Reduced stereolithographic part distortion through isolation

Abstract: An improved stereolithography system for generating a three-dimensional object by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium capable of altering its physical state in response to appropriate synergistic stimulation by impinging radiation, particle bombardment or chemical reaction, using information defining the object which is specially processed to reduce curl, stress, birdnesting and other distortions, the successive adjacent laminae, representing corresponding successive adjacent cross-sections of the object, being automatically formed and integrated together to provide a step-wise laminar buildup of the desired object, whereby a three-dimensional object is formed and drawn from a substantially planar surface of the fluid medium during the forming process. Reducing stereolithographic distortion through isolation of stress is described.

Description: This invention relates to a method of and an apparatus for forming of a three-dimensional object according to the precharacterising part of claim 1 and according to the precharacterising part of claim 18 respectively. This technology has come to be referred to as "stereolithography" and this term will be used herein. More particularly in stereolithography apparatus the desired configuration of the object is defined by stored data that may be acted on to generate all the successive laminae. The following source documents and terms are referred to in the description given below: U.S. Patent 4,575,330 (Hull) which describes a stereolithographic system from which the present invention has been developed. EP-A-0 354 637 (3D Systems, Inc.) published 14th February, 1990 which, inter alia, describes data processing techniques for converting CAD (computer aided design) data to data controlling stereolithographic processing. "Understanding PHIGS" published by Template, Megatek Corp., San Diego, California, U.S.A. which explains an ANSI Standard relating to PHIGS, the Programmer's Hierarchical Interactive Computer Graphics System. "Smalley" - the name given herein to an opening (gap) specially introduced into a desired object configuration with a view to avoiding various distortions that may occur in building the object by stereolithography. "slicing" - a term generally applied to the conversion of the data representing the desired object configuration into sets of data controlling the formation of successive laminae or slices of the object being built. This invention is generally concerned with a method and apparatus for forming three-dimensional objects from a fluid medium and, more particularly, is applied in an improved stereolithography system involving the application of enhanced data manipulation and lithographic techniques to production of three-dimensional objects, whereby such objects can be formed more rapidly, reliably, accurately and economically. Specifically, this invention is concerned with measures to reduce distortion of the object. Such distortions arise from stress or the failure of one layer to locate accurately with respect to an underlying layer in certain circumstances. It is common practice in the production of plastic parts and the like to first design such a part and then painstakingly produce a prototype of the part, all involving considerable time, effort and expense. The design is then reviewed and, oftentimes, the laborious process is again and again repeated until the design has been optimized. After design optimatization, the next step is production. Most production plastic parts are injection molded. Since the design time and tooling costs are very high, plastic parts are usually only practical in high volume production. While other processes are available for the production of plastic parts, including direct machine work, vacuum-forming and direct forming, such methods are typically only cost effective for short run production, and the parts produced are usually inferior in quality to molded parts. Very sophisticated techniques have been developed in the past for generating three-dimensional objects within a fluid medium which is selectively cured by beams of radiation brought to selective focus at prescribed intersection points within the three-dimensional volume of the fluid medium. Typical of such three-dimensional systems are those described in U.S. Pat. Nos. 4,041,476; 4,078,229; 4,238,840 and 4,288,861. All of these systems rely upon the buildup of energization to solidify the fluid at selected points deep within the fluid volume, to the exclusion of all other points in the fluid volume. Unfortunately, however, such three-dimensional forming systems face a number of problems with regard to resolution and exposure control. The loss of radiation intensity and image forming resolution of the focused spots as the intersections move deeper into the fluid medium create rather obvious complex control situations. Absorption, diffusion, dispersion and diffraction all contribute to the difficulties of working deep within the fluid medium on an economical and reliable basis. In recent years, "stereolithography" systems, such as those described in US-A-4,575,330, which is hereby incorporated by reference as though set forth in full herein, have come into use. Basically, stereolithography provides a method for automatically building complex plastic parts by successively printing cross-sections of transformed or cured photopolymer (such as liquid plastic) on top of each other until all of the thin layers are joined together to form a whole part. With this technology, the parts are literally grown in a vat of liquid plastic. This method of fabrication is extremely powerful for quickly reducing design ideas to physical form and for making prototypes. Photocurable polymers change from liquid to solid in the presence of light and their photospeed with ultraviolet light (UV) is fast enough to make them practical model building materials. The material that is not polymerized when a part is made is still usable and remains in the vat as successive parts are made. An ultraviolet laser generates a small intense spot of UV. This spot is moved across the liquid surface with a galvanometer mirror XY scanner. The scanner is driven by computer generated vectors or the like. Precise complex patterns can be rapidly produced with this technique. The laser scanner, the photopolymer vat and the elevator, along with a controlling computer, combine together to form a stereolithography apparatus, referred to as an "SLA". An SLA is programmed to automatically make a plastic part by drawing a cross section at a time, and building it up layer by layer, so that the object built comprises superposed laminae Stereolithography represents an unprecedented way to quickly make complex or simple parts without tooling. Since this technology depends on using a computer to generate its cross sectional patterns, there is a natural data link to CAD/CAM. However, such systems have encountered difficulties relating to shrinkage, curl and other distortions, as well as resolution, accuracy and difficulties in producing certain object shapes. Objects built using stereolithography have a tendency to distort from their CAD designed dimensions. This distortion may or may not appear in a specific object, based on how much stress is developed by the specific cure parameters and on the object's ability to withstand stress. The stress that causes distortion develops when material that is being converted from liquid to solid comes into contact with and bonds to previously cured material. When material is converted from liquid to solid it shrinks slightly. This shrinking causes stress and has two primary physical causes: 1) density of the liquid is less than that of the solid plastic; and 2) the chemical reaction that causes the change of state is strongly exothermic causing the curing material to thermally expand and contract. This stress causes a distortion known as curl. Certain sections of an object will be able to resist stresses without any apparent warp (stress is at a tolerable level). On the other hand, other sections may distort considerably as the stress and structural strength balance each other. Since stress is caused by contact between curing material and cured material it can be propagated along the entire length of contact between the curing line and cured material. Most contact of curing to cured material occurs from one layer to the next as opposed to along a single layer. This implies most distortions will be vertical in nature as opposed to horizontal. Therefore, there has been a need for a technique to reduce vertical distortions. "Birdnesting" is a phenomena that can occur on parts that require down-facing, near-flat skin by the stereolithographic apparatus' SLICE program. Commercial implementations of SLICE programs for converting a CAD/CAM representation of a part into vectors are described EP-A-0 354 637. The different vector types, including near-flat skin, cross-hatch, and boundary vectors are also discussed in this specification. Areas that require down-facing, near-flat skin are problematic because their boundary vectors do not have any support when they are drawn. By