Abstract:
A stereolithography apparatus having a resin vat with computer controlled heating elements responsive to thermistors for controlled preheating of the resin vat, an elevator assembly for supporting and releasably retaining a build platform removably attached to the stereolithography apparatus frame such that elevator forks supporting the build platform can be released into the vat and removed from the stereolithography apparatus with the vat, and a recoater blade that can be removed and accurately installed by hand, thus providing that all wetted components can be quickly and efficiently removed and installed and enabling rapid changeover from spent resin to fresh resin and minimizing apparatus downtime between stereolithography operations.

Description:
[0001]     This application is a continuation-in-part of pending U.S. application Ser. No. 11/240,821, filed Sep. 30, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to methods and apparatus for rapid prototyping and manufacturing (“RP&amp;M”) to produce three-dimensional objects, and more particularly to improving the productivity and efficiency of RP&amp;M systems.  
         [0003]     RP&amp;M is the name given to a field of technologies that can be used to form three-dimensional objects or solid images. In general, RP&amp;M techniques build three-dimensional objects, layer-by-layer, from a building medium using data representing successive cross-sections of the object to be formed. Computer Aided Design and Computer Aided Manufacturing systems, often referred to as CAD/CAM systems, typically provide the object representation to an RP&amp;M system. The three primary modes of RP&amp;M include stereolithography, laser sintering, and ink jet printing of solid images.  
         [0004]     Laser sintering builds solid images from thin layers of heat-fusible powders, including ceramics, polymers, and polymer-coated metals to which sufficient energy is imparted to solidify the layers. Ink jet printing builds solid images from powders that are solidified when combined with a binder. Stereolithography, to which the subject matter herein is primarily addressed, builds solid images from thin layers of polymerizable liquid, commonly referred to as resin.  
         [0005]     Stereolithography and laser sintering systems typically supply the energy for creating and building up the thin cross-sections of three-dimensional objects through modulation and precise directional control of lasers. The laser applies energy to a targeted area of the layer of powder or liquid building medium. The thin targeted layer is called the working surface of the building medium. Conventional RP&amp;M laser systems position the laser beam using a scanning system having galvanometer-driven mirrors that are directed by a control computer. The mirrors deflect a laser beam in response to a CAD/CAM program that has been tessellated into the STL format and sliced into cross-sectional data files that are merged into a build file.  
         [0006]     In stereolithography, three-dimensional objects result from successive solidification of a plurality of thin layers of a polymerizable liquid, one on top of another, until all of the thin layers join together to form the three-dimensional object. Each layer represents a thin cross-section of the desired three-dimensional object. Polymerizable liquids are generally referred to as “resins,” and solidified layers of resin are said to be cured. Practical building media typically include resins that cure sufficiently fast, usually with ultraviolet light. An ultraviolet laser generates a small and intense spot of light that is moved across the liquid surface with a galvanometer mirror in an x-y scanner in a predetermined pattern. The scanner is driven by computer generated vectors or the like. This technique rapidly produces precise complex patterns.  
         [0007]     A typical stereolithography system includes a laser scanner, a vat for containing the resin, an object support platform, which is capable of being raised and lowered in the vat, and a controlling computer. The computer controls the system automatically to make a plastic part, forming one thin cross-section of cured resin at a time on the object support platform and building the desired three-dimensional object up layer-by-layer. The object support platform supports the cured layers and rests beneath the surface of the liquid resin the distance of one layer thickness to define a working surface. The laser cures selected portions of liquid resin at the working surface to cure the next layer. The computer controls the system to recoat the surface of the cured resin with fresh resin and repeats the steps thousands of times until completing the desired object. The object or multiple objects being built and the completed sequence of steps is sometimes referred to as a “build.” An operator removes the build from the vat of resin for cleaning and further curing as needed. The liquid resin remaining in the vat remains usable so long as it is not too contaminated with suspended bits of cured resin.  
         [0008]     One method of recoating the cured resin layers with fresh resin requires “deep dipping” the platform in the liquid resin. The platform vertically drops below the surface of the bath of resin a distance greater than the desired layer thickness to coat the cured layers with fresh liquid resin. The system raises the platform to one layer thickness beneath the resin surface. Excess liquid resin runs off to level the resin by gravity to a single layer thickness. Thereafter, the laser applies energy to the working surface.  
         [0009]     The waiting period for the thin layer to level varies depending on several factors, including the viscosity of the polymerizable liquid, the layer thickness, part geometry, cross-section, and the like. Some recent resins level more quickly than prior resins. Leveling can be assisted by the use of a doctor blade or vacuum assisted doctor blade, sometimes referred to as a Zephyr blade, to sweep across the surface of the resin, applying fresh resin and removing the excess much more quickly than by gravity settling and leveling the working resin surface in the vat containing the resin. The blade is said to recoat the solidified layers and is often referred to as a “recoater.”  
         [0010]     Various improvements have been proposed to increase the efficiency with which RP&amp;M techniques are accomplished, including improvements to laser systems for more efficient use of the laser and for more precise imaging, improvements to building media, reduction of curing time, control of resin level in the vat, and the like. It would be desirable to make additional improvements that enable stereolithography systems to produce more objects in less time, and to do so with greater precision and less human intervention.  
       SUMMARY  
       [0011]     This invention provides several improvements to rapid prototyping and manufacturing systems that enable an unattended building of a three-dimensional object. Two three-dimensional object builds can be accomplished in sequence, one after the other, from the same location in a single building medium, without requiring a human operator present after the first build starts. The system does not require an operator to attend the completion of the first build and its removal from the building medium, the start of the second build, or the completion of the second build. While the system can be used for a single build, the system allows the return of an operator to a system having two objects built in sequence and awaiting unloading, cleaning, and further curing as needed.  
         [0012]     The system of the invention can be applied to multiple chamber units having a single energy source so that more than one build can be completed at a time, each followed by a second unattended build. The objects completed in a single first build and those completed in a second build can be of the same or different design, and the building medium is the same for the second build as for the first. The objects completed simultaneously in adjacent chambers, which will be either the first or second build in an unattended build sequence, will usually be prepared from the same building medium, but need not be so long as the appropriate machine and process adjustments are made to enable curing.  
         [0013]     The invention includes the capability of reducing the time required to change resins and provides apparatus and methods for removing all resin-wetted components from the system and installing fresh components dedicated to a fresh resin maintained at or near operating temperature, which can be a different resin from the resin of the removed components. Thus, the invention avoids significant down-time for exchanging resin vats and initiating another stereolithography build sequence.  
         [0014]     In more specific detail of an embodiment of the invention, the invention provides apparatus and methods for stereolithography that include a housing having an elevator for supporting, raising, and lowering a support platform for an object to be built, a vat for containing a liquid resin from which an object is built, a source of energy for solidifying selected laminae of the liquid resin, a cart for unattendedly removing a first build from the elevator, and control systems for controlling the elevator, the energy source, the cart, and the resin level in the vat.  
         [0015]     In a still more specific embodiment, the elevator includes first and second subassemblies of components. The first subassembly is associated with the resin vat and can be installed into and removed from a chamber in a stereolithography housing with the vat. The second subassembly of elevator components is fixed in the stereolithography housing chamber. The first and second subassemblies interconnect to form the entire elevator assembly and can also be referred to respectively as the vat elevator subassembly and the chamber elevator subassembly.  
