Thermoplastic composite material and press die thereof

Abstract

Methods and material compositions are disclosed for applying additive manufacturing of polymers to manufacture press dies, such as sheet metal forming. As disclosed in various embodiments, the composite material includes a thermoplastic, a first filler having particles with a low aspect ratio, and a second filler having a high aspect ratio. In at least one embodiment, the composite material according to the disclosed teachings has a compressive modulus of more than 3500 MPa and a compressive strength of more than 70 MPa, and the composition has sufficient mechanical properties for press tools and is suitable for extrusion type additive manufacturing processes. In at least one embodiment, the use of the disclosed composite material with additive manufacturing allows the overall weight of the tool to be reduced by including voids in the die.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 63 / 302,814, filed January 25, 2022, which is incorporated herein by reference. [Technical field]

[0002] The present disclosure relates to the manufacture of dies for use in presses for reshaping malleable sheet stock, and more particularly to the use of additive manufacturing processes in combination with adapted thermoplastic compositions to efficiently create press dies characterized by low density and high compression properties. [Background technology]

[0003] High pressure presses with forming dies have long been used to form sheet metal or other malleable sheet stock material into final parts, such as body parts for automobiles and aircraft. Fixed shape dies can be used for mass production of identical sheet metal parts, which once formed, have complex shapes and curvatures, such as shapes with double curvatures. Summary of the Invention [Problem to be solved by the invention]

[0004] Forming a large-sized die or a pair of complementary dies from a sufficiently robust solid metal can result in a very heavy die that is extremely costly and difficult to modify. The time and cost associated with creating a large-format die, for example to form an automobile body door or quarter panel, makes designing small batch dies or iterative improvements cumbersome and infeasible. While conventional techniques such as CNC machining and electrical discharge machining (EDM) are typically applied to cut dies from solid blocks, metal additive manufacturing techniques can also be used somewhat advantageously to form press die dies, but often still require surface machining, resulting in dies with significant mass and significant cost. Thus, there remains a need for improvements in the field of manufacturing large press dies for pressing and stamping malleable sheet stock. [Means for solving the problem]

[0005] The present teachings disclose processes and material compositions that allow for the manufacture of large press dies by extrusion of thermoplastics with enhanced mechanical properties. According to some embodiments, it is found that thermoplastics using a mixture of fillers with different aspect ratios exhibit strength, modulus and other properties suitable for making press dies. According to some embodiments, the use of extrudable filled thermoplastics expands the achievable linear dimensional size, area or volume of a press die beyond the limits of unfilled plastics. In some embodiments, filled thermoplastics may be used as feedstock materials for extrusion additive manufacturing processes, allowing for passages, internal frameworks, truss structures, internal voids or inclusions that lighten the die without compromising strength, with proper design and slicing.

[0006] Various illustrative embodiments are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which like reference numerals refer to similar elements. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram of an additive manufacturing system in which a polymer composite material may be usefully applied as a feedstock, in accordance with an embodiment of the present teachings. [Diagram 2] FIG. 1 is a schematic diagram of an automated manufacturing system with multiple tool heads for use in conjunction with an exemplary manufacturing process in accordance with a preferred embodiment of the present teachings. [Diagram 3] FIG. 1 is a diagram of a direct pellet extruder in which a polymer composite material may be usefully applied as a feedstock, according to an embodiment of the present teachings. [Figure 4] FIG. 1 is a diagram of a filament-fed extruder in which a polymer composite material may be usefully applied as a feedstock, according to an embodiment of the present teachings. [Figure 5A] FIG. 1 is a diagram of an overall process for preparing and using a polymer composite material to manufacture a part formed in a sheet metal pressing process, in accordance with a preferred embodiment of the present teachings. [Figure 5B] FIG. 1 is a diagram of an overall process for preparing and using a polymer composite material to manufacture a part formed in a sheet metal pressing process, in accordance with a preferred embodiment of the present teachings. [Figure 5C] FIG. 1 is a diagram of an overall process for preparing and using a polymer composite material to manufacture a part formed in a sheet metal pressing process, in accordance with a preferred embodiment of the present teachings. [Figure 6A] Schematic diagram showing possible distributions of multiple types of filler particles within a polymer matrix. [Figure 6B] Schematic diagram showing possible distributions of multiple types of filler particles within a polymer matrix. [Figure 6C] Schematic diagram showing possible distributions of multiple types of filler particles within a polymer matrix. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The following detailed description describes the preparation of various polymer composites according to the present teachings, and then describes how the composites are applied as a feedstock in an additive manufacturing process to produce durable press die molds for forming materials such as aluminum, stainless steel, or mild steel sheets. As shown below, certain novel compositions of mixtures of polymer matrices and fillers with predetermined attributes significantly facilitate the use of additive and subtractive processes in forming cost-effective, rapidly performed forming tools that exhibit favorable physical properties such as strength, stiffness, and durability.

[0009] In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the exemplary and preferred embodiments of the present invention. However, it will be apparent that some embodiments may be practiced without these specific details or with alternative equivalent configurations. In some cases, more general structures and devices are left out of the figures or shown in block diagram form in order to avoid unnecessarily obscuring the more essential components for explaining the embodiments of the present invention and its operating principles.

[0010] At the outset, it is important to note that, unless the context indicates otherwise, the following terms shall have the following meanings as used herein and are applicable to both the singular and plural forms:

[0011] The terms "a," "an," "the," "at least one," and "one or more" are used interchangeably. Thus, for example, a high performance composite composition containing "a" amorphous polymer and reinforcing fibers means that the composite composition can include "one or more" amorphous polymers and reinforcing fibers.

[0012] The terms "additive manufacturing," "three-dimensional printing," or "3D printing" refer to any process used to create three-dimensional objects in which successive layers of material are formed under computer control (e.g., electron beam melting (EBM), fused deposition modeling (FDM), direct pellet extrusion, inkjet, additive manufacturing (LOM), selective laser sintering (SLS), and stereolithography (SL)).

[0013] The term "feedstock" refers to a form of material that can be utilized in an additive manufacturing process (e.g., as a build material or soluble support). Non-limiting examples of feedstocks include, but are not limited to, pellets, powders, filaments, billets, liquids, sheets, molded profiles, and the like.

[0014] The term "melt processing techniques" refers to techniques for applying thermal and mechanical energy to reshape, blend, mix, or otherwise modify a polymer or composition, such as compounding, extrusion, injection molding, blow molding, rotoforming, or batch mixing. For clarity, 3D printing processes useful for printing thermoplastic and elastomeric melt processable materials are examples of melt processing techniques.

[0015] The terms "polymer" and "polymeric" refer to molecules of high relative molecular weight whose structure essentially comprises repeat units actually or conceptually derived from molecules of lower relative molecular weight covalently bonded together.

[0016] The term "amorphous polymer matrix" generally refers to a polymer with a crystallinity of less than 50% as measured by differential scanning calorimetry (DSC). In the context of the present teachings, it is preferred to use an amorphous polymer matrix that exhibits a flexural modulus (determined by ASTM D790) of more than 2,000 MPa.

[0017] The term "reinforcing fibers" refers to a plurality of fibers having an average diameter of less than 20 microns and an average aspect ratio (fiber length / fiber diameter) of greater than 10.

[0018] The term "polymer composite" refers to a melt-processable composition of an amorphous polymer matrix and reinforcing fibers.

[0019] The terms "preferred" and "preferably" refer to embodiments that may provide certain advantages, under particular circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the claims.

[0020] The recitation of numerical ranges using endpoints includes all numbers subsumed within that range (eg, 1 to 5 includes 1, 1.5, 3, 3.95, 4.2, 5, etc.) and any subset ranges subsumed within that range.

[0021] As used herein, the term "molten" applies loosely to any state of a material when it is heated or otherwise softened and has a viscosity low enough to flow through a small nozzle under pressure. It is recognized that for some materials, more precise terms such as "plastic" or "amorphous" states may be more commonly used, and the term "molten" is intended to encompass these situations. When a material is referred to as "molten" herein, it is generally at a temperature substantially above its glass transition temperature, if applicable.

[0022] Polymer composites have utility in many commercial applications, including what is known as "additive manufacturing" or "three-dimensional printing" (3D printing).

