Stereolithographic exposure head

Abstract: A stereolithographic exposure head (20) for use with a stereolithographic apparatus comprises a plurality of mirrors individually responsive to input signals, the mirrors operable to reflect incident radiation along a plurality of optical pathways, the mirrors forming a nxm matrix; an illumination source for emitting radiation incident upon the mirrors, the radiation operable to harden a stereolithographic liquid (14); and control circuitry responsive to input data operable to select one of the plurality of pathways along which each of the mirrors will reflect incident radiation to form an image (26).

Description: TECHNICAL FIELD OF THE INVENTION This invention relates in general to electro-optical devices, and, more particularly, to a stereolithographic exposure head. BACKGROUND OF THE INVENTION Stereolithography is a process by which three dimensional objects are fabricated from thin layers of hardened or cured liquid polymers. The liquid polymers or resins are selectively hardened by an intense light source of suitable wave length. Typically, an ultraviolet ("UV"), argon-ion, or other type of laser is used to harden the liquid polymer. The particular laser is directed to a thin film of the polymer by an x-y scanner interpreting, typically, computer-aided design ("CAD") data. The CAD data mathematically represents the shape of the object to be produced as a series of sequential thin layers of laminas, typically 1/4th of a millimeter (1/100th of an inch) in thickness. The object may thereby be produced by curing portions of a film of resin that correspond to the first layer of the object, by coating the hardened layer with a uniform thin liquid film, and by curing portions of the second film that correspond to the next lamina of the object. This process is repeated until each lamina of the model has been reproduced as a layer of hardened polymer. The model produced by stereolithography may be used for testing and evaluation of engineering designs, for tooling purposes and for low volume manufacturing applications. Examples of stereolithographic processes are described in WO-A-91/12120 and WO-A-89/09687. WO-A-91/12120 discloses an optic system for exposing successive layers of a liquid material to form a solid three-dimensional article. The optic system includes a laser, a beam shutter, bending mirrors, an adjustably movable Z-axis correction focussing lens, an objective lens and a scanner head having a x scanning mirror and a y-scanning mirror. A control computer controls operation of the optic system. WO-A-89/09687 discloses a laser based plastic model making work station including a system computer, a laser beam generator, a double mirror laser beam diverting system, and a laser beam scanner. The laser beam scanner can be moved in two dimensions. Known stereolithographic process devices and methods are limited by at least two characteristics. First, the use of an x-y scanner limits the speed by which a laser beam may be scanned on to a film of liquid resin to approximately 1.4 m (55 inches) per second. This speed, coupled with the high vertical resolution of the process, results in lengthy production time and low product output. Second, the use of a particular laser as the illumination source limits the type of liquid resin that may be used in the system. Each resin has an individual reaction to various wavelengths of electromagnetic radiation. Typically, each resin may be hardened by only one type of laser. This results in little or no ability to change the resin composition without expensive hardware changes. Also, not all wavelengths of radiation may be produced as laser light. This precludes the use of certain resins because no laser exists to cure them. From EP-A-0 391 529 a visual display system is known having a spatial light modulator and a source of random polarity light. The spatial light modulator includes an array of deformable mirrors, each of said mirrors operable to reflect incident light along a first path or a second path, wherein said light reflected along the second path is directed toward a display surface. IEEE Transactions On Electron Devices, Vol. ED-30, No. 5, May 1983, pages 539 to 545, describes a deformable mirror device which is an x-y array of deformable mirror elements addressed by an underlying array of MOS transistors. It is reported that applications include optical information processing and projection displays. Therefore, a need has arisen for a stereolithographic exposure head which allows for high production output and which is compatible with a wider range of stereolithographic resins. SUMMARY OF THE INVENTION In accordance with the present invention, a stereolithographic epxosure head according to claim 1 is provided. Particular embodiments of the invention are the subject of the dependent claims. The first technical advantage of the disclosed invention is its speed. The plurality of mirrors which comprise the deformable mirror device allow an entire layer of resin to be hardened in one exposure interval. This technique allows much greater throughput. A second technical advantage