Abstract:
A method and apparatus for additive manufacturing wherein a fiber composite filament having an arbitrarily shaped cross section is softened and then flattened to tape-like form factor for incorporation into a part that is being additively manufactured.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This case claims priority of U.S. patent application Ser. No. 62/314,000 filed Mar. 28, 2016, which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to an additive manufacturing process using fiber composites. 
       BACKGROUND OF THE INVENTION 
       [0003]    Additive manufacturing—so called “3D printing”—is the term given to processes that manufacture objects via sequential-layer material addition/joining throughout a 3D work envelope under automated control. ISO/ASTM52900-15 defines seven categories of additive manufacturing processes: binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and vat polymerization. 
         [0004]    Composite 3D printing processes produce an object or part (hereinafter collectively “part”) by depositing composite filaments made of continuous fiber-reinforced thermoplastics, which quickly harden to form a layer. Successive layers of material are deposited to build the object. 
         [0005]    A fiber composite consists of fibers as the discontinuous phase, a matrix (typically a resin) as the continuous phase, and an interface region (the interface). The strength of a fiber composite is determined by the wettability of the fiber within the resin matrix. Wettability refers to the degree of adhesion and bonding between the resin and the fibers. A high degree of wetting means the resin has good surface adhesion to the fibers, coating the fibers uniformly. A low degree of wetting means the fibers are not completely coated by resin, resulting in void space or bubbles between the fiber and resin. 
         [0006]    When subjected to stress, a composite part with a high degree of wetting will effectively transfer the stress to the fibers through the resin matrix. This composite part then has a much higher tensile strength as compared to a part made only of resin material. In comparison, a composite part with a low degree of wetting will experience fiber pullout when subjected to stress. Fiber pullout results in ineffective stress transfer from the resin matrix to the fibers due to the void space at the fiber/resin interfaces. The void space acts as nucleating sites for cracks, resulting in premature failure of the part via crack propagation. The composite part with a low degree of wetting has a lower tensile strength as compared to a part made only of the resin material due to the higher void space with this composite part. 
         [0007]    To ensure a high degree of wetting, the composites industry mostly uses thermoset (e.g., epoxy, etc.) resin systems. Thermosets typically have a low viscosity compared to other polymers, resulting in high melt flow and a high degree of fiber wetting. While thermoset resins provide sufficient wetting, they cannot tolerate higher operating temperatures, often limited to 100° C. or less. As compared to thermosets, thermoplastics generally have a higher viscosity, making it more difficult to achieve sufficient fiber wetting; however, thermoplastics offer higher tensile strength and stiffness, higher toughness, and a higher operating temperature as compared to thermoset resins. 
         [0008]    U.S. patent application Ser. No. 14/184,010 discloses an apparatus for manufacturing an object having a deposition head including a nozzle for heating thermoplastic composite material having one or more fiber strands. In some embodiments, the apparatus also includes a turntable, a robotic arm for moving the deposition head, and spools of composite filament. 
         [0009]    That invention involved the use of a heated extrusion nozzle to melt and deposit thermoplastic fiber-composite filament. The composite feed filament comprises thousands (e.g., 1K, 3K, 6K, 12K, 24K, etc.) of continuous fibers impregnated with thermoplastic resin. The continuous fiber includes, without limitation, carbon, fiberglass, aramid (AKA Kevlar), and carbon nanotubes (CNT). The composite filament has a cylindrical, elliptical, or rectangular cross section. In the case of a rectangular cross section, the aspect ratio (width-to-thickness) is about 3:2; that is, it is distinguishable from a tape, which is significantly wider than it is thick. 
         [0010]    That deposition process completely melts the thermoplastic filament as it contacts the nozzle. As the continuous filament is pushed through the nozzle, the pressure of the nozzle forces the polymer underneath the continuous carbon fiber tow, which can result in exposed fiber on the top surface. As a result, there is a loss of fiber adhesion within the polymer matrix and the wetting of the fibers by the polymer is reduced. 
