Abstract:
An apparatus and method for printing an object via additive manufacturing is disclosed. In accordance with an illustrative embodiment, one or more inks are prepared, including a thermo-polymer ink, a nano-filler ink, and a thermo-polymer/nano-filler ink. In some embodiments, an object is printed by depositing alternating layers of thermo-polymer ink and nano-filler ink and exposing the layers to microwave radiation. In some other embodiments, an object is printed by depositing alternating layers of thermo-polymer/nano-filler ink and nano-filler ink and exposing the layers to microwave radiation. In some additional embodiments, an object is printed by depositing successive layers of thermo-polymer/nano-filler ink and exposing them to microwave radiation.

Description:
STATEMENT OF RELATED CASES 
       [0001]    This case claims priority of U.S. Patent Applications Ser. No. 62/144,417 filed Apr. 8, 2015 and Ser. No. 62/155,916 filed May 1, 2015, both of which applications are incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to additive manufacturing (or 3D printing) of composite structures. 
       BACKGROUND OF THE INVENTION 
       [0003]    The additive manufacturing process is widely known as “3D printing.” Numerous 3D-printing methodologies have been described in prior art, the most common being selective-laser sintering (SLS), stereolithography (SLA), and extrusion-based 3D printing or fused filament fabrication (FFF). 
         [0004]    In FFF, a 3D object is produced, in accordance with a mathematical model thereof, by extruding a succession of small, flattened strands of molten material that harden as soon as they leave the extrusion nozzle. The object is built from the bottom up, one layer at a time. 
         [0005]    In SLS, a laser selectively fuses powdered material by scanning, on the surface of a powder bed, cross-sections generated from a  3 -D digital description of the object. After scanning a cross-section, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the object is completed. 
         [0006]    In stereolithography, an ultraviolet (UV) laser is directed toward a vat of photopolymer resin. Using computer aided design software (CAM/CAD), the UV laser draws a design or shape on the surface of the photopolymer vat. Due to its photosensitivity to UV light, the resin solidifies and forms a single layer of the nascent 3D object. This process is repeated for each layer of the design until the 3D object is complete. 
         [0007]    Although suitable for prototyping, most 3D printed objects are typically not robust enough to be used as structural parts, such as for use in automotive, aerospace, medical or other aggressive-use applications. This is the case regardless of the methodology (FFF, SLS, SLA) used. 
         [0008]    The excellent mechanical properties of carbon nanotubes (“CNTs”) and graphene suggest that incorporation of a very small amount of either of these materials into a polymer matrix can lead to structural materials having exceedingly high durability and high strength as well as low weight. To date, however, the production of high-strength CNT polymer composites has proven to be rather difficult; CNT-infused polymers produced thus far show some improvement in strength, but far below expectations. 
         [0009]    Research has indicated that poor adhesion between the polymer and CNT is the limiting factor for imparting the mechanical properties of CNTs to polymer composites. Moreover, van der Waals interactions cause CNTs to form stabilized bundles, making them very difficult to disperse and align in a polymer matrix. 
         [0010]    Researchers have focused on ways to effectively disperse CNTs into a polymer matrix. Thus far, techniques that have found at least some success for dispersing CNTs in the polymer matrix include: solution mixing, melt mixing, electrospinning, in-situ polymerization, and chemical functionalization of the CNTs. 
         [0011]    Although 3D-printed objects have been made using CNT/polymer composites, the resulting parts do not demonstrate any significant enhancement in mechanical properties (c.a., no more than about a three-fold increase, which in the context of CNTs for example, is negligible compared to the potential). Rather, CNT/polymer-composite printed objects are currently being used for their electrical properties, such as to control electrostatic discharge. 
       SUMMARY 
       [0012]    The present invention provides apparatus, method, and compositions for printing objects using thermo-polymer and CNT or other nanomaterials, which avoids the shortcomings of the prior art. 
         [0013]    In accordance with embodiments of the invention, 3D objects comprising a thermo-polymer/nanomaterial material are printed via from specially prepared “inks.” In one embodiment, a 3D object is printed from (i) a thermo-polymer ink and (ii) a nano-filler (CNTs or other nano-scale material) ink. In a second embodiment, a 3D object is printed from a thermo-polymer/nano-filler ink. In a third embodiment, a 3D object is printed from (i) a thermo-polymer/nano-filler ink and (ii) a nano-filler ink. 