the time cross-hatch is finally drawn, to secure the boundaries, the boundary vectors may have moved away from their proper positions and, therefore, may not be secured at particular locations. These unsecured boundaries can move up and down and give a rough surface finish to the object, similar to a bird's nest. Hence, workers in the art have recognized the need for a solution to the aforedescribed problems encountered in stereolithographics, and there continues to be a long existing need in the design and production arts for the capability of rapidly and reliably moving from the design stage to the prototype stage and to ultimate production, particularly moving directly from the computer designs for such plastic parts to virtually immediate prototypes and the facility for large scale production on an economical and automatic basis. Accordingly, those concerned with the development and production of three-dimensional plastic objects and the like have long recognized the desirability for further improvement in more rapid, reliable, economical and automatic means which would facilitate quickly moving from a design stage to the prototype stage and to production, while avoiding the problems of stress, distortion and poor part finish. The present invention is founded on a recognition of various distortions that can occur in the building of objects by stereolithography and the development of an understanding of the causes of such distortions as will be discussed in detail below. The practice of the invention involves the introduction into solid portions of the desired object configuration of openings or gaps which may be called "Smalleys". In particular aspects of this invention these openings are introduced in a special way to inhibit the transmittal of distortion-causing stresses between layers or along a layer, although as indicated above stress is not the only cause of distortion. The provision of such openings may be incorporated into the data processing by which basic CAD data, for example, is converted into the slice data for forming each successive lamina. It is to be acknowledged that a desired object configuration may include an internal hollow entirely enclosed by solid material. An example is shown in EP-A-0 250 121 (Scitex Corporation). In order to allow the unsolidified fluid medium to drain from the internal hollow opposed small vertical conduits are formed which intersect the hollow interior and extend to external surfaces of the object such that the ends of the drainage conduits are entirely open. EP-A-0 250 121 discloses the processing of CAD data specifically to form film masks for exposing successive layers of transformable fluid. The prior specification recognizes the effects of shrinkage in the individual fluid layers s they are selectively solidified by exposure through the respective masks. To mitigate shrinkage, the use of a pair of complementary masks for each layer is proposed. Each mask contains part of the layer pattern and the pair together contain the whole layer pattern. The pair of complementary masks may be supplemented by exposure through a third mask containing the whole pattern. This procedure is repeated for each layer. According to a first aspect of the present invention, there is provided a method of forming a three-dimensional object by selectively solidifying successive layers of a transformable fluid medium by subjecting each layer to a selected pattern of a prescribed stimulation whereby the object is built of a succession of superposed laminae, wherein the selected pattern for each layer is obtained by processing stored data defining a desired configuration of the object, characterised by: said processing step comprising modifying the stored data defining the desired object configuration by introducing openings in solid portions of the desired configuration, each of said openings as defined by the modified data existing in at least one lamina and having at least a portion which is vertically bounded by a solid portion of the lamina immediately preceding the earliest-formed lamina in which the opening exists. According to a second aspect of the present invention there is provided an apparatus for forming a three-dimensional object from stored data defining a desired configuration of the object, the object being built of a succession of superposed laminae each of which is formed by solidifying a layer of a transformable fluid medium by selective application thereto of a prescribed stimulation, the apparatus including a data processing means for generating from the stored data the pattern of stimulation applied to each fluid layer, characterized in that: said data processing means comprises data-modifying means responsive to said stored data to produce modified data introducing openings into solid portions of the desired object configuration defined by said stored data, each of said openings as defined by the modified data existing on at least one lamina and having at least a portion which is vertically bounded by a solid portion of the lamina immediately preceding the earliest formed lamina in which the opening exists. Particular embodiments of the invention are the subject of the dependent claims 2 to 17 and 19 to 33. The introduced openings provided in accordance with the invention may be subject to dimensional modification in the actual processing of the transformable fluid medium. In the preferred processing technique particularly discussed herein there are two curing stages. The first is the application to the layer of transformable fluid medium, in the form of a photopolymer, of a beam of UV radiation which scans the layer and irradiates it in accordance with the selected pattern. There is a combination of dimensional changes due to the curing of portions of the layer and to a phenomenon known as cure width mentioned further below. The second curing stage is known as post-cure and may incur further dimensional changes. These changes tend to reduce the size of openings and for certain types of small openings may close the openings altogether. However, as will be explained below, the openings will by then have served the structural purpose for which they were introduced. The invention may be put into practice so as to reduce distortion by isolating sections of an object so that stress cannot be transmitted from one section to another. This isolation technique (Smalleys) limits the distortion in a given section to that which can be caused by the stress developed within that section only, not from other sections. Lamina sections prone to curling may be isolated by designing small holes or gaps at stress points in the CAD design of the part. These gaps, called "Smalleys", block propagation of stresses along layer sections. This reduces the stresses acting on a part to only those created within the section. If the Smalleys are properly designed, these localized stresses will be below the threshold value which would curl the layer section. Smalleys are also used to reduce birdnesting. The width of Smalleys, for this application, is generally less than cure width, so that after curing they are completely filled in and so no structural integrity is lost through their use. Smalleys are placed periodically in regions of down-facing near-flat triangles with heights appropriate to extend vertically through the near-flat triangles. The placement of Smalleys is based on several factors that affect the likelihood of having bird-nesting problems: the radius of curvature of the boundaries, the length of near-flat zones, the likelihood of boundaries moving, etc. Smalleys do not need to penetrate completely through a wall, as they do in their other application, but they do need to penetrate deep enough to insure a contact point with the boundaries on the previous layer. Thus the practice of the present invention enables us to harness the principles of computer generated graphics in combination with stereolithography, i.e., the application of lithographic techniques to the production of three-dimensional objects, to simultaneously execute computer aided design (CAD) and computer aided manufacturing (CAM) in producing three-dimensional objects directly from computer instructions. The invention can be applied for the purposes of sculpturing models and prototypes in a design phase of product development, or as a manufacturing system, or even as a pure art form. The making of solid objects by successively "printing" thin layers of a curable material, e.g., a UV curable material, one on top of the other, may be performed by using a programmed movable spot beam of UV light, e.g. generated by a laser, shining on a surface or layer of UV curable liquid used to form a solid cross-section of the object at the surface of the liquid. The object is then moved, in a programmed manner, away from the liquid surface by the thickness of one layer, and the next cross-section is then formed and adhered to the immediately preceding layer defining the object. This process is continued until the entire object is formed. Essentially all types of object forms can be created with such a technique and the practice of the present invention will be particularly described with reference to building