         [0011]    In addition to a loss of wetting, this type of deposition involves a 90-degree bend between the nozzle and part being manufactured, which can cause fiber breakage and damage. More specifically, as the fibers bend around a 90-degree angle in the melt phase, the fibers become twisted and tangled together. The twisting and tangling of the fibers can cause them to break, reducing the overall strength and stiffness of the deposited material. The twisting and tangling further reduces the wetting of the fibers and also produces an uneven, rough surface finish. 
         [0012]    Therefore, a need exists for way to improve the mechanical properties, surface fidelity, and finish of additively manufactured thermoplastic fiber composite parts. 
       SUMMARY 
       [0013]    The present invention provides a way to improve properties of additively manufactured parts in comparison to the prior art. 
         [0014]    In accordance with the illustrative embodiment, a fiber composite filament (towpreg) having an arbitrarily shaped cross section, which is typically circular, ellipsoidal, or rectangular (hereinafter “substantially circular”) is softened and then flattened, increasing its aspect ratio (width-to-thickness). In particular, whereas the feed filament will have an aspect ratio of about 1 (for circular filaments) to about 1.5 (typical for rectangular composite filaments), the flattened filament will typically have an aspect ratio of at least about 5. That is, the flattened filament will have a tape-like form factor, wherein its width is noticeably greater than its thickness. 
         [0015]    In the illustrative embodiment, the filament is flattened via a shaper, such as a roller, which applies a compressive force to the filament. A focused heat source, which in the illustrative embodiment is a laser (e.g., diode, fiber, etc.), first softens and eventually melts at least a portion of the composite filament. 
         [0016]    Compared to the prior-art processes, embodiments in accordance with the present provide certain advantages. For example, with respect to processing fiber-composite feedstocks via a heated nozzle, the loss of wetting and fiber breakage issues that plague such processes are substantially avoided. In particular, because the filament material is not completely melted, resin does not flow out of the carbon fiber. This prevents or reduces the loss of wetting associated with complete melting as in the prior art. 
         [0017]    Furthermore, the flattened shape is desired for a deposited composite filament, because this shape is more conducive to the additive manufacturing process and produces parts with better material properties as compared to deposited composite having a circular or elliptical cross section. Notwithstanding the tape-like form that results after flattening, embodiments of the invention do not suffer from the shortcomings, discussed below, of prior-art molding processes that use a tape-based feedstock, such as AFP (automated fiber placement), ATL (automated tape layup), and FW (filament winding). 
         [0018]    In AFP, thin tapes having a rectangular cross section (c.a. 0.250 inches wide by 0.006 inches thick) are sequentially placed onto a form or mold to create an object. Among any other reasons, the tape form factor is used so that tapes may be placed adjacent to each other, thereby covering the entire surface of the mold while maintaining conformability of the individual strips. The cross section of the tape is not typically changed during processing (typically, there is no way to do so). Furthermore, it would be undesirable to alter the cross section, because this would change the positioning of the tape. The positioning of the tape must be maintained so as to minimize gaps or overlaps between strips of tapes. These tapes are limited in their ability to be “steered” or turned in-plane, driving the industry towards narrower and narrower tapes (to enable a measure of steering) while maintaining their thickness. 
         [0019]    ATL is an architecturally identical process to AFP, but uses relatively wider tapes (&gt;3″) than AFP while maintaining the typical thickness (c.a. 0.006 inches). This process is functionally limited to producing objects with relatively large curvatures, wherein only minimal steering of the tape is possible. 
         [0020]    In the FW process, a fiber is wound around a cylindrical mandrel. The consolidation force is applied via fiber tension. This limits objects produced via this process to convex, closed-section geometries and prohibits in-plane steering (all fiber paths must typically follow a geodesic path). Furthermore, the ability to apply process heat to the incoming material is severely limited, making in-situ consolidation of high-temperature thermoplastics exceedingly difficult. 
