Patent Application
20160346997
The present disclosure is related generally to three-dimensional printing (3D printing) and more particularly to 3D printed composite structures.
With the growing need for lightweight, high-performance structural materials, cellular materials have become increasingly more relevant over the past several decades because of their low density, high specific properties, and potential for multifunctionality (e.g., structural, transport, electrical and magnetic applications). Such materials are utilized in high stiffness sandwich panels, energy absorbers, catalytic materials, vibration damping, insulation, and other products. In this class of materials, the properties of the bulk may depend on i.) the base material from which the cellular structure is made, ii.) the topology and shape of the cells (i.e., the architecture), and iii.) the relative density of the material, that is, the density of the cellular structure relative to the density of the base material. Therefore, the development of high performance base materials amenable to fabrication into cellular structures with controlled architecture is of paramount importance. When the architecture can be controlled, properties can be optimized to the desired application. Materials which exhibit ordered architecture and hierarchy may achieve properties far superior to equivalent composites with random architecture (La, random composites or foams containing the same constituents at the same volume fractions). For example, nacre has a work of fracture value ˜150 times higher than the simple average of the individual constituents, and wood still rivals the best engineering materials in terms of specific bending stiffness (E1/2/ρ) and specific bending strength (σ2/3/ρ). Advances in the fabrication of synthetic cellular materials, which enable finer control over architecture at multiple length scales, could lead to drastic increases in material properties, wider commercial use and substantial improvements in mass efficiency over existing engineering materials and systems.
As a prime example of a natural material with complex architecture, wood utilizes microscopic bundles of highly oriented cellulose nano-fibrils in a multi-orientation layup within the walls of its cellular structure to achieve extremely high specific stiffness and strength. To demonstrate the importance of controlling fiber orientation in a similar engineering system, a series of finite element analyses were conducted using Abaqus software (Dassault Systèmes, France) on a fiber composite in a triangular honeycomb geometry. Referring to
A 3D printable composite ink formulation comprises an uncured polymer resin, filler particles, and a latent curing agent, where the composite ink formulation comprises a strain-rate dependent viscosity and a plateau value of elastic storage modulus G′ of at least about 103 Pa.
A filamentary structure extruded from a nozzle during 3D printing comprises a continuous filament including filler particles dispersed therein. At least some fraction of the filler particles in the continuous filament comprise high aspect ratio particles having a predetermined orientation with respect to a longitudinal axis of the continuous filament.
A filamentary structure extruded from a nozzle during 3D printing comprises a continuous filament including high aspect ratio particles dispersed therein. At least some fraction of the high aspect ratio particles in the continuous filament have a helical orientation comprising a circumferential component and a longitudinal component, where the circumferential component is imparted by rotation of a deposition nozzle and the longitudinal component is imparted by translation of the deposition nozzle.
A 3D printed composite structure comprises a polymer composite including a thermoset polymer matrix and filler particles dispersed therein, where the polymer composite is made by the following process: a continuous filament is deposited on a substrate in a predetermined pattern layer by layer. The continuous filament comprises a composite ink formulation including an uncured polymer resin, filler particles, and a latent curing agent. The filler particles include high aspect ratio particles that are at least partially aligned along a longitudinal axis of the continuous filament when deposited. The composite ink formulation is cured, preferably after deposition, to form the polymer composite, and the high aspect ratio particles have a predetermined orientation in the thermoset polymer matrix.
A 3D printed composite structure comprises a polymer composite including a polymer matrix and oriented high aspect ratio particles dispersed therein, wherein the polymer composite is made by: extruding a continuous filament from a nozzle while the nozzle rotates about a longitudinal axis thereof and translates with respect to a substrate, the continuous filament comprising a composite ink formulation including high aspect ratio particles in a flowable matrix material; depositing the continuous filament in a predetermined pattern on the substrate, where at least some fraction of the high aspect ratio particles in the continuous filament have an orientation comprising a circumferential component due to rotation of the nozzle and a longitudinal component due to translation of the nozzle; and processing the continuous filament to form the polymer matrix with oriented high aspect ratio particles dispersed therein.
A 3D printed lattice structure comprises a microlattice comprising a plurality of layers of extruded filaments arranged in a crisscross pattern. The extruded filaments comprise a polymer composite including a polymer matrix and high aspect ratio particles dispersed therein. The high aspect ratio particles are at least partially aligned with a longitudinal axis of the respective extruded filament along a length thereof.
A 3D printed cellular structure comprises a cellular network comprising cell walls separating empty cells, where the cell walls comprise a polymer composite comprising filler particles dispersed in a polymer matrix. The filler particles comprise high aspect ratio particles having a predetermined orientation within the cell walls.
A 3D printed cellular structure comprises 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. The filler particles may comprise high aspect ratio particles that are at least partially aligned with the cell walls along a length thereof.
A 3D printed cellular structure comprises a network of cell walls separating empty cells, where the cell walls comprise a polymer composite including high aspect ratio particles dispersed in a polymer matrix. At least about 20% of the high aspect ratio particles have a long axis oriented within about 80 degrees of a height direction of the cell walls.
A method of making a 3D printed composite structure may comprise depositing a continuous filament, which comprises a composite ink formulation including an uncured polymer resin, filler particles, and a latent curing agent, on a substrate in a predetermined pattern layer by layer. The filler particles include high aspect ratio particles at least partially aligned along a longitudinal axis of the continuous filament when deposited. The composite ink formulation is cured to form a polymer composite comprising a thermoset polymer matrix and the filler particles dispersed therein, where the high aspect ratio particles have a predetermined orientation in the thermoset polymer matrix.
