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United States Patent5609813
Allison , ; et al.March 11, 1997
Title
Method of making a three-dimensional object by stereolithography
Abstract
An improved method for stereolithographically making an object by alternating the order in which similar sets of vectors are exposed over two or more layers. In another method, a pattern of tightly packed hexagonal tiles are drawn. Each tile is isolated from its neighboring tiles by specifying breaks of unexposed material between the tiles. Using an interrupted scan method, vectors are drawn with periodic breaks along their lengths. In another method, modulator and scanning techniques are used to reduce exposure problems associated with the acceleration and deceleration of the scanning system when jumping between vectors or changing scanning directions.
Inventors:Allison; Joseph W. (Valencia, CA), Smalley; Dennis R.  (Baldwin Park, CA), Hull; Charles W.  (Santa Clarita, CA), Jacobs; Paul F.  (La Crescenta, CA)
Assignee:3D Systems, Inc. (Valencia, CA)
Appl. No.:475714
Filed:June 7, 1995
Current U.S. Class:264/401 345/419 365/106 365/107 425/174.4 427/510 427/512 427/553 427/554 427/555 700/120 118/620 156/272.8 156/273.3 156/273.5 156/275.5 156/307.1 156/379.6 250/492.1 264/308 
Field of Search:264/308,401 425/174.4 156/272.8,273.3,273.5,275.5,307.1,379.6 427/510,512,553,554,555 118/620 250/492.1 364/468,476 365/106,107 395/119
U.S. Patent Documents
2775758December 1956Munz
4575330March 1986Hull
4665492May 1987Masters
4752498June 1988Fudim
4844144July 1989Murphy et al.
4863538September 1989Deckard
4915757April 1990Rando
4938816July 1990Beaman et al.
4961154October 1990Pomerantz et al.
5130064July 1992Smalley et al.
5151813September 1992Yamamoto et al.
5182055January 1993Allison et al.
Foreign Patent Documents
0250121Jun., 1987EP
0322257Dec., 1988EP
0388129Mar., 1990EP
61-225012Oct., 1986JP
63-145015Jun., 1988JP
Other References
Kodama, Rev. Sci. Instrum. 52(11), Nov. 1981, "Automatic Method For Fabricating A Three Dimensional Plastic Model with Photo-Hardening Polymer" pp. 1770-1773. .
Herbert, A. J., Journal of Imaging Technology, vol. 15, No. 4, Aug. 1989, "A Review of 3D Solid Object Generation", pp. 186-190. .
Herbert, A. J., Journal of Applied Photographic Engineering, vol. 8, No. 4, pp. 185-188, Aug. 1982, "Solid Object Generation". .
Requicha, A. A. G. et al. "Solid modeling: A Historical Summary and Contemporary Assessment", Institute of Electrical and Electronics Engineers Computer Graphics and Applications, Mar. 1982, pp. 9-24. .
Fudim, E. V. "Sculpting Parts With Light" Machine Design (Mar. 6, 1986), pp. 102-106. .
Fudim, E. V. "A New Method of Three-Dimensional Micromachining," Mechanical Engineering (Sep. 1985) pp. 54-59..~
Primary Examiner: Tentoni; Leo B.
Attorney, Agent or Firm:Smalley; Dennis R. Vradenburgh; Anna M.
Parent Case Text


This application is a continuation of U.S. patent application Ser. No. 08/121,846, filed Sep. 14, 1993, now allowed; which is a continuation of U.S. patent application Ser. No. 07/906,207, filed on Jun. 25, 1992, now U.S. Pat. No. 5,256,340; which in turn is a continuation of U.S. patent application Ser. No. 07/702,031, filed on May 17, 1991, now U.S. Pat. No. 5,182,055; and which is a continuation-in-part of U.S. patent application Ser. No. 07/516,145, filed Apr. 27, 1990, now abandoned; which in turn is a continuation-in-part of U.S. patent application Ser. No. 07/429,435, filed Oct. 30, 1989, now U.S. Pat. No. 5,130,064; which in turn is a continuation-in-part of U.S. patent application Ser. No. 07/331,644, filed Mar. 31, 1989, now U.S. Pat. No. 5,184,307; which in turn is a continuation-in-part of U.S. patent application Ser. No. 07/269,801, filed Nov. 8, 1988, now abandoned; which in turn is a continuation-in-part of U.S. patent application Ser. No. 07/182,830, filed Apr. 18, 1988, now U.S. Pat. No. 5,059,359; U.S. patent application Ser. No. 07/429,435 is also a continuation-in-part of U.S. patent application Ser. No. 07/339,246, filed Apr. 17, 1989, now U.S. Pat. No. 5,104,592; which in turn is a continuation-in-part of U.S. patent application Ser. No. 07/182,823, filed Apr. 18, 1988, now abandoned.
Claims

We claim:
1. A method of forming at least a portion of a three-dimensional object on a stacked layer-by-layer basis using stereolithography comprising:
a) applying a layer of flowable material capable of solidification upon exposure to synergistic stimulation;
b) generating and sequencing a pattern of exposure paths, in a prescribed order and direction, for said layer corresponding to a cross section of said object;
c) exposing said exposure paths to synergistic stimulation according to said sequencing of said exposure paths to form a layer of said three-dimensional object; and
d) repeating steps b) and c) for subsequent layers until said three-dimensional object is formed, wherein the step of sequencing the exposure of said exposure paths is altered with a different sequence of exposure on at least one subsequent layer.

2. The method of claim 1 wherein step of sequencing is periodically alternated with a different sequencing on subsequent layers.

3. The method of claim 1 wherein the step of sequencing is altered with a different sequencing on every other layer.

4. The method of claim 1 wherein the step of sequencing is altered with a different sequencing on each of four subsequent layers then repeated such that a pattern of alternate sequencing occurs every four layers.

5. The method of claim 1 wherein the step of sequencing is altered with a different sequencing on each of eight subsequent layers then repeated such that a pattern of alternate sequencing occurs every eight layers.

6. The method of claim 1 wherein the sequencing is altered by varying the direction of scanning of at least one similar exposure path between at least two layers.

7. The method of claim 1 wherein the sequencing is altered between at least two layers by varying the order in which similar exposure paths are exposed to synergistic stimulation.

8. The method of claim 1 wherein the sequencing is altered by varying the order of propagation of similar exposure paths between at least two layers.

9. The method of claim 1 wherein the exposure paths are defined by vectors.

10. The method of claim 1 wherein the step of generating a pattern of exposure paths comprises generating a pattern of at least two sets of paths and wherein the sequencing of said at least two sets of paths are altered between at least two layers.

11. The method of claim 10 wherein the step of sequencing of said at least two sets of paths are altered by varying the order of propagation in which similar exposure paths are exposed to synergistic stimulation between at least two layers.

12. The method of claim 11 wherein the step of sequencing is further altered by varying the direction of scanning of at least one similar exposure path between at least two layers.

13. An improved method for forming at least a portion of a three dimensional object on a layer by layer basis from a medium solidifiable upon exposure to synergistic stimulation, of the type wherein successive layers of the medium are applied and selectively solidified to form layers of the three-dimensional object, and wherein the solidified layers of the three-dimensional object are adhered together to form said three-dimensional object, each layer of the medium being selectively solidified through the scanning of patterns of synergistic stimulation corresponding to a cross-section of the object, the scanning patterns including exposure paths being scanned in a sequenced order and direction, the improvement comprising:
altering the sequence of exposure of exposure paths to synergistic stimulation between at least two layers to be formed.

14. The method of claim 13 wherein at least one scanning pattern comprises two sets of exposure paths and the altering of the sequence of exposure comprises altering the order in which the two sets of exposure paths are scanned with synergistic stimulation.

15. The method of claim 13 wherein the altering of the sequence of exposure comprises altering the direction of scanning of at least one similar exposure path between said at least two layers to be formed.

16. The method of claim 13 wherein the altering of the sequence of exposure comprises altering the direction of propagation of exposure paths.

17. The method of claim 13 wherein altering the sequence of exposure on subsequent, layers occurs periodically.

18. The method of claim 13 wherein altering the sequence of exposure on subsequent layers occurs non-periodically.

19. The method of claim 13 wherein the scanning pattern is an x and y hatch pattern.

20. The method of claim 13 wherein the scanning pattern utilizes a skintinuous method of building objects.

21. The method of claim 13 wherein the scanning pattern additionally includes the improvement of exposing at least a portion of a layer to at least two sets of non parallel paths for forming outward facing skin surfaces.

22. The method of claim 13 wherein the altering of the sequence of exposure on subsequent layers includes exposing a portion of a layer to a first set of non consecutive parallel exposure paths, and exposing the portion to a second set of nonconsecutive parallel exposure paths substantially interposed between and parallel to the first set of nonconsecutive parallel paths.

23. An improved apparatus for forming at least a portion of a three-dimensional object on a layer-by-layer basis from a medium solidifiable upon exposure to synergistic stimulation, of the type including means for applying successive layers of the medium, means for selectively solidifying the layers to form layers of the three-dimensional object, and for adhering the layers of the three-dimensional object together to form said at least portion of the three-dimensional object, wherein the means for solidifying includes means for scanning patterns of synergistic stimulation corresponding to a cross-section of the object, and wherein the scanning patterns include exposure paths being scanned in a sequenced order and direction, the improvement comprising:
means for altering the sequence of exposure of exposure paths to synergistic stimulation between at least two layers to be formed.

24. The apparatus of claim 23 wherein the means for altering the sequence of exposure comprises means for altering the order of propagation of exposure paths.

25. The apparatus of claim 23 wherein the means for altering the sequence of exposure comprises means for altering the direction of scanning of at least one similar exposure path between at least two layers.

26. The apparatus of claim 23 wherein the means for altering the sequence of exposure further comprises means for periodically altering the sequence of exposure on subsequent layers.

27. The apparatus of claim 23 wherein the means for altering the sequence of exposure additionally comprises means for randomly altering the sequence of exposure on subsequent layers.

28. The apparatus of claim 23 wherein the means for altering the sequence of exposure additionally comprises means for non-periodically altering the sequence of exposure on subsequent layers.

29. The apparatus of claim 23 wherein the means for altering the sequence of exposure additionally comprises means for utilizing x and y hatch patterns.

30. The apparatus of claim 23 wherein the means for altering the sequence of exposure further comprises means for utilizing skintinuous building techniques.
Description
BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to the field of stereolithography, which is a technique for making solid, three-dimensional objects (or "parts") from solidifiable materials (e.g. fluid or fluid-like material such as photopolymers, sinterable powders, and bindable powders).