         [0021]    Because the composite filament is not flattened prior to deposition in embodiments of the invention, it is easy to steer or otherwise place the filament in any location in space, enabling the printing of parts that include both open and closed cross section with highly contoured geometries and small radii corners that are not possible with tape based methods. Thus, the tape-like form of the composite filament is acquired after each segment of the filament is positioned. Consider, for example, the forms depicted in  FIGS. 1A through 1D . 
         [0022]      FIG. 1A  depicts a composite filament being steered, on edge, in a spiral path (3 dimensions), such as for forming ducts or coil springs, in accordance with some embodiments of the invention.  FIG. 1B  depicts a composite filament being steered in a serpentine path (2 dimensions), such as for forming a flat spring, in accordance with some embodiments of the invention.  FIG. 1C  depicts a composite filament being steered to form objects having “T-” or “L-” shaped profiles, such as objects with sharp inside or outside corners. And  FIG. 1D  depicts an undulating surface, which can be a surface of part formed via the ability to steer composite filaments as described herein. 
         [0023]    At the same time, embodiments of the invention avoid the problems associated with depositing a composite filament through a heated nozzle (loss of wetting, etc.). Additionally, better inter-laminar strength is achieved than in the prior art since additional polymer-bonding mechanisms are utilized. In particular, the polymer is more thoroughly intermixed with the substrate improving heat transfer and bonding. Furthermore, controlling the compaction force (via positioning of a robotic arm, etc.) in accordance with embodiments of the invention enables the width (and thickness) of a deposited filament to be varied, as needed, rather than being limited by a fixed tape width, as in the AFP, ATL, or FW processes. 
         [0024]    The prior-art processes discussed above cannot manufacture parts in such fashion or with such surface features or, as in the case with additive manufacturing through a heated nozzle, are likely to result in objects having compromised final properties. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIGS. 1A -1D  depict various paths that can be followed, and profiles and surfaces that can be formed, via embodiments of the present invention. 
           [0026]      FIG. 2  depicts a block diagram of the salient components of an additive manufacturing system that employs composite filament shaping in accordance with an illustrative embodiment of the present invention. 
           [0027]      FIG. 3  depicts an illustrative embodiment of the additive manufacturing system of  FIG. 2 . 
           [0028]      FIG. 4  depicts a method for additive manufacturing in accordance with an illustrative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]      FIG. 2  depicts the salient functional elements of deposition system  200  in accordance with the present invention. Included in system  200  are positioning subsystem  202 , feed subsystem  204 , focused heat source  206 , and shaper  210 . 
         [0030]    In the illustrative embodiment, positioning subsystem  202  comprises a multi-axis end effector (e.g., robotic arm, etc.). In the illustrative embodiment, the multi-axis end effector has sufficient degrees of freedom (i.e., six DOF) to enable true three-dimensional printing. That is, the positioning subsystem is capable of delivering a feed filament to an arbitrary location in space, as specified in accordance with the build instructions. This enables system  200  to print along the natural loading contours of an part. Printing with such a multi-axis end effector is described, for example, in Ser. No. 14/184,010, previously referenced and incorporated by reference herein. 
         [0031]    In some other embodiments, positioning subsystem  202  comprises a gantry having one or two translational degrees of freedom (x and/or y axes). In some of such embodiments, a build plate, on which the part is printed, is also considered to be part of the positioning subsystem. In such embodiments, the build plate is movable in the z direction (and possibly the x or y direction depending on gantry capabilities), such that three degrees of freedom are provided for the build. In some further embodiments, a robotic arm can be supported by a gantry. It is within the capabilities of those skilled in the art to design or specify a robotic arm, other multi-axis end effector, or gantry system to provide the requisite functionality for system  200 . 
         [0032]    Feed subsystem  204  delivers a filament to a build surface (e.g., a build plate, previously deposited layers of filament, etc.). In the illustrative embodiment, the composite filament is a cylindrical towpreg consisting of a continuous fiber (e.g., 1K, 3K, 6K, 12K, 24K, etc.) impregnated with thermoplastic resin. The continuous fiber includes, without limitation, carbon, fiberglass, aramid (AKA Kevlar), or carbon nanotubes (CNT). 
