Abstract:
An apparatus and a method of using the apparatus to deliver metered quantities of powder to a target area in a laser sintering process from a single sided bi-directional powder delivery system to ensure fresh powder is preheated prior to fusing the powder with a laser beam. Metered quantities of powder are deposited for preheating adjacent the target area and then are spread by a mechanism that traverses the target area.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to the field of freeform fabrication, and more specifically is directed to the fabrication of three-dimensional objects by selective laser sintering.  
         [0002]     The field of freeform fabrication of parts has, in recent years, made significant improvements in providing high strength, high density parts for use in the design and pilot production of many useful articles. Freeform fabrication generally refers to the manufacture of articles directly from computer-aided-design (CAD) databases in an automated fashion, rather than by conventional machining of prototype articles according to engineering drawings. As a result, the time required to produce prototype parts from engineering designs has been reduced from several weeks to a matter of a few hours.  
         [0003]     By way of background, an example of a freeform fabrication technology is the selective laser sintering process practiced in systems available from 3D Systems, Inc., in which articles are produced from a laser-fusible powder in layerwise fashion. According to this process, a thin layer of powder is dispensed and then fused, melted, or sintered, by laser energy that is directed to those portions of the powder corresponding to a cross-section of the article. Conventional selective laser sintering systems, such as the Vanguard system available from 3D Systems, Inc., position the laser beam by way of galvanometer-driven mirrors that deflect the laser beam. The deflection of the laser beam is controlled, in combination with modulation of the laser itself, to direct laser energy to those locations of the fusible powder layer corresponding to the cross-section of the article to be formed in that layer. The computer based control system is programmed with information indicative of the desired boundaries of a plurality of cross sections of the part to be produced. The laser may be scanned across the powder in raster fashion, with modulation of the laser affected in combination therewith, or the laser may be directed in vector fashion. In some applications, cross-sections of articles are formed in a powder layer by fusing powder along the outline of the cross-section in vector fashion either before or after a raster scan that “fills” the area within the vector-drawn outline. In any case, after the selective fusing of powder in a given layer, an additional layer of powder is then dispensed, and the process repeated, with fused portions of later layers fusing to fused portions of previous layers as appropriate for the article, until the article is complete.  
         [0004]     Detailed description of the selective laser sintering technology may be found in U.S. Pat. Nos. 4,863,538; 5,132,143; and 4,944,817, all assigned to Board of Regents, The University of Texas System, and in U.S. Pat. No. 4,247,508 to Housholder, all hereby incorporated by reference.  
         [0005]     The selective laser sintering technology has enabled the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including polystyrene, some nylons, other plastics, and composite materials such as polymer coated metals and ceramics. Polystyrene parts may be used in the generation of tooling by way of the well-known “lost wax” process. In addition, selective laser sintering may be used for the direct fabrication of molds from a CAD database representation of the object to be molded in the fabricated molds; in this case, computer operations will “invert” the CAD database representation of the object to be formed, to directly form the negative molds from the powder.  
         [0006]      FIG. 1  illustrates, by way of background, a rendering of a conventional selective laser sintering system currently sold by 3D Systems, Inc. of Valencia, Calif.  FIG. 1  is a rendering shown without doors for clarity. A carbon dioxide laser and its associated optics are shown mounted in a unit above a process chamber that includes a powder bed, two feed powder cartridges, and a leveling roller. The process chamber maintains the appropriate temperature and atmospheric composition for the fabrication of the article. The atmosphere is typically an inert atmosphere, such as nitrogen.  
         [0007]     Operation of this conventional selective laser sintering system is shown in  FIG. 2  in a front view of the process with the doors removed for clarity. A laser beam  104  is generated by laser  108 , and aimed at target surface or area  110  by way of scanning system  114  that generally includes galvanometer-driven mirrors which deflect the laser beam. The laser and galvonometer systems are isolated from the hot chamber  102  by a laser window  116 . The laser window  116  is situated within radiant heater elements  120  that heat the target area  110  of the part bed below. These heater elements  120  may be ring shaped (rectangular or circular) panels or radiant heater rods that surround the laser window  116 . The deflection and focal length of the laser beam are controlled, in combination with the modulation of laser  108  itself, to direct laser energy to those locations of the fusible powder layer corresponding to the cross-section of the article to be formed in that layer. Scanning system  114  may scan the laser beam across the powder in a raster-scan fashion, or in vector fashion. It is understood that scanning entails the laser beam intersecting the powder surface in the target area  110 .  
