1. Field of the Invention
The present invention is directed to a process of fabricating a three-dimensional object from a light curable liquid resin, and more particularly to an improvement in such process for fabricating the three-dimensional object with increased accuracy and efficiency.
2. Description of the Prior Art
Such a three-dimensional object forming process from a light curable resin has been proposed in the art to be advantageous in fabricating a small quantity of product models or prototypes without relying upon molds or machining tools, as disclosed in Japanese Patent Early Publication [Kokai] No. 56-144478, and 62-35966. As schematically illustrated in FIGS. 1 and 2 of the attached drawings, the typical prior art process utilizes a container 10 supplied with a light curable liquid resin 20 and a light beam 30 such as a laser beam which is directed to the surface of the liquid resin 20 for solidification of the liquid resin 20 to a certain depth from the surface. It is therefore possible to form a cured layer 1 at the liquid surface which has a desired pattern or cross-sectional configuration by moving the light beam 30 across the liquid resin surface. Thus formed layer 1 is held on a movable carrier plate 12 and is lowered into the liquid resin 20, at which condition, new liquid flows over the layer 1 and is subjected to a like beam irradiation to form on that layer another cured layer 1 of the same or different configuration. In this manner, successive cured layers are build up to fabricate a three-dimensional object of any desired configuration. In the illustrated prior art system, a lens 33 is incorporated to focus the light beam 30 at a point immediately adjacent the surface of the liquid resin 20 to give an intense light energy thereto for efficient solidification of the liquid to a fixed depth. The carrier plate 12 is connected through an elevator arm 14 to a drive source so as to be vertically movable in a step-wise fashion within the container 10. The thickness of the individual layers 1 can be suitable controlled by adjusting the strength and focusing spot of the light beam 30, and the vertical distance for each step-wise movement of the carrier plate 21.
As known from the above, a three-dimensional object is expected to have a more accurate and smoother outer configuration as the successive layers are made thinner. This is because, as shown in FIG. 2, the two-dimensional configurations of the successive layers 1 will have to change over a differing cross-sectional portion of the desired object to thereby eventually leave apparent steps 4 between the ends of successive layers 1, which steps 4 appear on the outer surface of the three-dimensional object detracting from a smoothly curved surface, and therefore degrading the overall appearance of the object. Accordingly, there is a need in the above process to form the successive layers as thin as possible for fabrication of an accurately outlined three-dimensional object. However, as the layers are made thinner, the overall number of the layers is increased with an attendant increase in the step-wise operations of lowering the plate 12, thereby requiring more fabrication time and therefore lowering fabrication efficiency.
From a standpoint of improving fabrication efficiency, it has been a practice to use a light beam having an intensity strong enough to expedite the solidification and to move or scan the light beam quickly across the surface of the liquid resin for rapid solidification of the liquid resin. However, such rapid solidification inevitably incurs a correspondingly large amount of curing shrinkage and therefore residual stress. In fact, when the solidification is carried out slowly or steadily by moving the light beam slowly across the liquid surface, the layer being cured can be supplied with the liquid resin from the surroundings so as to compensate for the curing shrinkage. In addition shrinkage stress is relieved during slow solidification, thereby reducing the overall amount of the curing shrinkage as well as the residual stress in the cured layer. Nonetheless, with rapid solidification as required for increased fabrication efficiency, the supply of the liquid from the surrounding cannot keep with the shrinkage occurring at a rapid rate. In addition, the solidification proceeds before the shrinkage stress is sufficiently relieved, thereby leaving a relative large shrinkage as well as residual stress in the cured layer. Such large curing shrinkage and residual stresses are not acceptable and should be avoided for fabrication of a three-dimensional object, because of the fact that the curing shrinkage results in dimensional variations or in inaccurate reproduction of the three-dimensional object and that the residual stress will be relieved gradually over a long period after the fabrication of the three-dimensional object, this in turn can result in dimensional variation. Further, in the case when the layer is formed by continuously moving the light beam across the surface of the liquid resin, there may be develop local deformation or localized residual stress due to possible difference in the shrinkage between the previously and later cured portions, which increases the possibility of leaving remarkable local strain or uneven residual stress in the cured layer. Accordingly, it is also demanded in the fabrication of three-dimensional objects to minimize the curing shrinkage and the residual stress without considerable sacrifice in the fabrication efficiency, for accurate fabrication of the three-dimensional object. In the prior art, an attempt has been made also to obtain an accurate three-dimensional object by delicately controlling penetration depth and/or moving range of the light beam with the use of a computer. However, even with such delicate control, it is still difficult to eliminate the curing shrinkage and therefore obtain an accurately shaped three-dimensional object.