A method of making a 3D printed cellular structure may comprise depositing a continuous filament, which comprises a composite ink formulation including an uncured polymer resin, filler particles, and a latent curing agent, on a substrate in a predetermined pattern layer by layer to form stacks of the continuous filament. The filler particles include high aspect ratio particles at least partially aligned along a longitudinal axis of the continuous filament when deposited. The composite ink formulation is cured to form a polymer composite comprising a thermoset polymer matrix and the filler particles dispersed therein. Upon curing, the stacks of the continuous filament form cell walls of a cellular structure comprising the polymer composite, and the high aspect ratio particles are at least partially aligned with the cell walls along a length thereof.
A method of making a 3D printed composite structure comprises extruding a continuous filament from a nozzle while the nozzle rotates about a longitudinal axis thereof and translates with respect to a substrate. The continuous filament comprises a composite ink formulation including high aspect ratio particles in a flowable matrix material. The continuous filament is deposited in a predetermined pattern on the substrate, where at least some fraction of the high aspect ratio particles in the continuous filament have a helical orientation comprising circumferential and longitudinal components due to rotational and translational motion of the nozzle.
An apparatus for 3D printing comprises: a 3D positioning stage for implementing translational motion; a nozzle assembly mounted on the 3D positioning stage, the nozzle assembly comprising a hollow stationary portion connected to a hollow rotatable portion; a motor mounted on the 3D positioning stage, the motor being operatively connected to the hollow rotatable portion to implement rotational motion thereof; and a controller electrically connected to the 3D positioning stage and to the motor for independently controlling the translational motion and the rotational motion of the nozzle assembly.
The terms “comprising,” “including,” and “having” are used interchangeably throughout this disclosure as open-ended terms to refer to the recited elements (or steps) without excluding unrecited elements (or steps).
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.
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
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.
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.
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
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:
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:
where h is the structure height. Time-dependent behavior, such as viscoelastic creep or solvent evaporation, are not considered by these equations.
As shown by the data of
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.
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. %.
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.
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.
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
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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. %.
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. %.
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.
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.
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. Representative examples of various cellular structures—including square, hexagonal and triangular honeycomb structures—that can be formed by 3D printing are shown in
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
For example, the 3D printed composite structure may be a cellular structure, as shown in
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 microns to about 500 microns. The length of each cell wall may range from 0.5 mm to about 50 mm. As shown in
Given the high rest storage modulus and shear yield strength of the continuous filament, the cell walls may be built to heights of up to 100 layers (e.g., from 2 layers to 100 layers). The height of each of the cell walls may depend on the size of the continuous filament and the number of layers. Generally speaking, the maximum height may be up to about 100 times the thickness of the cell wall. For example, the height may be at least about 5 times, at least about 10 times, at least about 20 times, at least about 50 times, or at least 80 times the thickness of the cell wall.
Relative density may be defined as the density of the cellular structure relative to the density of the polymer composite making up the cell walls. Using a composite ink formulation engineered to provide good rheological properties as well as to form a polymer composite exhibiting high stiffness and strength, the length of the cell walls and size of the cells may be increased to minimize the relative density of the cellular structure. As illustrated in
Another example of a 3D printed composite structure is the exemplary microlattice shown in the scanning electron microscope image of
Generally speaking, a microlattice structure such as that shown in
Returning to the exemplary cellular structures of
The exemplary cellular structure shown in
As evidenced by
To quantify the mechanical properties of the printed composite structures, printed tensile bars and triangular honeycomb structures were tested on an Instron 5566 load frame in tension and compression, respectively. The effects of build direction were probed by using two separate print paths for the tensile bars, one oriented longitudinally along the tensile direction, and one oriented transverse to the tensile direction, as illustrated in
The epoxy composites containing SiC whiskers and carbon fiber rods show significant anisotropy and print path dependence due to the high degree of alignment of the fillers during deposition. The printed composite structures show a substantial increase in Young's modulus, E, over the unfilled epoxy resin from 2.66±0.17 GPa to 8.06±0.45 and 10.61±1.38 GPa for the transverse specimens with and without carbon fibers, respectively, and 24.5±0.83 and 16.10±0.03 GPa for the longitudinal specimens with and without carbon fibers, respectively. This represents up to a 9-fold increase in modulus over the cast epoxy.
Failure strength values, σf, for the printed composite structures are comparable to that of the cast epoxy (71.1±5.3 MPa), with the longitudinal specimens exhibiting somewhat higher strengths (66.2±6.1 and 96.6±13.8 MPa, with and without carbon fiber, respectively) than the transverse specimens for both ink formulations containing rods or whiskers (43.9±4.1 and 69.8±2.9 MPa, with and without carbon fiber, respectively).
