Patent Publication Number: US-2019184630-A1

Title: Three dimensional printer resin replenishment method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This non-provisional patent application is a continuation-in-part of U.S. Ser. No. 15/899,452, entitled “THREE DIMENSIONAL PRINTER RESIN REPLENISHMENT METHOD,” filed on Feb. 20, 2018 which claims priority to U.S. Provisional Application Ser. No. 62/460,947, filed on Feb. 20, 2017, incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure concerns an apparatus and method for fabrication of solid three dimensional (3D) articles of manufacture from energy curable materials. More particularly, the present disclosure concerns a way of optimizing the speed and output quality of a three dimensional (3D) printer that utilizes photocurable resins. 
     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 containment vessel holding the curable 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. 
     In one system embodiment the vessel includes a transparent sheet that forms part of a lower surface of the vessel. The support surface is positioned above and in facing relation with the transparent sheet. The following steps take place: (1) The movement mechanism positions the support surface whereby a thin layer of the photocurable resin resides between the support surface and the transparent sheet. (2) The light engine transmits pixelated light up through the transparent sheet to selectively cure a layer of the photocurable resin onto the support surface. (3) The movement mechanism then incrementally raises the support surface. Steps (2) and (3) are repeated to form a three dimensional (3D) article of manufacture having a lower face in facing relation with the transparent sheet. 
     One challenge is the “fouling” of the transparent sheet with cured polymer. Ideally the polymer would only cure on the support surface and the lower face of the 3D article of manufacture. However, some polymer may cure upon the lower window. Over time, the cured polymer on the window will interfere with proper operation of the 3D printer. Also, the lower face may stick to the lower window. To overcome this problem, various solutions have been deployed including providing a release coating on the lower window and/or using chemical inhibitors to prevent the resin from curing on or near the lower window. 
     Another challenge with such a system is how to maintain a supply of fresh resin at a build plane proximate to the lower face. When the lower face of the 3D article of manufacture has a solid and large cross sectional area, the resin can become depleted at the build plane. Up and down motion of the lower face of the 3D article of manufacture might be used to replenish the thin layer of resin. This up and down motion can impart stresses upon the lower face of the 3D article of manufacture, resulting in a decrease in quality. Slowing down the up and down motion can reduce these stresses but will result in longer processing times. What is needed is a system that provides high speed operation without a reduction in quality of the 3D article of manufacture. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  is a schematic diagram depicting a first embodiment of a three dimensional (3D) printing system using a non-contact sensor having an emitter and detector. 
         FIG. 1B  is a schematic diagram depicting a second embodiment of a three dimensional (3D) printing system using “contact” sensor that is in physical contact with or mounted to a transparent membrane. 
         FIG. 2  is a flowchart depicting an embodiment of a method for operating a three dimensional (3D) printing system to manufacture a three dimensional article. 
         FIG. 3  is an exemplary schematic timing diagram depicting a height H(t) versus time t of a lower face of a three dimensional (3D) object of manufacture above a transparent sheet. 
         FIG. 4  is an exemplary schematic timing diagram depicting a height H(t) and a transient displacement s(t) versus time t. H(t) is a height of a lower face of a three dimensional (3D) object of manufacture above a reference plane which may correspond to an equilibrium position of a transparent sheet. The transient displacement s(t) is a dynamically varying displacement or vertical position of a portion of the transparent sheet from an equilibrium position. 
         FIG. 5  is a top view of an exemplary vessel containing resin and a three dimensional (3D) object of manufacture having a solid circular cross section. 
         FIG. 6  is a side cross sectional view of an exemplary vessel containing resin and a three dimensional (3D) object of manufacture. 
