Abstract:
A computer-controlled apparatus and method for fabricating three-dimensional articles in layerwise fashion is disclosed. Upon dispensing a layer of a fusible powder, a laser irradiates selected locations of that layer to fuse the powder into a cross-section of the article to be formed in that layer, such that the fused cross-sections fuse together into the article. The laser is controlled in a raster scan fashion across the selected locations of the powder layer. The parallel raster scan lines are separated from one another, centerline-to-centerline, according to a selected pitch, or fill scan spacing value. The positions of the parallel scan lines are determined with respect to a coordinate system at the powder layer, rather than with respect to boundaries of the cross-section being formed; in alternating layers, the parallel scan lines are offset from one another by one-half the pitch. This arrangement of the scan lines optimizes the structural strength of the article being formed, while minimizing the number of scans required to form the article.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is related to copending and commonly assigned application Ser. No. __/__,__, entitled “Selective Laser Sintering With Optimized Raster Scan Direction”, filed contemporaneously with this application, and incorporated herein by this reference. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    This invention is in the field of rapid prototyping, and is more specifically directed to the fabrication of three-dimensional objects by selective laser sintering.  
           [0004]    The field of rapid prototyping 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. “Rapid prototyping” generally refers to the manufacture of articles directly from computer-aided-design (CAD) data bases 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.  
           [0005]    By way of background, an example of a rapid prototyping technology is the selective laser sintering process practiced in systems available from 3D Systems, Inc. of Valencia, Calif., 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 SINTERSTATION 2500plus 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 laser may be scanned across the powder in raster fashion, with modulation of the laser effected 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.  
           [0006]    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 assigned to 3D Systems, Inc., all incorporated herein by this reference. Laser power control systems for selective laser sintering systems are described in U.S. Pat. No. 6,085,122, issued Jul. 4, 2000, and in U.S. Pat. No. 6,151,345, issued Nov. 21, 2000, both assigned to 3D Systems, Inc., and also incorporated herein by reference. By way of further background, U.S. Pat. No. 5,352,405, issued Oct. 4, 1994 assigned to 3D Systems, Inc., and incorporated herein by reference, describes a method of scanning the laser across the powder in a selective laser sintering apparatus to provide a uniform time-to-return of the laser for adjacent scans of the same region of powder, thus providing uniform thermal conditions over the cross-section of each of multiple parts within the same build cylinder.  
           [0007]    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, NYLON, 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.  
           [0008]    [0008]FIG. 1 illustrates, by way of background, the construction and operation of a conventional selective laser sintering system  100 . As shown in FIG. 1, selective laser sintering system  100  includes a chamber  102  (the front doors and top of chamber  102  not shown in FIG. 1, for purposes of clarity). Chamber  102  maintains the appropriate temperature and atmospheric composition (typically an inert atmosphere such as nitrogen) for the fabrication of the article.  
           [0009]    The powder delivery system in system  100  includes feed piston  114 , controlled by motor  116  to move upwardly and lift a volume of powder into chamber  102 . Two powder pistons  114  may be provided on either side of part piston  106 , for purposes of efficient and flexible powder delivery, as used in the SINTERSTATION 2500plus system available from 3D Systems, Inc. Part piston  106  is controlled by motor  108  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  118  is a counter-rotating roller that translates powder from feed piston  114  to target surface  104 . Target surface  104 , 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  106 ; the sintered and unsintered powder disposed on part piston  106  will be referred to herein as part bed  107 . Another known powder delivery system feeds powder from above part piston  106 , in front of a delivery apparatus such as a roller or scraper.  
           [0010]    In conventional selective laser sintering system  100  of FIG. 1, a laser beam is generated by laser  110 , and aimed at target surface  104  by way of scanning system  142 , generally including galvanometer-driven mirrors that deflect the laser beam. The deflection of the laser beam is controlled in combination with modulation of laser  110  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  142  may scan the laser beam across the powder in a raster-scan fashion, or in vector fashion. Cross-sections of articles are often formed in a powder layer by scanning the laser beam in vector fashion along the outline of the cross-section in combination with a raster scan that “fills” the area within the vector-drawn outline.  
