Method of incremental object fabrication

An incremental object or part fabrication method includes the step of determining the dimensional boundaries of the part, the dimensional boundaries including x, y, and z dimensions. A reference surface is provided and a first incremental layer of mold material is formed on the reference surface. The first incremental layer of mold material is selectively interrupted to define at least one dimensional boundary of the part. A first incremental layer of part material is formed upon at least the reference surface, wherein a second reference surface is defined by the first incremental layers of mold and part material. The steps of forming incremental layers of mold and part material are repeated until the object is fully fabricated and all dimensional boundaries are defined.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to methods for fabricating 
three-dimensional objects. More specifically, the present invention 
relates to a method for fabricating three-dimensional objects in 
incremental layers employing data generated in a CAD/CAM system, the 
method steps being controlled by a central computer or distributed data 
processing system. 
2. Background Information 
Recently, there have been great strides in technology relating to rapid 
prototyping and manufacturing ("RP&M"), particularly to the integration of 
CAD/CAM ("Computer Aided Design/Computer Aided Manufacturing") systems 
into the object fabrication process beyond simple object design and 
drafting. A goal of this technology is to obtain a final or near-final 
three-dimensional object or part from CAD/Cam data with as little human 
intervention as possible. A comprehensive discussion of the state of the 
RP&M art is found in Rapid Prototyping and Manufacturing, P. Jacobs et al. 
(Society of Manufacturing Engineers, 1992). 
One technology that has received a great deal of attention in this regard 
is known as "stereolithography," which literally means "three-dimensional 
printing." In stereolithography, dimensional data generated in a CAD/CAM 
system is employed to "print" an object in a fully or near-fully automated 
fabrication system. The stereolithography method and apparatus is 
disclosed in a number of patents assigned to 3-D Systems Inc. of Valencia, 
Calif. The 3-D Systems method and apparatus employs CAD/CAM data to 
control a beam of radiant energy. The beam of radiant energy is directed 
into a bath of radiantly cured liquid polymer to selectively cure the 
polymer and thus build an object through accumulation of incremental 
layers of cured polymer. Thus, the three-dimensional printing is 
accomplished by selective curing of substantially two-dimensional layers 
of polymeric material. 
Another method and apparatus for RP&M is disclosed in a series of patents 
assigned to the University of Texas. In this method, a CAD/CAM system is 
employed to control a laser, which selectively sinters particles of 
material, typically a plastic powder, to form the object through 
accumulation of incremental layers of sintered material. 
Other known RP&M systems adhesively laminate together layers of polymeric 
or paper tape. Each layer is trimmed by a laser or other means to the 
cross-sectional dimensions of the object, wherein accumulation of layers 
of tape forms the fully fabricated object. 
A drawback to all of these methods is that the ultimate part, while in some 
respects satisfactory for model building, generally lacks the material 
properties desired of a satisfactory prototype part. For example, the 
parts resulting from the cured liquid polymer processes tend to be 
brittle. The parts resulting from the laser sintering process suffer from 
the porosity and strength problems typical of sintered parts. 
One attempted solution to this problem is disclosed in a series of patents 
assigned to the University of Southern California. In this process, liquid 
metal is deposited in droplet form and the droplet stream is manipulated 
to form the object through accumulation and solidification of the metal 
droplets. This process results in a satisfactory metallic prototype part, 
but is extremely complex, even in the context of this complex 
technological area, is extremely expensive, and the control systems are 
not sufficiently developed to accurately and repeatably produce prototype 
parts with satisfactory dimensional tolerances. 
A need exists, therefore, for a method of fabricating objects, employing a 
CAD/CAM system, that is capable of fabricating an object with satisfactory 
material properties and dimensional tolerances, and with the ability to 
fabricate complex and intricate shapes. 