     
    
    
     SUMMARY 
     In a first aspect of the disclosure a three dimensional (3D) printer includes a vessel, a light engine, a fixture, a movement mechanism, a sensor, and a controller. The vessel is for containing a liquid photocurable resin and includes a tensioned transparent sheet defining at least part of a lower surface through which the resin can be illuminated. The light engine is disposed and configured to selectively harden the resin at a build plane above the transparent sheet. The fixture is for supporting a three dimensional (3D) article of manufacture whereby a lower face of the 3D article of manufacture is immersed in the resin in facing relation with the transparent sheet. The movement mechanism is configured for controllably translating the fixture whereby a vertical height H(t) of the lower face of the 3D article of manufacture can be controlled. The sensor is configured to sense a transient displacement of the transparent sheet from an unperturbed position. The controller is configured to: (a) activate the movement mechanism to position the lower face of the 3D article of manufacture at the build plane; (b) activate the light engine to selectively harden a layer of resin onto the lower face; (c) repeat (a) and (b) N−1 times, N is two or more, during repeating includes an incremental vertical motion of a distance d (described below); (d) activate the movement mechanism to translate the lower face upwardly by a distance of D and then downwardly to the build plane pursuant to a pump cycle, D is at least two times d; (e) concurrent with the motion in (a) and (d), monitor a signal from the sensor that is indicative of the vertical displacement; (f) update a motion parameter based at least partly upon the signal from the sensor, the motion parameter defines motion in one or more of steps (a) and (d); and (g) repeat steps (a) to (f) until the 3D article of manufacture is completed. 
     Step (a) includes incrementally raising the lower face by a distance d to offset a thickness of material solidified onto the lower face of the three dimensional article. D can be equal to at least four times d, at least 10 times d, at least 50 times d, or at least 100 times d. The distance d can be in range of 10 to 100 microns or about 30 microns in particular embodiments. 
     In one implementation the sensor is a non-contact sensor that does not directly physically contact the transparent sheet. The sensor can include a proximity sensor having an emitter and detector. The sensor can include an interferometer. The sensor can be an individual sensor or a plurality or array of sensors. 
     In another implementation the sensor contacts or is mounted upon the transparent sheet. The sensor can be an accelerometer. The sensor can be an LVDT (linear variable differential transformer) sensor. The sensor can be an individual sensor or a plurality or array of sensors. 
     In yet another implementation the light engine includes a light source and a spatial light modulator. The spatial light modulator operates to selectively control pixel elements across a build plane over which the resin is selectively cured. In one embodiment the spatial light modulator is a digital mirror device. The spatial light modulator receives unprocessed light from the light source and reflects or transmits pixelated and processed light. The light engine includes optics that deliver the processed light to the build plane within the resin and proximate to the lower face of the 3D article of manufacture. 
     In a further implementation the controller is electrically and/or wirelessly coupled to the light engine, the movement mechanism, and to the sensor. The controller includes a processor coupled to an information storage device. The information storage device includes a non-transient or non-volatile storage device that stores instructions that, when executed by the processor, process signals from the sensor and control the light engine and the movement mechanism. The controller can be contained in a single IC (integrated circuit) or multiple ICs. The controller can be disposed at one location or distributed in multiple locations within the three dimensional printing system. 
     In a yet further implementation updating the motion parameter includes updating motion when step (a) is repeated. Updating the motion parameter can include updating the number N (total number of times (a) and (b) are executed before a pump cycle). 
     In another implementation updating the motion parameter includes updating motion when step (d) is executed. This can include updating the pump distance D. This can include updating a peak vertical velocity during pumping. 
     In yet another implementation a translation velocity of the lower face of the 3D article of manufacture is adjusted in real time based upon a signal from the sensor. The translation velocity can be decreased if the signal from the sensor indicates that the transient displacement has a magnitude that exceeds a designated upper control limit. The translation velocity can be decreased if the signal from the sensor indicates that the transient displacement versus time has a magnitude that exceeds a designated upper control limit. The translation velocity can be increased if the signal from the sensor indicates that the transient displacement has a magnitude that is less than a designated lower control limit. The translation velocity can be increased if the signal from the sensor indicates that a slope of the transient displacement versus time is less than a lower control limit. 
     In a further implementation the transient displacement versus time rises and then falls while the lower face of the 3D article of manufacture is being translated upwardly away from the transparent sheet. The upward translation can be halted in real time in response to a real time analysis of the transient displacement versus time. When the magnitude of the transient displacement begins to rapidly fall the upward translation can be halted. 