[0023] Polymeric materials may generally be considered "3D printable" if they exhibit several important attributes. 3D printable feedstocks are preferably thermoplastics that can be softened or melted at high temperatures, extruded through a relatively small nozzle, and then cooled to solidify without significant chemical or structural degradation. Because 3D printing involves building a solid object by successive additions of extruded polymer, preferred feedstock materials also exhibit substantial bonding between newly applied and previously applied materials. Another useful attribute in some contexts is that the feedstock material is supplied and handled in the form of flexible or semi-rigid filaments wound on reels, as used by many types of 3D printers, although certain other types of printers allow bulk pellets as an alternative to filaments. Some of the many other factors that affect suitability as a 3D printing feedstock relate to heat dissipation, avoidance of nozzle clogging and voids, and achieving isotropic material strength properties in the finished object.

[0024] "3D printable" polymer composites, typically containing a thermoplastic binder and a percentage of fibrous fillers to add tensile and shear strength, have been developed to meet the needs of applications in many industries. However, one unmet industrial need is a 3D printable polymer composite that can meet the performance requirements of mold applications, particularly sheet metal mold applications. Such molds or forms have a specific shape, and the raw sheet metal piece is reshaped by being conformed to the shape of the mold. Under substantial forces that cause deformation, the sheet metal is bent or stretched to conform to the mold, and then retains the shape imparted by the mold. Sheet metal forms are typically manufactured from steel or aluminum. Steel and aluminum have extremely high stiffness and compressive properties, such as are required to forcefully and permanently bend, deform, or stretch a sheet metal blank into a finished shape. Unfortunately, despite the availability of several technologies for 3D printing metal parts in general, doing so to create metal molds for sheet metal forming is difficult and often prohibitively expensive, especially for large workpiece sizes. Thus, there is a recognized need for cost-effective, easily 3D printable polymer composite materials for creating this type of mold.

[0025] Polymer composites have found significant industrial utility in many markets and applications. However, the number of polymer composites that can be effectively 3D printed is limited as a result of the requirements specific to the 3D printing method (i.e., rheology, adhesion properties, particle size, shrinkage / warpage, etc.). According to the teachings described herein, polymer composites are formulated to be 3D printable and have superior properties that make them suitable for specific mold applications. In one embodiment, the high performance composites introduced herein include an amorphous polymer matrix, reinforcing fibers, and preferably lightweight fillers. In one embodiment, composites formulated according to the present teachings exhibit a compressive modulus of greater than 3500 MPa and a compressive strength of greater than 70 MPa. The high performance composites of the present invention have utility in many applications, including but not limited to additive manufacturing feedstocks (e.g., fused deposition modeling and direct pellet extrusion). In a preferred embodiment, the amorphous polymer matrix is ​​polycarbonate (PC), polylactic acid (PLA) or polycycloolefin copolymer (COC). In a preferred embodiment, the reinforcing fibers are carbon or glass fibers. In various preferred embodiments, the lightweight filler comprises hollow glass microspheres.

[0026] The thermoplastic composites described herein have applications in 3D printable molds, specifically sheet metal molds. Such applications require extremely high resistance to permanent deformation under high speed and load. If the mold deforms under load, it needs to be sufficiently elastic to return to its original shape when the load is removed. The advantage of 3D printed molds is that they simplify mold design and prototyping, allowing for rapid iteration of experimental runs. They are also well suited for small batch production, as the cost of creating molds in this manner is much lower than for traditional metal forms.

[0027] The present disclosure describes high performance polymer composites comprising a substantially amorphous polymer matrix, reinforcing fibers, and optionally hollow glass microspheres.

[0028] Various amorphous polymers can be "melt processed" (essentially by heating and thorough mixing) with reinforcing fibers to create various composite materials according to the present teachings. Amorphous polymers for use with the present teachings preferably exhibit a flexural modulus of greater than 2000 MPa and a crystallinity of less than 50%. Non-limiting examples of amorphous polymers that can be used to make such compositions of interest include acrylonitrile-butadiene-styrene copolymers (ABS), polyacrylates, polymethacrylates, polyesters, polyvinyl chloride (PVC), polycarbonates, polystyrenes, cycloolefin copolymers (COC), or combinations thereof. Preferred amorphous polymers include polylactic acid, polycarbonates, and polycycloolefin copolymers. Additives as disclosed herein to impart other useful attributes can also be optionally included in the polymer composition.

[0029] The reinforcing fibers are melt processed into the amorphous polymer matrix. The present teachings may be practiced using organic or inorganic reinforcing fibers. According to a preferred embodiment, the reinforcing fibers have an aspect ratio (length / diameter) of at least 10:1 on average. Non-limiting examples of fibers that may be suitable include carbon fibers, basalt fibers, glass fibers, wollastonite, cellulose fibers, carbon nanotubes, and graphene. Preferred reinforcing fibers are glass fibers and carbon fibers. In one embodiment, the reinforcing fibers comprise 1-50% of the polymer composite composition. In a preferred embodiment, the reinforcing fibers comprise 5-30% of the composite composition. In a most preferred embodiment, the reinforcing fibers comprise 10-25% of the composite composition.

[0030] According to some compositions made according to the present teachings, the polymer composite includes a lightweight filler. Lightweight fillers useful in applying the present teachings are those that can withstand the melt processing described above. Non-limiting examples of lightweight fillers useful in implementing the variations within the present teachings include hollow glass microspheres, polymeric microspheres, cenospheres, perlite, and pumice. A preferred lightweight filler is a hollow glass microsphere having a crush strength of greater than 10,000 psi, such as IM30K hollow glass microspheres available from 3M Company, St. Paul, Minnesota. In one embodiment, the lightweight filler comprises 1-30% by weight of the polymer composite. In a preferred embodiment, the lightweight filler comprises 5-20% by weight of the polymer composite. In a most preferred embodiment, the lightweight filler comprises 5-15% by weight of the polymer composite.

[0031] In another embodiment, additional fillers can be added to impart specific performance attributes to the composite, such as flexural modulus and strength. Non-limiting examples of fillers include mineral and organic fillers, including carbonates, silicates, talc, mica, wollastonite, clay, silica, alumina, carbon black, carbon nanotubes, graphite, graphene, solid glass microspheres, ceramics, and traditional cellulosic materials, including wood flour, wood fiber, sawdust, wood chips, newsprint, paper, flax, hemp, straw, rice husk, kenaf, jute, sisal, peanut shells, soybean husks, or any cellulose-containing material.

[0032] The present teachings describe polymer composites that include novel combinations of filler types with substantially diverse aspect ratios and maximum dimensions. According to preferred embodiments of the present teachings, the polymer composites combine both rigid linear fibers and rigid spherical fillers in a polymer to achieve advantageous properties for certain applications, such as the creation of 3D printed molds for sheet metal forming presses. The rigid linear fibers may have a length to width ratio of 10:1 or more, while the spherical fillers are essentially characterized by nearly uniform dimensions in all directions. The rigid linear fibers may have a length of, for example, 6 mm and a diameter of less than 8 microns, while the spheres may have an average diameter of, for example, 18 microns.

[0033] Such mixtures of fillers with diverse morphologies result from the interaction of the two fillers, resulting in non-obvious advantages in various attributes beyond the simple superposition of their respective effects. It is believed that other combinations of fillers with substantially diverse morphologies may exhibit similar interactions and improve the physical and mechanical properties of amorphous polymers, especially following the aforementioned feedstock properties that make the polymer suitable for forming high compressive strength molds by 3D printing. According to a preferred embodiment, the aspect ratios of the fillers preferably differ by at least a factor of 2, but are believed to be preferred by factors of 5, 10 or more. For example, in conjunction with a spherical filler with an aspect ratio of 1:1, the second filler may include elongated rigid filaments with an aspect ratio of 5:1. In this case, the aspect ratios differ by a factor of 5.

[0034] Table 1 provides a complement of specific polymers, reinforcing fibers, and lightweight fillers that have been demonstrated to achieve benefits in accordance with the present teachings and principles. It should be emphasized that these listed types should not be considered as limiting the scope of the claimed invention. [Table 1]

[0035] Various formulations of polymer composite materials suitable for use as described herein can be prepared using procedures and equipment utilized in the reduction to practice, as outlined below. However, neither the present teachings nor the claimed subject matter should be construed as being limited in any way by the supplementation of the equipment, parameters and procedural steps described below.

