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 raster scan lines are defined, for each cross-section, to achieve an optimal fill scan time. The optimal fill scan time is determined, by the computer estimating the fill scan time by rotating the cross-section over a plurality of rotational angles, and estimating the fill scan time for each of the rotated cross-sections for at least a sample of the fill scans necessary to form the article. The actual fill scan vectors to be used in selective laser sintering of the article are rotated, from a coordinate axis at the target plane, according to the rotation of the cross-section providing the lowest estimated fill scan time.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is related to copending and commonly assigned application Ser. No. ______, entitled “Selective Laser Sintering With Interleaved Fill Scan”, 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 relatively new field of rapid prototyping has provided significant improvements in providing high strength, high density, parts useful for design verification and in pilot production. “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 at those portions of the powder layer that correspond to a cross-section of the article in that layer. 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, 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, the next layer of powder is then dispensed, and the process is 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]    More detailed descriptions of the selective laser sintering technology are provided by U.S. Pat. No. 4,863,538, U.S. Pat. No. 5,132,143, and U.S. Pat. No. 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 “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 which are 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 , which is controlled by motor  116  to move upwardly and lift a volume of powder into chamber  102 . Two feed 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 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]    [0011]FIG. 2 illustrates a portion of target surface  104  at which four cross-sections  50  of one or more articles are being formed in a top layer of powder according to a conventional selective laser sintering method. In this example, cross-sections  50  are equally-sized rectangles, but at different angular orientations from one another relative to the x-y plane of target surface  104 . As shown in FIG. 2, each of these cross-sections  50  are formed by raster scans of the laser beam across the powder of target surface  104 , along scan lines  62 . Also as shown in FIG. 2, each of the scan lines  62  are parallel to the x-axis in the coordinate system of target surface  104 ; as such, the x-axis is the “fast” scan axis for the raster scan of the laser beam, while the y-axis is the “slow” axis as it is the direction in which the raster scans advance upon completion of each scan.  
           [0012]    According to the conventional technique illustrated in FIG. 2, the number of uniformly spaced raster scan lines  62  required to form a given cross-section  50  depends upon the orientation of the cross-section  50  in the x-y coordinate plane of target surface  104 . In this example, four scan lines  62  are required to scan horizontally-oriented cross-section  50   a . Eighteen and fifteen scan lines are required for angularly oriented cross-section  50   c  and  50   b , respectively. Vertically oriented cross-section  50   d  requires thirteen scan lines  62 . The spacing of scan lines  62  is selected by the operator of the selective laser sintering system, depending upon factors such as the desired structural strength of the resulting article, thickness of the powder layers, surface texture, and build speed. Thermal factors related to the scanning of the laser beam in the selective laser sintering process are described in the above-incorporated U.S. Pat. No. 5,352,405.  
           [0013]    It has been observed, in connection with the present invention, that the number of raster scans (e.g., scan lines  62 , in FIG. 2) is a significant factor in the overall time required to build an article by way of selective laser sintering. It has been discovered, in connection with the present invention, that a reduction in the number of scans performed in a given layer will therefore translate into a reduced build time, even if the scans are longer as a result. In addition, considering that the selective laser sintering apparatus must store vectors corresponding to the raster scan lines, in its memory, a reduction in the number of raster scan lines will result in fewer vectors to be stored in computer memory, and thus in more efficient use of computer resources.  
         BRIEF SUMMARY OF THE INVENTION  
         [0014]    It is therefore an object of the present invention to provide a selective laser sintering method and apparatus in which the build time of an article is reduced by reducing the number of raster scan lines required for each cross-section of the article.  
           [0015]    It is a further object of the present invention to provide such a method and apparatus for which the computer memory requirements for storage of control vectors can be optimized.  
           [0016]    It is a further object of the present invention to provide such a method and apparatus for fabricating an article with optimized tensile strength along its major axes.  
           [0017]    It is a further object of the present invention to provide such a method and apparatus for fabricating an article for which rough surface area due to vector ends can be minimized.  
           [0018]    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.  