of the device is its flexibility. Almost any illumination source may be used with the deformable mirror device. The illumination source can therefore be tailored to the particular resin so used and may, in fact, be an inexpensive incandescent light bulb. A third technical advantage of the disclosed invention is its cost. Deformable mirror devices are currently available in mass produced quantities at costs far below that of an x-y laser scanner. Another technical advantage of the invention is its suitability to large model fabrication. Multiple exposure heads may be combined to cure a large layer of a liquid. Resolution will meanwhile remain high. BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which: FIGUREs 1a through 1e illustrate one sequence of steps for producing a model using the stereolithographic process; FIGURE 2 is a partially perspective, partially schematic illustration of an exposure head containing one embodiment of the disclosed invention for use in the process depicted in FIGUREs 1a through 1e; FIGURE 3 depicts perspectively an area array deformable mirror device useful in the exposure head of FIGURE 2; FIGURE 4 depicts schematically how the exposure head of FIGURE 2 may be combined with similar exposure heads to increase possible model size; and FIGURE 5 depicts perspectively how the exposure head of FIGURE 2 may be articulated to cure a larger film size. DETAILED DESCRIPTION OF THE INVENTION The preferred embodiment of the present invention and its advantages are best understood by referring to FIGUREs 1 through 4 of the drawings, like numerals being used for like and corresponding parts of the various drawings. FIGUREs 1a through 1e illustrate one sequence of steps for producing a model using the stereolithographic process. (a) Stereolithographic apparatus ("SLA") 10 comprises vat 12 for receiving a stereolithographic liquid 14, a working surface 16, an elevator 18 for controlling the level of the liquid relative to the working surface 16, and an exposure head 20 for curing selected portions of the resin as will be described more fully below. The stereolithographic liquid may be a resin or a polymer, as is well known to those skilled in the art. SLA 10 may also comprise a liquid applicator 22 for ensuring a uniform liquid thickness at each modeling stage. In operation, a thin film of liquid is applied to working surface 16 by, for instance, raising elevator 18 within vat 12. Elevator 18 will cause liquid 14 to rise through perforations 24 to form a thin film on working surface 16. Exposure head 20 then cures selected portions of the liquid film by directing electromagnetic radiation thereto. Exposure head 20 is controlled by circuitry (shown in FIGURE 2) that interprets data of the object to be so modeled. The data represents the object as a series of thin sequential layers or laminas which, when stacked on top of each other form the complete object. Typically, computer-aided design ("CAD") data may be most easily converted into sequential layers or laminas of the model to be produced. (b) Elevator 18 again rises to cause liquid 14 to cover the previously cured layer 26 ("workpiece") resting on working surface 26 with a second film of liquid. This second layer of liquid will cover both the cured and uncured portions of the first layer. Applicator 22 may also spray a small amount of liquid onto workpiece 26 to ensure uniform film coverage of the workpiece 26. Each film is approximately 0.25 mm (one 1/100th of an inch) thick. (c) Exposure head 26 cures the second layer of liquid applied in FIGURE 1b as described in connection with FIGURE 1a. (d) The steps in FIGURES 1a through 1c are then repeated until the entire part 28 has been produced. (e) Complete part 28 may then be removed from SLA 10. Under certain conditions using certain liquids, it may be necessary to cure part 28 in an oven (not shown) prior to its use. An SLA, similar to that depicted in FIGURES la through le, may be purchased from 3D Systems, Inc. of Valencia, California or from Quadrax Laser Technologies, Inc. of Portsmouth, Rhode Island. The 3D Systems' SLA employs a HeCd laser with a wavelength of 325 nm. The 3D System differs from the process described above. It successively lowers the working surface below the surface of liquid instead of raising the liquid relative to the working surface. There, the formed object is slowly submerged as it is produced on the surface of the liquid. The Quadrax Laser Technologies' SLA uses an argon-ion visible light laser in the process described in connection with FIGURES la-lc. Other SLA designers use ultraviolet ("UV") lasers to harden stereolithographic liquids. FIGURE 2 is a partially perspective, partially schematic illustration of an exposure head 20 containing one embodiment of the disclosed invention for use in the process depicted in FIGURES 1a through 1e. The exposure head 20 comprises an illumination source 30 an area arra deformable mirror device ("DMD") 32, two lenses 34 and 36, and control circuitry 38. Illumination source 30 emits radiation that is operable to harden a particular SLA liquid. Lens 34 more uniformly