The present invention is directed to methods for forming three-dimensional objects on a layer-by-layer basis and, more particularly, is directed to a method of creating supports for stereolithographic objects by using an improved method of curing the photopolymer liquid from which the three-dimensional build object is generated.
In recent years, many different techniques for the fast production of three-dimensional models have been developed for industrial use. These are sometimes referred to as rapid prototyping and manufacturing (xe2x80x9cRPandMxe2x80x9d) techniques. In general, rapid prototyping and manufacturing techniques build three-dimensional objects layer by layer from a working medium 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, presently the most common RPandM technique, may be defined as a technique for the automated fabrication of three-dimensional objects from a fluid-like material utilizing selective exposure of layers of the material at a working surface to solidify 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. Layers of material are successively formed and selectively transformed or solidified (i.e. cured) using a computer controlled laser beam of ultraviolet light (UV) light 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.
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 CAD/CAM. However, such systems have encountered difficulties relating to shrinkage, curl and other distortions, as well as resolution, accuracy, and difficulties in producing certain object shapes.
Although stereolithography has shown itself to be an effective technique for forming three-dimensional objects, various improvements addressing the technology""s difficulties have been desired for some time. Many improvements have addressed the aforementioned difficulties and have been made to object accuracy, speed and appearance of the build object over the years. However, there still remains a need for further improving the build object appearance and ease of making certain object shapes. Various aspects of the stereolithographic building process can impact the build object appearance and ease of generation of the three-dimensional object. For instance, one aspect and area for needed improvement is with build object supports.
The fabrication of supports is the first step in the stereolithographic build process since the build object initially must be supported on the elevator platform in the vat of photopolymer of the stereolithographic systems. Supports are required for a plurality of reasons. The supports act like fixtures in conventional machining and hold the build object or part in place during the build process. Supports also provide a means of securing isolated segments or islands that would otherwise float away and they are used to restrain certain geometries that would likely experience distortion, such as curl, during laser curing of the photopolymer liquid. Support structure must be removed from the build object in a manual and labor-intensive post-processing step. Supports are shown in the figures in U.S. Pat. No. 4,575,330. The supports shown in this patent attach the object to the platform.
The original type of posts or supports used were actually formed by curing single points. These points were cured for specific lengths of time to give appropriate cure depths, with a corresponding cure width. This type of post is limited by its strength and the associated cure time required to achieve this strength level, if it is possible to obtain the desired strength at all.
Another type of post or support structure is based on the need to increase the adhesion strength between layers. The adhesion strength is proportional to the area of contact between layers. When curing a point, the cure width quickly reaches a limit where additional cure width is unpractical. Therefore, another method of increasing contact area was implemented. Instead of curing supports that are point vectors in cross-section, this new method uses supports that are polygons in cross-section. These polygons can be triangles, rectangles, octagons, etc. These structures give much greater contact area between layers and much greater adhesion strength, along with much greater structural strength against horizontal translation. These supports work reasonably well, but they still present difficulties because: (1) they are hard to remove from the object; (2) they support only a limited number of object vectors; and (3) they require the use of a base to support the polygons to insure attachment to the perforated build platform.
Ease of removal of the support structure is especially critical as it affects both speed of the build process and appearance of the final build object. Supports traditionally have been difficult to remove once the support material has been cured to support the object, either on the build platform or to support a hollow structure within the build object. Removal of supports that leave marred up-facing or down-facing surfaces on the build object detract from the appearance of the build object and the utility of the stereolithographic process.
One form of three-dimensional object modeling that addresses the issue of support removal is the fused deposition modeling technique. U.S. Pat. No. 5,503,785 discusses the need to remove support structure with minimal effort so as to obtain quick and easy removal. This patent disclosed a method of using a first material to build the modeling object in the support structure, and a second release material that is formed of a different composition than the material for the object and the support structure. The second release material is deposited in the space between the build object and the support structure and is selected so that it forms a weak, breakable bond with the first material in a readily separable joint along the object, support structure interface. The disadvantage to this approach, however, is that the apparatus and method requires a pair of dispensers, each having its own discharge tip for different material and material reservoirs for the build object/support structure material and the release layer material. This approach unnecessarily adds to the complexity of the apparatus.
These problems are solved in the method and support structure of the present invention.
It is an aspect of the present invention that a smoother surfaced build object or part is obtained that requires decreased processing because of the ease of removal of the support structure from the build object due to the use of a method of curing the photopolymer liquid to form the build object that employs differential curing of both the build object and the support structure.
It is another aspect of the present invention that differential curing of the support structure at the up-facing and down-facing surfaces where the supports intersect the part makes removal of the supports easier, less time consuming, and less detrimental to the appearance of the finished build object or part.
It is a feature of the present invention that less curing of the support structure occurs at the up-facing and down-facing surfaces to create weak points at the build object or part support boundary to permit the supports to easily be broken away.
It is another feature of the present invention that the stereolithographic build method identifies the geometric polygons support that actually touch the build object.
It is yet another feature of the present invention that the support structure has less contact surface area with the build object than prior support structure.
It is still another feature of the present invention that different cure depths of the photopolymer material forming the build object are employed for different boundaries of the build object.
It is an advantage of the present invention that the initial preparation is faster due to employing more automation and less user analysis of the supports to be built.
It is another advantage of the present invention that better build object support is provided that leads to higher yields from the use of the stereolithographic process and apparatus since the build object can be over supported, resulting in less failures due to lack of support.
It is still another advantage of the present invention that the method involves less post-processing of the build object since it is easier to remove the support structure that contacts both the support platform or elevator and the build object.
It is yet another advantage of the present invention that the build object has better appearance because there is less interface or contact surface with the support structure.
It is yet another advantage of the present invention that the weak points at the interface between the support structure and the build object make it easier to remove the support structure.
It is still a further advantage of the present invention that the support structure does not retain trapped resin between the supports in the part being built.
It is still another advantage of the present invention that very delicately featured parts may be constructed because the support structure may be removed without harming the part.
These and other aspects, features, and advantages are obtained by the present invention through the use of a method of differential curing of the support structure that interfaces between up- and down-facing surfaces and the build object to create a faster generated, better appearing final part that requires less post-processing.