Patent Publication Number: US-10766193-B2

Title: Three dimensional printing system with resin containment solution

Description:
RELATED APPLICATIONS 
     The present application is a continuation-in-part of patent application Ser. No. 15/720,707 entitled “THREE DIMENSIONAL PRINTING SYSTEM WITH IMPROVED RELIABILITY, SAFETY, AND QUALITY” filed on Sep. 29, 2017 which is a continuation in part of U.S. patent application Ser. No. 15/612,228 entitled “THREE DIMENSIONAL PRINTING SYSTEM WITH IMPROVED MACHINE ARCHITECTURE” filed on Jun. 2, 2017. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure concerns an apparatus and method for fabrication of solid three dimensional (3D) articles of manufacture from radiation curable (photocurable) resins. More particularly, the present invention provides a solution for avoiding catastrophic failures in a three dimensional printing system. 
     BACKGROUND 
     Three dimensional (3D) printers are in rapidly increasing use. One class of 3D printers includes stereolithography printers having a general principle of operation including the selective curing and hardening of radiation curable (photocurable) liquid resins. A typical stereolithography system includes a resin vessel holding the photocurable resin, a movement mechanism coupled to a support surface, and a controllable light engine. The stereolithography system forms a three dimensional (3D) article of manufacture by selectively curing layers of the photocurable resin. Each selectively cured layer is formed at or proximate to a “build plane” within the resin. 
     One challenge with stereolithography systems is the reliability and maintenance in working with liquid resins. In particular, resin containment failures can cause catastrophic damage to a three dimensional printing system. What is desired is a system with an improved resin containment solution. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  is an isometric drawing depicting an exemplary three dimensional printing system. 
         FIG. 1B  is a side view of an exemplary three dimensional printing system. 
         FIG. 1C  is a rear view of an exemplary three dimensional printing system. 
         FIG. 2A  is a schematic block diagram of an exemplary three dimensional printing system. 
         FIG. 2B  is an illustration depicting a “build plane” which represents a thin slab of resin being selectively cured by a light engine. 
         FIG. 3A  is a top view of an exemplary resin vessel. 
         FIG. 3B  is a side view of an exemplary resin vessel. 
         FIG. 3C  is an isometric view of a lower side of an exemplary resin vessel. 
         FIG. 3D  is a bottom view of an exemplary resin vessel. 
         FIG. 4  is a top view of an exemplary support plate. 
         FIG. 5A  is a top view of an exemplary support fixture. 
         FIG. 5B  is a side view of an exemplary support fixture. 
         FIG. 5C  is a diagram illustrating an “inflow distance” of resin flowing through openings in the support fixture. 
         FIG. 6  is a flowchart depicting an exemplary method of manufacturing a three dimensional article of manufacture. 
         FIG. 7A  is an isometric drawing depicting loading a resin vessel onto a support plate with an interface mechanism in a non-operating state. 
         FIG. 7B  is an isometric drawing depicting loading a resin vessel onto a support plate with an interface mechanism in an operating state. 
         FIG. 7C  is an isometric drawing depicting a fluid spill containment vessel about to be loaded onto a lower side of a support plate. 
         FIG. 7D  is an isometric drawing depicting a fluid spill containment vessel loaded onto a lower side of a support plate. 
         FIG. 8A  is a simplified cross sectional schematic illustration of tensioning of a transparent sheet. 
         FIG. 8B  is a simplified diagram of force exerted on the transparent sheet of  FIG. 8A . 
         FIG. 8C  is a simplified cross sectional schematic illustration of an alternative method of tensioning a transparent sheet. 
         FIG. 9  is an isometric drawing depicting loading a support fixture onto a receiving arm. 
         FIG. 10  is a top view of a portion of an exemplary three dimensional printing system with a resin vessel and support fixture installed. 
         FIG. 11  is an isometric drawing illustrating an improved design for securing a fluid spill containment vessel to a three dimensional printing system. 
         FIG. 12  is an isometric drawing of an embodiment of a fluid spill containment vessel. 
         FIG. 12A  is a cross-sectional view taken from AA′ of  FIG. 12 . 
     
    
    
     SUMMARY 
     In a first aspect of the disclosure, a three dimensional printing system includes a resin vessel, a light engine, and a fluid spill containment vessel. The resin vessel is for containing resin and has a transparent sheet on a lower side that provides a lower bound for the resin. The light engine is disposed below the resin vessel for projecting pixelated light up through the transparent sheet. The fluid spill containment vessel is disposed between the resin vessel and the light engine to capture resin that spills from the resin vessel. The fluid spill containment vessel includes a housing for containing the spilled resin, a transparent window for allowing the pixelated light to project up through the fluid spill containment vessel and to the transparent sheet, and a fluid barrier wall surrounding at least part of the clear window and configured to reduce an amount of spilled resin required to occlude the pixelated light. 
