The process of liquid composite molding (LCM), including resin transfer molding, to produce structural composites has gained considerable attention over the last decade. One barrier to the process gaining further acceptance has been the lack of adequate knowledge and expertise in the cost-effective production of reinforcement preforms. If the LCM process is to remain economically viable, low-cost methods of preform production must be further advanced. At present, two basic input forms of fiberglass are available to the LCM molder for producing a stiff three-dimensional (3-D) preform: a thermoformable continuous strand mat and a multi-end roving.
Three basic routes are available for fabricating LCM preforms from the two basic input forms of reinforcement (mats and rovings). These are cut-and-sew preforming, directed fiber spray-up, and stamping of thermoformed mats. Cut-and-sew preforming is utilized in aerospace and low-volume applications. In a cut-and-sew preform, areas of material are defined based on the requirements determined in a finite element analysis. In this process, the general size and shape of each area is cut from a conformable material and fit to the part mold or a part model; this is then cut, trimmed and sewn to fit the desired dimensions. A final template is built and the actual reinforcement is cut and sewed on the preform. This process is slow and labor intensive.
The directed fiber spray-up process utilizes an air-assisted chopper/binder gun which conveys glass and binder to a perforated metal screen shaped identical to the part to be molded. The chopped fibers are held in place on the screen by a large blower drawing air through the screen. Once the desired thickness of reinforcement has been achieved, the chopping system is turned off and the preform is formed by polymerizing or curing the binder. Once stabilized, the preform is cooled and removed from the screen. A pre-shaped screen or perforated mold is required in this process.
The thermoformed mat process requires an oven to heat the mat, a frame to hold it while being stretched into shape, and a tool to form the mat into a preform. In a typical process, several plies of mat would be cut to the approximate desired shape of the molded part, allowing extra material to be held in a frame. The frame containing the material is then placed in an oven to be heated (up to 170.degree. C.) and then quickly transferred to the forming tool. The tool is closed, forming and cooling the mat for a short period of time. After removing the frame and trimming the waste fibers clamped in the frame, the preform is ready for molding. Both thermoplastic and thermoset binder systems are available to retain the formed shape. Again, a pre-shaped tool or mold is required in this process.
The preparation of fiber preforms for metal matrix composites (MMCs) or ceramic matrix composites (CMCs) is often accomplished by machining blocks or sheets of fibers (e.g., preforms used in U.S. Pat. No. 4,141,948, Feb. 27, 1979 to W. Laskow and C. Morelock). A preform can also be made by pouring a curable mixture of carbon fiber and binder into a mold, followed by the removal of excess binder by the application of reduced pressure or vacuum pumping (e.g., U.S. Pat. No. 4,320,079, Mar. 16, 1982 to W. Minnear and W. Morrison). In a similar approach, a fiber preform precursor is impregnated with a colloidal suspension of inorganic material. This impregnated preform precursor is cooled to precipitate the inorganic material from the suspension and then dried to form a rigidized fiber preform (U.S. Pat. No. 4,902,326, Feb 20, 1990 to D. Jarmon). Other methods of making composite preforms may be found in the following U.S. Patents: U.S. Pat. Nos. 5,346,774 (Sep. 13, 1994 to K. Burgess), U.S. Pat. No. 5,350,545 (Sep. 27, 1994 to H. Streckert, et al.), U.S. Pat. No. 5,456,981 (Oct. 10, 1995 to P. Olry, et al.), U.S. Pat. No. 5,571,628 (Nov. 5, 1996 to L. Hackman), U.S. Pat. No. 5,529,620 (Jun. 25, 1996 to W. Gorbett, et al.), U.S. Pat. No. 5,705,008 (Jan. 6, 1998 to D. Hecht), and U.S. Pat. No. 4,659,610 (Apr. 21, 1987 to S. George, et al.). A common shortcoming of these preform making methods is the need to have a pre-shaped mold or tool against which a preform structure of a desired shape is made. Otherwise, the preform must be made into a larger-than-necessary shape and then machined down to the desired shape.
Solid freeform fabrication (SFF) or layer manufacturing (LM) is a new fabrication technology that builds an object of any complex shape layer by layer or point by point without using a pre-shaped tool (die or mold). This process begins with creating a Computer Aided Design (CAD) file to represent the geometry or drawing of a desired object. As a common practice, this CAD file is converted to a stereo lithography (.STL) format in which the exterior and interior surfaces of the object is approximated by a large number of triangular facets that are connected in a vertex-to-vertex manner. A triangular facet is represented by three vertex points each having three coordinate points: (x.sub.1,y.sub.1,z.sub.1), (x.sub.2,y.sub.2,z.sub.2), and (x.sub.3, y.sub.3,z.sub.3). A perpendicular unit vector (i,j,k) is also attached to each triangular facet to represent its normal for helping to differentiate between an exterior and an interior surface. This object image file is further sliced into a large number of thin layers with the contours of each layer being defined by a plurality of line segments connected to form polylines. The layer data are converted to tool path data normally in terms of computer numerical control (CNC) codes such as G-codes and M-codes. These codes are then utilized to drive a fabrication tool for building an object layer by layer.