of objects in this manner. Complex forms are more easily created by using the functions of a computer to help generate the programmed commands and to then send the program signals to the stereolithographic object forming subsystem. Of course, it will be appreciated that other forms of appropriate synergistic stimulation for a curable fluid medium, such as particle bombardment (electron beams and the like) chemical reactions by spraying materials through a mask or by ink jets, or impinging radiation other than ultraviolet light, may be used in the practice of the invention. By way of example, in the practice of the present invention, a body of a fluid medium capable of solidification in response to prescribed stimulation is first appropriately contained in any suitable vessel to define a designated working surface of the fluid medium at which successive cross-sectional laminae can be generated. Thereafter, an appropriate form of synergistic stimulation, such as a spot of UV light or the like, is applied as a graphic pattern at the specified working surface of the fluid medium to form thin, solid, individual layers at the surface, each layer representing an adjacent cross-section of the three-dimensional object to be produced. In accordance with the invention, information defining the object is specially processed to reduce distortion, and is generally intended to increase resolution, strength, accuracy, speed and economy of reproduction. Superposition of successive adjacent layers on each other is automatically accomplished, as they are formed, to integrate the layers and define the desired three-dimensional object. In this regard, as the fluid medium cures and solid material forms as a thin lamina at the working surface, a suitable platform to which the first lamina is secured is moved away from the working surface in a programmed manner by any appropriate actuator, typically all under the control of a microcomputer or the like. In this way, the solid material that was initially formed at the working surface is moved away from that surface and new liquid flows into the working surface position. A portion of this new liquid is, in turn, converted to solid material by the programmed UV light spot to define a new lamina, and this new lamina adhesively connects to the material adjacent to it, i.e., the immediately preceding lamina. This process continues until the entire three-dimensional object has been formed. The formed object is then removed from the container and the apparatus is ready to produce another object, either identical to the first object or an entirely new object generated by a computer or the like. The data base of a CAD system can take several forms. One form consists of representing the surface of an object as a mesh of polygons, typically triangles. These triangles completely form the inner and outer surfaces of the object. This CAD representation also includes a unit length normal vector for each triangle. The normal points away from the solid which the triangle is bounding and indicates slope. Means are provided for processing CAD data, which may be in the form of "PHIGS" or the like, into layer-by-layer vector data that can be used for forming models through stereolithography. Such information may ultimately be converted to raster scan output data or the like. The implementation of stereolithography in conjunction with a CAD system enables a designer to create a design on the CAD system, applying the concepts of this invention, to reduce curl, stress, birdnesting and other distortions and build an accurate plastic model in a few hours. By way of example, a stereolithographic process may include the following steps. First, the solid model is designed in the normal way on the CAD system, without specific reference to the stereolithographic process. Model preparation for stereolithography involves selecting the optimum orientation, adding supports, building in appropriate stress relief, and selecting the operating parameters of the stereolithography system. The optimum orientation will (1) enable the object to drain, (2) have the least number of unsupported surfaces, (3) optimize important surfaces, and (4) enable the object to fit in the resin vat. Supports must be added to secure unattached sections and for other purposes, and a CAD library of supports can be prepared for this purpose. The stereolithography operating parameters include selection of the model scale and layer (slice) thickness. The surface of the solid model is then divided into triangles, typically "PHIGS". A triangle is the least complex polygon for vector calculations. The more triangles formed, the better the surface resolution and hence, the more accurate the formed object with respect to the CAD design. Data points representing the triangle coordinates and normals thereto are then transmitted typically as PHIGS, to the stereolithographic system via appropriate network communication such as ETHERNET. The software of the stereolithographic system then slices the triangular sections horizontally (XY plane) at the selected layer thickness. The stereolithographic unit (SLA) next calculates the section boundary, hatch, and horizontal surface (skin) vectors. Hatch vectors consist of cross-hatching between the boundary vectors. Several "styles" or slicing formats are available. Skin vectors, which are traced at high speed and with a large overlap, form the outside horizontal surfaces of the object. Interior horizontal areas, those within top and bottom skins, are not filled in other than by cross-hatch vectors. The SLA then forms the object one horizontal layer at a time by moving the ultraviolet beam of a helium-cadmium laser or the like across the surface of a photocurable resin and solidifying the liquid where it strikes. Absorption in the resin prevents the laser light from penetrating deeply and allows a thin layer to be formed. Each layer is comprised of vectors which are typically drawn in the following order: border, hatch, and surface. The first layer that is drawn by the SLA adheres to a horizontal platform located just below the liquid surface. This platform is attached to an elevator which then lowers the elevator under computer control. After drawing a layer, the platform dips a short distance, such as several millimeters into the liquid to coat the previous cured layer with fresh liquid, then rises up a smaller distance leaving a thin film of liquid from which the second layer will be formed. After a pause to allow the liquid surface to flatten out, the next layer is drawn. Since the resin has adhesive properties, the second layer becomes firmly attached to the first. This process is repeated until all the layers have been drawn and the entire three-dimensional object is formed. Normally, the bottom 6.3 mm (0.25 inch) or so of the object is a support structure on which the desired part is built. Resin that has not been exposed to light remains in the vat to be used for the next part. There is very little waste of material. Post processing typically involves draining the formed object to remove excess resin, ultraviolet or heat curing to complete polymerization, and removing supports. Additional processing, including sanding and assembly into working models, may also be performed. The stereolithographic system to be described herein has many advantages over currently used apparatus for producing plastic objects. Stereolithography avoids the need of producing design layouts and drawings, and of producing tooling drawings and tooling. The designer can work directly with the computer and a stereolithographic device, and when he is satisfied with the design as displayed on the output screen of the computer, he can fabricate a part for direct examination. If the design has to be modified, it can be easily done through the computer, and then another part can be made to verify that the change was correct. If the design calls for several parts with interacting design parameters, the method of the invention becomes even more useful because of all of the part designs can be quickly changed and made again so that the total assembly can be made and examined, repeatedly if necessary. Moreover, the data manipulation techniques which are now proposed enable production of objects with reduced stress, curl and distortion, and increased resolution, strength accuracy, speed and economy of production, even for difficult and complex object shapes. After the design is complete, part production can begin immediately, so that the weeks and months between design and production are avoided. Stereolithography is particularly useful for short run production because the need for tooling is eliminated and production set-up time is minimal. Likewise, design changes and custom parts are easily provided using the technique. Because of the ease of making parts, stereolithography can allow plastic parts to be used in many places where metal or other material parts are now used. Moreover, it allows plastic models of objects to be quickly and economically provided, prior to the decision to make more expensive metal or other material parts. There will be described below stereolithographic methods and apparatus embodying the present invention