     In one implementation the transparent window is substantially horizontal. The fluid barrier feature is a vertical barrier wall that surrounds the clear window on four sides to define a constrained volume within which spilled resin will accumulate. The constrained volume is less than ten percent (10%) of a volumetric containment capacity of the fluid spill containment vessel. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1A-C  are views of an exemplary three dimensional (3D) printing system  2 .  FIG. 1A  is an isometric view,  FIG. 1B  is a side view, and  FIG. 1C  is a rear view. In describing printing system  2  axes X, Y, and Z are used to illustrate positions, directions, and motions. Axes X, Y, and Z are mutually orthogonal. Axes X and Y are “lateral” or “horizontal” axes. Axis Z is a “vertical” axis. Axis Z is typically aligned or nearly aligned with a gravitational reference. In describing directions the following conventions will be used: +Y is to the “right” and −Y is to the “left”+Z is generally upward and −Z is generally downward. 
     Three dimensional printing system  2  includes a vertical support  4  having a front side  6  and a back side  8 . Vertical support  4  generally provides a “vertical backbone” from which other components of three dimensional printing system  2  are mounted. 
     A support plate  10  is mounted to the vertical support  4 . Support plate  10  has a proximal end  12  that is proximate to the front side  6  of vertical support  4 . Support plate  10  extends from proximal end  12  to distal end  14  along the lateral axis X. Support plate  10  has an inner surface  16  facing inwardly and defining a central opening  18 . 
     A resin vessel  20  is supported by the support plate  10 . The resin vessel  20  has a rear portion  22  that is proximate to the proximal end  12  of the support plate  10 . The resin vessel  20  has a front portion  24  that is proximate to the distal end  14  of the support plate  10 . Resin vessel  20  has an inner edge  26  that surrounds a central opening  28 . The central openings  18  and  28  are laterally aligned with respect to each other to enable an optical path for vertically projected pixelated light. Central opening  28  is laterally contained within central opening  18 . 
     A resin fluid outlet  30  is positioned over the rear portion  22  of resin vessel  20 . A fluid level sensor  32  is positioned over the rear portion  22  of the resin vessel  20 . The resin fluid outlet  30  and fluid level sensor  32  are separated from each other along the lateral axis Y. 
     A fluid spill containment vessel  34  is releasably mounted to a lower side  36  of the support plate  10 . Fluid spill containment vessel  34  is for capturing any resin spills resulting from damage to or overfilling of the resin vessel  20 . The fluid spill containment vessel  34  includes a window (to be discussed below). The window is laterally aligned with the central openings  18  and  28  to enable the aforementioned optical path for vertically projected pixelated light. 
     Mounted to the rear side  8  of vertical support  4  is a vertical track  38 . A carriage  40  is mounted in sliding engagement with the vertical track  38 . A motorized lead screw  42  is configured to drive the carriage  40  along vertical axis Z. The lead screw  42  is coupled to motor system  44  which rotates the lead screw  42  to drive the carriage  40  vertically along the vertical track  38 . A pair of fixture receiving arms  46  extend from the carriage  40  along the lateral axis X. Supported between the receiving arms  46  is a support fixture  48 . 
     A light engine  50  is mounted to the vertical support  4  via a support bracket  52 . Support bracket  52  extends away from the front side  6  of vertical support  4  along lateral axis X. Pixelated light from light engine  50  is projected vertically upwardly. The pixelated light passes through the fluid spill containment vessel  34 , the support plate  10 , and the vessel  20  to a build plane within the resin vessel  20 . 
       FIG. 2A  is a block diagram schematic of the three dimensional printing system  2  including some mechanical features and a simplified electrical block diagram. The resin vessel  20  is shown containing resin  54 . Resin vessel  20  includes a transparent sheet  55  which defines a lower bound for the resin  54  in vessel  20 . The resin is being supplied from resin supply  56  and along a resin supply path  58  to the resin fluid outlet  30 . An interface mechanism  60  is configured to controllably latch the resin vessel  20  to the support plate  10  and to position the resin fluid outlet  30  and the fluid level sensor  32  over the resin vessel  20 . 