         [0016]     The vat elevator subassembly includes an elevator attachment bracket for attachment to an elevator drive plate in the stereolithography housing chamber elevator subassembly. The attachment bracket has hooks for releasably engaging attachment to a support rod on the elevator drive plate and a receiver for receiving a centering pin on the elevator drive plate that locates the attachment bracket precisely in alignment with the horizontal x, y plane of the working surface of the resin.  
         [0017]     The attachment bracket is fixedly supported on an elevator frame that extends vertically so as to be readily lowered into the resin vat and raised out of the vat. The elevator frame also extends generally horizontally for providing a pair of elevator forks to support and secure an object support platform. The object support platform is supported by the forks, and by arms extending horizontally outwardly from each side of the rear of the forks. The platform is secured to the forks by releasably engaging latch members at the front of the forks. The latch members are actuated by a spring-biased latch linkage. The latch linkage is operable to engage a ramp on the elevator support when the elevator is raised sufficiently high above the resin so as to release the latch members, and thus the platform, from latching engagement.  
         [0018]     The cart for removing a first build from the elevator can, if desired, be operated by computer control to install a fresh object support platform on the elevator. The support platform can be lowered into the vat for a second, unattended build. After the build is completed, the elevator rises to remove the completed three-dimensional object or objects in the build and the platform from the resin to drain. The cart, referred to below as an auto off-load cart, is equipped to dock precisely into the housing and with the resin vat. Telescoping arms extend on computer-controlled command to engage and remove the first build and associated support platform and can be extended to install a fresh platform for a second build, if desired.  
         [0019]     In still further embodiments, the invention includes a recoater assembly for leveling the resin that can be computer controlled for remaining parallel to the working surface across the surface of the resin. The recoater assembly includes a recoater blade and a carrier for the blade that makes adjustments in any of three directions (y, z, and theta): 1) the horizontal y-axis direction of travel of the recoater assembly across the resin surface, 2) the vertical z-axis of travel up and down, providing for blade gap between the bottom of the recoater blade and the working surface of the resin and for removal of the recoater assembly from the vat, and 3) the rotational theta axis, parallel to the y-axis, for maintaining the blade parallel to the resin surface throughout the y-axis direction of travel. The x, y plane corresponds to the working surface of the resin.  
         [0020]     The recoater blade is kept at the same distance from the working surface of the resin throughout the length of travel of the recoater. The recoater blade travels vertically along an axis “z” and also rotates about a longitudinal axis, theta, that is parallel to and spaced from the axis “y” of travel of the recoater so that the ends of the recoater are always the same distance from the resin surface and the blade is parallel to the resin surface. This embodiment of the invention corrects for machine errors and reduces inaccuracies in the three-dimensional products. Machine errors arise from unevenness in the mechanical systems that in the past have required tedious adjustments to the recoater systems.  
         [0021]     Computer control of the recoater is provided in response to data sets for the distance between the bottom of the recoater and working surface of the resin obtained prior to initiating laser contact with the resin surface. A sensor contained within the recoater carrier housing provides this data to the computer. The sensor is on a motion system that moves along the length of the blade (x-axis). The sensor operates above the horizontal x, y plane of the working surface of the resin at two fixed locations x, one on each side of the recoater adjacent the edge of the vat, to obtain data at multiple points y of travel of the recoater. The recoater has thinned-down feet at each end, blade gap sensing feet, to which the sensor determines the distance. The distance of the sensor to the bottom of the foot can be accurately determined since the distance to the bottom of the foot is known and can be added to the sensor determination of the distance to the top of the thinned-down portion of the foot. The sensor is displaced a slight distance x to obtain a reading of distance to the working surface of the resin. The difference between the distance to the working surface and the bottom of the recoater is calculated and this data is stored for each side of the resin vat. The computer sets the blade gap for the z axis based on empirical data for the particular resin in use. The recoater is rotated about the theta axis and is raised or lowered along the z axis to maintain a constant distance at each end from the working surface so that the blade gap remains fixed. Thus, machine and positioning errors, including errors in the tracks along which the recoater travels, can be taken into account and corrected.  
         [0022]     The recoater does not need to be changed between builds in the unattended sequential build mode and is designed for precise positioning and easy removal and replacement by hand and without tools whenever a fresh vat of resin is installed. The recoater is fixedly attached to the carrier at each end. Magnets may be used.  
         [0023]     Correct orientation of the blade is confirmed in two ways. Differently shaped alignment pins are included on each end of the blade for placement in corresponding receptacles on the recoater carrier housing. Contacts are included on each end of the recoater carrier, all of which must be activated to result in a signal from a proximity switch to show the blade is correctly positioned on the carrier housing. The recoater normally is vacuum-assisted and is provided with a vacuum receptacle in the blade, a countersink for a soft fitting extending from the carrier, for which vacuum communication is established simply by correctly positioning the blade on the carrier and turning on the vacuum.  
         [0024]     The apparatus and process of the invention can be applied to a single vat of resin or to two or more vats operated with a single laser in which one layer is solidified in selected vats while others are recoated. Typically, two vats will be lased, one after the other, and it is possible to lase more than two using appropriate scanners and beam splitters.  
         [0025]     The resin vat includes containers of supplemental resin for supplying additional liquid to the vat as needed in response to a level sensor. During a build, it is desirable to maintain a precisely controlled level of liquid in the vat. The resin level fluctuates as some of the resin is solidified and as the platform lowers the build into the resin to complete additional layers at the surface. It is also necessary to add resin to the vat between builds to maintain the level of resin sufficient for a second build.  
         [0026]     In a specific embodiment of the invention, the vat and supplemental resin containers include tags for radio frequency identification (RFID). The resin in the supplemental container can readily be screened and identified prior to entering into the resin in the vat so as to avoid contamination of the resin in the vat by the wrong resin.  
         [0027]     Resin vats and wetted parts can be conveniently and rapidly exchanged for fresh resin and wetted parts after a vat is spent or when it may otherwise be desirable to change resins. The recoater is wetted in use and is easily removed and replaced in precise position by hand and without tools. The wetted elevator components include the attachment bracket, elevator frame, and elevator forks referred to above, which together form a vat-dedicated elevator subassembly that can be removed from the elevator drive plate fixed in the stereolithography chamber and wheeled out of the chamber in the vat-to-be-exchanged. The wetted platform supporting a build will have already been removed with the build and a fresh platform installed on the forks of a fresh vat elevator subassembly. The fresh vat and elevator subassembly can be wheeled into place and attached to the elevator drive plate with the resin preheated to a controlled temperature.  
         [0028]     In a more specific embodiment, the vat of the invention includes at least one heating element located on the outside bottom surface of the vat and at least one thermistor for sensing resin temperature. Heating of the resin is computer controlled to cause the working surface of the resin to approach a set point without overheating the remainder of the resin, and heating control is continued throughout the stereolithography build sequence. The vat may include a lid for use in heating the resin and maintaining a controlled temperature prior to introducing the vat into the stereolithography chamber.  