         [0033]    The thermoplastic can be a semi-crystalline polymer or a mixture of a semi-crystalline polymer and an amorphous polymer. The semi-crystalline material can be, for example and without limitation, a polyaryletherketone (PAEK), such as polyetherketone (PEK), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetheretherketoneketone (PEEKK), and polyetherketoneetherketoneketone (PEKEKK). The semi-crystalline polymer can also be other semi-crystalline thermoplastics, for example and without limitation, polyamide (PA), polybutylene terephthalate (PBT), poly(p-phenylene sulfide) (PPS). 
         [0034]    In embodiments in which the feed is a blend of an amorphous polymer with a semi-crystalline polymer, the semi-crystalline polymer can be one of the aforementioned materials and the amorphous polymer can be a polyarylsulfone, such as polysulfone (PSU), polyethersulfone (PESU), polyphenylsulfone (PPSU), polyethersulfone (PES), polyetherimide (PEI). In some additional embodiments, the amorphous polymer can be, for example and without limitation, polyphenylene oxides (PPOs), acrylonitrile butadiene styrene (ABS), methyl methacrylate acrylonitrile butadiene styrene copolymer (ABSi), polystyrene (PS), and polycarbonate (PC). 
         [0035]    In the blend, the weight ratio of semi-crystalline material to amorphous material is in a range of about 50:50 to about 95:05, inclusive, or about 50:50 to about 90:10, inclusive. Preferably, the weight ratio of semi-crystalline material to amorphous material in the blend is between 60:40 and 80:20, inclusive. The ratio selected for any particular application is primarily a function of the materials used and the properties desired for the printed part. 
         [0036]    Focused heat source  206  provides accurate control of the processing temperature of the composite filament. In the illustrative embodiment, the focused heat source is a laser. The beam spot size of a laser is precisely controllable. In accordance with the illustrative embodiment, such precise control of beam-spot size enables the laser to be aligned to heat at least a portion of a previously deposited (underlying) layer of filament to melting while, at the same time, softening (but not melting) or only partially melting the filament being deposited. 
         [0037]    In the illustrative embodiment, focused heat source  206  precisely heats the composite filament above its glass transition temperature up to its melting temperature. In the illustrative embodiment, the resin in the composite filament is either not melted, or not completely melted, such that resin does not flow (or flow minimally) out of the filament (e.g., carbon fiber, etc.). This prevents or reduces the loss of wetting associated with movement of the polymer resin out of the fiber, as previously discussed. The temperature at which any particular composite filament feedstock softens is a function of its composition. Those skilled in the art can readily determine a desired “softening” temperature by simple experimentation. 
         [0038]    After softening, the composite filament is reshaped. In particular, the composite filament is flattened so that it acquires a more tape-like form. As previously noted, the flattened form is desirable for a composite filament because this shape is conducive to the additive manufacturing process and produces parts with better material properties as compared to those built from composite filaments having substantially circular cross sections. 
         [0039]    In some embodiments of the invention, the underlying filament is at least partially melted. By doing so, the bonding and adhesion between the underlying filament and the just-deposited, overlying filament increases, enhancing the overall mechanical properties of the nascent part. 
         [0040]    In some embodiments in which the underlying filament is at least partially melted, the portion that is partially melted is the portion adjacent to the overlying filament. In some embodiments, a portion of the just-deposited (overlying) filament is melted. In some of such embodiments, the melted portion is the “lower” portion of the composite filament. Thus, in some embodiments, an “upper” portion of the underlying filament is melted and a “lower” portion of the overlying filament is melted. It is to be understood that the fiber doesn&#39;t melt; rather, the thermoplastic in and around the fiber melts. 
         [0041]    In some embodiments, about 10 percent to about 50 percent of the original (unflattened) thickness of the overlying composite filament is melted. In other words, in some embodiments, up to about one-half of the composite fiber (the lower half) is melted. 
