Patent Publication Number: US-2005131570-A1

Title: Procedures for rapid build and improved surface characteristics in layered manufacture

Description:
RELATED APPLICATIONS  
      The present application is related to co-pending U.S. patent application Ser. No. ______ [1100.1103101], titled TOOL PATH PLANING PROCESS FOR COMPONENT BY LAYERED MANUFACTURE, filed on date even herewith. 
    
    
     FEDERAL SPONSORSHIP  
      This invention was made with Government support under ______ contract number N00014-94-C-0115, entitled “______”. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION  
      The present invention is related generally to machine manufacturing of components. In particular, the present invention is related to rapid prototyping manufacturing including layered manufacturing and solid freeform fabrication.  
     BACKGROUND OF THE INVENTION  
      Using conventional techniques, a desired article to be made can initially be drawn, either manually or automatically utilizing a computer-aided design (CAD) software package. The article can be formed by removing material from material stock to form the desired shape in a machining operation. The machining operation may be automated with a computer-aided machining (CAM) process. The design and manufacture process may be repeated multiple times to obtain the desired part. A common practice is to mechanically remove material to create three-dimensional objects, which can involve significant machining skills and turn around time.  
      One process for making three-dimensional objects builds up material in a pattern as required by the article to be formed. Masters, in U.S. Pat. No. 4,665,492, discusses a process in which a stream of particles is ejected and directed to coordinates of the three-dimensional article according to data provided from a CAD system. The particles impinge upon and adhere to each other in a controlled environment so as to build up the desired article.  
      Processes and apparatus also exist for producing three-dimensional objects through the formation of successive laminae which correspond to adjacent cross-sectional layers of the object to be formed. Some stereo lithography techniques of this type use of a vat of liquid photocurable polymer which changes from a liquid to a solid in the presence of light. A beam of ultraviolet light (UV) is directed to the surface of the liquid by a laser beam which is moved across the liquid surface in a single plane, in a predetermined XY pattern, which may be computer generated by a CAD system. In such a process, the successive layers may be formed in a single, horizontal plane, with successive layers solidifying together to form the desired object. See, for example, U.S. Pat. No. 4,575,330 to Hull. Arcella et al., in U.S. Pat. No. 4,818,562, discuss a method for forming an article by directing a laser beam on a fusible powder which is melted by the beam, and which solidifies to form the desired shaped object.  
      Recently, various solid freeform fabrication techniques have been developed for producing three-dimensional articles. One such technique, used by Stratasys, Inc. (Eden Prairie, Minn.), is referred to as Fused Deposition Modeling (FDM). See, for example, U.S. Pat. No. 5,121,329 to Crump, herein incorporated by reference. FDM builds solid objects, layer by layer, from polymer/wax compositions according to instructions from a computer-aided design (CAD) software program. In one FDM technique, a flexible filament of the polymer/wax composition is heated, melted, and extruded from the nozzle, where it is deposited on a workpiece or platform positioned in close proximity to the dispensing head. The CAD software controls the movement of the dispensing head in the horizontal X-Y plane and controls the movement of the build platform in the vertical Z direction. By controlling the processing variables, the extruded bead or “road” can be deposited layer by layer in areas defined by the CAD model, leading to the creation of the desired three-dimensional object. Other examples of layered manufacturing techniques include multi-phase jet solidification techniques and/or laser-engineered net shaping. The extruded bead can be a ceramic suspension or slurry, a molten plastic, a powder-binder mixture, a polymeric material ready for curing or hardening, a molten metal, or other suitable materials which harden with time and/or exposure to an external stimulus. The bead can also be a curable strip of polymer or pre-polymer with polymerization initiated by radiation.  
      In conventional layered manufacturing techniques, the layers are formed or deposited in a flowable state which can be in the form of a series of long beads of extruded material. The beads can have a rounded, oblong, or circular transverse cross-sectional profile, where the external side faces of the bead can bulge outward. The conventional material layers are typically rounded at the periphery, forming layer surfaces having convex intra-layer regions and sharp, mechanically weak concave inter-layer regions. In particular, where the stacked bonded layers form the manufactured part side surfaces, the concavities can form sharp crevices having poor properties with respect to crack propagation and fracture.  
      In conventional layered manufacturing, cavities, either external or internal, are often found in product designs. The cavities may have upper structures such as ceilings or overhangs. The upper structures may be cantilevered structures having one end or edge free or structures only unsupported in the middle, between supports on either side or edge. The structures are unsupported in the sense that during deposition or formation of the still flowable main material, the material will drop down through the cavity without a structure previously established to support the main material during hardening. The cavities below have a volume which can be defined by a downward projection of the unsupported portion of the main material above.  
      In conventional layered manufacturing, a support structure of secondary material is built, layer by layer, to provide a support structure for the material to be formed or deposited in the layer above. The secondary material forms layers which also require support from the layer below for their deposition. Using conventional methods, an unsupported structure is supported by secondary material, layer under layer, from top to bottom, until the bottom of the cavity is reached, or until the workpiece platform being used to build the article is reached. The secondary material is later removed by mechanical, chemical, or thermal means, leaving the main material article. A large amount of secondary material can be required to build the removable structure, as well as a large build time required to form the secondary material layers.  
      What would be desirable are methods suitable for making parts using layered manufacturing which provide superior crack resistant surfaces. Methods which require less time to build support structures would also be advantageous.  