that satisfy a long existing need for an improved CAD and CAM system capable of rapidly, reliably, accurately and economically designing and fabricating three-dimensional parts and the like with reduced stress, curl, birdnesting or other distortions. This invention and its practice will be further described in conjunction with the accompanying drawings of illustrative embodiments, in which: FIG. 1 is an overall block diagram of a stereolithography system for the practice of the present invention; FIGS. 2 and 3 are flow charts illustrating the basic concepts employed in practicing the method of stereolithography of the present invention; FIG. 4 is a combined block diagram, schematic and elevational section view of a system suitable for practicing the invention; FIG. 5 is an elevational sectional view of a second embodiment of a stereolithography system for the practice of the invention; FIG. 6 is a software architecture flowchart depicting in greater detail the overall data flow, data manipulation and data management in a stereolithography system; FIG. 7 illustrates perspectives of a distorted part and one with distortion minimized through the use of "Smalleys." FIG. 8 schematically depicts the application of Smalleys to a typical curling situation; FIG. 9 illustrates, in section, the application of Smalleys for inhibiting curl in thick interior structures; FIG. 10 is a side view of a CAD designed cone without Smalleys; FIG. 11 and 12 are views of the sliced CAD designed cone and what it might look like after building; FIG. 13 is a top view of a CAD designed cone showing possible locations in the XY plane where Smalleys might be inserted; FIG. 14 is a side view of a sliced CAD designed cone with Smalleys and what it might look like after building; FIG. 15 is a top view of two cross-sections of a cone with no Smalleys; FIG. 16 is a top view of two cross-sections of a cone with the second layer only showing the boundary vectors drawn; FIG. 17 is a top view of two cross-sections of a cone with the second layer showing the boundary vectors not making contact with cross-hatch in a particular location; FIG. 18 is a top view of two cross-sections of a cone with Smalleys; FIG. 19 is a top view of a two cross-sections of a cone with Smalleys with the second layer only showing the boundary vectors drawn; and FIG. 20 is a top view of two cross-sections of a cone with Smalleys with the second layer showing the boundary vectors making contact with cross-hatch everywhere. The description that follows explains how distortion is reduced by isolating sections of an object built by stereolithography so that stress cannot be transmitted from one section to another. This isolation technique (Smalleys) limits the distortion in a given section to that which can be caused by the stress developed within that section only, not from other sections. Referring now to the drawings, and particularly to FIG. 1 thereof, there is shown a block diagram of an overall stereolithography system suitable for practicing the present invention. A CAD generator 2 and appropriate interface 3 provide a data description of the object to be formed, typically in PHIGS format, via network communication such as ETHERNET or the like to an interface computer 4 where the object data is manipulated to optimize the data and provide output vectors which reduce stress, curl and distortion, and increase resolution, strength, accuracy, speed and economy of reproduction, even for rather difficult and complex object shapes. The interface computer 4 generates layer data by slicing, varying layer thickness, rounding polygon vertices, filling, generating flat skins, near-flat skins, up-facing and down-facing skins, scaling, cross-hatching, offsetting vectors and ordering of vectors. More detail about the vector types are available in EP-A-0 354 637 Briefly, boundary vectors are used to trace the outline of each cross-section, hatch vectors are used to provide internal structure between the boundary vectors, and skin vectors are used to define the outer surfaces of the object. They are traced in the following order: boundary, hatch, skin. The vector data and parameters from the computer 4 are directed to a controller subsystem 5 for operating the system stereolithography laser, mirrors, elevator and the like. FIGS. 2 and 3 are flow charts illustrating a basic system of the present invention for generating three-dimensional objects by means of stereolithography. Many liquid state chemicals are known which can be induced to change to solid state polymer plastic by irradiation with ultraviolet light (UV) or other forms of synergistic stimulation such as electron beams, visible or invisible light, reactive chemicals applied by ink jet or via a suitable mask. UV curable chemicals are currently used as ink for high speed printing, in processes of coating or paper and other materials, as adhesives, and in other specialty areas. Lithography is the art of reproducing graphic objects, using various techniques. Modern examples include photographic reproduction, xerography, and microlithography, as is used in the production of microelectronics. Computer generated graphics displayed on a plotter or a cathode ray tube are also forms of lithography, where the image is a picture of a computer coded object. Computer aided design (CAD) and computer aided manufacturing (CAM) are techniques that apply the abilities of computers to the processes of designing and manufacturing. A typical example of CAD is in the area of electronic printed circuit design, where a computer and plotter draw the design of a printed circuit board, given the design parameters as computer data input. A typical example of CAM is a numerically controlled milling machine, where a computer and a milling machine produce metal parts, given the proper programming instructions. Both CAD and CAM are important and are rapidly growing technologies. Referring now more specifically to FIG. 2 of the drawings, the stereolithographic method is broadly outlined. Step 8 calls for generation of CAD or other data, typically in digital form, representing a three-dimensional object to be formed by the system. This CAD data usually defines surfaces in polygon format, triangles and normals perpendicular to the planes of those triangles, e.g., for slope indications, being presently preferred, and in a presently preferred embodiment of the invention conforms to the Programmer's Hierarchial Interactive Graphics System (PHIGS) now adapted as an ANSI standard. This standard is described, by way of example in the publication "Understanding PHIGS", published by Template, Megatek Corp., San Diego, California, which is hereby fully incorporated into this disclosure by reference as though set forth in full herein. In Step 9, the PHIGS data or its equivalent is converted, in accordance with the invention, by a conversion system to a modified data base for driving the stereolithography output system in forming three-dimensional objects. In this regard, information defining the object is specially processed to reduce stress, curl and distortion, and increase resolution, strength and accuracy of reproduction. Step 10 in FIG. 2 calls for the generation of individual solid laminae representing cross-sections of a three-dimensional object to be formed. Step 11 combines the successively formed adjacent lamine to form the desired three-dimensional object which has been programmed into the system for selective curing. Hence, the stereolithographic system generates three-dimensional objects by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium, e.g., a UV curable liquid or the like, capable of altering its physical state in response to appropriate synergistic stimulation such as impinging radiation, electron beam or other particle bombardment, or applied chemicals (as by ink jet or spraying over a mask adjacent the fluid surface), successive adjacent laminae, representing corresponding successive adjacent cross-sections of the object, being automatically formed and integrated together to provide a step-wise laminar or thin layer buildup of the object, whereby a three-dimensional object is formed and drawn from a substantially planar or sheet-like surface of the fluid medium during the forming process. The aforedescribed technique illustrated in FIG. 2 is more specifically outlined in the flowchart of FIG. 3, where again Step 8 calls for generation of CAD or other data, typically in digital form, representing a three-dimensional object to be formed by the system. Again, in Step 9, the PHIGS data is converted by a conversion system to a modified data base for driving the stereolithography output system in forming three-dimensional objects. Step 12 calls for containing a fluid medium capable of solidification in response to prescribed reactive stimulation. Step 13 calls for application of that stimulation as a graphic pattern, in response to data output from the computer 4 in Fig. 1, at a designated fluid surface to form thin, solid, individual layers at that surface, each layer representing an adjacent cross-section of a three-dimensional object to be produced. In the practical application of the invention, each lamina will be a thin