具体实施方式：
[0056]3D printing techniques offer unparalleled flexibility in achievable geometric shape and complexity over existing manufacturing techniques. These methods, also called additive manufacturing, build components incrementally by adding material through a deposition process. A new 3D printable composite ink formulation has been developed that can be used to fabricate strong and lightweight composite structures, such as open or closed cellular structures inspired by wood and other natural materials. The composite ink formulation can maintain a filamentary shape and span large gaps without sag after being extruded through a nozzle. A new method of 3D printing that allows control over the orientation of high aspect ratio particles in the deposited filament and in the printed composite structure has also been developed. Printed and cured polymer composites prepared from the new ink formulation using the methods described herein have been shown to exhibit an order of magnitude higher Young's modulus than competing materials while retaining equivalent (or higher) strength.
[0057]FIGS. 2A and 2B show schematics of the 3D printing process, which may also be referred to as 3D deposition, direct-write fabrication or direct-write robocasting. 3D printing entails flowing a rheologically-tailored ink composition through a deposition nozzle integrated with a moveable micropositioner having x-, y-, and z-direction capability. In the present method, the ink composition may include high aspect ratio particles that have a significant length-to-width aspect ratio, as shown schematically in FIG. 2B. As the nozzle is moved, a filament comprising the ink composition may be extruded through the nozzle and continuously deposited on a substrate in a configuration or pattern that depends on the motion of the micropositioner. In this way, 3D printing may be employed to build up 3D structures layer by layer, such as the exemplary cellular structures shown in FIGS. 2C-2F. The high aspect ratio particles may have a predetermined orientation in the deposited filament and in the printed composite structure.
[0058]The new method to control the orientation of high aspect ratio particles or fibers during 3D printing may involve introducing a rotational shear component to a composite ink formulation as it is being extruded through the deposition nozzle. This approach is enabled by the development of a 3D printing apparatus comprising a rotatable deposition nozzle that can be rotated at a specified rate about its axis, as set forth in greater detail below. The rotational motion may be controlled independently of the translational motion used to advance the deposition nozzle over a substrate to print a continuous filament, as shown schematically in FIGS. 2A and 2B.
[0059]High aspect ratio (or anisotropic) particles preferentially align along the direction of extension and shear in extensional and shear flows, respectively. In an extrusion process, this promotes particle alignment along the axis of extrusion; in an extrusion-based 3D printing process (e.g. direct-write printing or fused deposition modeling), the shear field between a translating nozzle and a stationary substrate may facilitate particle alignment along the print direction and within the plane of the printed layer. By introducing rotation to the nozzle during deposition, an additional shear field may be generated between the nozzle and the stationary substrate.
Composite Ink Formulation
[0060]The new 3D printable composite ink formulation includes a flowable matrix material and filler particles dispersed therein. The 3D printable ink formulation may comprise a mixture of an uncured polymer resin, filler particles and a latent curing agent. The composite ink formulation may have a strain-rate dependent viscosity (and thus can be said to be shear-thinning or viscoelastic) and may exhibit a plateau value of shear storage elastic modulus G′ of at least about 103 Pa. As is discussed in further detail below, the filler particles may include isotropic and/or anisotropic particles.
[0061]FIG. 3A shows viscosity as a function of shear rate and FIG. 3B shows moduli data (storage modulus G′ and loss modulus G″) for several exemplary composite ink formulations in comparison with an (unfilled) epoxy resin. The composition of each composite ink formulation is set forth in Table 1. Referring to FIG. 3A, the epoxy resin (without reinforcement or filler particles) exhibits rate-independent Newtonian flow behavior, while all of the composite ink formulations show a clear dependence of viscosity on shear rate. FIG. 3B reveals that the composite ink formulations exhibit significant shear thinning and yield stress behavior, again in contrast to the unreinforced epoxy resin. As can be seen, the plateau value of the storage elastic modulus G′ may in some cases be at least about 104, Pa or at least about 105 Pa, and may approach 106 Pa. The composite ink formulation may also exhibit a shear yield stress of at least about 100 Pa.