     SUMMARY OF THE INVENTION  
      The present invention includes improved methods for making objects using layered manufacturing techniques, as well as the objects made possible through use of these methods. One group of methods forms objects having improved surface properties made possible by forming a mold layer of a second material prior to forming a main part layer of a first material. Another group of methods forms objects requiring less time and material to build. This group of methods includes methods for building minimized secondary material support structures having less volume than conventional support structures.  
      More particularly, the present invention includes methods for forming a mold layer of a second material along the periphery of the object surfaces to be improved. The second material layers can be convexly rounded at the periphery, forming a rounded mold layer to receive the later formed first material. The first material layer can thus form an impression of the second material layer along the periphery of the first material layer. The impression formed along the first layer side face can have a rounded, concave, middle intra-layer region and a convex, inter-layer region where the multiple layers stack together. The inter-layer convexities have superior mechanical strength and superior crack resistance relative to the concave inter-layer regions of the conventionally made parts.  
      In one method, a data file containing representations of a three-dimensional object is accepted as input. The data file can be a three-dimensional CAD file, for example, a stereo lithographic (STL) file. The three-dimensional data can be partitioned into horizontal slices or layers, which can be represented by two-dimensional closed curves or poly-line segments having an associated layer thickness. The curves can define the outside and/or inside of areas to be filled with the main material. The curves can later be filled with raster tool paths generated to fill the area with material. The user can identify surfaces of the three-dimensional object to receive surface improvement and, directly or indirectly, identify the curves or curve portions corresponding to the surfaces to be improved.  
      A set of secondary curves can then be generated, the secondary curves corresponding to secondary material areas to abut the main material areas. The secondary curves thus formed preferably correspond to layer areas having at least two bead widths of secondary material. Some embodiments form secondary material layers with no voids, while other embodiments form secondary material layers having voids to reduce material usage and build time. The secondary material curves can then be used to generate tool paths for the secondary material. The secondary and main material tool paths can be checked for consistency and lack of interference before being integrated and the processing completed.  
      In the manufacturing phase, the part can be built up, bottom to top, by depositing the secondary and main materials, layer by layer. If secondary material is called for in the current layer, a secondary material nozzle can deposit a bead of secondary material of the desired bead width along the previously calculated path. A main material nozzle can then deposit a bead of main material of the desired bead width and along the previously calculated tool path. The flowable main material, formed along the previously formed secondary mold layers, can form an impression of the mold layers convex edge shape, thereby attaining a concave intra-layer shape and a convex inter-layer shape, where the stacked layers join each other. The secondary material can be later removed, exploiting differential mechanical, chemical, or thermal properties. In a preferred embodiment, the main and secondary materials are not the same, but are the same material in other embodiments. Improved surfaces provided by the present invention can have improved mechanical properties due to the lack of sharp, inter-layer convexities.  
      The present invention also includes methods for building removable support structures that form the secondary structures using substantially less volume than the cavity volume. The support structures can have at least one sloping side surface having a substantial deviation from vertical. In one group of structures, the support forms an angle or corner brace, supporting the cavity ceiling from a side wall. The angle piece can have a width decreasing with depth, indenting or offsetting until the support piece has no width. In another group of structures, the support forms a column or interior wall having a wide topmost layer and less wide middle and bottom layers. The wide top layers support the main material layer above, with the lower layers decreasing in width. The lower layers can be indented or offset inward by a small amount at each layer. The indent amount is preferably less than about one-half of the bead width of the layer above.  
      One method for generating the minimized support structures accepts two-dimensional curves for each layer as input. The two-dimensional curves represent the inner and outer perimeters of the main material layers for the part to be built. The unsupported or overhanging structures can be identified by processing the layers of the main structure from top to bottom, beginning with the second to top layer. The layers can be processed as pairs having an upper and lower layer. The upper layer can be reduced in one or more dimensions by an indent or offset amount ultimately corresponding to the slope of the side surface of the minimized support structure. In some embodiments, certain dimensions are automatically or manually selected as not to be reduced in extent. The difference of the reduced projected upper layer and the lower layer corresponds to an unsupported upper area, which will require support prior to formation. New secondary support material curves can be generated at the current lower level to provide the missing support, and these newly added secondary support material curves added to the main material curves for the current, lower layer. The newly added curves will also require support from below during formation, and are added to the set of main material curves, but are identified as secondary material curves.  
      The current layer can be set to be the next lower layer, making the previous lower layer of the pair the upper layer, and the process repeated. The new calculation will now take into account any curves representing either unsupported main material or secondary support material. The process can be repeated for all layers of the part to be made.  
      One output of the method can be a set of secondary material curves to be filled with secondary support material. The secondary material curves can be further processed by raster filling the areas within the curves using conventional rasterizing techniques. The curves and tool paths generated can be checked for consistency and lack of interference, both within the secondary material and between the secondary and main materials. The rasters can be used as tool paths to control the formation or deposition of main and secondary material.  
