Abstract:
3D printing systems and methods avoid build-compromising misalignments through the use of a self-leveling assembly that maintains a constant and typically fully parallel orientation between a build platform and the bottom surface of a resin tank. As a result, contact between the floor of the resin tank and the build platform surface may be uniformly flat and even, and perpendicular to the z-axis motion of the deposition source.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to, and the benefits of, U.S. Provisional Application Ser. Nos. 61/792,053, filed on Mar. 15, 2013, and 61/704,937, filed on Sep. 24, 2012, the entire disclosures of which are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Three-dimensional (3D) printers build a solid object based on a digital model. One approach to 3D printing is “stereolithography,” in which solid objects are created by successively “printing” thin layers of a curable polymer resin, first onto a substrate and then one atop another. In traditional systems, a layer is pointwise deposited and then hardened by exposure to actinic radiation, following which the next layer of liquid resin is deposited thereover. While the technology has improved in many ways over the years, there exist many hurdles that have not been overcome, specifically in the areas of cost and accessibility. 3D printers remain for the most part expensive to manufacture and sell. They may also be complicated to operate. 
         [0003]    The steps involved in a 3D printing operation typically begin with user selection of a 3D model in a .STL or other supported format. The object represented by the selected model may be configured or optimized for a specific 3D printer using, for example, a personal computer. Configuration can involve, e.g., locating and orienting the part in space and creation of support structures needed for the object to be printed successfully. Often multiple parts can be placed in the 3D build volume of the printer. Driver software transfers the print job—i.e., the modified digital model—to the 3D printer itself. Before printing begins, the user inserts or cleans a “build platform” on which the object is printed, and provides material for printing. During printing, user interaction with the printer is usually limited, although s/he may monitor progress by, for example, looking through a window. After the object has been printed, the build platform is removed from the printer, and the printed object is separated from the build platform and from any support structure. The removal process can be delicate, requiring the use of various of tools in order not to damage the printed object. A cleaning process is usually required to obtain a high-quality print. In stereolithography, for example, the printed object may be subjected to a wash solution to remove excess resin and, in some instances, a post-cure exposure step whereby the object is bathed in actinic radiation to promote full cure. 
         [0004]    One common source of error in 3D printing is misalignment of the build platform with respect to the resin source, resulting in error in the directional travel vector of the build platform or the resin source; this, in turn, compromises the ability to print objects that are dimensionally accurate and without accumulating error along the x and y axes. Similarly, imperfections in the flatness of the build platform surface compromise the accuracy of deposition and jeopardize adhesion of the resin to the build platform. 
       SUMMARY 
       [0005]    The present invention relates to 3D printing systems and methods that avoid build-compromising misalignments. Embodiments of the invention utilize a self-leveling assembly that establishes and maintains a constant and typically fully parallel orientation between a deposition mechanism and the build platform. In some embodiments, the deposition mechanism is an inkjet or other nozzle-terminated ejection system configured for two-dimensional (2D) scanning in a plane parallel to the build platform. In other embodiments, the system is configured for “reverse stereolithography,” in which a liquid resin surrounding the build platform is pointwise hardened thereagainst. In this case, the parallel orientation is maintained between the build platform and an opposed surface, e.g., the bottom of a resin tank. Implementations in accordance herewith compensate for error in the directional travel vector of either or both of the opposed surfaces as well as for errors in the flatness of either surface. 
         [0006]    As used herein, the term “substantially” or “approximately” means ±10% (e.g., by weight or by volume), and in some embodiments, ±5%. The term “consists essentially of” means excluding other materials that contribute to function, unless otherwise defined herein. The term “light” refers to any form of electromagnetic radiation and not merely, for example, to visible light. Reference throughout this specification to “one example,” “an example,” “one embodiment,” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the present technology. Thus, the occurrences of the phrases “in one example,” “in an example,” “one embodiment,” or “an embodiment” in various places throughout this specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, routines, steps, or characteristics may be combined in any suitable manner in one or more examples of the technology. The headings provided herein are for convenience only and are not intended to limit or interpret the scope or meaning of the claimed technology. 
         [0007]    The following detailed description together with the accompanying drawings will provide a better understanding of the nature and advantages of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a perspective view of a system environment in which embodiments of the present invention may be deployed. 
           [0009]      FIG. 2  is a partially cut-away elevation of the system illustrated in  FIG. 1 . 
           [0010]      FIG. 3  is a close-up elevation of certain components of a self-leveling tank in accordance with embodiments of the present invention. 
           [0011]      FIG. 4  is a close-up perspective view showing the operation of a series of ball spring plungers in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Refer first to  FIG. 1 , which illustrates a representative stereolithography system  100 . The system includes a base housing  105  containing various mechanical, optical, electrical and electronic components that operate the system  100 . A transparent upper housing  107  surrounds the build platform and a resin tank  115 , which is sized to receive the build platform  107  therein, as discussed below. The build platform  110  is secured to a carriage  120  configured for vertical movement along a gantry  122 ; movement of the carriage  120  along the gantry  122  is controlled by drive components (not shown) within the gantry  122  and the base housing  105 . 