TABLE 1Exemplary Ink FormulationsEpoxy + clay + Epoxy + clay + Epoxy + Epoxy + clay + Epoxy + clay + Epoxy + claySiC inkSiC + CFInkclay inkSiC inkSiC + CFink (weight(weightink (weightconstituents(g)(g)ink (g)fraction)fraction)fraction)Epoxy resin3030300.6320.480.455Acetone000.5000.008DMMP3330.0630.0480.045VS03 curing1.51.51.50.0320.0240.023agentNano-clay13880.2740.1280.121SiC0202000.320.303whiskersCarbon003000.045fibers
[0062]During printing, the rheology of the composite ink formulation influences the printability, height, and morphology of structures that can be fabricated. At rest, the ink formulation ideally has a sufficiently high elastic storage modulus, G′, and shear yield strength (as indicated by the shear stress value at which the storage and viscous moduli cross for a given composition as shown for example in FIG. 3B) to maintain the printed shape. Under a shear stress, the ink formulation ideally exhibits significant shear thinning to allow flow through small diameter nozzles without requiring prohibitively high driving pressures. When an ink formulation is properly designed, self-supporting structures can be made with filaments that span many times their diameter in free space.
[0063]An estimate of the storage modulus, G′, required for a filament to span a given distance with less than 5% sag is given by the following equation:
G′>1.4ρgL4D3,
[0064]where ρ is the mass density, g is the gravitational constant, L is the span length, and D is the filament diameter. The shear yield stress, TY, required to achieve a self-supporting structure with a given build height can be calculated as follows:
τY=ρgh3,
[0065]where h is the structure height. Time-dependent behavior, such as viscoelastic creep or solvent evaporation, are not considered by these equations.
[0066]As shown by the data of FIGS. 3A and 3B, filler particles may be incorporated into the ink formulation to alter the rheological properties of the uncured polymer resin. They may also be used to influence the mechanical properties of the printed composite structure, as discussed further below. The uncured polymer resin selected for the ink formulation may be a thermosetting polymer resin, such as an epoxy resin, a polyurethane resin, a polyester resin, a polyimide resin, or a polydimethylsiloxane (PDMS) resin that undergoes a cross-linking process when cured.
[0067]The latent curing agent used in the ink formulation prevents premature curing of the polymer resin; typically, curing is activated by heat exposure after the composite structure has been printed. In conventional 3D printing methods, drying, solidification and/or curing may occur during the printing process such that a deposited layer is partially or fully solidified before the next layer of ink is deposited. Such “on the fly” curing approaches may be required when the printing inks are not engineered with the rheological properties to withstand the layer-by-layer construction of large components. However, premature curing of the ink may lead unsatisfactory bonding between adjacent layers, thereby diminishing the mechanical integrity of the 3D printed structure and/or leading to component warpage due to differential shrinkage. The latent curing agent incorporated in the composite ink formulation may be activated by elevated temperatures in the range of 100° C. to about 300° C. and may have a long pot life, allowing a prepared ink formulation to print consistently over a long time period (e.g., up to about 30 days). Some latent curing agents that may be suitable for the composite ink formulation may be activated by UV light instead of heat. One example of a suitable latent curing agent for epoxy resin is an imidazole-based ionic liquid, such as VSO3 from BASF Group's Intermediates Division. Other commercially available latent curing agents may also be used.
[0068]The composite ink formulation may include the uncured polymer resin at a concentration of from about 30 wt. % to about 95 wt. % and the filler particles at a concentration of from about 5 wt. % to about 70 wt. %. The latent curing agent may be present in the ink formulation at a concentration of from greater than 0 wt. % to about 5 wt. %.
[0069]The concentration of the latent curing agent is more typically specified in terms of weight relative to the weight of the uncured polymer resin. Thus, the latent curing agent may be present at a weight concentration of from greater than 0 to about 15 parts per hundred parts of the uncured polymer resin.
[0070]The volume fraction of filler particles may be a stronger predictor of the rheology of the composite ink formulation than the weight fraction of particles. In other words, the rheology of a composite ink formulation including a high weight fraction of a very dense reinforcement may be similar or identical to that of a composite ink formulation containing a low weight fraction of a low density reinforcement—if the volume fraction of the filler particles is about the same for the two formulations. It is useful for this reason to specify a suitable volume fraction of filler particles for the composite ink formulation. Typically, a suitable range of solids loading (particle loading) is from about 5 vol. % to about 60 vol. %, independent of the weight fraction of the particles.
[0071]The composite ink formulation may further comprise an antiplasticizer such as, for example, dimethyl methyl phosphonate (DMMP). By including the antiplasticizer, the initial viscosity of the epoxy resin may be reduced to allow a higher concentration of filler particles. The antiplasticizer may also contribute to an increased stiffness and strength in the cured composite structure. The antiplasticizer may be present in the ink composition at a concentration of from about 0 wt. % to about 15 wt. %. As with the latent curing agent, the concentration of the antiplasticizer is more typically specified in terms of weight relative to the weight of the uncured polymer resin. Thus, the antiplasticizer may be present at a weight concentration of from greater than 0 to about 20 parts per hundred parts of the uncured polymer resin. All of the composite ink formulations as well as the epoxy ink used to prepare the data shown in FIGS. 3A and 3B included a small amount of DMMP.