           [0019]    The present invention may be implemented into a method and apparatus for fabricating an article by selective laser sintering, in which the direction of raster scanning is optimized for each cross-section of the article. A computer associated with the selective laser sintering apparatus derives scan vectors for each article cross-section. In deriving the vectors, the scan time of the article cross-section is simulated or otherwise calculated, using several trial orientations of the cross-section. The scan vectors are then derived for the optimal orientation of the cross-section, and then stored in computer memory. The process is repeated separately for each cross-section to be formed in a given layer of powder, and for each layer of powder in the build cycle.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0020]    [0020]FIG. 1 is a schematic diagram, in perspective view, of a conventional selective laser sintering apparatus.  
         [0021]    [0021]FIG. 2 is a plan view of a portion of a layer of powder, in which multiple cross-sections of articles are formed according to the prior art.  
         [0022]    [0022]FIG. 3 is a schematic diagram, in a perspective view, of a selective laser sintering apparatus utilizing the preferred embodiment of the invention.  
         [0023]    [0023]FIG. 4 is a flow diagram illustrating a method of fabricating an article in layerwise fashion according to the preferred embodiment of the invention.  
         [0024]    [0024]FIG. 5 is a flow diagram illustrating a method of generating the fill vectors for each cross-section of an article in each layer, in the method of FIG. 4 according to the preferred embodiment of the invention.  
         [0025]    [0025]FIG. 6 is a coordinate plane diagram illustrating an example of a set of rotational angles as used in the method of FIG. 5 according to the preferred embodiment of the invention.  
         [0026]    [0026]FIG. 7 is a flow diagram illustrating a method of testing sampled rotations of an article cross-section to determine the optimal scan orientation in the method of FIG. 5 according to the preferred embodiment of the invention.  
         [0027]    [0027]FIGS. 8 a  through  8   d  are plan views illustrating an example of the method of FIG. 7 according to the preferred embodiment of the invention.  
         [0028]    [0028]FIG. 9 is a plan view of a portion of a layer of powder, in which multiple cross-sections of articles are formed according to the prior preferred embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    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 one of a number of thermal mechanisms, including 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.  
         [0030]    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.  
         [0031]    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 data base 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.  
         [0032]    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.  
         [0033]    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 (not shown in FIG. 4) 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.  
         [0034]    The process begins with the enabling of scan direction optimization by the human user of selective laser sintering system  100  in process  200 . The optimization of scan directions is contemplated to be an optional feature in selective laser sintering, and as such is expected to be selectable for each build cycle by the user. If scan direction optimization is not enabled, generation of the fill vectors for each layer of powder will be performed in the conventional manner. Following enabling of the optimization of the scan direction in process  200 , certain build parameters are set by the human user, and stored in the memory of computer  140 , in process  201 . These build parameters include those that are necessary to define the scanning of laser beam  105  in the build cycle, including the thickness of the powder layers to be dispensed, and the scan fill spacing, or pitch, between adjacent raster scan lines. Other related parameters such as laser spot size, scan speed, and the like may also be set in process  201 , if not previously stored in computer  140 . In process  202 , the human user of system  100  arranges, with the assistance of computer  140 , the article or articles to be fabricated within part bed  107  in this build cycle. Of course, if multiple articles are to be fabricated, these articles are arranged to fit within part bed  107 . It will be apparent to those skilled in the art having reference to this specification that processes  200 ,  201 ,  202  may be performed in any order.  
         [0035]    Individual layers, and individual cross-sections of the articles to be formed in those individual layers, are now processed beginning with process  204 . In 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, and generates the set of outline and fill vectors to be traced in each layer of powder, referenced to a coordinate system of target surface  104  at the top layer of powder in part bed  107 . The generation of the vectors in process  204  may be performed in real-time during the build itself, for example by generating the vectors for the next layer during the selective laser sintering of a prior layer, or alternatively process  204  may be carried out 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 a real-time process, performed by computer  140 . Referring now to FIG. 5, process  204  for generating the fill vectors for one or more cross-sections of articles to be formed in each of the layers of the build cycle, according to the preferred embodiment of the invention, will be described in detail.  