The epoxy composite containing only clay platelets displays nearly identical longitudinal and transverse properties (E=5.86±0.62 and 6.23±0.24 GPa and σf=37.5±5.3 and 47.7±2.7 MPa, for longitudinal and transverse specimens, respectively), indicating isotropic properties independent of build direction. Mechanical properties for all three composite formulations, epoxy reinforced with clay, epoxy reinforced with clay and silicon carbide (SiC), and epoxy reinforced with clay, SiC and carbon fibers (CF), are summarized in Table 4 in comparison with data for a cast epoxy, and plotted in
The printed polymer composites may have a Young's modulus from about 6 GPa to about 25 GPa and a failure strength of from about 40 MPa to about 100 MPa. The Young's modulus may be at least about 10 GPa, at least about 15 GPa, or at least about 20 GPa, and may be up to about 25 GPa or about 30 GPa. The failure strength may be at at least about 60 MPa, at least about 70 MPa, at least about 80 MPa, at least about 90 GPa, and up to about 100 MPa.
Referring to
Representative stress-strain data for the honeycomb structures are shown in
Scaling laws governing the strength and modulus of these cellular structures are well established and follow the following relationships:
where Es, σTS, and ρs are the Young's modulus, tensile strength, and density of the base solid material, respectively, and E and σc are the Young's modulus and strength, respectively, of the cellular structure. For a triangular lattice, B=C=⅓ and b=c=1. These model predictions are also plotted in
where e≡w0/t, and w0 is the amplitude of waviness and t is the wall thickness. Predictions for reduced modulus values are plotted in
To put the properties of the 3D printed polymer composites into context, data for commercially available printed polymers and polymer composites, as well as data for balsa wood and properties of the wood cell wall material alone, are included in
When printed into lightweight cellular structures, such as the honeycomb structures shown in
3D Printing of Composite Structures without Nozzle Rotation
A method of making a 3D printed composite structure, such as those described above, may include depositing a continuous filament comprising a composite ink formulation on a substrate in a predetermined pattern layer by layer, where the composite ink formulation includes filler particles in a flowable matrix material. For example, the composite ink formulation may include an uncured polymer resin, filler particles, and a latent curing agent. The filler particles may comprise high aspect ratio particles that are 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 a polymer composite comprising the filler particles dispersed in a polymer matrix, where the high aspect ratio particles have a predetermined orientation in the polymer composite. The polymer matrix is typically a thermoset polymer matrix, but may be a thermoplastic polymer matrix in some embodiments.
The method may be employed to fabricate stiff and lightweight structures, such as cellular structures. In one example of cellular structure fabrication, the method may comprise depositing the continuous filament on a substrate in a predetermined pattern layer by layer, as described above, to form stacks or layers of the continuous filament. The filler particles may include high aspect ratio particles that are at least partially aligned along a longitudinal axis of the continuous filament when deposited. The composite ink formulation may be cured to form a polymer composite including the filler particles dispersed in a polymer matrix. Upon curing, the stacks of the continuous filament form cell walls of a cellular structure comprising the polymer composite. The high aspect ratio particles of the polymer composite may be at least partially aligned with the cell walls along a length thereof.
Depending on the characteristics of the filler particles and the size of the nozzle used for deposition, the high aspect ratio particles may also be highly aligned (as opposed to just partially aligned) with the longitudinal axis of the continuous filament and/or the cell walls, where the degree of alignment is as explained above.
The “continuous filament” deposited on the substrate may be understood to encompass a single continuous filament of a desired length or multiple extruded filaments having end-to-end contact once deposited to form a continuous filament of the desired length. In addition, two or more continuous filaments in a given layer of a structure may be spaced apart, as end-to-end contact may not be required. A continuous filament of any length may be produced by halting deposition after the desired length of the continuous filament has been reached. The desired length of the continuous filament may depend on the print path and/or the geometry of the structure being fabricated. Generally speaking, the desired length is at least as large as the inner diameter of the nozzle and may be many times the inner diameter (ID) of the nozzle (e.g., at least about 10·ID, at least about 100·ID, at least about 1000·ID, or at least about 10000·ID).
As shown in
Curing of the composite ink formulation may be carried out after deposition of the continuous filament. That is, the curing may be carried out only after deposition is completed. For example, when the method is applied to form a cellular structure or network, the curing may be carried out after all of the stacks or layers have been formed. As discussed above, premature curing (e.g., during printing of the continuous filament) may lead to unsatisfactory bonding between adjacent layers, thereby diminishing the mechanical integrity of the 3D printed structure and/or leading to component warpage. Because a latent curing agent is employed in the composite ink formulation, premature curing can be avoided. Generally, the curing may entail heating the composite ink formulation at a temperature of from about 100° C. to about 300° C. The curing may also entail more than one heating step, such as a first heat treatment at a temperature from about 100° C.-150° C. and a second heat treatment at a temperature of from about 200° C.-300° C.
The printed composite structure formed by 3D printing and curing, including the cellular structure and polymer composite comprising the polymer matrix and filler particles, may have any of the characteristics described elsewhere in this disclosure.
The method is applicable to extrusion-based printing processes including direct-write printing, as described above, and fused deposition modeling. In the former case, flow through the nozzle and deposition of the continuous filament may be facilitated by using a composite ink formulation with a strain-rate dependent viscosity (and which may be said to be shear-thinning or viscoelastic). In the latter case, extrusion and deposition may rely on the temperature-dependent flow behavior of a thermoplastic polymer, as discussed in more detail below.
