Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
1999-11-05
2001-01-09
Lee, John D. (Department: 2874)
Coherent light generators
Particular beam control device
Nonlinear device
C264S308000, C264S401000, C425S174400, C425S375000
Reexamination Certificate
active
06172996
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the layer-by-layer formation of three-dimensional objects according to the principles of stereolithography. More particularly, it relates to stereolithographic exposure systems utilizing pulsed exposure of the medium by electromagnetic radiation.
BACKGROUND OF THE INVENTION
In recent years, many different techniques for the fast production of three-dimensional models have developed for industrial use, which are sometimes referred to as Rapid Prototyping and Manufacturing (RP&M) techniques. In general, RP&M techniques build a three-dimensional object, layer-by-layer, from a working material utilizing a sliced data set representing cross-sections of the object to be formed. Typically an object representation is initially provided by a Computer Aided Design (CAD) system.
Stereolithography, the presently dominant RP&M technique, may be defined as a technique for automated fabrication of three-dimensional objects from a fluid-like material utilizing selective solidification of thin layers of the material at a working surface to form and adhere successive layers of the object (i.e. laminae). In stereolithography, data representing the three-dimensional object is input as, or converted into, two dimensional layer data representing cross-sections of the object. Thin layers of material are successively formed and selectively transformed (i.e., cured) into successive laminae according to the two-dimensional layer data. During transformation the successive laminae are bonded to previously formed laminae to allow integral formation of the three-dimensional object.
A preferred material typically used in a Stereolithographic Apparatus (SLA) is a liquid photopolymer resin. Typical resins are solidifiable in response to selected wavelengths of electromagnetic radiation (e.g., selected wavelengths of ultraviolet (UV) radiation or visible light). This radiation of selected wavelength may be termed “solidifying radiation”. The electromagnetic radiation is typically in the form of a laser beam which is directed to a target surface of the resin by way of two orthogonal computer controlled scanning mirrors. The scanning speed of the beam across the liquid surface is controlled so as to provide a desired exposure and associated depth of cure. A more detailed description of stereolithographic techniques (i.e. methods and apparatus) is found in the following patents and applications which are hereby incorporated by reference:
U.S. Pat. No. 4,575,330 to Hull:
Describes the fundamentals of stereolithography.
U.S. Pat. No. 5,058,988 to Spence et al.:
Describes the use of beam profiling techniques in stereolithography.
U.S. Pat. No. 5,059,021 to Spence et al.:
Describes the use of scanning system drift correction techniques for maintaining registration of exposure positions on the target surface.
U.S. Pat. No. 5,104,592 to Hull et al.:
Describes the use of various scanning techniques for reducing curl-type distortion in objects that are being formed stereolithographically.
U.S. Pat. No. 5,123,734 to Spence et al.:
Describes a technique for calibrating a scanning system on a stereolithographic apparatus.
U.S. Pat. No. 5,133,987 to Spence et al.:
Describes the use of a large stationary mirror in the beam path between the scanning mirrors and a target surface.
U.S. Pat. No. 5,182,056 to Spence et al.:
Describes the simultaneous use of multiple wavelengths to expose the resin.
U.S. Pat. No. 5,184,307 to Hull et al.:
Describes the use of slicing techniques for converting three-dimensional CAD data into cross-sectional data for use in exposing the target surface to appropriate stimulation.
U.S. Pat. No. 5,321,622 to Snead et al.:
Describes the use of Boolean operations in deriving cross-sectional data from three-dimensional object data
U.S. Pat. No. 5,999,184, to Smalley et al.:
Describes the use of solidification techniques to simultaneously cure multiple layers.
U.S. Pat. No. 5,965,079, to Gigl et al.:
Describes various scanning techniques for use in stereolithography.
Commercially available photopolymers for use in Stereolithography are typically of acrylate, epoxy or combined chemistry. Typically, resins contain a plurality of components. These components may include one or more photoinitiators, monomers, oligomers, inert absorbers, and other additives. The usefulness of resins for stereolithography is in part determined by the photospeed of the resin and the ability of the resin to form adequately cohesive laminae of appropriate thickness. It is desired that the photospeed be high enough to enable rapid solidification of cross-sections with available power levels of solidifying radiation. Further, since the depth of polymerization in the resin is linked to the locations at which photons are absorbed, absorption of photons by the resin must be sufficient to form adequately thin layers. Examples of preferred photopolymers include, but are not limited to, SL 5170, SL 5180, SL 5081, SL 5154 and SL 5149 (manufactured by Ciba Specialty Chemicals Corporation North America of Los Angeles, Calif. and as sold by 3D Systems, Inc. of Valencia, Calif.), SOMOS 6100, 6110, 5100, 5110, 2100 and 2110 (manufactured by Du Pont Company, New Castle, Del.).
The photoinitiators are the component of the resin that determines the photosensitivity of the resin at a given wavelength. Radiation absorption by the photoinitiator leads to chemical changes in the photoinitiator which can cause polymerization of the monomers and oligomers. Thus, radiation of appropriate wavelengths to be absorbed by the photoinitiator is known as solidifying radiation. At some wavelengths the monomers/oligomers can absorb electromagnetic radiation. As absorption by the monomers/oligomers typically do not yield an efficient polymerization reaction, the absorption of solidifying radiation by the monomers/oligomers is typically undesired. Thus, the most effective wavelengths for use in stereolithography are those which are strongly absorbed by the photoinitiator (high coefficient of absorption) and only weakly absorbed by the monomers and oligomers (low coefficient of absorption). Examples of preferred photoinitiators include, but are not limited to, triarylsulfonium salts, mixtures of triarylsulfonium salts with phosphate salts or antimonate Salts; 2,2-dimethoxy-2-phenyl acetophenone (BDK) C 16H
16
O
16
; 2,4,6-trimethyl benzoyl diphenyl phosphine oxide (TPO); and 1-hydroxycyclohexyl phenyl ketone (HCPK) C
13
H
16
O
2
.
The useable wavelength range is bounded at the low wavelength end by monomer/oligomer absorption properties and at the upper wavelength end by photoinitiator absorption. As such, the reactive (i.e., actinic) spectral sensitivity of a photopolymer resin may be described as the product of the photoinitiator absorption spectrum and the monomer/oligomer transmission spectrum, as shown in FIG.
1
.
FIG. 1
depicts plots of photoinitiator absorption
11
, monomer/oligomer transmission
13
, and reactive sensitivity or reactive response
15
of the resin. As the absorption and transmission coefficients not only depend on the specific chemical composition of each component, but also on the concentrations of each component within the resin, shifts in wavelength for peak reactive response may result due to changes in either composition or concentration. For a given resin composition this peak can be readily determined by one of skill in the art.
In other words, the absorption by the monomer/oligomer, which depends upon the wavelength of radiation, affects the effectiveness of activitating the photopolymers as, in some instances, monomer/oligomer absorption competes with the absorption by the photoinitiator.
In the example of
FIG. 1
, the peak reactive response occurs within a range of about 328 nm-337 nm and the half maximum response falls within the range of about 320 nm to about 345 nm. As such, in this example electromagnetic radiation within the range of 320 to 345 nm is preferred and electromagnetic radiation within the range of 328 to 337 nm is even more preferred. The more preferred
Hug William F.
Partanen Jouni P.
3-D Systems, Inc.
D'Alessandro Ralph
Laurenson Robert
Lee John D.
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