     In a yet further implementation the controller is configured to define the pump cycle based upon a parametric correlation that correlates pump cycle parameters with at least a geometry of the lower face of the 3D article of manufacture. The pump cycle parameters include a pump distance D and a translation velocity of the lower face of the 3D article of manufacture. In a first embodiment a lookup table correlates a pump cycle parameter with a lower face geometric parameter. In the first embodiment the lookup table is updated based upon an analysis of the signal from the sensor. In a second embodiment a functional relationship relates a pump cycle parameter to a lower face geometric parameter. In the second embodiment the functional relationship is updated based upon an analysis of the signal from the sensor. 
     In another implementation a resin flow regime can be defined between the lower face of the 3D article of manufacture and the transparent sheet. The resin flow regime can be laminar or turbulent based upon a Reynolds number. The pump cycle is updated to maximize the Reynolds number while maintaining laminar flow and avoid turbulent flow. The pump cycle can also have an upper limit on the dynamic force for a very viscous resin. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1A  is a schematic diagram representation of a first embodiment of a three dimensional (3D) printing system  2 . Indicated in  FIGS. 1A and 1B  are mutually perpendicular axes X, Y, and Z. Axes X and Y are lateral axes. In some embodiments X and Y are also horizontal axes. Axis Z is a central axis. In some embodiments Z is a vertical axis. In some embodiments the direction +Z is generally upward and the direction −Z is generally downward. 
     Three dimensional printing system  2  includes a vessel  4  containing a photocurable resin  6 . Vessel  4  includes a transparent sheet  8  that defines at least a portion of a lower surface  10  of vessel  4 . A light engine  12  is disposed to project light up through the transparent sheet  8  to solidify layers of photocurable resin  6  to progressively form a three dimensional article of manufacture  14 . The three dimensional article of manufacture  14  is attached to and thereby supported by a fixture  16 . A movement mechanism  18  is coupled to fixture  14  for translating the fixture  16  along the vertical axis Z. 
     The three dimensional printing system  2  includes a sensor  20  that is configured to output a signal indicative of a transient vertical displacement or position s(t) of a portion of the transparent sheet  8  from an equilibrium position. When the three dimensional article  14  is moved up and down in resin  6 , a transient force is generated upon the transparent sheet  8  which causes the transient displacement s(t). The signal outputted by sensor  20  can be indicative of a position, a velocity, and/or an acceleration. Alternatively, the signal outputted by sensor  20  can be a processed signal that characterizes or quantifies the transient displacement s(t) in some way. 
     In the illustrated embodiments of  FIGS. 1A and 1B , the sensor  20  can be a single sensor  20 . Alternatively, the sensor  20  can include a plurality or array of sensors  20  which can sense s(t) for a plurality or array of locations upon the transparent sheet  8 . 
     In the illustrated embodiment, the sensor  20  includes an emitter E and detector DT. The emitter E emits radiation that partially reflects from the transparent sheet  8  and is then received by the detector DT. In one particular embodiment, the sensor  20  includes components of a proximity sensor. In another particular embodiment, the sensor  20  includes components of an interferometer. 
     A controller is  22  is electrically or wirelessly coupled to the light engine  12 , the movement mechanism  18 , and the sensor  20 . Controller  22  includes a processor  24  coupled to an information storage device  26 . The information storage device  26  includes a non-transient or non-volatile storage device that stores instructions that, when executed by the processor  24 , analyze and process signals from the sensor  20  and control the light engine  12  and the movement mechanism  18 . Controller  22  is contained in a single IC (integrated circuit) or multiple ICs. Controller  22  can be disposed at one location or distributed among multiple locations in three dimensional printing system  2 . 
     The three dimensional article of manufacture  14  has a lower face  28  that faces the transparent sheet  8 . As light engine  12  selectively applies light energy through the transparent sheet  8  it selectively polymerizes resin proximate to a “build plane”  29  which can be coincident or proximate to the lower face  28 . This has the effect of selectively building or forming a layer of resin onto lower face  28 . The movement mechanism, under control of controller  22 , controls a height H(t) of the lower face  28 . In the illustrated embodiment, H(t) is approximately a distance between the lower face  28  and the transparent sheet  8  in an equilibrium position when it is nearly flat or planar. 