         [0014]    Unlike the approaches of the prior art, wherein various techniques have been used to improve the dispersion of nanomaterial in polymer, applicant has avoided suspending nanomaterial in the polymer. Rather, the nanomaterial is suspended in a liquid solution, therefore ensuring wetting and dispersion. This has enabled applicant to produce compositions with much higher concentrations of nanomaterial in thermo-polymer than the prior art. 
         [0015]    The aforementioned inks are dispensed under pressure layer-by-layer in accordance with build instructions. In some embodiments, an object is printed via alternating layers of thermo-polymer ink and nano-filler ink. In some other embodiments, an object is printed via successive layers of a thermo-polymer/nano-filler composite ink. And in yet further embodiments, an object is printed via alternating layers thermo-polymer/nano-filler composite ink and nano-filler ink. 
         [0016]    In a departure from the prior art, rather than simply using laser light or UV, embodiments of the invention utilize microwave radiation. The use of microwave radiation physically moves some types of nano-filler, such as CNTs, back and forth, which heats up and (indirectly) melts nearby thermo-polymer. Neither UV nor laser light can do this. 
         [0017]    Compared to the prior art, the use of microwave radiation alone (or in combination with a laser) results in:
       Faster deposition speeds. Global, rapid heating and melting of high-temperature thermoplastics is not possible via SLS or SLA techniques.   Better printed-object material properties. Microwave radiation penetrates deeper than UV enabling better adhesion between layers and rasters through the printed object compared to SLS or SLA techniques. Furthermore, microwave radiation has been shown to remove defects within CNTs thereby increasing strength.   Improved surface finish. Elimination of z-layer appearance through surface-targeted microwave radiation at relatively low power.   Higher crystallinity. Tighter control is possible over the melting and fusing of polymer nanoparticles with controlled microwave radiation as compared to UV or laser. Relatively low power microwave radiation can be used during or after the build to anneal the object and reduce the presence of voids/defects, which would otherwise compromise materials properties.       
 
         [0022]    In some embodiment, the invention provides a method for printing an object via additive manufacturing, wherein the process comprises: 
         [0023]    depositing a first layer, the first layer comprising droplets of a first ink, wherein the first ink comprises thermo-polymer particles in a first suspension medium; 
         [0024]    depositing a second layer on at least a portion of the first layer, wherein the second layer comprises droplets of a second ink, wherein the second ink comprises first nano-filler in a second suspension medium; 
         [0025]    depositing a third layer on at least a portion of the second layer, wherein the third layer comprises droplets of the first ink; and 
         [0026]    exposing the first, second and third layers to microwave radiation. 
         [0027]    In some embodiments, the invention provides an apparatus for printing an object via additive manufacturing, the apparatus comprising: 
         [0028]    a print head having at least one nozzle, wherein the print head dispenses at least one of:
       (i) a first ink comprising a thermo-polymer in a suspension medium, and   (ii) a second ink comprising a nano-filler in a suspension medium;       
 
         [0031]    a gantry, wherein the print head is movably coupled to the gantry to enable the print head to move in at least one direction; 
         [0032]    a source of microwave radiation; and 
         [0033]    a build platform, wherein the at least first ink or second ink is dispensed from the print head to the build platform, and wherein the build platform is movable in a vertical direction. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0034]      FIG. 1  depicts a 3D printer in accordance with an illustrative embodiment of the present invention. 
           [0035]      FIG. 2A  depicts a first embodiment of a print head for use in conjunction with the 3D printer of  FIG. 1 . 
           [0036]      FIG. 2B  depicts a second embodiment of a print head for use in conjunction with the 3D printer of  FIG. 1 . 
           [0037]      FIG. 3A  depicts a first layer structure, printed by the 3D printer of  FIG. 1 , for printing nano-filler/polymer composite parts in accordance with an embodiment of the invention. 
           [0038]      FIG. 3B  depicts a second layer structure, printed by the 3D printer of  FIG. 1 , for printing nano-filler/polymer composite parts in accordance with an embodiment of the invention. 
           [0039]      FIG. 3C  depicts a third layer structure, printed by the 3D printer of  FIG. 1 , for printing nano-filler/polymer composite parts in accordance with an embodiment of the invention. 
