of about six feet, an inside diameter of about two feet, and a depth of about two feet. The size of the build drum  12  can vary to suit a particular application. In addition, the apparatus  10  can be situated within an enclosure and can include air handling equipment for cleaning the work environment. The enclosure can include windows for monitoring operation of the apparatus  10 . 
   Additionally, the apparatus  10  may include multiple build drums  12  and printbar assemblies  18 . In one possible configuration, the apparatus  10  includes multiple build drums  12  spaced about a centrally located gantry that carries the printing components, i.e., material dispenser, spreader, and the printheads. The gantry can be rotated into position above one of the build drums  12 . In this configuration, the user can be printing on one build drum  12  while removing parts from another build drum  12 , thereby allowing for continuous operation. In another embodiment, the build drum  12  can be radially stationary, but vertically movable. In this embodiment, the printing components are configured to move radially about the build drum  12 . In a particular embodiment, the gantry supporting the printing components rotates radially about the build drum  12  while the printheads move back and forth along a radius of the build drum  12 . This configuration allows for printing over substantially the entire surface area of the build drum  12 . 
     FIGS. 15A and 15B  depict an alternative embodiment of a 3D printing apparatus  300  in accordance with the invention. As shown in  FIG. 15A , the apparatus  300  includes three build drums  312  disposed on a carousel  313 . The printing hardware is stationary as the carousel  313  rotates the build drums  312  around a carousel pivot shaft  314  into alignment with the printing hardware. The build drums  312  and printing hardware are essentially the same as previously described. 
     FIG. 15B  depicts the carousel  313  rotating counter-clockwise (arrow  315 ) to move one build drum  312 A out of alignment with the printing hardware and a second build drum  312 B into alignment with the printing hardware. The carousel can rotate in either the clockwise or counter-clockwise direction. One advantage to this arrangement is that the apparatus  300  can be printing on one build drum  312 C, while one set of printed objects can be curing in the second build drum  312 B and another set of printed objects are being removed from the third build drum  312 A. 
     FIGS. 6A and 6B  depict systems  70 ,  92  for 3D printing utilizing two different build material feed systems  74 ,  96 . Referring to  FIG. 6A , the system  70  includes a 3D printing apparatus  72 , similar to that previously described with respect to  FIGS. 1-3 , and the build material feed system  74  remotely connected to the 3D printing apparatus  72 . The build material feed system  74  includes a storage bin, or hopper  80 , for holding the build material and structure for conveying the build material to the 3D printing apparatus  72 . The hopper  80  may include multiple internal compartments for holding multiple build material components that are mixed before being conveyed to the three-dimensional printing apparatus  72 . Additionally, the multiple compartments might hold different types of build materials, with the build material feed system  74  including structure for delivering one or more different materials to the apparatus  72 . 
   The build material feed system  74  shown in  FIG. 6A  includes a supply duct  82 , a supply pump  84 , a return (or overflow) duct  88 , and a return (or overflow) pump  90 . These components  82 ,  84 ,  88 ,  90  connect the hopper  80  with the 3D printing apparatus  72  and are capable of conveying a continuous or intermittent flow of material to the 3D printing apparatus  72 , as needed. The ducts  82 ,  88  can be rigid or flexible or combinations thereof. For example, a flexible hose can be used at the connection points between the ducts  82 ,  88  and the 3D printing apparatus  72 , while the portion of the ducts  82 ,  88  running between the build material feed system  74  and the 3D printing apparatus  72  can be rigid pipe. In alternative embodiments, the build material feed system  74  could include a conveyer belt system, a carousel, a feed screw, a gravity feed system, or other known components for transporting loose powder materials. The systems could be operated manually or driven pneumatically, hydraulically, or electrically. Additionally, the build material feed system  74  may include a main fill port or duct  86  on the hopper  80 . Further, the build material feed system  74  may include one or more sensors connected to the controller  73  to monitor and control material levels in the hopper  80  and/or the amount and the rate of the materials being delivered to the 3D printing apparatus  72 . 
   The hopper  80  is filled with build material, typically in powder form, via the duct  86 . Alternatively, the hopper  80  may include a removable cover for filling. The material is directly fed to the 3D printing apparatus  72  via the supply duct  82  exiting the bottom of the hopper  80 . The supply pump  84  is located in the supply duct  82  to facilitate transportation of the material to a build material dispenser assembly  76  on the 3D printing apparatus  72 . In the embodiment shown, the excess material is collected in a material overflow tray  78  located on the 3D printing apparatus  72  and returned directly to the hopper  80  via the return duct  88  and the return pump  90  located in the return duct  88 . The material is returned to the top of the hopper  80 . In an alternative embodiment, the return material is processed before being returned to the hopper  80 . In a particular embodiment, the build material feed system  74  may include an agitation component to maintain the build material in a powder form. Alternatively or additionally, the build material feed system  74  may include components for handling build materials supplied in other than powder form. 