In recent years, stereolithography systems, such as those described in U.S. Pat. No. 4,575,330, issued Mar. 11, 1986 and entitled "Apparatus for Production of Three-Dimensional Objects by Stereolithography," have come into use Basically, stereolithography is a method for automatically building complex three-dimensional parts by successively solidifying thin cross-sectional layers. These layers may be composed of photopolymer resin, powdered materials, or the like. Some types of powder materials are converted from a fluid-like medium to a cohesive cross-section by melting and solidification. The layers are solidified on top of each other consecutively until all of the thin layers are joined together to form a whole part. Photocurable polymers change from liquid to solid upon exposure to synergistic stimulation. Many photopolymers exist whose photospeed (rate of transformation from liquid to solid) upon irradiation with ultraviolet light (UV) is fast enough to make them practical model building materials. In a preferred system, a radiation source (e.g., an ultraviolet laser) generates a beam which is focused to a small intense spot which is moved across the liquid photopolymer surface by galvanometer or servo type mirror x-y scanners. The scanners are driven by computer generated vectors or the like. The material that is not polymerized when a part is made is still functionable and remains in the vat for use as successive parts are made. With this technology, the parts are literally grown from a vat of fluid-like material (e.g. resin or powder). Specifically, the parts are grown from a thin layer near a surface of the vat of fluid-like material. In this manner precise complex three dimensional patterns can be rapidly produced. This method of fabrication is extremely powerful for quickly reducing design ideas to physical form for making prototypes.

This technology typically utilizes a stereolithography apparatus, referred to as an "SLA," which generally includes a laser and scanner, a photopolymer vat, an elevator, and a controlling computer. The SLA is programmed to automatically make a three-dimensional part by forming it as a sequence of built-up cross-sectional layers.

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 computer aided design and manufacture (CAD/CAM). However, such systems have presented challenges relating to structural stress, shrinkage, curl and other distortions, as well as resolution, speed, accuracy and difficulties in producing certain object shapes.

RELATED PATENTS AND APPLICATIONS

The following U.S. patents, U.S. patent applications, and PCT patent application are incorporated herein by this reference as though fully set forth herein:

______________________________________ Pat. App. Status ______________________________________ U.S. 06/638,905 U.S. Pat. No. 4,575,330 U.S. 07/429,435 U.S. Pat. No. 5,130,064 U.S. 07/331,644 U.S. Pat. No. 5,184,307 U.S. 07/269,801 Abandoned U.S. 07/182,830 U.S. Pat. No. 5,059,359 U.S. 07/415,134 Abandoned U.S. 07/339,246 U.S. Pat. No. 5,104,592 U.S. 07/182,823 Abandoned U.S. 07/182,801 U.S. Pat. No. 4,999,143 U.S. 07/183,015 U.S. Pat. No. 5,015,424 U.S. 07/182,012 Abandoned U.S.
07/268,428 Abandoned U.S. 07/429,911 U.S. Pat. No. 5,182,056 U.S. 07/415,168 Abandoned U.S. 07/265,039 Abandoned U.S. 07/249,399 Abandoned PCT/US89/04096 WO 90/03255 U.S. 07/429,301 Abandoned ______________________________________

U.S. patent application Ser. No. 07/429,435, now U.S. Pat. No. 5,130,064, describes some main features of the present invention. U.S. patent application Ser. No. 07/331,644, now U.S. Pat. No. 5,184,307, describes in great detail the presently preferred stereolithographic apparatus, as well as various methods to form parts therewith. This application is incorporated herein by reference, including its appendices, as though fully set forth herein to facilitate handling due to its relatively lengthy disclosure. Two reference manuals, The SLA-250 User Reference Manual and The SLA-500 Reference Manual are hereby incorporated into this disclosure by reference as though fully set forth herein. These manuals accompanied U.S. patent application Ser. No. 07/429,435, as Appendices B and C respectively.

U.S. Pat. No. 4,575,330 to Hull discusses stereolithography in general. It teaches complete polymerization of each cross-section in the formation of a stereolithographically-formed object.

U.S. patent application Ser. No. 07/415,134, now abandoned, describes off-absorption-peak wavelength post curing of parts which were formed based on the primary approach to building stereolithographic parts.

U.S. Patent Application Ser. No. 07/339,246, now U.S. Pat. No. 5,104,592, describes several methods of reducing curl distortion.

U.S. Pat. No. 4,999,143 describes the use of web supports to support and minimize curl in a part being formed.

U.S. Pat. No. 5,015,424 describes the use of "smalleys" to minimize curl.

U.S. patent application Ser. No. 07/429,911, now U.S. Pat. No. 5,182,056, describes the use of multiple penetration depths in the stereolithographic process, along with the use of beam profile characteristics in combination with resin parameters to predict various cure parameters associated with the creation of stereolithographic parts. This application also describes the role of beam profile information in the creation of skin fill and discusses various multiple wavelength curing methods for reducing part distortion.

U.S. patent application Ser. Nos. 07/415,168, 07/268,428, and 07/183,012 all of which are now abandoned, disclose various methods of finishing a stereolithographic part surface to smooth out discontinuities in a post-processing step.

U.S. patent application Ser. No. 07/429,301 discusses post-processing techniques.

In the normal practice of stereolithography, objects or "parts" are built on a layer-by-layer basis, where each layer represents a thin cross-section of the part to be formed. Initial approaches to stereolithographic part building were based on the complete filling (e.g. substantial polymerization of all regions of a cross-section to a thickness at least as deep as the layer thickness) of layers. This filling was either done by the scanning of a pencil of light, by a focused or defocused pencil of light, or by flood exposure of an appropriate cross-sectional image. The pencil of light approach strictly used complete filling of cross-sections based on the scanning of adjacent overlapping vectors until the entire cross-sectional pattern was cured. These initial approaches suffered from several drawbacks, including distortion, curl, inaccurate sizing, lack of structural integrity, and lack of uniformity in down-facing surface appearance.

Later stereolithographic techniques used an internal lattice of partially cured building material ("cross-hatch" or "hatch") in place of completely filling the successive cross-sections. The internal structures primarily consisted of cross-hatch separated by untransformed building material (e.g. liquid photopolymer or the like). In this approach, the outer and inner edges of each layer are solidified by scanning of what are called "boundary vectors" (also termed, "boundaries" or "border vectors" or "borders"). These vectors separate the interior solid regions of a cross-section from exterior untransformed building material. Cross-sections or portions of cross-sections that bound external regions of the part are completely filled with skin fill (termed "fill" or "skin") after being cross-hatched. The hatch insured adequate support for the "skin" as it is being created, thereby minimizing distortion.

The skin, crosshatch, and borders trap untransformed building material (e.g. liquid photopolymer) internally in the part structure and hold it in place while the part is being created. The trapped untransformed building material (e.g. liquid photopolymer) and at least partially transformed building material (e.g. at least partially cured polymer) which forms the boundaries, hatch, and skin are brought to full transformation (e.g. polymerization) in a later process known as "post curing". For additional information on post-curing, see U.S. patent application Ser. No. 07/415,134, now abandoned.

Fairly extensive post-curing can be required when the internal cross-hatch lattice only defines discrete x-z and y-z, planes, or the like, which are separated from each other by more than the width cured by a beam, as in such cases long vertical corridors of unpolymerized material remain substantially uncured until post-processing. It is an object of the invention to provide a method of reducing or eliminating post-processing time and associated distortions while increasing structural integrity of the stereolithographically formed part.

Stereolithographic building techniques have upon occasion resulted in down-facing features having a "wafflish" appearance and texture. This appearance and texture are due to inappropriate curing techniques being used on regions of layers that contain down-facing features. When down-facing features are given both hatch and skin fill, there can be overexposure of the regions where the hatch and fill coincide. Similarly, overexposure can occur at the points of intersection of cross-hatch vectors. In the past, it has been possible to ignore the requirement of uniform cure depth for down-facing features, since other accuracy-related errors overshadowed this effect. However, as the stereolithography art strives for and attains increasingly higher levels of accuracy, imperfections such as these can no longer be overlooked. It is an object of the invention to correct these imperfections in combination with improved building techniques.

It is also an object of the invention to obtain accurate skin thicknesses without the need of periodically building test parts and without the need of being concerned with energy distribution in the beam (beam profile). Traditionally, the methods used to estimate skin depth were only guesses that had remote connection to actual experimental data or theoretical expectations. The actual skin thicknesses obtained by these traditional approaches were strongly dependent upon beam profile characteristics, skin vector spacing, drawing speed, and resin characteristics. However, these parameters were not coordinated to yield a particular skin thickness. For example, skin thicknesses intended to be 20 mils could easily range from 15 to 25
mils. In the past, this type of thickness range has been tolerated, but as the art of stereolithography advances, there is an increasing need for more accurate and less cumbersome methods of predicting the required exposure to obtain a desired skin thickness.

SUMMARY OF THE INVENTION

To these ends, a stereolithographic method comprises the steps of constructing stacked layers to form an object having external boundaries, internal cross hatch, and skinned up- and down-facing features. Skin fill is provided in less than all regions of the stacked layers, but in association with more than the up- and down-facing features of the object.

To these ends, all cross-sectional layers may be provided with skin fill and crosshatch.

According to another aspect of the invention, a method provides all cross-sectional layers with boundaries and unidirectional fill or multidirectional fill and no cross hatch.

According to another aspect of the invention, a method provides boundaries appropriate to each cross-section and provides cross-sections with at least two types of non-parallel hatch vectors wherein effective adhesion (that capable of transmitting significant curl) only occurs at the overlapping points between the vectors of the two hatch types. Additionally the hatch vectors of each type are spaced as close together as possible without being spaced so close that they can induce curl into adjacent vectors or have curl induced in them by adjacent vectors.

According to another aspect of the invention, a method comprises the steps of providing boundaries appropriate to each cross-section and providing cross-sections with at least two types of non-parallel hatch vectors that are offset from their corresponding types on the previous layer wherein effective adhesion between cross-sections (that capable of transmitting significant curl) only occurs near the overlapping points between the vectors of the two hatch types of the present layer. Additionally, the hatch vectors of each type are spaced as close together as possible without being spaced so close that they can induce curl into or have curl induced in them by adjacent vectors or transmit curl from one vector to another vector.

According to another aspect of the invention, each region of a cross-section that is internal to the object (i.e. not forming a down-facing or up-facing region), is cured in the form of point exposures, "bullets". The point exposures on successive layers are offset one from another, and the bullets are cured to approximately a depth of one layer thickness. The spacing of the point exposures on a single layer are as close together as is reasonable without the material cured in association with each bullet affecting the material cured in association with adjacent bullets. In this approach the up-facing and down-facing features may be formed by a variety of techniques.

According to yet another aspect of the invention, each region of a cross-section that is internal to the object (i.e. not forming a down-facing or up-facing region), is cured in the form of point exposures, "bullets", wherein the bullets are cured to a depth substantially equal to two layer thicknesses, and wherein bullets on successive layers are offset one from another. The positioning pattern is repeated every other layer, and the spacing of the bullets on each cross-section is such that their cured separation is greater than zero but less than their cure width one layer thickness below their upper surface. The bullets cured on the present layer substantially fill in the gaps left in the previous cross-section when it was formed. In this approach the up-facing and down-facing features may be formed by a variety of techniques.