       
    
    
     DESCRIPTION 
       [0040]    The present invention provides a way to produce printed parts or other objects (hereinafter collectively “objects”) from a composite material comprising: (i) polymer and (ii) “nano-filler,” such as CNT and/or other nano-scale materials. Unlike printed objects formed from CNT/polymer composites using prior-art techniques, those formed in accordance with the present teachings exhibit substantially enhanced mechanical properties. 
         [0041]      FIG. 1  depicts 3D printer  100  for use in printing objects from a nano-filler/polymer composite, in accordance with an illustrative embodiment of the present invention. 3D printer  100  is capable of printing objects from a nano-filler/polymer composite via either (i) a selective laser sintering methodology, (ii) a modification thereof that uses microwaves, or (iii) a methodology unlike any existing process using microwaves. 
         [0042]    3D printer  100  includes housing or enclosure  102 , gantry system  108 , print head  116 , focused heat sources  118  and  120 , build platform  122 , and computer/controller  128 . 
         [0043]    Housing  102  defines build chamber  106 . Housing  102  provides an environmentally controlled environment; during printing, build chamber  106  is under an inert-gas atmosphere (e.g., nitrogen, argon, helium, etc.). The temperature in build chamber  106  rises during the build (since heat is being input to the system via focused heat sources  118  and  120  and not being removed). This is beneficial since the elevated temperature reduces warping of the printed object. Temperature in build chamber starts out near room temperature and can rise to as much as about 280° C. The inert-gas atmosphere in build chamber  106  also prevents the inclusion of debris or other foreign particles into the object being produced. In some embodiments, housing  102  includes microwave radiation (Faraday) shielding  104 . 
         [0044]    In the illustrative embodiment, notional gantry system  108  enables print head  116  and coupled focused heat sources  118  and  120  to move in the X and Y directions. Gantry  108  includes rail(s)  110 , arm  112 , and rail  114 . Rail(s)  110  are oriented in the X direction (left/right in  FIG. 1 ) and rail  114  is oriented in the Y direction (front/back in  FIG. 1 ). Arm  112  movably couples to rail(s)  110  and supports rail  114 . Print head  116  movably couples to rail  114 . Movement of arm  112  along rail  110  and movement of print head  116  along rail  114  is computer controlled (drive systems not depicted). 
         [0045]    Print head  116  deposits material on build platform  122  to sequentially build object  126 . Build platform  122  is heated and is movable in the Z direction (up/down in  FIG. 1 ), such that 3D printer  100  provides 3 degrees-of-freedom for building object  126 . 
         [0046]    3D printer  100  also includes controller  128 . The controller reads and executes build instructions generated by an outboard computer (not depicted) based on a 3D model of the object that is to be printed (i.e., object  126 ). For example, controller  128  orchestrates the build by controlling movements of print head  116 , the rate at which material is deposited on to build platform  122 , and the operation of focused heat sources  118  and/or  120 . 
         [0047]      FIGS. 2A and 2B  depict embodiments of print head  116 . The print head is somewhat analogous to an “ink-jet” print head. 
         [0048]      FIG. 2A  depicts print head  116 A, which includes two internal “ink” reservoirs  230 A and  230 B, ultrasonic dispersion system  234 , and nozzle  236 .  FIG. 2B  depicts print head  116 B which does not include internal reservoirs; rather ultrasonic dispersion system  234  in this embodiment is fed by external reservoirs  232 A and  232 B. 
         [0049]    As discussed in further detail below, during printing, one of the reservoirs contains a thermo-polymer/nano-filler composite ink and the other reservoir contains a nano-filler ink. In some other embodiments, one of the reservoirs contains a thermo-polymer ink and the other reservoir contains a nano-filler ink. And in yet some further embodiments, both reservoirs contain a thermo-polymer/nano-filler composite ink. In the latter embodiment wherein only a thermo-polymer/nano-filler composite ink is used for printing, print head  116 A can contain a single reservoir and print head  116 B can be fed by a single reservoir. 
         [0050]    The reservoirs  230 A/ 230 B or  232 A/ 232 B feed ultrasonic dispersion system  234 . The ultrasonic dispersion system, well known to those skilled in the art, is capable of dispersing the particles of thermo-polymer and/or nano-filler within the suspension medium of the ink (e.g., water, methanol, etc., as discussed further below). Ultrasonic dispersion system  234  and nozzle  236  are commercially available (as an integrated unit) from Sono-Tek of Milton, N.Y. and others. 