     The light engine  50  includes a light source  62  and a spatial light modulator  64 . The light engine  50  projects pixelated light  66  up to a “build plane”  68  which is coincident with or proximate to a lower face  70  of a three dimensional article of manufacture  72  being fabricated. Build plane  68  is depicted in  FIG. 2B  as a two dimensional array of pixels  74 . Each pixel  74  corresponds to a pixel element of the spatial light modulator  64 . 
     Build plane  68  defines a lateral addressable extent of the light engine  50  within the resin vessel  20 . The build plane  68  is actually a very thin slab or “slice” of resin with lateral dimensions in X and Y and a small vertical thickness. This slab of resin is selectively cured based upon a “slice” of data that is processed and sent to the spatial light modulator  64 . The build plane  68  slab does not touch the transparent sheet  55  because an oxygen, chemical, or other inhibitor is utilized to block polymerization on an upper surface of transparent sheet  55 . Each time a portion of the build plane  68  slab is selectively cured, it provides another accretive layer onto the lower face  70  of the three dimensional article of manufacture  72 . 
     The thickness of resin between the lower face  70  and the transparent sheet  55  is important because it provides an optical path for the pixelated light  66 . The weight of the resin  54  and other factors can cause the transparent sheet  55  to bulge between a center  76  and edges  78  of build plane  68 . Such a bulge will result in variable curing and dimensional variations as a function of a distance from the center  76 . To reduce this factor, a unique tensioning mechanism is provided to maintain flatness of the transparent sheet  55 . 
     A controller  80  is controllably coupled to fluid level detector  32 , motor system  44 , light engine  50 , resin supply  56 , and interface mechanism  60 . Controller  80  includes a processor (not shown) coupled to an information storage device (not shown). The information storage device includes a non-transient or non-volatile storage device that stores software instructions that, when executed by the controller  80 , operate (and/or receive information from) fluid level detector  32 , motor system  44 , light engine  50 , resin supply  56 , interface mechanism  60 , and other portions of three dimensional printing system  2 . The controller  80  can be located on one circuit board or distributed among multiple circuit boards throughout the three dimensional printing system  2 . 
       FIGS. 3A-D  depict views of the resin vessel  20 .  FIG. 3A  is a top view,  FIG. 3B  is a side view,  FIG. 3C  is an isometric view, and  FIG. 3D  is a bottom view of resin vessel  20 . The construction of resin vessel  20  includes resin vessel body  82 , transparent sheet  55 , and retainer  84  that clamps the transparent sheet  55  to the resin vessel body  82 . 
     The resin vessel body  82  has an outer peripheral edge  86  and inner edge  26 . Inner edge  26  defines the central opening  28  that is closed on a lower side by the transparent film  55 . Resin vessel body  82  includes a sloped surface  88  surrounded by a peripheral wall  90  partly defining the outer peripheral edge  86 . Peripheral wall  90  helps to contain the resin  54  contained by the resin vessel  20 . The sloped surface  88  allows resin to drain toward the central opening  28 . 
     Resin vessel body  82  has a pair of opposing latch features  92  that are in opposing locations with respect to the lateral axis Y. Formed into the sloped surface  88  of the vessel body  82  is a channel  96  for receiving resin  54  from the resin fluid outlet  30 . 
       FIG. 4  is a top view depicting the support plate  10 . Support plate  10  includes an upper surface  98  including a recessed surface  100  bounded by an inwardly facing wall  102 . When the resin vessel  20  is loaded onto the upper surface  98  of support plate  10 , a lower portion of resin vessel  20  is partially received into a recess  101  defined between the inwardly facing wall  102  and a raised ridge  104  that rises above the recessed surface  100 . The resin vessel  20  is aligned to the support plate  10  by engagement between the outer peripheral edge  86  and the inwardly facing wall  102 . 
     Extending above the recessed surface  100  is raised ridge  104 . When the resin vessel  20  is loaded onto the support plate  10 , the raised ridge  104  engages a lower surface of the transparent sheet  55 , thereby laterally tensioning the transparent sheet  55 . The engagement between the peripheral edge  86  and the inwardly facing wall  102  aligns the raised ridge  104  relative to the inner edge  26  of central opening  28  of resin vessel  20 . Aligned, the raised ridge  104  is disposed at a substantially constant distance from the inner edge  26 . Simultaneously the central opening  28  of the resin vessel is aligned relative to the central opening  18  of the support plate  10 . In the illustrated embodiment the raised ridge  104  defines at least part of the inwardly facing surface or edge  16  that bounds and defines the central opening  18 . 