SUMMARY OF THE INVENTION 
It is a general object of the present invention to provide an improved 
method of three-dimensional object or part fabrication. This and other 
objects of the present invention are achieved by determining the 
dimensional boundaries of the part, the dimensional boundaries including 
x, y, and z dimensions. A reference surface is provided and a first 
incremental layer of mold material is formed on the reference surface. The 
first incremental layer of mold material is selectively interrupted to 
define at least one dimensional boundary of the part. A first incremental 
layer of part material is formed upon at least the reference surface, 
wherein a second reference surface is defined by the first incremental 
layers of mold and part material. The steps of forming incremental layers 
of mold and part material are repeated until the part is fully fabricated 
and all dimensional boundaries are defined. 
According to a preferred embodiment of the present invention, the 
incremental layers of mold material are formed of a UV-cured liquid 
polymer by a stereolithography apparatus, the incremental layers of part 
material are electroformed of nickel or other metal, and the method is 
implemented and controlled in a computer system. 
Other objects, features, and advantages of the present invention will 
become apparent to those having skill in the art with reference to the 
detailed description, which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the Figures, and particularly to FIG. 1, functional 
components of an apparatus for use in implementing the method according to 
the present invention are illustrated in block diagram form. The 
functional components include a computer system 11. Computer system 11 is 
depicted as a desk top personal computer, but could include a workstation, 
mainframe, distributed data processing system, or the like, so long as the 
system is adapted for centralized control and implementation of other 
functional components of the apparatus. 
These functional components include a stereolithography apparatus 13, an 
electroform apparatus 15, a finishing apparatus 17, and a transport 
apparatus 19. All are coupled to computer 11 for control of each 
respective functional component 13, 15, 17, 19 by computer 11 in 
implementation of the method according to the present invention. The 
operation of and interrelation between the functional components will 
become apparent as the method of the present invention is described in 
greater detail below. 
FIG. 2 is a perspective view of a three-dimensional object or part 21 that 
is fabricated according to the method of the present invention. Part 21 
comprises a pair of end pieces 23, which are connected by four columns 25. 
A spherical ball 27 is contained within the space defined by end pieces 23 
and connecting columns 25. Part 21 is integrally formed, preferably of a 
metallic material. The intricate, part-within-a-part, nature of part 21 
illustrates the intricacy of parts that can be formed with the method 
according to the present invention and the versatility of the method of 
the present invention. As with any three-dimensional object, part 21 has 
x, y, and z dimensional boundaries. 
Before part 21 is fabricated, these dimensional boundaries can be obtained 
from data contained in a conventional CAD/CAM system with 
three-dimensional solid modeling capability, such as Pro/ENGINEER.TM., a 
CAD/CAM system sold by Parametric Technology Corporation of Waltham, 
Massachusetts. Given a three-dimensional, solid model description of part 
21 in a CAD/CAM system, the x, y, and z dimensional boundaries of the part 
can be determined appropriately for the method of the present invention in 
a conventional manipulation of the data. As is conventional in 
stereolithography, the x and y dimensional boundaries of part 21 are 
determined at each of a selected number of points along the z axis. The 
incremental distance dz between each of the selected points on the z axis 
may be selected by the operator and in large part determines the intricacy 
of the part 21 that can be fabricated, and the quality of dimensional 
tolerance and surface finish that can be obtained. For example, to obtain 
a relatively smooth surface on a rounded object such as spherical member 
27, increment dz should be selected to be relatively small, on the order 
of 0.004 inch or less. For generally straight-sided objects, the increment 
dz is less critical. 
With reference to FIGS. 1-3E, the method according to the present invention 
is described. After determining and generating the dimensional boundaries 
of part 21 in the CAD/CAM system of computer 11, the dimensional data is 
downloaded from computer 11 to stereolithography apparatus 13. The 
downloaded data is manipulated and processed to a form appropriate for use 
by stereolithography apparatus 13. 
Stereolithography apparatus 13 is provided with a generally flat reference 
surface or build datum 31, as illustrated in FIG. 3A. Stereolithography 
apparatus 13 is employed to form a first incremental layer 33 of mold 
material of thickness dz, determined previously along with other 
dimensional boundaries of part 21. Incremental layer of mold material 33 
is selectively interrupted, wherein one or more interruptions 35 in layer 
33 define various x and y dimensional boundaries of part 21. 