According to another aspect of the invention, each region of a cross-section that is internal to the object and exists on the present cross-section as well as the N-1 succeeding cross-sections, is cured in the form of point exposures, "bullets". Each cross-section is divided into a pattern of slightly overlapping bullets. These bullets are divided up into N groups where the bullets associated with successive groups are used to expose material in association with the successive cross-sections beginning with the present cross-section. Each bullet is cured to a depth substantially equal to N layer thicknesses. In this approach, regions of cross-sections within N-2 layers of a down-facing feature are handled by modified techniques similar to those described above, and the up-facing and down-facing features may be formed by a variety of techniques.

According to yet another aspect of the invention, at least up- and down- facing features are provided with skin fill that is created by scanning in a first pass using nonconsecutive fill vectors, followed by scanning in at least one additional pass that completes the exposing process by filling in between the originally drawn vectors.

In another aspect of the invention, regions of intersecting vectors at least in down-facing surfaces are determined, and exposure of one or more of the respective intersecting vectors at these intersecting regions is reduced, such that at least the down-facing features have a uniform exposure.

In another aspect of the invention, a region that contains a combination of hatch and fill vectors is created and cured to a uniform depth. The creation of this region comprises the steps of creating the desired hatch vectors, and then creating corresponding skin fill types that do not contribute to additional exposure of the regions of their corresponding hatch vectors.

In another aspect of the invention, an improved stereolithographic method comprises determining necessary exposure and vector spacing and scanning parameters in order to obtain a known thickness of skin fill.

In still another aspect of the invention, a stereolithographic method comprises the steps of constructing stacked layers to form an object having external boundaries, internal cross hatch, and skinned up- and down-facing features. Specifically, this method comprises the steps of: (a) selecting layers to be provided with skinned surfaces; (b) providing means for calculating the amount of total exposure required to obtain skin curing of a preselected depth at the layers selected to have skinned surfaces; (c) providing means for determining the number of vectors that will be exposing each region in the layers, and (d) providing means for at least partially transforming (e.g. polymerizing) the layers by exposing them first to boundary vectors, then to hatch vectors, and then to skin vectors, each vector providing an exposure sufficient to cure to the preselected depth calculated in step (b), divided by the number of other vectors that will intersect the vector at a given region as determined in step (c).

According to other aspects of the invention, these improvements are used in combination with one another and/or in combination with curl reduction techniques as described in: U.S. patent application Ser. No. 07/339,246, now U.S. Pat. No.
5,104,592; U.S. patent application Ser. No. 07/183,015, now U.S. Pat. No. 5,015,424; U.S. patent application Ser. No. 07/182,801, now U.S. Pat. No. 4,999,143 and the other applications cited previously all of which are fully incorporated herein by reference. For example, according to yet another aspect of the invention, an improved stereolithographic method is disclosed comprising the combined use of hatch with nonconsecutive skin fill in more than the up- and down- facing features. As another example, an improved stereolithographic method is disclosed comprising the method of reducing exposure where vectors intersect and providing discontinuities in skin fill to avoid multiple vector exposure in regions where hatch vectors have been provided.

Other aspects of the invention, together with objects and attendant advantages of the various embodiments, will best be understood from an examination of the drawings along with the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURES 1a-1d collectively show a top view of a layer showing boundaries, hatch and skin without compensation for multiple exposures of building material at the various regions of the cross-section. FIGURES 1a-1d represent respectively: a) boundaries only; b) crosshatch only; c) skin only; and d) combined vectors.

FIGS. 2a-2d collectively show a side view of FIGURE 1d as intersected by various vertical planes. FIGS. 2a-2d represent, respectively: a) repeat of FIGURE 1d with vertical planes; b) a view of the edge of the layer along plane 1 showing the various depths obtained in different regions; c) a view of the edge of the layer along plane 2 showing the various depths obtained in different regions; and d) a view of the edge of the layer along plane 3 showing the various depths obtained in different regions.

FIGS. 3a-3e collectively illustrate a top view of a layer showing boundaries, hatch and skin as created by a presently preferred technique. FIGS. 3a-3e represent respectively: a) boundaries only; b) crosshatch only; c) skin type 1; d) skin type
2; and e) combined vectors.

FIGS. 4a-4d collectively show a side view of FIG. 3e as intersected by three different vertical planes. FIGS. 4a-4d represent, respectively: a) repeat of FIG. 3e with vertical planes; b) a view of the edge of the layer along plane 1 showing the various depths obtained in different regions; c) a view of the edge of the layer along plane 2 showing the various depths obtained in different regions; and d) a view of the edge of the layer along plane 3 showing the various depths obtained in different regions.

FIGS. 5a and 5b illustrate the profiles of a cured "string," corresponding to the cure produced by a single vector.

FIGS. 6a-6d show down-facing surface profiles of parts made in accordance with Example IV below.

FIGS. 7a-7c show a comparison between traditional vector ordering techniques and examples of various vector ordering techniques of some of the preferred embodiments of the present invention. FIG. 7a depicts a top view of a cross-section of an object showing a consecutive drawing order for the vectors. FIG. 7b depicts the same cross-section but filled using a non-consecutive order that fills the cross-section in two passes. FIG. 7c depicts the same cross-section but filled using a non-consecutive order that fills the cross-section in three passes.

FIGS. 8a-8i collectively illustrate the vectors used and cure depths obtained, for a sample cross-section, by utilizing the "weave" building embodiment. FIGS. 8a, 8c, and 8e respectively represent top views of boundary vectors, X hatch vectors, and Y hatch vectors. FIGS. 8b, 8d, and 8f depict side views of the material cured in association with FIGS. 8a, 8c, and 8e respectively. FIG. 8g represents a top view of the combination of material cured by the individual vector types of FIGS. 8a, 8c, and 8f. FIGS. 8h and 8i represent side view of the cure depths associated with two different vertical planes intersecting the cross-section of FIG. 8g.

FIGS. 9a and 9b collectively show a side view indicating the difference between stacking one direction of crosshatch on top of each other from layer to layer and staggering the hatch from layer to layer. FIG. 9a depicts a side view of hatch being stacked on top of each other from layer to layer. FIG. 9b depicts a side view of hatch being staggered from layer to layer.

FIGS. 10a-10c collectively show the configuration of bullets cured in association with practicing the seventh embodiment of the present invention. FIG. 10a depicts a top view of the boundary and bullets cured on a first cross-section. FIG. 10b depicts a top view of the boundary and bullets cured on a second cross-section. Comparison of FIGS. 10a and 10b indicate that the bullets are staggered from the first to the second layer. FIG. 10c depicts a side view of the boundaries and bullets of five cross sections stacked one on top of the other.

FIG. 11 illustrates a side view of the overlapping nature of the bullets formed in practicing the eighth embodiment of the present invention.

FIG. 12 depicts the three-dimensional object used in Example 1 to test the improvements of "skintinuous" building.

FIG. 13 depicts a top view of the uppermost cross-section of the part of FIG. 12, along with indications as to what measurements were made on the part; and

FIGS. 14a and 14b collectively depict a side view of a CAD designed object and the object as reproduced according to the teaching of the first preferred embodiment of the present invention. FIG. 14a depicts the CAD designed object, and FIG. 14b depicts the reproduction.

FIGS. 15a, 15b, and 15c collectively depict the part used in the experiment of Example VI. FIG. 15a depicts a three-dimensional view of the part. FIG. 15b depicts a top view of the part. FIG. 15c depicts an exaggerated top view of the distortion of the part after post curing.

FIGS. 16a, 16b, and 16c collectively depict sample cross-sections of an object for the purpose of distinguishing up-facing and down-facing features of an object and the relationship of such features to subregions of each cross-section. FIG. 16a depicts a top view of the bounded and unbounded regions of a single sample cross-section. FIG. 16b depicts a side view of three sample cross-sections of an object. FIG. 16b' depicts a side view of the subregions of the middle cross-section.

FIGS. 17a-g illustrate various patterns and shapes for tiling and the weak bending axes, if any, associated with each pattern/shape combination.

FIGS. 18a and 18b are top views of vectors scanned respectively according to a first pass of an offset weave square tiled exposure pattern and according to both passes of an offset weave square tiled exposure pattern.

FIGS. 19a, 19b, and 19c are top views of vectors scanned respectively according to a first pass of an offset-weave hexagonal-tiled exposure pattern and according to both passes of an offset-weave hexagonal-tiled exposure pattern and according to both passes of an alternative offset-weave hexagonal-tiled exposure pattern.

FIG. 20 depicts the currently utilized conventional scanning order for fill and hatch vectors.

FIGS. 21a to 21d depict respectively each of the scanning patterns of a four-layer sequence of cross-sections utilizing the drawing order of alternate sequencing example #2.

FIGS. 22a to 22h depict respectively each of the scanning patterns for an eight-layer sequence of cross-sections utilizing the drawing order of alternate sequencing example #3.

FIGS. 23a, 23b, and 23c depict the gaps between tiling on a first layer being filled in on a second layer with slightly offset tiles followed by grouting between the offset tiles.

FIG. 24 depicts closing gaps between tiles on a first layer with an at least partially floating member on a second layer which floating portion is riveted to an adjacent tile on the first layer.

FIGS. 25a to 25g depict various orientations of vectors n and n+1 along with various virtual scanning paths that can be followed to scan from vector n to vector n+1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The instant invention addresses alone or in combination four improvements in stereolithographic methods. These are, first: methods of increasing structural integrity while reducing the need for post-curing; second: methods of obtaining uniform exposure in regions of intersecting vectors of different types; third: methods of determining cure depth; and fourth: methods intended to reduce distortion due to shrinkage, curl, and post cure. Although these three aspects of the invention are closely inter-related and are often cross-dependent, they will be addressed in sequence in this detailed description, and will also be illustrated in the examples below.

Definitions

"Beam profiles" represent the energy distribution of irradiation in a beam of ultraviolet light or the like, used to cure photopolymer or other curable material in accordance with stereolithography practices.

"Building materials" are materials that can be used in the present invention for forming three-dimensional parts. The acceptable building materials are materials that can transform from one state to another state in response to exposure to synergistic stimulation. The two states are then separable after exposure of a single layer to synergistic stimulation or separable after completion of a plurality of layers. The most preferred materials are materials that transform from a fluid-like state to a cohesive state or solid state. These materials include liquid photopolymers, sinterable powders, bindable powders or the like. Prior to exposure to appropriate synergistic stimulation (e.g. IR radiation from a Carbon Dioxide laser or the like) the sinterable powders are in a fluid-like state since the powder particles can flow past one another whereas after sintering the powder particles are joined to form a cohesive mass. Similarly the bindable powders are in a fluid-like state prior to exposure to appropriate synergistic stimulation (e.g. a chemical binder dispensed into the powder in a selective and controlled manner) whereas after exposure the binder sets and the powder (and binder) form a cohesive mass. The most preferred of the above materials for the present invention are the photopolymer type materials. Other acceptable materials include relatively solid sheets of material that are transformable from one state to another. These sheet like materials include "dry film" type photopolymer materials that can be solidified upon exposure to appropriate synergistic stimulation wherein after exposure the exposed and unexposed materials can be separated by differential solubility in an appropriate solvent.