           [0011]    Referring now to FIGS. 2 a  through  2   c , the relationship of successive fill scans among multiple parts in the same build cylinder, and among successive scanned layers, in conventional selective laser sintering processes will be described. FIG. 2 a  is a plan schematic view of a portion of a layer of powder at target surface  104  at which cross-sections  152   a ,  154   a ,  156   a , are being formed in the current layer of powder, for three different parts or objects being fabricated in the build cycle. These cross-sections  152   a ,  154   a ,  156   a  are formed, in this example, by a combination of vector outlining and raster scan fills, as discussed above. As shown in the cross-sectional view of FIG. 2 c , vector outline scans  160  define the outer boundaries of each of cross-sections  152   a ,  154   a ,  156   a , and fill scans  162  fill the interior of each of cross-sections  152   a ,  154   a ,  156   a  in a raster scan manner. The vector outlines  160  are not shown in FIG. 2 a  (and FIG. 2 b ), for the sake of clarity. FIG. 2 b  illustrates, in plan view, the scanning of cross-sections  152   b ,  154   b ,  156   b  in the next layer of powder.  
           [0012]    As shown in FIGS. 2 a  through  2   c , the rastering of fill scans  162  are carried out in an “x-fast” manner, in which each scan of the laser beam is parallel to the x-axis. Conversely, the “slow” axis in this example is the y-axis, as the scan path is incremented in the y-direction after completion of each x-direction scan. Typically, the direction in which the scans increment alternate from layer to layer. In this example, the slow axis direction in cross-sections  152   a ,  154   a ,  156   a  (FIG. 2 a ) is the +y direction, while the slow axis direction in the next cross-sections  152   b ,  154   b ,  156   b  (FIG. 2 b ) is the −y direction.  
           [0013]    The spacing between adjacent fill scans  162  is defined by a distance L between adjacent fill scans  162 , as shown in FIGS. 2 a  and  2   c . Distance L, or at least its maximum specification value, is defined according to a tradeoff between structural strength of the sintered article (which increases with decreasing L) and the speed of manufacturing (which of course increases with increasing L). It is contemplated that distance L will depend upon the particular application of the resulting article, upon the specific powder material used, and other factors.  
           [0014]    The spacing between adjacent fill scans  162  of course only partially defines the location of scans  162 ; the absolute positioning of fill scans  162  within a layer also depends upon the location of the initial scan in the cross-section. According to this conventional method, the position of fill scans  162  within a given cross-section  152   a ,  154   a ,  156   a  is determined relative to the outer boundary of that cross-section, and depends upon distance L. This positioning is based on the outer boundary, even if this outer boundary is not vector traced by the laser beam. FIG. 2 c  illustrates, for cross-section  154   a  in which the slow axis incremental direction is the +y direction, that the first fill scan is set at a position that is distance L from the right-most vector scan  160  (measured centerline-to-centerline). Each successive fill scan  162  in cross-section  154   a  is then separated by distance L from the previous fill scan  162  (also measured centerline-to-centerline), continuing until the last fill scan  162  is made within cross-section  154   a . In FIG. 2 c , fill scans  162  are shown schematically in a non-overlapping manner for clarity; actually, adjacent fill scans  162  will overlap one another so that the powder at adjacent fill scans  162  will fuse together into a mass.  
           [0015]    Referring back to FIG. 2 a , the definition of the position of fill scans  162  within cross-sections  152   a ,  154   a ,  156   a  based upon the boundaries of each cross-section results in the fill scans  162  not necessarily aligning collinearly with one another. For example, fill scans  162  of cross-section  154   a  are offset, in the y-dimension, from fill scans  162  of cross-sections  152   a ,  156   a . As evident from FIG. 2 a , this offset among cross-sections  152   a ,  154   a ,  156   a  causes a large number of scan lines to be traced in the fabrication of these articles.  
           [0016]    Referring now to FIG. 2 b , according to this conventional method, cross-sections  152   b ,  154   b ,  156   b  are next formed, after the dispensing and spreading of the next layer of powder over that in which cross-sections  152   a ,  154   a ,  156   a  were formed. Cross-sections  152   b ,  154   b ,  156   b  are then formed by way of vector scans  160  (FIG. 2 c ) and fill scans  162 . In this example, as shown in FIG. 2 c , cross-sections  152   b ,  154   b ,  156   b  are identical (in the x and y dimensions) to cross-sections  152   a ,  154   a ,  156   a , and as such vector scans  160  overlie one another in these two layers.  
