Plastic and nonmetallic article shaping or treating: processes – Stereolithographic shaping from liquid precursor
Reexamination Certificate
1999-02-17
2001-03-27
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Stereolithographic shaping from liquid precursor
Reexamination Certificate
active
06207097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains generally to methods for fabricating three dimensional objects, and more particularly to methods for generating such objects using stereolithographic techniques.
2. Description of the Background Art
Stereolithography is a technique that was originally developed for rapidly producing prototypes of three-dimensional physical parts that are designed using computer-aided design (CAD) methods. This technique is basically a “three-dimensional printing process” that produces a solid plastic model of the CAD part by using a laser beam to draw cross sections of the part on the surface of a photo-curable liquid plastic or resin. The apparatus for carrying out this method, termed Stereolithography Apparatus (SLA), includes a computer program for slicing the CAD model into thin cross sections. One embodiment of this method includes focusing a laser beam to a small intense spot on the surface of the liquid resin by means of a computer-controlled optical scanning system. The beam cures liquid resin to a solid at the locations on the surface corresponding to the cross section of a given layer in the CAD model. A computer-controlled elevator system then lowers the newly-formed layer into the liquid for a distance comprising the layer thickness, and another layer is formed on top in a similar manner, such that each layer adheres to the layer below it. The solid model is thus built up, layer by layer, from successive cross sections of the CAD model, starting from the bottom up to the top of the model. The entire process is controlled by a computer, which effectively converts the CAD data for a physical three-dimensional object into a solid model realization.
This technique has been described in detail in the technical literature, including a series of patents that disclose this technique and associated SLA devices. Reference is made to U.S. Pat. No. 4,575,330 (Hull), issued Mar. 11, 1986, which is one of the earlier patents describing this technology. This technique has been further discussed extensively in the text entitled
Rapid Prototyping and Manufacturing; Fundamentals of Stereolithography
by Paul F. Jacobs, published in 1992 by the Society of Manufacturing Engineers. The foregoing references are incorporated herein by reference. Apparatus for carrying out this technique is currently manufactured and sold by 3D Systems, Inc. of Valencia, Calif.
Previously, the known stereolithographic systems have been used to fabricate relatively large prototype parts of limited precision. The above-identified reference, “
Rapid Prototyping and Manufacturing; Fundamentals of Stereolithography
”, p. 314, reports that the overall dimensional accuracy of prototype parts fabricated by SLA approaches 5 mils (0.005 inches), compared with the CAD model values. This 5-mil resolution value is the extreme limit, and SLA-fabricated parts more typically have a resolution in the 10 mil region. The SLA system manufactured by 3D Systems, Inc. produces parts in which the minimum horizontal feature size is limited to about 10 mils, and the laser beam positioning accuracy is limited to 5 mils (SLA-250 User's Manual, Page B-1). Therefore the stereolithography technique that is known to persons of ordinary skill in the art is unsatisfactory for production of very small parts with high precision, where the above resolution limits exceed the acceptable tolerances for such high precision parts. It is desirable to adapt and modify the known stereolithography techniques for fabricating small parts with accuracies and tolerances at least 10 times smaller than the above limits (i.e. 10 times higher precision).
SUMMARY OF THE INVENTION
The present invention comprises an improved method for stereolithographic fabrication of small precision plastic parts. This method is an adaptation and modification of the stereolithographic methods disclosed previously and known to persons of ordinary skill in the art. In a preferred embodiment, the method is carried out using the SLA 250-40 stereolithography system manufactured by 3D Systems, Inc. of Valencia, Calif., with an epoxy-based resin material sold under the trademark CIBATOOL SL 5170, and manufactured by Ciba-Geigy, Ltd. of Marly, Switzerland. The SLA-250 system is sold with a user's manual which describes in detail the method specified by the manufacturer for fabricating parts with the system. Therefore, the method of the present invention comprises additional steps with reference to the manufacturer's specified method, and also modifications of these steps. In short, this invention is an improvement of previously known techniques for stereolithography, as will be understood by persons of ordinary skill in the art who are familiar with the above user's manual, together with the other references identified above.
The first step in the improved method is to level the SLA machine. The surface of the liquid resin should achieve horizontal accuracy to ensure uniformity in the prototype layers.
The second step is to modify the laser used in the SLA-250 system. This modification procedure is described in detail in the article entitled “
Rapid Prototyping: New Lasers Make Better Parts, Faster
” by Kenneth Ibbs and Norma Jean Iverson (the present applicant), published in the June, 1997 issue of
Photonics Spectra
by Laurin Publishing Co., Inc. This article is included in the above-identified provisional application as “Appendix A” and is incorporated herein by reference. The preferred laser is a 30 mw He—Cd laser operating at a wavelength of 325 nm in the pure TEM00 mode, such as the Model 3630NX laser system manufactured by LiCONiX, Inc. of Santa Clara, Calif. The beam from this laser may be focused to a spot of approximately 3 mils diameter at the resin surface.
The next step is to measure the actual diameter of the beam spot. This value is then entered into the data files for the software controlling the computer that is included in and operates the SLA 250 system. This parameter is called the “beam width compensation value”, identified in the software as “COMP”.
Next, it is desirable to determine the shrink compensation factors (SCF) for both the “x” and “y” directions by building a test part using the stereolithographic process. A preferred structure for this step is the “AT Part”, described and illustrated in Section 11.3, pages 302-306 of the above-identified reference, “
Rapid Prototyping and Manufacturing; Fundamentals of Stereolithography
”, wherein the side length of the outer square is approximately 8 inches, and the inner square boxes have sides approximately 1 inch in length. The SCF parameters are entered also into the software data files.
The slice layer thickness for the stereolithographic process must also be entered into the software data files. In a preferred embodiment of the invention this parameter may range from 6 mils down to 3 mils or less.
For these values of the slice thickness, and the variables determined as described above, other software parameters for the SLA system are further modified. Changed parameters include “Zdip velocity”, “contour”, “minimum full widths”, “hatch type” (set equal to “box”), and “hatch spacing” (set to 2-4 mils for 6 mil layer thickness). In addition, the parameters governing “pre-dip” and “post-dip” delays are changed, as well as the parameters determining compensation for jagged layer buildup. In a perferred embodiment of the method, parameter listings are included in the above-identified provisional application as “Appendix B”, which is incorporated herein by reference. These parameters are for the “ACES” (“Accurate Clear Epoxy Solid”) part building style, described in the SLA-250 User's Manual, and differ from the parameter settings normally furnished by the manufacturer in the SLA-250 system.
It should be noted that the laser used in the preferred method produces a smaller beam power than that used in previous stereolithographic fabrication processes. This reduced beam power arises from the reduced focused beam spot size in the pu
Lewis Francis H.
Tentoni Leo B.
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