"Bullets" are volumes of a building material which are solidified in response to a beam of synergistic stimulation exposing the material in substantially single non-overlapping point irradiations. The usual shape of the cured material is similar to that of a bullet. FIG. 5b depicts a cross-sectional view of a line or vector of material cured by a beam of radiation. It can equally well be interpreted as depicting a two dimensional view of a bullet, wherein the three-dimensional bullet would be the volume of revolution formed by rotating the object about a vertical axis through its center.

"Effective Cure Width" (ECW) is a distance equal to twice the closest possible spacing of two vectors from one another that will render a given individual cure depth (i.e., a cure depth associated with each vector) without measurably increasing the cure depth of the combination. For the preferred beam profiles and cures, the Effective Cure Width (ECW) is always less than the Maximum Cure Width (MCW) (i.e., the width of the solidified string at the building material surface), such that different lines of solidified material can be adhered without an increase in cure depth. For example, in FIG. 5b, the horizontal separation between lines 118 and 120 might represent the ECW for string 100. Typically, one half the ECW represents the closest point that a similar line of material can approach string 100 without measurably increasing its maximum cure depth. More generally, the ECW is a zone that surrounds the center line of a string, such as string 100, that represents the closest position that another string (of arbitrary thickness and direction) of solidified material or set of strings of material can approach the first string without resulting in the maximum cure thickness of the combination being measurably greater than the maximum thickness of either string. As two non-parallel vectors approach an intersection point, the excess exposure point, "EEP" (the point at which the combination will cause a measurable increase in cure depth) is determined by the beam profile and angle of approach of the two vectors. If the vectors are perpendicular the excess exposure point is the 1/2 the ECW. If the vectors approach each other at a 45 degree angle the excess exposure point is at 1/2.times.1,414.times.ECW. An approximate relationship between ECW, the angle of approach, and the EEP is

where A is the angle between the vectors. A more accurate relationship can be derived from information regarding the beam profile, the depth of cure, the building material response characteristics, and the intersection direction of the vectors.

"Layers" are the incremental thicknesses between successive cross-sections into which an object is divided. These layers form the basis for the thicknesses of building material (e.g. photopolymer). They must receive sufficient exposure to synergistic stimulation (e.g. ultraviolet light or other polymerizing radiation) to transform from their fluid-like state into a cohesive structure. The layers are constructed to adhere to one another and collectively form a solidified (e.g. polymerized or partially polymerized) stereolithographically produced part.

"Maximum Cure Depth (MCD) " and "Maximum Cure Width (MCW)" refer, respectively, to the deepest and widest cure that is obtained when exposing a single line or bullet of uncured building material to synergistic stimulation. The maximum cure depth is generally what is referred to as the cure depth of boundary and hatch lines. Since a beam of light is not generally of constant intensity across its width, the cure depth and width caused by a beam tracing across a line one or more times does not produce a uniform depth and width of cure. The maximum depth of cure generally occurs near the middle of a cross-section of the trace but it can actually occur anywhere depending on the distribution of intensity in the beam. It may also depend on the direction of scanning of the beam in forming the trace. The maximum width of cure generally occurs at the top (surface) of the cured line of material. An example of the maximum depth and width of cure are depicted in FIG. 5a which shows a line (sometimes called a string) of cure material 100. Vector 102 indicates the scanning direction used in creating the string of material 100. Surface 104 represents the solidified material that was created from the fluid-like material that formed part of the surface of the curable material. FIG. 5b represents an end-on view of string 100. Line 106 indicates the position of the top of the cured string 100, while line 108 represents the bottom of cured string. The vertical distance between 106 and 108
is the maximum cure depth of string 100. Line 112 represents the left-most edge of string 100, while line 114 represents the right-most edge of string 100. The horizontal separation between 112 and 114 is the maximum cure width of string 100. Such a string 100 of solidified building material may be used for several purposes: 1) to insure adhesion between the layer associated with its creation and the preceding layer, 2) to form a down-facing feature of a part being created, or 3) as an element of a series of such strings of cured material, where the series will be used for one of the above two purposes. An up-facing feature is not included in the above since it can be fit into one of the above categories depending on the situation. For the first purpose listed above, maximum cure depth may preferably be greater than the layer thickness. The vertical separation between line 106 and line 110 represents the layer thickness in such a case. For the second purpose the MCD represents the layer thickness, and for the third purpose the vertical separation between line 106 and line 116 might represent the layer thickness since the net thickness of the cured material might increase from the segments overlapping each other.

"Overlapping" refers to two or more exposures being given to a region so that an increase in maximum cure depth occurs. Since cure profiles are not necessarily step functions, two separately exposed areas can touch and bind to one another without changing the maximum cure depth of either. When two lines are exposed beside one another their maximum widths may overlap resulting in a larger exposure in this region, and a corresponding increase in depth. But if this additional exposure does not occur in the region near the maximum cure depth of the individual lines, their combined maximum cure depth will not generally be measurably deeper than their individual maxima. Overlapping sometimes refers to situations when two side by side exposures affect the curing of each other whether or not they result in an increase in the maximum cure depth of either one. The context in which the term "overlapping" is used will generally make its meaning clear.

"Step Period" (SP) is a part-building parameter that defines the period of time between each laser step.

"Step Size" (SS) is a part-building parameter that defines the spatial size of the step moved by the laser spot on the building material surface.

"Vectors" are data that represent the length and the direction and maybe the period of irradiation (exposure) in the process of solidifying the building material in the preferred embodiment of the present invention (e.g. a scanning beam of ultraviolet radiation, on a liquid photopolymer, or other fluid-like solidifiable medium).

"Skin" vectors are horizontal surface vectors that are typically traced from one boundary to an opposing boundary at relatively high speed and with a substantial overlap between successive vectors which are generally traced in opposite directions, and typically form "skin fill" which defines at least the upper and lower horizontal exterior surfaces of a stereolithographically-formed part in traditional stereolithography and in several of the preferred embodiments of the present invention. Typically, skin vector spacing is from about 1 to about 4 mils with a maximum cure width of a single exposed skin vector being about 14 to 15 mils. Of course, these exemplary and illustrative parameters can be varied as needed based upon such considerations as the desired smoothness of the layers, the power of the irradiating source (e.g., laser), the possible speed range of the irradiating source (i.e., the maximum drawing speed), the layer thickness desired, and the number of vectors that are desired to be stored. According to certain aspects of this invention, however, skin fill is provided in more than the exterior surfaces of the part. According to other aspects of the invention, skin vectors can be drawn non-consecutively and/or nonoverlapping (e.g., a first pass at 7-8 mil intervals and a subsequent pass at intervening intervals). These aspects and others are described in detail below.

"Boundary" vectors are traced to define the vertical exterior surfaces of the stereolithographically-formed part (therefore to define the range of each cross-section). These vectors generally are scanned more slowly than skin vectors such that a greater cure depth is obtained. Boundaries, unlike skin fill, generally do not rely on overlapping offset passes to attain their full cure depth. In situations where regions on a given layer overlap regions of the previously formed layer (non-down-facing regions), it is preferred that the cure depth exceed the layer thickness, so that improved adhesion between layers results. In regions of down facing features, it is preferred that net cure depth be substantially equal to the layer thickness.

"Hatch" vectors are similar to boundary vectors, except that they are traced in a substantially uniform, criss-cross type pattern, to define the internal lattice structure of the stereolithographically-formed part. Again, it is preferred that the cure depth exceed the layer thickness, if being drawn in a non-down-facing region, so that improved adhesion between layers results. If being drawn in a down-facing region, then layer thickness cure depth is preferred. In several preferred embodiments of the present invention, adhesion between layers is obtained by the extra cure depth that is obtained from the intersections of two or more crosshatch vectors wherein the cure depth of the individual hatch lines is insufficient to cause curl inducing adhesion between the layers.

"Skintinuous" in general terms describes any building technique which generates a substantially solid fill pattern over a substantial portion of the cross-sectional area of a part.

"Multipass" refers to a drawing technique which utilizes more than one pass to expose a region (e.g. a line) so that the building material is substantially reacted before direct adhesion with surrounding structures takes place. The purpose of this method is to minimize pulling forces between layers and therefore to reduce curl.

"Interrupted scan" or "Bricking" refers to scanning a vector with recurring gaps to relieve transmitted stress.

"Tiling" is an interrupted scanning technique that applies to relatively wide regions as opposed to vectors (the wide regions may be made up of vectors). This scanning results in distinct shapes which fit together very snugly but are not adhered to each other. The intention of this method is to maximize percentage of build process curing while reducing transmitted stresses that generate curl.

"Log Jam" refers to a scanning technique where some internal hatch (or fill) vectors are retracted from the layer borders to avoid adhesion, wherein after exposure of the hatch or fill an offset border or the like is scanned to attach the hatch and original border.

"Quilting" refers to a drawing technique which first partitions each layer into patches by scanning a relatively large crosshatch structure. Each patch is then treated as an individual region to be scanned. This method relieves problems that can arise when drawing relatively large regions with floating material techniques (e.g. log jam).

"Strongarm" refers to a scanning technique wherein a downfacing region is given extra exposure to make it extra rigid thereby increasing its ability to resist distortion caused by adhesion with material from the next higher layer.

"Weave" generally refers to any drawing pattern which generates a near solid fill pattern, wherein vectors on the first pass (threads) are spaced slightly further apart than the maximum cure width (MCW) and have exposures that are less than that necessary for adhesion (i.e. under cured). Adhesion is obtained on a second pass or higher order pass by cumulative exposure resulting at the intersecting regions of threads. These intersecting regions are sometimes called stitches.

"Interlace" is a particular type of "non-consecutive scanning" wherein every other vector is scanned on a first pass of a region and the other vectors are scanned on a second pass.

"Staggered" refers to a building method where different drawing patterns are used on alternating layers. For example staggered hatch refers to offsetting or shifting the hatch vectors on every other layer so that the hatch vectors on adjacent layers do not overlay each other. The intended purpose of this method is to produce a more homogeneous structure and possibly to reduce curl in some instances.

"Smalleys" refer to a building technique where holes or gaps are placed at critical locations on a given cross-section (generally implemented through the CAD design but they can be implemented from a Slice type program onto individual cross-sections). They reduce curling by interrupting the propagation of stresses from one region of a layer to another region of the layer.

"Riveting" or "Stitching" refers to an exposure technique that applies different levels of exposure to a given layer, wherein some of the exposures are less than that necessary for adhesion and some of the exposures are sufficient to cause adhesion thereby creating discrete locations of adhesion which might resemble rivets.

"Webs" are support structures that are not a portion of a desired final reproduction of a CAD designed object but they are formed along with the object by the stereolithography apparatus to give support to various features of the object and to allow easy separation of the object from the building platform.

"Up-facing and Down-facing Features of an Object" are regions or subregions on particular cross-sections that represent an upper or lower extent of the object.