lamina, but thick enough to be adequately cohesive in forming the cross-section and adhering to the adjacent laminae defining other cross-sections of the object being formed. Step 14 in FIG. 3 calls for superimposing successive adjacent layers or laminae on each other as they are formed, to integrate the various layers and define the desired three-dimensional object. In the practice of the invention further described below, as the fluid medium cures and solid material forms to define one lamina, that lamina is moved away from the working surface of the fluid medium and the next lamina is formed in the new liquid which replaces the previously formed lamina, so that each successive lamina is superimposed and integral with (by virtue of the natural adhesive properties of the cured fluid medium) all of the other cross-sectional laminae. The process of producing such cross-sectional laminae is repeated over and over again until the entire three-dimensional object has been formed. The object is then removed and the system is ready to produce another object which may be identical to the previous object or may be an entirely new object formed by changing the basic configuration data controlling the stereolithographic system. FIGS. 4-5 of the drawings illustrate various apparatus suitable for implementing the stereolithographic methods illustrated and described by the systems and flow charts of FIGS. 1-3. The data base of a CAD system can take several forms. One form, as previously indicated, consists of representing the surface of an object as a mesh of triangles (PHIGS). These triangles completely form the inner and outer surfaces of the object. This CAD representation also includes a unit length normal vector for each triangle. The normal points away from the solid which the triangle is bounding. This invention provides a means of processing such CAD data into the layer-by-layer vector data that is necessary for forming objects through stereolithography. For stereolithography to successfully work, there must be good adhesion from one layer to the next. Hence, plastic from one layer must overlay plastic that was formed when the previous layer was built. In building models that are made of vertical segments, plastic that is formed on one layer will fall exactly on previously formed plastic from the preceding layer, and thereby provide good adhesion. As one starts to make a transition from vertical to horizontal features, using finite jumps in layer thickness, a point will eventually be reached where the plastic formed on one layer does not make contact with the plastic formed on the previous layer, and this causes severe adhesion problems. Horizontal surfaces themselves do not present adhesion problems because by being horizontal the whole section is built on one layer with side-to-side adhesion maintaining structural integrity. Therefore, means are provided for insuring adhesion between layers when making transitions from vertical to horizontal or horizontal to vertical sections, as well as providing a way to completely bound a surface, and ways to reduce or eliminate stress and strain in formed parts. A presently preferred embodiment of a stereolithographic apparatus is shown in elevational cross-section in FIG. 4. A container 21 is filled with a UV curable liquid 22 or the like, to provide a designated working surface 23. A programmable source of ultraviolet light 26 or the like produces a spot of ultraviolet light 27 in the plane of surface 23. The spot 27 is movable across the surface 23 by the motion of mirrors or other optical or mechanical elements (not shown in Fig. 4) used with the light source 26. The position of the spot 27 on surface 23 is controlled by a computer control system 28. As previously indicated, the system 28 may be under control of CAD data produced by a generator 20 in a CAD design system or the like and directed in PHIGS format or its equivalent to a computerized conversion system 25 where information defining the object is specially processed to reduce stress, curl and distortion, and increase resolution, strength and accuracy of reproduction. A movable elevator platform 29 inside container 21 can be moved up and down selectively, the position of the platform being controlled by the system 28. As the device operates, it produces a three-dimensional object 30 by step-wise buildup of integrated laminae such as 30a, 30b, 30c. The surface of the UV curable liquid 22 is maintained at a constant level in the container 21, and the spot of UV light 27, or other suitable form of reactive stimulation, of sufficient intensity to cure the liquid and convert it to a solid material is moved across the working surface 23 in a programmed manner. As the liquid 22 cures and solid material forms, the elevator platform 29 that was initially just below surface 23 is moved down from the surface in a programmed manner by any suitable actuator. In this way, the solid material that was initially formed is taken below surface 23 and new liquid 22 flows across the surface 23. A portion of this new liquid is, in turn, converted to solid material by the programmed UV light spot 27, and the new material adhesively connects to the material below it. This process is continued until the entire three-dimensional object 30 is formed. The object 30 is then removed from the container 21, and the apparatus is ready to produce another object. Another object can then be produced, or some new object can be made by changing the program in the computer 28. The curable liquid 22, e.g., UV curable liquid, must have several important properties. (A) It must cure fast enough with the available UV light source to allow practical object formation times. (B) It must be adhesive, so that successive layers will adhere to each other. (C) Its viscosity must be low enough so that fresh liquid material will quickly flow across the surface when the elevator moves the object. (D) It should absorb UV so that the film formed will be reasonably thin. (E) It must be reasonably insoluble in that same solvent in the solid state, so that the object can be washed free of the UV cure liquid and partially cured liquid after the object has been formed. (F) It should be as non-toxic and non-irritating as possible. The cured material must also have desirable properties once it is in the solid state. These properties depend on the application involved, as in the conventional use of other plastic materials. Such parameters as color, texture, strength, electrical properties, flammability, and flexibility are among the properties to be considered. In addition, the cost of the material will be important in many cases. The UV curable material used in the presently preferred embodiment, is DeSoto SLR 800 stereolithography resin, made by DeSoto, Inc. of Des Plains, Illinois. The light source 26 produces the spot 27 of UV light small enough to allow the desired object detail to be formed, and intense enough to cure the UV curable liquid being used quickly enough to be practical. The source 26 is arranged so it can be programmed to be turned off and on, and to move, such that the focused spot 27 moves across the surface 23 of the liquid 22. Thus, as the spot 27 moves, it cures the liquid 22 into a solid, and "draws" a solid pattern on the surface in much the same way a chart recorder or plotter uses a pen to draw a pattern on paper. The light source 26 for the presently preferred embodiment of a stereolithography is typically a helium-cadmium ultraviolet laser such as the Model 4240-N HeCd Multimode laser, made by Liconix of Sunnyvale, California. In the system of FIG. 4, means may be provided to keep the surface 23 at a constant level and to replenish this material after an object has been removed, so that the focus spot 27 will remain sharply in focus on a fixed focus plane, thus insuring maximum resolution in forming a high layer along the working surface. In this regard, it is desired to shape the focal point to provide a region of high intensity right at the working surface 23, rapidly diverging to low intensity and thereby limiting the depth of the curing process to provide the thinnest appropriate cross-sectional laminae for the object being formed. The elevator platform 29 is used to support and hold the object 30 being formed, and to move it up and down as required. Typically, after a layer is formed, the object 30 is moved beyond the level of the next layer to allow the liquid 22 to flow into the momentary void at surface 23 left where the solid was formed, and then it is moved back to the correct level for the next layer. The requirements for the elevator platform 29 are that it can be moved in a programmed fashion at appropriate speeds, with adequate precision, and that it is powerful enough to handle the weight of the object 30 being formed. In addition, a manual fine adjustment of the elevator platform position is useful during the set-up phase and when the object is being removed. The elevator platform 29 can be mechanical, pneumatic, hydraulic, or electrical and may also be optical or electronic feedback to precisely control its position. The elevator platform 29 is typically fabricated of either glass or aluminum, but any material to which