   As shown in  FIG. 6B , the system  92  includes a 3D printing apparatus  94 , similar to that previously described with respect to  FIGS. 1-3 , and the build material feed system  96  remotely connected to the 3D printing apparatus  94 . The build material feed system  96  is similar to the system  74  described with respect to  FIG. 6A  and includes a hopper  102 , a supply duct  106 , a supply pump  108 , a return (or overflow) duct  114 , and a return (or overflow) pump  116 . The build material feed system  96  further includes a blending assembly  110 . In the embodiment shown, the blending assembly  110  is disposed in the supply duct feeding the 3D printing apparatus  94 ; however, the blending assembly  110  could be located in the hopper  102  to blend the materials before they leave the hopper  102 . 
   The blending assembly  110  includes multiple component hoppers  112 . In this configuration, the main hopper  102  holds one or more of the major constituents of the build material that are supplied to the blending assembly  110 , such as sand. One or more additional constituents are introduced to the blending assembly  110  via the component hoppers  112 . The blending assembly  110  controls the feed rate and blending of the various constituents to create the final build material. Additionally, the blending assembly  110  can blend the excess material received from the return duct  114  into the build material supplied to the 3D printing apparatus  94 . In a particular embodiment, the blending assembly  110  meters the excess material into the blended build material in such a manner as to not effect the quality of the material being delivered to the 3D printing apparatus  94 . 
     FIGS. 7A and 7B  depict the removal of three-dimensional objects or printed parts  126  from one embodiment of a 3D printing apparatus  120  in accordance with the invention. In  FIG. 7A , the build drum  124  is shown in partial section to illustrate the positioning of the printed parts  126 . Layers of the build material accumulate in the build drum  124  and the printed parts  126  are surrounded by non-printed (unbound) build material  128 . There are various ways of removing the parts  126 ; however, in the embodiment shown, the parts  126  are removed though a top opening  122  of the build drum  124 . Specifically, the unbound build material  128  is evacuated from the build drum  124  by, for example, vacuuming. Alternatively, the unbound material  128  could be drained through bottom or side openings in the build drum  124 . Once the unbound material  128  is removed, the parts  126  can be manually or automatically removed from the build drum  124 . In one embodiment, the top opening  122  is partially covered. The parts  126  may be further processed, as needed. 
     FIGS. 8A and 8B  illustrate the diagnostic station  38  of  FIG. 1 . Other diagnostic systems are possible; for example detecting drops of binder or printing a test pattern on the build material. The diagnostic station  38 , as shown in detail in  FIG. 8B , includes chart paper  130  mounted between a paper supply roll  132  and a paper take-up roll  134 , an optical scanner  138 , a fixed reference printhead  140 , and a paper drive capstan  136 . The capstan  136  is used to accurately feed and position the chart paper  130 . To run a diagnostic test, a portion of the printhead array  40  is moved in position over the diagnostic station  38  (arrow  142  in  FIG. 8A ). A clean section of chart paper  130  is positioned below the printhead array  40  (arrow  144  in FIG.  8 A). The printheads  48 , including the reference printhead  140 , print on the chart paper  130 . The printed test pattern is passed under the optical scanner  138  for analysis. In one embodiment, the optical scanner  138  is a CCD camera that reads the test image. The apparatus controller  73 , via the diagnostic station  38 , is able to determine if the printheads  48  are printing correctly or are in need of cleaning or replacement. In an alternative embodiment, the chart paper  130  may move continuously while the printhead array  40  moves continuously over it, printing a test pattern on the paper. 
     FIGS. 9A-9J  illustrate a system  146  for cleaning a printhead  150 . The system  146  is located in the service station  34  ( FIG. 1 ). In one embodiment, the system  146  includes a cleaning station  148  made up generally of a latch pawl  152 , a spring  154 , a squeegee  156 , a printhead cap  158 , a cap carrier  192 , a second spring  162 , and a cam track  164 . Only a single cleaning station  148  is shown for descriptive purposes; however, multiple stations  148  may be disposed in the service station  34 . Alternatively, a single cleaning station  148  may service multiple printheads  150  by, for example, successively positioning the printheads  150  relative to the cleaning station  148 . 