Each cross-section is formed from a combination of bounded and unbounded regions. Bounded regions are those that form a portion of the solid structure of an object (regardless of whether the region is formed as a completely solidified region or as a cross-hatched region). Unbounded regions are those that form an empty or hollow portion of an object. These concepts are depicted in the example of FIG. 16a. FIG. 16a depicts a top view of a sample cross-section of an object. This sample cross-section can be divided into three bounded regions and two unbounded regions. Boundary 700 bounds region 705, boundary 710 bounds region 715, and boundary 720 and 725 bound region 730. Regions 735 and 740 are unbounded regions.

Each of the bounded regions of a cross-section may be divided into subregions, which are determined by relationships between bounded regions on a given cross-section and bounded and unbounded regions on the two adjacent (one higher and one lower) cross-sections. Up-facing regions of cross-section "i" are those bounded subregions of cross-section "i" that are underneath unbounded subregions of cross-section "i+1". The down-facing subregions of cross-section "i" are the bounded subregions of cross-section "i" which overlay unbounded subregions of cross-section "i-1". Some subregions may represent both up-facing and down-facing features; in this case the subregion is generally considered to be a down-facing subregion since appropriate curing of down-facing features is generally more critical than curing of up-facing features. This concept is depicted in the example of FIG. 16b and 16c. Cross-section "i" 750 is above cross-section "i-1" 755 and is below cross-section "i+1" 760. FIG. 16c is a repeat of FIG. 16b but with cross-section "i", 750, divided into subregions. The up-facing bounded subregions of cross-section "i" are labeled as 761, 764 and 768. The down-facing bounded subregions of cross-section "i" are 761, 762, and 769. The bounded subregions that are neither up-facing nor down-facing are 763, 765, and 767. The unbounded regions of cross-section "i" are subregions 766 and 770. It can be seen that subregion 761 is both up-facing and down-facing and thus it would generally be processed as a down-facing feature. If a cross-section "j" is above a completely unbounded cross-section, then all of cross-section "j" is a down-facing feature (e.g., the bottom of the part). If a cross-section "j" is below a completely unbounded cross-section, cross-section "j" is an up-facing feature (e.g. the top of the part).

Other definitions can be obtained as needed from remaining disclosure and the manuals attached as Appendices B and C to U.S. patent application Ser. No. 07/429,435, now U.S. Pat. No. 5,130,064 incorporated herein by reference. Moreover, the specifications of the SLA hardware, the resin and laser types, and the generally preferred parameters with respect to the stereolithographic processes described and improved upon herein are set forth in those Appendices.

Preferred Methods of Obtaining Improved Structural Integrity

Several preferred embodiments of this invention relate to methods of obtaining improved structural integrity, lower post cure distortion, lower overall horizontal distortion, and in many cases overall lower vertical distortion (e.g. vertical curl) by effectively providing skin on more than just the up- and downfacing surfaces of the part being formed. For example, the effect of providing skin at only the up- and down-facing surfaces, and supplying cross hatch in x-z (X hatch) and y-z (Y hatch) planes, is to create an internal structure consisting essentially of relatively long columns of substantially untransformed material trapped by at least partially transformed crosshatch and boundary material on the sides and skin on the up-facing and down-facing surfaces. Accordingly, a leak in any portion of a down-facing or up-facing skin or cross hatch would have the potential to cause distortion and unwanted drainage of untransformed building material. However, if skin is provided in the x-y (horizontal) plane, at more than the up- and down-facing surfaces, then the compartments of untransformed material trapped by cross-hatch, boundary, and skin would be much smaller and better-contained. Other advantages emanating from providing additional skinned surfaces within the internal structure of the part can include improved structural integrity, less distortion during formation, reduced post-curing times, and reduced postcure distortion. Additionally, surface finishing can be performed before post-curing, and in some circumstances, post curing can be completely avoided. There are various preferred embodiments that employ different approaches in obtaining this additional fill.

A first group of embodiments utilize exposures analogous to traditional skin filling techniques in that the fill is generated by a series of overlapping exposures. These embodiments may or may not employ the use of what is traditionally known as cross-hatch and fill in the same region of a cross-section.

In a first preferred embodiment an object is formed on a layer by layer basis initially by the exposure of building material to boundary vectors on a cross-section, followed by exposure of crosshatch vectors on the cross-section, and finally followed by exposure of skin fill vectors on any up-facing and down-facing regions on the cross-section. Additionally, on periodic or random (with a certain probability of occurrence) cross-sections even in non-down-facing and in non-up-facing regions skin fill vectors are provided and exposed. For example, at every 1/2 inch vertical interval through the part, which at 10 mil layers corresponds to every fifty layers, skin vectors are generated that provide for skinning of the entire cross-section. These skin vectors are provided in a form in which areas that are down facing can be distinguished from areas that are not down-facing so that different cure parameters can be used if necessary. It is possible to distinguish other regions but it has been found unnecessary to do so. The advantages of this approach have been previously described.

Of course other vertical spacings of skin fill are possible including geometry selective spacing. That is some geometric features may be better handled by one spacing of skins while others require a different spacing of skins. In this embodiment the boundary vectors and crosshatch vectors that are used to achieve adhesion between layers are generally given some overcure to insure adequate adhesion. However, the skin vectors that are used in non-downfacing regions can be given a cure depth that is less than, equal to, or greater than the layer thickness. It has generally been found that a skin depth greater than the layer thickness causes excessive curl and therefore isn't optimal. The skin vectors (combined with all other vector types) in a down facing region are, on the other hand given, only a one layer thickness cure depth. This embodiment can be combined in all, or in part, with the uniform skin thickness methods to be described hereinafter.

This method of building can be substantially implemented by Slicing the desired CAD object file, or the like, twice and then editing and Merging the resulting .sli files together. The first Slice is done with normal Slicing parameters. For example by using X and 60/120 crosshatch with a 50 mil spacing and using X skin fill with a 3 mil spacing. The second Slice is done without the use of skin fill but with the use of closely spaced cross-hatch (which will function as skin fill) of type and spacing equivalent to the skin spacing of the first Slice. For example, continuing with the previous example, the second Slice would be done with the same layer thickness but with only X type cross-hatch spaced at 3 mils. After creation of the second Slice file, it is edited by hand or by a program that can go in and remove the skinning cross-hatch associated with the cross-sections not using fill in the non-down-facing and non-up-facing regions. Next the two files are Merged together using merging options that keep all the vectors from the first Slice and that keep only the remaining X layer crosshatch vectors from the second Slice (all other vector types are removed including near-flat down-facing crosshatch). These hatch and fill vectors are still distinguished by block headers that indicate which Merge object they came from. Therefore the combined file can be built as a single object.

One must be sure to give the proper exposure values to each vector type. Therefore, the hatch vectors from the second slice object are given associated exposure values equivalent to skin fill. This procedure will produce an object substantially like that described above. However, there are several differences between this implementation and that desired. First the regions of down-facing features and up-facing features might be given a double exposure (and therefore extra undesired cure depth) depending on whether the crosshatch from the second Slice of the object is still included in the combined file or not. Second, since the present Slice program doesn't generally separate non-down facing hatch from down-facing hatch (except in the near-flat regions), there will be an additional cure in the down-facing regions since the crosshatch must be overcured somewhat to insure adhesion between cross-sections.

FIGS. 14a and 14b depict a side view of an object built according to the techniques of this first embodiment. FIG. 14a represents a side view of the CAD designed object. The dotted regions indicate solid regions. FIG. 14b represents a side view of the object as built according this first preferred embodiment wherein every third layer is skinned to help increase the structural integrity of the object. The regions labeled with forward slashes, "/" indicated regions that are skinned because, they are down-facing. The regions labeled with back slashes, ".backslash." indicate regions that are skinned because they are up-facing. The regions that are labeled with X's indicate regions that are to be skinned according to the teaching of the present embodiment that would not otherwise be skinned. Layers 1, 4, 7, and 10 are to be skinned according to this embodiment.

In a second preferred embodiment an object is built by providing and exposing boundary vectors on each layer, crosshatch on each layer and skin fill vectors on each portion of each layer. As with the previous embodiment and the following embodiments, this second embodiment is not restricted to part building with the use of vector data. The vector data is simply used as an implementation of the concepts of the invention and other methods of implementation could be used. Certain concepts of the invention deal with amount of solidification on each cross-section and/or the order of material solidification on each cross-section and/or the depth of solidification of each region on each cross-section. This second embodiment is similar to the first described embodiment except that now skin-fill is supplied on every region of every cross-section not just with down-facing features, up-facing features, and with periodic cross-sections. This second embodiment therefore results in green parts that have little or no substantially untransformed material trapped internal to their boundaries. There will be no substantially untransformed material if the effective skin depth thickness is equal to or greater than the layer thickness. There will be, to a greater or lesser extent, some substantially untransformed material if the effective skin cure depth is less than the layer thickness. As with the previous embodiment, it is desired to get some net overcure between regions of a cross-section that overlap with regions of the previous cross-section to insure adequate adhesion, but it is desired in down-facing regions that the net cure depth be uniform and be of only a one layer thickness depth. It has been found that with embodiments like the present one, where substantially all material on each cross-section is substantially transformed, that vertical curl can and generally does go up significantly but that horizontal distortion goes down significantly. It is known that the amount of curl (both horizontal and vertical) can vary tremendously depending on the amount of overcure between layers; the amount of overcure between adjacent lines on the same cross-section; the extent of the area over which the overcure takes place; the thickness of the layers; and the order of intercross-sectional solidification as well as the order of intracross-sectional solidification. If parts are to be built that contain few unsupported critical features, or in which the unsupported features can be supported by Webs, the direct application of this embodiment can lead to substantial improvements in part accuracy.

If the part to be built does contain critical regions that cannot be well supported then modifications to this embodiment can be helpful in reducing the vertical "curl" type distortion that may result. These modifications might include the use of the techniques of this embodiment (or "continuous skinning" or "skintinuous") on only the regions that are or can be adequately supported and continuing to use the standard building methods of boundaries, widely spaced crosshatch, down-facing feature skinning routines, and up-facing feature skinning routines on the other regions of the part. "Strongarm" building techniques can be effectively used in these other regions of the part. The result of this modified approach would be substantially increased horizontal accuracy in the supported regions with no sacrifice in vertical accuracy in the unsupported regions.

Other modifications, to avoid increased vertical distortion include the use of Smalleys, described in U.S. patent application Ser. No. 07/183,015, now U.S. Pat. No. 5,015,424; the use of Multipass drawing techniques, described in U.S. patent application Ser. No. 07/339,246, now U.S. Pat. No. 5,104,592 the use of Rivet type layer to layer adhesion techniques, described in U.S. patent application Ser. No. 07/339,246, now U.S. Pat. No. 5,104,592; the use of "strongarm", "log jam" and "quilting"; as well as other techniques to be described hereinafter; and similar techniques; and combinations thereof.

