Abstract:
A method of obtaining uniform thermal distribution imaging in a thermally initiated and thermally based laser sintering process whereby a three-dimensional object is formed layer-by-layer in which the scanning sequences in successive layers is varied to more uniformly control the build up of heat within a three-dimensional object being formed. An improved method of image scanning multiple parts in a single build process is also employed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     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 utilizing an improved scanning technique. 2. Description of the Related Art  
         [0003]     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 manual 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.  
         [0004]     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.  
         [0005]     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 incorporated herein by this reference.  
         [0006]     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.  
         [0007]     Laser sintering is a thermally based process that is dependent upon good thermal control of the process in the powder bed to obtain good three-dimensional parts. The sources of thermal energy are the radiant heaters for the part bed, the cylinder heaters to preheat the powder in the powder feed cylinders, the part bed heater, and the laser. The laser is typically a CO 2  laser that scans the fresh powder layer to selectively fuse powder particles in the desired areas. Unequal build-up of heat in one portion of the part bed during the process results in parts being fabricated having unequal properties or for larger individual parts, having different properties within the same part.  
         [0008]     Existing laser sintering systems increasingly have the need to produce parts with greater strength and improved physical properties to meet the demands of the increasing number of applications for laser sintering products. Although the design of the present commercial systems has proven to be very effective in delivering both powder and thermal energy in a precise and efficient way, there is a need to improve the physical properties of the parts produced by the exposure of the powder layers at the target surface by the scanning laser. This need is successfully addressed by the scanning techniques of the present invention.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     It is an aspect of the present invention that the powder bed for solidification is cross scanned by vectors that cross the target surface at 90 degree rotations on successive scans.  
         [0010]     It is another aspect of the present invention that the cross-scanning pattern scans at 90 degree or orthogonally offset patterns from the prior scanning and goes through a four-pattern sequence before being repeated, regardless of whether all scanning is in one layer or successive layers of powder for solidification.  
         [0011]     It is a feature of the present invention that the first or zero degree scan pattern starts from one corner of the target surface and the subsequent 90 degree rotations of the scans progress sequentially from there.  
         [0012]     It is another feature of the present invention that the first and third scans and the second and forth scans may be selectively interleaved.  
         [0013]     It is an advantage of the present invention that more uniform physical properties are obtained from laser sintered fabricated parts using the enhanced scanning techniques of the present invention.  
         [0014]     It is another advantage of the present invention that less distortion resulting from heat build-up occurs when freeform fabrication parts are made utilizing the enhanced scanning techniques of the present invention.  
         [0015]     It is a further advantage of the present invention that there is greater repeatability of freeform fabrication part properties from part-to-part when using the enhanced scanning techniques of the present invention.  
         [0016]     It is yet another advantage of the present invention that parts with improved surface finish, quality, and easier break out from the part cake are obtained when using the enhanced scanning techniques of the present invention.  
         [0017]     These and other aspects, features, and advantages are obtained from improved scanning techniques in a freeform fabrication powder-based technology such as laser sintering where powder particles are fused together from a scanning pattern that employs successive orthogonally offset and alternating opposed sweeping directions of the scanning energy beam.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     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:  
         [0019]      FIG. 1  is a diagrammatic illustration of a prior art selective laser sintering machine with portions cut away;  
         [0020]      FIG. 2  is a diagrammatic front elevational view of a conventional prior art selective laser sintering machine showing some of the mechanisms involved;  
         [0021]      FIG. 3  is a diagrammatic illustration of the laser scanning pattern employed for the first quantity of powder that has been spread on the powder bed to be exposed on the powder bed;  
         [0022]      FIG. 4  is a diagrammatic illustration of the laser scanning pattern employed for the second quantity of powder that has been spread to form a fresh powder layer to be exposed on the powder bed;  
         [0023]      FIG. 5  is a diagrammatic illustration of the laser scanning pattern employed for the third quantity of powder that has been spread to form a fresh powder layer to be exposed on the powder bed;  
         [0024]      FIG. 6  is a diagrammatic illustration of the laser scanning pattern employed for the fourth quantity of powder that has been spread to form a fresh powder layer to be exposed on the powder bed; and  
         [0025]      FIG. 7   a  and  7   b  is a diagrammatic illustration of the laser scanning sequence order employed for the scanning of multiple parts being built on a powder bed.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]      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 is 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. It is also possible to use a vacuum in the process chamber.  
         [0027]     Operation of this conventional selective laser sintering system  100  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 process 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 is 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 .  
         [0028]     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  125 , 127  as described immediately hereafter. Feed system  126  first pushes up a measured amount of powder from the powder in feed cylinder  123  and a counter-rotating roller  130  picks up and spreads the powder over the part bed  132  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 process chamber  102  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  104  to pass through the optical element or 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 system  124  pushes up a prescribed amount of powder from the powder in feed cylinder  129 . The roller  130  then dispenses powder over the target  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 system the center part bed piston  128  drops by the desired layer thickness to make room for additional powder.  
         [0029]     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 process 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 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 powder 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.  
         [0030]     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.  
         [0031]     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.  