     The transparent sheet  8  is under tension to maintain planarity of the build plane  29 . As the lower face  28  is translated vertically, the pressure of the resin  6  will tend to deflect the transparent sheet  8 . This deflection is transient and the transparent sheet  8  will tend to move back to the equilibrium position when there is no motion of the lower face  28 . The distance s(t) is the transient vertical displacement of the transparent sheet  8  from an equilibrium position. The displacement s(t) is transient because the tension in the transparent sheet  8  returns the transparent sheet to its equilibrium position. 
     In an alternative embodiment, a second sensor  21  is also coupled to the controller  22 . The second sensor  21  emits a second signal received by controller  22  that is indicative of a dynamic force F(t) experienced by the fixture  16  during vertical motion of the lower face  28  in the resin  6 . 
       FIG. 1B  is a schematic diagram representation of a second embodiment of a three dimensional (3D) printing system  2  which is similar to the first embodiment of  FIG. 1A  except for the illustrated sensor  20 . In the illustrated embodiment, sensor  20  is attached or mounted to the transparent sheet. The illustrated sensor  20  can be referred to as a “contact sensor” because it physically contacts the transparent sheet  8 . Sensor  20  can include an accelerometer. In another embodiment, sensor  20  is an LVDT (linear variable differential transformer) having a movable portion that is urged against the transparent sheet  8 . In some embodiments, sensor  20  can include a plurality or array of sensors  20  attached to or contacting the transparent sheet  8 . In yet other embodiments, sensor can includes a combination of a non-contact sensor  20  (as illustrated in  FIG. 1A ) and a contact sensor  20  (as illustrated in  FIG. 1B ). 
       FIG. 2  is a flowchart depicting an exemplary method  30  of forming a three dimensional article of manufacture  14  with a three dimensional (3D) printing system  2 .  FIG. 3  is an exemplary graph of H(t) versus time t that can correspond to a part of method  30 . The two will be discussed at the same time. 
     According to step  32  the lower face  28  of the three dimensional article of manufacture  14  is positioned at an operating height H op  above the transparent sheet  8 . Step  32  is initially depicted by the left side of the  FIG. 3  graph at t=0. 
     According to step  34 , the light engine  12  is activated to photo polymerize a layer of resin at the build plane  29  and onto the lower face  28 . Step  34  corresponds to the dashed line between t=0 and t=t 1  in  FIG. 3 . After the polymerization of step  34 , the distance H(t) is reduced by a thickness of photocure resin that has been added onto lower face  28 . The method then proceeds back to step  32  in which the lower face is raised back to an operating height H op . 
     Steps  32  and  34  are repeated one or more (N−1 in which N is at least equal to two) times before proceeding to step  36 .  FIG. 3  depicts these as being repeated twice but the number of times steps  32  and  34  are repeated can vary. They may be not be repeated at all or can be repeated 1, 2, 3, 4, 5, 6, 7, or more times before proceeding to step  36 . The number of repeats can be a function of various factors such as the geometry of the lower face  28 , resin  6  rheology, and cure depth into the resin. At or around time t 1  and time t 2  the lower face  28  is incrementally raised by a distance d to provide the proper operating height H op  before operating the light engine. 
     When the lower face  28  has a relatively large geometry the resin  6  may not completely refill between the lower face  28  and the transparent sheet  8  between t=0 and t=t 3 . At some point the quality of added layers of resin is impaired. Then a “pump cycle” can be performed. An exemplary pump cycle is illustrated according to  FIG. 3  between time t=t 3  and t=t 7 . The lower face  28  is raised to H(t)=H pump  above the transparent film  8 . The distance that lower face  28  is vertically raised between t=t 3  and t=t 6  will be referred to as pump distance D. 