           [0017]    However, as noted above, the direction of slow axis incrementing is opposite for cross-sections  152   b ,  154   b ,  156   b  relative to cross-sections  152   a ,  154   a ,  156   a . In this example, cross-sections  152   b ,  154   b ,  156   b  are incremented in the −y direction, while cross-sections  152   a ,  154   a ,  156   a  are incremented in the +y direction. As shown in FIG. 2 c , the first fill scan  162  proceeding from the left-most vector scan  160  along the −y axis is separated from this vector scan  160  by distance L. Each successive fill scan  162  is then separated from the preceding fill scan  162  by distance L, as in the previous case of cross-sections  152   a ,  154   a ,  156   a.    
           [0018]    It has also been observed, in connection with this invention, that this conventional definition of the location of fill scans  162  based upon the outer boundaries results in fill scans  162  that have no relation to one another, when considered among layers. For example, as evident from FIG. 2 c , the spacing between a fill scan  162  in an upper layer and the adjacent fill scans  162  in the layer immediately below is not uniform. In the example of FIG. 2 c , fill scan  162  in cross-section  154   b , is separated from one adjacent fill scan  162  in cross-section  154   a  by a distance d 1 , and from the other adjacent fill scan  162  in cross-section  154   a  by a distance d 2  that is much smaller than distance d 1 . The strength of bonding between cross-sections  154   a ,  154   b  is therefore limited by the larger distance d 1 . The worst case of this spacing will occur when fill scans  162  in adjacent layers exactly line up with one another, such that distance d 2  will be at a minimum and distance d 1  will be at a maximum.  
           [0019]    Through geometric analysis, it has been observed, in connection with the invention, that the distance L between adjacent scans in the same layer defines the maximum possible distance d 1  that may occur in the fabrication of a given article. Conversely, the structural strength of the sintered article, which depends in large part on the maximum distance d 1  between adjacent fill scans  162  in adjacent layers, limits the spacing distance L between fill scans  162  in the same layer. In order to guarantee the desired structural strength, the spacing distance L must be selected assuming the worst case condition of fill scans  162 , namely where fill scans  162  in successive layers overlie one another. However, many articles will be formed in which the worst case condition is not present, and therefore the actual distances d 1  will be less than the maximum. In these cases, therefore, the spacing distance L between fill scans  162  in the same layer, defined according to the worst case condition, will be smaller than necessary, resulting in a longer build time for each cross-section of the article than is necessary to achieve proper structural strength.  
           [0020]    By way of further background, U.S. Pat. No. 5,711,911 describes numerous techniques for ordering vector scans in the formation of an object from a liquid photopolymer by way of stereolithography. The techniques disclosed in this document address various limitations in the texture and thickness of photocured liquids. One of these disclosed techniques involves the interleaving of scans within the same layer of liquid photopolymer. Specifically, the reference discloses a layer of liquid photopolymer that is scanned, in a first pass, using non-consecutive fill scan vectors; a second pass completes the photoexposing process by scanning those scan lines between the scans of the first pass.  
         BRIEF SUMMARY OF THE INVENTION  
         [0021]    It is therefore an object of the present invention to provide a method of fabricating one or more articles by selective laser sintering in which the build time in each layer is minimized.  
           [0022]    It is a further object of the present invention to provide such a method in which the structural strength of the fabricated articles is not degraded despite a reduction in the number of fill scans.  
           [0023]    It is a further object of the present invention to provide such a method in which the structural strength of the fabricated articles is uniform.  
           [0024]    Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.  
           [0025]    The present invention may be implemented into the selective laser sintering of a three-dimensional article, in which the article is formed in layerwise fashion by the sintering, or melting and resolidification, of a powder. According to this invention, the cross-sections of the articles formed in a given layer are raster-scanned, with a selected line-to-line spacing between fill scans, and beginning from an arbitrary position in the layer. In the next layer of powder that is dispensed over the prior layer, the cross-sections of the articles are raster-scanned with the same spacing, but with the location of the scan lines substantially centered between the locations of the scan lines in the previous layers. By locating scan lines in successive layers relative to one another, rather than relative to the boundaries of the object cross-section in that layer, the number of scans required for the formation of the article or articles can be reduced, perhaps by as much as a factor of two, without degrading the structural strength of the article so formed.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0026]    [0026]FIG. 1 is a schematic diagram, in perspective view, of a conventional selective laser sintering apparatus.  
         [0027]    [0027]FIGS. 2 a  and  2   b  are plan views that schematically illustrate the scanning of successive layers of powder in conventional selective laser sintering.  
         [0028]    [0028]FIG. 2 c  is a cross-sectional view of one of the articles illustrated in FIGS. 2 a  and  2   b , according to conventional selective laser sintering.  