Ink Preparation: Composite ink formulations were prepared by incorporating the additives into the epoxy resin via Thinky Planetary Centrifugal Mixer (Thinky USA, Inc., Laguna Hills, Calif.) using 125 mL glass containers and a custom adaptor. Batches started with 30 grams of Epon 826 resin (Momentive Specialty Chemicals, Inc., Columbus, Ohio). 3 grams of DMMP (Sigma Aldrich, St. Louis, Mo.) were added first, followed by 5 minutes of mixing and 2 minutes of defoam cycle in the Thinky. Next, SiC whiskers (SI-TUFF™ SC-050, ACM, Greer, SC 29651) were added in 5 or 10 gram increments, followed by the nano-clay platelets (Cloisite 30b, Southern Clay Products, Inc., Gonzales, Tex. 78629), in 2 gram increments, and, when used, the milled carbon fibers (Dialead K223HM, Mitsubishi Plastics, Inc., Tokyo, Japan), in 1 gram increments. Finally, the ink is allowed to cool to room temperature (the mixing causes significant heating), and then the curing agent, Basionics VS03 (BASF, Ludwigshafen, Germany), was added at 5 parts per hundred, relative to the epoxy resin. When carbon fibers are used, 0.5 g of acetone was added along with the curing agent. Each material addition was followed by 5 minutes of mixing and 2 minutes of defoaming in the Thinky mixer.
Rheology: Rheological properties of the composite ink formulation were characterized using an AR 2000ex Rheometer (TA Instruments, New Castle, Del.) with a 40 mm flat plate geometry and a gap of 500 μm or 1000 μm, when the ink formulation contained carbon fibers. All measurements were preceded by a one minute conditioning step at a constant shear rate of 1/s, followed by a ten minute rest period to allow the ink structure to reform.
Printing: The composite ink formulation was loaded into 3 cc, luer-lock syringes (Nordson EFD, Westlake, Ohio) and centrifuged at 3900 rpm for 10 minutes to remove bubbles. Loaded syringes were then mounted in an HP3 high-pressure adaptor (Nordson EFD) and the assembly was mounted on an Aerotech 3-axis positioning stage (Aerotech, Inc., Pittsburgh, Pa.) for deposition. The ink formulation was driven pneumatically and controlled via an Ultimus V pressure box (Nordson EFD), which interfaces with the Aerotech motion control software. Luer-lock syringe tips (Nordson EFD) were used to dictate filament diameter, and filaments were deposited onto glass slides covered with Bytac®, PTFE-coated aluminum foil (Saint Gobain Performance Plastics, Worcester, Mass.) to prevent adhesion. Print paths for each geometry were written as parameterized g-code scripts and were designed to maximize continuity within each printed layer. Printed composite structures were then pre-cured at 100° C. for 15 hours, cooled, removed from the substrate, and cured for 2 hours at 220° C.
Characterization of Printed Composites: Density measurements on fully cured polymer composites were made using the Archimedes method, and the relative densities of honeycombs specimens were calculated from the measured mass and volume of each specimen. Prior to testing, surfaces of the cellular structures were ground flat to ensure good contact with the compression platens. Printed specimens were tested in an Instron 5566 load frame (Instron, Norwood, Mass.) at a strain rate of about 2×10−4 1/s for the tensile and compression specimens, respectively. Strain in the samples was measured using the Instron Advanced Video Extensometer (AVE). Reported tensile properties represent an average of at least three samples.
3D Printing of Composite Structures with Nozzle Rotation
Referring to
At least some fraction of the high aspect ratio particles in the continuous filament have an orientation comprising a circumferential component and a longitudinal component due to the rotational and translational motion of the nozzle, respectively. This orientation is defined with respect to a longitudinal axis of the continuous filament and may be referred to as a helical orientation. Preferably, 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% and up to 100% of the high aspect ratio particles in the continuous filament are helically oriented. The continuous filament may be processed (e.g., cured or cooled) to form a polymer composite comprising a polymer matrix and oriented high aspect ratio particles dispersed therein, as described in greater detail below.
The rotational motion of the nozzle may be controlled independently from the translational motion. The rotation of the nozzle (which may also be referred to as the “nozzle portion”) may occur continuously during translation of the nozzle, or the rotation may occur intermittently during translation of the nozzle. Also or alternatively, the rotational speed of the nozzle may be varied during printing while the translation speed of the nozzle remains the same or is also varied. These approaches may be useful to form continuous filaments having a variation in high aspect ratio particle orientation along the length of the filament, as described further below.
Rotation rates ω of from about 1 rad/s to about 1000 rad/s, and translation speeds (or deposition rates) of from about 1 mm/s to about 500 mm/s are typical. The relative magnitude of the translation speed ν to the rotation rate ω may influence the degree of rotational shear experienced by the composite ink formulation during extrusion, and hence the preferred angle of orientation of the high aspect ratio particles with respect to the longitudinal axis of the continuous filament. This angle of orientation may be referred to as the helical angle φ, where 0°<φ<90° for a non-zero rotation rate ω and translation speed ν, as illustrated in
Influenced by the rotational and translational shear fields during extrusion, the high aspect ratio particles may follow (roughly or precisely) a helical path of helical angle φ along a length of the continuous filament. For example, at least about 40% of the high aspect ratio particles at a radial position rmax, where rmax is approximately equivalent to an inner radius R of the nozzle, may have a long axis oriented within about 40 degrees of the helical path. Preferably, at least about 50%, at least about 60%, at least about 70%, at least about 80%, or at least about 90% of the high aspect ratio particles at the radial position rmax may have a long axis oriented within about 40 degrees of the helical path.