         [0013]    Operation of the system  100  may be understood with reference to  FIGS. 1 and 2 . The illustrated system utilizes a reverse stereolithography process by which an object is built up in layers on a downwardly facing receiving surface  210  of the build platform  110 . In an initial configuration, the build platform  110  is fully submerged within the resin tank  115  so that the surface  210  is in contact with the bottom surface  215  of the tank  115 . Typically the surface  215  is made of a compliant elastomeric material, such as a silicone (e.g., polydimethysiloxane, or PDMS). The bottom surface  215 , and indeed all surfaces between the tank  115  and the internal components within the bottom housing  107 , are transparent to actinic radiation, generally provided by a laser, capable of curing liquid resin within the tank  115 . For example, a conventional ultraviolet laser and drive components within the bottom housing  107 , collectively indicated at  217 , may cooperate with movable mirrors that scan the beam from below over the bottom surface  210  of the build platform  210 . The beam is selectively activated during movement of the mirrors so that pulses are delivered in a pointwise or “imagewise” pattern corresponding to the bottom layer of the object to be printed. The beam, where activated, cures the resin to create a solid element of material against, and adhering to, the receiving surface  210 . When this layer is completed, the height of the build platform  110  is raised slightly along the gantry  122  so that another solid layer can be cured by the laser to adhere to the previously deposited layer. The process is repeated until the 3D object is fully formed, suspended upside-down from the surface  210 . 
         [0014]    Embodiments of the present invention are directed to retaining the tank  115 —in particular its bottom surface  215 —in parallel relation with the build surface  210  and, as well, with the optical components  217  directing the laser beam. It should be understood, however, that the principles hereof may be applied to other 3D printing architectures, e.g., utilizing a deposition print head that must be maintained in parallel relation with a build surface. 
         [0015]    In the representative embodiment shown in  FIGS. 2-4 , the resin tank  115  is secured to a carrier tray  220  by force applied by a series of ball-spring plungers  225  as described below. The carrier tray  220 , in turn, is suspended above the top surface of a larger support tray  230  by a series of spring-loaded connectors  235 ; in the illustrated embodiment, there are four such connectors each located at a corner of the tank carrier tray  220 . With particular reference to  FIG. 3 , each of the connectors  235  may be a threaded stud  310 . The head of each threaded stud  310  is mechanically or adhesively affixed to the tank carrier tray  220 . The shanks of the threaded studs  310  pass through orifices in the support tray  220 , and are free to slide vertically through these orifices. Vertical travel of the shanks through the respective orifices is limited by lock nuts  315  located below the support tray  230 ; as a result, the tank carrier tray  220  and the support tray  230  are loosely connected with a gap G between them. This gap is bridged by springs  320  along the shanks of the studs  310  intervening between the trays  220 ,  230  and urging them away from each other. The springs  320  apply a preload force that keeps the trays  220 ,  230  apart (with tension against the lock nuts  315 ) and are compressible by vertical movement of the build platform  110 . 
         [0016]    As explained above, when the 3D printer  100  begins printing a new part, the build platform  110  descends until its build surface  210  presses against the floor elastomeric floor  215  of the tank  115 , compressing the springs  320  separating the carrier and support trays  220 ,  230 . With the springs  320  fully compressed, further downward force is applied to the build platform to squeeze any resin out from between the contacting surfaces. This provides an even flat surface between the resin tank and the build platform, which is necessary for accurate printing, even if errors in flatness exist between the tank floor  215  and the build surface  210 ; in such circumstances, the springs  320  will not compress evenly but instead have sufficient stiffness to conform the surfaces  210 ,  215  to each other so as to compensate for error arising from misalignment or small imperfections in flatness. 
         [0017]    When the build platform  110  is raised, its surface eventually loses contact with the floor  215  of the tank  115 . The studs  235  and lock nuts  315  are preferably uniformly sized so that the gap G between the trays  220 ,  230  is constant across the opposed areas; that is, the trays remain precisely parallel even if the springs  320  have slightly different stiffnesses (or if the stiffnesses vary over time with use), since as long as the springs have enough force to urge the trays apart, the identical connectors enforce a uniform distance between them. As a result, the gap G and the spatial orientation of the resin tank  115 —which are established by the studs  235  and lock nuts  315 —remain fixed as the build platform  110  rises. Any necessary adjustment can be accomplishing by tightening or loosening the lock nuts  315 . 