      In manufacture, the main and secondary material tool paths can be fed to a layered manufacturing machine for each layer. The minimized support sloping side faces, which were likely calculated top down, are built bottom up. The sloping side faces of the support structures can be built with a slight overhang at each higher level, the overhang preferably not exceeding one-half (½) a bead width. The secondary material support structures can thus be built to have large dimensions at the topmost layer. In some objects, the next layer up will consist of a main material layer deposited on the now solidified secondary material layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a top, cross-sectional view through a single layer of a prior art object formed by a bead deposited along a tool path;  
       FIG. 2  is a top, cross-sectional view through a single layer of a prior art object formed by a bead deposited along a tool path, including a perimeter contour tool path;  
       FIG. 3  is a side view of a prior art object formed by layered manufacturing techniques having serrated outer surfaces and interlayer surface concavities;  
       FIG. 4  is a detailed view of a surface interlayer of  FIG. 3 ;  
       FIG. 5  is a side view of an object formed during layered manufacture having the main material abutted by a removable secondary surface improvement material;  
       FIG. 6  illustrates the object of  FIG. 5  after removal of the secondary surface improvement material;  
       FIG. 7  illustrates in detail the object of  FIG. 6 , including an intra-layer surface rounded concavity and an interlayer surface convexity;  
       FIG. 8A  is a highly diagrammatic side cross-sectional view of a design object having a surface curve, the design having been sliced into layers for layered manufacturing;  
       FIG. 8B  is a highly diagrammatic side cross-sectional view of the object of  
       FIG. 8A  in the process of manufacture, with some main and secondary material layers having been deposited;  
       FIG. 8C  is a highly diagrammatic side cross-sectional view of the object of  
       FIG. 8B  after all main and secondary material layers have been deposited;  
       FIG. 8D  illustrates a highly diagrammatic side cross-sectional view the object of  FIG. 8C  after removal of the secondary material;  
       FIG. 9A  is a highly diagrammatic top view of a design object layer to be manufactured by layered manufacturing, the design having an internal rounded cavity surface and an external perimeter surface;  
       FIG. 9B  illustrates the manufacture of the  FIG. 9A  design after the secondary surface improvement material layers have been deposited as a mold for the main material layer;  
       FIG. 9C  illustrates the manufacture of the  FIG. 9B  object after deposition of the main material layer abutting the secondary surface improvement material;  
       FIG. 9D  illustrates the manufacture of the  FIG. 9C  object after removal of the secondary material;  
       FIGS. 10A and 10B  illustrate a highly diagrammatic top view of a secondary material layer, wherein the secondary material layer has substantial voids within;  
       FIGS. 11A and 11B  illustrate a highly diagrammatic top view of a secondary material layer, wherein the secondary material layer has substantial voids within and no contour bead;  
       FIG. 12A  illustrates a highly diagrammatic side cross-sectional view of an object having a cavity defined beneath an external overhang, the overhang requiring support during deposition;  
       FIG. 12B  is a highly diagrammatic side cross-sectional view of a conventional secondary material support used to support the overhang of  FIG. 12B ;  
       FIG. 12C  is a highly diagrammatic side cross-sectional view of the composite component formed by the deposition of the main and secondary material layers of  FIGS. 12A and 12B ;  
       FIG. 13A  is a highly diagrammatic side cross-sectional view of a component having an interior cavity having an unsupported layer requiring support during deposition;  
       FIG. 13B  is a highly diagrammatic side cross-sectional view of a conventional secondary support for supporting the unsupported layer of  FIG. 13A ;  
       FIG. 13C  is a highly diagrammatic side cross-sectional view of the composite component formed by the deposition of the main and secondary material layers of  FIGS. 13A and 13B ;  
       FIG. 14A  is a highly diagrammatic side cross-sectional view of a component having an exterior cavity defined by an overhang;  
       FIG. 14B  is a highly diagrammatic side cross-sectional view of a secondary support structure formed according to the present invention, requiring less material and deposition time;  
       FIG. 14C  is a highly diagrammatic side cross-sectional view of the objects of  FIGS. 14A and 14B  deposited layer by layer;  
       FIG. 15A  is a highly diagrammatic side cross-sectional view of an object having an internal cavity requiring support during manufacture;  
       FIG. 15B  is a highly diagrammatic side cross-sectional view of a secondary support structure having two sloping side faces requiring less material and deposition time;  
       FIG. 15C  is the object of  FIG. 15A  deposited over the secondary support material of  FIG. 15B , on a layer-by-layer basis;  
       FIG. 16  is a high level flow chart of a process used to generate tool paths from a CAD drawing;  
       FIG. 17  is a flow chart describing a procedure for generating curves for all the layers requiring surface improvement according to the present invention;  
       FIG. 18  is a flow chart describing a procedure for generating a minimized secondary support structure;  
       FIG. 19  is flow chart describing a procedure for generating tool paths for improved surface characteristics; and  
       FIG. 20  is a flow chart describing a procedure for tool path generation for minimized support. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  illustrates a top view of a single layer of an object  40  made using layered manufacturing techniques. Object  40  is formed from a single bead  42  laid along a tool path  44 , having a zigzag pattern to substantially fill a rectangular area. Bead  42  has a diameter or width indicated at D/W and a length indicated at L. Bead  42  may be seen to flow together at inter-bead region  46  where adjacent sections of the bead abut one another. Bead  42  and object  40  may be formed using any suitable manufacturing technique, for example, fused deposition techniques, multi-phase jet solidification techniques, or laser-engineered net shaping techniques. Bead  42  can be a ceramic suspension or slurry, a molten plastic, a powder-binder mixture, a polymeric material ready for curing or hardening, a molten metal, or other suitable materials which harden with time and/or exposure to an external stimulus. Bead  42  can also represent a curable strip of polymer or pre-polymer with polymerization initiated with UV radiation.  