The high aspect ratio particles may also more precisely follow the helical path of helical angle φ along a length of the continuous filament. For example, at least about 40% of the high aspect ratio particles at the radial position rmax may have a long axis oriented within about 20 degrees of the helical path. Preferably, at least about 50%, at least about 60%, at least about 70%, at least about 80%, or at least about 90% of the high aspect ratio particles at the radial position rmax may have a long axis oriented within about 20 degrees of the helical path.
The above-described helical alignment of the high aspect ratio particles may occur over an entire length of the continuous filament or over only a portion of the length (e.g., over a given distance or cross-section).
As would be recognized by one of ordinary skill in the art, the helical angle φ is a linear function of radial position within the nozzle, with zero shear due to rotation at the center of the nozzle and maximum shear due to rotation at the nozzle perimeter, assuming the rotation occurs about a central longitudinal axis of the nozzle. Also assuming a uniform shear field, the magnitude of the rotational shear rate may be given by
where r is the radial position, w is the rotation rate, and h is the distance between the substrate and the nozzle. The magnitude of the translational shear rate may be given by
where ν is the translation speed. Assuming that the high aspect ratio particles are substantially aligned along the shear direction, this leads to a helical angle given by
In actuality, the theoretical fiber orientation may depend on the shear rate, rheological properties of the ink, particle aspect ratio, particle loading fraction, and shear history of the composite ink formulation, but (3) provides a best case scenario for highly aligned high aspect ratio particles. Because the rotational shear rate depends on r, some fraction of the high aspect ratio particles may orient along the longitudinal axis of the continuous filament at the center, where r=0, and high aspect ratio particles at the perimeter (where r=rmax=R) may have the maximum helical angle.
The 3D printing methods described herein (with or without rotational motion of the nozzle) are applicable to extrusion-based printing processes including direct-write printing and fused deposition modeling. In the former case, flow through the nozzle and deposition of the continuous filament may be facilitated by using a composite ink formulation with a strain-rate dependent viscosity (and which may be said to be shear-thinning or viscoelastic). In the latter case, extrusion and deposition may rely on the temperature-dependent flow behavior of a thermoplastic polymer.
In the case of direct-write printing, the flowable matrix material may comprise an uncured polymer resin. The composite ink formulation may further include a latent curing agent to prevent premature curing of the polymer resin (e.g., during deposition), as described above. Typically, curing is activated by heat exposure after the continuous filament has been deposited. Upon curing, a polymer composite comprising a thermoset polymer with oriented high aspect ratio particles dispersed therein may be formed. Suitable composite ink formulations may show a clear dependence of viscosity on shear rate, as described above. Any or all parts of the description of the composite ink formulation as set forth above may be applicable here.
Alternatively, the flowable matrix material may comprise a thermoplastic polymer at an elevated temperature (e.g., above Tm), and the polymer composite may be formed by cooling the continuous filament during deposition (e.g., in the case of fused deposition modeling). Suitable thermoplastic polymers may include one or more of acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), ULTEM™, polyether ether ketone (PEEK), polyether ketone ketone (PEKK), Nylon, and polycarbonate (PC). The polymer may be heated to a temperature of between about 100° C. and 400° C. prior to or during extrusion, and cooling may occur at room or elevated temperature as the continuous filament is deposited on the substrate. In this case, the polymer composite that is formed may comprise a thermoplastic polymer matrix with oriented high aspect ratio particles dispersed therein.
Generally speaking, whether the flowable matrix material comprises an uncured polymer resin or a thermoplastic polymer, a filamentary structure extruded from a nozzle as described herein may comprise a continuous filament including filler particles dispersed therein, where at least some fraction of the filler particles in the continuous filament comprise high aspect ratio particles having a predetermined orientation with respect to a longitudinal axis of the continuous filament.
When the nozzle is translating without rotation, the filamentary structure may include high aspect ratio particles that are at least partially aligned along the longitudinal axis of the continuous filament, as defined previously. The high aspect ratio particles may also be highly aligned along the longitudinal axis of the continuous filament.
When the nozzle is translating and rotating, the filamentary structure extruded from the nozzle may be described as a continuous filament including high aspect ratio particles dispersed therein, where at least some fraction of the high aspect ratio particles have a helical orientation comprising a circumferential component and a longitudinal component with respect to a longitudinal axis of the continuous filament. The circumferential component is imparted by rotation of a deposition nozzle and the longitudinal component is imparted by translation of the deposition nozzle.
The continuous filament may have a generally cylindrical shape due to extrusion through the deposition nozzle, although deviations from a perfectly cylindrical shape are possible due to settling of the continuous filament after deposition and/or use of a nozzle having a non-circular cross-section.
The continuous filament may have any or all of the features described elsewhere in this disclosure. For example, 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% and up to 100% of the high aspect ratio particles in the continuous filament may be helically oriented (in the case of nozzle rotation) or may be oriented such that the long axis of the particle is within about 40 degrees of the longitudinal axis of the continuous filament (when there is little or no nozzle rotation). The continuous filament may comprise a composite ink formulation having any or all of the features described elsewhere in this disclosure. For example, the continuous filament may comprise a thermoplastic polymer or an uncured polymer resin with the high aspect ratio particles dispersed therein, as described above.