According to the preferred embodiment of the present invention, 
stereolithography apparatus 13 is the 3D-Systems model SLA-250, 
manufactured and sold by 3-D Systems Inc. of Valencia, Calif. This 
apparatus employs the previously mentioned CAD/Cam data to selectively 
form incremental layers of UV-curable liquid polymer. Specifically, the 
SLA-250 employs an argon laser to at least partially solidify or cure the 
UV-curable polymer on a build datum (31 in FIG. 3A-3E), which is moved up 
and down in a bath of the UV-curable polymer. The SLA-250, because of the 
nature of the liquid polymer employed therein, requires an additional 
post-cure step in an ultraviolet oven to fully cure incremental layer 33. 
Photopolymer resins used in stereolithography apparatus 13 include both 
acrylate and epoxy formulations. A preferred photopolymer resin is 
Cibatool.TM. XB 5170 epoxy resin, which is available from Ciba-Geigy A.G., 
Fribourg, Switzerland, but other resins are available and improved resins 
are becoming available. Thus, prior to subsequent processing, reference 
surface or build datum 31 and incremental layer 33 must be removed from 
stereolithography apparatus 13 by transport apparatus 19 and subjected to 
the post-cure process. 
Transport apparatus 19 could be one of a variety of robotic or automated 
devices that is computer controllable to move a pallet consisting of at 
least build datum 31 and incremental layers of partially formed part 21 
between the various functional components 13, 15, 17, of the apparatus for 
implementation of the method according to the present invention. 
FIG. 3B depicts, in fragmentary section view, the result of the next step 
of the method according to the present invention, in which a first 
incremental layer of part material 37 is formed on at least interruption 
35 in layer of mold material 33. Again, this step necessitates that at 
least build datum surface 31 and incremental layer of mold material 33 be 
moved from stereolithography apparatus 13 to electroform apparatus 15. 
According to a preferred embodiment of the present invention, incremental 
layer of part material 37 is formed by an electroforming process. 
Electroforming is a conventional process that is similar to electroplating 
in that it occurs in an electrolytic solution containing ions of the metal 
to be deposited and is driven by an anode and cathode arrangement in which 
the cathode, or negatively charged object in the solution, bears a 
negative image of the object to be formed. An electrical charge on the 
cathode attracts the metallic ions, which are deposited on the cathode and 
accumulate over time to form a layer of metal having a positive relief of 
the contours of the cathode. Because electroforming occurs on a molecular 
level, the resulting layer of metal has excellent material properties. 
Typically in electroform operations, the cathode is known as a mandrel. In 
the present invention, incremental layers of mold material 33 serve as the 
cathode or mandrel, and electroformed incremental layers of part material 
37 adopt the configuration of incremental layers of mold material 33, 
including at least one interruption 35. 
According to a preferred embodiment of the present invention, incremental 
layers of part material 37 are formed of nickel. Nickel is preferred 
because it is readily adapted for electroform procedures and generally has 
satisfactory material properties for most prototype part applications. 
Alternative materials include copper and iron and may be more appropriate 
for certain applications. 
Because the preferred first incremental layer of mold material 33, which 
serves as the mandrel, is formed of polymeric material, the mold material 
must be metalized for layer 33 to perform its role as a cathode in 
electroform apparatus 15. This step is conventional in electroforming and 
permits the appropriate surfaces of mold material 33 to become 
electrically charged, thus facilitating the formation of layer of part 
material 37 on the appropriate surfaces of layer of mold material 33 and 
interruption 35. 
Metalizing can be accomplished in a number of ways, including spraying a 
metallic compound or solution on the area to be metalized. According to a 
preferred embodiment of the present invention all of the uppermost 
surfaces of incremental layer of mold material 33 are metalized, including 
the sidewalls defined by interruption 35. Metalizing is removed from the 
uppermost surface of layer 33 prior to electroforming in a process similar 
to the finishing step described below to avoid unnecessary formation of 
part material thereon. Alternatively, metalizing is applied selectively 
only to the sidewalls defined by interruption 35 because it is redundant 
to metalize the metallic surface provided by layer of part material 37. 