illuminates DMD 32 than would otherwise occur without it. Lens 36 focuses and magnifies the light reflected off of DMD 32 onto the working surface 16 (shown in FIGUREs 1a-1e). DMD 32 is an electro-optical device containing a regular n x m array of mirrors manufactured by Texas Instruments, Inc. of Dallas, Texas. Each mirror may be electronically controlled to reflect incident radiation along one of a plurality of optical pathways. In the preferred embodiment, DMD 32 comprises a matrix of bistable mirrors, i.e., there are two optical pathways for each mirror. Illumination source 30, DMD 32, and lenses 34 and 36 are positioned such that light impinging upon DMD 32 from illumination source 30 may be focused onto a thin film of SLA resin if, and only if, one of the two optical pathways is selected. The optical pathway of radiation emitted from illumination source 30 is depicted by the converging and diverging dashed lines. Each bistable mirror on DMD 32 is controlled by circuitry 38 which interprets data from a processor (not shown). Circuitry 38 may be located elsewhere or may be integrated completely with a standalone processor. Illumination source 30 may be tailored to the individual stereolithographic liquid used in the SLA process and might be an ordinary tungsten-halogen incandescent light bulb or any number of visible or non-visible light lasers. FIGURE 3 depicts perspectively an area array deformable mirror device 32 illustrated in the exposure head 20 of FIGURE 2. DMD 32 comprises a body 40 having a planar active surface 42 and a number of electrical pins 44. The active surface 42 of DMD 32 may contain an n x m matrix of individually addressable bistable mirrors. Each mirror is typically a square or diamond having sides of 12 to 20 microns. This small size allows a single DMD having a footprint of approximately 1290 mm 2 (2 square inches) to have over 2 million individually addressable mirrors in, for instance, a 1920 x 1080 matrix. This small mirror size allows exposure head 20 (shown in FIGURE 2) to cure a 101 x 202 mm 2 (4 x 8 square inch) area in a single exposure interval with the same resolution as achieved by prior x-y scanner/laser exposure head combinations. Typically, these prior exposure heads achieve resolutions of ± 0.13 mm (± 0.005 inches). In such a case, lens 36 of FIGURE 2 would also magnify the image of DMD 32 by a factor of 4x-6x. FIGURE 4 depicts how multiple exposure heads 20 may be combined or "tiled" to cover an area of, for instance, 203 x 406 mm 2 (8 x 16 square inch). This allows the user to maintain the same high degree of resolution described in connection with FIGURE 3. In such a tiled configuration, each exposure head 20 cures one quadrant of the film resin. Each quadrant exposes a 101 x 202 mm 2 (4 x 8 square inch) area. Any number of exposure heads 20 can be combined to increase maximum model size. FIGURE 5 depicts how a single exposure head 20 may be mounted on a positioning device 46 with freedom of movement in two dimensions. The single exposure head 20 may thereby be made to cure a lamina of resin larger in size than the projected image of DMD 32 (FIGURES 2 and 3). This allows the user to maintain the same high degree of resolution described in connection with FIGURE 3. Positioning devices used in lithographic processes are known in the art to have resolutions comparable with the SLA process. These positioning devices are used to align photomasks with silicon wafers in the production of devices such as integrated circuits. Although the present invention has been described in detail, it should be understood that various changes, substitutions and operations can be made hereto without departing from the scope of the invention as defined by the appended claims.

Claims: A stereolithographic exposure head (20) for forming a three-dimensional article from a liquid medium (14) capable of solidification when subjected to prescribed energy, the prescribed energy provided by exposure of successive layers of the liquid material by an illumination source, the exposure head (20) comprising: a micromirror device (32) having a plurality of mirrors arranged in a n x m matrix on a common plane, each of said mirrors operable to reflect incident light along a first path or a second path; a light source (30) illuminating a plurality of said mirrors; control circuitry (38) for controlling said mirrors in response to input data received by said control circuitry; and a lens (36) for receiving light directed along said first path and focusing said light onto an exposure plane, such that light from multiple mirrors simultaneously exposes portions of a layer of the liquid material. The stereolithographic exposure head of claim 1, wherein said lens (36) is also operable to magnify the image of the plurality of mirrors. The stereolithographic exposure head of claim 1, further comprising a positioner (46) for moving the head (20) in two dimensions.

Inventors: MITCHAM LARRY D, NELSON WILLIAM E, MITCHAM, LARRY D., NELSON, WILLIAM E.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1992
Main classification: B29C67
IPC codes: G03F7, B29C67, B29C35, G02B26