     FIG. 9A  represents a starting position of the cleaning system  146 . As shown in  FIG. 9B , the printhead  150  approaches the cleaning station  148  and engages the latch pawl  152 . The latch pawl  152  is actuated as the printhead  150  passes over the latch pawl  152 . The printhead  150  continues to move past the latch pawl  152  and engages the squeegee  156  ( FIG. 9C ). The printhead  150  passes over squeegee  156 . As shown in  FIG. 9D , the printhead  150  contacts the cap carrier  192 , which is driven along the cam track  164  and compresses the spring  162 . The printhead cap  158  is positioned against a printhead face  160  ( FIGS. 9E and 9F ). As shown in  FIG. 9F , the printhead cap  158  seals against the printhead face  160  while the face  160  is rinsed with wash fluid (see  FIG. 10 ). 
   After the printhead face  160  is cleaned, the printhead  150  begins to move out of the cleaning station  148  ( FIG. 9G ). The latch pawl  152  engages the cap carrier  192 , halting its movement. As shown in  FIG. 9H , the printhead  150  engages the squeegee  156 , which wipes the printhead face  160 . In an alternative embodiment, the squeegee  156  vibrates to further clean the printhead face  160 . The printhead  150  continues its forward movement, actuating the latch pawl  152  ( FIG. 91 ), which, in turn, releases the cap carrier  192  ( FIG. 9J ). The cap carrier  192  snaps back to the start position. The system  146  is now ready to clean another printhead  150 . 
     FIG. 10  depicts the action of  FIG. 9F  in greater detail. The printhead  150  is positioned with the printhead face  160  against the printhead cap  158 , which in this embodiment is made of rubber. The cap includes a seal lip  172  for sealing about the printhead face  160 . The cleaning station  148  is coupled to a wash fluid supply container  182  via a supply duct  184  and a wash fluid return container  186  via a return duct  188 . The wash fluid return container  186  is in communication with a vacuum source  180 , in this case a vacuum pump, via a vacuum duct  190 . Additionally, a valve  178  is located in the return duct  188 . The valve  178  may be manually or automatically actuated. 
   In operation, the vacuum source  180  creates a vacuum within a cavity  174  in the printhead cap  160 . The vacuum pulls wash fluid from the supply container  182  through the supply duct  184 . The wash fluid enters the cavity  174  as a spray  176  against the printhead face  160 . The spray  176  washes debris, such as excess build material and dried binder, off the printhead face  160 . The used wash fluid and debris are drawn out of the cavity  174  by the vacuum source  180  and into the return container  186  via the return duct  188 . Additionally, the negative pressure created in the cavity  174  by the vacuum source  180  prevents the wash fluid from entering the jet nozzles and, in fact, may cause a small amount of binder to flow out of the nozzles to flush any powdered build material out of the nozzle. Blockages or obstructions in the jet nozzles can cause the jets to fire in the wrong direction. Once the operation is complete, the system  148  moves onto the step depicted in  FIG. 9G . 
     FIG. 11  depicts an alternative embodiment of a cleaning station, also referred to as a reconditioning station  406 . The reconditioning station  406  is shown removed from the printing apparatus  10 ; however, the reconditioning station  406  can be included on the printbar assembly  18  or in the service station  34 . The reconditioning station  406  includes a plurality of wiping elements  408  and a plurality of lubricators  410 . The wiping elements  408  and the lubricators  410  are mounted on a plate  412  that can be actuated to travel, as indicated by arrow  401 . The engaging surfaces  414  of the wiping elements  408  and the lubricators  410  are disposed upwards so that when the printhead  476  is in the reconditioning station  406 , the wiping elements  408  and the lubricators  410  clean the printheads  476  from below ( FIGS. 12A-12C ). Also, in the illustrated embodiment, one wiper  408  and one lubricator  410  acting as a pair  416  are used to clean each printhead  476 . Further, in the illustrated embodiment, each wiper and lubricator pair  416  are offset from each other to correspond with the offset spacing of the printheads  476  (see, for example, printheads  48  in  FIG. 8A ). In other embodiments, however, any number of wiping elements  408  and lubricators  410  can be used to clean the printheads  476 , and the wiping elements  408  and lubricators  410  can be spaced using any desirable geometry. 
     FIGS. 12A-12C  depict one method of using the reconditioning station  106 . The printhead(s)  476  is disposed above the reconditioning station  406  ( FIG. 12A ). The plate  412  on which the wiping elements  408  and lubricators  410  are mounted is then actuated into alignment with the printheads  476 , and the printheads  476  are wiped and lubricated from beneath to remove any accumulated grit and to improve the flow of binding material out of the printheads  476 . Specifically, the lubricator  410  applies a lubricant to the printhead face  477  to moisten any debris on the printhead face  477 . Then, the printhead  476  is moved to pass the printhead face  477  over the wiping element  408  (e.g., a squeegee), which wipes the printhead face  477  clean. Alternatively, the printhead face  477  could be exposed to a vacuum source to remove any debris present thereon. 