the cured plastic material will adhere is suitable. A computer controlled pump (not shown) may be used to maintain a constant level of the liquid 22 at the working surface 23. Appropriate level detection system and feedback networks, well known in the art, can be used to drive a fluid pump or a liquid displacement device, such as a solid rod (not shown) which is moved out of the fluid medium as the elevator platform is moved further into the fluid medium, to offset changes in fluid volume and maintain constant fluid level at the surface 23. Alternatively, the source 26 can be moved relative to the sensed level 23 and automatically maintain sharp focus at the working surface 23. All of these alternatives can be readily achieved by appropriate data operating in conjunction with the computer control system 28. Fig. 6 of the drawings illustrates the overall software architecture of a stereolithography system in which the present invention may be practiced. As an overview, the portion of our processing referred to as "SLICE" takes in the object that you want to build, together with any scaffolding or supports that are necessary to make it more buildable. These supports are typically generated by the user's CAD. The first thing SLICE does is to find the outlines of the object and its supports. SLICE defines each microsection or layer one at a time under certain specified controlling styles. SLICE produces a boundary to the solid portion of the object. If, for instance, the object is hollow, there will be an outside surface and an inside one. This outline then is the primary information. The SLICE program then takes that outline or series of outlines and says, but if you build an outside skin and an inside skin they won't join to one another, you'll have liquid between them. It will collapse. So let us turn this into a real product, a real part by putting in cross-hatching between the surfaces or solidifying everything in between or adding skins where it's so gentle a slope that one layer wouldn't join on top of the next, remembering past history or slope of the triangles (PHIGS) whichever way you look at it. SLICE does all those things and uses some lookup tables of the chemical characteristics of the photopolymer, how powerful the laser is, and related parameters to indicate how long to expose each of the output vectors used to operate the system. That output consists of identifiable groups. One group consists of the boundaries or outlines. Another group consists of cross-hatches. A third group consists of skins and there are subgroups of those, upward facing skins, downward facing skins which have to be treated slightly differently. These subgroups are all tracked differently because they may get slightly different treatment, in the process the output data is then appropriately managed to form the desired object and supports. More detail about the different vector types produced by SLICE are contained in EP-A-0 354 637. After the three-dimensional object 30 has been formed, the elevator platform 29 is raised and the object is removed from the platform for post processing. In addition, there may be several containers 21 used in the practice of the invention, each container having a different type of curable material that can be automatically selected by the stereolithographic system. In this regard, the various materials might provide plastics of different colors, or have both insulating and conducting material available for the various layers of electronic products. As will be apparent from FIG. 5 of the drawings, there is shown an alternate configuration of a stereolithograph wherein the UV curable liquid 22 or the like floats on a heavier UV transparent liquid 32 which is non-miscible and non-wetting with the curable liquid 22. By way of example, ethylene glycol or heavy water are suitable for the intermediate liquid layer 32. In the system of FIG. 4, the three-dimensional object 30 is pulled up from the liquid 22, rather than down and further into the liquid medium, as shown in the system of FIG. 3. The UV light source 26 in FIG. 5 focuses the spot 27 at the interface between the liquid 22 and the non-miscible intermediate liquid layer 32, the UV radiation passing through a suitable UV transparent window 33, of quartz or the like, supported at the bottom of the container 21. The curable liquid 22 is provided in a very thin layer over the non-miscible layer 32 and thereby has the advantage of limiting layer thickness directly rather than relying solely upon adsorption and the like to limit the depth of curing since ideally an ultra-thin lamina is to be provided. Hence, the region of formation will be more sharply defined and some surfaces will be formed smoother with the system of FIG. 5 than with that of FIG. 4. In addition a smaller volume of UV curable liquid 22 is required, and the substitution of one curable material for another is easier. A commercial stereolithography system will have additional components and subsystems besides those previously shown in connection with the schematically depicted systems of FIGS. 1-5. For example, the commercial system would also have a frame and housing, and a control panel. It should have means to shield the operator from excess UV and visible light, and it may also have means to allow viewing of the object 30 while it is being formed. Commercial units will provide safety means for controlling ozone and noxious fumes, as well as conventional high voltage safety protection and interlocks. Such commercial units will also have means to effectively shield the sensitive electronics from electronic noise sources. The present invention addresses some additional problems encountered in the previous practice of stereolithography. Each new layer of a stereolithographic part tends to pull upward on the next lower layer while it is being formed. This is a direct result of stresses created by the curing layer as the liquid is converted to solid. This action may cause both layers to curl upward, dependent on the geometry of the layers and whether or not the lower layer is securely held in place either by supports or by strong adhesion to the next lower layer. Certain of these shapes are more susceptible to curling, and may require special design features known as Smalleys in order to inhibit or minimize curl. Stresses are created in the curing layer in two ways. First, the liquid plastic used in stereolithography is less dense than as a solid. This means that the solid will take up less volume and will tend to pull on the lower layer as it shrinks. Second, the plastic expands when it is heated by the polymerization process and subsequently contracts as it cools. Since the new layer formed by the laser is firmly bonded to the lower layer, it tends to pull upward on the lower layer as it cools. To ensure that stresses are maintained at a level that will not cause curling it may be possible to use resins whose properties minimize thermal expansion and contraction. These resins are in development, but may still not solve the curling problem for all applications. In accordance with one aspect of this invention, it is proposed to isolate sections of a part so that the stresses cannot propagate over large distances and will not be transmitted beyond certain stress points in the part. Layer sections prone to curling may be isolated by designing small holes or gaps at stress points in the CAD design of the part. These gaps, called "Smalleys", block propagation of stresses along layer sections. This reduces the stresses acting on a part to only those created within the section. If the Smalleys are properly designed, these localized stresses will be below the threshold value which would curl the layer section. Hence, Smalleys inhibit the transmission of stress from one section to another. They also serve to limit the stress to an amount that will minimize distortion in a given section (by limiting the stress before it gets large enough to cause distortion). Smalleys are generally designed on the CAD to be 0.38 to 0.76 mm (15 to 30 mils) wide (depending on the expected cure width). They are also generally designed to be 1 to 2 mm (40 to 80 mils) tall (depending on the strength of the material and part geometry). When the material is cured, the Smalleys narrow by a full cure width of material. Hence, the right choice of design width can yield Smalleys that are almost completely hidden after post curing. It must be noted, however, that Smalleys must be designed so that when boundary vectors are drawn the Smalleys do not completely close. This is typically accomplished during the CAD design of the object. When we implement the ability to offset vectors to account for finite cure width of material, the width of design of Smalleys can be reduced to a few mils. Note that in the above discussion, the Smalleys will narrow by a full cure width of material only if beam width compensation is not being performed when the material around the Smalleys is being cured. Beam width compensation is described in more detail in EP-A-0 354 637. Briefly, beam width compensation moves the border vectors for a cross section inwards by one half the beam width, so that cross section, once cured, will more