         [0051]    After ultrasonic dispersion, the ink(s) are expelled from nozzle  236  under pressure and deposited, layer-by-layer, as directed by controller  128  in accordance with the build instructions. Further description of the build process is provided later in this specification. 
         [0052]    With continued reference to  FIG. 1 , in the illustrative embodiment, focused heat source  118  and focused heat source  120  are coupled to print head  116 . In the illustrative embodiment, focused heat source  118  is a laser (hereinafter “laser  118 ”) and focused heat source  120  is a maser (hereinafter “maser  120 ”). 
         [0053]    In some other embodiments, only a single focused heat source, either a laser or a maser, is present. As discussed further below, in some embodiments, sources  118  and/or  120  apply heat to the ink immediately after it is deposited (and only to that ink). In some embodiments, non-focused microwave generator  124 , which applies microwave radiation at a relatively lower power than laser  118  or maser  120 , provides an unfocused application of heat to all material that has been deposited on build platform  122 . In various embodiments, focused heat sources  118  and/or  120  are used with or without microwave generator  124 . In yet a further embodiment, microwave generator  124  is used without focused heat sources  118  and/or  120 . 
         [0054]    In the embodiment depicted in  FIG. 1 , a single print head  116  is used and laser  118  and maser  120  are affixed thereto such that the print head and the focused heat sources move in concert. However, a variety of other arrangements are contemplated:
       (i) Print head  116  with only laser  118  affixed thereto; maser  120  supported separately by a fixture for movement independently of print head  116 /laser  118 ;   (ii) Print head  116  with only maser  120  affixed thereto; laser  118  supported separately by a fixture for movement independently of print head  116 /maser  120 ;   (iii) Print head  116  supported by itself; laser  118  and/or maser  120  supported separately by a fixture for movement independently of print head  116 ;   (iv) Multiple print heads  116  each with laser  118  and/or maser  120  attached and each print head supported by its own arm for movement independently of the other print heads; and   (v) Versions of (i) through (iii) wherein there are multiple print heads  116  and multiple focused heat sources supported separately therefrom.       
 
         [0060]    In yet some further embodiments, print head  116  includes a sufficient number of nozzles arrayed in a  1 D array in the Y direction, for example, such that the print head only moves in the X direction, wherein not all of the nozzles are necessarily printing at the same time. Of course, the nozzles could be array in the X direction such that the print head only moves in the Y direction. In still further embodiments, the  1 D array of nozzles is not sufficient to cover the full print range in the direction of the array such that there is some limited movement in the direction of array of nozzles. In yet some additional embodiments, there are multiple nozzles in a single print head, wherein the nozzles are arrayed in a 2D array. 
         [0061]    Ink composition. One ink comprises a mixture of thermo-polymer particles, at least one nano-filler, and a suspension medium. A second ink comprises thermo-polymer particles and a suspension medium. A third ink comprises nano-filler, a suspension medium, and optionally a surfactant. 
         [0062]    An important distinction between embodiments of the invention and the prior art is that unlike the prior art, in the inventive ink compositions, the nano-filler is not simply mixed or compounded in the polymer. Rather, nano-filler, whether alone in a nano-filler ink or as a composite in the thermo-polymer/nano-filler ink, is suspended in a liquid solution. This ensures wetting and dispersion, which is believed to be critical to realizing the promise of polymer/nano-filler composite materials. 
         [0063]    The polymer, which is typically a thermoplastic resin, is present in the ink as particles having a size that is preferably in the range of about 2 to 5 microns. Suitable thermoplastic resins include, without limitation, polyaryletherketone (PAEK), polyethertherketone (PEEK), polyetherketoneketone (PEKK), polyethylene (PE), polyetherimide (PEI commonly known as Ultern), polyethersullone (PES), polysullone (PSU), polyphenylsullone (PPSU), polyphc-mylenc-3 oxides (PPOs), acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), polyglycolic acid (PGA), polyarnide-imide. (PAI), polystyrene (PS), polyamide (PA), polybutylene terephthalate (PBT), poly(p-phenylene sulfide) (PPS), polyethersulfone (PESU), polyphenylene ether (commonly known as PrimoSpire), and polycarbonate (PC). These thermoplastic resins are readily commercially available from companies such as Solvay Plastics or others. 