         [0032]     The laser scanning techniques used in system  100  can have a marked effect on the heat distribution within the part bed  132  and the part cake  106 . If the laser starts its scan at the same location in each layer of powder, there can be an unequal build-up of heat at that location in the part cake as the powder part bed  132  is repeatedly renewed with a fresh layer of powder. This can be true whether the particular powder material is a multi-material, such as a polymer coated metal, for example steel, or a single component powder such as nylon or polycarbonate. It has been a more frequently noted problem with multi-material powders, such as polymer coated steel.  
         [0033]     To address this potential non-uniform thermal distribution during laser imaging the laser scanning system  114  employs specific scanning patterns and paths to minimize the build-up of heat in any one particular location in the part bed  132  and the part cake  106 .  FIGS. 3-6  show the scanning pattern followed by the laser beam  104  in successive layers of fresh powder that are spread on the part bed  132  by the counter-rotating roller  130 , or other appropriate spreading mechanism. Alternatively  FIGS. 3-6  show the scanning pattern on successive scans, such as where multiple scans are used to expose each fresh layer of powder. The initial scanning or exposure of the powder layer is shown in  FIG. 3  wherein the laser beam  104  starts its scanning at the lower left corner of the target area  110  and moves horizontally across the target area  110  in the process chamber  102  in opposing sequential parallel zig-zagged paths at a 0 20  angle. The next scan or exposure of the powder part bed  132  is shown in  FIG. 4  wherein the laser beam  104  starts its scanning at the lower right corner of the target area  110  and moves vertically across the target area  110  in the process chamber  102  (appearing as upward and downward movements in  FIG. 4 ) in opposing sequential parallel zig-zagged paths at a 90° angle or orthogonally offset from the scanning pattern employed in the exposure of the first layer of powder. The starting location for the laser beam  104  is also moved from the starting location of the scanning pattern in the previous layer of powder in the part bed  132 . The next or third scan or exposure of the third layer of powder in the target area  110  is shown in  FIG. 5  wherein the laser beam  104  starts its scanning at the upper right corner of the target area  110  and moves horizontally across the part bed  132  in the process chamber  102  in opposing sequential parallel zig-zagged paths at a 180° angle or orthogonally offset from the scanning pattern employed in the exposure of the second layer of powder. The starting location for the laser beam  104  is also moved from the starting location of the scanning pattern in the previous layer of powder in the target area  110 . The last scan or exposure in the 4-step patterned laser scanning technique of the powder in target area  110  is shown in  FIG. 6  wherein the laser beam  104  starts its scanning at the lower right corner of the target area  110  and moves vertically across the part bed  132  in the process chamber  102  (appearing as downward and upward movements in  FIG. 6 ) in opposing sequential parallel zig-zagged paths at a 270° angle or orthogonally offset from the scanning pattern employed in the exposure of the third layer of powder. Again, the starting location for the laser beam  104  is moved from the starting location of the scanning pattern in the previous layer of powder in the target area  110 .  
         [0034]     This orthogonally offset scanning pattern helps reduce the build-up of heat in the part cake  106  as it increases in height layer by layer of powder that is scanned by not starting the scanning in the same location in the target area  110  or following the same scanning path each time, thereby allowing more time to pass and more of the laser heat energy to be spread throughout the part cake  106  facilitating uniform thermal distribution in the part cake  106  and the target area  110  during imaging. The four steps in the laser scanning pattern are continually repeated until the three-dimensional object being fabricated is completed. It is also desirable for selective parts to interleave the scanning paths during the exposure of the second and fourth quantities of powder with the scanning paths followed during the exposure of the first and third quantities of powder.  
         [0035]      FIGS. 7A and 7B  show the technique employed when multiple parts are being fabricated in a single build process in the laser sintering system  100 .  FIGS. 7A and 7B  reflect 4 distinct parts being built in the process chamber  102  and their relative location in the generally geometrically shaped target area  110 . As seen in the Figures, the target area  110  in part bed  132  is preferably rectangular or square in shape. Each of the four parts are numbered in their order of scanning so that the part numbered 1 is always the first part to be scanned and the others are scanned in ascending order. While only four parts are shown, it is to be understood that any multiple number of parts can illustrate the pattern whether greater of lesser in number. As shown between  FIG. 7A  and  FIG. 7B  the first part scanned is varied in each layer of powder scanned, moving from the upper left hand comer to the lower left hand comer. Subsequent layers of powder will have the start point for the laser scanning commence in the lower right hand comer and move to the upper right hand comer. This varied start point for the laser scanning operation by part helps reduce the build-up of heat in one part by not starting the scanning in the same part each time thereby allowing more time to pass and more of the laser heat energy to be spread through out the target area  110  and the part cake  106 , further facilitating uniform thermal distribution during imaging when multiple parts are being fabricated. Alternatively, as was stated with respect to the orthogonally offset scanning pattern, the varied start point for the laser scanning operation can be utilized for each scan where there are multiple laser scans employed with each powder layer. In this alternative approach each scan would be accomplished in the ascending order shown in the Figures, regardless of the layer of powder being scanned and only with reference to the number of parts being built.  
         [0036]     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. For example and as previously mentioned, the improved scanning techniques can be employed for each scanning exposure of the laser beam and not just be limited by the number of layers of fresh powder deposited regardless of whether a single part or multiple parts are being fabricated in a build cycle. 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. All patent applications, patents and other publications cited herein are incorporated by reference in their entirety.