A 3D printed composite structure may comprise a polymer composite including a polymer matrix and oriented high aspect ratio particles dispersed therein, where the polymer composite is made by extruding a continuous filament from a nozzle while the nozzle rotates about a longitudinal axis thereof and translates with respect to a substrate. The continuous filament may comprise a composite ink formulation including high aspect ratio particles in a flowable matrix material. The continuous filament may be deposited in a predetermined pattern on the substrate, where at least some fraction of the high aspect ratio particles in the continuous filament have an orientation comprising a circumferential component due to rotation of the nozzle and a longitudinal component due to translation of the nozzle. The continuous filament may be further processed to form the polymer matrix with oriented high aspect ratio particles dispersed therein. The processing may comprise curing or cooling. Any of the composite ink formulations set forth anywhere in this disclosure may be employed to form the 3D printed composite structure.
The continuous filament may be deposited layer by layer to form a stack of layers of the continuous filament. The stack of layers may form a dense solid or a porous structure comprising one or more pores or cells. For example, the stack of layers may define a cellular structure comprising a network of cell walls separating empty cells, as shown for example in
Because the printed composite structure is 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. The height of the cell wall may correspond approximately to the average diameter of the continuous filament multiplied by the number of layers in the stack.
When a continuous filament is stacked up layer by layer, the high aspect ratio particles on an upper surface of a bottom layer may be oriented at +φ with respect to the print direction, while high aspect ratio particles on a lower surface of the adjacent upper layer may be oriented at −φ with respect to the print direction. This leads to a situation akin to traditional laminate composites with +/−φ layups. At the same time, high aspect ratio particles on the left and right “sides” of the continuous filament may be oriented at an angle φ from the horizontal, thus achieving out-of-plane fiber orientation. By directing particle orientation in this fashion and integrating variable nozzle rotation with translation, printed composites may be able to achieve previously unattainable properties, including higher strength and stiffness in the z-direction (or the “height direction” of a stack of filaments), tailored shear moduli in printed cellular structures, spatial gradients in fiber orientation, and, potentially, fully isotropic properties with fiber reinforcement.
As explained above, a high rate of rotation and a low translation speed may result in the alignment of the high aspect ratio particles being dictated predominantly by the rotational shear, with the particles orienting nearly perpendicular to the print direction (e.g., close to the height direction) at any point along the circumference of the continuous filament. At sufficiently high rates of rotation and translation, the high aspect ratio particles may protrude from the continuous filament, as shown in FIG. 17 and discussed in more detail below. Alternatively, with a low rotation rate and high translation speed, high aspect ratio particle orientation may be predominantly dictated by the shear field due to translation, and the high aspect ratio particles may align closer to the print direction.
Thus, depending on the rotational component of the nozzle motion relative to the translational component, at least about 20% of the high aspect ratio particles in the 3D printed composite structure may have a long axis oriented within about 80 degrees of a height direction of the stack of layers (or the cell walls, if the 3D printed composite structure is a cellular or honeycomb structure as described above). Preferably, at least about 30%, at least about 40%, at least about 50%, or at least about 60% of the high aspect ratio particles may have a long axis oriented within about 80 degrees of the height direction of the stack of layers or the cell walls. The height direction may be understood to be parallel to the z-direction as defined above.
Accordingly, a 3D printed cellular structure may comprise a network of cell walls separating empty cells, where the cell walls comprise a polymer composite including high aspect ratio particles dispersed in a polymer matrix, and where at least about 20% of the high aspect ratio particles have a long axis oriented within about 80 degrees of a height direction of the cell walls.
Because the relative rates of rotation and translation of the nozzle are controllable, the particles may be more highly oriented in the height direction. For example, at least about 20%, at least about 30%, at least about 40%, at least about 50%, at least about 60%, or at least about 80% of the oriented high aspect ratio particles may have a long axis oriented within about 60 degrees of the height direction of the stack of layers (or the cell walls of a cellular structure). It is also contemplated that a considerable volume fraction of the high aspect ratio particles may have a long axis oriented within about 40 degrees of the height direction of the stack of layers or the cell walls. For example, at least about 20%, at least about 30%, at least about 40%, at least about 50%, or at least about 60% of the oriented high aspect ratio particles may have a long axis oriented within about 40 degrees of the height direction of the stack of layers or the cell walls.
Again, depending on the rotational component of the nozzle motion relative to the translational motion, the high aspect ratio particles in the stack of layers or cell walls may be even more highly oriented in the height direction (e.g., within about 20 degrees of the height direction). For example, at least about 20%, at least about 30%, at least about 40%, at least about 50%, at least about 60%, or at least about 80% of the oriented high aspect ratio particles may have a long axis oriented within about 20 degrees of the height direction of the stack of layers or the cell walls.
The above-described alignment of the high aspect ratio particles may occur over an entirety of the stack of layers or cell walls, or over only a portion thereof (e.g., over a given layer or cross-section).
Achieving a controlled out-of-plane orientation of the high aspect ratio particles during deposition of the continuous filament, as described herein, may allow composites with improved mechanical properties to be fabricated.