More background information on electroforming technology can be found in 
Electroplating Engineering Handbook, A. Kenneth Graham, (Van Nostrand 
Reinhold, 3d. Ed. 1971) and Practical Electroplating Handbook, N.V. 
Parthasaradhy (Prentice Hall, 1989). 
After incremental layer of part material 37 is formed over and on first 
layer of mold material 33, its uppermost surface is machined, ground, or 
otherwise rendered dimensionally flat and parallel to build datum 31 in 
finishing apparatus 17. The finishing step is important because it 
provides a new reference surface 41 on which subsequent layers of part and 
mold material can be formed. For dimensional accuracy, second reference 
surface 41 must be flat and parallel to build datum 31. 
Finishing apparatus 17 is one of any number of conventional numerically 
controlled grinders, mills, or other machine tools. Again, this finishing 
step necessitates that a pallet consisting of at least build datum 31 and 
layers of mold and part material 33, 37 be moved by transport apparatus 19 
between electroform apparatus 15 and finishing apparatus 17. 
The steps represented by FIGS. 3A-3C are repeated, substantially 
identically as disclosed before, until all dimensional boundaries of part 
21, including the x, y, and z dimensions, are fully defined, and object 21 
is fully fabricated, as illustrated in FIG. 4. FIGS. 3D and 3E illustrate 
the result of subsequent formation of layers of part and mold material. 
FIG. 3D illustrates the formation of the columns of the part, and FIG. 3E 
illustrates formation of a portion of the spherical ball of the part. 
In the method steps subsequent the formation of the first incremental 
layers of mold and part material 33, 37, the metallic and electrically 
conductive layer of part material 37 will form a portion of second build 
datum 41 and largely eliminates the necessity of the metalizing step of 
the electroforming process. However, metalizing may remain advantageous 
for certain part features, particularly at the sidewalls defined by 
interruption 35. 
FIG. 4 illustrates fully formed part 21, including spherical member 27 and 
end pieces 23, encased in mold material 33 produced by the method 
according to the present invention. Thus, as a final step, mold material 
33 must be removed to permit access to and full function of part 21. 
Depending upon the polymer employed in stereolithography apparatus 13, a 
number of methods of removal may be employed. For example, if mold 
material 33 is a polymer that degrades upon exposure to heat, mold 
material 33 may simply be melted away. Other polymeric materials may 
necessitate acid-bath removal, bead or sand blasting, grinding, or other 
similar material removal operations. When mold material 33 is removed, a 
fully formed and functional nickel part 21, substantially as illustrated 
in FIG. 2, remains. 
In summary, FIG. 5 is a flow chart depicting the principal operative steps 
of the method according to the present invention. Block 51 represents the 
step of determining the dimensional boundaries of the object employing 
CAD/CAM system in computer 11. A first incremental layer of mold material 
is formed in stereolithography apparatus 13, as represented by block 53. 
Next, a first incremental layer of part material is formed in electroform 
apparatus 15, as denoted by block 55. Then, the layers of part and mold 
material are finished to form a new reference surface in finishing 
apparatus 17, as represented by block 57. The steps represented by block 
53-57 are repeated until all dimensional boundaries of the part are 
defined, as indicated by block 59. Finally, the incremental layers of mold 
material encasing the part are removed to reveal a fully fabricated part 
having a surface finish, dimensional tolerances, and material properties 
that are generally improved over the prior art, as represented by block 
The principal advantage of the present invention is that it provides an 
improved method of rapid prototyping and manufacturing of metallic parts. 
Because of the iterative and repetitive nature of the method according to 
the present invention, it is particularly adapted to automated computer 
control. Additionally, because individual functional components of the 
apparatus for implementing the method are presently subject to computer 
control, the method according to the present inventions is particularly 
adapted to be centrally controlled by a single computer or a distributed 
data processing system. 
The invention has been described with reference to a preferred embodiment 
thereof. It will be apparent to those skilled in the art that the present 
invention is susceptible to variation and modification without departing 
from the scope of the invention.