As with the previously described first embodiment method of building, this method of building can be substantially implemented by Slicing the desired CAD object file, or the like, twice and then Merging the files together. The first Slice is done with relatively normal Slicing parameters, except no skin fill is used. One example is by using X and 60/120 crosshatch with a 50 mil spacing. The second Slice is done, again, without the use of skin fill but with the use of closely spaced crosshatch of type and spacing equivalent to that which is desired for forming skin fill on each layer. For example the second Slice may be done with the same layer thickness but with only X type crosshatch spaced at 3 mil. After creation of the second Slice file, the two files are Merged together using Merging options that keep all the vectors from the first Slice (except any skin fill vectors that were used) and that keep only the X crosshatch (including near-flat down-facing crosshatch) from the second Slice file (all other vector types are removed). The hatch-from the first Slice and the fill vectors from the second Slice (actually hatch vectors from the second Slice) are still distinguished by block headers that indicate which Merge object they are from. Therefore the combined file can be built as a single object being sure to give the proper exposure values to each vector type. Therefore the hatch vectors from the second slice object are given associated exposure values equivalent to skin fill. This procedure will produce an object substantially like that described above as the preferred method of this embodiment. However, there is a difference between this implementation and the desired one described above. Since the present Slice program doesn't generally separate non-down facing hatch from down-facing hatch (except in the near-flat regions), there will probably be an additional cure in the down-facing regions since the crosshatch may need to be somewhat overcured to insure adhesion between cross-sections.

With the second embodiment and the first embodiment as well as with embodiments described hereinafter, there are many ways to use the existing commercial software or to modify the outputs from the present commercial software to at least partially implement the various embodiments. The implementations herein are only meant to be examples of such techniques.

In a third preferred embodiment each cross-section is supplied with boundaries and skin fill vectors only (this embodiment does not utilize crosshatch. In this third embodiment the boundaries may be cured to an effective depth equal to the layer thickness or greater than the layer thickness depending on whether they are to be used to obtain adhesion with the previous cross-section or whether they are to be used to form a downfacing feature. As with the previous embodiment the skin vectors in non-down-facing regions may be cured to a depth less than, equal to, or greater than the layer thickness. It has been found that if skin vectors are cured to an effective depth greater than the layer thickness, by usual curing techniques, vertical curl will be greater. Therefore, if it is desired to cure the skin vectors to such a depth it is advisable to utilize a drawing method that will help to reduce curl, such as multipass. Multipass is a method of solidifying material in at least a two step process, wherein a first exposure of material to the synergistic stimulation leads to a depth of cure less than the layer thickness and the second pass (or higher order pass) results in a net cure depth that insures adhesion. Multipass is an effective way of reducing curl. An additional enhancement of multipass scanning is described in U.S. patent application Ser. No. 07/429,911, now U.S. Pat. No. 5,182,056 regarding the use of multiple wavelengths during the multiscanning process. A short penetration depth exposure is given on the first one or more passes to insure that substantial transformation of building material occurs prior to one or more additional exposures using long penetration depths that are used to obtain and insure adequate adhesion between cross-sections.

An additional problem that might occur with this third embodiment is that of excessive horizontal curl. In the previous embodiments horizontal curl was kept to a minimum by the exposing of cross-hatch prior to the exposing of skin, wherein the crosshatch would act as a stabilizing frame upon which skin could be formed. Since this third embodiment doesn't contain cross-hatch it may be necessary to utilize a horizontal curl reduction technique also.

Such techniques include the use of nonconsecutive vector drawing, the use of non-overlapping fill vectors (e.g. "weave" which is the topic of embodiments to be described later), and the filling of non-consecutively drawn vectors by intermediate vectors (in many respects this is a horizontal version of the multipass technique described above). The non-consecutive ordering of vectors refers to a technique of supplying fill or hatch vectors with a particular spacing and then exposing the vectors in a nonconsecutive manner. In traditional stereolithography fill vectors are cured in a consecutive order.

An example illustrating the differences between consecutive ordering and non-consecutive ordering is depicted in FIG. 7. FIG. 7a illustrates a cross-section of boundary 200 and containing unidirectional fill vectors 201 to 209. In traditional stereolithography the order of drawing is from vector 201 to vector 209. The direction of scanning each of these vectors has generally been such that the amount of jumping between vectors is minimized. The odd numbered vectors have generally been drawn from left to right and the even numbered vectors have been drawn from right to left. Therefore the entire fill can be drawn with minimal jumping between the head of one vector and the tail of the next.

FIG. 7b illustrates a similar cross-section but wherein an example of nonconsecutive drawing order is used to minimize any horizontal curl that might have a tendency to occur. The cross-section is surrounded by boundary 220 and it is filled with vectors 221 to 229. The drawing order is from 221 to 229, therefore every other vector is skipped on a first pass of drawing and then the vectors skipped on the first pass are scanned on a second pass. This technique is especially useful for minimizing curl when two consecutively scanned vectors are separated by a distance so that the material cured by each vector individually doesn't connect to the material cured by the consecutively scanned vectors. Then on a second pass (or later pass) the gaps between the material exposed by the first pass are filled in by the additional pass which scans vectors intermediate to those of the first pass. If the width of cure of each vector is relatively wide compared to spacing between vectors it may be necessary to skip more than just every other vector. For example it may be necessary on a first pass to cure one vector and skip three vectors then cure another vector and skip the next three vectors, and so forth. On the second pass one may then cure the intermediate vector of each set of 3 vectors not drawn on the first pass, and then finally on a third pass the remaining unexposed vectors are scanned. This is illustrated in FIG. 7c. The boundary 240 is filled with vectors 241 to 249 wherein the scanning order is 241 to 249.

If the skin vectors will only be given an effective cure depth less than or equal to the layer thickness it will likely be necessary to supply additional exposure in the form of point rivets, or the like, on the portion of the present cross-section that overlaps the previous cross-section. The proper use of rivets will lead to adequate adhesion between layers but will also tend to keep vertical curl to a minimum. As with the deeper cure depth methods this approach may also require the use of horizontal curl reduction techniques.

As with the previously described methods of building, this method of building can be substantially implemented by a user by Slicing the desired CAD object file, or the like, a single time. The part is sliced with cross-hatch but without skin vectors. The crosshatch vectors are spaced with a separation typical for skin fill. One or more crosshatch types may be simultaneously used. For example, one can use both X and Y hatch with a spacing of four mils each. If the maximum cure width of cure for a single pass along one vector is equal to or greater than the spacing between vectors (e.g. 12 mil MCW) and one doesn't want consecutively cured vectors to effect one another then a drawing pattern similar to that described for FIG. 7c can be used for each type of crosshatch. This procedure will produce an object substantially like that which would be produced by the preferred methods of the third embodiment. However, there is a difference between this implementation and the desired one described above. Since the present Slice program doesn't generally separate non-down facing hatch and boundaries from down-facing hatch (except in the near-flat regions) and boundaries, there will probably be an additional cure in the down-facing regions since the crosshatch may be somewhat overcured to insure adhesion between cross-sections.

A fourth embodiment is similar to the third just described but it doesn't use boundary vectors. Therefore, this embodiment supplies and exposes only fill type vectors. Since there are no boundaries associated with each cross-section in this embodiment, and therefore nothing other than surface tension and viscosity to hold the vectors in place as they are drawn except where there is horizontal contact to adjacent vectors and vertical contact with the previous cross-section, the vectors of this embodiment must be drawn in a highly ordered manner. The vectors must be drawn in an order and/or to a depth that assures adequate structural support to insure that each vector stays in place until the entire cross-section is drawn. If the vectors are drawn in an improper order, then it is possible that some of them will drift out of position or be distorted out of position prior to the completion of the exposure and solidification of the cross-section. Since vertical curl generally occurs between material cured on the presently drawn cross-section and material cured on the previously drawn cross-section, vectors in this region can be drawn in a nonconsecutive order and can also be cured using two pass multipass to insure minimal curl. Subsequently, the vectors that occur in down-facing regions can be cured in a non-consecutive interlaced manner with vectors from the other hatch types. For example, one or more nonconsecutive X type vectors can be scanned followed by the scanning of one or more Y type vectors and then repeating the exposure of other X type and Y type vectors until all the vectors have been scanned. In this region the direction of scanning can be just as important as the order of scanning. To insure the most appropriate positioning of vectors they may need to be scanned from the supported region towards the unsupported region.

Other embodiments, described hereinnext, create at least a substantial amount of fill on a cross-section in a manner more analogous to standard approach crosshatch vectors. That is by supplying and exposing vectors which are spaced such that they do not effect each other during their exposure. They are spaced at or slightly above the expected maximum cure width of the individually exposed vectors. Thereby, after exposure of all vectors a substantially transformed cross-section results with only minimal untransformed material between the vectors. The various embodiments of this approach are broadly known by the name "Weave".

The name "Weave" particularly applies to the first preferred embodiment of this concept (the fifth embodiment of this application). This embodiment is the presently most preferred embodiment of the various Skintinuous building techniques. This embodiment consists of supplying and exposing boundary vectors, next supplying and exposing at least two types of non-parallel cross-hatch, wherein the exposure of the first cross-hatch type is insufficient to produce a cure depth that results in enough adhesion to induce vertical curl to the previous cross-section and wherein the exposure of the second cross-hatch type is equivalent to the first type, thereby resulting in sufficient exposure in the overlapping regions to cause adhesion between cross-sections. The spacing of the crosshatch vectors is such that they are spaced slightly further apart than the maximum cure width of the individual vectors when given the appropriate exposure to result in the desired cure depth.

As with the previously described methods of building, this method of building can be substantially implemented by using SLA software to Slice the desired CAD object file, or the like, a single time. The part is Sliced with cross-hatch but without skin vectors. The crosshatch vectors are spaced with a separation slightly greater (e.g. 10%) than the expected maximum cure width. The presently preferred system for building parts using this embodiment is the SLA-250 manufactured by 3D Systems, Inc. of Valencia, Calif. The presently preferred building material is XB 5081 stereolithographic resin (liquid photopolymer) manufactured by Ciba-Geigy. The presently preferred system utilizes a HeCd laser operating a 325 nm which typically results in a width of cure of approximately 10-11 mils or less for a cure depth of 8-9 mils. Therefore, the crosshatch vectors are spaced at approximately 12 mils. The presently preferred fill vectors are combined X and Y crosshatch. The presently preferred SLA software is version 3.60. When using the presently preferred software, an object is built by exposing boundary and hatch vectors. As stated earlier, adhesion between cross-sections is obtained at the intersection points between the two hatch types. The cure depth of these intersection points is approximately 12 to 14 mils when parts are built with 10 mil layers. This method of building results in substantially less horizontal distortion and equivalent or less vertical distortion than when equivalent parts are built with standard techniques. Measured post cure distortion is substantially less than for parts built using conventional methods.