[0072]In some cases, a solvent such as acetone may be added to the composite ink formulation. The solvent may be effective in lowering the viscosity of the ink formulation prior to deposition, thereby enabling higher printing speeds and reducing the propensity of the extruded filament to “curl up” against the nozzle during deposition. The solvent may have a concentration of from 0 wt. % to about 20 wt. % in the composite ink formulation.
[0073]A number of different types of filler particles may be incorporated into the composite ink formulation for rheology control and/or to influence the mechanical or other (e.g., electrical, thermal, magnetic etc.) properties of the printed composite structure. In one example, the filler particles may be carbon-based, and thus may comprise carbon. For example, the filler particles may comprise silicon carbide particles and/or particles of another carbide, such as boron carbide, zirconium carbide, chromium carbide, molybdenum carbide, tungsten carbide or titanium carbide. It is also envisioned that the filler particles may comprise substantially pure carbon particles. In other words, the filler particles may comprise carbon particles consisting of carbon and incidental impurities. Examples of suitable carbon particles may include diamond particles, carbon black, carbon nanotubes, carbon nanofibers, graphene particles, carbon whiskers, carbon rods, and carbon fibers, which may be carbon microfibers. The filler particles may also or alternatively comprise clay particles, such as clay platelets; oxide particles, such as silica, alumina, zirconia, ceria, titania, zinc oxide, tin oxide, iron oxide (e.g., ferrite, magnetite), and/or indium-tin oxide (ITO) particles; and/or nitride particles, such as boron nitride, titanium nitride, and/or silicon nitride. As one of ordinary skill in the art would recognize, the filler particles may be electrically conductive, semiconducting, or electrically insulating.
TABLE 2Constituent properties of exemplary filler particles and epoxy resinModulusDensityMor-Characteristic(GPa)(g/cc)phologydimensionsEpoxy resin2.71.16——(e.g., Epon 826)Clay platelets1701.98platelet<10 μm(e.g., Cloisiteagglomerates* ofnano-clay)1 × 100 nmplatelets;SiC whiskers4503.21rod0.65 μm × 12 μmCarbon fibers9002.2rod 10 μm × 220 μm*Agglomerates may at least partially exfoliate during mixing.
[0074]The constituent properties of some exemplary filler particles and epoxy resin are provided in Table 2. Clay platelets are believed to act predominantly as a rheology modifier, imparting the desired shear thinning and shear yield stress to the uncured composite ink formulation, but they also contribute to stiffening of the cured epoxy matrix. The silicon carbide whiskers impart a high storage modulus to the ink formulation, but they may not provide a sufficient shear yield strength for the printed filament to maintain its shape. In small quantities, the carbon fibers may have a small effect on the rheology of the ink formulation. However, high aspect ratio whiskers and fibers, when used, may become highly aligned in the shear and extensional flow field within the nozzle during deposition, as shown schematically in FIG. 2B, and may result in very effective stiffening in the cured composite structure along the direction of printing.
[0075]The filler particles may thus include high aspect ratio particles that have aspect ratio of greater than 1, or greater than about 2, where the aspect ratio may be a length-to-width ratio. In some cases, the aspect ratio may refer to a length-to-thickness ratio. If the filler particles are agglomerated, the aspect ratio relevant to the properties of the ink formulation and the printed composite may be the aspect ratio of the agglomerated particles. If the width and the thickness of a particle are not of the same order of magnitude, the term “aspect ratio” may refer to a length-to-width ratio. The filler particles may comprise, for example, whiskers, fibers, microfibers, nanofibers, rods, microtubes, nanotubes, or platelets. At least some fraction of, or all of, the high aspect ratio particles may have an aspect ratio greater than about 2, greater than about 5, greater than about 10, greater than about 20, greater than about 50, or greater than about 100. Typically, the aspect ratio of the high aspect ratio particles is no greater than about 1000, no greater than about 500, or no greater than about 300. Such high aspect ratio particles may be at least partly aligned during 3D printing of the ink formulation, depending in part on the size and aspect ratio of the particles in comparison to the diameter of the deposition nozzle.