         [0029]     Thus, the invention provides for an unattended stereolithographic build from a single vat of resin after a first build has been completed. The invention includes a number of improvements to stereolithography apparatus so as to enable unattended builds and provides several features that can be subject to automated computer control to greatly simplify obtaining the precision required for accurate production of three-dimensional objects. These improvements include the automated off-load cart for removing a first build from the elevator and providing a fresh object support platform, switching the laser between vats for simultaneous builds, coupling of supplemental resin containers directly to the stereolithography system for automated determination of resin supply sufficiency to support an unattended build, RFID identification of resin containers for maintaining integrity of the resin, automated leveling of the resin working surface level during a build and automated refilling of the vat between builds, automated determination of the distance between the working surface and the recoater and the mapping of this distance over the axis of travel of the recoater for automated control of the rotation of the recoater and correction of machine errors, automated release of the object support platform from the elevator and replacement with a fresh platform, and installation and removal of the recoater blade entirely by hand and in the absence of tools. The invention reduces the time required to change resins between build sequences by providing for removal and accurate installation of the recoater blade by hand and in the absence of tools and providing for removal and installation of wetted elevator components with the resin vats. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]     Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:  
         [0031]      FIG. 1  is a perspective view of a dual-chamber housing of the invention for producing objects by stereolithography and showing the resin vats associated therewith, one in shadow and one in perspective;  
         [0032]      FIG. 2  is a perspective view of a resin vat showing disposed thereon in an exploded view, an object support platform and a subassembly of the elevator for raising and lowering the platform in the resin;  
         [0033]      FIG. 3  is a perspective view of a portion of the elevator subassembly of  FIG. 2  having portions removed therefrom to show various details of the elevator subassembly;  
         [0034]      FIG. 4  is a perspective view of a recoater assembly of the invention;  
         [0035]      FIG. 5  is an exploded partial perspective of a recoater carrier and recoater blade for one end of the recoater assembly of  FIG. 4 ;  
         [0036]      FIG. 6  is a partial perspective view of the underside of the recoater blade portion of  FIG. 5 ;  
         [0037]      FIG. 7  is a partial perspective view of the interior rear of a stereolithography chamber of the invention and showing a portion of a subassembly of an elevator;  FIG. 8  is a partial perspective and isolated view of the elevator subassembly of  FIG. 7  and showing its relationship to a portion of the elevator subassembly of  FIG. 2 ;  
         [0038]      FIG. 9  is a partial perspective view of the elevator subassemblies of  FIGS. 2 and 8  shown assembled;  
         [0039]      FIG. 10  is a partially cut-away view of a portion of the chamber housing showing the axes of movement of the elevator, which is the vertical z axis, of the recoater blade and carrier, which is the horizontal front-to-rear y axis, and of a blade gap sensor, which are the y axis and the horizontal side-to-side x axis;  
         [0040]      FIG. 11  is a sectional side view showing the resin cart entering into the process chamber;  
         [0041]      FIGS. 12 through 14  are a series of sectional side views showing the resin cart in position in the process chamber and attached to the elevator, and elevation of the platform;  
         [0042]      FIG. 15  is a side view showing operation of the auto off-load cart, including completion of the first build which is removed to the cart, installation of a fresh platform, completion of the second build and the removal of the second build from the vat;  
         [0043]      FIG. 16  is a perspective view of a resin vat of the invention with resupply resin containers and a level maintenance container mounted thereon;  
         [0044]      FIG. 17  is a perspective view of a resin vat similar to that of  FIG. 16  showing the lid removed and interior features of the vat;  
         [0045]      FIG. 18  is a perspective view of the underside of the resin vat of  FIG. 17 ; and  
         [0046]      FIG. 19  is a graph showing temperature profiles for a resin contained in the vat of  FIG. 17 . 
     
    
     DETAILED DESCRIPTION  
       [0047]     The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. These embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.  
         [0048]     Turning now to  FIG. 1 , shown generally at  10  is a dual-chamber housing for housing two chambers  12 ,  13  for stereolithography. The housing has two chambers for increased efficiency of laser usage. While the object surface in one chamber is recoated, the laser can be applied to the recoated object surface in the other chamber so as to build objects in both chambers in a single run. The laser and the system for using the beam in multiple chambers is addressed in detail below.  
         [0049]     The housing has view windows  14 ,  15  on opposite sidewalls, one in each chamber  12 ,  13 , respectively. Each chamber has a door  16 ,  17  with a hingedly openable and removable window. The windows are used for operating an automated system for unattended removal of a support platform and completed object and placement of a fresh object support platform for a second and unattended build.  
         [0050]     Vat  21  contains a resin  18  from which the stereolithography apparatus creates three-dimensional objects. Vat  20  is shown in shadow disposed in chamber  12 . Vat  21  is shown ready to introduce into chamber  13  through open chamber door  17 . An elevator attachment bracket  23  is located adjacent the rear of the vat  21  for attachment to an elevator lift plate  82  ( FIG. 8 ) enabling an object support platform  30  ( FIG. 2 ), which is the platform on which the build takes place, to be raised and lowered in the vat with respect to the working surface of the resin. The elevator attachment bracket has hooks  86  ( FIG. 8 ) by which attachment to the lift plate is secured.  
         [0051]     The elevator attachment bracket and elevator lift plate cooperate as part of an elevator assembly that comprises several components all of which cooperate to lift and lower the platform.  FIG. 2  shows disposed above vat  21  a subassembly  22  of the elevator components that can be introduced into and removed from the chamber with the vat. If desired, the removable subassembly shown in  FIG. 2  can be dedicated to a single vat. These components include the elevator attachment bracket  23  rigidly fixed to a supporting elevator framework  24  and elevator forks  25  supported on frame  24 . Frame  24  extends vertically so as to be capable of reaching the bottom of the vat. The forks on the frame cooperate with spaced supports  27 ,  28  extending laterally from the rear of the frame to support build platform  30 . Latch  36  works with tabs  33  and  35  on the forward end of the elevator framework to secure and release the platform for automated installation and removal of the platform once the build has been completed and the elevator has been lifted from the vat.  
         [0052]     It should be recognized that the above discussion of  FIGS. 2 and 8  with respect to the elevator assembly and the subassembly  22  associated with vat  21  and chamber  13  applies equally to vat  20  and chamber  12  and that vat  20  will have similar elevator components and an object support platform. In this regard, the discussion below of the elevator assembly and subassemblies, recoater assembly, vat, process steps, and operation of the auto off-load cart in the context of one vat or chamber applies equally to another vat or chamber.  
         [0053]      FIG. 3  shows the frame  24  and elevator forks  25  for supporting platform  30  with sheet metal covering removed and exposes a latch linkage  37 . Latch linkage  37  is operable to actuate latch  36  to secure the platform in conjunction with tabs  33  and  35  on installation and to release the platform from the forks for removal from the system. The latch linkage is actuated by a spring biased rod  38  to cause latch  36  to secure the platform to the forks when installed on the forks and then release the platform for removal when the platform is sufficiently elevated above the vat.  
         [0054]     Actuation of the latch linkage for release of the build platform from the elevator forks is illustrated in  FIG. 9  with reference to  FIG. 15 .  FIG. 9  shows the platform  30  secured to the elevator forks  25  within the chamber  12  and within vat  20  within the chamber. When the elevator assembly is raised sufficiently high, rod  38  engages a ramp surface  90  on a chamber component frame  92  ( FIGS. 3, 9 , and  15 ). As the elevator continues to rise, the ramp forces the latch release rod outwardly toward the platform and actuates the latch linkage to release latch  36 . Similarly, when a platform is installed on empty forks and the elevator assembly is lowered so that the ramp does not engage the release rod, then a spring biases the latch release rod and linkage to close the latch and secure the platform against tabs  33  and  35  on the forks, as illustrated in  FIG. 15 .  