         [0036]    For purposes of this description, the current layer of powder for which computer  140  is generating the fill vectors will be referred to as layer k, and the current article cross-section upon which computer  140  is operating will be referred to as cross-section j. It is of course understood that multiple article cross-sections within the same layer may belong to different articles, or to the same article, depending upon the particular geometries of the desired articles. As will become apparent from the following description, according to the preferred embodiment of the invention, the vectors for each layer are generated substantially one layer at a time, for the current (or next) layer k of powder that is to be selectively sintered by laser beam  105 . To conserve memory, therefore, process  220  is first performed, according to the preferred embodiment of the invention, to discard the stored vectors for a previous layer (e.g., k-1 or earlier) from the memory of computer  140 , once those stored vectors have been used in selective laser sintering. The layer for which the vectors 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.  
         [0037]    According to this preferred embodiment of the invention, a set of contour vectors are then defined for current cross-section j in current layer k, in process  222 . These contour vectors will be defined relative to an x-y coordinate system applied to the target plane of system  100 , with the origin of this coordinate system (and the assignment of the x and y axis directions) being arbitrarily assigned. Typically, in a two-mirror scanning system as described above relative to system  100  in FIG. 3, the fast scan axis (i.e., the x-axis) will be parallel to the line traced by laser beam  105  when mirror  147  is rotated and mirror  146  is held fixed, while the slow scan axis (e.g., the y-axis) will be parallel to the line traced by laser beam  105  when mirror  146  is rotated and mirror  147  is held fixed.  
         [0038]    Once the contour of cross-section j in layer k is defined by computer  140  in process  222 , process  225  is next performed to evaluate and test the fill scan time for this cross-section at multiple rotational angles, so that the scan time of this cross-section can be optimized according to the preferred embodiment of the invention. In summary, computer  140  will effectively simulate the scanning of the current cross-section j in current layer k at several rotations from its position as arranged in process  202 . The results of these simulations will determine an optimal orientation for the fast scan axis in the scanning this cross-section j; this optimal orientation is that at which the shortest scan fill time is obtained. Attention is now directed to FIGS. 6 and 7, relative to which the steps of process  225  according to the preferred embodiment of the invention will be described in detail.  
         [0039]    [0039]FIG. 6 illustrates an exemplary set of angles of rotation in the x-y plane corresponding to target surface  104  at the top layer of powder in part bed  107  of system  100 . In this example, eight possible rotations are shown, ranging from a rotation of +90° to a rotation of −67.5° including no rotation (i.e. rotation of 0°). The sample rotations in this example are separated by 22.5°, or one-sixteenth of a rotation. The other angles (rotations from +112.5° to −90°) opposing the eight rotations from +90° to −67.5°, and need not be included in process  224 , because their associated scan times will be identical to those of their corresponding opposites.  
         [0040]    Referring now to FIG. 7, process  225  begins with the selection of certain parameters to be used in the testing or simulation. Because process  225  is a simulation or calculation, the actual structural strength of the object is not pertinent, and thus the spacing of the fill scans in process  225  can be expanded from that which will actually be used. It is therefore contemplated that a fair comparison of the scan times can be derived by simulating the time required to scan a fraction of the actual scans that will be carried out. In process  234 , therefore, the user selects a sample ratio, which is the ratio of the number of scans to be evaluated in process  225  for the current cross-section, relative to the number of scans that selective laser sintering system  100  will actually carry out in fabricating the cross-section. It is contemplated that a ratio of 1:2 will often provide an accurate relative measure of the fill scan times of the various rotations, while still reducing the computational time and resources of the simulation. Also in process  234 , the user of computer  140  selects the incremental angle of rotation between simulated rotations; in the example of FIG. 6, the incremental angle Δ is 22.5°.  