      Referring now to  FIG. 2 , another object  41  is illustrated, also formed using layered manufacturing. Object  41  is similar to object  40  of  FIG. 1 , but has an outer contour bead  43  formed of a first bead  45  which surrounds an internal second bead  47 . Both  FIGS. 1 and 2  illustrate conventional layered manufacturing techniques.  
       FIG. 3  illustrates a prior art object  50  formed of three vertical layers  51  abutting one another along interlayer planes  56 . Object  50  includes a sloping surface  52  and a substantially vertical surface  54 . A bead height is indicated at “H” for layer  51 . Numerous interlayer serrations may be seen along sloping face  52  at interlayer regions  56 . Serrations are formed having concave regions  58  between layers  51  and convex, rounded regions near the intralayer regions indicated at convex surface  60 . Vertical surface  54  may also be seen to have numerous sharp concave regions  62  disposed along interlayer regions  56 . Sloping face  52 , in particular, has sharp serrations along the staircased face. Concavities  58  and  62  may be seen to have sharp notches which are stress risers having low mechanical strength.  
      Referring now to  FIG. 4 , prior art concavity  62  of  FIG. 3  is illustrated in greater detail. Concavity  62  may be seen to lie along interlayer region  56  between two beads or layers  51 . Layers  51  extend to an outermost convex and rounded region  60 , and come together along a sharp acute angle  64  formed between the two layers. In the limiting case, the acute angle  64  approaches zero degrees (0°) as a limit. Concave region  62  acts as a region likely to cause crack propagation and weaken the structure.  
      Referring now to  FIG. 5 , an object  90  is illustrated, showing one method of layered manufacture according to the present invention. Object  90  is shown to be formed of three vertically stacked layers  97 ,  98 , and  100 . Object  90  includes a sloped external surface  91  and a substantially vertical external surface  93 . Object  90  is formed of a first or main material, which is abutted in  FIG. 5  by a secondary or supporting material  92  and  94 . The structural material of structures  92  and  94  can serve as a scaffolding or mold for forming the outside of object  90  so as to have improved surface properties. As will be later discussed, support structures  92  and  94  are preferably laid down or deposited prior to the deposition of the main material. For example, a secondary material layer  95  may be first deposited, followed by a secondary material layer  96 , thereby forming convex regions inwardly directed. First main material layer  97  may then be deposited in between support layers  95  and  96 , thereby flowing to assuming the shape of the support layers  95  and  96 . This may be repeated layer by layer, with the main material deposition following the surface improvement material deposition.  
      Referring now to  FIG. 6 , object  90  is shown after removal of support structures  92  and  94 . Support structures  92  and  94  are preferably formed of an easy-to-remove material which differs from the main material. In a preferred method, the structural material does not mix with the main material, and is easy to separate. In one embodiment, the alternate material is physically separable, which can include tearing apart of material and/or use of a non-sticking material. In another embodiment, the alternate material has a lower melting point than the main material and can be separated by heating. In yet another embodiment using chemical separation methods, the alternate material is soluble in a solvent that does not dissolve or damage the main material. Sloping side face  91  and vertical side face  93  may both be seen to lack the sharp concave features of object  50  illustrated in  FIG. 3 . In particular, interlayer regions  108  may be seen to form convex features  110 , while the intra-layer regions form smooth concave regions  104 .  
      Referring now to  FIG. 7 , concave region  104  is illustrated in greater detail. Concave region  104  may be seen to lie in an intra-layer region of object  90 . In the embodiment illustrated, a shallow angle  112  may be seen to be formed by concave rounded regions  104 . In the limiting case, a tangent along a semicircular or concave surface may be seen to approach an angle of 180 degrees as a limit. In comparing the objects of  FIGS. 4 and 7 , it may be seen that object  90  of  FIG. 7  lacks the sharp serrations and crevices present at inter-layer region  56  in the formation of object  50  of  FIG. 4 .  
      Referring now to  FIG. 8A , an object  120 , as designed, is illustrated. Designed object  120  may be the object as modeled in a CAD drawing or other design tool. Object  120  includes a curved surface region  124  and a straight surface region  122 . Object  120  has been divided into numerous slices  126 , denoted by lines in  FIG. 8A . In  FIG. 8A , slices  126  are demarcated by the center line of each layer. While  FIG. 8A  shows all of the slices having the same thickness, it is contemplated that the slices may have different thicknesses, if desired. Some embodiments of the invention have layer thicknesses of between about 0.001 inches and about 0.030 inches. In one embodiment of the invention, the layer thickness is between about 0.005 inches and about 0.015 inches.  
       FIG. 8B  illustrates an object being manufactured to form design object  120 . A first surface support material  130  has been deposited, followed by a first main material layer  132 , followed by a second surface support layer  136 , followed by a second main layer  138 , followed by a third surface support material layer  140 , followed by a third main material layer  142 , followed by a fourth surface support material layer  144 . An interface region  145  between the structural material and the main material may be seen.  FIG. 8C  illustrates a main structure or part  148  abutting a support structure  146  after completion of the support structure.  FIG. 8D  illustrates main structure  148  after removal of support structure  146 , thereby exposing side surface  150 .  