         [0008]     Two feed systems ( 124 , 126 ) feed powder into the system by means of push-up piston systems. A part bed  132  receives powder from the two feed pistons as described immediately hereafter. Feed system  126  first pushes up a measured amount of powder and a counter-rotating roller  130  picks up and spreads the powder over the part bed in a uniform manner. The counter-rotating roller  130  passes completely over the target area  110  and part bed  132 . Any residual powder is deposited into an overflow receptacle  136 . Positioned nearer the top of the chamber are radiant heater elements  122  that pre-heat the feed powder and a ring or rectangular shaped radiant heater element  120  for heating the part bed surface. Element  120  has a central opening which allows a laser beam to pass through the laser window  116 . After a traversal of the counter-rotating roller  130  across the part bed  132  the laser selectively fuses the layer just dispensed. The roller then returns from the area of the overflow receptacle  136 , after which the feed piston  124  pushes up a prescribed amount of powder and the roller  130  dispenses powder over the target area  110  in the opposite direction and proceeds to the other overflow receptacle  138  to deposit any residual powder. Before the roller  130  begins each traverse of the part bed  132  the center part bed piston  128  drops by the desired layer thickness to make room for additional powder.  
         [0009]     The powder delivery system in system  100  includes feed pistons  125  and  127 . Feed pistons  125  and  127  are controlled by motors (not shown) to move upwardly and lift, when indexed, a volume of powder into chamber  102 . Part piston  128  is controlled by a motor (not shown) to move downwardly below the floor of chamber  102  by a small amount, for example 0.125 mm, to define the thickness of each layer of powder to be processed. Roller  130  is a counter-rotating roller that translates powder from feed systems  124  and  126  onto target surface  110 . When traveling in either direction the roller carries any residual powder not deposited on the target area into overflow receptacles ( 136 , 138 ) on either end of the process chamber  102 . Target surface  110 , for purposes of the description herein, refers to the top surface of heat-fusible powder (including portions previously sintered, if present) disposed above part piston  128 ; the sintered and unsintered powder disposed on part piston  128  will be referred to herein as part cake  106 . System  100  of  FIG. 2  also requires radiant heaters  122  over the feed pistons to pre-heat the powders to minimize any thermal shock as fresh powder is spread over the recently sintered and hot target area  110 . This type of dual piston feed system, providing fresh powder from below the target area, with heating elements for both feed beds and the part bed is implemented commercially in the Vanguard™ selective laser sintering system sold by 3D Systems, Inc. of Valencia, Calif.  
         [0010]     Another known powder delivery system uses overhead hoppers to feed powder from above and either side of target area  110  in front of a delivery apparatus such as a wiper or scraper.  
         [0011]     There are advantages and disadvantages to each of these systems. Both require a number of mechanisms, either push-up pistons or overhead hopper systems with metering feeders to effectively deliver metered amounts of powder to each side of the target area and in front of the spreading mechanism which typically is either a roller or a wiper blade.  
         [0012]     Although a design such as system  100  has proven to be very effective in delivering both powder and thermal energy in a precise and efficient way there is a need to do so in a more cost effective manner by reducing the number of mechanisms and improve the pre-heating of fresh powder to carry out the selective laser sintering process.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     It is an aspect of the present invention to provide a method and apparatus for fabricating objects by selective laser sintering employing fewer mechanisms.  
         [0014]     It is another aspect of the present invention to provide a method and apparatus for fabricating objects by selective laser sintering which deposits all of the powder from an overhead feed system that is needed to form two successive cross-sectional layers on one side of a target area and which concurrently levels the powder for the first successive layer while transporting the powder for the second successive layer to an opposing second side of the target area.  
         [0015]     It is a feature of the present invention that a method and apparatus for fabricating objects via selective laser sintering are provided without sacrificing good thermal control and good powder delivery.  
         [0016]     It is another feature of the present invention that a modified process and an apparatus that utilize only one overhead feed hopper and no feed pistons with radiant heaters are provided.  
         [0017]     It is another feature of the present invention that the second powder wave used to form the second layer of powder is preheated in a parked position within the process chamber while the laser beam scans the first layer of powder.  
         [0018]     It is an advantage of the present invention that an apparatus and a method for employing that apparatus are provided for fabricating objects with a selective laser sintering system having a smaller machine footprint.  
         [0019]     It is another advantage of the present invention that the method and apparatus are achieved at a lower cost than prior laser sintering systems.  