       FIGS. 5A and 5B  depict support fixture  48 .  FIG. 5A  is a top view and  FIG. 5B  is a side view. Support fixture  48  includes an upper portion  108 , a lower planar portion  110 , and a side wall  112  coupling the upper portion  108  to the lower planar portion  110 . 
     The upper portion  108  includes portions  108 X that extend along the lateral X axis and portions  108 Y that extend along the lateral Y axis. The portions  108 Y are for supporting the support fixture  48  between the receiving arms  46 . Each  108 Y portion includes a datum feature  114  for receiving and aligning to pins that extend upwardly from the receiving arms  46 . The portions  108 Y are also made of a magnetic material that is held down by magnets embedded in receiving arms  46 . In an illustrative embodiment the entire support fixture  48  is formed from a magnetic material. When the support fixture  48  is being raised, the receiving arms  46  provide support in an upper direction because the receiving arms  46  press upwardly on the portions  108 Y. When the support fixture is lowered whereby lower planar portion  110  is passing into resin  54 , the magnetic interaction between the upper portion  108  and the receiving arms  46  provides a downward force that secures the support fixture  48  to the receiving arms  46 . 
     The lower planar portion  110  has a lower surface  116  upon which the three dimensional article of manufacture  72  is formed. Formed into the lower planar portion  110  is a dense array of small openings  118 . A primary purpose of the small openings  118  is to reduce a fluid drag just before and at the start of forming the three dimensional article of manufacture  72 . Before forming the three dimensional article of manufacture  72 , the lower surface  116  is moved through the resin  54  and very close to the transparent sheet  55 . As the lower surface  116  approaches the transparent sheet  55 , resin is displaced and must flow laterally from between the lower surface  116  and the transparent sheet  55 . Without such openings  118  the force exerted on the transparent sheet  55  can be large enough to bulge or even damage the transparent sheet  55 . 
     The openings  118  allow the resin  54  to escape vertically through the lower planar portion  110  of the support fixture  48 . But there is still a vertical force being exerted upon the transparent sheet  55 . This vertical force varies positively with an “inflow distance” D.  FIG. 5C  illustrates the inflow distance D between three small openings  118 . The inflow distance D is a geometric parameter that varies monotonically with a vertical force that is indirectly exerted between the lower surface  116  and the transparent sheet  55  by the thin layer of resin  54  therebetween. 
     The inflow distance  120  is geometrically defined as the distance that resin must flow out of an opening  118  between the transparent sheet  55  and the lower surface  116  before the entire lower surface  116  is covered with resin. This occurs when the dashed circles representing a “resin front” flowing out of the circles close all uncovered gaps. This therefore occurs when the resin fronts intersect at a midpoint between the arrangement of the three openings  118 . 
     Defining some terms: S=the center to center distance between the openings along Y. R=opening radius. D=the inflow distance between an edge of the opening and the midpoint between the openings. Using geometry, the result is that D=S/√3−R for this arrangement of openings. 
     For a particular example, the center to center distance S is 4.5 millimeters. The opening radius R is 1.5 millimeters. Then the inflow distance D is about 1.1 millimeters (rounding to the first significant figure). 
     Preferably the dense array of small openings  118  cover the entire area of the build plane  68  in order to minimize a vertical force exerted on the transparent sheet  55 . In one embodiment the dense array of small openings  118  includes at least 100 small openings  118 . In another embodiment the dense array of small openings  118  includes at least 200 small openings  118 . 
     In some embodiments the inflow distance is less than 3 millimeters. In other embodiments the inflow distance is less than 2 millimeters. In yet other embodiments the inflow distance is less than 1.5 millimeters. 
     The small openings  118  are the primary feature in reducing fluid drag and force on the transparent sheet  55  just before and at the beginning of forming the three dimensional article of manufacture  72  (and/or when lower surface  116  is moving vertically through resin proximate to the upper surface of the transparent sheet  55 ). As the three dimensional article of manufacture  72  is being formed, the distance between the lower surface  116  and the transparent sheet  55  increases and the effect of the openings  118  decreases. 
     Formed along the side wall  112  are a plurality of large openings  120 . The large openings  120  reduce the fluid drag of the resin  54  as the lower planar portion  110  of the support fixture  48  is being raised or lowered in the resin  54 . During fabrication of a three dimensional article of manufacture  72  the large openings  120  become a greater factor than the smaller openings  118  in reducing fluid drag when a sufficient portion of the three dimensional article of manufacture  72  is formed. The large openings  120  also provide the function of allowing residual resin  54  to drain from the support fixture  48  when the lower planar support portion  110  is lifted out of the resin  54  in resin vessel  20 . 