         [0041]    The iterative simulation and calculations of sample fill times begins with process  236 , in which current x-section j is “rotated” by angle φ m  according to a rotation index m. As noted above, the angle of rotation φ m  includes no rotation, or φ m =0°. In process  238 , computer  140  calculates the time that would be required to fill carry out a fill scan of cross-section j of layer k, when rotated by the current angle φ m . This calculation of process  238  is performed for the number of scans determined by the sample ratio selected in process  234 , but otherwise is intended to sum the time required for the particular scan itself along the fast scan direction (x-direction), plus the incrementing in the slow scan axis (y-direction). For improved accuracy, additional time for each scan line can include acceleration and deceleration times at the ends of the vectors, as described in the above-incorporated U.S. Pat. No. 6,085,122, and any time required for scanning the laser beam outside of the outline of cross-section j. These factors are important in the actual simulation and calculation of process  238 , considering that approximately the same scan fill area is subsumed by cross-section j, regardless of the orientation of the fill scan lines, which indicates that the actual true scan time is constant over the various rotations. However, because of the overhead time involved in connection with each linear scan, regardless of its length, the number of scans is preferably minimized in determining the optimum cross-section orientation for selective laser sintering.  
         [0042]    [0042]FIG. 8 a  illustrates an exemplary cross-section  252   0 , in its position in the x-y plane as arranged in process  220 ; in other words, at a rotational angle φ m  of 0°. In this representation of FIG. 8 a , the sampled fill scan lines are parallel to the x-axis. These sampled fill scan lines are separated relatively widely from one another, as described above, considering that process  238  is a simulation only, and therefore the derived scans need not be so closely spaced as to form a unitary cross-section. Indeed, it is preferred to not store the actual simulated fill scans themselves in the memory of computer  140  for cross-section  252   0 , beyond the completion of process  238 , to conserve the memory resources of computer  140 . In addition, there is no need to center or otherwise translate the position of cross-section  252  for purposes of the calculation, thus saving additional computational resources. In process  238 , computer  140  calculates an estimate of the time that would be required to scan cross-section  252   0  in the manner illustrated in FIG. 8 a , and stores this estimate in its memory.  
         [0043]    Computer  140  next determines whether any remaining rotational angles are to be estimated for current cross-section j. If so (decision  239  is YES), index m for rotational angle φ m  is incremented (or decremented, if appropriate), in process  240 , and control returns to processes  236 ,  238  for the rotation of current cross-section j and the calculation of its estimated sample fill scan time, respectively.  
         [0044]    [0044]FIGS. 8 b  and  8   c  illustrate two other exemplary rotated cross-sections  252   +1 ,  259   −1 , for comparison with non-rotated cross-section  252   0 . In FIG. 8 b , cross-section  252   +1  is rotated in a positive angular direction by one incremental angle Δ, which in this example is 22.5°. As evident from FIG. 8 b , the simulated scan lines are again parallel to the x-axis. In FIG. 8 c , cross-section  252   1  is illustrated as rotated from non-rotated cross-section  252   0  by one incremental angle Δ in the negative angular direction, which in this example is −22.5°. Again, its sampled simulated scan lines are parallel to the x-axis.  
         [0045]    It is of course contemplated that, in performing process  225  to test the scan fill time of the rotated cross-sections, computer  140  may alternatively rotate the angle of the simulated scan lines while maintaining the orientation of the cross-section fixed, rather than rotating the angular orientation of the cross-section and maintaining the scan direction fixed. According to this embodiment of the invention, and using currently available CAD routines, iteratively rotating the cross-section itself is more computationally efficient.  
         [0046]    Upon determining that all desired rotations of current cross-section j have been simulated and their estimated fill scan time stored (decision  239  is NO), one additional simulation is performed according to the preferred embodiment of the invention. In process  242 , computer  140  refers to the contour of current cross-section j as generated in process  222 , and determines the angle at which the longest outline vector segment is oriented in that current cross-section. Once this angle is found, computer  140  then rotates cross-section j by the arithmetic inverse of this angle, so that a rotated cross-section is derived in which the longest outline vector segment is parallel to the simulated scan lines (in this example, parallel to the x-axis). Referring to the example of FIG. 8 a , the longest outline vector of cross-section  252  is the longer side of the rectangle, which is at an angle approximately −18° from the x-axis; in this example, therefore, process  242  will rotate cross-section  252  by an angle of +18°. The sampled simulated fill scan time is then calculated for this rotation, and the result stored in memory, by computer  140  in process  242 .  