         [0029]    [0029]FIG. 3 is a schematic diagram, in a perspective view, of a selective laser sintering apparatus utilizing the preferred embodiment of the invention.  
         [0030]    [0030]FIG. 4 is a flow diagram illustrating a method of selective laser sintering according to the preferred embodiment of the invention.  
         [0031]    [0031]FIG. 5 is a flow diagram illustrating the generation of vectors for cross-sections of articles to be built in the method of selective laser sintering according to the preferred embodiment of the invention.  
         [0032]    [0032]FIGS. 6 a  and  6   b  are plan views, and FIG. 6 c  is a cross-sectional view, illustrating operation of the selective laser sintering method of the preferred embodiment of the invention upon successive layers of powder.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]    As will become apparent from the following description, the present invention is beneficial when applied to rapid prototyping systems that utilize lasers in the fabrication of articles from computer readable representations of those articles, such as those created by computer-aided-design (CAD) or computer-aided-manufacturing (CAM) systems. It is contemplated that the present invention is particularly beneficial when applied to rapid prototyping methods that are based upon a thermal mechanism. As is well-known in the art, selective laser sintering is a rapid prototyping approach that uses a thermal mechanism to form the article, in that particles of powder in selected locations of each of a sequence of layers are fused to one another at locations that receive laser energy. The fusing or binding of particles at the selected locations takes place through sintering (in its traditional sense), melting and resolidification, initiation of a chemical reaction (including thermosetting), or some other thermally based mechanism; for purposes of this description, and as consistent in the rapid prototyping field, all of these mechanisms will be referred to as “sintering”. Accordingly, the following description will be directed to a selective laser sintering system. It is of course to be understood that the present invention may be used to benefit in other types of rapid prototyping systems that involve a thermal mechanism.  
         [0034]    Fabrication of a cross-section of the desired article or articles is effected by laser  110 , which provides a beam which is directed by scanning system  142  in the manner described in the U.S. patents referred to hereinabove and as will now be described relative to FIG. 3. Laser  110  includes, in addition to a laser itself, such conventional control elements as described in the above-referenced U.S. Pat. No. 4,863,538, including for example a front mirror assembly, and focusing elements such as diverging and converging lenses. The type of laser  110  used depends upon many factors and in particular upon the type of powder that is to be sintered. For many types of conventional powders, a preferred laser is a 100 watt CO 2  type laser with controllable power output, although lasers having as low as 25 watt power output are useful with some materials. Laser  110 , when on, emits laser beam  105  that travels generally along the path shown by the arrows in FIG. 3.  
         [0035]    Computer  140  and scanning system  142  control the direction of laser beam  105  as it impinges target surface  104 . In this preferred embodiment of the invention, computer  140  includes a controlling microprocessor for scanning system  142  and further includes a system for storing a computer readable representation of the article or articles being produced, such as a CAD/CAM database or data files generated from such a database, at least in slice-by-slice form if not in entirety, to define the dimensions of the article or articles being produced. A conventional personal computer workstation, such as a microprocessor-based personal computer that includes floating point capabilities, is suitable for use as computer  140  in the preferred embodiment of the invention. Computer  140  generates signals on lines AIM to scanner processor  103 , in scanning system  142 , to direct laser beam  105  across target surface  104  according to the cross-section of the article to be produced in the current layer of powder. Laser power control system  150  controls the power of laser  110  in response to control signals from computer  140  and feedback signals from scanner processor  103 ; an example of advanced laser power control that may be used in connection with this embodiment of the invention is described in the above-incorporated U.S. Pat. No. 6,151,345.  
         [0036]    Scanning system  142  includes prism  144  for redirecting the path of travel of laser beam  105 ; the number of prisms  144  necessary for directing laser beam  105  to the proper location is based on the physical layout of the apparatus. Alternatively, as is well known in the art, one or more fixed mirrors can be used in place of prism  144  for directing laser beam  105 , depending upon the particular layout of system  100 . Scanning system  142  further includes a pair of mirrors  146 ,  147 , which are driven by respective galvanometers  148 ,  149 . Galvanometers  148 ,  149  are coupled to their respective mirrors  146 ,  147  to selectively orient the mirrors  146 ,  147  and control the aim of laser beam  105 . Galvanometers  148 ,  149  are mounted perpendicularly to one another so that mirrors  146 ,  147  are mounted nominally at a right angle relative to one another. Scanner processor  103  in scanning system  142  controls the movement of galvanometers  148 ,  149  to control the aim of laser beam  105  within target surface  104 , in response to the signals on lines AIM from computer  140  that are generated according to the computer readable representation produced from a CAD/CAM data base, which defines the cross-section of the article to be formed in the layer of powder at target surface  104 . Other scanning systems may alternatively be used in connection with this invention, including, for example, an x-y gantry system that delivers energy beams by way of a fiber optic cable.  
