Plastic and nonmetallic article shaping or treating: processes – Stereolithographic shaping from liquid precursor
Reexamination Certificate
2000-07-13
2002-12-31
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Stereolithographic shaping from liquid precursor
C264S308000, C359S298000, C425S174400, C425S375000
Reexamination Certificate
active
06500378
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to the field of rapid prototyping (RP) manufacturing or processes.
2. Related Art
RP is a technology which has seen great advances since its initial application in the 1980's. In one common embodiment known as stereolithography, RP manufacturing comprises a bath of curable liquid wherein some movable point within the bath is subjected to stimulation by a prescribed curing source. As the source is moved with respect to the bath or as the bath is moved with respect to the source, the point which undergoes solidification or curing is constantly made to move. The result is the construction of a solidified mass of cured material contained within the otherwise liquid bath. The region commonly solidified occurs at or very near the surface of the bath in most practical applications. As the liquid is solidified, the solid structure is progressively lowered into the bath allowing the uncured liquid to flow over the surface which is in turn subjected to the same process. By continuing to solidify these very thin layers or laminae, the solid object is built up into its final shape. Bonding of one layer to a previous layer is an inherent property of the process as is known in the art. An adequate description of this process can be found in U.S. Pat. No. 4,575,330 issued to Charles W. Hull, which is hereby incorporated by reference.
The main advantages of the RP process are its ability to drastically reduce the time between product conception and final design, and its ability to create complex shapes. More traditional modeling or prototyping is obtained from an iterative generation of a series of drawings which are analyzed by the design team, manufacturing, the consumer, and perhaps others until a tentative final design results which is considered viable. This agreed upon design is then created by casting and/or machining processes. If molds are needed, these must be fabricated as well. The finished prototype is then tested to determine whether it meets the criteria for which the part was designed. The design and review process is often tedious and tooling for the creation of the prototype is laborious and expensive. If the part is complex, then a number of interim components must first be assembled. The prototype itself is then constructed from the individual components.
Use of RP significantly reduces the expense and time needed between conception and completion of the prototype. Commonly, the concept is rendered in CAD (computer aided design). As this process is fully electronic, drawings are not required for fabrication. The CAD system is used to generate a compatible output data file that contains information on the part's geometry. This file is typically converted into a “sliced” data file that contains information on the part's cross-section at predetermined layer depths. The RP control system then regenerates each cross-section sequentially at the surface of the curable resin. The fabricated part can be analyzed by the team or used for various form, fit, and functional tests. Due to the rapid speed and low cost of the process, several designs can be fabricated and evaluated in a fraction of the time and for significantly less than it would take to machine each concept. Since the RP process creates the structure by the creation of very thin laminae, complex components with internal complexities can be easily rendered without requiring the assembly of a plurality of individual components.
A disadvantage of RP other than its initial cost for the technology is that the time associated with the creation of each part can be longer than desired. Since creation of the part occurs in a point-by-point, layer-by-layer process, the time necessary to produce a single part can become excessive. For instance, an arbitrary part of six cubic inches with a 50 percent fill ratio will require approximately six hours to image utilizing current stereolithographic techniques having a 0.005 inch layer build, a laser spot size of 0.010 inches, and a 100 inch per second laser rate assuming no losses. This estimate comprises imaging time alone and does not account for platform movement, sweeping of the resin surface, resin setting time, and mirror inertias that take considerable time between formation of each lamina. Reduction in fabrication times continue to be a desirable goal. Though the above description pertains to the process of stereolithography; the process, as well as the general advantages and disadvantages are similar for other RP technologies.
Another disadvantage of RP specific to stereolithography is that parts produced by this process leave the bath in a very soft state requiring a post-cure process. This too takes time, typically a minimum of 20 minutes. In this soft state, the part is very deformable. Since the part is removed from the bath in a fragile state, supports are often needed to assist in the part's creation and to ensure proper post-curing without significant deformation. In fact, these supports are often vital to parts created by stereolithography especially those parts having overhanging or other unsupported features. The soft state associated with conventional stereolithography is inherently unavoidable for at least two reasons. The first is that the stereolithography though not as rapid as the present invention is still optimized for speed. Thus, increasing the exposure time of the laser at each point on the surface of the resin would significantly increase the processing time of the part. Secondly, to create the part, the laser is rastered without overlapping and the energy at the forming lamina has a gaussian distribution. This traps uncured photopolymer between cured lines. This could be avoided by overlapping the laser paths; however, this would also greatly increase processing time.
Accordingly, the development and production of a faster method to create prototypes and finished parts using RP technology is a desirable goal. Improvement to the existing processes would greatly increase the use of RP and would result in the continued advancement of technology in general due to the increased ease in the creation of complex parts.
SUMMARY OF THE INVENTION
Briefly, the method and apparatus comprises processing an entire cross-section of the object at one time. By reducing the time needed to stimulate the bath surface to form the laminae, the entire object could be formulated more quickly. Increasing the quantity of material stimulated at each time interval is a preferred way to perform this function. The invention therefore comprises a method of solidifying a discrete quantity of a curable medium by subjecting the surface of said medium to a prescribed energy source and controlling that source in such a fashion to cure only the portion desired while leaving the remainder of the medium uncured. To accurately image an entire cross section at any one time in this manner requires accurate control of the energy source. The means currently preferred to accomplish this is through use of reflective digital light switch technology. This technology was created by Texas Instruments (TI) and is currently referred to as deflectable beam spatial light modulation (SLM). TI refers to the process when applied to its typical applications under their common law trademark as Digital Light Processing (DLP). More specifically, they refer to the critical mechanism used as a Digital Micromirror Device (DMD). U.S. Pat. No. 5,061,049 for a “Spatial light modulator and method” issued on Oct. 29, 1991 to L. Hornbeck of Texas Instruments provides the basic configuration of such a device. Further descriptions of this technology can be found in numerous white papers by TI as well as issued patents including among others, a presentation placed in writing originally given by Larry J. Hornbeck entitled “Digital Light Processing for High-Brightness, High-Resolution Applications” on Feb. 10-12, 1997 in San Jose Calif. A history of the development of the DMD can be found by the same author in an article titled “Fro
EOM Technologies, L.L.C.
Schmeiser Olsen & Watts
Tentoni Leo B.
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