The formation of a cross-section by this fifth embodiment is depicted in FIG. 8a to 8i. FIG. 8 represents a square cross-section which is to be cured to a uniform depth. FIG. 8a depicts a top view of the material cured by scanning of the boundary vectors. FIG. 8b depicts a sectional view of the material cured in FIG. 8a along line b. The cure depth of the boundary vectors is the layer thickness plus some overcure amount (e.g. 10 mil layer thickness+6 mil overcure). FIG. 8c depicts a top view of the material cured in response to the scanning of the X cross-hatch on a dashed background of material cured in response to boundary vectors. FIG. 8d depicts a sectional view of the material cured in FIG. 8c along line d--d. The cure depth of the X crosshatch is less than one layer thickness (e.g. 8 mil cure depth for a 10 mil layer thickness). The exposed regions are relatively wide compared to the unexposed regions. That is, the spacing between hatch vectors was only slightly more than the maximum cure width of the hatch vectors (e.g. 12 mil spacing of hatch vectors and an 11 mil maximum cure width).

FIGS. 8e and 8f show similar cured material for Y cross-hatch vectors. FIG. 8g depicts a top view of the superposition of material cured as depicted in FIGS. 8a, 8c, and 8e. The small square zones in the FIGURE represent uncured material. The size of these squares is about 1 mil on edge or less whereas the solidified material between them is about 11 mils on edge. FIG. 8h represents a side view of the cured shape of material along the line h--h of FIG. 8g. Line h--h is directly above the maximum cure from an X hatch vector. FIG. 8i represents a side view of the cured shape of material along line i--i of FIG. 8g. The cure depth of the regions where the X and Y hatch vectors overlap has increased to something greater than the layer thickness. Line i-i is located between the cure of two adjacent X hatch vectors. Most of the area of the cross-section is more like FIG. 8h than FIG. 8i. The exposure in FIG. 8h isn't uniform but the nonuniformity is less than that produced in the traditional approach to skinning a surface during part building. The main reason for this reduction is that there isn't a superposition of discrete hatch vectors with a six mil over cure each which results in up to an 11 mil overcure, or more, at their intersection points combined with skin vectors that form a uniform layer thickness cure depth. Instead there is simply a double exposure of closely spaced hatch that produces a substantially uniform cure depth with points of approximately 5 mil overcure superimposed on it.

A variation of this fifth embodiment is to use weave in all non-down-facing regions and to use other more traditional approaches to skinning (including the uniform skinning methods described below) on the down facing features and to give these down-facing features a layer thickness cure depth.

A sixth embodiment of the present invention is similar to the fifth embodiment just described except in the sixth embodiment the crosshatch (or fill) vectors are offset or "staggered" from layer to layer. One implementation of this method is to offset the vectors on adjacent layers by 1/2 the hatch spacing. Therefore, the hatch vectors on every other layer overlay the same hatch paths. Other forms of layer to layer offset are possible wherein the overlaying of hatch paths (hatch paths are lines on a given cross-section that have the potential of being crosshatched) is repeated at some other period than on every other layer. For example hatch paths may not overlay each other for 3 or more layers.

Offset or "staggered" crosshatch may be utilized with standard building techniques as well as with the various embodiments of the present invention. The advantages of using offset crosshatch with standard building techniques (that is widely spaced hatch) involve the production of smoother vertical surfaces of an object, more uniform volumetric properties, and possibly less curl between layers since adhesion between layers is due to points instead of lines.

In building parts for investment casting, hollow parts lead to less structural strain on the casting molds as the building material is burned away. Hollow parts can be built with crosshatch but without skins, thereby tending to allow untransformed material between the cross-hatches to drain from the object. Solid parts tend to expand and crack investment casting molds, whereas hollow parts have less of a tendency to do so. However, building hollow parts can be a problem even when skin fill isn't used if the hatch vectors on successive cross-sections lie on top of each other. Untransformed building material can be trapped between the cross-hatch and the boundaries. The trapped material can later become solidified, thereby losing the desired hollow part characteristics of the object. However, if the centering-to-centering distance between crosshatch vectors is greater than approximately twice the maximum cure width, then offsetting the vectors by 1/2 the spacing on every other layer will result in a part where substantially all of the internally untransformed material can flow through various gaps and thus can be removed prior to using the part to make an investment casting mold. This advantage of offset crosshatch is illustrated in FIGS. 9a and 9b.

FIGS. 9a and 9b illustrate the sixth embodiment and depict side views of an object whose boundary vectors are offset from one another but are not offset sufficiently to allow drainage of untransformed material between them (these are nonvertical but steep boundaries as opposed to flat or near-flat boundaries). FIG. 9a depicts a part built with overlaying crosshatch and therefore pockets of untransformed material trapped within it. FIG. 9b depicts the part built with offset crosshatch therefore allowing pathways for removal of internal untransformed material. Assuming the top of each partially depicted object reconverges so that building material cannot be removed from the top, as shown in FIG. 9a, only pockets 306 and 308 can drain whereas
302, 304, 310, and 312 cannot drain. In FIG. 9b, the entire internal area of the part forms one interconnected pocket from which substantially all untransformed material can drain. If using a photopolymer, drainage can be enhanced by utilizing elevated temperatures to reduce resin viscosity. Since a primary principle of the present invention is to solidify as much internal material as possible this sixth embodiment of offset crosshatch diverges from the skintinuous building techniques. But it is a useful building method in its own right. Its ability to reduce post cure distortion is discussed in Example 6.

Other embodiments of the nonoverlapping approach to building can be developed from appropriate combinations with techniques discussed in conjunction with overlapping exposure techniques as well as with the additional embodiments of the present invention to be described hereinafter.

Besides the two main approaches described previously, that is overlapping fill or non-overlapping fill on at least a portion of the cross-sections, there is an additional class of embodiments that are used to increase structural integrity. This next class of skintinuous embodiments are based on the curing of discrete points of material called "Bullets" instead of the curing of overlapping or non-overlapping lines of material. The bullets are cured in association with a single cross-section as a plurality of substantially nonoverlapping exposures. The embodiments of this approach comprise methods of solidifying primarily internal regions of objects whereas the down-facing and up-facing regions of the object may be cured by the other approaches described herein.

The seventh embodiment of Skintinuous (the first embodiment of this class) involves curing the internal portions of desired cross-sections by exposing material within the boundaries of the cross-sections as a series of discrete points. The points that are exposed on a given cross-section are spaced from each other by a distance slightly greater than the maximum diameter of the cured material formed upon exposure of the material to synergistic stimulation. In other words, on a single internal cross-section a substantial portion of the material is cured in the form of point exposures with a small gap separating each point exposure from its neighbors. This separation stops the transmission of stress and therefore diminishes curl. Each point is cured to a depth equal to or slightly greater than the layer thickness to insure adhesion between cross-sections. On the next cross-section the pattern of exposure is "staggered" or shifted so that the point exposures on this next cross-section are centered above the gaps between the points on the previous cross-section. This shifting pattern of bullet exposures is continued on alternating cross-sections until the desired region of the part is complete. This allows substantial structural integrity to be developed between cross-sections while decreasing the amount of curl when building with a given layer thickness.

Two consecutive overlapping sample cross-sections are shown in FIGS. 10a and 10b. These sample cross-sections depict a cross-section boundary 400 enclosing a series of point exposures. FIG. 10a depicts point exposures 402 located on a particular grid while FIG. 10b depicts point exposures offset (staggered) from those in FIG. 10a. A comparison of the two FIGURES indicates that the bullets on one cross-section are centered in the middle of the space between bullets on the previous cross-section. FIG. 10c depicts substantially a side view of the combined FIGS. 10a and 10b along line c--c. This FIGURE, at least in a 2 dimensional view, illustrates how the bullets are staggered from layer to layer. The illustrations of FIG. 10a and
10b depict a particular arrangement of points on a given cross-section but other arrangements are possible. For example to get a tighter fit of bullets (higher ratio of transformed to untransformed material on a given cross-section) one could locate the points in a hexagonal pattern. This hexagonal pattern when combined with higher cross-sections could form a hexagonal close-pack structure. Therefore each bullet would have 6 nearest neighbors on the cross-section as opposed to only 4 as shown in FIGS.
10a and 10b.

This method of building with bullets can only be partially implemented from present 3D Systems software. This implementation is only partially satisfactory. Proper utilization of this embodiment requires modified software.

The implementation from the present software is done by slicing the object with a single type of crosshatch vectors (e.g. X vectors) with a spacing slightly larger than the diameter of the bullets of material that will be formed. The object is sliced a second time with the part offset by 1/2 the spacing along the direction perpendicular to the chosen crosshatch direction (e.g. the Y-axis). The two objects are merged together using options that shift the second file by - 1/2 the crosshatch spacing along the perpendicular direction (e.g. Y-axis) and 1/2 the crosshatch spacing along the X-axis. Also the merging options used remove all the vectors from the second slice file except for the cross-hatch vectors. Then the resulting file is edited to remove the hatch vectors from alternating slice files on alternating layers.

When vectors are cured using the present software they are not cured by a continuously sweeping beam but by a beam that jumps a small distance (e.g. integer multiples of 0.3 mils) known as the stepsize or SS, and then it waits at each allowed SS position a period of time known as the step period or SP (e.g. integer multiples of 10 microseconds). The particular positioning for both timing and jumping is based on the beginning point of each vector that is drawn. When the object is built, the SS value used is equivalent to the spacing between vectors (e.g. 12 mils or approximately an SS value of 40). Since the two files were offset when they were sliced along the direction perpendicular to the hatch direction and then shifted back together during merge, the Y-value associated with each bullet will be offset by 1/2 the crosshatch spacing on alternating layers. Since the files were merged then offset from each other by 1/2 the crosshatch spacing along the direction of the crosshatch the X-value associated with each component will be offset by 1/2 the crosshatch spacing between alternating layers. This method of implementation is useable but not always satisfactory since in some of the extreme values, the X-component of each vector fall slightly (1/2 the crosshatch spacing) outside the boundary of the cross-section. Other software based methods of implementation using the present software are based on slicing the part twice as described above but here each part is sliced with a layer thickness twice as large as desired and one of the parts is shifted by 1/2 the layer thickness prior to slicing and then shifted back during merging.

A useful tool for implementing this embodiment from the present software without a major change would be to include a parameter that would allow crosshatch vectors to be reduced in length at each end by a specified amount. This would allow shifting along the direction perpendicular to the hatching direction, along with the reduction of the vectors created by the second slice by 1/2 the crosshatch spacing, followed by the reregistration of the files during merging. Also of use would be an editing program that could read through the merged file and remove selected vector types from appropriate cross-sections.

If one is building an object with an orientation that is substantially more susceptible to curl than a perpendicular orientation, this embodiment may be modified to that of an embodiment of offset unidirectional crosshatch. Here the direction of offsetting would be the direction most susceptible to curl and the direction of the vectors would be the less susceptible direction.

The eighth embodiment of the present invention is similar to the seventh embodiment but the bullet's are cured so as to solidify material in association with the previous cross-section as well as the present cross-section. Therefore the cure depth of each bullet is typically equal to or somewhat larger than two layer thicknesses. Accordingly, in this embodiment, it is important, when working with a particular cross-section, to know not only the internal regions of the present cross-section but to also know the internal overlapping regions of the previous cross-section. The spacing of the bullets is somewhere between what it was in the previous embodiment and the diameter of the bullet at one layer thickness below the present cross-section. The adhesion between layers is gained substantially by adhesion between the sides of the bullets on the present cross-section at a position one layer thickness below their upper surface and the sides of the bullets on the previous cross-section at their upper surface. A side view of the bullet positions on adjoining layers is depicted in FIG. 11. It is noted that the present embodiment is primarily for regions of the object two or more layers from down facing features.