[0036] The following procedures were carried out to prepare the material samples and obtain the data shown in Table 2 below: Each of Samples 1-6 was prepared according to the weight ratios in Table 2. Samples 1-11 were separately gravimetrically extruded into a 27 mm twin screw extruder (52:1 L:D, Model QC from Entek Extruders, Lebanon, Oregon). 3 The melt was fed into a 3-strand die (commercially available as 3-14-15-16). Compounding was completed using the following conditions: throat 100C, barrel 2-14, die 220C for samples 1-6. Screw speed was 300 rpm and throughput was 10 Kg / hr. For compounds 7-11, throat 100C, barrel 2-14, die 180C. Screw speed was 300 rpm and throughput was 10 Kg / hr. For examples 1-11, reinforcing fibers and lightweight fillers were fed separately downstream in a side stuffer in zone 5. The melt was extruded through a 3-strand die (3 mm diameter), conveyed onto a continuous cooling belt and pelletized into 3 mm x 2 mm pellets. The pellets were then injection molded using ASTM tooling and an Arburg 40 ton press. Barrel temperatures were 220C for examples 1-6 and 200C for examples 7-11. The resulting specimens were characterized for compression properties according to ASTM 1621 and specific gravity according to the Archimedes method. [Table 2]

[0037] In Table 2, the column labeled "Blends" is simply an index to reference samples prepared with different amorphous polymers and blend components listed in Table 1. The next four columns list the weight percent of the polymer matrix and fillers. The final three columns refer to the empirically measured physical properties of each blend, namely, compressive strength (CS), compressive modulus (CM) and specific gravity (SG).

[0038] One pattern evident in Table 2 is observed by comparing Mix 5 to Mix 2, which represents a consistent proportion of reinforcing fiber at 15% by weight, while replacing a portion of the polymer with 10% by weight hollow glass microspheres. In response attributes, compressive strength and specific gravity remain essentially unchanged, but the compressive modulus increases significantly by approximately 16%.

[0039] Another pattern evident in Table 2 comes from comparing Mix 11 with Mix 8, where compression quality similarly increases with the addition of certain hollow glass microspheres as lightweight fillers.

[0040] Furthermore, when examining the trends along mixes 4-5-6 and 9-10-11 in Table 2, it appears that the percentage of spherical filler particles increases continuously and that both compressive strength and compressive modulus generally increase as a function of the addition of low aspect ratio filler material, even as the overall density of the material decreases. In other words, by replacing some of the volume with spherical particles of lightweight filler, it is possible to reduce the consumption of amorphous polymer and achieve acceptable strength and stiffness, but without compromising extrudability properties as would be the case if only fibrous fillers were used in place of the polymer.

[0041] It should be reaffirmed that compressive strength and compressive modulus are not the only important response variables when applying materials as feedstocks for additive manufacturing as taught herein. A trade-off in strength and stiffness may be acceptable for ease of additive manufacturing and improved physical attributes of the objects so formed. Some of these attributes may be difficult to quantify. Thus, some exemplary mixtures shown in Table 2, as well as other proportions and other ingredients than those listed, may be preferred in a given manufacturing situation for reasons that are not apparent from the compressive strength and modulus data alone. Mix 3 shows that it may be possible to achieve even higher strength and stiffness with the addition of reinforcing fibers alone, but other attributes related to ease of 3D printing, loss of elasticity, lubricity, and machinability may be affected by employing this mixture. Considering that additive manufacturing allows for tool designs that include internal voids, trabecular structures, internal supports "where-needed," etc., dramatically improving build times and reducing tool costs and material consumption, the trade-off in strength and stiffness to yield a better feedstock for additive manufacturing may be well justified.

[0042] Several important attributes of the finished object formed from the composite material are influenced by the ratio between the polymer matrix, the substantially elongated fillers and the substantially round fillers. These include, for example, compressive modulus at the time of molding, tensile and compressive strength, resilience after each load cycle, coefficient of thermal expansion (TCE), surface lubricity, ease of machinability, surface durability and isotropic strength properties.

[0043] Still other important attributes relate to facilitating the 3D printing process, some of which differ from or exceed material requirements in other applications such as traditional machining or injection molding. If a filament-fed extruder is used, some important properties include the ability to extrude the material into durable filaments for use as a feedstock. If a DPE is used, certain qualities may facilitate the ability to form into pellets and feed reliably to the DPE head. For both extruder types, the extruded material exhibits good cohesion between printed layers, is stable at extruder temperatures, is resistant to nozzle clogging, and can maintain homogeneity of filler concentration while undergoing melting, extrusion, deposition, and cooling.

[0044] It is believed that excessive proportions of any of the above filler types can be detrimental. High concentrations of elongated (high aspect ratio) or fibrous filler particles can impair extrudability and interlayer adhesion during additive manufacturing, as well as machinability and lubricity of the finished part. On the other hand, high concentrations of spherical (low aspect ratio) particles can lead to localized brittleness and "chip-outs" on the part surface during machining or during use cycles, especially if homogeneity is not achieved. Also, the use of spherical additives alone does not reduce "cold flow" or improve the tensile or flexural strength of the composite to the extent that fibers do. The design of the finished part may require some members of the part to withstand tensile and flexural forces by elasticity.

[0045] Apart from these concerns regarding the use of only one or the other filler type, a rationale is contemplated for using both types in a complementary manner to achieve better overall attributes of the polymer composite.

[0046] 6A-6C provide visualizations of fillers dispersed within a matrix, such as within a polymer, to illustrate how combinations of fillers having various aspect ratios may complement each other and alleviate some of the problems exhibited when using only one or the other. In all of these sketches (not to scale), the fillers shown are presumed to be immersed in the polymer matrix and to substantially occupy all of the interstitial volume not occupied by the filler, while the polymer itself is not shown or is considered invisible. While advantages are empirically realized by the present teachings, the depictions of FIGS. 6A-6C are not intended to bind the claimed invention to any particular confirmed theory of operation, but rather to set forth a conceptual explanation of how various aspect ratios may interact to provide observed features or advantages.

[0047] FIG. 6A shows the presence of linear filler particles with high aspect ratios. FIG. 6A shows a situation where only linear filler materials, such as reinforcing fibers, are present to improve strength properties. These may be relatively rigid and straight as shown, or may be somewhat flexible or arcuate over much longer lengths. FIG. 6A reflects that while particle orientation and proximity are generally scattered, there may also be local clustering between similarly aligned particles. This effect can be initially minimized by thorough mixing and shear agitation during compounding, but some alignment and aggregation may reoccur when the melt containing the fibers is extruded through a tube or orifice, for example, when the melt is extruded into a filamentary feedstock or extruded from a nozzle in a DPE system. During the process of melting and reflowing the composite, the elongated fibers may become aligned and close together as shown, and the resulting clusters 610, once formed, may be energetically favorable or difficult to break under random, mild shear conditions. On a macroscopic scale within a bulk material, some of the consequences of this coalescence can be regions of brittleness or directional variation in strength and stiffness, higher filler content to achieve a given overall strength, and increased likelihood of clogging when the composite is forced through narrow passages such as the extruder nozzle of a 3D printer. In the latter case, incompressible tightly packed clusters are potentially more problematic than entanglements of randomly oriented fibers that can nevertheless change conformation to accommodate flow.

[0048] FIG. 6B shows an alternative situation where only more or less spherical fillers such as hollow glass beads are added. These tend to improve the compressive strength and various other qualities. This type of filler can also reduce the density of the composite, especially if hollow spheres are used. The beads are preferably distributed in a homogeneous manner, but in practice they may drift into arrangements while the matrix is ​​substantially "melted" and coalesce into clusters 612 that resemble hexagonal close-packed arrays. It is possible for the spheres to fit into a close-packed array, especially when using very uniform spheres in an environment that provides some mobility within the amorphous polymer matrix. FIG. 6B also shows that the spherical fillers may not be uniformly distributed during the initial melt processing into the polymer matrix, or may coalesce into localized clusters during the subsequent melting and extrusion processing.

[0049] Similar to the elongated particles in Figure 6A, these localized clusters are unlikely to disperse once formed. Clusters of particles that are spherical (or otherwise have near unity aspect ratios) create similar problems with inhomogeneous material quality. This phenomenon reduces the efficiency of the filler and also locally dilutes the polymer matrix, forming small domains within the composite that have completely different behavior than the bulk material.