     FIGS. 13A-13D  depict an alternative embodiment of a reconditioning station  506  in accordance with the invention. The reconditioning station  506  may also be mounted in the service station  34 . The reconditioning station  506  includes a reservoir  542  that holds a washing solution  543  and a pump  545  that delivers the washing solution  543  under pressure to at least one nozzle  540  and preferably an array of nozzles  540 . The nozzles  540  are capable of producing a high velocity stream of washing solution  543 . In operation, the nozzles  540  are directed to the printhead face  577  of the printhead  576 . When directed onto the printhead face  577 , the washing solution  543  loosens and removes contaminants, such as build material and binding material, from the printhead face  577 . The orientation of the nozzles  540  may be angled with respect to the printhead face  577 , such that a fluid flow is induced across a plane of the printhead face  577 . For example, the washing solution can contact the printhead  576  at the side nearest the nozzles  540  and drain from the side of the printhead  576  furthest from the nozzles  540 . This approach improves the efficacy of the stream of washing solution  543  by reducing the accumulation of washing solution on the printhead face  577 , as well as the amount of washing solution  543  and debris that would otherwise drain near and interfere with the nozzles  540 . A splash guard may also be included in the reconditioning station  506  to contain splashing resulting from the streams of liquid washing solution  543 . 
   It is desirable to remove a large portion of the washing solution  543  that remains on the printhead face  577  after the operation of the nozzles  540  is complete. This is conventionally accomplished by drawing a wiping element  408  across the printhead face  477 , as shown in  FIG. 12C . A disadvantage of this approach is that contact between the wiping element  408  and the printhead face  477  may degrade the performance of the printhead  476  by, for example, damaging the edges of the inkjet nozzle orifices. Accordingly, it is an object of this invention to provide a means of removing accumulated washing solution from the printhead face  577 , without contacting the delicate region around the inkjet nozzles. In one embodiment, a wicking member  544  may be disposed such that the printhead face  577  may pass one or more times over its upper surface  546  in close proximity, without contact, allowing capillary forces to draw accumulated washing solution  543  away from the printhead face  577 . The wicking member  544  may be made from rigid, semi-rigid, or compliant materials, and can be of an absorbent or impermeable nature, or any combination thereof. 
   For the wicking member  544  to effectively remove accumulated washing solution  543  from the printhead face  577 , the gap between the upper surface  546  of the wicking member  544  and the printhead face  577  must be small, a desirable range being between about 0 inches to about 0.03 inches. A further object of this invention is to provide a means for maintaining the gap in this range without resort to precise, rigid, and costly components. 
   In another embodiment, the wicking member  544  may consist of a compliant rubber sheet oriented approximately orthogonal to the direction of relative motion  547  between the wicking member  544  and the printhead  576  and with a portion of its upper edge  546  disposed so that it lightly contacts or interferes with the printhead face  577  only in non-critical areas away from the printhead nozzle orifices. The upper edge  546  of the wicking member  544  may include one or more notches  548  at locations where the wicking member  544  might otherwise contact delicate components of the printhead face  577 . System dimensions are selected so that the wicking member  544  always contacts the printhead face  577 , and is deflected as the printhead  576  passes over it, independent of expected variations in the relative positions of the printhead  576  and the reconditioning station  506 . The upper edge  546  accordingly follows the position of the printhead face  577 , maintaining by extension a substantially constant space between the printhead face  577  and the relieved surface notch  548 . To further prolong the life of the printhead  576 , a bending zone of the wicking object  544  can be of reduced cross-section to provide reliable bending behavior with little deformation of the upper edge  546  of the wicking member  544 . 
     FIGS. 13B-13D  illustrate a reconditioning cycle in accordance with the invention.  FIG. 13B  shows the printhead  576  approaching the reconditioning station  506  along the path  547 . When the printhead  576  lightly contacts the wiping member  544 , as shown in  FIG. 13C , motion stops along the path  547  and the washing solution  534  is directed at the printhead face  577  by the nozzle array  540 . When the spraying operation is complete, the printhead  576  continues to travel along the path  547 , as shown in  FIG. 13D . The wiping member  544  is further deflected to allow passage of the printhead  576 , and the accumulated washing solution  543  is wicked away from the printhead face  577 . After being sprayed and wiped, the printhead  576  may print a plurality of droplets to eject any washing solution that may have been ingested during the reconditioning process. 
   Additional cleaning methods are contemplated, such as wiping the printhead face with a cylindrical “paint roller” that cleans and moistens itself by rolling in a reservoir of wash fluid. In another embodiment, a cleaning system could include a continuous filament that carries wash fluid up to a printhead face and carries debris away to a sump. The system may include a small scraper that can be run over the filament to remove built up debris. 
   Having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. The described embodiments are to be considered in all respects as only illustrative and not restrictive.