         [0055]      FIGS. 7 and 8  illustrate in detail a second subassembly  39  of components of the elevator assembly.  FIG. 7  illustrates the subassembly in the context of the stereolithography chamber and  FIG. 8  illustrates the subassembly in relation to the elevator attachment bracket  23  of the first subassembly. These components of the second subassembly are fixed in stereolithography chamber  12  along the back wall  80  ( FIG. 7 ) opposite door  16  ( FIG. 1 ) and do not enter or exit with a vat as do the components of the subassembly  22  shown in  FIG. 2 . These fixed elevator components of the second subassembly of  FIGS. 7 and 8 , the chamber elevator components, receive the elevator components of the first subassembly, the vat elevator components ( FIG. 2 ), and specifically the elevator attachment bracket  23  of  FIG. 8 , to form, in combination, an entire elevator assembly. The elevator assembly with the two subassemblies connected is shown in  FIG. 9  in a perspective view.  
         [0056]     Elevator lift plate  82  includes a locating pin  83  for fitting into a receiver  96  of  FIG. 9  on elevator attachment bracket  23 , locating attachment bracket  23  and thereby the elevator frame  25  and forks  24  in the horizontal x, y plane. The elevator lift plate  82  includes a rod  84  that engages the hooks  86  on the attachment bracket  23  ( FIG. 8 ) for raising and lowering the attachment bracket and associated frame and forks within and out of a vat of resin. A lift screw  85  is shown in  FIGS. 7 and 8  for the elevator lift plate  82 . Lift screw  85  is turned by motor  91  to lift and lower the elevator lift plate, attachment bracket, frame, and forks along the vertical z axis. Also included is a locating pin  88  for locating the resin vat  20  in a horizontal plane parallel to the x,y plane of the surface  112  of resin  18  (see briefly  FIG. 12 ). Locating pin  88  inserts into receiver  200  on the vat (see briefly  FIG. 18 ).  
         [0057]      FIG. 10  illustrates in partial perspective the elevator lift plate  82  positioned on the z axis to receive a vat of resin in stereolithography chamber  12  and the positions in relation thereto of the laser scanner  100  and recoater blade and carrier,  42  and  44  respectively. The sequence of steps of rolling a vat into a stereolithography chamber, preparing for, and removing a build are shown in  FIGS. 11 through 15 . As a vat with its elevator subassembly and secured platform are rolled into a stereolithography chamber of the invention and centered on locating pin  88 , then the attachment bracket  23  is aligned vertically with the elevator lift plate  82  and lift rod  84  ( FIGS. 11 and 12 ). Motor  91  turns lift screw  85  to raise the elevator lift plate ( FIG. 13 ). As the lift plate rises, the lift rod engages and seats in hooks  86  on the attachment bracket  23  ( FIG. 13 ) and locating pin  83  engages and seats in receiver  96  (see briefly  FIG. 8 ) on the attachment bracket, thereby joining the chamber elevator and vat elevator subassemblies and centering the vat elevator subassembly within the vat. As the lift plate rises farther still, the vat elevator subassembly and platform rise within the resin ( FIG. 14 .). Raising the lift plate sufficiently releases the latch  36 , as discussed above. Lowering the lift plate sufficiently releases the lift rod from engagement with the attachment bracket so that a vat and vat elevator subassembly can be removed, if needed, as is the case when installing a fresh resin. Normally, the vat and vat elevator subassembly are removed as a unit separately from a platform having a build upon it.  
         [0058]     Turning now to a discussion of the recoater assembly and its use for mapping the blade gap prior to a build, the recoater blade  42  and carrier  44  traverse the resin working surface  112  in the y direction through chamber  12 , best seen in  FIG. 14  The recoater assembly is identified generally at  40  ( FIG. 4 ). The recoater assembly includes a recoater blade  42  and a carrier  44  to which the recoater blade is attached and which provides for movement of the recoater blade. The recoater blade is computer controlled to move along the axes as shown: 1) horizontally back and forth across the surface of the resin, in the y axial direction, 2) vertically up and down in the z axial direction, and 3) rotationally about the center of the blade, which is the axis theta, parallel to and spaced from the y axis. A conventional function of a recoater blade and of the one illustrated is to speed up leveling of fresh resin layers between laser scanning exposures of the working surface  112 , which typically provides parts of greater accuracy in a shorter period of time than deep dipping and gravity settling.  
         [0059]     The carrier for the recoater blade of  FIG. 4  is mounted to a vertical motion stage  47 . Vertical motion stage  47  sits in a track  49  for translating the blade in the up and down direction along the vertical z axis. Track  49  in turn mounts into track  50  and travels in track  50  to advance the recoater blade across the surface of the resin, in the horizontal y axial direction. Cable drives and associated stepper-based linear actuator motors have been determined to be suitable for use in these aspects of the practice of the invention.  
         [0060]     The recoater assembly of the invention includes a sensor  45  for providing readings to a controller to keep the recoater blade parallel to the resin surface throughout its length of travel. Each end of the recoater blade is kept at the same distance from the resin surface. The sensor is contained within carrier  44  and is translatable in the x axial direction along the length of the carrier so that distance readings can be obtained in different locations along the x axis of the surface of the resin. A cable drive contained in the carrier can be used to translate the sensor in the carrier, powered by a motor in the vertical motion stage  47 .  
         [0061]     The distance of the recoater blade from the resin surface when sweeping across the surface is termed “blade gap.” The blade gap typically depends on the resin chosen for the particular build and its physical characteristics, and is a quantity that is empirically predetermined for the build and stored in the stereolithography control computer&#39;s memory. The fiction of the sensor  45  is to provide the data necessary to keep the blade gap as specified throughout the range of travel of the recoater blade across the resin surface. Variances in the tracks in which the recoater assembly travels and other sources of machine error can change the blade gap. The computer controlled recoater assembly of the invention substantially resolves these problems, taking what has been a hardware problem in the industry and providing a software solution.  
         [0062]     Sensor  45 , best seen in  FIG. 4 , is a laser diode sensor and is a high resolution sensor with a narrow measurement range. An Omron optical sensor Model No. ZXLD30, available from Omron Electronic Components in Schaumberg, Ill., has been determined to be useful as the sensor  45 . The Omron sensor is highly sensitive and works by emitting a focused energy beam to contact the target and then receiving the reflected beam, from a comparison of which distance to the target can be determined with a degree of accuracy sufficient for stereolithography.  
         [0063]     The recoater blade  42  is kept parallel to the resin surface in response to data obtained by the Omron sensor  45  prior to the start of a build. The sensor obtains data from which the computer controller determines the distance from the bottom  76  ( FIG. 6 ) of the recoater blade to the top or working surface  112  ( FIG. 12 ) of the resin along a variety of points y at two points x along each side of the resin vat corresponding to the two ends of the recoater blade.  FIGS. 4, 5 ,  6 , and  14  illustrate the relative locations of the Omron sensor  45  and the recoater assembly  44  and the working surface  112  as the recoater carrier  44  with sensor  45  traverse the working surface  112  to obtain this data and map the adjustments to the recoater blade that will be made during a build. During a build, the computer controller rotates the recoater blade, in response to the map obtained prior to the build, about its axis of travel, theta, which is an axis parallel to and spaced from the y axis of the resin surface, to keep the two ends of the recoater blade the same distance from the resin surface at spaced points x along the y axis. The data based on which the blade is rotated is not obtained in real time, and the difference between real time data and a map obtained prior to the start of a build has not been determined to be significant.  
         [0064]      FIGS. 5 and 6  show an end portion of the recoater blade  42 , taken along the x axis and corresponding to the right hand end of the recoater blade illustrated in  FIG. 4 .  FIG. 6  shows the same end as  FIG. 5  and from the bottom of the blade to fully illustrate the blade&#39;s features. Feet  56 , only one of which is shown, extend laterally outwardly from the bottom of the blade on each end, the bottom surface of which, surface  60 , defines the bottom of the blade for measurement purposes. Each foot is cut down or precision ground to a thin surface  58  on the end of the foot away from the blade at the top surface for obtaining the Omron sensor readings for the bottom of the blade. The distance from the bottom of the foot,  60 , to the top surface of the cut down portion,  58 , is fixed and stored in the computer. This foot depth distance is small owing to the operating range of the Omron sensor.  