[0050] FIG. 6C shows the presence of two fillers with significantly different shapes and aspect ratios, in this example, both elongated fibers and relatively small spheres. FIG. 6C depicts the hypothesized phenomenon that the presence of interstitial spheres prevents the tendency of elongated fibers to align and coalesce. (See, for example, the vicinity indicated by reference number 614.) To make the two fibers parallel and close together along most of their overlapping length requires displacing any interstitial spheres and fibers that happen to be caught between them. Similarly, the presence of elongated fibers tends to prevent the formation of organized clusters of spheres. At the very least, it can be observed that a stable, regular arrangement (if it exists) that includes both elongated fibers and spheres would form spontaneously and would be unlikely to cause problems of its own. Both fillers impart desirable changes to strength, modulus or other properties, and further dilute each other's concentrations, so that the incidence of homogeneous particle clustering is much reduced while achieving comparable or even improved strength and modulus properties, which may further provide additional benefits, for example, in terms of flowability and machinability properties during additive / subtractive manufacturing.

[0051] In addition to the main components mentioned above, various polymer composite compositions according to the present teachings can also use various additives that can impart specific attributes and functionality to the resulting polymer composite. Non-limiting examples of suitable additives include antioxidants, light stabilizers, fibers, blowing agents, foaming additives, antiblocking agents, heat reflective materials, heat stabilizers, impact modifiers, biocides, antimicrobial additives, compatibilizers, plasticizers, tackifiers, processing aids, lubricants, coupling agents, thermal conductors, electrical conductors, catalysts, flame retardants, oxygen scavengers, fluorescent tags, inert fillers, minerals, and colorants. Additives can be incorporated into the polymer composite polymer composition as powders, liquids, pellets, granules, or in any other extrudable form.

[0052] Having described various polymer composite materials as examples embodying the present teachings and principles, it is now appropriate to describe examples of manufacturing situations and processes that influence the compounding of the composite material and lead to the construction of strong, durable components. With a clear understanding of the equipment and processing steps, one can better appreciate how the attributes of the feedstock materials are engineered to aid in the overall manufacture of the mold in this manner.

[0053] FIG. 1 illustrates an exemplary additive manufacturing system 100 that may advantageously utilize polymer composite materials formulated according to the present teachings as input feedstocks. The system 100 is shown to include a motor-driven multi-axis motion control system 120 that controllably moves one or more tool heads, such as an extruder head 150, relative to a build plate 130. The build plate 130 may refer to either the direct surface from which additive building begins or a plate overlaid with an intermediate sheet of material or chemical coating. Thus, the build plate 130 may also be referred to herein as a "build surface," particularly with reference to the top surface of the build plate and any intermediate material placed thereon. The motion control components, in combination with the extruder head 150, constitute an additive manufacturing system, more specifically in the form of a 3D printer. The illustrated multi-axis motion control system 120 generates motion along three orthogonal axes in an arrangement known as a Cartesian coordinate system, with any point in the build space being referenced by a unique triplet of scalar values ​​corresponding to displacements along the three mutually orthogonal axes. Other coordinate systems can be used to specify positions within the "build space" of the system.

[0054] The extruder head 150 is shown mounted on a carriage 151 that is controllably moved along the longitudinal axis of a cross beam 125 by rotation of the shaft of the X-axis motor 124. Typically, the beam 125 includes one or more linear bearings that facilitate smooth movement of the carriage 151 parallel to the longitudinal axis of the beam 125. Additionally, the beam 125 may house a lead screw (not clearly visible in the figure) that is coupled to the carriage 151 by a precision nut and secured within the beam 125 by rotation and thrust bearings and coupled to the shaft of the X-axis motor 124. Rotation of the shaft of the X-axis motor 124 may rotate the lead screw, which causes the carriage 151 to move toward or away from the motor 124 in a controlled manner. The X-axis motor 124 is often a stepper motor, but may also be an AC or DC servo motor with a shaft position encoder and / or tachometer operating in a closed loop control mode to facilitate movement to very precise positions. Many such arrangements of motors, lead screws, bearings, and associated components are possible.

[0055] The arrangement of motor 124 and beam 125 achieves controlled movement of extruder head 150 in what may be referred to as the horizontal X-axis in the print space coordinate system, while motors 122A, 122B and their respective columns 123B, 123A can use a similar arrangement of linear guides, bearings, and lead screws to allow Z-axis motors 122A, 122B to controllably move extruder head 150 vertically, i.e., toward or away from build plate 130. More specifically, beam 125 can be mounted on a carriage (not visible in this view) that couples to lead screws in columns 123A and 123B. As Z-axis motors 122A and 122B synchronously rotate their respective lead screws, the entire beam 125, X-axis motor 124, and extruder head 150 are moved upwards or downwards.

[0056] To achieve further movement of the build plate 130 relative to the extrusion head 150, a third motor, which may be referred to as a Y-axis motor 126, may act on a lead screw 127 to which the build plate 130 is coupled. Rotation of the shaft of the motor 126 controls the position of the build plate 130. The build surface 130 may be supported by a linear bearing rail, such as rail 128, and may slide or rotate along it.

[0057] It should be understood that the arrangement of motors, bearings, etc. shown in FIG. 1 is merely one example for achieving controlled relative motion between the extruder head 150 and the build plate 130 such that an object is formed by extrusion of material through the nozzle 158. Various other arrangements are known and are equally suitable as embodiments to which the present invention may be applied. For example, in some arrangements, the build plate may move in two horizontal axes and the extruder head may move only vertically. Alternatively, the build plate may move only vertically and the extruder head may move in two horizontal axes. In still other arrangements, the extruder head may be coupled to a motor-driven gantry that achieves motion in all three axes and the build plate may remain stationary. The present invention is equally applicable to a wide variety of configurations of motion control devices, including those described above as well as the so-called "Core XY", "H-bot" and "delta" arrangements, independently. Robotic arms that achieve angular motion at a series of pivoting joints may also be utilized.

[0058] Further, it should be understood that for simplicity, Figure 1 excludes many fasteners, brackets, cables, cable guides, sensors, and myriad other components that may be employed in the manufacture of such a system, but which are not necessary to explain the principles of the invention or its best mode. Where linear guides and lead screws are described, it should be understood that the present teachings are not limited to application to machines using such mechanisms, and that belt-driven and gear-driven systems, for example, are equally suitable for use and susceptible to the challenges addressed by the present invention.

[0059] The extrusion head 150 is described in further detail below in connection with FIG. 3. In summary, the role of the extrusion head 150 is to receive plastic in pellet form driven by bursts of air through a feed tube 152, "melt" the plastic, and extrude it out the end of a nozzle 158 in a continuous stream. Typically, the plastic pellets are stored in a large external pellet reservoir 102 and fed to the extruder head 150 in small amounts as needed. A detector included in the extruder head 150 determines when additional pellets are needed and electrically controls the operation of an air valve 154 which switches on a momentary burst of compressed air provided by a compressed air inlet 155.

[0060] To coordinate the moving parts of the system to form a three-dimensional solid object on the build plate 130 from the extruded material emerging from the tip of the nozzle 158, a control box 160 contains electronics, such as a microprocessor and motor drive circuitry, which are coupled to the X, Y, and Z motors, as well as the numerous sensors and heating elements in the system 120, as described above. The electronics in the control box 160 also control the extruder motors, described below, whether the extrusion is of the direct pellet extrusion type (FIG. 3) or the filament-fed type (FIG. 4).

[0061] A wide variety of 3D printer control boards can be used. Some examples of suitable control electronics that may operate within the control box 160 are the RAMBo™ control board manufactured by UltiMachine, which runs the Marlin firmware, and the so-called "Smoothie boards," which run the open source Smoothieware firmware.

[0062] The primary role of such a controller board is to interpret a sequential list of position commands, such as a so-called G-code file, and output signals to drive motors to perform the commanded movements. A G-code file or the like describing the coordinate movements required to form a particular object may be supplied to the controller through the controller's connection to a wired data communications network, for example, via TCP / IP communications over an Ethernet connection, or through a wireless network connection such as a "WiFi" or IEEE 802.11 connection. The G-code file (or a data file, such as a file in STL format from which a G-code file can be prepared) may also be supplied on a removable flash memory card, such as an SD card, which may be inserted into the SD card slot 165 on the control box 160.