[0076]The high aspect ratio particles may have at least one short dimension (e.g., thickness and/or width) that lies in the range of from about 1 nm to about 50 microns. The short dimension may be no greater than about 20 microns, no greater than about 10 microns, no greater than about 1 micron, or no greater than about 100 nm. The short dimension may also be at least about 1 nm, at least about 10 nm, at least about 100 nm, at least about 500 nm, at least about 1 micron, or at least about 10 microns.
[0077]The high aspect ratio particles may have a long dimension (e.g., length) that lies in the range of from about 5 nm to about 10 mm, and is more typically in the range of about 1 micron to about 5 microns, or from about 100 nm to about 500 microns. The long dimension may be at least about 10 nm, at least about 100 nm, at least about 500 nm, at least about 1 micron, at least about 10 microns, at least about 100 microns, or at least about 500 microns. The long dimension may also be no greater than about about 5 mm, no greater than about 1 mm, no greater than about 500 microns, no greater than about 100 microns, no greater than about 10 microns, no greater than 1 micron, or no greater than about 100 nm.
[0078]If the filler particles are substantially isotropic particles, then they may have an aspect ratio of about 1 and a linear size (e.g., diameter) that lies within any of the above-described ranges.
[0079]The composite ink formulation and the printed composite structure may include filler particles of more than one type, size and/or aspect ratio, allowing for optimization of the rheology of the composite ink formulation as well as enhancement of the mechanical properties of the printed composite structure. For example, the filler particles may comprise a first set of particles added primarily to refine the flow properties of the composite ink formulation, and a second set of particles added primarily to improve the stiffness of the printed composite part. In one example, the second set of particles may include high aspect ratio particles, such as silicon carbide whiskers or carbon fibers, while the first set of particles may be more isotropic in morphology with an aspect ratio lower than the second set of particles, such as clay platelets or oxide particles, which may include agglomerates. The particles (or agglomerates) of the first set may have, for example, an aspect ratio in the range of about 1 to about 4, and the particles of the second set may have an aspect ratio of about 5 to about 20 (e.g., at least about 10, or at least about 15). The aspect ratio of the particles of the second set may also be greater than 20, greater than 50, or greater than 100, for example.
[0080]It should be noted that when a set of particles—or more generally speaking, more than one particle—is described as having a particular aspect ratio, size or other characteristic, that aspect ratio, size or characteristic can be understood to be a nominal value for the plurality of particles, from which individual particles may have some deviation, as would be understood by one of ordinary skill in the art.
[0081]The filler particles may further comprise a third set of particles having a different chemical composition, size and/or aspect ratio from each of the first and second sets of particles. FIGS. 3A and 3B show an exemplary shear-thinning, high-yield stress epoxy ink formulation including three different sets of particles (clay platelets, silicon carbide whiskers and carbon fibers) that can be used to produce a printed composite structure having anisotropic mechanical properties and an extremely high Young's modulus (see FIG. 7A, which is discussed further below). It is contemplated that the composite ink formulation may include up to 5 different sets of particles, where the particles of each set differ from the particles of the other sets based on their composition, size and/or aspect ratio. Assuming the rheological requirements are met, the number and amount of different types of particles may be tuned to optimize the properties of the printed composite part.
[0082]It should be noted that the particles of the first, second, third and/or higher sets may have a chemical composition, size and/or aspect ratio as described in any of the examples and embodiments in this disclosure. Also, as would be recognized by one of ordinary skill in the art, particles of one set are physically intermixed with particles of the other set(s) in the composite ink formulation. In fact, it is typically advantageous to have a homogeneous mixture of all of the types of particles.
[0083]It is beneficial to control the relative amounts of the various types of filler particles to optimize the mechanical properties of the printed composite structure without sacrificing the rheological properties of the composite ink formulation. Exemplary concentration ranges are provided in Table 3 below.
TABLE 3Exemplary ranges of possible composite ink constituentsExemplaryPreferredConcen-Concen-trationstrationsPossible Ink ConstituentsExamples(wt. %)(wt. %)Polymer resinEpoxy resin30-95 40-60SolventAcetone0-200-2AntiplasticizerDMMP0-150-5Latent curing agentVS030-102-4Filler particlesClay platelets5-5010-30(e.g., AR* from about 1-4)Filler particlesSiC whiskers0-5010-30(e.g., AR from about 5-20)Filler particlesCarbon fibers0-40 2-10(e.g., AR > 20)*AR = aspect ratio
[0084]As set forth above, the composite ink formulation may include the polymer resin at a concentration of from about 30 wt. % to about 95 wt. %. For example, the concentration of the polymer resin in the composite ink formulation may be at least about 30 wt. %, at least about 40 wt. %, at least about 50 wt. %, at least about 60 wt. %, at least about 70 wt. %, or at least about 80 wt. %. The concentration of the polymer resin in the composite ink formulation may also be no greater than about 95 wt. %, no greater than about 90 wt. %, no greater than about 80 wt. %, no greater than about 70 wt. %, or no greater than about 60 wt. %.