         [0065]     To obtain readings, the Omron sensor is set within the carrier at a point along the x axis, adjacent to one end of the recoater blade.  FIG. 4 , shows the sensor  45  in shadow in the carrier adjacent the left end. The blade does not touch the resin when obtaining readings and no resin obscures the top of the foot from the Omron sensor. As can be understood from  FIGS. 4, 6  and  14  in the context of the chamber  12 , at a fixed location x, y, the Omron sensor  45  takes a reading of the distance from the sensor to the top  58  of the foot  56  to assign a value for the foot position corresponding to the bottom  60  of the foot and based on the predetermined depth of the foot between the surface  58  and bottom  60 . The Omron sensor is displaced a small distance x to approximate the same location x as the foot and sufficient to enable the Omron sensor to take a reading from the sensor to the top of the resin surface,  112 . The computer control determines the difference between these two readings for the bottom of the foot and the resin surface and stores the datum. The recoater assembly then translates a distance y (not shown) to obtain additional data points until the entire surface along one side of the vat has been mapped at a variety of points y for one x. Thereafter, the Omron sensor translates to the opposite side of the carrier, the right side, to obtain data mapping that side of the vat at different locations x and the same locations y to complete the map. The entire map is obtained by computer control and stored for use during the subsequent build.  
         [0066]     The recoater blade  42  can be attached to and removed from the carrier  44  entirely by hand. The stereolithography system&#39;s computer controls alignment of the recoater blade and substantially reduces the tedious procedures associated with prior apparatus. Turning now to  FIGS. 5 and 6  and a discussion of the features of the recoater blade and carrier that provide for ease of installation and removal of the blade, knurled handles  55 , only one of which is shown on the recoater blade of  FIG. 6  are used for installing the recoater blade and removing the recoater blade from the carrier by hand. It should be recognized that there is a corresponding handle on the end of the blade not illustrated in  FIG. 6 , as can be seen in  FIG. 4 . A receiver  64  on the recoater blade ( FIG. 5 ) receives a corresponding alignment pin  66  on the recoater carrier. Pin  66  is illustrated round in cross-section and this shape can be varied. It may be desirable to provide a second alignment pin of a different shape on the opposite end of the recoater carrier and a corresponding receiver on the blade. These alignment pins assist the operator to make sure the blade is correctly oriented on the carrier. Magnets  70  and  71  or other attachment means, one at each end of the blade carrier and one at each end of the recoater blade, respectively, secure the recoater blade on the carrier. Other attachment means can be used, although these may require tools for installation or removal of the blade. Contacts  72  can be provided to activate a proximity switch  75  for indicating that the blade is properly secured in place on the carrier. It is useful to provide three such contacts, one on the end of the carrier as shown, and two on the opposite end so that a three-point contact is required to activate the proximity switch to signal correct position for the recoater blade on the carrier.  
         [0067]     The recoater blade includes a vacuum channel  77  on its bottom surface  76  seen in inverted position in  FIG. 6 . The vacuum channel aids in leveling the fresh resin layers in a conventional manner. The blade includes a centrally located sight window  78  ( FIG. 4 ) for sighting by an operator whether the vacuum is activated. The vacuum connection between the blade and carrier is not hard plugged and requires no tools to complete. The vacuum connection is “soft” in that connection is provided between cooperating and sealing vacuum ports located centrally of the blade and carrier, typically a vacuum cup on the blade and a cooperating member on the carrier.  
         [0068]     Prior to installation of the vat in a stereolithography chamber, the recoater assembly is “parked,” which is to say the recoater blade is located on the y axial direction nearest the door of the chamber and is raised in the z axial direction out of the way of the vat which has wheels affixed to its bottom so it can be maneuvered like a cart ( FIGS. 11 and 12 ). The vat can be rolled into the chamber without striking the recoater assembly. Once the vat is installed, the recoater assembly can be lowered adjacent to and spaced from the surface of the resin for mapping the relationship between the resin surface and the bottom of the recoater blade ( FIG. 17 ). The invention accomplishes mapping with the recoater blade held a greater distance above the resin working surface than the blade gap, a distance sufficient to make sure the feet on the blade do not become covered with resin, which would negatively impact the ability of the Omron sensor to develop the data needed to control the blade gap.  
         [0069]     When a build begins, the recoater blade vacuum is turned on and the blade is lowered to the predetermined blade gap for the resin ( FIG. 14 ). The vacuum pulls resin up into the vacuum channel  77  in the blade and into the sight window  78 . The feet typically will have resin over them as the blade sweeps the surface, which can occur for each application of a fresh resin layer to the build on the platform.  
         [0070]     The accuracy of the build is very sensitive to maintaining a precise level of the resin in the vat. The build plane is established prior to the start of a build. The laser scanning system  100  ( FIG. 19 ) is rigidly mounted to the chamber and is controlled to strike the resin working surface at a particular point in space, termed the “build plane,” which establishes the range of z values in which the x, y plane of the working surface can be located. The ability of the invention to automate effective leveling of the recoater blade depends on keeping the resin at the same level as when the map of the resin surface was established prior to the build.  
         [0071]     A second Omron liquid level sensor is shown at  87  ( FIGS. 7 and 9 ) and is rigidly fixed to the elevator frame in the rear of the chamber housing for determining the level of resin in the vat and whether additional resin should be added to the vat. Sensor  87  determines whether resin needs to be added or removed to maintain a build plane during a build. During the build, the sensor  87  determines the level of resin in the vat so that resin can be added or removed to maintain the same level at all times and thus at the same distance from the blade as that obtained during mapping. The Omron sensor is a laser diode sensor operating closed loop in that the sensor operates between successive scans by the laser of the working surface and shuts off when the appropriate resin level is reached.  
         [0072]     A vat  21  with supplemental resin containers  127 ,  128 ,  129  for use in controlling the resin level is illustrated in  FIGS. 16 and 17  with a removable cover  141 . Resin containers  127 ,  128  and  129  ( FIG. 16 ) are retained in tiltable receptacles  133 ,  134  and  135  respectively which are pivotable away from vat  21  by engaging handles  136 ,  137  and  138  to facilitate ease of installation and removal. Each container  127 ,  128 , and  129  connects to vat  21  via a quick disconnect double shut off coupling. Each container  127 ,  128 , and  129  has a nozzle that mates with a coupling (both not shown) inside the corresponding receptacle  133 ,  134 , and  135  on vat  21  so that when connected resin is able to flow from the container through the coupling into the vat  21 .  
         [0073]     It is useful to mark the resin containers with radio frequency identification tags (RFID technology) so as to ensure accurate resin replacement and to avoid the costly error of mixing different resins. An operator can be alerted prior to connecting the resin vat to the supplemental container if the resins are different and can obtain confirmation of the correct resin. Each nozzle has molded or otherwise integrated into it an RFID tag. Each coupling is a “smart coupler” because it has molded or otherwise integrated into it a reader to sense and pass on to the stereolithography system&#39;s control computer data about the container and resin in it, such as resin type, resin batch number, expiration date, resin volume and potentially the vat and stereolithography system identities in which the container is being used. The readers are proximity readers so the stereolithography system control computer can alert the operator with an alarm before the container nozzle is couplingly connected to the vat via the receptacle coupling if the incorrect resin or an expired resin is being installed in the container. This data flow between the vat and the stereolithography system&#39;s computer occurs through the data and power port  101  when the vat and the stereolithography system are connected via appropriate cabling.  