         [0037]    Referring now to FIG. 4, the operation of computer  140  in controlling laser beam  105  according to the preferred embodiment of the invention will be described in detail. This operation begins with several processes in which parameters are established for a given build cycle. As known in the art, a build cycle refers to a cycle of operation of the system of FIG. 3, in which one or more articles are formed in layerwise fashion in one instance of part bed  107 . For this build cycle, as conventional in the art, the desired powder material is selected by the human user. Computer  140  in turn reads a file to load various build parameters such as laser power, part bed temperature, and the like.  
         [0038]    In process  196 , the user of system  100  enables the interleaved fill process of the preferred embodiment of the invention, for example by setting a flag in computer  140 . Upon enabling interleaved fill, the user of system  100  then sets, in process  198 , a value for fill scan spacing parameter L, which is then stored by computer  140  for the build cycle. Fill scan spacing parameter L is the spacing between adjacent fill scan traces in the raster scanning of the powder in this build cycle. According to this embodiment of the invention, fill scan spacing parameter L is the pitch, or distance between corresponding points of adjacent ones, of the raster scans; for example, fill scan spacing parameter L corresponds to a centerline-to-centerline distance between adjacent scans. Alternatively, fill scan spacing parameter L may be specified as a spacing (generally negative spacing, indicating overlap) between adjacent scan lines, considering the spot size of laser beam  105 . As will also be described in further detail below, fill scan spacing parameter L is selected to ensure adequate structural strength of the resulting article, and depends upon many parameters of the build cycle, but primarily upon the layer thickness. Other important parameters that can affect fill scan spacing parameter L are the characteristics of the powder material, the laser energy being delivered (i.e., the laser power and scan rate), the chamber temperature, and the desired density of the resulting article. For example, some materials, such as NYLON-based powders, are fully melted in the selective laser sintering process, while other materials, such as polymer-coated steel and amorphous polymers, are fused only at the perimeters of the powder particles. It is contemplated that those skilled in the art having reference to this specification can readily derive the fill scan spacing parameter L, for a given material and set of sintering conditions, through rudimentary experimentation. It is contemplated that the fill scan spacing parameter L will generally have a value of on the order of a few tenths of a millimeter.  
         [0039]    In process  200 , the human user of system  100  arranges, with the assistance of computer  140 , the articles to be fabricated within part bed  107 . Typically, the articles to be fabricated are arranged to maximize the number of articles that can be fabricated in a single build cycle. In process  202 , computer  140  loads the desired layer thickness based on an input from the user. These preparatory processes  196 ,  198 ,  200 , and  202  may, of course, be performed in any order.  
         [0040]    Prior to process  204  of FIG. 4, computer  140  receives a computer readable representation of the articles to be formed in the current selective laser sintering build cycle. In process  204 , computer  140  generates the set of outline and fill vectors to be traced in a given layer of powder, referenced to a coordinate system of part bed  107 . According to the preferred embodiment of the invention, process  204  is carried out in real-time during the build itself, for example by generating the vectors for each layer immediately prior to its selective laser sintering, or alternatively in a pipelined manner preparing the vectors for the next layer during the selective laser sintering of a prior layer. Further in the alternative, the generation of the vectors in process  104  may be performed as a batch operation, for all layers in the build cycle prior to initiating selective laser sintering. In addition, all or part of process  204  may be performed by computer  140  in system  100 , or alternatively by a separate off-line computer. For purposes of this description, process  204  will be described as performed by computer  140  for each layer k, immediately prior to the selective laser sintering of that layer  
         [0041]    [0041]FIG. 5 illustrates, in further detail, the operation of process  204  according to the preferred embodiment of the invention. As will be noted below, according to the preferred embodiment of the invention, the fill scans for each layer are generated substantially one at a time, for the current (or next) layer of powder that is to be selectively sintered by laser beam  105 . To conserve memory, therefore, process  222  is first performed, according to the preferred embodiment of the invention, to discard the stored fill scans for a previous layer from the memory of computer  140 , once those stored fill scans have been used in selective laser sintering. The layer for which the fill scans are discarded may be the immediately prior layer to the current layer, or may be for a layer even further back in the process, depending upon the memory resources of computer  140  and any “pipelining” of the vector generation that is implemented. In process  224 , computer  140  then effectively “slices” the arranged volumetric CAD representations of the articles to be formed in the build cycle for a given layer of powder to be selectively sintered (the current layer having the index k), thus defining the cross-sections of the articles in that layer (k), and defining the positions of those cross-sections within the corresponding layer (k) with respect to the coordinate system of part bed  107 .  