      Referring now to  FIG. 9A , a single design layer  160  is illustrated in a top view. Design layer  160  includes a main material layer or region including an interior cavity  166 . Arrow  170  indicates an out direction from main material region  178  on the external surface, while arrow  172  indicates the out direction from the interior surface within cavity  166 . The term “out” thus refers to a direction away from the main material and toward the non-material region or air space near the surface.  
      Referring now to  FIG. 9B , an object layer being created according to design layer  160  is illustrated.  FIG. 9B  illustrates the object after deposition of the secondary material within a peripheral region and an interior region. Secondary material may be seen to have been deposited within an exterior margin  164  and interior margin  168 . Region  178  is indicated as not yet filled by any main material.  FIG. 9C  shows the object layer under construction after deposition of main material within region  178 . The main material within region  178  may be seen to abut secondary material at regions  164  and  168 , thereby being formed between the two secondary material regions. In this way, interior surfaces may have the surface characteristics improved as well as the exterior surfaces.  FIG. 9D  illustrates the object after removal of the secondary support material, leaving main material region  178  surrounding cavity  166 .  
       FIGS. 10A and 10B  illustrate a main material layer  180  similar to main material layer  178  of  FIG. 9C . The embodiment illustrated includes secondary material layers using less material. Secondary material has been deposited as an internal layer  182  and as an external layer  185 , similar to layers  168  and  164  of  FIG. 9C . External secondary material layer  185  is illustrated in greater detail in  FIG. 10B , illustrating a support structure having a large void contribution. Exterior support layer  185  may be seen to include a contour bead  186  disposed along the exterior of the object and a second bead  188  formed in a zigzag or squarewave pattern, thereby leaving a number of voids  190 . External layer  184  thus provides support for forming main material layer  180 , while using less material and requiring less time to form the secondary support layer.  
      Referring now to  FIGS. 11A and 11B , a main material layer  180  similar to main material layer  178  of  FIG. 9C  may be seen to be surrounded by an external structural support or surface improvement layer  192  using less secondary material to construct. Support layer  192  is formed of a single bead  194  configured in a zigzag or squarewave pattern having a number of voids  196 . Support region  192  may be seen to have an even larger void contribution than region  185  of  FIG. 10B . External support layer  192  may be appropriate where a less fine external finish in required or allowed for the main material portion.  
       FIG. 12A  illustrates a highly diagrammatic side view of a prior art part  200  having a cantilevered or unsupported ceiling structure  202  and a side wall, or supported structure  204 , thereby defining a side cavity  206  under the overhang of the cantilevered portion  202 . Cantilevered portion  202  can define a cavity volume by projecting the cantilevered portion vertically downward.  FIG. 12B  illustrates a secondary material support  208  suitable for supporting cantilevered portion  202 .  FIG. 12C  illustrates support region  208  supporting cantilevered region  202 . Current methods teach forming support region  208  near, but not touching, side wall  204 , leaving a space  201  to ease removal of support structure  208 .  FIGS. 12A through 12C  illustrate a prior art method for generating support for a part during manufacture. In particular, it may be seen that cantilevered region  202 , when formed by the deposition of not-yet-solid bead, would require support during solidification of the bead over cavity region  206 .  
      Referring now to  FIG. 13A , another part  210  is illustrated having a supported region  214 , a supported region  216 , and an unsupported region  212  suspended therebetween. Unsupported region  212  defines a cavity  218  thereunder. Unsupported region  212 , when formed using many layered manufacturing techniques, requires support during the solidification of the bead over interior cavity region  218 .  FIG. 13B  illustrates a secondary support structure  220  suitable for use in manufacturing object  210 .  FIG. 13C  illustrates object  210 , after manufacture, being supported by secondary supporting material  220 . Support material  220  may be seen to support unsupported region  212  during the solidification of the bead. Support region  220  may be removed after solidification of the main material. Current methods teach forming support region  220  near, but not touching, supported regions  214  and  216 , leaving spaces  211  to ease removal of support structure  220 .  FIGS. 12A through 12C  and  13 A through  13 C illustrate prior art methods of providing secondary material support for a part according to layered manufacturing techniques.  
      Referring now to  FIG. 14A , structure  200  of  FIG. 12A  is again illustrated.  FIG. 14B  illustrates a minimized support structure  209  suitable for support of cantilevered region  202 .  FIG. 14C  illustrates minimized support structure  209  disposed within cavity  206 . As may be seen from inspection of  FIG. 14C , a substantial void volume  211  is left within cavity  206 . Void volume  211  results in less material being used for formation of support structure  209 , as well as less time required to form support structure  209 . Support structure  209  does not extend to the bottom of cavity  206 , but rather abuts main structure portion  204  along an interior wall region  207 , ending at a base layer  205 .  FIG. 14C  thus illustrates a support structure that fills less than half of the cavity volume it is disposed within. Support structure  209  may be seen to have a sloping side face  215 .  
      Referring again to  FIG. 14B , support structure  209  may be seen to have a length for each layer indicated at “L” and an indent or offset for each layer beneath the immediately disposed upper layer. The indent is indicated at  213 . In one embodiment, the indent varies between about one-tenth ({fraction (1/10)}) of a bead diameter and about one-half (½) of a bead diameter. In a preferred embodiment, the indent does not exceed one-half (½) of a bead diameter, so as to minimize the unsupported region of the bead during bead solidification. In another embodiment, not requiring separate illustration, base layer  205  is repeated downward to the floor of the cavity, and can be several beads wide.  