This SFF technology enables direct translation of the CAD image data into a three-dimensional (3-D) object. The technology has enjoyed a broad array of applications such as verifying CAD database, evaluating design feasibility, testing part functionality, assessing aesthetics, checking ergonomics of design, aiding in tool and fixture design, creating conceptual models and sales/marketing tools, generating patterns for investment casting, reducing or eliminating engineering changes in production, and providing small production runs. The potential of adapting SFF technology for the preparation of reinforcement preforms from fibers and/or particulates for composite applications has been largely ignored.
The SFF techniques that potentially can be used to fabricate short fiber- or particulate-reinforced composite parts or their precursor preforms include fused deposition modeling (FDM), laminated object manufacturing (LOM) or related lamination-based process, and powder-dispensing techniques. The FDM process (e.g., U.S. Pat. No. 5,121,329; 1992 to S. S. Crump) operates by employing a heated nozzle to melt and extrude out a material such as nylon, ABS plastic (acrylonitrile-butadiene-styrene) and wax in the form of a rod or filament. The filament or rod is introduced into a channel of a nozzle inside which the rod/filament is driven by a motor and associated rollers to move like a piston. The front end, near a nozzle tip, of this piston is heated to become melted; the rear end or solid portion of this piston pushes the melted portion forward to exit through the nozzle tip. The nozzle is translated under the control of a computer system in accordance with previously sliced CAD data to trace out a 3-D object point by point and layer by layer. In principle, the filament may be composed of a fiber or particulate reinforcement dispersed in a matrix (e.g., a thermoplastic such as nylon). In this case, the resulting object would be a short fiber composite or particulate composite. The FDM method has been hitherto limited to low melting materials such as thermoplastics and wax and has not been practiced for preparation of metallic parts, possibly due to the difficulty in incorporating a high temperature nozzle in the FDM system.
A more recent patent (U.S. Pat. No. 5,738,817, April 1998, to Danforth, et al.) reveals a FDM process for forming 3-D solid objects from a mixture of a particulate composition dispersed in a binder. The method involves additional operations of preparing a mixture of particles and binder and forming the mixture into a filament or rod form. The mixture in this filament or rod form is then introduced into a nozzle with the leading portion of the filament/rod melted and extruded to deposit onto a work surface point by point and layer by layer for forming a 3-D shape. The binder in this 3-D shape is later burned off with the remaining particulate composition densified by re-impregnation or high-temperature sintering. A large amount of binder, up to 60.about.80% by volume, must be burned off and this represents a significant waste of material and requires a long duration of time to accomplish.
Other melt extrusion-type processes include those disclosed in Valavaara (U.S. Pat. No. 4,749,347, June 1988), Masters (U.S. Pat. No. 5,134,569, July 1992), and Batchelder, et al. (U.S. Pat. No. 5,402,351, 1995 and U.S. Pat. No. 5,303,141, 1994). These melt extrusion based deposition systems, if adapted for forming short fiber or particulate composite object would suffer from the same shortcomings as in FDM. Furthermore, the incorporation of 20.about.40% short fibers or particulates in a matrix melt would further increase the viscosity of the material in a flow channel, making it more difficult to operate the FDM or related extrusion-based process.
In a series of U.S. Patents (e.g., U.S. Pat. No. 5,204,055, April 1993), Sachs, et al. disclose a 3-D powder printing technique that involves using an ink jet to spray a computer-defined pattern of liquid binder onto a layer of uniform-composition powder. The binder serves to bond together those powder particles on those areas defined by this pattern. Those powder particles in the un-wanted regions remain loose or separated from one another and are removed at the end of the build process. Another layer of powder is spread over the preceding one, and the process is repeated. The "green" part made up of those bonded powder particles is separated from the loose powder when the process is completed. This procedure is followed by binder removal and metal melt impregnation or sintering. This technique is limited to one type of powder particles in one layer and is useful for fabricating uniform-composition material only. The technique does not lend itself for varying the powder composition from point to point for the preparation of heterogeneous materials.