         [0047]    Referring back to FIG. 5, following the calculation of estimated fill scan times for each of the desired trial rotations of current cross-section j in process  225 , computer  140  interrogates these stored estimated fill scan times to identify the minimum estimate, and the rotation associated with that minimum estimated fill scan time, in process  226 . Referring back to the example of FIGS. 8 a  through  8   c , among the three exemplary rotated cross-sections  252 , cross-section  252   +1  has the minimum fill scan time, considering that it is intersected by fewer scan lines than the others. However, because cross-section  252  is rectangular, the rotation parallel to the longest side as used in process  242 , will provide the true minimum, and therefore, in this example, the optimum angle of rotation is approximately 18°.  
         [0048]    In process  228 , the actual fill scans are generated for current cross-section j, based upon the rotation selected in process  226 , and stored in the memory of computer  140  for use in the actual selective laser sintering process. These fill scans are rotated from the nominal fast scan axis (e.g., the x-axis) by the arithmetic inverse of the angle of rotation φ m  associated with the rotation selected in process  226 , so that, in the selective laser sintering process, current cross-section j will be scanned at the optimum angle of rotation φ m  determined in process  226 . In addition, the spacing of the fill scans derived in process  228  corresponds to the value of the fill scan spacing parameter L selected and stored in process  218 .  
         [0049]    [0049]FIG. 8 d  illustrates the result of process  228  as applied to cross-section  252 , based upon its optimal rotation. As shown in FIG. 8 d , cross-section  252 ′ is at a non-rotated position, in other words corresponding to non-rotated cross-section  252   0  shown in FIG. 8 a . However, the fill scans in cross-section  252 ′ are not parallel to the x-axis as in the estimates of FIGS. 8 a  through  8   c , but instead are rotated from the x-axis by the inverse of its optimum rotation. In this example, because the selected optimal cross-section  252   +1 , was at a rotation of +18°, each of the fill scans in cross-section  252 ′ of FIG. 8 d  are rotated from the x-axis by −18°.  
         [0050]    Preferably, as described in the above-incorporated application Ser. No. ______ entitled “Selective Laser Sintering With Interleaved Fill Scan”, or alternatively in United Kingdom Patent Application 0118652.7, filed Jul. 31, 2001, also incorporated herein by this reference, the fill scans for current cross-section j that are generated in process  228  are not only separated from one another by the value of fill scan spacing parameter L, but are positioned relative to the x-y coordinate plane and not to the boundary of cross-section j itself. In addition, the positions of the fill scans are offset from one another, in successive layers k, by one-half the value of fill scan spacing parameter L to maximize the structural strength of the object as formed by selective laser sintering.  
         [0051]    Upon generating and storing the fill scans for current cross-section j in layer k at the optimal rotation in process  228 , decision  229  is performed by computer  140  to determine whether additional cross-sections remain for which fill scans are to be generated. If so (decision  229  is YES), index j for cross-sections within the current layer is incremented in process  230 , and control returns to process  222  for the generation of the outline contour vectors and optimized fill scans for the next cross-section.  
         [0052]    The skilled reader will recognize that, if multiple cross-sections are present in the current layer k, each cross-section has its own optimized scan direction derived independently from that of the other cross-sections in that layer k. FIG. 9 illustrates, in plan view, rectangular cross-sections  250   a ,  250   b ,  250   c ,  250   d  at target surface  104 , with their fill scans  62  shown as generated according to the preferred embodiment of the invention described above. Cross-sections  250  of FIG. 9 correspond to the same shape, size, and position as cross-sections  50  of FIG. 2. Comparison of fill scans  62  for cross-sections  250  with fill scans  62  of cross-sections  50  illustrate the optimization provided by the present invention. Each of cross-sections  250  have the minimum number of scans (e.g., four), regardless of their orientation. In contrast, for cross-sections  50  of FIG. 2, which each have fill scans  62  in the same fast scan direction, parallel to the x-axis, only cross-section  50   a  (which happens to be oriented with its longest sides parallel to the x-axis) is optimized; cross-sections  50   b ,  50   c ,  50   d  all require many more than the optimized, minimum, number of fill scans  62 .  