In still other embodiments, transformation of material that is located on one cross-section may occur from exposures given in association cross-sections two or more layers higher up.

As discussed above, several new skinning techniques can advantageously be used in connection with this invention, based on non-consecutive ordering of skin vectors. Traditionally, skin vectors are ordered head-to-tail, such that a first vector pass is made along a fill path from one boundary to an opposing boundary, and a pass along the next vector is then made, slightly offset (e.g., typically from 1 to 4 mils from the first), from the latter boundary back to the first. However, it has been found, in accordance with some of the preferred embodiments of this invention, that distortion can be reduced by appropriate, non-consecutive scanning and therefore nonconsecutive formation order of skin fill. Specifically, the offset between vectors can be advantageously increased (e.g., doubled or tripled, or more), such that the successive skin vectors have less impact, or do not impact, upon adjacent lines of cured building material for a given series of passes across the surface of the region of the part being formed. Additionally, in one or more successive series of passes, additional skin vectors can be drawn between those that had been drawn in the earlier series of passes. These embodiments preferably have crosshatch vectors on each layer as well as skin vectors.

Yet another embodiment according to which distortion can be minimized involves skinning in different directions for different layers. For example, in a part having x- and y- hatch on each layer, odd layers can be skinned in the x-direction and even layers in the y-direction, or vice versa.

In still another embodiment, skin fill can be provided in both x- and y- directions on a given layer having x- and y-cross-hatch.

According to a most preferred embodiment, however, x, 60.degree. and 120.degree. cross-hatch is provided with skin fill in at least one of the x, 60.degree. or 120.degree. directions, and preferably, in each of the directions. In a preferred variation of this embodiment, discussed in more detail below, the skin vectors of a given direction are not traced directly over the hatch vectors of the same direction, thereby avoiding excess exposure of any given location. Moreover, since exposure is provided in three directions over any given point in a skinned layer, the vector scanning speed can be increased by a factor of three to yield one-third of a normal exposure per vector, resulting in a uniform exposure after all three directional passes are made.

Another embodiment is that of "tiling". In this embodiment one of the previously mentioned approaches is utilized in exposing individual "tile-like" regions wherein small spacings of material between the individual tiles are left untransformed to act as stress relief zones. The size of the individual tiles can range from that of a point exposure to that of an entire cross-section.

Tiling is a method of forming a layer of an object produced by stereolithography, wherein the layer is divided into a series of area elements or tiles. Each area element is isolated from adjacent area elements by spacings. The spacings around each area element remain untransformed, at least until all neighboring area elements or tiles are transformed or solidified. The spacings between the individual tiles are left untransformed to act as stress relief zones. The width of the spacing is typically small compared to the width of the individual tiles. Individual tiles can be drawn with borders or without; however, it is presently preferred to draw tiles without individual borders.

Tiling can also be used as another technique of curl reduction when implemented on a second or higher layer above a down-facing feature. Generally no curl is generated on a downfacing feature so there is no need for tiling as a curl reduction technique on a down-facing feature. It should also be noted that tiling does not generally apply to down-facing features because there is no underlying structure to attach individual tiles to during the transformation process, i.e., tiling may be applied only to an at least partially supported area as opposed to a completely unsupported area.

Since the tiles are individual and discreet relatively small areas, the use of tiles limits shrinkage to the boundary of the tile. This reduces stress and curl on the tiled layer, an especially important consideration in the first few layers immediately above a down-facing feature. Curl generally occurs mainly in down-facing features. These features curl upwardly as a result of the formation of the next several overlying layers. On the other hand, a potential disadvantage of tiling is that it possibly provides less strength.

The spacings between the tiles can be transformed or solidified (referred to as grouting or mortar) usually after all of the tiles have been formed. An entire object can be made by tiling to reduce post treating. This grouting is usually transformed to a lesser degree than the tiles (a lower exposure is used).

By way of example, when using a presently preferred material such as XB5081 and 5 mil layers, tiling may be used in forming the first through twentieth layers above a first layer of a down-facing feature and especially to form layers 1 through 10
(assuming the first layer is supported). If 10 mil layers are used, tiling would preferably be applied in the range of the 1st through 10th layer and especially in the 1st through 5th layer above a down-facing feature.

Preferably, the tile sizes range from the width of a laser beam (about 0.010 inches, 1/4 mm) up to about 0.120-0.150 inch, with the most preferred range being from 3/4-2 millimeters on edge.

The spacings or gaps between the tiles should be as small as possible, within the limits of accuracy of placement and cure width of a beam of synergistic stimulation. The typical width of these gaps is in the range of 1-10 mils after exposure and cure. It is important that the material in the spacings or gaps not be transformed or solidified sufficiently to transmit stress.

After forming one or more layers with tiling and without grout it is generally desirable to start offsetting tiles from layer to layer or to begin grouting between tiles or to stop the tiling process altogether in order to insure adequate structural integrity of the part. As stated earlier, a potential problem with forming parts utilizing tiling is the tendency towards structurally weak parts. However if tiling is to be discontinued, grouted, or offset, it is important to minimize any tendency toward reintroducing curl that may result from closing the gaps. As the gaps are closed, any shrinkage of material that occurs above the gap, while the shrinking material is adhered to both ends of the gap, can cause curl distortion by tending to bring the top ends of the gap closer together (closer than the spacing between the bottom edges of the gap). Since gaps result in relatively weak axes this scenario of reintroducing curl is very likely. Additionally, it has long been suspected and recently experimentally verified that shrinkage of curing material can still be occurring several seconds after exposure of an area is suspended. This means that closing a gap with a line of curing material which is adhered to the first side, and extended from the first side to the second side, and adhered to the second side within a few seconds can induce stresses into the part which can eventually lead to distortion.

A preferred method of closing gaps, and thereby insuring adequate structural integrity of a part, while avoiding reintroduction of curl, is based on insuring that at least a substantial portion of the material cured over the gap is allowed to shrink prior to adhesion to both sides of the gap.

A first embodiment of this method slightly offsets the tiles on a second layer from the corresponding positions of tiles on the previous (first) layer, wherein the offset is such that the tiles on the second layer substantially cover the gaps between the tiles on the first layer without completely bridging the gap thereby avoiding the simultaneous adhesion of a single cured shrinking mass to both sides of the gaps on the first layer. Thereafter, allowing for sufficient time for the tiles and the tile material spanning the gaps to complete their shrinking, grouting between the tiles on the second layer is formed. This grouting completes the bridging of the gap with only minimal shrinkage of material over the gap while adhesion to both sides exists. Since this grouting is offset from the gaps on the first layer, and since shrinkage of material on the second layer over the gaps occurs before the grouting is formed substantially less curl is introduced. Any desired exposure can be used in forming the grouting, without necessarily being limited to grouting of lower exposure than that used to cure the tiles.

This first embodiment is depicted in FIGS. 23a to 23c. FIG. 23a depicts a side view of a portion of an object comprising a first layer which is formed with tiles and a second layer with slightly offset tiles and offset grout. The tiles of the first layer are indicated with numeral 800. The tiles of the second layer are offset sufficiently from tiles on the first layer to substantially cover the gap area of the first layer without being offset so far as to adhere to the adjacent tiles on the first layer. These tiles of the second layer are depicted with numeral 804. The grouting between the tiles on the second layer is cured after the tiles 804 on the second layer have been allowed to shrink (e.g., generally at least a 3 to 5 second delay between completing neighboring tiles and beginning to grout). The grouting is indicated with numeral 808. FIG. 23b depicts a top view of the tiles 800 of the first layer and FIG. 23c depicts a top view with superimposed tiles 800 of the first layer and tiles 804 of the second layer and the grouting 808 of the second layer.

A second embodiment of this method forms the second layer, on which the gaps will be closed by floating at least one end of the solidified material which spans the gap until after at least a substantial portion of the shrinkage has occurred. After allowing for shrinkage to occur, the floating end(s) can be tacked down with rivets, or multipass, or the like.

FIG. 24 depicts a side view of a gap 810 between tiles 800 on a first layer being closed off on a second layer by floating at least one end of bridging material 814 until the bridging material has completed shrinking. The rivet completing the closure is depicted with numeral 818.

Additional embodiments of this type involve the progressive partial closure of the gaps over a plurality of layers. For example a gap can be partially narrowed from one or both sides on a second layer followed by additional narrowing or complete closure on the third or higher layer.

These methods of closing gaps between tiles are applicable to the closure of the stress relief gaps disclosed in U.S. Pat. No. 5,015,424. As will be apparent to those of skill in the art these methods can be modified or/and combined with themselves or other curl reduction techniques to effectively close gaps without reintroduction of substantial curl.

Where a scanning mirror-directed laser beam is used to transform the material, the "jumping" speed from tile to tile across the spacings must be considered. The mirror(s) directing the laser beam has a moment of inertia which limits its rate of angular acceleration. If the laser is to jump from the edge of one tile to the adjacent edge of another tile, the jumping speed is limited since there is only a very small distance in which the mirror can accelerate before it must begin to decelerate to properly direct the laser onto the edge of the next adjacent tile. Since the jumping speed is limited, the material in the gap may be inadvertently partially cured by the jumping laser beam. In tiling methods wherein the laser is frequently jumping back and forth between tiles, this can become especially problematic.

Inadvertent curing of the material in the gaps during jumping between tiles can be overcome in several ways. The mirrors can be made to accelerate faster, although a practical upper limit is quickly reached. Alternatively, a shutter can be provided to block the laser beam before reaching the scanning mirror. However, mechanical shutters also suffer from inertial lag and are considered too slow to be effective. Electrically driven crystal acousto-optic shutters can be considered. A third technique, i.e., the "long jump" technique is the most preferable. In the long jump technique, the laser is jumped from a far edge of a tile accelerating over the tile (the launch tile) then crossing the gap at maximum velocity to a distant point on the adjacent tile (the landing tile) decelerating and beginning to transform the area of the tile near its far edge. By making the long jump, the laser has a sufficient distance in which to accelerate such that it passes over the gap at a high speed and decelerates to the landing point distant from the gap.

Tiles can be formed in various patterns and shapes. One basic way is to form square or rectangular tiles in a straight grid pattern, i.e., with the gaps or grout lines extending continuously in the two directions of the layer (x and y directions). However, with this grid pattern, the grout lines themselves are relatively long lengths of material which when cured are subject to curling. Moreover, the tiles forming the simple grid pattern are not structured to resist curling in either direction caused by shrinkage of the grout lines during curing. Thus, the straight grid tile pattern has two axes of weakness.

An improved tile pattern is an offset or staggered grid wherein the tiles are staggered in alternate rows, like bricks in a horizontal wall. With the tiles in this staggered grid pattern, the grout lines extend continuously only in one axis, rather than in two axes as with the straigh