     The large openings  120  reduce a fluid pressure difference between the resin  54  inside the side wall  112  of the support fixture  48  and outside the side wall  112  as the lower planar portion is being raised and lowered within the resin  54 . As the lower planar portion  110  is being lowered into the resin  54 , the large openings  120  allow resin to flow into the space above the lower planar portion  112 . As the lower planar portion  110  is being raised, the large openings allow the resin to flow out of the space above the lower planar portion  112 . 
     According to the illustrated embodiment the large openings  120  are distributed to surround the dense array of small openings  118 . The large openings are at least partially formed into the side wall  112 . In some embodiments, individual large openings  120  span the side wall  112  and an edge of the lower support portion  110 . In one embodiment a large opening  120  has a cross sectional area equal to at least a plurality of the cross sectional area of one small opening  118 . In another embodiment the large opening  120  has a cross sectional area equal to at least five times the cross sectional area of one small opening  118 . In another embodiment the large opening  120  has a cross sectional area equal to at least ten times the cross sectional area of one small opening  118 . 
     The side wall  112  is preferably angled relative to vertical axis Z to enable a nested stacking of the support fixtures  48 . This enables a stack of support fixtures  48  to be loaded into a magazine for automated loading into a printing system  2 . In one embodiment the angle of the side wall  112  relative to the vertical axis Z is in a range of 10 to 50 degrees. In another embodiment the angle of the side wall  112  relative to the vertical axis Z is in a range of 20 to 40 degrees. In yet another embodiment the angle of the side wall  112  relative to the vertical axis Z is in a range of 25 to 35 degrees. In a further embodiment the angle of the side wall  112  relative to the vertical axis Z is about 30 degrees. There is a tradeoff in the angle. As the angle increases, a required area of the support fixture  48  and resin vessel  20  increases for a given area of a build plane  68 . Thus, a minimal angle may seem optimized. However as the angle decreases, vertical stacking efficiency of the support fixtures  48  decreases. Therefore an angle of about 30 degrees from vertical is roughly an optimal tradeoff for vertical stacking efficiency versus size. 
     The portions  108 X of the upper portion  108  that extend along the X axis include a plurality of bent tabs  122  that extend above an upper planar surface  124  of the portions  108 X. The bent tabs  122  are for engaging a lower planar surface  126  of the portions  108 X to provide a controlled vertical spacing between stacked support fixtures  48 . In the illustrative embodiment an individual bent tab  122  is bent into a U-shape whereby an end of the tab  122  faces inwardly. 
       FIG. 6  is a flowchart representing a manufacturing method  130  for using printing system  2  to fabricate a three dimensional article of manufacture  72 . Some individual steps of method  130  will also be described and/or illustrated with respect to subsequent figures in added detail. Also, some of the earlier figures pertain to method  130 . Most or all of the steps of method  130  can be under control of the controller  80 . For steps  132 ,  136 , and  146  any or all of these can be performed either manually or with a robotic arm under control of the controller  80 . Remaining steps  134 ,  138 - 144 , and  148  can be controlled by the controller  80 . 
     According to step  132 , the resin vessel  20  is loaded onto support plate  10 . According to step  134 , the interface mechanism  60  is activated to secure the resin vessel  20  to the support plate  10  and to position the resin fluid outlet  30  and the fluid level sensor  32  over the resin vessel  20 . According to step  136 , the support fixture  48  is loaded onto the fixture receiving arms  46 . In some embodiments step  136  is performed before step  134  and/or before step  132 . 
     According to step  138 , the resin supply  56  is activated whereby the resin supply  56  supplies resin to the resin vessel  20 . According to this step the controller  80  utilizes the fluid level sensor  32  to monitor a fluid level of the resin  54  in the resin vessel  20 . The controller  80  activates the resin supply  56  to pump resin  54  through the supply path  58  and out the resin fluid outlet  30  until a proper level of resin  54  is present in resin vessel  20 . During subsequent steps, the controller  80  can continue to monitor information from the fluid level sensor  32  and operate the resin supply  56  to maintain a proper level of resin in the resin vessel  20 . 