This same drawback is true of the selected laser sintering or SLS technique (e.g., U.S. Pat. No. 4,863,538, Sep. 5, 1989 to C. Deckard) that involves spreading a full-layer of powder particles and uses a computer-controlled, high-power laser to partially melt these particles at desired spots. Commonly used powders include thermoplastic particles or thermoplastic-coated metal and ceramic particles. The procedures are repeated for subsequent layers, one layer at a time, according to the CAD data of the sliced-part geometry. The loose powder particles in each layer are allowed to stay as part of a support structure. The sintering process does not always fully melt the powder, but allows molten material to bridge between particles. Commercially available systems based on SLS are known to have several drawbacks. One problem is that long times are required to heat up and cool down the material chamber after building. In addition, the process has not been successfully applied to fabrication of fiber composite parts.
Most of other prior-art layer manufacturing techniques also have been largely limited to producing parts with homogeneous material compositions. Furthermore, due to the specific solidification mechanisms employed, many other LM techniques are limited to producing parts from specific polymers. For instance, Stereo Lithography and Solid Ground Curing (SGC) rely on ultraviolet (UV) light induced curing of photo-curable polymers such as acrylate and epoxy resins. Additionally, most of the current RP systems are not effective in adding fibers into a RP material or varying the fiber type of a composite object from layer to layer and from spot to spot.
Modified laminated object manufacturing (LOM) has been used to prepare polymer matrix and ceramic matrix composites (D. Klosterman, et al, in Proceedings of The 7.sup.th International Conference on Rapid Prototyping--1997, Mar. 31-Apr. 3, 1997, San Francisco, Calif., USA, ed. By R. P. Chartoff, et al.; pp.43-50 and pp. 283-292). The process involves, for instance, feeding, laminating and cutting thin sheets of prepregs (pre-impregnated fiber preform) in a layer-by-layer fashion according to computer-sliced layer data representing cross sectional layers of a 3-D object. The process cycle typically consists of laminating a single sheet of prepreg to an existing stack, laser cutting the perimeter of the part cross section, and laser-dicing or "cubing" the waste material. After all layers have been completed, the part block is removed from the platform, and the excess material is removed to reveal the 3-D object. This process results in large quantities of expensive prepreg materials being wasted.
In U.S. Pat. No. 5,514,232, issued May 7, 1996, Burns discloses a method and apparatus for automatic fabrication of a 3-D object from individual layers of fabrication material having a predetermined configuration. Each layer of fabrication material is first deposited on a carrier substrate in a deposition station. The fabrication material along with the substrate are then transferred to a stacker station. At this stacker station the individual layers are stacked together, with successive layers being affixed to each other and the substrate being removed after affixation. One advantage of this method is that the deposition station may permit deposition of layers with variable colors or material compositions. In real practice, however, transferring a delicate, not fully consolidated layer from one station to another would tend to shift the layer position and distort the layer shape. The removal of individual layers from their substrate also tends to inflict changes in layer shape and position with respect to a previous layer, leading to inaccuracy in the resulting part.
In U.S. Pat. No. 5,301,863 issued on Apr. 12, 1994, Prinz and Weiss disclose a Shape Deposition Manufacturing (SDM) system. The system contains a material deposition station and a plurality of processing stations (for mask making, heat treating, packaging, complementary material deposition, shot peening, cleaning, shaping, sand-blasting, and inspection). Each processing station performs a separate function such that when the functions are performed in series, a layer of an object is produced and is prepared for the deposition of the next layer. This system requires an article transfer apparatus, a robot arm, to repetitively move the object-supporting platform and any layers formed thereon out of the deposition station into one or more of the processing stations before returning to the deposition station for building the next layer. These additional operations in the processing stations tend to shift the relative position of the object with respect to the object platform. Further, the transfer apparatus may not precisely bring the object to its exact previous position. Hence, the subsequent layer may be deposited on an incorrect spot, thereby compromising part accuracy. The more processing stations that the growing object has to go through, the higher the chances are for the part accuracy to be lost. Such a complex and complicated process necessarily makes the over-all fabrication equipment bulky, heavy, expensive, and difficult to maintain. The equipment also requires attended operation.
Therefore, an object of the present invention is to provide an improved layer-additive process and apparatus for producing a 3-D reinforcement preform shape to be used for making a composite material part.
Another object of the present invention is to provide a computer-controlled process and apparatus for producing a multi-material preform shape, for use in a 3-D composite part, on a layer-by-layer basis.
It is a further object of this invention to provide a computer-controlled composite preform-building process that does not require heavy and expensive equipment.
It is another object of this invention to provide a process and apparatus for building a CAD-defined object in which the reinforcement composition pattern can be predetermined.
Still another object of this invention is to provide a layer manufacturing technique that places minimal constraint on the range of reinforcement materials that can be used in the fabrication of a 3-D composite object.