      Referring now to  FIG. 15A , object  210  of  FIG. 13A  is once again shown.  FIG. 15B  illustrates a minimized support structure  221  having two sloping or curved side faces  219 .  FIG. 15C  illustrates support structure  221  disposed within cavity  218 , thereby supporting overhanging region  212 . Support structure  221  may be seen to have a base portion  217  much smaller than top portion  229 . Base  217  may be supported by a workpiece platform or the cavity floor. Sloping faces  219  may be seen to provide void areas  223  and  225  within cavity  218 . Minimized support structure  221  thus provides support while requiring substantially less material and deposition time for the support structure.  
      Qualitatively, the structures of FIGS.  14 A-C and  15 A-C are generated using methods which plan the layers from top to bottom and which build the layers from bottom to top. Each layer to be minimized can be indented at each level, not more than the bead width, otherwise the bead would drop down. The indent is preferably not more than one-half (½) of a bead width. The indenting can continue at each level until no more support material is required, or until the minimum allowable support layer dimension is reached. Some embodiments continue forming the minimum allowable support layer dimension all the way to the bottom.  
      The indents form a local slope which can be defined as the change in local height per the change in local width. In one embodiment, the local slope is less than about ten (10). In another embodiment, the local slope is less than about two (2). One embodiment has a side face having a deviation from vertical of at least forty degrees (40°). The resulting support structures occupy less than about twenty percent (20%), forty percent (40%), and fifty percent (50%) of the main material cavities in various embodiments of the invention.  
      The methods used to create the structures of FIGS.  14 A-C and  15 A-C preferably operate on curves generated by slice programs which slice 3D CAD objects into two-dimensional curves having a thickness. The two-dimensional curves can be approximated as poly-lines or a series of ordered points. The curves define the outer perimeters to be filled, as well as the inner void perimeters to be left unfilled, for each layer of the part to be made. For each curve, the curve immediately above that curve can be projected downward onto the curve, and the difference taken to determine any unsupported areas that would allow deposited beads to fall through. The upper curves should first be reduced by the indent amount to allow for the support structure sloping side faces and reduced secondary material usage previously described. Any unsupported areas can be handled by creating new curves to form support areas, and the new curves added to the current level curves, as the new curves in turn require support from the level below. This process can continue until the bottom most layer is reached. The result is a set of additional curves defining areas to be filled with secondary support material for each layer.  
      The secondary material curves and the main material curves can be used as input by a rasterizing program which generates rasters to be used as tool paths to fill the areas within the curves with material. The tool paths can be followed for each layer by a layered manufacturing tool head, such as an extruder nozzle, in generating the support structures from the bottom up. In one embodiment, two nozzles are used, one for the alternate material and one for the main material.  
      Referring now to  FIG. 16 , a high level method or algorithm  300  is illustrated. Method  300  and the subsequent methods can be implemented on a computer using any suitable programming language. Suitable languages include, without limitation, Fortran, C, C++, Java, BASIC, and Pascal. Method  300  can operate on a CAD file containing a representation of an object to be manufactured, and can output data files describing curves to be filled in, and tool paths to be followed to fill in the curves. The present invention explicitly includes computer programs inputting and outputting data, where the output data will ultimately be used to drive layered manufacturing tools. The computer programs can exist as human readable source code and/or as compiled and ready to execute machine code. The computer programs can reside on machine readable media, including magnetic and optical discs.  
      Method  300  can begin with a CAD drawing input step  302 , which can include input of a 3D CAD drawing file, for example a 3D Auto-CAD® drawing file. The 3D CAD file can include primitives such as solid polygons with holes and extruded two-dimensional solids. The 3D file can also include a 3D model which has been converted into a set of triangles, such as is found in a stereo lithographic (STL) file. In some embodiments, surfaces have been marked or tagged by a human or machine user to indicate that selected surfaces are to be improved or used to abut support structures. The curved surfaces of the 3D object may be represented or approximated by a surface formed of the straight line segments of triangles.  
      Proceeding to step  304 , the 3D CAD model can be sliced into numerous equal thickness slices along the X-Y or horizontal plane. One example of slicing technology is the QuickSlice program, available from Stratasys, Incorporated (Eden Prairie, Minn.). The X-Y plane is typically horizontal due to the importance of gravity in determining the placement of flowable, semi-solid beads requiring solidification. The slices typically correspond to the layers formed in layered technology and may be one bead thickness in height. In an illustrative example, in a vertically disposed cylindrical solid having a vertically disposed interior cylindrical cavity or annulus, a slice could be modeled as a large circle having a smaller circle within, where the solid material portion corresponds to the area between the two circles. The two circles, along with a depth or height, could represent the slice. In one embodiment, the cylinder and interior cavity is modeled using an STL format, and the circles are actually represented by poly-lines or series of points approximating the circles.  
      In one method, the slice thicknesses are not equal, and step  306  is executed in place of step  304 . Step  304  utilizes adaptive slicing, which can vary the slice thickness according to the geometry and desired surface properties of the part being made. In the example of the vertically disposed cylinder having a vertical cavity, the slice could have a large thickness, as the vertical cross section may not vary with height. Tata et al. discuss an adaptive slicing technique in U.S. Pat. No. 5,596,504.  
