Plastic and nonmetallic article shaping or treating: processes – Stereolithographic shaping from liquid precursor
Reexamination Certificate
2000-03-21
2002-07-23
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Stereolithographic shaping from liquid precursor
C264S435000, C264S494000
Reexamination Certificate
active
06423260
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for producing ordered parts from liquid crystal monomers. More particularly, it relates to a method and apparatus for providing ordered parts from non-ordered liquid crystal monomers by stereolithography, photolithography, ink jet deposition, or other systems.
The applications for built parts, such as those produced by stereolithography, have grown from simple visualization of engineering designs to fabrication of functional end-use prototypes. With the growth of applications has come a need for higher performance resins. In particular, the upper-use temperatures of cured resins needs to approach or exceed 200° C. for applications such as directly formed molds for injection molding, and under-the-hood automotive applications. Improved mechanical properties such as modulus and impact strength are also important for these applications. Dimensional accuracy has been a key issue for rapid building of functional parts. Dimensional accuracy is a function of resin shrinkage, draw style patterns, beam diameter compensation, galvanometer calibration, etc. While part accuracy has improved dramatically since the inception of stereolithography, further improvements are possible.
Accordingly, the need exists for an improved method and apparatus which can be used to build parts having anisotropic properties and having upper use temperatures exceeding 100° C.
SUMMARY OF THE INVENTION
That need is met by the present invention which provides an apparatus and process for aligning liquid crystal (LC) monomers and then photopolymerizing to produce parts having glass transition temperatures exceeding those possible with commercially available stereolithography resins, and having optimized mechanical properties. Parts with isotropic, anisotropic, or a combination (as a function of location in the part) of isotropic and anisotropic properties may be produced by varying the layer-to-layer alignment of the LC monomer or by varying the cure temperature (i.e., varying the mesogenic state). Thermosetting liquid crystal monomers contain rod-like mesogenic cores connected by alkane spacers to reactive end-groups. Like crystalline solids, LC materials have some kind of long range molecular order, however, they lack the three-dimensional translational order found in truly crystalline materials.
The simplest type of mesogenic phase is called nematic. In nematic phases, the molecular axis are on average parallel but lack any kind of translational order as indicated in FIG. IA. Smectic phases have both uniaxial molecular orientation and some degree of translational order as indicated in FIG. IB. Many different types of smectic phases have been identified. Some are more fluid in nature (e.g., smectic A) while others are more solid in nature (e.g., smectic D). At the clearing temperature, order disappears and the phase structure becomes isotropic as indicated in FIG. IC. The viscosity of nematics can be considerably lower than those of smectics, on the other hand, smectics have a higher degree of order.
Macroscopic alignment of the LC monomer in a preferred direction can be induced by a variety of means such as by rubbed substrates, magnetic field, electric field, and shear. Photopolymerization, such as by UV laser or visible light laser, of the LC monomer in the aligned state “locks” in the anisotropic structure resulting in materials with anisotropic physical and mechanical properties. Alternatively, a broad band UV light source such as a mercury or xenon lamp or fiber optic light source can be used. A photoinitiator is added in an amount of between about 0.1 and about 4% by weight and preferably between about 0.5% and about 2.0% by weight prior to photopolymerization. Still as a further alternative, a high intensity visible light source, and as a halogen lamp, may be used. In that instance, a visible light photoinitiator would be used. Mechanical strength and stiffness are greater in the molecular alignment direction than in the transverse direction. Also, because the reactive end groups are more tightly packed, cure in an aligned state results in lower shrinkage than is obtained with conventional resins.
Layered objects can be “built” using LC monomers where the layers or regions within the layers are aligned using an external force such as shear, electrical field, or magnetic field forces or combinations thereof. Thus, a magnet may be used to create layers in which the molecular alignment within sections of each layer may be altered by controlling the angle between the magnetic poles and the build axis. In a preferred embodiment, the layers or areas within the layers may be aligned using a magnet on a rotating platform.
Thus, the preferred apparatus of the present invention is a conventional stereolithography apparatus with the addition of a magnet outside the vat in order to align the monomer before cure. The magnet is positioned on a rotatable platform so that alignment can be established at any angle relative to the galvanometer axis. The vat is temperature controlled over a wide range of from about 25° C. to about 150° C. This is desirable so that it is possible to work with all LC phases: smectic, nematic and isotropic. Optionally, the apparatus may contain a molecular alignment measurement device such as an ellipsometry device. Ellipsometry is analogous to birefringence except that reflectance measurements are used instead of transmitted light.
Alternatively, the part can be built with an ink jet deposition apparatus. As in stereolithography a magnet on a movable swivel surrounding the object would be used to align the LC monomer which is deposited by drop-on-demand jets. Layer by layer, a partial layer curing proceeds using a lamp, fiber optic light source, or laser.
As yet another alternative embodiment, the part can be built with a photolithographic apparatus. As in stereolithography a magnet on a movable swivel surrounding the object would be used to align the LC monomer. Layer by layer, partial layer curing, proceeds by exposing the LC monomer to a light source through a photolithographic mask. The photomask can be generated xerographically or by liquid crystal display. Each layer is exposed with a different photomask.
Accordingly, it is an object of the present invention to provide a method for producing ordered objects from initially non-ordered liquid crystal monomers. It is also an object of the present invention to provide parts produced by such a method. It is a further object of the present invention to provide an apparatus for performing, such a process.
These and other objects and advantages of the present invention will become apparent from the detailed description of the preferred embodiments and claims presented below.
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Chartoff Richard P.
Schultz John W.
Ullett Jill S.
Killworth, Gottman Hagan & Schaeff, L.L.P.
Tentoni Leo B.
University of Dayton
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