         [0053]    Upon determining that no additional layers remain to be processed (decision  229  is NO), the generation of fill vectors for the current build cycle is complete. Control then passes to the first instance of process  206  (FIG. 4), to begin the actual layerwise fabrication of the article or articles by way of selective laser sintering. Alternatively, as discussed above, process  204  may be performed as a batch operation for all layers of the build cycle prior to initiating the actual build, or may alternatively be performed in real-time during the 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. 5,076,869. 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. Once the powder is dispensed, laser beam  105  is 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  208 , if vector outlining is to be performed.  
         [0055]    In process  210 , one of the article cross-sections in 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 generated for that cross-section in that layer in process  204 . As discussed above, the direction of fill scan for each article cross-section in the current layer is optimized for the fastest scan time. Because of the independence in scan direction optimization among the various cross-sections, multiple cross-sections in any given layer are likely scanned in different fast-axis directions. Decision  211  is therefore performed to determine whether additional article cross-sections in this current layer remain to be raster scanned; if so (decision  211  is YES), control passes back to process  210  to perform the fill scan for that next cross-section.  
         [0056]    Upon completion of the raster, or fill, scanning for each cross-section within the current layer in process  210  (decision  211  is NO), process  212  is optionally performed to vector-outline each article cross-section in the current layer. Alternatively, each cross-section may be outlined immediately prior to or after its fill scanning, after the fill scanning of another cross-section in that layer, or prior to the fill scanning of any one of the cross-sections in the current layer. Computer  140  then executes decision  213  to determine whether additional layers remain to be selectively sintered in the current build cycle. If so (decision  213  is YES), control passes to process  214 , in which computer  140  increments index k of the layers, and initializes index j to refer to a first cross-section within that next layer. Control then passes back to process  204 , for generation of the optimized fill vectors for the cross-sections in this next layer.  
         [0057]    As is well-known in the art, extremely thin layers, for example on the order of a few tenths of a millimeter thick, are typically used in selective laser sintering. With such thin layers, in many cases the shape of article cross-sections often does not drastically change from layer to layer. Accordingly, it is contemplated that the optimized fill scan calculations according to the preferred embodiment of the invention need not necessarily be performed for every layer, but may alternatively be applied periodically to layers in the sequence. Such sampling of the layers in optimizing the scan direction will be especially useful in systems that have limited computational capacity for computer  140 , or in which computer  140  is burdened by other tasks involved in the selective laser sintering build cycle. Further in the alternative, computer  140  may analyze the article or articles involved in the build cycle, and apply a rule-based or other adaptive algorithm to determine which specific layers in the build ought to be processed for scan direction optimization; for example, computer  140  may identify those layers at which significant changes in the article cross-sections are present.  
         [0058]    Following the generation of optimized fill vectors for the next layer, process  206  is next performed, to dispense the next layer of powder over the previously sintered layer. Processes  208  and  210  then control laser beam  105  to fill scan and vector outline the cross-sections in this new layer, in the manner described above.  
         [0059]    The process continues, via decision  213 , until the build cycle is complete (decision  213  is NO). Cool down of part bed  107  including the sintered article or articles 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, completing the fabrication of the articles.  
         [0060]    The present invention provides important advantages in selective laser sintering, particularly in improving the efficiency and manufacturing capacity of selective laser sintering systems. It is contemplated that the overall build cycle time can be significantly reduced through the implementation of this invention. This improvement in build cycle time is available at little cost, other than computational resources of the system computer. Considering the significant advances in the capability of modern microprocessors and computer systems, it is therefore contemplated that adequate computational resources are available for use according to this invention.  
         [0061]    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.