         [0020]     The invention includes a method for forming a three dimensional article by laser sintering that includes at least the steps of: depositing a quantity of powder on a first side of a target area; spreading the powder with a spreading mechanism to form a first smooth surface; directing an energy beam over the target area causing the powder to form an integral layer; depositing a quantity of powder on an opposing second side of the target area; spreading the powder with the spreading mechanism to form a second smooth surface; directing the energy beam over the target area causing powder to form a second integral layer bonded to the first integral layer; and repeating the steps to form additional layers that are integrally bonded to adjacent layers so as to form a three-dimensional article, wherein the depositing step includes at least depositing all of the powder required for two successive layers on the first side of the target area and concurrently spreading the powder for the first successive layer while transporting the powder for the second successive layer to the opposing second side of the target area on the spreading mechanism, the powder for the second successive layer being dislodged by an appropriate device during a second depositing step.  
         [0021]     The invention also includes an apparatus for producing parts from a powder comprising a chamber having a target area at which an additive process is performed, the target area having a first side and an opposing second side; a means for fusing selected portions of a layer of the powder at the target area; a powder feed hopper located above and on the first side of the target area for feeding desired amounts of the powder; a means for spreading a first layer of powder over the target area while carrying a second quantity of powder to the opposing second side of the target area to be used for a second layer of powder; and a means for depositing the second quantity of powder on the opposing second side of target area.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0022]     These and other aspects, features and advantages of the invention will become apparent upon consideration of the following detailed disclosure, especially when taken in conjunction with the accompanying drawings wherein:  
         [0023]      FIG. 1  is a diagrammatic illustration of a prior art selective laser sintering machine with portions cut away;  
         [0024]      FIG. 2  is a diagrammatic front elevational view of a conventional prior art selective laser sintering machine showing some of the mechanisms involved;  
         [0025]      FIG. 3  is a diagrammatic front elevational view of the system of the present invention showing the metering of the powder in front of the roller mechanism;  
         [0026]      FIG. 4  is a second diagrammatic front elevational view of the system of the present invention showing the parking of the powder wave near the part bed;  
         [0027]      FIG. 5  is a third diagrammatic front elevational view of the system of the present invention showing the retraction of the roller mechanism and the parking of the roller mechanism under the feed mechanism while the laser is selectively heating the part bed and the part bed heater is pre-heating the parked powder wave;  
         [0028]      FIG. 6  is a fourth diagrammatic front elevational view of the system of the present invention showing the dispensing of the second layer of powder onto the top of the roller mechanism;  
         [0029]      FIG. 7  is a fifth diagrammatic front elevational view of the system of the present invention showing the first layer of powder being distributed across the target area and the second layer of powder being carried on top of the roller mechanism to the other side of the target area;  
         [0030]      FIG. 8  is a sixth diagrammatic front elevational view of the system of the present invention showing the depositing of the second layer of powder adjacent to the roller on the opposing side of the powder bed and depositing of residual powder from the first layer in an overflow receptacle;  
         [0031]      FIG. 9  is the seventh diagrammatic front elevational view of the system of the present invention showing the parking of the second powder wave near the part bed;  
         [0032]      FIG. 10  is an eighth diagrammatic view of the system of the present invention showing the parking of the roller to the side while the laser is selectively heating the part bed and the part bed heater is pre-heating the parked powder wave;  
         [0033]      FIG. 11  is a ninth diagrammatic front elevational view of the system of the present invention showing the second layer of powder being distributed across the target area; and  
         [0034]      FIG. 12  is a tenth diagrammatic front elevational view of the system of the present invention showing the roller completing one cycle by depositing residual powder in a second overflow receptacle.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]     An apparatus for carrying out the present invention can be seen in  FIG. 3  and is shown generally as  150 . The process chamber is shown as  152 . The laser beam  154  enters through a laser window  156  that isolates the laser and optics (not shown) of the same type as described with respect to  FIG. 1  from the higher temperature environment of the process chamber  152 . Radiant heating elements  160  provide heat to the part bed and to the areas immediately next to the part bed. These radiant heating elements can be any number of types including, for example, quartz rods or flat panels. A preferred design is fast response quartz rod heaters.  
         [0036]     A single powder feed hopper  162  is shown with a bottom feed mechanism  164  controlled by a motor (not shown) to control the amount of powder dropped onto the bed below. The feed mechanism can be of several types including, for example, a star feeder, an auger feeder, or a rotary drum feeder. A preferred feeder is a rotary drum. A part piston  170  is controlled by a motor  172  to move downwardly below the floor of the chamber  152  by a small amount, for example 0.125 mm, to define the thickness of each layer of powder to be processed.  
         [0037]     Roller mechanism  180  includes a counter-rotating roller driven by motor  182  that spreads powder from powder wave  184  across the laser target area  186 . When traveling in either direction the roller mechanism  180  carries any residual powder not deposited on the target area into overflow receptacles  188  on opposing ends of the chamber. Target area  186 , for purposes of the description herein, refers to the top surface of heat-fusible powder, including any portions previously sintered, disposed above part piston  170 . The sintered and unsintered powder disposed on part piston  170  will be referred to as part bed  190 . Although the use of counter-rotating roller mechanism  180  is preferred, the powder can also be spread by other means such as a wiper or a doctor blade.  