To test the 3D printing apparatus shown in
To demonstrate out-of-plane orientation (e.g., in the height direction or z-direction), a hexagonal honeycomb structure is printed 5 mm high (approximately 18 layers) using the rotating nozzle. The cellular structure is shown in
Ink Preparation: Exemplary composite ink formulations are prepared by mixing an epoxy resin (Epon 826 epoxy resin, Momentive Specialty Chemicals, Inc., Columbus, Ohio) with appropriate amounts of dimethyl methyl phosphonate (DMMP, Sigma Aldrich, St. Louis, Mo.), nano-clay platelets (Cloisite 30b, Southern Clay Products, Inc., Gonzales, Tex.), and milled carbon fibers (Dialead K223HM, Mitsubishi Plastics, Inc., Tokyo, Japan) using a Thinky Planetary Centrifugal Mixer (Thinky USA, Inc., Laguna Hills, Calif.) in a 125 mL glass container using a custom adaptor. An imidazole-based ionic liquid is employed as a latent curing agent (Basionics VS03, BASF Intermediates, Ludwigshafen, Germany). Batches start with 30 grams of Epon 826 resin. 3 grams of DMMP are added first, followed by 2 minutes of mixing in the Thinky. Next, the milled carbon fibers are added in 1 gram increments. Each material addition is followed by 3-5 minutes in the Thinky mixer. Finally, the ink formulation is allowed to cool to room temperature prior to the addition of the curing agent, Basionics VS03, at 5 parts per hundred by weight, relative to the epoxy resin. After the addition of the curing agent, the composite ink formulation is mixed for 3 minutes.
3D Printing: An exemplary composite ink formulation is loaded into 3 cc, luer-lock syringes (Nordson EFD, Westlake, Ohio) and centrifuged at 3900 rpm for 10 minutes to remove bubbles. Loaded syringes are then mounted in an HP3 high-pressure adaptor (Nordson EFD) in the rotating nozzle mount, and the assembly is mounted on an Aerotech 3-axis positioning stage (Aerotech, Inc., Pittsburgh, Pa.) for deposition. The nozzle is rotated using a JameCo electric motor, part number 164786 (JameCo Electronics, Belmont, Calif.). The composite ink formulation is was driven pneumatically and controlled via an Ultimus V pressure box (Nordson EFD), which interfaces with the Aerotech motion control software. Luer-lock syringe tips (Nordson EFD) are used to dictate filament diameter, and a continuous filament is deposited onto glass slides covered with Bytac®, PTFE-coated aluminum foil (Saint Gobain Performance Plastics, Worcester, Mass.) to prevent adhesion. The print path for a cellular structure having a honeycomb geometry is written as parameterized g-code scripts, and are designed to maximize continuity within each printed layer. Printed composite structures are pre-cured at 100° C. for 15 hours, cooled, removed from the substrate, and cured for 2 hours at 220° C.
To test the 3D printing apparatus shown in
Rotation rates may range from greater than 0 deg/s to 3000 deg/s with the current motor (or about 0 to 52.4 rad/s). Depending on the desired fiber alignment and the translation speed of the nozzle, the rotation rate may be at least about 10 deg/s, at least about 100 deg/s, at least about 200 deg/s, at least about 300 deg/s, at least about 500 deg/s, at least about 700 deg/s, or at least about 1000 deg/s. Typically, the rotation rate is no more than about 3000 deg/s, no more than about 2500 deg/s, or no more than about 2000 deg/s.
In these examples, a stepper motor connected directly to the axis control of the printer is employed to drive the rotation. Consequently, the rotation of the nozzle may be controlled as precisely as the translation of the nozzle. In addition, fiber alignment may be programmed according to location in the filament. For example,
As explained above, only the nozzle portion of the 3D printing apparatus shown in
At sufficiently high rotation rates and translation speeds (e.g., about 1500 deg/s and 10 mm/s, or higher), fibers may emerge from the filament, resulting in a “spiky” printed structure with protruding fibers, as shown for example in
Ink Preparation: Several ink variations are prepared for printing. Each of these begin with 60 g of an epoxy resin (Epon 826, Momentive Specialty Chemicals) and 6 g of dimethyl methyl phosphonate (DMMP, Sigma Aldrich). A translucent ink (“Ink 1”) is made by adding 18 g of nanoclay (Nanocor) to the base (above) in order to impart a shear-thinning response. 2 g of milled carbon fibers (Dialead K223HM, Mitsubishi) with approximate lengths of 220 μm and diameters of 10 μm are added. Another translucent ink (“Ink 2”) is made as described for Ink 1, but substituting 2 g of longer, chopped carbon fibers (Dialead K223HE, Mitsubishi) instead of the milled carbon fibers. An additional translucent ink (“Ink 3”) is made by including a larger quantity of the milled carbon fibers (14 g instead of 2 g). A separate ink (“Ink 4”) is made by adding 16 g of nanoclay to the base (above) in order to impart a shear-thinning response. 40 g of silicon carbide whiskers (SI-TUFF SC-050, ACM) are added to improve the mechanical response, followed by the addition of 6 g of milled carbon fibers (Dialead K223HM, Mitsubishi). After mixing the above ink compositions in a SpeedMixer (FlackTek, Inc.) for 5 minutes at 1800 rpm, 3 g of Basionics VS03 latent curing agent (BASF) is added, followed by 2 minutes of additional mixing.