         [0042]    Once the positions of the cross-sections within part bed  107  are known for current layer k, computer  140  then derives the vectors to be traced by laser beam  105  in that layer. As known in the selective laser sintering art, each cross-section of the article may be formed by a raster scan of the interior region only, or by a raster scan of the interior in combination with a vector tracing of the outline of the cross-section. Such vector outlining can improve the accuracy of the article and also provide a smoother surface texture of the article, particularly for some materials. If vector outlining is to be performed for one or more articles in the build cycle, process  226  is performed by computer  140  to derive and store the vectors in layer k to be traced by laser beam  105  when outlining each cross-section.  
         [0043]    According to the preferred embodiment of the invention, the positions of the fill scans, for the raster scanning of the interior of article cross-sections to be sintered, are interleaved from layer to layer. Accordingly, decision  227  is next performed to determine whether layer k is odd-numbered or even-numbered in the overall build cycle. Of course, the layer numbering is arbitrary, as the operation of decision  227  will simply ensure interleaved fill scan generation for successive layers, as will become apparent.  
         [0044]    For an odd-numbered layer k, process  228 O is performed to set the slow scan direction for the generated fill scan vectors. In this example, considering the sintered layer to be in the x-y plane, the +x-direction will be the so-called “fast scan” direction, which is the direction in which laser beam  105  travels in making a single raster scan. The “slow scan” direction, namely the direction in which laser beam  105  increments from scan-to-scan, will be either in the +y direction or −y direction, depending upon the layer k. In process  228 O, the slow scan direction is set to the +y direction, for odd-numbered layer k.  
         [0045]    Process  230 O is then performed by computer  140 , to arrange the position of fill scans within odd-numbered layer k. According to this exemplary implementation, these positions are based on the coordinate reference of y=0, as will now be described relative to FIG. 6 a . As shown in this FIG. 6 a , three article cross-sections  252   a ,  254   a ,  256   a  are being formed in layer k=a (a being odd-numbered). The outlines of article cross-sections  252   a ,  254   a ,  256   a  are shown as dashed lines in this example. Fill scans  262  are defined by computer  140  in process  230 O. In the example of FIG. 6 a , fill scans  262  are located at multiples of fill scan spacing parameter L from the y=0 axis, with fill scans  262  potentially located at y=0, y=L, y=2L, . . . , y=mL (of course, negative multiples of L are also permitted, depending upon the arbitrary location of y=0 in the x-y coordinate plane of part bed  107 ). Of course, if no article cross-section is intersected along a line that is a multiple of L in the coordinate plane, as in the case of y=0 in FIG. 6 a , then no fill scan  262  will be defined for that line. Each fill scan  262  is not only defined, in process  230 O, with respect to its line position in the y-dimension, but also with its start and stop points along that line in the x-dimension, with the start and stop points corresponding to the boundaries of the article cross-section intersected by that line.  
         [0046]    Because fill scans  262  are placed relative to an arbitrary axis y=0, the locations of fill scans  262  have no relation to the boundaries of cross-sections  252   a ,  254   a ,  256   a  in that layer. For example, the first (in the +y direction) fill scan  262  of cross-section  252   a  is very near the boundary of that article, while the first fill scan  262  of cross-section  254   a  is spaced apart from its boundary. These fill scans  262  are stored, in the memory of computer  140 , in an ascending order corresponding to the +y slow scan direction, so that laser beam  105  increments in that direction during the actual build.  
         [0047]    One important benefit of the present invention is apparent from FIG. 6 a . Because each of fill scans  262  are defined relative to an arbitrary axis in the plane of layer k, the fill scans  262  are collinear with one another even if associated with different cross-sections and articles. As shown in FIG. 6 a , each of cross-sections  252   a ,  254   a ,  256   a  have fill scans  262  that are located at, and only at, multiples of fill scan spacing parameter L relative to the y=0 axis.  