     According to step  140 , the motor system  44  operates the lead screw  42  to translate the carriage  40  whereby the lower surface  116  of support fixture  46  is positioned at an operating distance from the transparent sheet  55 . According to step  142 , the light engine  50  is activated to selectively polymerize a layer of the resin onto the lower surface  116 . Steps  140  and  142  are repeated until the entire three dimensional article of manufacture  72  is formed. As a note, when step  140  is repeated, it is the lower face  70  of the three dimensional article of manufacture  72  that is positioned at the operating distance from the transparent sheet  55 . 
     According to step  144 , the motor system  44  is operated to raise the three dimensional article of manufacture  72  out of the resin  54 . According to step  146  the support fixture  48  is unloaded from the receiving arms  46 . According to step  148 , the interface mechanism  60  is operated to move the resin fluid outlet  30  and the fluid level sensor  32  from above the resin vessel  20 . Also according to step  148  the resin vessel  20  is unlatched so that it can be removed from the support plate  10 . 
     As a note various alternative embodiments are possible. For example, step  148  can be skipped and the process can proceed to step  136  whereby another support fixture  48  is loaded for forming another three dimensional article of manufacture  72  with the same resin vessel  20 . Thus, the depicted method  130  is illustrative and lends itself to certain variations. 
       FIGS. 7A-D  are isometric views depicting loading and securing the resin vessel  20  and the fluid spill containment vessel  34  to the support plate  10 .  FIG. 7A  depicts step  132  of  FIG. 6 . The resin vessel  20  has been loaded onto the support plate  10 . A lower portion of resin vessel  20  has been received into the recess  101  (see also  FIG. 4 ). Engagement of the peripheral edge  86  and the inwardly facing wall  102  has provided lateral (X and Y) alignment of the resin vessel  20  with respect to the support plate  10 . 
     Also shown in  FIG. 7A  is a resin handling module  150  that supports both the resin fluid outlet  30  and the fluid level sensor  32  with arms  154 . The resin handling module  150  is configured to rotate about an axis parallel to lateral axis Y. In  FIG. 7A  the resin handling module  150  is shown in a non-operating position whereby the resin fluid outlet  30  and the fluid level sensor  32  are not in position over the resin vessel  20 . Latches  152  are also shown in a non-engaged position. 
       FIG. 7B  depicts step  134  of  FIG. 6 . Between  FIGS. 7A and 7B  the resin handling module  150  has been rotated about an axis parallel to lateral axis Y from a non-operating position ( FIG. 7A ) to an operating position ( FIG. 7B ). In the operating position the resin fluid outlet  30  and the fluid level sensor  32  are both positioned over the resin vessel  20 . Also the latches  152  are engaged with latch features  92  at opposing ends of the resin vessel. The latches  152  exert a downward (−Z) vertical force on the latch features  92  to increase a tension in the transparent sheet  55 . 
     The resin handing module  150  includes two arms  154  that are linked together whereby they rotate together in unison between the non-operating position and the operating position of the resin handling module  150 . The interface mechanism  60  that actuates the resin handling module  150  and the latches  152  is configured to simultaneously actuate them to move them back and forth between a non-operating state (non-operating position of resin handling module  150  and latches  152  not engaged) to an operating state (operating position of resin handling module  150  and latches  152  engaged). 
     In the illustrative embodiment, the interface mechanism  60  includes pneumatic actuators  156 .  FIG. 7C  depicts a more complete view of the pneumatic actuators  156  (shown without air “plumbing”). There is a pneumatic actuator  156  coupled to each latch  152  and a pneumatic actuator  156  coupled to the resin handling module  150 . The air pressure applied to the pneumatic actuators  156  enables motion of the resin handling module  150  and latches  152  to be simultaneous. 
     The resin vessel  20  is unloaded in reverse order of being loaded. This includes (1) changing the interface mechanism  60  from an operating to a non-operating state—going from  FIG. 7B  to  FIG. 7A , and then (2) unloading the resin vessel  20  from the support plate  10 . 
       FIGS. 7C and 7D  depict the fluid spill containment vessel  34  being slidingly mounted to the lower side  36  of the support plate  10 . The fluid spill containment vessel  34  includes a transparent window  158  for allowing light to pass from the light engine  50  to the resin vessel  20 . The fluid spill containment vessel  34  has a generally tapering profile from a distal end  160  to a proximal end  162 . The tapering profile provides an internal slope whereby resin can drain away from the transparent window  158  and into a trough  164 . This minimizes a tendency for a light path from the light engine  50  to the build plane  68  to be occluded by spilled resin that has accumulated in the fluid spill containment vessel  34 . 