         [0042]    It is very difficult or impossible to exercise the precise control over temperature profiles that is required to maintain wettability and provide the partial melting described above when using a conventional heat source (e.g., a conductive heating element, directed hot air, etc.). Hence, in the illustrative embodiment, focused heat source  206  is used to provide precisely controlled heating of a just-deposited composite filament as well as an underlying composite filament. 
         [0043]    In some alternative embodiments, other focused heated sources may suitably be used, such as, without limitation, a concentrated microwave source (MASER), focused ultrasonic sound, focused infrared radiation, ion beam, and electron beam. 
         [0044]    Shaper  210  applies “downward-directed” pressure to the softened/partially melted filament, thereby controlling its position/location and altering its cross section from substantially circular to a flattened form. As used in this disclosure and the appended claims, the term “flattened” means that the width of the composite fiber is at least 5 times greater than its thickness. Furthermore, the term “flattened” includes cross sections that are not literally “flat-rectangular,” including, without limitation, plano-convex, plano-concave, bi-concave, and meniscus. Such not-literally-rectangular forms may result, for example, from the shape of an underlying build surface. As previously mentioned, alteration of the cross section in the aforementioned fashion facilitates consolidation of the composite filament into the geometry of the desired object. 
         [0045]      FIG. 3  depicts deposition system  200 ′ in accordance with an illustrative embodiment of invention. System  200 ′ is an implementation of system  200  depicted in  FIG. 2 . 
         [0046]    In system  200 ′, positioning subsystem  202  is embodied as notional robotic arm  302 . The robotic arm is coupled to support plate  318 , which supports the various subsystems and elements of system  200 ′. Robotic arm  302  moves support plate  318 , and all subsystems/elements attached thereto, so as to position the system to deliver a composite filament to a desired point in space consistent with the build instructions for the part. 
         [0047]    In the illustrative embodiment, robotic arm  302  is appropriately configured with rigid members  314  and joints  316  to provide six degrees of freedom (three in translation: x, y, and z axes; and three in orientation: pitch, yaw, and roll). 
         [0048]    In system  200 ′, feed subsystem  204  includes spool  320 , feed motor  326 , feed tube  328 , and cutter  330 . Spool  320  is rotatably coupled to member  322 , the latter of which is attached (e.g., via bolts  324 , etc.) to support plate  318 . Composite filament  336  is wound around spool  320 . The filament passes through motor  326 , feed tube  328 , and cutter  330 . Motor  326  draws composite filament  336  from spool  320 . As it passes through cutter  330 , filament  336  is sized, as appropriate, in accordance with build instructions. Feed tube  328  is attached to support plate  318 , such as via clamps  334 . 
         [0049]    Filament  336  is delivered to build plate  350  from delivery end  332  of feed tube  328 . It will be appreciated that if manufacture of a part has already begun, filament  336  might be delivered to a previously deposited layer of composite filament. The term “build surface,” as used in this disclosure and the appended claims, refers to either a build plate, etc., such as build plate  350 , or a previously deposited layer of material, or anything else that filament  336  might be deposited upon. 
         [0050]    In the illustrative embodiment, feed tube  328  is used to simply deliver and guide filament, in its original form (i.e., no change in shape, etc.) to the build plate  350 . In some embodiments, feed tube  328  is not heated. 
         [0051]    In some embodiments, delivery end  332  of the feed tube  328  is appropriately configured and/or positioned to deliver the composite filament directly underneath shaper  210 , which is embodied in the illustrative embodiment as roller  340 . 
         [0052]    Roller  340  rotates about pin  341  but is otherwise rigidly coupled to support plate  318  via member  342  and bolts  344 . In other words, roller  340  is free to rotate about pin  341  along the x-direction, but is rigidly coupled to support plate  318  with respect to movements along the y-direction and the z-direction. 
         [0053]    In system  200 ′, focused heat source  206  is embodied as laser  346 , such as a diode or fiber laser, although other types of lasers may suitably be used. Laser  346  is rigidly coupled to support plate  318 , such as via clamps  348 . 