         [0048]    Following the completion of the appropriate one of processes  230 , control then passes to process  206  for the selective laser sintering of this layer k, as will be described below. It is useful to now consider how the fill scans in the next layer k+1 will be arranged relative to the fill scans of the current layer k. Where the previous index (k−1) was odd-numbered, the next instance of decision  227  will of course determine that the current index k is even-numbered. In process  228 E, computer  140  will then set the slow scan direction to the −y direction, which is of course the opposite direction of that set by process  228 O for odd-numbered layers k. In process  230 E, computer  140  then derives the positions of fill scans  262 . For this even-numbered layer k, however, these fill scan positions are not based on the y=0 axis, but instead are staggered by one-half the value of fill scan spacing parameter L, or L/2. This staggering places the positions of fill scans  262  for even-numbered layers k directly between (in the x-y plane) fill scans  262  for the odd-numbered layer below (and above) the current layer k.  
         [0049]    [0049]FIG. 6 b  illustrates the placement of fill scans  262  in even-numbered layer k=b (where b=a+1). In FIG. 6 b , article cross-sections  252   b ,  254   b ,  256   b  are illustrated; these cross-sections  252   b ,  254   b ,  256   b  correspond to the next cross-section in the build from corresponding previous cross-sections  252   a ,  254   a ,  256   a , respectively. Fill scans  262  in FIG. 6 b  are located at multiples of fill scan spacing parameter L relative to an axis y=L/2. In other words, fill scans  262  are located at y=L/2, y=3L/2, y=5L/2, . . . , y=mL+(L/2). Again, negative positions of fill scans  262  may be used, depending upon the location of the y=0 axis. As before, start and stop points along the x-dimension are also defined and stored for each fill scan  262 . In addition, considering the −y slow scan direction, the vectors of fill scans  262  are ordered negatively, to be scanned in that incrementing direction during the build of the articles.  
         [0050]    As in the case of even-numbered layer k=a, the placement of fill scans  262  in layer k=b is made without regard to the location of the boundaries of cross-sections  252   b ,  254   b ,  256   b . This lack of correspondence between fill scans  262  and cross-section boundaries is evident, in FIG. 6 b , by fill scans  262  being near a boundary of cross-sections  254   b ,  256   b , but not near boundaries of cross-section  252   b . In addition, fill scans  262  in different ones of cross-sections  252   b ,  254   b ,  256   b  are collinearly aligned with one another, because of their placement relative to the y=L/2 axis.  
         [0051]    Referring now to FIG. 6 c , the relationship of fill scans  262  for two layers k=a and k=b within a single article  252  will now be described. In this cross-sectional view, both fill scans  262  and outline vectors  264  are illustrated; in this example, the boundaries of the cross-sections  252   a ,  252   b  overlie one another, as indicated by the overlying outline vectors. As noted above, FIG. 6 c  shows fill scans  262  schematically in a non-overlapping manner for clarity; in practice, adjacent fill scans  262  will overlap one another so that the powder at adjacent fill scans  262  will fuse together. In lower layer k=a, fill scans  262  are spaced apart from one another by the value of fill scan spacing parameter L, but the spacing of the outer fill scans  262  relative to outline vectors  264  is not specified (except that this spacing is necessarily less than the value of fill scan spacing parameter L, otherwise another fill scan  262  would be inserted). In next layer k=b, fill scans  262  are spaced apart from one another again by the value of fill scan spacing parameter L, but are at locations that are substantially between the y locations of fill scans  262  in the underlying layer k=a. Again, the locations of fill scans  262  have no relation to the boundaries of cross-section  252   b , besides being within the value of fill scan spacing parameter L.  
         [0052]    According to the present invention, in which fill scans  262  are positioned relative to an arbitrary axis in the coordinate system of part bed  107 , and not relative to article boundaries, and in which fill scans  262  are staggered from layer-to-layer, the distance between adjacent fill scans  262  in successive layers is uniform. FIG. 6 c  illustrates fill scan  262  in layer k=b is distance d x  between both of its adjacent fill scans  262  in layer k=a. This distance d x  is maintained in this relationship between all adjacent fill scans  262  in successive layers. Distance d x  depends only upon the layer thickness and the value of fill scan spacing parameter L, and does not depend upon the location of outline vectors  264  or the boundaries of any cross-section. The structural strength of the article depends upon fill scan distance d x , as this distance determines the extent to which the selectively sintered powder in one scan fuses to that in an adjacent scan. Because of the uniformity in fill scan distance d x , and its dependence upon the selectable parameters of layer thickness and fill scan spacing parameter L, the user can now directly select a value of fill scan spacing parameter L from the desired strength, without requiring large margins on this fill scan spacing parameter L to allow for a worst case inter-scan distance, as in the conventional methods. Therefore, the number of scans can be reduced from conventional methods, without affecting the structural strength of the article.  