[0063] To provide a human-accessible control interface, essentially all of the available control boards support an LCD display and user interface 164, as shown as part of the control box 160 in Figure 1. Power to drive the control box 160 and the motors, sensors and heating elements of the system 120 is obtained from a connection to a power line 162.

[0064] The build plate 130 is preferably heated to a controlled temperature, most commonly using electrical resistance heating elements (not visible in the figures) that may be mounted underneath the bed and thermally coupled to the bed. For this purpose, it is common to use a high temperature rated silicone rubber heating mat with conductive paths embedded therein and bonded to the bottom of the build plate. A temperature sensor such as a thermistor is typically included to provide feedback to a proportional-integral-derivative (PID) controller that maintains a set build plate temperature by controlling the application of heating current to the heating mat. Such elements for heating the build plate are common and need not be described further here.

[0065] The temperature within the enclosure 110 may be further increased above typical room ambient temperatures by the addition of other heating elements (not shown), or simply by the heat incidentally dissipated from the build plate 130. In a properly insulated enclosure 110, the heat from the build plate 130 may be sufficient to heat the interior of the enclosure to a useful level by convection alone.

[0066] FIG. 2 presents a simplified diagram of a manufacturing system compared to FIG. 1 by removing the visibility of enclosures and other components to facilitate the description of an exemplary context in which the present teachings may be advantageously applied. For convenience, system 100, labeled above as an "additive manufacturing system," is preserved as such in FIG. 2, although it should be noted that system 100 may be augmented with other tool heads to constitute a system that is at least essentially "additive," although optionally other, possibly non-additive processes may be applied as well. Hereinafter, system 100 may be referred to as an example of a more general "automated manufacturing system" or variations based on an "automated additive manufacturing system."

[0067] As a depiction of the newer system, FIG. 2 shows a multiple tool head assembly, one of which is a direct pellet extrusion (DPE) head 150, coupled to the cross beam 125 by attachment to a carrier 151, as introduced in FIG. 1. Additionally, FIG. 2 shows a filament extrusion head 261 attached to the carrier 251 as another material addition mechanism. Yet another tool head, a multi-axis machining head 262, is coupled via the carrier 252 and is operable within the same build space as the two additive tool heads 150 and 261, and functions as a material removal head capable of acting on the same build object. The machining head 262 may have many axes of motion and may provide a turning spindle or chuck at its distal end that holds a cutting tool, such as a drill bit, router bit, or end mill. While FIG. 2 happens to show three tool heads (two additive and one subtractive), any number and mix of tool heads may be realized, subject only to the practical limitations of the motion system and the available space. Additionally, while the three tool heads shown can be configured such that all three carriages 151, 251, 252 move together or are combined into a single carriage as driven by the motor 124, it is possible to build a system in which each carrier moves independently by a different mechanism or at separate times by a shared mechanism. Via mechanical or magnetic latches (not shown), each of the carriers 151, 251, 252 can be selectively coupled and decoupled from a lead screw or belt inside the cross beam 125 at any time to be driven along the cross beam 125 or remain "parked" at either end of the range of motion, allowing the motor 124 to selectively drive only one of the carriers at any given time. Another alternative arrangement could include having the tool heads move along separate cross beams or carried as payloads at the distal ends of multiple independent robotic arms. Typically, a single robotic arm, having multiple linkages connected by articulated joints and capable of programmable movement to specific coordinates in the build space, can be equipped to latch onto a specific tool head and apply its motion to the object being built.

[0068] Next, the operation of both direct pellet extruders and filament-fed extruders will be briefly described, with regard to how the attributes of the composite material may affect the suitability of these respective techniques, particularly where it may be practical to accommodate both types of extrusion using a single undifferentiated blend.

[0069] Direct pellet extruders of the type shown in FIG. 3, recently adapted to function as a moving 3D print head, offer many advantages over extruders of the type that accept a fixed diameter solid filament from a spool (described in FIG. 4). As a main advantage in industrial printing, pellet extruders can cope with long, uninterrupted builds that would otherwise consume multiple conventionally sized spools when using filament extruders. For example, pellet extruders can be used to form large 3D printed objects with a mass of 100 Kg over continuous printing periods of several hours or days. In contrast, filament-type extruders building the same object require numerous interruptions to replace spools that are commonly supplied in sizes of 1 Kg, 10 Kg or 25 Kg. At each point during the build where an empty spool of material must be replaced, there is a risk of affecting the quality of the object, and in some cases, the interruptions can even jeopardize the entire printing process in which many hours of print time and considerable costs have already been invested.

[0070] Another factor in favor of extrusion directly from pellets is cost reduction. Plastic in pelletized form is significantly less expensive than plastic already formed into filaments of precise diameter. Furthermore, because of the direct conversion from bulk pellets to molten form, pellet extruders can print materials that cannot be intermediately formed, stored and manipulated as solid filaments, such as polypropylene, glass-filled polycarbonate, polyethylene and PVC. Direct pellet extruders can also accommodate high throughput and large diameter ejection, such as through a 6 mm nozzle. This is significantly larger than the maximum practical filament size of about 3 mm, which is often ejected through nozzle openings of 1 mm or less. Thus, direct pellet extruders can print large objects hundreds of times faster than practical filament-fed printheads.

[0071] To illustrate the general operating principles of a direct pellet extrusion (DPE) head 150, Figure 3 of the drawings shows a typical design of a pellet extruder. While Figure 3 shows the desired or expected operation of such an extruder head, it must be emphasized that without the benefit of the present teachings, successful functioning may be hindered by undesirable effects as described herein. As introduced above, the role of the pellet type extruder head 150 is to convert solid pellets of plastic into a controlled stream of molten plastic and deposit the plastic in specific locations to form an object on a build plate.

[0072] In FIG. 3, the extruder head 150 includes a lead screw 312 disposed within a cylindrical barrel 310. Rotation of the lead screw 312 by the action of a motor 330 via a reduction gearbox 332 to achieve high torque at a low controlled speed causes a general downward movement of plastic pellets 301 entering the barrel from a pellet hopper 320. The barrel 310 is heated by multiple heating stages 360, 370, and 380 that are controlled to achieve a desired temperature profile along the length of the barrel. For example, as the pelletized material enters the top of the barrel and is compressed and forced down by the lead screw, the upper zone, referred to as the "transition zone" 368, is desirably set at 180°C. Further down the barrel 310, the material may experience temperatures approaching the final melt temperature at 220°C as it passes through the melt zone 378. These temperatures work well for ABS, but may be different for other materials. The heating stage to achieve this temperature profile may comprise electrical resistance heating elements nested against or surrounding the barrel 310 to transfer heat to the barrel 310. The top heating stage 360 ​​comprises heating elements 362 such as resistive wires (Nichrome) that achieve the desired temperature of the barrel by passing a controlled average current through them. A common heating tape or heating jacket is one example of a heater that may be applied.

[0073] The heating stage 360 ​​also includes a temperature sensing element 364, such as a thermocouple, that provides actual temperature information to a control unit that gates the flow of current to the heating element 362 to maintain a constant desired temperature. This temperature control function may be integrated into or collocated with the other control electronics in the control box 160 introduced above. Many of the commonly available motor control boards have integrated temperature controller capabilities, but this may also be handled by a separate commonly available closed loop controller.

[0074] Heating stages 370 and 380 operate similarly to stage 360, each having a respective heating element 372, 382 and temperature sensor 374, 384 to achieve independent local temperature control along the extruder barrel 310. Each heating element 362, 372, 382 typically has a power rating of several hundred watts.

[0075] The effect of heat applied in this manner to the barrel 310 is evident in that loose pellets 301, shown roughly in position and tending to settle downward under gravity in pellet hopper 320, are contacted and driven downward by the lead screw 312, along with the compression and propulsion of the material through the barrel by the lead screw 312. As the pellets are driven downward and enter the transition zone 368, they soften and begin to flow into one another and any interstitial air begins to be expelled by the increasing pressure. As the softened material moves into the hotter melt zone 368, the molten plastic becomes homogenous and free of any voids or bubbles. As it travels downward through the barrel, the material reaches a maximum temperature and is forced down into the nozzle 158, providing a pellet that can be expelled as a continuous bead in the form of an extrusion output 390, although it should be noted that in a more typical build, the ejection of extruded material can also be abruptly stopped and restarted by control of the extruder motor 330, within the scope of being motion controlled according to just the same G-code script as the XYZ motors. In other words, the extruder can generate a continuous stream or short segments of ejected material as needed for a given build process. Controlling the extruder motor 330 in coordination with the motion provided by the motors 122, 124, 126 deposits controlled amounts of material in specific locations and patterns to build a three-dimensional solid object on the build plate 130.