accurately represent the object. A floating or unsupported line of plastic does not distort from its drawn shape. It distorts only when another curing line or plastic comes into contact with it. This second line of plastic shrinks as it is drawn, so if it contacts the first (previously cured) line, the first line will be bent towards the second. If we consider the first line to be constrained in some manner, the distortion caused by the second line will be affected by the constraints to the extent that distortion will only occur in the areas of least resistance. If small gaps are made in this second line, then any stress that develops from the contact with the first line will be isolated between the gaps. If the gaps are used to separate regions of strong structural strength from weaker regions, the stresses from the strong regions cannot propagate to the weaker ones and cause distortion there. Distortion at any point will be less because the stress at that point is less. Vertical distortions are a primary problem so we are generally concerned with placing Smalleys in regions on layers above a critical layer, such as above the first layer of an unsupported region. Generally Smalleys are used to isolate regions from stress until they build up enough structural integrity to withstand the stress induced by curing successive layers. This will generally require that the Smalleys be several layers in height. After sufficient strength is developed, the Smalleys can be removed. There are several ways that Smalleys can be used to reduce distortion in an object: 1) Smalleys can be used to reduce distortion (separation of layers) and curl in solid areas of objects. This is especially true for cylindrical objects, but also true for other object geometries that have problems with distortion. 2) Smalleys can be placed at the ends of unsupported regions to reduce the distortion of the unsupported regions. This is especially true for upper edges of windows which are curved, and at the edge of a cantilever beam. Smalleys used in this way must be placed one layer (no more, no less) above the unsupported region. 3) Smalleys can be used to reduce distortion in objects with wide internal regions by hollowing out these regions. Designing Smalleys The key to effective use of Smalleys is their proper placement at stress points in the CAD design. The following examples describe layer sections where Smalleys are typically used. Example A: The continuous layer borders (LBs) of the cylindrical part shown in Figure 7 generate relatively large stresses. These stresses may cause curling if the layers are not adequately adhered to one another. As shown in Fig. 7, Smalleys should be placed at 90 degree intervals around the circumference of the part, with each Smalley typically being 4 to 5 slice layers tall. Each successive set of Smalleys should be offset about 45 degrees to maintain the structural integrity of the part. Example B: The unsupported upper edges of the curved windows shown in Figure 8 are highly susceptible to curling. Design Smalleys at the ends of the windows, as shown, but leave one continuous LB under each Smalley. Example C: The thick interior structure of the part shown in Figure 9 will tend to curl the exterior flanges and other unsupported surfaces. A large Smalley, one that hollows out the interior of the part, as shown, will minimize these stresses. Smalleys are typically designed on the CAD to be 0.38 to 0.76 mm (15 to 30 mils) wide and 1 to 2 mm (40 to 80 mils) tall. They generally decrease in size as the part is formed due to the viscous liquid filling the small gaps during dipping. Thus, if designed properly, Smalleys will prevent curl and then effectively disappear or reduce in size to narrow slits or slight indentations on the surface of the part during post curing. Smalleys are also used to reduce birdnesting. The width of Smalleys, for this application, is generally less than cure width, so that after curing they are completely filled in and so no structural integrity is lost through their use. Smalleys are placed periodically in regions of down-facing, near-flat triangles with heights appropriate to extend vertically through the near-flat triangles. The placement of Smalleys is based on several factors that affect the likelihood of having birdnesting problems. The radius of curvature of the boundaries, the length of near-flat zones, the likelihood of boundaries moving, etc. are significant. Smalleys do not need to penetrate completely through a wall, as they do in their other application, but they do need to penetrate deep enough to insure a contact point with the boundaries on the previous layer. Birdnesting can occur in objects that do not have near-flat triangles, but only when there are adhesion problems between layers (for example, when an object is built using dip delays that are too short). Smalleys can be used in these situations to help eliminate birdnesting also. Smalleys can be used in a variety of situations that have down-facing near-flat skin. FIG. 10 is a side view of a CAD designed cone without Smalleys. FIGS. 11 and 12 are views of the sliced CAD designed cone and what it might look like after building. FIG. 13 is a top view of a CAD designed cone showing possible locations in the XY plane where Smalleys might be inserted. FIG. 14 is a side view of a sliced CAD designed cone with Smalleys and what it might look like after building. Boundary vectors can move out of position for a couple of reasons: 1) convection currents within the liquid that can cause floating items to drift, 2) distortions of boundary vectors from making contact with already cured (but floating material), 3) newly cured material contacting, and distorting, boundary vectors before they are secured into position, and 4) shrinking of hatch as it starts to secure one side of the boundary causing the boundary to be pulled out of position. A couple of these causes can affect boundary vectors that are not associated with near-flat triangles, so if problems are found in non-near-flat regions, Smalleys may be useful. Boundaries can only birdnest when they can move or sections of them can move far enough out of position so that when cross-hatching is drawn, it does not contact the boundaries. Smalleys avoid this problem by having the boundaries cut in over the top of the boundaries from the previous layer, on a periodic basis. This cutting in over the top of previously cured boundaries prevents the present boundaries from moving out of position. FIG. 15 is a top view of two cross-sections of a cone with no Smalleys. FIG. 16 is a top view of two cross-sections of a cone with the second layer only showing the boundary vectors drawn. FIG. 17 is a top view of two cross-sections of a cone with the second layer showing the boundary vectors not making contact with cross-hatch in a particular location. FIG. 18 is a top view of two cross-sections of a cone with Smalleys. FIG. 19 is a top view of two cross-sections of a cone with Smalleys with the second layer only showing the boundary vectors drawn. FIG. 20 is a top view of two cross-sections of a cone with Smalleys with the second layer showing the boundary vectors making contact with cross-hatch everywhere. The stereolithographic method and apparatus that has been described has many advantages over currently used methods for producing plastic objects. The method avoids the need of producing tooling drawings and tooling. The designer can work directly with the computer and a stereolithographic device, and when he is satisfied with the design as displayed on the output screen of the computer, he can fabricate a part for direct examination, information defining the object being specially processed to reduce curl, stress, birdnesting and other distortions, and increase resolution, strength and accuracy of reproduction. If the design has to be modified, it can be easily done through the computer, and then another part can be made to verify that the change was correct. If the design calls for several parts with interacting design parameters, the method becomes even more useful because all of the part designs can be quickly changed and made again so that the total assembly can be made and examined, repeatedly if necessary. After the design is complete, part production can begin immediately, so that the weeks and months between design and production are avoided. Ultimate production rates and parts costs should be similar to current injection molding costs for short run production, with even lower labor costs than those associated with injection molding. Injection molding is economical only when large numbers of identical parts are required. Stereolithography is particularly useful for short run production because the need for tooling is eliminated and production set-up time is minimal. Likewise, design changes and custom parts are easily provided using the technique. Because of the ease of making parts, stereolithography can allow plastic parts to be used in many places where metal or other material parts are now used. Moreover, it allows plastic models of objects to be quickly and economically provided, prior to the decision to make more expensive metal or other material parts.