     The fluid spill containment vessel  34  has a pair of opposing upper lips  166  that extend outwardly along the lateral Y axis. Mounted to the lower side  36  of support plate  10  are two rails  168  that are aligned with lateral axis X and spaced apart with respect to lateral axis Y. The fluid spill containment vessel  34  is mounted to the support plate  10  by slidingly engaging the rails  168  with the upper lips  166  along the lateral axis Y. 
       FIGS. 7C and 7D  depict disengaged and engaged positions respectively of the fluid spill containment vessel  34  with respect to the support plate  10 . In the engaged state, the resin vessel central opening  28 , the support plate central opening  18 , and the fluid spill containment vessel  34  transparent window  158  are all aligned whereby the light engine  50  can project pixelated light up through them and to the build plane  68 . 
       FIG. 8A  is a cross sectional view depicting interaction of components involved in tensioning the transparent sheet  55  during steps  132  and  134  of  FIG. 6 . When the resin vessel  20  is loaded onto the support plate  10 , the raised ridge  104  engages the transparent sheet  55 . When the latches  152  engage the latch features  92 , they exert a combined downward latch force F L  upon the resin vessel. This has the effect of tensioning the transparent sheet  55 . The tension in the transparent sheet  55  can be controlled by controlling the latch force F L . 
       FIG. 8B  depicts the forces involved: T=Tension in Transparent Sheet  55 , F H =Horizontal Force Exerted on Transparent Sheet by Vessel Body  82 , f=horizontal frictional force exerted on transparent sheet  55  by raised ridge  104 , F V =vertical force exerted on transparent sheet  55  by Vessel Body  82 , and F R =Vertical Force Exerted by Raised Ridge  104  on Transparent Sheet  55 . Now, F V =W V +F L , where W V  is the weight of the resin vessel  20  and F L  is the downward force of both latches. These forces are known. 
     Summing the forces in X: T+f=F H . Summing the forces in Y: F V =F R . From geometry the tangent of 8 equals F V  divided by F H . From the above relationships, and from computing the frictional force f based a coefficient of friction and F R , the tension T can be approximated in terms of known variables. This diagram is a simplified approximation of the actual system because it is in two dimensions and the actual system would consider the sheet in three dimensions. If the angle θ is small, then the tension T can be quite large relative to the vertical forces applied. There may be a need to increase the vertical force over time to compensate for an increase in angle θ if the transparent sheet  55  stretches. The configuration of  FIG. 8A  has an advantage that the latch force F L  can be programmably controlled by the controller  80  controlling the air pressure applied to pneumatic actuators  156 . Thus, the tension T can be indirectly programmably controlled by the controller  80 . 
       FIG. 8C  illustrates an alternative embodiment in which the resin vessel  20  “bottoms out” on the support plate  10 . With this embodiment, the tension T in the transparent sheet  55  is governed by a vertical position of the vessel body  82  in relation to the raised ridge  104 . In this embodiment, any compression set in the transparent sheet  55  will reduce the tension T. While this embodiment is viable, it would be less desirable than the embodiment of  FIG. 8A  if the transparent sheet  55  stretches over time and/or if dimensional tolerances are not precisely controlled. 
       FIG. 9  depicts a mechanical interaction between the portion  108 Y of the support fixture  48  and receiving arm  46  as the support fixture is loaded according to step  136  of  FIG. 6 . The receiving arm  46  includes upstanding pin  170  that is received by datum feature  114  for providing lateral alignment of the support fixture  48 . Between both support arms the lateral alignment provided includes X, Y, and rotation about the axis Z. The portion  108 Y is formed from a magnetic material and receiving arm contains a magnet. The magnetic interaction and mechanical interaction along the vertical axis Z between portions  108 Y and the receiving arms provide support along the vertical axis Z and against rotation about the horizontal axes. 
       FIG. 10  is a top view of an embodiment of three dimensional printing system  2  with the resin vessel  20  and the support fixture  48  installed. The vertical support  4  has a front side  6  and a back side  8 . Extending from the front side  6  is the support plate  10 . The support plate  10  extends along lateral axis X from a proximal end  12  (proximate to the front side  6 ) to a distal end  14 . 
     Resin vessel  20  is disposed above a portion of the upper surface  98  of support plate  10  (held above a recessed portion  100  of the upper surface by the force of the raised ridge  104  upon the transparent sheet  55 ). The resin vessel  20  has a rear portion  22  that is laterally proximate to the proximal end  12  of the support plate. The resin vessel  20  has a front portion  24  that is laterally proximate to the distal end  14  of the support plate. The resin vessel  20  has a pair of opposed latch features  92  including left and right latch features  92  at opposed ends with respect to the lateral axis Y. Corresponding to the left and right latch features  92  are left and right latches  152 . 