     According to step  36  a pump cycle is determined or defined. This can be based upon factors such as a geometry of the lower face  28 , a geometry of the three dimensional article  14  proximate to or near the lower face  28 , and rheological properties of the resin  6 . The pump cycle defines D, a velocity profile or curve including H(t) versus t between t=t 3  and t=t 7 . In another embodiment of step  36 , the pump cycle is determined based upon a conservative “nominal” curve of H(t) versus t or a prior stored profile. 
     According to step  38  the movement mechanism  18  begins the pump cycle. According to  40  a signal from sensor  20  is monitored during all motion of the lower face  28  during steps  32 ,  38 , and  44  ( 40  can occur either continuously or multiple times during the process  30 ). The signal is indicative of the transient displacement s(t) of the transparent sheet along axis Z. An “idealized” transient displacement s(t) versus time is illustrated as s(t) versus time t in  FIG. 4 . This is idealized because an actual graph may have non-linear segments and added reverberations. 
     The top graph of  FIG. 4  depicts H(t) versus time t between t=t 3  and t=t 6 . While  FIGS. 3 and 4  depict motions as linear (constant slope) between various times, this is for illustrative purposes. The actual motion may be non-linear in order to optimize a resin flow regime between the lower face  28  and the transparent sheet  8 . 
     The bottom graph of  FIG. 4  depicts the transient displacement s(t) versus time inferred from sensor  20  between times t=t 3  and t=t 6 . Between t=t 3  and t=t 4  a viscous drag of the resin causes the transparent sheet to be rapidly flexed upwardly as the lower face  28  is raised. Then, at t=t 4 , the transparent sheet  8  “breaks free” as resin rushes in between the lower face  28  and the transparent sheet  8 . The sheet  8  may overshoot as at t=t 5  before returning to an equilibrium position at t=t 6 . 
     According to step  42 , motion for steps  32 - 44  can be adjusted in real time in response to and concurrently with analyzing the step  40  signal. Step  42  does not necessarily occur unless the analysis determines that transient displacement curve s(t) is not optimal. There are a number of embodiments for which step  42  is invoked and what follows are some exemplary embodiments. 
     In some embodiments, there is an upper control limit for the transient displacement s(t) versus time t between t 3  and t 4 . Too high of a displacement can result in damage to the three dimensional article of manufacture  14  and possibly even to the transparent sheet  8 . If the analysis shows that a magnitude of s(t) has exceeded the upper control limit then the slope of H(t) versus t (which equals the upward translation velocity of the fixture  16  is reduced in order to reduce a peak displacement s(t) for the motion (shown as a peak of the curve in  FIG. 4 ). 
     In some embodiments, there is a lower control limit for the transient displacement s(t) at a particular time t. Too low of a transient displacement s(t) indicates an opportunity to increase a build rate for the three dimensional article of manufacture  14  without adverse effects. If the analysis of step  40  indicates that a magnitude of the transient displacement s(t) is below the lower control limit, then the rate of upward translation of the lower face  28  (which equals the slope of height H(t) versus time t) is increased. 
     In some embodiments, the pump cycle being executed is based upon an expected graph of transient displacement s(t) versus time t. Based on the analysis it may be determined that the time t 4  occurs earlier than expected—in other words, the transient displacement s(t) begins to decline rapidly earlier than expected. Then the pump cycle can be temporally shortened by reducing times t=t 6  and t=t 7 . Doing so increases the build rate for the three dimensional article of manufacture  14  while still assuring a complete reflow of resin between the lower face  28  and the transparent sheet  8 . 
     In some embodiments, the number N (number of positioning and light engine activation) can be adjusted based upon the transient displacement s(t) during the incremental motion of repeating step  32 . If s(t) is above a control limit, the number N can be decreased. If s(t) is below a control limit, the number N can be increased. 
     According to step  44 , the pump cycle from t=t 3  to t=t 7  is completed. Step  44  actually coincides with step  32  at which the lower face is again positioned at the operating distance H op . The cycle of steps  32  to  44  can be repeated until the three dimensional article of manufacture  14  is completed. 
     According to step  46  a new parametric correlation is stored that correlates the pump cycle with at least the geometry of the lower face  28  and possibly other parameters. This new parametric correlation can be used for subsequent repeats of the steps  32  to  44 . This parametric correlation can take on various embodiments. 