         [0074]     Each container can also track the amount of resin that flows from it into the vat. Two of the supplemental resin containers,  127  and  128 , refill the vat between builds and operate through bellows pump  102  to supply a sufficient stroke volume of about a liter of resin per minute for a  420  liter capacity vat. Two Omron ultrasonic sensors  186  and  188  ( FIG. 17 ) are mounted on a wall of the vat  21  determine whether the resin is within preselected minimum and maximum values prior to a build to signal whether resin needs to be added. Each vat  21  also has an RFID tag  19  on the outside wall on which the elevator attachment hooks  86  of  FIGS. 11 and 12  are supported, which is read by a reader  81 , and passes the data about vat identity, initial resin quantity, and date of installation on to the stereolithography system&#39;s control computer.  
         [0075]     Resin is supplied first from one container, for example  128 , via line  106 , and when that container is emptied, from the other container, for example  127 , via line  107 , by the valve assembly  131 . The stereolithography control computer opens the appropriate valve in response to sensing by the ultrasonic sensor that the resin is below the minimum level prior to the start of a build. Replenishing resin flows from the supply container to vat  21  through valve assembly  131  and supply line  111  to the bellows pump  102  via an inlet port  182  and through an outlet port  184  ( FIG. 17 ).  
         [0076]     The other of the containers,  129 , acts as a reservoir, via inlet line  108  and outflow line  109 , to lower or increase the level in the vat in response to fluctuations during a build by means of a two-way flow valve in the valve assembly  131 . The stereolithography system control computer actuates the two-way flow valve by a command in response to liquid level sensor  87 . Sensor  87  takes a reading of the precise level in the vat between each layer. The resin may shrink when solidified. Displacement of resin by the platform and build as these are lowered may impact the level of resin in the vat. A useful pump for controlling resin level during a build is a metering pump, such as peristaltic pump  104 , for delivery of small, precisely controlled amounts of fluid and may take several strokes to add or remove resin so that the level in the vat can be closely controlled. A peristaltic pump can supply about  1  micron of fluid volume over several strokes to provide precise control.  
         [0077]     Valve assembly  131  can also circulate resin from the vat  21  through line  110  through valve assembly  131 , line  111 , bellows pump  102 , line  113 , and back into vat  21 , if needed. This circulation feature can help preserve the quality of the resin in the vat  21  and prevent viscosity increases. It can best be employed between builds either automatically or operator initiated through the stereolithography control system&#39;s software.  
         [0078]     As illustrated in  FIG. 16 , resin containers  127  and  128  have been removed and titlable receptacles  133  and  134  are shown in a tilted open position for receipt of resin containers. Resin supply line  111  and the cover for bellows pump  102  have been removed in  FIG. 17  revealing supply fitting  180  on valve assembly  131  and inlet port  182  on bellows pump  102  to which the resin supply line is connected for supplying resin to replenish the vat between builds in a sequential build mode. Outlet port  184  conveys the resin to the vat from the bellows pump via the portion of line  111  shown above titled receptacle  134 .  
         [0079]     Turning now to a discussion of the laser system used in the practice of the invention, it should be recognized that the focal plane of the laser beam emitted by the Omron sensor is the same whether at the resin surface or the plane of the recoater blade. A conventional beam profiler system employing a detector array and establishing a Gaussian beam distribution determines the beam location and width of the laser beam. The system can change the focal length of the beam using a 3-axis scanner that is self calibrating and permits customized blade gap settings for different resins. Storage of this information over time will establish a library of data log files for particular resins in individual systems of the invention.  
         [0080]     Laser systems of the type typically used for stereolithography are useful in the practice of the method and apparatus of the invention. An x, y scanning laser employing mirrors controlled by galvanometers to position the laser beam are useful. A scanning system  100  is illustrated in a highly schematic view in  FIG. 15  applying energy to the working surface  112  of the resin, the build plane, in a predetermined path to solidify a layer  115  of an object  117 . A laser window normally isolates the laser and galvanometer systems from the process chamber, which is heated.  
         [0081]     Dynamic beam deflectors can be used to generate more than one sequential path for the laser beam so that the laser can be used more effectively. To increase efficiency, a single laser can be used in connection with the practice of the invention to provide energy to two or more separate stereolithography chambers  12 ,  13  ( FIG. 1 ) and galvanometer systems for simultaneous builds. While a three-dimensional object in one chamber is being recoated with a layer of fresh resin, the laser can be conducting a scanning exposure in the adjacent chamber so that the laser does not sit idle between recoats of a build.  
         [0082]     The laser control system is capable of dynamically changing the laser focus so that larger objects can be produced without a loss of precision. As shown in  FIG. 9  , detector cells  89  located in the rear of the chamber and mounted to the chamber elevator subassembly frame  92  provide information for controlling the intensity, focal length, and spot size of the laser beam that is provided by scanner  100  ( FIG. 15 ) and is used to solidify the resin. As with the Omron sensor, a 3-axis scanner is useful to change the focal length and spot size of the laser so that the build is the same quality and precision whether in the middle of the resin or at the outer edges of the vat.  
         [0083]     The laser conducts a scanning exposure in the x, y plane of the surface of the resin. The progress of the build continues layerwise until the three-dimensional object is completed and the elevator assembly raises the support build platform  30  out of the resin  18  and above the working surface  112 . It should be recognized that the support layers for the desired object are the first to be solidified. The support platform is gradually lowered as cross-sectional layers are solidified by application of the laser beam. The laser solidifies a layer, the elevator lowers the platform to provide a fresh layer of resin and the recoater levels the resin to provide one layer thickness. After multiple sequences of scanning exposure of the laser and recoating with resin, the platform  30  has been lowered to a greater depth in the resin  18  and a single build object  117  has been completed. The elevator then removes the build and platform from the resin to the unload position.  
         [0084]     Automated offloading of a completed build and supporting platform and the completing a second build involved a sequence of steps. A completed build object  117  is supported on a platform  30  and elevated for release of the platform latch (not shown) as discussed above. It should be recognized that the recoater blade  42  and carrier  44  are parked prior to elevation of the build out of the resin. An auto offload cart  120  is connected to a computer control (not shown) for carrying out the unattended platform swap by which a second build can occur. Cart  120  has telescoping arm segments  123  and  125  (best seen in  FIG. 15 ) for, respectively, supporting and conveying a drip tray  122  for offloading the build object  117  and platform  30  and for supplying a fresh platform  124  for the vat.  
         [0085]     The side sectional view of  FIG. 15  illustrates that the offload cart  120  is rolled into contact with the door  16  or  17  of  FIG. 1  of a stereolithography chamber from which the window has either been manually removed or swung to the open position, and under a portion of the resin vat. As shown in  FIG. 1 , the doors have brushes defining an opening at the bottom through which the rollers of an auto offload cart enter. The auto offload cart should be docked with the vat so that the telescoping arms coordinate with the vat and elevator for a flawless platform exchange.  FIG. 16  illustrates fittings  132  and  139  that can be used to ensure docking to the resin vat. Docking is not automated, and is performed by an operator who also connects the cart to the computer controller system (not shown) for the stereolithography system. The window in the chamber door,  16 ,  17 , ( FIG. 1 ) is hingedly opened or removed by the operator so that the automated build removal can proceed.  