     The interface mechanism  60  (depicted in block diagram form in  FIG. 2A ) is activated whereby the resin handling module  150  and latches  152  are in an operating state. In the operating state the resin handling module  150  is in an operating position whereby the resin fluid outlet  30  and the fluid level sensor  32  are positioned over the resin vessel  20  and the latches  152  are engaged with the latch features  92  of resin vessel  20 . The resin fluid outlet  30  and the fluid level sensor  32  are disposed over the rear portion  22  of the resin vessel  20 . The resin fluid outlet  30  and the fluid level sensor  32  are also spaced apart and on either side of the vertical support  4  with respect to the lateral axis Y and each are supported by an arm  154  of the resin handing module  150 . 
     Extending from the back side  8  of vertical support  4  is carriage  40 . Extending forwardly (+X) along lateral axis X from carriage  40  are the receiving arms  46 . The receiving arms  46  are spaced apart along the lateral axis Y to an extent that the arms  154  are between the receiving arms  46 . Installed between receiving arms  46  is the support fixture  48  that spans a space between the receiving arms  46  along the lateral axis Y. 
       FIG. 11  depicts an alternative embodiment for the rails  168 . The rails  168  depicted in  FIGS. 7C and 7D  are formed from extruded or bent sheet metal. In contrast, the rails  168  depicted in  FIG. 11  are formed by injection molding. This allows for added features to enhance the ergonomics of engaging the opposing upper lips  166  with the rails  168 . 
     While previous figures depict the fluid spill containment vessel  34  as mounted directly to the lower side  36  of support plate  10 , other arrangements are possible. In an alternative embodiment, the fluid spill containment vessel  34  can be directly coupled to the vertical support  4 . In yet another alternative, vertical support can have a pair of support brackets extending therefrom. The fluid spill containment vessel  34  can be mounted to the brackets. 
     Fluid spill containment vessel  34  is quite effective in capturing a catastrophic spill of resin from the resin vessel  20 . One challenge is a slow leak or overflow of the resin vessel  20  that may not be noticed by a user of the three dimensional printing system  2 . If a slow leak is not noticed it is possible that the fluid spill containment vessel  34  could be completely filled and then begin to spill onto the light engine  50 . This could result in a very costly catastrophic failure. 
       FIGS. 12 and 12A  are isometric and cross-sectional views of an alternative embodiment of the fluid spill containment vessel  34 .  FIG. 12A  is a cross-sectional view taken from section AA′ of  FIG. 12 . In comparing  FIGS. 12 and 12A  with prior figures, like elements indicate like or similar features of fluid spill containment vessel  34 . These include transparent window  158 , distal end  160 , proximal end  162 , trough  164 , and upper lips  166 . However, these features do differ in their implementation from those illustrated and described in earlier figures. 
     In the illustrated embodiment, the fluid spill containment vessel  34  includes a housing  172  that defines a volumetric containment capacity for spilled resin. The transparent window  158  is generally horizontal. A fluid barrier wall  174  surrounds the transparent window  158  and in doing so defines a constrained volume  176  defined as a parallelepiped volume that is bounded by an upper surface of the transparent window  158  and a parallel plane at an upper end of the fluid barrier wall  174 . The constrained volume  176  is less than ten percent (10%) of the volumetric containment capacity of the housing  172 . As illustrated, the fluid barrier wall extends two to three millimeters above the transparent window  158 . In describing the transparent window  158  as “generally horizontal”, it is designed or intended to be horizontal but may not be exactly horizontal due to mechanical tolerances in the printing system  2 . 
     When the fluid spill containment vessel  34  is slidingly and releasably installed into the printing system  2 , the transparent window  158  aligns with the resin vessel central opening  28 , the support plate central opening  18 , and an optical axis of the light engine  50  so that the light engine can define the build plane  68  above the transparent sheet  55 . In the event of a slow spill or leak of resin, the constrained volume  176  is quickly filled with the spilled or leaked resin. This effectively eliminates an ability of the printing system  2  to properly fabricate three dimensional articles  72 , which quickly becomes apparent to a user of the printing system  2  before resin can fill the housing  172 . The user can then take action to eliminate a source of the spill and to replace the resin vessel  20  and the fluid spill containment vessel  34  as required. 
     The specific embodiments and applications thereof described above are for illustrative purposes only and do not preclude modifications and variations encompassed by the scope of the following claims.