     In a first embodiment, the parametric correlation is defined by a lookup table. Step  46  can include modifying the lookup table correlating the pump cycle to various input variables such as the geometry of lower face  28 . Alternatively, step  46  can include pointing to a new lookup table. 
     In a second embodiment the parametric correlation is defined by one or more functional relationships correlating the pump cycle to the geometry of lower face  28 . The functional relationship includes function parameters such a multiplicative constants. According to the second embodiment, step  46  includes modifying one or more of the function parameters and thereby modifying the functional relationship. The functional relationship can be a multiplicative factor, and updating the parameter can be updating the multiplicative factor. 
     In a third embodiment, the parametric correlation is defined by a combination of one or more lookup tables and one or more functional relationships. Step  46  can include modifications to a lookup table and/or a functional relationship. 
       FIGS. 5 and 6  are simplified diagrams for illustrating a simple geometry of a cylindrical three dimensional article of manufacture  14  for the purpose of discussing the geometry of lower face  28 .  FIG. 5  is a top view and  FIG. 6  is a side view of the vessel  4  containing resin  6  and three dimensional article of manufacture  14 . The required pump distance D correlates increasingly with an inflow radius R which is a lateral radius of the circular three dimensional article of manufacture  14 . Thus for such a simple geometry a lookup table can correlate the pump distance D and/or N with R. More complicated geometries won&#39;t have such a simple correlation, but for such geometries an equivalent inflow radius R can be estimated. By utilizing the method  30 , the correlation (D and/or N versus R) can be automatically improved over time for a given resin  6 . 
     Generally speaking, as R increases, N decreases and D increases. As the resin viscosity increases, N decreases and D increases. As R increases, pump velocities are decreased. As resin viscosity increases, pump velocities are decreased. 
     As the lower face  28  of the three dimensional article of manufacture  14  is raised above the transparent sheet  8 , resin inflows as indicated by arrows  48 . If the translation velocity H(t) versus t is of a low enough magnitude, the resin flow  48  regime will be laminar and flow vectors will tend to be uniformly radially inward. But as H(t) versus t is increased above a certain threshold, the resin flow  48  regime becomes turbulent and irregular. Such a flow regime is not optimal. The optimal resin flow  48  rate is an upper limit for laminar flow before the flow becomes turbulent. The method  30  including steps  42  and  46  can be performed to provide the highest magnitude of translation velocity (height H(t) versus time t) for which the resin flow  48  is laminar and an upper limit on the transient displacement s(t) is not exceeded. 
     Variables that define the pump cycle can vary dramatically. Operating with optimal laminar flow requires the correlation of a number of variables. The effective radius (inflow distance) R of a given layer of material can vary dramatically. A smaller effective radius can be in the range of 1 to 5 millimeters. A larger effective radius range can be in a range of 5 to 500 millimeters or more. 
     Rheological properties of the resin  6  combine with the geometry to determine an optimal pump cycle. The pump cycle may include a pump distance D as small as 100 to 300 microns or as large a 500 to 5000 microns depending on lower face  28  geometry, resin  6  rheology, and possibly other factors. The translation velocity (slope of height H(t) versus time t) can also vary widely. The acceptable maximum transient displacement s(t) is a function of factors including the resin  6  rheology and the lower face  28  geometry. For a given resin rheology this can be determined by finding the boundaries of the transient displacement s(t) for a given geometry. 
     For geometries other than a circle, the inflow distance R can be computed by analyzing how far resin must flow from various boundaries before covering a particular two dimensional shape of a cross-section. For a rectangle, 2R is generally equal to the width along the minor axis. This would also be true for a simple oval. For more complex shapes, an “erosional” method can be used to compute R. The distance of erosion at which the simple shape is gone equals the inflow distance. Yet other shapes like a comb-shape or shapes that have multiple portions can be estimated with algorithms. In one embodiment, step  36  is initially based upon a correlation of the pump cycle (D and vertical speed of pumping) with the computed inflow distance R and rheological properties of the resin being used. Then, step  42  is a correction based upon the transient displacement. 
     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.