Claims: A method of forming a three-dimensional object (30) by selectively solidifying successive layers (23) of a transformable fluid medium (22) by subjecting each layer (23) to a selected pattern of a prescribed stimulation (27) whereby the object is built of a succession of superposed laminae (30a, 30b, 30c), wherein the selected pattern for each layer (23) is obtained by processing stored data (20) defining a desired configuration of the object (30), characterized by: said processing step comprising modifying the stored data (20) defining the desired object configuration by introducing openings in solid portions of the desired configuration, each of said openings as defined by the modified data existing in at least one lamina and having at least a portion which is vertically bounded by a solid portion of the lamina immediately preceding the earliest-formed lamina in which the opening exists. A method as claimed in Claim 1 in which the opening is not completely closed upon solidification of the lamina containing said opening. A method as claimed in Claim 1 or 2 in which said stored data (20) is modified to produce openings that isolate adjacent sections of the same lamina from the transmission of stress therebetween (Fig. 8). A method as claimed in any one of Claims 1 to 3 in which said opening extends through a wall of the object in said desired configuration (Fig. 7). A method as claimed in any one of Claims 1 to 3 in which the desired configuration of the object contains a cantilever and said opening is placed at the supported end of said cantilever (Fig. 8). A method as claimed in Claim 5 in which said opening is separated by a single lamina from the unsupported region preceding the cantilever. A method as claimed in Claim 1, 2 or 3 in which said opening provides an enclosed hollow within a portion of the object that is solid in the desired object configuration (Fig. 9). A method as claimed in any one of Claims 1, 2 or 3 including introducing an opening in one lamina that extends from a portion of said one lamina unsupported by the immediately preceding lamina into a portion of said one lamina that is supported by the immediately preceding lamina (Figs. 18-20). A method as claimed in any one of Claims 1 to 8 in which said stored data (20) is modified to define an opening extending through a plurality of laminae which opening is 1-2 mm in the direction normal to the laminae and is 0.05-0.75 mm wide in the planes of the laminae. A method as claimed in any one of Claims 1 to 9 which is performed to build said laminae in an upward direction whereby said earliest-formed lamina is the lowermost lamina into which an opening extends (Fig. 4). A method as claimed in any one of Claims 1 to 10 in which said transformable fluid medium is a liquid. A method as claimed in Claim 11 in which said liquid is a polymer. A method as claimed in any one of Claims 1 to 12 in which said prescribed stimulation is radiation or particle bombardment. A method as claimed in Claim 13 in which the radiation is a beam of ultraviolet radiation. A method as claimed in Claim 13 or 14 in which said radiation is generated by a laser. A method as claimed in any one of Claims 1 to 12 in which said prescribed stimulation is a chemically reactive material selectively dispensed onto said layer of transformable fluid medium. A method as claimed in any of Claims 1 to 16 in which said prescribed stimulation is selectively applied to each layer of transformable fluid medium by scanning the surface of the layer. An apparatus for forming a three-dimensional object (30) from stored data (20) defining a desired configuration of the object, the object being built of a succession of superposed laminae (30a, 30b, 30c) each of which is formed by solidifying a layer (23) of a transformable fluid medium (22) by selective application thereto of a prescribed stimulation (27), the apparatus including a data processing means for generating from the stored data (20) the pattern of stimulation applied to each fluid layer (23), characterized in that: said data processing means (21) comprises data-modifying means (4) responsive to said stored data to produce modified data introducing openings into solid portions of the desired object configuration defined by said stored data (20), each of said openings as defined by the modified data existing on at least one lamina and having at least a portion which is vertically bounded by a solid portion of the lamina immediately preceding the earliest formed lamina in which the opening exists. Apparatus as claimed in Claim 18 in which said data-modifying means (4) is operable to produce openings that isolate adjacent sections of the same lamina from the transmission of stress therebetween (Fig. 8). Apparatus as claimed in Claim 18 or 19 in which said opening extends through a wall of the object in said desired configuration (Fig. 7). Apparatus as claimed in Claim 18 or 19 in which said data-modifying means is operable to produce an opening placed at the supported end of a cantilever in said desired configuration of the object (Fig. 8). Apparatus as claimed in Claim 21 in which said opening is separated by a single lamina from the unsupported region immediately preceding said cantilever (Fig. 8). Apparatus as claimed in Claim 18 or 19 in which said opening provides an enclosed hollow within a portion of the object that is solid in the desired object configuration (Fig. 9). Apparatus as claimed in any one of Claims 18 or 19 in which said data-modifying means (4) is operable to introduce an opening in one lamina that extends from a portion of said one lamina unsupported by the immediately preceding lamina into a portion of said one lamina that is supported by the immediately preceding lamina (Fig. 18-20). Apparatus as claimed in any one of Claims 18 to 24 in which said data-modifying means (4) is operable to define an opening which extends through a plurality of laminae and which is 1-2 mm in the direction normal to the laminae and is 0.05-0.75 mm wide in the planes of the laminae. Apparatus as claimed in any one of Claims 18 to 25 which operates to build said laminae in an upward direction whereby said earliest-formed lamina is the lowermost lamina into which an opening extends (Fig. 4). Apparatus as claimed in any one of Claims 18 to 26 comprising a container (21) for said transformable fluid medium and wherein said fluid medium is a liquid. Apparatus as claimed in Claim 27 in which said liquid is a polymer. Apparatus as claimed in any one of Claims 18 to 28 in which said prescribed stimulation is radiation or particle bombardment. Apparatus as claimed in Claim 29 in which the radiation is a beam of ultraviolet radiation. Apparatus as claimed in Claim 29 or 30 in which said radiation is generated by a laser. Apparatus as claimed in any one of Claims 18 to 28 in which said prescribed stimulation is a chemically reactive material selectively dispensed onto said layer of transformable fluid medium. Apparatus as claimed in any one of Claims 18 to 32 in which said prescribed stimulation is selectively applied to each layer of transformable fluid medium by scanning the surface of the layer.

Inventors: SMALLEY DENNIS ROLLETTE, SMALLEY, DENNIS ROLLETTE
Applicants: 3D SYSTEMS INC, 3D SYSTEMS, INC.
Application year: 1989
Main classification: B29C39
IPC codes: B29C35, B29C67, B29K105