         [0054]    In the illustrative embodiment, laser  346  is aligned to illuminate the portion of filament delivered to build plate  350 . The laser heats the filament to softening for incorporation into the build object. 
         [0055]    As previously noted, a laser is preferentially used as focused heat source  208  because it enables precise and accurate control of the processing temperature. Because the laser spot size can be precisely controlled, the laser can be directed to melt a previously deposited, underlying layer while simply heating the currently deposited layer until it softens. Or the laser can be directed to partially melt both the underlying and overlying layer. Again, melting the underlying layer during the deposition process results in an increase in bonding and adhesion between the layers, enhancing the overall mechanical properties of the build object. 
         [0056]    n some other embodiments, one laser is used for softening or partially melting the just-deposited filament, while a second laser is used to at least partially melt a previously deposited layer. In some further embodiments, a focused heat source is used to melt the underlying filament, but a heat source other than a laser (not necessarily a focused heat source; for example, a hot air blower, etc.) is used to soften the filament being deposited. This can occur before directly before or after the filament is deposited. 
         [0057]    Robotic arm  302  positions support plate  318  such that roper  340  applies pressure to the deposited filament. The applied pressure ensures that the filament sticks and adheres to the underlying layer. In the absence of such pressure, only gravity is available to bond and adhere the filament to the underlying layer, providing a relatively weak interface. 
         [0058]    Furthermore, as previously discussed, the applied pressure reshapes the cross section of the filament from substantially circular to a flattened form. That is, the substantially cylindrical composite filament feed is transformed into a flattened substantially tape-like form. 
         [0059]    In the prior art, wherein gravity alone is applied during deposition, the composite filament&#39;s cross section morphs from circular/rectangular to elliptical. Elliptical-shaped filaments tend to leave gaps and interstices in the build object. These gaps and interstices can act as nucleation sites for crack propagation, negatively impacting the mechanical properties of the build part. On the other hand, the flattened form in accordance with embodiments of the invention results in parts having minimal void/interstitial space. This results in printed parts having relatively better material properties. 
         [0060]    In illustrative embodiment, there is a 1:1 input-to-output material feed rate (i.e., the cross-sectional area of the filament entering the system equals that of the filament exiting the system to create the build object). Thus, although the cross-sectional shape of the composite filament changes in accordance with the present teachings (i.e., it is flattened), the cross-section area of the filament does not change. 
         [0061]      FIG. 4  depicts method  400  for additive manufacturing. In accordance with operation S 401 , a filament is deposited on the build surface or previously deposited layer of filament. In the illustrative embodiment, the filament is softened or partially melted, per operation S 402 . In the illustrative embodiment, the filament is softened after deposition; however, in some alternative embodiments, the filament is softened immediately prior to deposition. 
         [0062]    The important point here is that the composite filament must retain the ability to be shapeable (i.e., re-shaped to a flattened form of desired width and/or thickness). It is notable that a liquid will take the shape of a vessel, etc., into which it is poured. That is not a contemplated embodiment; in embodiments of the invention, the composite filament retains form/shape without external constraints. 
         [0063]    In operation S 403 , at least a portion of the filament underlying the most recently deposited filament is melted. Per operation S 404 , the feed composite filament, which typically has a substantially circular cross section, is reshaped to a flattened form having a tape-like aspect ratio. As previously noted, the width of the reshaped composite filament is at least 5 times greater than its thickness. In the illustrative embodiment, reshaping is accomplished via applied pressure; that is, by compressing the composite filament. 
         [0064]    In some embodiments, operations S 402  and S 403  are performed concurrently. In some other embodiments, operation S 402  is performed before operation S 403 . In some embodiments, operations S 402 , S 403 , and S 404  are performed concurrently. 
         [0065]    It is to be understood that although this disclosure teaches many examples of embodiments in accordance with the present teachings, many additional variations of the invention can easily be devised by those skilled in the art after reading this disclosure. As a consequence, the scope of the present invention is to be determined by the following claims.