         [0053]    As shown in FIG. 5, upon generation of the vectors for current layer k=b, control passes to process  206  of FIG. 4 to carry out the actual article fabrication. Again, as discussed above, process  204  may be performed in real-time during the build, or may alternatively be performed as a batch operation for all layers of the build cycle prior to initiating the actual build.  
         [0054]    Referring back to FIG. 4 in combination with FIG. 3, the method of fabricating an article according to the preferred embodiment of the invention continues with the dispensing of a layer of powder at the surface of part bed  107  in process  206 , for example by the translation of counter-rotating roller  118  (FIG. 1) to form the powder layer with minimal shear stress, as described in the above-incorporated U.S. Pat. No. 5,132,143. Other systems for dispensing a layer of powder may alternatively be used, including, for example, the delivery of a volume of powder from above the surface of part bed  107 , and in front of a moving roller or scraper.  
         [0055]    Once the powder is dispensed, in process  208 , the current layer of powder at the surface of part bed  107  is raster scanned by laser beam  105 , under the control of computer  140  and scanning system  142 , according to the fill scan vectors  262  generated for that layer in process  204 . As discussed above, the position of fill scans  262  according to this embodiment of the invention are based upon an axis in the coordinate system of the surface of part bed  107 , and not upon the boundaries of the cross-section being formed. These scans of laser beam  105  are separated, centerline-to-centerline, by the value of fill scan spacing parameter L. Of course, considering the thermal effects of the selective laser sintering mechanism, the powder will be fused along fill scans  262  so as to form a cohesive cross-section in that layer, with this cross-section fused to previously scanned portions of the same article in underlying layers. In addition, raster scans of separate cross-sections in this current layer are collinear with one another, as described above, which assists in the rapid raster scanning of this layer. If desired, laser beam  105  is then directed by computer  140  and scanning system  142  to selectively sinter the outline of the cross-sections in the current layer of powder in process  210 , if vector outlining is to be performed. Raster scanning process  208  and vector outlining process  210  may alternatively be performed in the reverse order, with vector outlining process  210  preceding raster scanning process  208 .  
         [0056]    In decision  211 , following the completion of the raster scanning of all cross-sections within the current layer in process  208  and any desired vector outlining in process  210 , computer  140  determines whether additional layers remain to be selectively sintered in the current build cycle. If so (decision  211  is YES), control passes to process  204  to generate fill vectors for the next layer; alternatively, if the fill vectors are generated in a batch process, control will pass to process  106  to dispense the next layer of powder, over the previously sintered layer. Following the eventual dispensing of powder, process  208  is then performed, if desired, to raster scan the boundaries of the cross-sections in this new layer, by laser beam  105 .  
         [0057]    In the next instance of process  208 , the current layer of powder is scanned by laser beam  105  according to fill scans  262  derived in process  204  for that layer. As described above, the raster scans in this layer will be spaced between the scans in the previous layer, and will be carried out in the opposite slow-scan direction from that of the previous layer. This raster scanning ensures that the distance between adjacent scans in successive layers is uniform, as described above.  
         [0058]    The process continues, via optional vector outlining process  210  and decision  211 , until the build cycle is complete (decision  211  is NO). Cool down of part bed  107  with the sintered article or articles therein is then carried out as appropriate for the material used, followed by removal of the loose powder from around the articles. Post processing, such as an anneal or infiltration of the articles with another material to improve the properties of the article, is then performed as desired.  
         [0059]    The present invention provides important advantages in selective laser sintering. Because the fill scan spacing between adjacent raster scans can now be selected with a high degree of confidence relative to the resulting article strength, it has been observed, in connection with the present invention, that fewer fill scans will be required to form an article of the desired structural strength. In some cases, a reduction by a factor of two can be achieved. Not only does this provide a more uniformly fabricated article, but because the build time in each layer is dominated by the number of fill scans, this reduction in the number of raster scans greatly reduces the time required to build articles by selective laser sintering.  
         [0060]    While the present invention has been described according to its preferred embodiments, it is of course contemplated that modifications of, and alternatives to, these embodiments, such modifications and alternatives obtaining the advantages and benefits of this invention, will be apparent to those of ordinary skill in the art having reference to this specification and its drawings. It is contemplated that such modifications and alternatives are within the scope of this invention as subsequently claimed herein.