[0076] Pellets 301 are fed into the extruder head 150 from a remote location at pellet inlet 325. As previously described, a long pellet feed tube 152 may feed the airborne pellets using a blast of compressed air. As the burst of air carries the pellets into the extruder head 150, the pellets drop into a holding chamber 322 and the air carrying the pellets disperses upward through an air filter 328 that includes a fine screen or filter media 327, such as a fluted paper filter. The air filter 328 captures any pellets propelled upward by the incoming air blast and allows them to settle into the chamber 322 once the blast subsides. The air filter 328, and particularly the filter media 327, also captures any extraneous debris or powder that may arrive with the burst of pellets.

[0077] As the pellets in the feed hopper 320 are consumed by the extrusion process and drawn into the extruder barrel 310 by the lead screw 312, the pellets in the holding chamber 322 drop into the feed hopper 320, and the overall level of pellets in the chamber 322 slowly decreases. A pellet level sensor, such as a capacitive proximity sensor 324, detects the presence or absence of pellets above a given level in the holding chamber 322. When the sensor 324 senses that the fill level of pellets in the chamber 322 is below a threshold, it sends an electrical signal indicating that more pellets are needed, which (either directly or through control circuitry in the control box 160) momentarily turns on the electrically actuated air valve 145 (FIG. 1). This causes compressed air to enter the mouth of the feed tube 152, which is immersed in a reservoir of pellets. The pellets are caught in the resulting burst of air, transporting them into the chamber 322 until the sensor 324 determines that the chamber 322 is again sufficiently full. The frequency at which the sensor 324 requests a burst of pellets depends on the rate at which the pellets are being consumed by the extruder. This in turn depends on the print speed and nozzle size. The automatic refill mechanism described above attempts to ensure an adequate supply of pellets to the extruder over a wide range of consumption rates. To further ensure a reliable supply of pellets from the holding chamber 322 to the pellet hopper 320, a stream of compressed air is provided through tube 326 to constantly agitate the pellets in the holding chamber 322. This prevents the pellets from forming "bridges," i.e., standing still and crowding multiple pellets at the opening of the hopper, and dropping into the tube or getting in the way of each other. This agitation also attempts to prevent the pellets from remaining in contact long enough to form a clot, but its effectiveness decreases rapidly with increasing chamber temperature. The holding chamber 322 is preferably made of a transparent material so that a human operator can visually monitor the success of the pellet supply and refill.

[0078] To aid in the successful operation of a direct pellet extruder as just described, the feedstock composite must exhibit certain favorable characteristics such as the ability to be shaped into pellets, spheres, etc. It is highly desirable that the pellets of the candidate composite be light enough to be transported by air bursts and have little tendency to stick to each other or to stay too long in the path between the inlet 325 and the transition zone 368 where the pellets are fully engaged and forcibly driven by the extruder screw 312. Surface physics and glass transition dynamics may be important under some circumstances. The surface finish of the pellets such as recesses, flat surfaces, and barbs may affect the success of the feeding, depending on other factors. Finally, some of the tapered areas within the extruder, especially near the nozzle, may be prone to clogging as the filler particles orient and coalesce into semi-organized clumps or become entangled. The choice of filler material and size may affect how well the composite prints using DPE. In particular, in the context of the present teachings, a mixture of fillers with diverse shapes or aspect ratios may provide some improvement in one or more of these properties when DPE is used.

[0079] FIG. 4 shows a so-called filament extruder, which is very common among desktop 3D printers, but can also be applied as an alternative to direct pellet extrusion in large-scale 3D printing. FIG. 4 shows a typical filament extrusion arrangement 400 of a type that can serve as the filament extruder head 261 introduced earlier. A thermoplastic material with sufficient flexibility can be formed as a continuous strand of fixed diameter filament 401 and wound onto a reel 402 for storage, for transportation, and as a source from which the filament can be steadily drawn as it is consumed in the 3D printing process. The filament 401 is shown fed into an input end 404 of a series of components that make up the filament extrusion arrangement 400. A key design feature of the most common type of filament extruder is that the filament itself acts as a piston, forcing the molten plastic out of the nozzle under pressure through a heated passage. As part of this scheme, the inner diameter of the heated passage is only slightly larger than the diameter of the filament 401. FIG. 4 shows a passage in the form of a tube 410 joining a heating block 412 and a cooling block 414. Depending on the implementation, the tube 410 may be a continuous tube or the like, or may be a connection of abutting tubular sections, often formed by threaded tube sections whose ends meet within threaded holes through the blocks 412 and 414. The heating block 412 is typically maintained at a specific temperature by a thermostat or temperature controller 415, which includes one or more electrical resistance heating cartridges 413 and monitors the heating block temperature using a thermocouple or thermistor as a temperature sensing element 416 and controls the current supplied to the heating cartridges 413. The heating block 412 reaches a temperature sufficient to effectively melt or soften the thermoplastic material of the filament, allowing it to flow from the nozzle 418 as the extruded material 419. Additionally, the heating block 412 must receive sufficient heating power to maintain the temperature, even though the cooler mass of filament constantly moving down the tube 410 must be raised to the melting temperature in order to be timely ejected from the nozzle 418.

[0080] The force for driving the filament 401 into and through the passage 410 is provided by an extruder drive motor 430 which rotates a knurled or ribbed drive gear 431. The gear 431 firmly contacts the filament 401, the knurls or ribs provide the traction force, and pinch rollers 432 provide an opposing force to help grip the filament. The drive motor 430 is typically a computer controlled servo or stepper motor to provide precise filament advancement in synchronization with other movements of the extruder head relative to the build plate.

[0081] As the filament driven by the extruder drive motor 430 is forced through the tube 410 towards the heater block 414, a particular phenomenon occurs in which the filament softens just enough to expand against the inner wall of the tube 410 and form a complete seal. A localized stiffness gradient is formed within the material, forming a remarkably stable seal. When a hotter, softer portion of the material tries to seep upward past the "seal," it quickly cools and merges with the cooler, harder portion above.

[0082] Although it may seem at odds with the objective of heating and extruding the filament, a cooling block 414 is shown thermally coupled to the tube 418. Without the benefit of the cooling block 414, the heating block 412 may heat the entire length of the tube 410, especially if the device is left dormant for a period of time. In that case, the top end of the tube 410 may exceed the softening or glass transition temperature of the filament material, at which point the desired sealing effect described above will be lost and molten material will seep out to the top of the tube. The cooling block 414 is provided to ensure that the entire tube 410 does not rise to melting temperature, and also to control where the internal self-sealing phenomenon occurs. The transition area 422 generally indicates a small section along the tube 410 where the temperature crosses between the lower temperature of block 414 and the higher temperature of block 412. The actual location of where the seal is formed may be anywhere within the section 422, or may be slightly off within block 412 or block 414, depending on the flow rate, thermal conductivity, temperature settings, etc. of the material.

[0083] The selection of thin-walled tubes 410 allows for a reduction in the path of thermal conduction in the short segment between heating block 412 and cooling block 414, which reduces the thermal load each of these components faces as they attempt to maintain their respective temperatures.

[0084] The cooling block 414 is equipped to dissipate heat by conduction or convection from the nearby heating block 412. The most common technique is to provide the cooling block with heat dissipating fins made of a highly thermally conductive material such as aluminum, and to use a small electric fan 411 to blow ambient room air over the fins. The block 414 may alternatively be liquid cooled by circulating a cooling gas or liquid through passages in the block. This approach is preferred when a heated modeling enclosure is used and the nearby ambient temperature provides much less efficient cooling.