         [0064]    Suitable nanofiller(s) include (and term “nano-filler” is defined for use in this disclosure and the appended claims to mean), without limitation, CNTs, graphene, graphene nano-platlets, graphene nano-ribbons, metallic nanoparticles, aramid nanofibers, carbon nanofibers, and the like. The metallic nanoparticles may include, but are not limited to, nanometer-size particles of silver, copper, steel, gold, or the like. These metallic nanoparticles are added for enhanced thermal/electrical conductivity (not mechanical strength). 
         [0065]    CNTs can include all types of multi-wall or single-wall CNTs as well as CNT-polymer encapsulated flakes. The diameter of the CNTs can range from about 0.1 nanometer (nm) to 50 nm and the lengths can range from 1 nm to about 10 millimeters (mm). 
         [0066]    Suitable suspension materials include, without limitation, distilled water, an alcohol-based solvent (methanol, ethanol, etc.) methyl acetate, tetrahydrofuran, 2-dichlorobenzene (DCB), acetone, chloroform, N-methyl-pyrrolidone (NMP), dimethylformamide (DMF), or any other solvent used to stabilize nanoparticles or thermoplastic resins in solution. 
         [0067]    Surfactants suitable for use in conjunction with the preparation of inks include sodium dodecylbenzene, and other ionic surfactants including, without limitation, sodium dodecylsulfate (SDS), sodium cholate (SC), and sodium deoxycholate (SDOC). 
         [0068]    Example—Preparation of thermo-polymer ink. In this example, PEEK is used as the thermo-polymer. PEEK ink is prepared by mixing PEEK/carbon black in water. The carbon black enhances the near infrared absorption for laser heating and sintering. The number average molecular weight of the PEEK can be in a range from about 40,000 to about 60,000 and the weight average molecular weight of the PEEK can be in a range from about 90,000 to about 110,000. Carbon black concentration can be in a range from about 0.1 to about 10 weight percent of PEEK/carbon black mixture. 
         [0069]    For this example, PEEK with a number average molecular weight of 50,200 and a weight average molecular weight of 105,800 and carbon black (at 1 weight percent of the PEEK/carbon black mixture) is compounded using a twin extruder with a high-shear screw. The PEEK/carbon black pellets were then powderized to produce an ultra-fine powder with a mean particle size of D50 and a mean diameter of 10 microns. Typically, particle size distribution will range from D10 to about D100 with diameters in the range of about 1 micron to about 100 microns. 
         [0070]    The powder was added to 1 L of water at 50 weight percent loading. The concentration of the powder in the ink is typically in the range of about 10 to 99 percent by weight. The mixture was energetically agitated at room temperature (25° C.) using directed low frequency, high power ultrasonication at 700 W and 20 kHz for 24 hours. Ultrasonic mixing can be conducted at power in the range of about 100 W to 1000 W and 1 kHz to 100 gHz. The time needed can range from about 0.5 to about 48 hours. “Liquid Mixer” brand ultrasonic mixer commercially available from Aurizon Ultrasonics, LLC of Kimberly, Wis. and “Q1700 Sonicator” brand ultrasonic mixer commercially available from Hielscher USA, Inc. of Ringwood, N.J. may suitably be used for ultrasonic mixing. 
         [0071]    After mixing, the thermo-polymer ink is ready to be poured into a reservoir for the print head. 
         [0072]    Example—Preparation of Nano filler ink. In this example, CNT is used as the nano-filler. CNT ink was prepared by mixing CNT and a surfactant with methanol. The CNT was a multi-wall, long aspect ratio CNT with a diameter of 10 nm and a length of 50 nm. 
         [0073]    The CNT was added to 1 L of methanol at 50 weight percent loading. The weight loading can be in a range of about 0.5 weight percent to about 80 weight percent. The surfactant sodium dodecylbenzene is added at 1 weight percent of the CNT/methanol mixture to enhance dispersion and prevent agglomeration of CNT. 
         [0074]    The mixture was then energetically agitated at room temperature using directed low power, high frequency ultrasonication at 12 W and 55 kHz for 24 hours. The Q700 Sonicator and Q500 Sonicator brands of ultrasonic mixer, commercially available from Hielscher USA, Inc. of Ringwood, N.J., may be used for this purpose. 
         [0075]    After mixing, the nano-filler ink is ready to be poured into a reservoir for the print head. 
         [0076]    Example—Preparation of Thermo-Polymer/Nano-filler ink. In this example, PEEK is used as the thermo-polymer and CNT is used as the nano-filler. The PEEK/CNT ink is prepared by mixing PEEK/CNT with water. The PEEK and CNT are characterized as for above examples. 