      With the slices completed, step  308  can be executed to form a tool path within the slice to form that layer of the object by filling in the solid portion of the slice by traversing the area with an additive technology tool head, for example, by using a Fused Deposition Machine. In the vertical cylinder example, a zigzag pattern may be created to lay down the bead between the inner and outer circles or poly-lines of the slice. Standard tool path generation techniques can be used, well known to those skilled in the art. An improved tool path generation method, discussed in co-pending U.S. patent application Ser. No. ______, entitled TOOL PATH PLANING PROCESS FOR COMPONENT BY LAYERED MANUFACTURE [1100.1103101], herein incorporated by reference, can also be used in conjunction with the present invention. Step  308  can be executed before and/or after the generation of additional layers created to improve the surface properties or provide support for the deposition of the main material layer.  
      In step  310 , layers can be created to improve surfaces of the main material. In the example of the vertical cylinder having an interior cavity, the inner and/or outer surfaces may be improved by creating an inner and/or outer annular shell, respectively. The layers of the shell may be laid down first in the layer, followed by the deposition of the main material.  
      In step  312 , the minimized support structures of the present invention can be created on a layer-by-layer basis. In the example of the vertical cylinder, if the interior cavity did not extent entirely through the cylinder, but was a blind cavity having a ceiling, the deposition of the ceiling would require creation of a support structure prior to depositing the first beads of the ceiling. Step  312  allows for creation of a support structure that does not require filling the entire cavity, to minimize support formation time and support material usage.  
      In step  314 , the tool path for the minimized support structures created in step  312  can be generated. The tool path for the surface improvement layers created in step  310  can be generated in step  316 . Step  308  can also be executed at this time. In step  318 , the tool paths for the main part, the surface improvement layers, and the support layers can be integrated and checked for consistency and lack of interference. After execution of step  318 , the layers and tool paths are preferably completely generated.  
      Referring now to  FIG. 17 , a method  350  is illustrated for improving surfaces of the part being manufactured. In step  352 , layers can be selected for surface improvement either manually or automatically. In one embodiment, the surface to be improved can be selected by a human user interacting with a CAD program. In one example, a human user selects a surface on a CAD program and sets a property of the surface to indicate that surface improvement is desired. When the slice generation program operates on the 3D object in the CAD file, the slicing program can then propagate this property and mark or tag every slice or slice region with a tag, indicating that the slice is to be improved. In one embodiment, a human user acts on the slices in the database only after slice generation to manually tag every slice to be improved. As the output of a slice generation program may be a series of poly-lines, the user may select one or more poly-line segments in each slice for improvement, typically maintaining consistency vertically through the slices. This method does not require modification of the 3D CAD program and can operate on the output of a standard slice generation program. Intelligence is normally required to specify which surfaces require improvement, as the intended use may be known only to a human user. In one example, an internal bore may call for surface improvement if the intended use is to retain a load bearing round pin, but may not require surface improvement if the intended use is to pass cooling fluid.  
      In step  354 , the current layer being operated on is initialized to be the topmost layer requiring surface improvement. The current layer surface curves are copied to a new working set of curves in step  356 . In step  358 , the new set of working curves are offset by the minimum acceptable alternate material shell width, typically outward from the main material toward the air side. In one method, the minimum acceptable alternate material shell width is at least two of the alternate material bead widths. The new set of offset curves is added to the curve set.  
      Interference is checked for in step  360 . Interference means that two curves are intersecting. In one example, an alternate material curve overlaps a main material curve, which could cause alternate material, then main material to be deposited in the same location if the tool paths were generated using the overlapping curves. In another example, two alternate material curves may overlap, which would cause two tool paths to be generated for the same location, causing excess material to be deposited in that location. If an interference is detected, then step  362  is executed to clip the curves. In the example where the main material and alternate material curves intersect, the main material curves will be used to clip the alternate material curves, as the part integrity takes precedence of the surface improvement shell location.  
      In step  366 , alternate material is assigned to the new curves, which typically corresponds to a shell of alternate material being formed near the surface of the main material. The new set of curves is added to the support set in step  368 . If all layers have been processed, this is detected at step  364 , and method  350  is substantially complete. If all layers have not been processed, then step  370  is executed to advance the current layer to the next layer down in step  370 , and step  356  is executed again.  
      Method  350  illustrates but one way to form the surface improvement material layers. The present invention includes the formation of alternate material layers out a specified distance from the main material surface. The alternate material provides a mold at each main material layer surface to be improved. The distance specified is the offset distance previously referred to, and will likely be related to the final alternate material shell thickness. If the offset distance first selected does not interfere with the main material, it is left unchanged, otherwise it is clipped so as to not interfere with the main material. If the resulting alternate material curves do not interfere with other alternate material curves, they are left unchanged; otherwise, they are clipped so as to not interfere.  
      Referring now to  FIG. 18 , a method  400  is illustrated, suitable for generating curves for minimized support structures such as described in FIGS.  14 A-C and  15 A-C. In step  402 , the current layer is initialized to be the layer immediately below the top layer, as there is nothing to support above the top layer. The current layer is thus the lower layer of a pair and the layer immediately above is the upper layer of the pair. In step  404 , the upper curve is copied and offset or indented in, thus creating a new smaller area curve, which is projected onto the lower current layer. Thus, a curve that may be smaller or larger than the current layer is projected onto the current layer. In some embodiments, the upper curve is offset in along some curve perimeters but not others. In one embodiment, the user can specify certain layer edges as being anchored, such that when a copy is made of the curve, the offset is taken only inward from some edges, but not others. In one example, the top two layers of  FIG. 14C  could be anchored, such that the upper layers are never indented in from the right, which could call for the support material to form a pillar not abutting the main material portion at the right.  