         [0086]     At the end of the build, the platform  30  and build object  117  are elevated sufficiently high to release latch  36  securing the platform against tabs  33  and  35  to the forks and frame  24 . A telescoping arm  123  extends from the cart having a drip tray  122  thereon so that the elevator frame  24  is disposed above the drip tray. The elevator frame is lowered and the drip tray and forks are configured so that the platform and build rest on the drip tray and the frame passes through. Telescoping arm  123  is retracted and the build object and platform are removed from the chamber to rest on the cart. The elevator frame is then lowered to receive a fresh platform, latch  36  still in the released position.  
         [0087]     Telescoping arm  125  is extended and has a fresh platform  124  placed thereon. The elevator frame is then raised to engage and receive this fresh platform and the telescoping arm is retracted). It should be recognized that the latch  36  securing the platform to the elevator frame does not engage until the platform is lowered sufficiently, in reverse of the method by which the latch is opened.  
         [0088]     Once the fresh platform is in place and the latch secured, the elevator can lower the fresh platform into the vat of resin and below the resin surface for a determination, as discussed above, as to the amount of resin that is required to be added for the second build. The second build is completed as the first, providing, as shown in  FIG. 15 , an offloaded first build object  117  and a second build object  130  elevated on its platform above the resin vat.  
         [0089]     Having described the apparatus of the invention in some detail, and turning now to a consideration of the process steps, a basic flow diagram for accomplishing an unattended build in a single vat according to the invention will be described. In the unattended build mode, the apparatus builds a first three-dimensional object or objects, removes the completed build from the vat and elevator, and completes a second, unattended build. After the first build is removed, the apparatus installs a fresh platform on the elevator and adjusts the resin parameters as required, completes the second build, and removes the second build from the resin vat. It should be recognized that several objects can be built in a build simultaneously in a single vat or in adjacent vats and that the following description illustrates an unattended build of a single object in but one vat. Unattended builds can occur simultaneously in other vats in a multi-vat process.  
         [0090]     At the process start, the human operator will have performed several functions. Having selected an unattended build mode, the operator will first need to input the object representation, typically using a CAD/CAM program for the object representation. The operator then determines the volume of resin required for the first build and whether the build volume is within the design limitations of the apparatus. For example, if the capacity of the apparatus includes building parts using up to 20 kilograms resin and no more, then if the object selected requires more, the operator will need to select a different build mode. If the design capacity provides for making a first and attended build, but not an unattended second build, then the unattended mode cannot be used.  
         [0091]     The operator also verifies that a recoater blade is installed and is parked up and out of the way so as not to hit the resin vat when installed. Once the resin vat is installed, then the operator should verify that the vat is correctly installed and that the vat contains sufficient resin of the correct type. Normally, the vat will include an elevator subassembly, including an elevator attachment bracket, elevator supporting framework and forks, and a build platform secured by latch  36  and tabs  33  and  35  to the forks. The entire vat and elevator subassembly is rolled into the stereolithography chamber to engage the chamber elevator subassembly.  
         [0092]     The relationship between the resin surface and the recoater blade can be mapped at this point or another point prior to the build and the data stored for use during the build. Once a particular resin is identified, the blade gap for the resin selected, and the level of the resin reproducibly controlled within the vat, then the map for these conditions should be useful for the same apparatus over a period of time.  
         [0093]     The operator then installs the auto off-load cart having once satisfied the initial requirements and verified that an unattended build is supported. To install the auto off-load cart, the operator hingedly opens or removes the window on the chamber door so that the telescoping arms of the auto off-load cart can extend into the process chamber for retrieving the platform and first build. The operator docks the auto off-load cart into the vat with the chamber door closed. The chamber is heated and to avoid disturbances to the process, the chamber door is kept closed. Wheeled feet extend from the auto off-load cart into the chamber through a cutout in the bottom of the chamber door that is covered with brushes to minimize debris and heat losses. The vat and auto off-load cart are configured to maintain a consistent position when docked, one with respect to the other, for automated operation. The operator also makes sure the auto off-load cart is connected to the stereolithography system&#39;s computer for control of the automated operation.  
         [0094]     Operators may change shifts at any point during preparation of the apparatus of the invention for the unattended build mode. Consequently, successful operation of the system often requires that the operators verify information about the system more than once. Thus, it is useful to have the computer control prompt the operator to verify before the first build starts that an empty platform is in fact in place on the auto off-load cart for installation when the first build is completed. Alternatively, verification that a platform is properly in place can be accomplished using appropriate sensors.  
         [0095]     Next the operator then causes the recoater assembly and elevator to go to the start positions. The elevator is lowered into the vat and is brought to a level just under the surface of the resin so as to define the working surface. The recoater assembly is lowered to the resin to define the preselected blade gap between the bottom of the foot and the working surface of the resin.  
         [0096]     At this time, just prior to the actual start of the laser and recoater, it is useful to prompt the operator to verify the operating parameters. The operator should verify that a build platform is actually installed on the forks. If the build were to start without a platform in place, the results in lost productivity and resin could prove costly. If the platform is not in place on the elevator forks, then the operator should park the recoater, raise the elevator to the platform release position, and install a fresh platform. Once the presence of a platform is verified, then the resin and chamber temperatures should be checked. Typically, temperature control is a computer controlled function that is continuously performed. Nevertheless, it is useful for the operator to verify that the temperature is correct prior to initiating a build. The operator should also verify that the vat has sufficient resin. Even if the capacity of the system is adequate for the build, the system should be checked to verify that the vat contains the resin and that the resin level is between the preselected minimum and maximum levels in the tank necessary for fine level control of the build plane.  
         [0097]     The operator should also verify that the supplemental resin containers contain sufficient resin for refilling the vat between builds and for fine level control during the builds. If not sufficient, then the operator should be prompted by the system to replace the partially full containers with full containers and to verify that the new containers contain the same resin as is in the vat. One efficient method of verifying the resins are the same is to run a radio frequency identification routine, or “RFID” routine. RFID tags can be included in the containers for automated identification prior to completing connection to the vat, after which the operator can complete installation if the resins are the same. The use of RFID tags on the containers and the vats permits data collection on the system&#39;s resin and resin usage to occur via data flow from the particular RFID reader on the elevator assembly for the vat and the individual RFID readers on the smart couplers on the vat for each container.  
         [0098]     If the above parameters have been met, then the build can proceed. The operator should then turn on the recoater vacuum and adjust the resin level and resin and chamber temperatures. At this point, the resin level is within the preselected minimum and maximum levels and the level is adjusted within these levels to the precise level of the build plane that has been selected. Material is pumped into or removed from the vat by a metering pump interface with the two supplemental resin containers used for this purpose, and is automatically controlled in response to a sensor.  
         [0099]     The recoater blade prepares the working surface to receive the laser and the actual stereolithography build can now begin with preparation of the support layers. At this time, the operator&#39;s attendance to the process is no longer needed and the build proceeds based entirely on computer controlled functions. Typically, after each layer is solidified, the elevator will lower the platform to receive a fresh coating of resin and raise the platform sufficiently for lasing of the next layer. Resin level is adjusted as needed depending on the amount of shrinkage due to solidification and displacement by the platform and object below the surface of the resin. The recoater blade sweeps the surface between each layer to prepare the working surface and the build proceeds. The build may include one or more three-dimensional objects.  