         [0077]    The PEEK/CNT was compounded using a twin extruder with a high-shear screw at 3 weight percent CNT in the PEEK/CNT mixture. The PEEK/CNT pellets are then powderized and the ink is produced as discussed above for the production of PEEK ink. 
         [0078]    After mixing, the thermo-polymer/nano-filler ink is ready to be poured into a reservoir for the print head. 
         [0079]      FIGS. 3A through 3B  depict three different “layering” approaches for printing an object using thermo-polymer and nano-filler. 
         [0080]    In the embodiment depicted in  FIG. 3A , an object is printed via alternating layers of thermo-polymer ink and nano-filler ink. That is, a first layer  340  of thermo-polymer ink is deposited, then a second layer of nano-filler ink  342  is deposited thereon, followed by a third layer  340  of thermo-polymer ink, etc. 
         [0081]    In the embodiment depicted in  FIG. 3B , an object is printed via successive layers  344  of thermo-polymer/nano-filler ink. And in the embodiment depicted in  FIG. 3C , an object is printed via alternating layers thermo-polymer/nano-filler ink  344  and layers of nano-filler ink  342 . 
         [0082]    Example—Printing an object. Inks are prepared as discussed above and added to the internal ( 230 A,  230 B) or external ( 232 A,  232 B) reservoirs, depending on the embodiment. Print head  116  is pressurized for expelling the ink(s). Pressure will typically be in a range of 100 psi to 10,000 psi, using gas or liquid. In an exemplary embodiment, print head  116  is pressurized with argon to 2000 psi. 
         [0083]    Prior to expulsion from nozzle  236 , the ink(s) are ultrasonically dispersed. For use in conjunction with embodiments of the invention, the dispersion system will operate at a power in a range of 50 watts to 1000 watts and at a frequency that is typically in a range of 1 kHz to 100 gHz. In an exemplary embodiment, the ultrasonic dispersion system operates at 42 kHz and 50 watts. 
         [0084]    The ink(s) are deposited at a temperature that is typically in the range of 25° C. (room temperature) to 280° C. (i.e., the inks are not heated, but temperature rises in build chamber  106  during the course of the build, as previously discussed). The ink is typically deposited to a height that is in a range from 50 microns to 500 microns. The ink is deposited at a rate that is usually within a range of 0.1 milli-liter (mL) per sec to 50 mL/sec. In an exemplary embodiment, ink is deposited to a height of 100 microns at 10 mL/sec. 
         [0085]    3D printer  100  includes laser  118 , maser  120 , and a conventional microwave generator  124 . Not all of these heat sources are used in every embodiment. For example, in some embodiments, laser  118  is used in conjunction with maser  120 ; in some other embodiments, laser  118  is used in conjunction with microwave generator  124 ; in some other embodiments, maser  120  is used in conjunction with microwave generator  124 ; and in some embodiments, only microwave generator  124  is used. Focused heat sources  118  and  120  are mounted at a distance from nozzle  236  that is typically in a range of about 1 to 50 centimeters, although at the greater distances, the focused heat sources will typically supported independently of print head  116 . In an exemplary embodiment, focused heat sources  118  and  120  are 3 centimeters away from nozzle  236 . 
         [0086]    Laser  118  functions as a “sintering” laser (in the SLS technique, laser is used to sinter powdered material, binding the material together to create a solid structure). Laser  118  can be, without limitation, a CO 2  laser, a CO laser, a YAG laser, an Nd:YAG laser, a holmium laser, an argon laser, a fiber laser, a laser diode, etc., operating in a range of about 10 watts to about 1 Kw. The thermoplastic resin used in the ink dictates the energy required to fuse the ink. The particle size and absorbance spectrum of the ink dictates the operational wavelength of the laser. Typically, inks having relatively smaller particles (of polymer) require shorter wavelengths. 
         [0087]    In some embodiments, laser  118  is used for sintering the thermo-polymer and thermo-polymer/nano-filler inks. The laser rapidly heats the ink to sinter the powdered material and evaporate the solvent in the ink. In some other embodiments, maser  120  can be used to rapidly heat the thermo-polymer/nano-filler ink and evaporate solvent. The maser will typically operate at a power in a range of 1 watt to 1 kW and at a frequency that is usually within a range of 40 MHz to 40 GHz. Laser  118  and maser  120  typically heat the various layers to a temperature within a range of 150° C. to 390° C. (as a function of the material). 