[0085]The concentration of the filler particles in the composite ink formulation may be at least about 5 wt. %, at least about 10 wt. %, at least about 20 wt. %, at least about 30 wt. %, at least about 40 wt. %, at least about 50 wt. %, at least about 60 wt. %, or at least about 70 wt. %. The concentration of the filler particles may also be no greater than about 70 wt. %, no greater than about 50 wt. %, no greater than about 30 wt. %, no greater than about 20 wt. %, or no greater than about 10 wt. %. In terms of volume fraction, the amount of the filler particles may be at least about 5 vol. %, at least about 10 vol. %, at least about 20 vol. %, at least about 30 vol. %, at least about 40 vol. %, or at least about 50 vol. %. The amount may also be no greater than about 60 vol. %, no greater than about 50 vol. %, no greater than about 40 vol. %, no greater than about 30 vol. %, or no greater than about 20 vol. %.
[0086]The latent curing agent may be present in the ink formulation at a concentration of greater than 0 wt. %, such as about 0.1 wt. % or greater, about 1 wt. % or greater, or about 2 wt. % or greater. The concentration of the latent curing agent may also be as high as about 10 wt. %, as high as about 5 wt. %, or as high as about 3 wt. %. Specified in terms of weight relative to the weight of the uncured polymer resin, the latent curing agent may be present at a weight concentration of greater than about 2 parts, greater than about 4 parts, greater than about 8 parts, or greater than about 12 parts per hundred of the uncured polymer resin, and up to about 15 parts per hundred of the uncured polymer resin.
[0087]The antiplasticizer, which is optional, may be present in the composite ink formulation at a concentration of up to about 15 wt. %, or up to about 10 wt. %. For example, the concentration of the antiplasticizer may be from about 2 wt. % to about 8 wt. %. Specified in terms of weight relative to the weight of the uncured polymer resin, the antiplasticizer may be present at a weight concentration of greater than about 2 parts, greater than about 4 parts, greater than about 8 parts, greater than about 12 parts, or greater than about 16 parts per hundred of the uncured polymer resin, and up to about 20 parts per hundred of the uncured polymer resin.
3D Printed Composite Structures: First Examples
[0088]Lightweight and high-stiffness composite structures, such as cellular structures inspired by natural materials such as wood, may be 3D printed from the composite ink formulations described above.
[0089]Representative examples of various cellular structures—including square, hexagonal and triangular honeycomb structures—that can be formed by 3D printing are shown in FIGS. 2C-2F, where the scale bars are 2 mm. The cellular structures may be aperiodic or periodic, like the honeycomb structures shown here. Methods of forming 3D printed composite structures, including cellular structures and microlattice structures, are described in detail below.
[0090]A 3D printed cellular structure may comprise a cellular network of cell walls separating empty cells, where the cell walls comprise a polymer composite including filler particles dispersed in a polymer matrix (e.g., a thermoset polymer matrix). The filler particles may comprise high aspect ratio particles that have a predetermined orientation within the cell walls. For example, the filler particles may be at least partially aligned with the cell walls along a length thereof.
[0091]Because the printed composite structure may be fabricated from a continuous filament in a layer by layer deposition process, each cell wall may have a size and shape defined by a stack of layers of the continuous filament. The length of the cell walls may align with the direction of printing or print path, which may be referred to as a “length direction.” The height of the cell walls may correspond approximately to the average diameter of the continuous filament multiplied by the number of layers in the stack, assuming no settling occurs. A “height direction” may be substantially perpendicular to the length direction.
[0092]High aspect ratio particles may be understood to be “at least partially aligned” with the longitudinal axis of the continuous filament (or the cell walls of the cellular network) if at least about 25% of the high aspect ratio particles are oriented such that the length or long axis of the particle is within about 40 degrees of an imaginary line extending along the longitudinal axis of the continuous filament (or along the length of each cell wall, or along the length direction). This imaginary line may also coincide with the print direction or print path. In some cases, the long axis of at least about 30%, at least about 35% or at least about 40% of the high aspect ratio particles may be oriented within about 40 degrees of the imaginary line.