3D Printing: Inks are loaded into 10 cc luer-lock syringes and centrifuged to remove bubbles. Subsequently, rotating luer-lock adapters (Cole-Parmer) are connected to the luer-locks of the syringes. Luer-lock deposition nozzles are selected based on the desired diameter of the printed filaments; typically tapered plastic nozzles (Nordson EFD) of either 610 μm or 840 μm in inner diameter are employed and connected to the rotating luer-lock adapter. A custom 3D positioning stage (Aerotech) is used for printing, ensuring precise placement and translation of the deposition nozzle. During printing, the ink flow is controlled either via pressure, using a commercial pressure control box (Nordson EFD), or via volume, using a syringe pump. In the former case, a flexible plastic tube connected the pressure box (which is stationary) to the back of the syringe (which is mounted on the 3D positioning stage). In the latter case in which volume control is used, the syringe is attached to the (stationary) syringe pump, with a flexible plastic tube inserted between the (stationary) syringe barrel and the rotating luer lock (which is mounted on the 3D positioning stage).
Print paths, including commands for both translation and rotation, are produced using mecode, a coding library developed at Harvard University (Lewis group) for the facile generation of G code commands from within a Python environment. Translation speeds of 3, 10, and 15 mm/s are used for this set of experiments. These translation speeds corresponded to ink volume rates of approximately 60, 200, and 300 μL/min, respectively. These volume rates are prescribed directly by the syringe pump when volume control is used. When pressure control is used, the corresponding pressures varies dramatically based on the specific ink used, and appropriate pressures are determined empirically. Rotation rates from 0 to 2000 deg/s are applied in order to produce filaments with a large range of ratios of rotation to translation speed.
More complicated structures have also been printed while rotation is applied, including porous log pile (or crisscross) structures and honeycomb cellular structures. For these structures, rotation has also been applied differently in different locations, to demonstrate spatial control of fiber alignment (e.g., for optimally reinforcing different parts of the structure).
One nozzle or a plurality of nozzles may be employed for 3D printing in a serial or parallel printing process. The nozzles may or may not have rotational capabilities. A nozzle suitable for printing may have an inner diameter of from about 1 micron to about 15 mm in size, and more typically from about 50 microns to about 500 microns. The size of the nozzle may be selected depending on the desired filament diameter. Depending on the injection pressure and the nozzle translation speed, the deposited filament may have a diameter ranging from about 1 micron to about 20 mm, and more typically from about 100 microns (0.1 mm) to about 5 mm. Rotation of the nozzle about its longitudinal axis may be achieved using an electric motor.
The printing process may involve more than one composite ink formulation. The composite ink formulation(s) fed to the one or more nozzles may be housed in separate syringe barrels that may be individually connected to a nozzle for printing by way of a Luer-Lok™ or other connector. The extrusion of the continuous filament may take place under an applied pressure of from about 1 psi to about 200 psi, from about 10 psi to about 80 psi, or from about 20 psi to about 60 psi. The pressure during extrusion may be constant or it may be varied. By using alternative pressure sources, pressures of higher than 100 psi or 200 psi and/or less than 1 psi may be applied during printing. A variable pressure may yield a filament having a diameter that varies along the length of the filament. The extrusion is typically carried out at ambient or room temperature conditions (e.g., from about 18° C. to about 25° C.) for viscoelastic ink formulations.
During the extrusion and deposition of the continuous filament, the nozzle may be moved along a predetermined path (e.g., from (x1, y1, z1) to (x2, y2, z2)) with respect to the substrate with a positional accuracy of within ±100 microns, within ±50 microns, within ±10 microns, or within ±1 micron. Accordingly, the filaments may be deposited with a positional accuracy of within ±200 microns, within ±100 microns, within ±50 microns, within ±10 microns, or within ±1 micron. The nozzle may be translated and the continuous filament may be deposited at translation speeds as high as about 3 m/s (e.g., from about 1 cm/s to about 3 m/s), and more typically in the range of from about 1 mm/s to about 500 mm/s, from about 1 mm/s to about 100 mm/s, or from about 1 mm/s to about 10 mm/s.
Referring to
The hollow stationary portion 120 may include at least one ink source (e.g., a syringe barrel) 165 which may be in fluid communication with the hollow rotatable portion 125. The at least one ink source 165 may comprise one or more pressure-controlled ink dispensing devices and/or one or more volume-controlled ink dispensing devices.
The hollow rotatable portion 125 may include a nozzle portion 135 for extrusion of a continuous filament therethrough that is fixedly attached to a rotatable connector 140, which in turn is rotatably attached to the hollow stationary portion 120. Accordingly, the nozzle portion 135 and the rotatable connector 140 may rotate as a unit while the hollow stationary portion 120 remains in place. The apparatus 100 may also include a substrate 145 positioned adjacent to the nozzle portion 135 for deposition of the continuous filament thereon. Typically, the substrate 145 is uncoupled from the 3D positioning stage 105, and the substrate 145 remains in place while the nozzle assembly 110 is moved.
As shown in
Rotating Nozzle: The apparatus shown in
Although the present invention has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible without departing from the present invention. The spirit and scope of the appended claims should not be limited, therefore, to the description of the preferred embodiments contained herein. All embodiments that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein. Furthermore, the advantages described above are not necessarily the only advantages of the invention, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the invention.
The present patent document claims the benefit of priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application Ser. No. 61/937,818, filed Feb. 10, 2014, and to U.S. Provisional Patent Application Ser. No. 62/080,576, filed Nov. 17, 2014, both of which are hereby incorporated by reference in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US15/15148 | 2/10/2015 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61937818 | Feb 2014 | US | |
| 62080576 | Nov 2014 | US |