      A check is made in step  406  to determine if the offset, reduced area upper curve even exists after the offsetting, as it may have been reduced either to nothing or a size below a limit. In one illustrative example, a {fraction (1/10)}-inch diameter circle offset in by {fraction (1/10)}th inch will vanish. Step  408  finds the difference in projection by subtracting the current layer curve from the projected offset curve. For example, in a solid cylindrical region, the upper layer will be a circular layer the same size as the lower layer. The algorithm will make a copy of the upper layer and offset this upper layer curve inward by the offset or indent amount. In the cylindrical solid case, the upper offset circle will have the lower full size circle subtracted from it, leaving negative area, as there is no unsupported material above the current layer. In the case of an overhang, such as a cantilevered region, the overhanging curve, once reduced, will have the support member subtracted from it, leaving the reduced overhang area as the difference area.  
      Step  410  determines whether a difference exists, that is, whether any part of the offset upper curve is not supported by the lower layer. The projection, difference calculation, and check for a difference thus determines whether the upper layer, once reduced by the offset, is fully supported by the layer below. An unsupported portion of an upper curve corresponds to beads that will fall unless a support has been built immediately beneath those beads prior to their deposition.  
      If a difference exists, step  412  is executed, and the offset curve is added to the set of curves belonging to the set of support set curves. The newly added curve is tagged or identified as being a support layer curve, but will be later treated in many respects as a main material layer, as the support layer also requires support during deposition, even though the purpose of the support layer is different than the purpose of the main material layer.  
      Step  416  determines whether all layers have been processed. If true, this portion of the processing is complete for algorithm  400 . If more layers require processing, step  414  is executed to increment the layer, making the next lower layer the current layer. Execution proceeds again at step  404 .  
      If step  406  determines that one or more curves have disappeared, then step  418  is executed. Step  418  begins iterating through each missing or vanished curve. In step  420 , a check is made to determine whether the missing curve abuts another layer. If this is true, then it may be possible to completely eliminate the support material layer, as illustrated with support base layer  205  in  FIG. 14C , as the small support layer portion, for example one third of a bead width, is adequately supported by the layer below. In step  424 , the projected upper curve is eliminated. If the curve does not abut, then step  422  is executed, and the original upper curve or some minimally dimensioned upper curve is projected onto the current layer. In this way, a minimally dimensioned support column, as illustrated in  FIG. 15C , can be continued downward without further offsetting to provide support for the higher layers.  
      Method  400  thus operates by taking each layer, determining whether the layer above, when reduced inward by an offset, would be unsupported by the current layer, and if so, adding a support material layer level with the current layer. When the current layer is abutting another layer, the layer above may be eventually reduced to nothing. When the current layer is not abutting another layer, the layer above may be clamped such that it is never reduced below a minimum dimension, providing a minimum cross section column for the remainder of the vertical distance to the cavity floor or the workpiece platform.  
      Referring now to  FIG. 19 , a method  450  for creating support material layer tool paths is illustrated. Method  450  is only one example of a method suitable for creating minimized support layer tool paths. Method  450  can start with the support and main material layers or slices already calculated. The curves or outlines of the main material layers and the support layers have been calculated, but not the raster tool paths within.  
      In step  452 , the interference distance for the support material is determined. For example, the interference distance can be set to the bead width of the alternate material. In step  454 , the current layer is initialized to be the top layer. A decision step  456  checks whether the current layer requires surface improvement; if not, a check is made in step  478  as to whether all layers have been processed. If all layers have been processed, then the method is essentially finished, and the method proceeds to  479 . If all layers have not been processed, then the next layer is set to be the current layer at  480 , and step  456  is executed again.  
      If surface improvement is required for the current layer, then in step  458  the previously generated curve from the support set is retrieved. If the retrieved curve does not abut the main material, checked in step  460 , then the curve is copied to the fill set in step  470 , with further processing discussed below. If the retrieved curve does abut the main material, then the support material curve is offset in the inward direction by the support material bead width, toward the support material, in step  462 . The new curve can be used as the contour tool path for the alternate material. In step  464 , the alternate material is assigned to the curve which is stored in the tool path set in step  466 . The curve is also copied to the fill set in step  468 , with execution proceeding to step  472 . If all support curves for this layer have been processed, checked at step  472 , then the rasters are created in step  474  and stored in the tool path set in step  476 . Execution then proceeds as previously described at step  478 .  
      Referring now to  FIG. 20 , a method  500  is illustrated for generating tool paths for minimized support layers. Beginning at step  502 , the current layer is initialized to be the top layer. If the current layer is to receive minimized support, checked at step  504 , then the curve is selected from the support set at step  506 . In step  508 , rasters are created within the curve boundary to fill the layer with support material. Support material is assigned to the rasters in step  510 , and the rasters stored in the tool path set in step  512 . If all layers have been processed, checked in step  514 , then method  500  is essentially finished and execution proceeds to  515 . If all layers have not been processed, then the current layer is incremented to the next layer down in step  516 , and execution proceeds again to step  504 .  
      Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.