[0093]The high aspect ratio particles may be understood to be “highly aligned” with the longitudinal axis of the continuous filament (or the cell walls of the cellular network) if at least about 50% of the high aspect ratio particles are oriented such that the length or long axis of the particle is within about 40 degrees of an imaginary line extending along the longitudinal axis of the continuous filament (or along the length of each cell wall, or along the length direction). This imaginary line may also coincide with the print direction or print path. In some cases, the long axis of at least about 60%, at least about 70%, at least about 80%, or at least about 90% of the particles may be oriented within about 40 degrees of the imaginary line.
[0094]Depending on the high aspect ratio particles used and the processing conditions, it may be possible to produce printed composite structures having at least about 25% of the high aspect ratio particles oriented such that the length or long axis of the particle is within about 20 degrees of the imaginary line described above, or within about 10 degrees of the imaginary line. In some cases, at least about 30%, at least about 40%, at least about 50%, at least about 60%, at least about 70%, at least about 80%, or at least about 90% of the particles may have a long axis oriented within about 20 degrees or within about 10 degrees of the imaginary line.
[0095]The above-described partial or high alignment of the high aspect ratio particles with respect to the longitudinal axis of the continuous filament (or the length of the cell wall, or the length direction) may occur over an entire length of the continuous filament or cell wall(s), or over only a portion of the length (e.g., over a given distance or cross-section).
[0096]Like the composite ink formulation from which it is formed, the polymer composite can include more than one type and size of filler particle. Accordingly, the degree of alignment may be different for different sets of particles. The degree of alignment may depend in part on the aspect ratio of the particles. For example, particles that have an aspect ratio of about 1 or slightly greater than 1 may not be substantially aligned along the longitudinal axis of the continuous filament during printing. On the other hand, particles with an aspect ratio of greater than 10 or 20 may be highly aligned. A large factor in determining the degree of alignment is the length of the particles relative to the diameter of the nozzle. It is believed that particles having a length that is at least about 5% of the diameter of the nozzle may be particularly well suited to being aligned during printing, assuming that clogging of the nozzle can be avoided. For this reason, it may be advantageous for the particles to have both a length that is at least about 5% of the diameter of the nozzle and a large aspect ratio, such as an aspect ratio greater than about 10. The particles may also have a length that is at least about 10%, at least about 20%, at least about 30%, at least about 40%, or at least about 50% of the diameter of the nozzle, and the length of the particles is ideally no longer than about 200% or about 300% of the diameter of the nozzle.
[0097]The filler particles (or “high aspect ratio particles” or “particles”) of the polymer composite can have any of the characteristics (composition, size, aspect ratio, concentration, etc.) described above for the filler particles of the composite ink formulation. As one of ordinary skill in the art would recognize, the filler particles of the polymer composite are the same as the filler particles of the composite ink formulation.
[0098]The polymer matrix of the polymer composite may comprise a thermosetting polymer such as epoxy, polyurethane, polyimide, polydimethylsiloxane (PDMS), or polyester. It is also contemplated that the polymer matrix may comprise a thermoplastic polymer, as described further below.
[0099]The polymer composite may be fabricated by the following process: a continuous filament, which comprises a composite ink formulation including an uncured polymer resin, filler particles, and a latent curing agent, is deposited on a substrate in a predetermined pattern layer by layer. The filler particles include high aspect ratio particles that may be at least partially aligned along a longitudinal axis of the continuous filament when deposited. The composite ink formulation may be cured, preferably after deposition, to form the polymer composite, where the high aspect ratio particles have a predetermined orientation therein. The resulting 3D printed composite structure may have any size and shape that can be formed by depositing a continuous filament and curing, as described above. The composite structure may be a substantially fully dense solid or a porous structure comprising voids or porosity.
[0100]For example, the 3D printed composite structure may be a cellular structure, as shown in FIGS. 2C-2F. In such a case, the cellular structure (or cellular network) may take the form of a honeycomb structure having from 3 to 6 cell walls surrounding each cell. As mentioned above, each cell wall may be defined by a stack of one or more extruded filaments deposited layer-by-layer on a substrate as a continuous filament.
[0101]The thickness of each cell wall may be determined by the diameter of the continuous filament, which may be influenced by the size of the nozzle as well as the deposition pressure and speed. The continuous filament may have a substantially cylindrical shape as a consequence of being extruded through the nozzle. The thickness of each cell wall may be in the range of from about 20 microns to about 20 mm, and is more typically from about 100 micro