Plastic and nonmetallic article shaping or treating: processes – Stereolithographic shaping from liquid precursor
Reexamination Certificate
2001-08-31
2004-07-20
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Stereolithographic shaping from liquid precursor
C118S100000, C118S120000, C264S308000, C425S174400, C425S196000, C425S375000, C427S356000
Reexamination Certificate
active
06764636
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a manufacturing technique for three-dimensional parts based on computer data representing their shape, and a device embodying this technique. Creating parts by the transformation of successive volumes (generally thin layers) of a raw material in an initial and secondary state by means of a device inducing said transformation, through the repetition of a cycle specifically involving a transformation phase of the material in at least one part of the working field performed by the device inducing the transformation, and a coating phase of the transformed material by the material which is not yet transformed, is known art.
2. Description of the Prior Art
Most of the so-called “Rapid Prototyping” machines employ this type of technique, in particular stereolithography machines, which use a photosensitive liquid raw material (which may be polymerized or reticulated) combined with a lighting device (using ultraviolet laser scanning, for example), and the so-called powder sintering machines, using a raw material in the form of a powder, which may be locally sintered by thermal means (infrared laser scanning, for example).
Numerous patent applications have been filed in relation to this type of machine, in particular in the field of stereolithography. A complete description of the technique is provided in document EP 0361847, and similar techniques are also described in documents EP 0450762 and EP 0484182. In document EP 0287657 a detailed description is provided of a powder sintering technique, as well as a description of the device for the implementation of the technique.
One feature common to the stereolithography and powder sintering techniques is that they require the implementation of means to spread the raw material in the form of thin layers (to perform the coating phase), automatically, rapidly and homogeneously. In general, the coating phase consists in sweeping the surface of the working field with the aid of at least one elongated device called a recoater blade, which is placed in movement over the working field. Depending on the case, the recoater blade transports the material used for the coating, or simply levels off a previous deposit of material.
Numerous devices have been designed to embody the recoater blade. For example, in the field of powders, in document EP 0287657, there is a description of a recoater blade composed of a dynamic roller which rotates around it axis, and is placed in parallel to the working field; the lower part of the roller has the same elevation as the working field. This roller also travels carrying a volume of material located upstream and gradually spreads a new layer of material downstream, thus coating the previously transformed material.
With respect to liquid raw materials, two types of techniques can be distinguished: the specific coating process is ensured in advance on the working field by coating media (chute, spray, immersion, etc.), and the recoater blade simply levels the free surface of the liquid by means of a horizontal sweeping movement, or the recoater blade performs the coating and leveling functions simultaneously. The second mode of operation involved the transport by the recoater blade (or by associated attached media) of a volume of material sufficient to coat the zones, which have just been solidified.
Various devices have been designed for the creation of the recoater blade, for example, a simple rigid blade (cf. Document EP 0361847), whose cross-section may have a specific shape, or the association of two rigid elements (document EP 0484182), which are attached to brush elements laid out in staggered rows. These initial devices have not been completely satisfying.
In fact, when using a so-called “simple recoater blade,” such as, for example, a blade (or any other more or less flexible equivalent), whose lower part comes in contact with the free surface of a liquid, in parallel to said free surface, stress is generated in the area of contact between the blade and the liquid. This local shear stress is transmitted to the liquid located below the free surface, at a depth much greater than the thickness of the placed layer.
In the case of the resins traditionally used in stereolithography, whose behavior is similar to that of a Newtonian liquid, with viscosities of several thousand centipoises, the recoater blade may generate movement in the liquid at a depth which may range from several millimeters to several centimeters.
The portions of solid or semi-solid material transformed during the preceding cycles represent obstacles to the liquid in movement, which translate into strong variations of the liquid's flow condition. Therefore, near the edges of the section being produced, flatness defects appear in the layer that was just spread by the recoater blade. A schematic illustration of these faults is provided in document WO 95/15842, where meniscuses (concave or convex) are depicted near the liquid-solid transition areas.
These meniscuses cause geometric faults on the parts created, and, once solidified, represent an additional impediment for the placement of the following layer, which finally, translates into an amplification of defects, incompatible with the coating device (risk of pulling up the layers, material accidents, etc.).
In order to limit these problems, it is sometimes possible to slow the speed of movement of the recoater blade, or impose a setting period prior to proceeding with the following transformation phase, but these solutions result in a substantial increase in part manufacturing time, leading to increased production costs, which may be prohibitive.
The solution proposed in document WO 95/15842, does not call into question the use of so-called “simple recoater blade,” but rather consists in creating a covering structure which closely follows the shape of the parts and has the effect of distancing the meniscus problems from the areas of said parts. This clever technique allows for the use of a “simple recoater blade” composed of a thin, elongated, flexible part, and allows for very short time frames for the coating phases. The provision of the material necessary for the coating phase takes place by the transport of a type of wave of material, formed at the start of the recoater blade's movement. The volume of material included in said wave is progressively consumed by the effect of the combination of gravity and movement.
Another solution, described in document WO 96/23647 consists in using a so-called “active recoater blade,” in this case, a dynamic roller which moves in a “counter-rotation” direction, associated with a type of barrier, composed of a rectilinear blade, whose lower side is located a short distance from the upper part of the roller. A roller turning in a “counter-rotation” direction is such that any point located on its periphery when it passes the point of its trajectory closest to the working field, has a relative tangential speed with respect to the axis of rotation moving in the same direction as the speed of movement of the rotational axis of the roller with respect to the working field.
The operation of this device is illustrated in the figures attached to this document, for the case in which a prior deposit of material has take place downstream. The barrier forms an obstacle to the free circulation of the liquid located downstream of the roller, allowing for the regulation of the thickness of the film formed on the upstream portion of the roller. The film's free surface meets that of the layer formed upstream, thus forming a sharp lap-back point in proximity to the lower portion of the roller. This lap-back point defines the height of the free surface of the material placed upstream, and since it is constantly fixed with respect to the roller axis, the layer may be leveled.
A detailed analysis of the physical phenomena involved is proposed in the document in order to explain the advantages of such a device with respect to a single blade (simple recoater blade), in particular wi
Allanic Andre-Luc
Schaeffer Philippe
3-D Systems, Inc.
D'Alessandro Ralph
Tentoni Leo B.
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