         [0018]    A spring-loaded coupling system facilitates easy removal and switching of resin tanks. As illustrated in  FIG. 4 , a slot  410  is located on each side the tank carrier tray  220 . These slots  410  slidably receive complementary flanges  415 , which project from the bottom side edges of the resin tank  115 , as the tank slides into the carrier tray  220 . The flanges  415  have a plurality of (e.g., two each) holes or depressions  420  which, when the tank  115  is fully inserted into the slots  410 , align with the ball spring plungers  225  mounted to the tray  220 . The head  425  of each of the plungers  225  is urged by an internal spring  430  against one of the tank flanges  415 , and when the ball spring plungers  225  engage the holes  420 , the heads  425  are forced into the holes  420  with an audible click, ensuring that the resin tank  115  maintains its location securely. 
         [0019]    As will be appreciated by those having skill in the art, the inventive concepts in the above-described embodiment may be implemented in alternative ways. In one alternate embodiment, the mechanism depicted in  FIGS. 2-4  is modified so as to attach the build platform  110  using the spring-loaded connecting system described above such that springs connecting the build platform to the apparatus provide an even flat surface between the resin tank  115  and the build platform  110 . As above, the tank has a compliant layer  215  on its interior floor. The tank  115  is secured to a carrier tray  220  either in a conventional manner or using the spring-loaded connectors  225  described above. In this alternative embodiment, the build platform is attached to the retaining assembly  265  by means of one or more spring-loaded connectors. These connectors may be threaded studs. The head of each threaded stud is mechanically affixed to the build platform  110 . The shanks of the threaded studs pass through orifices in the build-platform retaining assembly  265 , and the shanks are free to slide vertically through these orifices. Vertical travel of the shanks through the respective orifices is limited by lock nuts; as a result, the build platform  110  and retaining assembly  265  are loosely connected with a gap between them. This gap is bridged by springs along the stud shanks intervening between the build platform  110  and the retaining assembly  265  and urging them away from each other. The springs apply a preload force that keeps the build platform  110  and retaining assembly  265  apart (with tension against the lock nuts) and are compressible by vertical movement of the build platform. As disclosed above, the build platform descends until it presses against the floor  215  of the tank  115 , now compressing the springs separating the build platform  110  and the build platform retaining assembly  265 . With the springs fully compressed, further downward force is applied to the build platform  110  that squeezes any resin out from between the contacting surfaces  210 ,  215 . This provides an even flat surface between the resin tank and the build platform, even if errors in flatness exist between the tank floor  215  and the bottom surface  210  of the build platform  110 ; in such circumstances, the springs will not compress evenly but instead have sufficient stiffness to conform the surfaces to each other so as to compensate for error arising from misalignment or small imperfections in flatness. Once again, this approach may be applied to a other types of 3D printing systems, e.g., in which a print head, rather than the build platform, is affixed to the retaining assembly  265 . 
         [0020]    In yet another embodiment, the build platform  110  is mounted on a central ball joint  150  (see  FIG. 1 ), which may be located within the retaining assembly  265 , such that the platform  110  is free to rotate in order to align with the floor  215  of the resin tray  115 . Springs or other elastic members may be attached at the corners of the build platform so as to provide a force restoring the orientation of the build platform  110  orientation when not pressed against the floor  215  of the resin tray  115 . The ball joint may be used to fix the orientation of the build platform  110  relative to the z-axis, while allowing the build platform  110  to pivot in order to compensate for misalignment between the build platform and the resin tray  115 . Alternatively, the ball joint may include an internal spring so as to also allow for movement in the z-axis direction. When the surfaces  210 ,  215  have been brought into proper alignment, the ball joint may be locked into place using a compression collar (or a simple clamp or screw); for example, the compression collar may be spring-loaded and operable by means of a grip or button, which the user releases to lock the ball joint. 
         [0021]    In each of the disclosed embodiments, individual springs and retaining lock nuts may be replaced by alternate mechanical elements to provide compliance within the printing system. Springs, for example, may be functionally replaced with an elastic sheet, flexure bearing or other flexure element adhered or otherwise attached between the support and carrier trays. The use of an adhesive material in connection with an elastic sheet may advantageously reduce or eliminate the need for lock nuts or shanks to limit the range of motion. Alternatively, structural elements such as the carrier  120  or other mounting components may be designed with a flexible material or living hinge such to allow the surface  210  to accommodate to (i.e., align with) the tank floor  215  by virtue of vertical movement of the build platform  110 . In such an alternative embodiment, the compressible structural elements function analogously to the springs in the embodiments disclosed above. As yet another embodiment, the mounting systems described above may be left free during an initial levelling and calibration step, but fixed after calibration such that the mounting points are substantially more rigid than during the calibration step. By increasing the rigidity of the mounting points during operation, the initial alignment and calibration can be advantageously preserved during operation. 
         [0022]    Thus, although the invention has been described with respect to specific embodiments, it will be appreciated that the invention is intended to cover all modifications and equivalents within the scope of the following claims.