[0085] In some equipment designs, the extruder motor drive 430 may be physically located directly above the cooling block 414 and the heating block 412, and may be mounted to move with these blocks as a unit (as on the carriage 251 in FIG. 2). Alternatively, the extruder drive assembly 433 may be located some distance away from the moving blocks and nozzle, and may remain stationary. To ensure that the thrust applied to the filament 401 by the extruder drive assembly 433 is transferred to push the filament 401 into the passageway 410 (rather than simply expanding the loop of filament that extends between the extruder drive assembly 433 and the passageway 410), an optional Bowden tube 440 that is laterally flexible but longitudinally rigid may be mounted as a feed tube between these members. The Bowden tube 440 preferably has an inner diameter large enough to allow the filament to pass easily. The inner diameter is also less than twice the filament diameter, minimizing play so that the filament is more responsive as forward and reverse motions are executed by the motor 430.

[0086] To support successful operation of the filament extruder arrangement as just described, the feedstock material provided in the form of filament 401 must exhibit sufficient flexibility to be able to be formed into a filament and then spooled onto reel 402. Temperature-related properties such as glass transition rheological properties and thermal stability may make some potential feedstock compositions more suitable than others for a filament-fed 3D printing process. An additional concern when considering filled thermoplastic composites is nozzle clogging due to filler material gathering and agglomerating. This is of particular concern when nozzle taper or other prevailing conditions encourage filler particles to align, gather, or entangle to form a filler-concentrated plug.

[0087] 5A-5C depict a process 500 for manufacturing sheet metal forming tools by using additive manufacturing as one exemplary environment in which the present teachings may be usefully applied. In FIG. 5A, the feedstock components are depicted as being fed into a melting process 510 for mixing the components to obtain a filled polymer composite material for use as a 3D printing feedstock. The components include an amorphous polymer 502 as the primary binding matrix, a first filler having substantially elongated particles (exemplified by reinforcing fibers 504 having an aspect ratio (length to diameter) of preferably greater than 10:1), and a lightweight second filler 506. Additives 508 may optionally be introduced into the mixture to alter certain properties of the composition.

[0088] According to a preferred embodiment, lightweight filler 506 may include particles that are less elongated and may be spherical in nature, unlike filler 504. In particular, lightweight filler 506 may comprise hollow glass spheres to reduce density and provide significant compressive strength and other benefits described herein.

[0089] The melting process 510 mixes these components into a homogenous composite material that can be drawn or extruded into a feedstock in the form of filaments 520 or can be calendered, chopped, cast, or pelletized into a feedstock in the form of pellets 530.

[0090] The filament 520 may then be provided, for example on a spool, to a filament-fed 3D printer 524, i.e., one that uses a filament extrusion head similar to that shown in FIG. 4. A three-dimensional design 542 representing the desired shape of a sheet metal forming tool 550 may be prepared in a computer workstation 540 and converted into a series of motion commands 523 that are conveyed to the printer 524. Using the composition prepared by the melting process 510, the printer 524 acts on the instructions 523 to create the required forming tool 550, having mechanical properties that make it suitable for sheet metal forming operations, in accordance with the present teachings.

[0091] Alternatively, the composite material in the form of pellets 530 may be provided in bulk form 532 and fed to a pellet-fed 3D printer 534 equipped with a direct pellet extrusion head of the type depicted in Figure 3. Operating based on continuous motion commands 533 from a workstation 540, and utilizing the composition prepared by the melting process 510, the printer 534 may build a forming tool 550.

[0092] Regardless of what type of printer is used, the resulting printed forming tool 550 benefits from additive manufacturing's unique ability to provide internal support 551 in the form of pillars, walls, truss structures, waffle fills, interstitial voids, and the like that would be difficult or impossible to form by more traditional means. Large tools in particular benefit from being additively manufactured to place material only where needed. At this stage, the specific properties of polymer composite materials can facilitate or reduce the burden associated with some of this post-processing. For example, a mold for forming sheet metal into a final part shape measuring 1 meter by 0.5 meters and having a relief depth of 0.25 meters can be quickly and efficiently manufactured from composite materials embodying the present teachings.

[0093] The preparation of press die molds by additive manufacturing of the recommended dual filler composites offers many important advantages. Large molds can be produced relatively quickly and inexpensively compared to all-metal molds, making even small batch molds cost-effective. Furthermore, because of the increased stiffness and compressive strength imparted by the dual filler, deep excursion molds containing internal truss structures, struts, and lightweight internal voids are possible using an extrudable thermoplastic matrix. This large mold can be applied to a general-purpose flat press jaw, and because it is lightweight compared to metal molds, mold changes can be made easier and safer than with metal molds. In the context of using thermoplastic materials to make press dies, large may refer to die dimensions, raw material sheet dimensions, or final part dimensions that exceed 0.25 square meters in surface area or at least one dimension exceeds 0.50 meters. Alternatively, the term "large format" may refer to a die size, raw material sheet size, or final part size having at least one linear dimension that exceeds one or more values ​​from the set consisting of 0.25 meters, 0.50 meters, 0.75 meters, 1 meter, 1.4 meters, 2.0 meters, and 3.0 meters. Additionally or alternatively, the term "large format" may refer to a die size, raw material sheet size, or final part size that exceeds one or more values ​​from the set consisting of 0.125 square meters, 0.25 square meters, 0.50 square meters, 0.75 square meters, 1 square meter, 1.4 square meters, 2.0 square meters, 4.0 square meters, and 9.0 square meters in area.

[0094] The depth of the impression made in the sheet metal may also distinguish a "large" die. Proportional to the lateral dimensions discussed above, the depth of the impression may be 10% or more, but even shallow reliefs may be very demanding on the die if there is significant local tilt. In absolute measurements, the depth at which the press permanently deforms the flat sheet to form a large part may range from 0.1 meters or more, 0.2 meters or more, 0.3 meters or more, to 0.5 meters or more. While some implementations have attempted to use polymers as a lining over a molded metal die, the present teachings encourage and enable the construction of press dies entirely by additive manufacturing using specific multi-filler thermoplastic compositions, adapting generic flat press jaws to the desired die cavity shape. This ability of the additively formed die to be fully completed and self-supporting may in some cases eliminate the need to prepare any part-specific or complementary metal die as a backing behind the polymer die face. This practice also makes it easy to exchange different die sets on a given multi-purpose press.

[0095] The relative proportions of amorphous polymer 502, filler 504 and filler 506, along with possible additives 508, can be adjusted to improve the ability to machine, polish or otherwise prepare the surface of forming tool 550. For example, the addition of fillers that harden the polymer or change its thermal conductivity can result in cleaner, more precise cuts from the milling bit with less stringing and attendant melting. However, excessive concentrations or non-uniform distribution of some fillers can cause voids or "chip outs" during machining, resulting in pits in the surface.

[0096] 5B represents the next step in the overall process for making and applying a polymer composition to the fabrication of a sheet metal forming tool. After being constructed by the process of 3D printing, the forming tool 550 may undergo some additional refinement by machining to precise dimensions, grinding, polishing, coating, or otherwise preparing the surface of the forming tool 550. As representative of machining to a final surface contour, a robotic multi-axis machining head 553 is depicted acting on the forming tool 550. In some implementations, this process may be performed by a tool head 262 integrated into the system 100.

[0097] Figure 5B also shows a complementary forming tool 555, which may be designed and formed by the same process as shown in Figure 5A. The complementary forming tool 555 is generally designed to accommodate and parallel the same contour of the shape of the forming tool 550, but may need to deviate somewhat from that shape, if necessary, to properly form the sheet metal blank. In preparation for working the sheet metal blank into the forming shape, the forming tools 550 and 555 are mounted in opposing jaws 559 of a press.

[0098] Although complementary press dies 550 and 555 are shown for illustrative purposes, single-sided press dies operating in conjunction with rollers or in a hydroforming process are equally possible. Depending on the design of the final part being formed, some press dies may be designed to impart only curved bends, creases, or curvatures in isolation, and even if there are multiple such turns on a single workpiece, the need to forcibly stretch portions of the sheet stock is minimal if the bends, creases, and curvatures do not cross or overlap within the boundaries of the sheet being formed. In more demanding applications, including doubly curved surfaces, intersecting bends, or deeper drawers, press dies may be designed to apply significant stretching forces to the sheet stock.

[0099] The operation of the press with the 3D printed forming tool positioned thereon is shown in FIG. 5C. The blank of material 570 being formed may represent a sheet of stainless steel, mild steel, aluminum alloy, plastic, leather, or other material being formed using the forming tool. Of course, the present teachings are not limited to only apply when sheet stock is being formed. It is contemplated that forming tools constructed as described herein may also be used when non-planar or partially pre-formed blanks are being reshaped under pressure or force.