         [0038]     Operation of the selective laser sintering system of this invention is shown beginning in  FIG. 3 . In a first powder dispensing step a quantity of powder is metered from above from the hopper  162  by the bottom feed mechanism  164  to a position in front of the roller mechanism  180 . The quantity of powder metered will depend upon the size of the target area  186  to be covered and the desired layer thickness to be formed. The deposited quantity of powder appears as a mound, but will be referred to hereinafter as a parked powder wave. Parked powder wave  184  shown in  FIG. 3  can contain from about 2.9 to about 8.0 cubic inches of powder when layer thicknesses of from about 0.003 inches (0.0762 mm) to about 0.008 inches (0.203 mm) are desired in each layer formed.  
         [0039]     In a second step, shown in  FIG. 4 , the counter-rotating roller mechanism  180  is activated to move the powder wave  184  slightly forward and park it at the edge of the target area  186  on a first side in view of the radiant heating elements  160 . In a third step, shown in  FIG. 5 , the roller mechanism  180  is moved back and parked directly under the feed hopper  162 . In iterations other than for the first quantity of powder metered from the feed mechanism  164 , the laser (not shown) is then turned on and the laser beam  154  scans the current layer to selectively fuse the powder on that layer. While the laser is scanning the roller mechanism  180  remains parked with its powder support surface or powder carrying structure  183  directly under the powder feeder hopper  162 . Also while the laser is scanning, the parked powder wave  184  adjacent the first side of the target area  186  is pre-heated by the action of the radiant heating elements  160 . This step can eliminate the need for separate radiant heaters to pre-heat the powder.  
         [0040]     In a next step, shown in  FIG. 6 , a second powder wave  185  is fed onto the powder support surface or powder carrying structure  183  on the top of the roller mechanism  180 . After scanning by the laser of the current layer the next step, shown in  FIG. 7 , begins. The roller mechanism  180  is activated and traverses completely across the system, spreading the first layer of pre-heated powder from the first parked powder wave  184  across the target area  186 , while carrying the second powder wave  185  for creating the second layer of powder on powder support surface  183  of the roller mechanism  180 . In the next step, shown in  FIG. 8 , a mounted stationary blade  192  dislodges the second powder wave  185  for creating the second layer of powder off of the powder support surface  183  of the top of roller mechanism  180  as the roller mechanism passes under the blade, depositing the second powder wave  185  on the floor of the process chamber  152  adjacent the second opposing side of the target area  186  while the roller mechanism  180  proceeds to feed any excess powder into the overflow receptacle  188 .  
         [0041]     In the next step, shown in  FIG. 9 , the roller mechanism  180  immediately reverses and moves to park the second powder wave  185  near the part bed  190  and in sight of the radiant heating elements  160  sufficiently close to receive heating effect from them. In the next step of this preferred embodiment shown in  FIG. 10  the roller mechanism  180  moves back and parks while the laser scanning action is completed and the quantity of powder in the second powder wave  185  is pre-heated by the radiant heating elements  160 . After the laser scanning action is complete the roller mechanism  180  is then activated and moves to spread the second quantity of powder in the second powder wave  185  over the surface of the target area  186  as shown in  FIG. 11 . After leveling the powder the roller mechanism  180 , as seen in  FIG. 12 , proceeds to the end of its run and drops any excess powder into the overflow receptacle  188 . This completes the cycle and the next cycle is ready to proceed as shown in  FIG. 3 .  
         [0042]     This inventive design concept reduces a laser sintering machine in both footprint (the horizontal width of the build chamber) and in mechanical mechanisms. The present invention now employs only one feed hopper, one piston, and preferably only one set of radiant heater elements. The reduced size of the build chamber improves the temperature control and temperature response of the system.  
         [0043]     While the invention has been described above with references to specific embodiments, it is apparent that many changes, modifications and variations in the materials, arrangement of parts and steps can be made without departing from the inventive concept disclosed herein. Accordingly, the spirit and broad scope of the appended claims is intended to embrace all such changes, modifications and variations that may occur to one of skill in the art upon a reading of the disclosure. For example any suitable device such as a skive, roller or brush can be used to dislodge or remove the quantity of powder in the second powder wave from the powder carrying surface or structure of the specific spreading mechanism employed, whether a roller, wiper blade or other suitable device. All patent applications, patents and other publications cited herein are incorporated by reference in their entirety.