         [0100]     Once the build with the number of objects being fabricated is completed, the build is stopped, and the laser is turned off, then the system proceeds without an operator in attendance to exchange platforms. The system parks the recoater assembly up and out of the way of the elevator forks so as to enable the elevator to move completely out of the resin vat with the build on the platform. The elevator is raised to the unload position in which the latch securing the elevator platform is released and the platform can be removed from the forks. The elevator forks, build platform, and build are now positioned over the vat and the unused resin still in contact with the forks, platform, and build object then drains into the vat. After a sufficient dwell time to provide an effective drain, the auto off-load cart removes the platform and completed and drained build object. Computer control extends a set of telescoping arms from the auto off-load cart underneath the elevator forks so that the forks can be lowered to deposit the platform and build on the telescoping arms of the auto off-load cart. A drain pan normally is desirable on the telescoping arms so that the platform and completed build object are deposited onto the drain pan on the telescoping arms. The telescoping arms retract to remove the platform and build object(s) from elevator forks and the stereolithography chamber.  
         [0101]     After the first build is removed from the chamber, the elevator moves the forks into position to receive a fresh platform. Telescoping arms again extend from the auto off-load platform. Depending on the configuration of the auto off-load cart, the cart may have one or two sets of telescoping arms. If two, then the first build and platform remain in place outside the chamber. If one set, then the first build object again enters the chamber and the area above the vat and is positioned above the forks. The elevator forks are raised to engage and receive a fresh platform from the telescoping arms and then the telescoping arms are removed, the first build object(s) and platform are stored with the auto off-load cart until the second build is completed and the operator returns to the system.  
         [0102]     Once the fresh platform is installed, the system returns to repeat several of the previous steps. The elevator lowers the fresh platform into the resin and brings the platform to the appropriate level. The system automatically and in response to sensors refills the vat and adjusts the resin levels and temperature and sweeps the working surface to prepare for the second build. The second build proceeds and, when completed, the recoater is parked and the elevator removes the second build and platform to an upper position out of the vat.  
         [0103]     When the operator returns, the first and second builds are complete, the first build is stored outside the chamber on the auto off-load cart, and the second build is within the chamber, drained above the vat and ready to unload. It should be recognized that a single build is similar in the steps of building an object, and that the auto off-load cart may or may not be installed as desired. In either case, for a single build the system is directed to shut down after the first build. The system of  FIG. 1  is a dual chamber system, and so two unattended builds can be performed using a single laser and separate scanners and auto off-load carts for each chamber to provide two builds outside the chamber, one on each cart, and two inside, one in each chamber.  
         [0104]     The invention also provides for rapid exchange of resin vats. The resin cart, the easily removable and replaceable recoater blade, and the elevator subassemblies all cooperate to enable a fresh vat of heated resin to be installed and components that have come into contact with the resin and been wetted by the resin to be exchanged, all with minimal downtime between stereolithography operations.  FIG. 17  shows resin vat  21  having the lid  141  ( FIG. 16 ) removed to illustrate features mounted on an interior wall of the vat, including those related to heating the resin prior to the resin vat being rolled into the stereolithography housing and fixed in place. It should be understood that lid  141  is normally in place on the top of vat  21  during heating and is removed when the vat is installed into the stereolithography housing. Lid  141  serves an important function in heating of the resin to maintain heat in the vat.  
         [0105]     Shown on the interior of vat  21  in  FIG. 17  and mounted along a wall thereof is a thermistor  190  for determining the temperature of the resin near the working surface of the resin. The working surface should be maintained at a steady temperature near the desired set point for the particular resin. A second thermistor is located centrally on the bottom of the vat behind a small cover  192  for indicating the temperature of the resin at the bottom of the vat where the resin is heated. The thermistor at  192  is directly indicative of the temperature of the wall of the bottom of the vat and indicates the highest temperature attained by the resin so as to preclude overheating and thermally degrading the resin. Thermistors  190  and  192  are electrically connected to the computer control for the stereolithography system through input/output connection  101  ( FIGS. 17 and 18 ) and provide temperature data to the controller that is used to control the application of heat to the resin, to maintain the working surface of the resin at the desired set point, and to avoid overheating the resin.  
         [0106]      FIG. 19  illustrates a graphical relationship of the temperature of the resin over time, as indicated at the working surface and at the bottom of the resin vat where heat is applied to the resin. For the resin used in connection with  FIG. 19 , the maximum temperature to which the resin is heated, which is adjacent the bottom of the vat, is 45 degrees Centigrade and this peak temperature can be held for at least 4 hours without harming the resin. Peak heat applied to the resin in this manner minimizes the time required to approach the set point for the working surface. For the resin selected in connection with  FIG. 19 , set point temperature can be reached in 7 hours with a degree of variance of 1 degree. As can be seen in  FIG. 19 , the set point was nominally reached within one degree within 5 hours. Maximum steady state temperature, after the initial heating cycle and for maintaining the resin at the bottom of the tank in a heated condition, and thereby the working surface within one degree of set point, is 34 degrees. Thus, the resin can easily be heated overnight to operating temperature or during a prior build after which it is anticipated that a fresh vat of resin would be desired. It should be recognized that the maximum peak and set point temperatures may vary from resin to resin.  
         [0107]     The resin is heated through silicone blanket heaters  194  and  196  encapsulating resistance wires and mounted by an adhesive to the exterior walls of the vat and normally the bottom wall. Additional blanket heaters may be mounted to the vertical side walls, if needed, and associated thermistors desirably would be located centrally adjacent these heaters. The blanket heaters are connected to electrical input/output connection  101  through wires and a temperature controller located behind a panel  198 . The blanket heaters are connected and operational through input/output connection  101  when the vat is installed in the stereolithography housing and for heating the resin to steady state prior to installing the vat into the housing. Once installed, heated air can be supplied in nonturbulent and laminar flow in the housing to flow around the bottom of the vat and across the top of the open surface of the vat to maintain the working surface of the resin at a precise temperature and to preclude temperature effects on the laser.  
         [0108]     Once a build sequence has been completed, then the completed build and platform are removed from the housing. The vat elevator subassembly, including the forks, is lowered back into the spent vat so that the spent vat and wetted elevator components can be removed from the housing. In the sequential build mode, illustrated in  FIG. 15 , cart  120  and build  117  are removed from contact with the housing and then the second completed build  130  and associated platform  124  can be removed. Elevator subassembly  22  ( FIG. 2 ) is then lowered back into the vat for removal with the vat. The elevator is lowered to the position illustrated in  FIG. 12 . Latching hooks  86  release rod  84  in the reverse of the procedure for installing the vat described with reference to  FIG. 8 . The operator then rolls the spent vat  20  out of the stereolithography housing where the vat can be cleaned and reconditioned for refilling with resin and reuse.  
         [0109]     In the meantime, a fresh vat of resin  21 , having its own dedicated vat elevator subassembly installed, has been heated to the desired set point during the completion of the prior build and is ready for immediate installation into the housing. The operator removes the wetted recoater blade and installs a fresh one. The operator removes the lid from vat  21  ( FIG. 16 ), severs the electrical connection to the vat for preheating at  101 , and rolls the fresh vat into place in the housing for connection of the dedicated vat elevator subassembly to the housing elevator subassembly in a manner similar to that illustrated in  FIGS. 11 through 14 . Typically, the resin in this fresh vat will be a different resin, although the same type of resin as previously used can also be used in the fresh vat. Thereafter, a build sequence can be repeated as described above. The entire vat exchange sequence can be accomplished in 10 minutes or less.  
         [0110]     Many modifications and other embodiments of the invention set forth herein will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. All patent applications, patents, and other publication cited herein are incorporated by reference in their entirety.