[0100] A blank 570 is inserted between the jaws of a press, specifically between opposing forming tools 550 and 555, which may be formed according to various aspects of the present teachings (step 572).

[0101] The compression stage 573 corresponds to pressing the jaws 559 together, capturing the blank 570 between them, and applying sufficient force to displace, deform, or reshape the blank according to the design of the forming tools 550, 555. In some applications, the compression step may involve heating the blank, the forming tools, or both. The compression process may also include the application of a vacuum, lubricant, or cover gas. In large presses, the overall force applied to the jaws may be several tons, and the overall cycle time may range from a fraction of a second to a few minutes, depending on the material being formed. In some processes, the blank 570 may be initially inserted in a somewhat pliable form, but then undergoes a chemical reaction, such as hardening, cooling, drying, phase change, or polymerization, to become more permanently shaped to fit the cavity defined by the forming tools. After the required molding cycle is completed, a separation stage 574 is performed, in which the jaws 559 are separated and the molded part is removed from the press.

[0102] The present teachings regarding 3D printing processes for creating polymer composites and polymer forming tools are not limited to the present examples involving sheet metal blanks, the application of which is believed to be particularly challenging and the present teachings are believed to be particularly advantageous in addressing these challenges. The present teachings may also be applicable to vacuum forming and layup forming, depending on the compatibility of the materials and solvents, if used.

[0103] Through steps 572-574, compressive strength and modulus of elasticity, lubricity and wear resistance may be at least some useful qualities when considering materials for making a tool and the proportions of amorphous polymer, elongated and non-elongated fillers, and optional additives to make a polymer composite suitable for a forming tool.

[0104] In the preceding description, various exemplary embodiments have been described with reference to the accompanying drawings. However, it will be apparent that various modifications and changes may be made and further embodiments may be implemented without departing from the scope of the invention as set forth in the following claims. For example, certain features of an embodiment described herein may be combined with or substituted for features of another embodiment described herein. Thus, the specification and drawings should be considered in an illustrative and not a restrictive sense.

Claims

1. 1. A method for manufacturing a press die formed according to a digital data model representing the outer shape of the die, comprising: additively fabricating a press die from a polymer feed composition by cumulatively extruding the feed composition to conform to a contour according to said digital data model. Including, The feed composition comprises: Amorphous thermoplastics; a first filler dispersed in the amorphous thermoplastic, the first filler comprising particles having a first average dimension aspect ratio; and a second filler dispersed in said amorphous thermoplastic material, said second filler comprising particles having a second average dimensional aspect ratio that differs from said first average dimensional aspect ratio by at least a factor of two. A method comprising:

2. The method of claim 1 , wherein the first filler material comprises elongated particles having a first average dimension aspect ratio of greater than 10:

1.

3. 3. The method of claim 2, wherein the first filler material comprises elongated fibrous particles made from at least one of the group consisting of glass, carbon, cellulose, mineral, and polymeric fibers.

4. The method of claim 2 , wherein the first filler material comprises substantially rigid fibers.

5. 10. The method of claim 1, wherein the second filler material comprises substantially circular particles having a second average dimensional aspect ratio of about 1:

1.

6. The method of claim 2 , wherein the second filler material comprises substantially circular particles having a second average dimensional aspect ratio of about 1:

1.

7. The method of claim 6 , wherein the second filler material comprises hollow spheres.

8. 7. The method of claim 6, wherein the second filler is made from at least one material from the group consisting of glass microspheres, pumice, expanded ash, cenospheres, and polymeric microspheres.

9. 2. The process of claim 1, wherein the amorphous thermoplastic material comprises one or more species selected from the group consisting of acrylonitrile-butadiene-styrene copolymer (ABS), polyacrylate, polymethacrylate, polyester, polyvinyl chloride (PVC), polycarbonate, polystyrene, cycloolefin copolymer (COC), polylactic acid, polycarbonate, and polycycloolefin copolymer.

10. 2. The method of claim 1, wherein the weight percentage of the first filler is 10% or more and the weight percentage of the second filler is 5% or more.

11. The method of claim 1 , wherein the composition exhibits a compressive strength of greater than 100 MPa.

12. The method of claim 1 , wherein the composition exhibits a compressive strength of greater than 70 MPa.

13. 10. The method of claim 1, wherein the composition comprises a first weight percent of the first filler and exhibits greater compressive strength than the amorphous thermoplastic alone, and the specific gravity of the composition is lower than the specific gravity of an alternative composition comprising only the amorphous thermoplastic and the first weight percent of the first filler.

14. 10. The method of claim 1, wherein the composition comprises a first weight percent of the first filler and exhibits a higher compressive modulus than the amorphous thermoplastic alone, and the specific gravity of the composition is lower than the specific gravity of an alternative composition comprising only the amorphous thermoplastic and the first weight percent of the first filler.

15. 10. The method of claim 1, wherein the composition comprises an amorphous polymer, a fixed first weight percent of the first filler, and a variable second weight percent of the second filler, and wherein increasing the second weight percent of the second filler increases at least one of the compressive strength or compressive modulus of the composition.

16. calculating at least one set of tool path instructions for directing an additive manufacturing system to extrude the polymer feedstock composition, the tool path instructions including at least one set of instructions for forming a contour-fitting boundary and at least one set of instructions for forming at least one unfilled void within an enclosed volume defined by a digital data model; The method of claim 1 further comprising:

17. calculating at least one set of tool path instructions for directing an additive manufacturing system to extrude the polymer feedstock at a specific location, the tool path instructions including at least a first set of instructions for forming a die face that conforms to a contour defined by a digital data model; The method of claim 1 further comprising:

18. 20. The method of claim 17, wherein the tool path instructions further include at least one second set of instructions for forming at least one unfilled passage disposed between the die face and a surface of the die that contacts the jaws of a press.

19. The method of claim 1 , wherein at least one linear dimension of the die is greater than 0.5 meters.

20. 1. A press die for forming a malleable material into a desired shape, the press die comprising: A first surface, which is molded to assume a particular shape by pressing the malleable material against the first surface. Including, The press die comprises: Amorphous thermoplastics; a first filler dispersed in the amorphous thermoplastic, the first filler comprising particles having a first average dimension aspect ratio; and a second filler dispersed in said amorphous thermoplastic material, said second filler comprising particles having a second average dimensional aspect ratio that differs from said first average dimensional aspect ratio by at least a factor of two. formed from a polymer composition comprising A press die comprising:

21. 21. The press die of claim 20, wherein the first filler material comprises elongated particles having a first average dimension aspect ratio of greater than 10:

1.

22. 22. The press die of claim 21, wherein the first filler material comprises elongated fibrous particles made from at least one of the group consisting of glass fibers, carbon fibers, cellulose fibers, mineral fibers, and polymeric fibers.

23. 22. The press die of claim 21, wherein the first filler material comprises substantially rigid fibers.

24. 21. The press die of claim 20, wherein the second filler material comprises substantially circular particles having a second average dimensional aspect ratio of about 1:

1.

25. 22. The press die of claim 21, wherein the second filler material comprises substantially circular particles having a second average dimensional aspect ratio of about 1:

1.

26. 26. The press die of claim 25, wherein the second filler material comprises hollow spheres.

27. 26. The press die of claim 25, characterized in that the second filler is made of at least one material from the group consisting of glass microspheres, pumice, expanded ash, cenospheres, and polymeric microspheres.

28. 21. The press die according to claim 20, characterized in that the weight ratio of the first filler is 10% or more and the weight ratio of the second filler is 5% or more.

29. 21. The press die of claim 20, wherein the composition exhibits a compressive strength of more than 70 MPa.

30. 1. A method for manufacturing a sheet metal forming tool for use in a sheet metal press, comprising the steps of: performing an additive manufacturing process using a thermoplastic feedstock material including an amorphous polymer, a first filler material having elongated particles, and a second filler material having substantially non-elongated particles, by heating, extruding, and depositing the thermoplastic feedstock material into successive layers to build a sheet metal forming tool; Including, The proportions of the first and second fillers are adjusted so that the sheet metal forming tool exhibits a compressive strength of greater than 70 MPa and a compressive modulus of greater than 4 GPa. A method comprising:

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