         [0088]    Furthermore, maser  120  or microwave generator  124  can be applied in an unfocused manner at a relatively lower power resulting in somewhat less of a temperature rise than for sintering. This lower power microwave radiation provides for bonding across deposited layers and surfaces and can be applied after a build is complete or periodically throughout the build. Maser  120  or microwave source  124  can also be applied at even lower power resulting in even less of temperature rise than for the bonding operation for the purpose of annealing the thermo-polymer to increase the crystallinity and strength of the object. For annealing, temperature is ramped-up slowly. Annealing ramp-up rates will typically fall within a range of 1° C./hr to 100° C./hr. In an exemplary embodiment, temperature is increased at about 10° C./hr and then held at the annealing temperature for a period of time (e.g., 6 hrs., etc.), and then slowly cooled (e.g., 5° C./hr). 
         [0089]    In an exemplary embodiment based on inks discussed above, for PEEK and PEEK/CNT inks, laser  118  can be a Nd:YAG laser with a wavelength of 1.064 microns and a power of 70 watts. The laser heats either of the inks to 330° C. to sinter the PEEK particles and evaporate the solvent within about 5 seconds of deposition. For the CNT and PEEK/CNT inks, and maser  118  operates at 120 W and 10 GHz. Maser  118  can be used to heat the PEEK/CNT ink to 330° C. to sinter the PEEK particles and rapidly evaporate the solvent. For PEEK/CNT ink, to bond across layers, the maser can be operated at 30 watts to reach a temperature of about 280° C. For annealing, the maser can be operated at 10 watts to reach a temperature of about 200° C. 
         [0090]    With reference to the embodiments depicted in  FIGS. 3A and 3C , nano-filler ink  342  interposes successive layers of thermo-plastic ink  340  and thermo-plastic/nano-filler ink  344 . Taking the embodiment of  FIG. 3A , for example, first layer of thermo-plastic ink is deposited and then sintered by laser  118  or maser  120 . There is a maximum time, typically in a range of about 0.1 to about 360 seconds, from when the ink is deposited to when the focused heat source  118  or  120  must sinter the material. The timing is dependent on the viscosity and other rheological properties of the inks. The time it takes to sinter/melt the material will typically be in a range of between about 0.1 to about 120 seconds. 
         [0091]    After a polymer layer is deposited, a second layer of nano-filler ink  342  is deposited. Although there is no polymer in the nano-filler layer (such that sintering/fusing is not required), laser  118  or maser  120  can optionally be applied to nano-filler layers (at low power) to accelerate the evaporation of solvent (i.e., the suspension medium) from the layer. 
         [0092]    Following the nano-filler layer, a third layer of thermo-polymer ink  340  is deposited and sintered. This fuses the first three layers together. In some other embodiments, microwave radiation, via maser  118  or microwave source  124  is used to selectively heat nanofiller layer  342 , thus heating and fusing the neighboring thermo-polymer surfaces together. This creates an inner core of directional nano-filler. The use of microwave radiation may result in faster fusing times compared to using the sintering laser (on the polymer layers) alone. This method may also enhance layer-to-layer adhesion thereby increasing the z-axis properties, including mechanical strength and electrical/thermal conductivity. The is embodiment can be used with or without the additional use of a sintering laser (to sinter the polymer layers). 
         [0093]    Then a fourth layer of nano-filler ink is deposited (with optional heating), followed by a fifth layer of thermo-polymer ink, focused heat application to sinter and fuse, and, optionally, broad application of relatively lower power microwave radiation. This fuses the 4 th  and 5 th  layer to the 3 rd  layer, and so forth. The thermo-polymer layers provide rigidity to the nano-filler layer sandwiched therebetween and help form the geometrical shape of the object being printed. 
         [0094]    The embodiment depicted in  FIG. 3C  proceeds in the same fashion as describe above, with thermo-plastic/nano-filler ink  344  replacing thermo-plastic ink  340 . 
         [0095]    In the embodiment depicted in  FIG. 3B , there is no intervening layer  342  of nano-filler ink. In this embodiment, each layer is itself a composite (of the polymer and nano-filler). As in the previous embodiment, the laser heat source sinters the deposited ink shortly after it is deposited. Maser  120  is not used. 
         [0096]    It is to be understood that although the 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.