Over the past two decades, additive manufacturing (AM) technology has become fully ingrained into pop culture, with Do it Yourself (DIY) applications for the home, schools and other locations in addition to industrial applications for the aerospace, automotive, and biomedical industries. While AM can be used to fabricate objects from metals, polymers, and ceramics, polymeric materials are currently the most common. ASTM Standard F2792-12a1 describes techniques that can convert polymeric materials into useful products: 1) sheet lamination; 2) material extrusion; 3) vat photopolymerization; 4) powder bed fusion (with polymers); 5) binder jetting; and 6) material jetting. The automotive industry was an early adopter of 3D printing of polymeric materials, for example in the early 1990s a Japanese manufacturers2,3 used a commercialized vat photopolymerization process (widely known as stereolithography (SL)) to manufacture prototype door panels. More recently, an extrusion-based AM system was used on the International Space Station (ISS)4.
Many different processes are used to process polymeric materials for additive manufacturing. The AM process of sheet lamination involves the layering of single sheets of material, cutting out a shape by either a laser or blade and then laying subsequent sheets which are bound by an adhesive. Material extrusion, also widely known as Fused Deposition Modeling (FDM), is the most common extrusion technology used for AM. It entails the extrusion of a thermoplastic monofilament, which is then deposited layer by layer to create a 3D object. In stereolithography, a 3D object is created by curing a photocurable resin, layer by layer, with an ultraviolet (UV) laser. In binder jetting, material in powder form is joined together by the deposition of an adhesive, while in material jetting, a photocurable resin is deposited in a manner similar to that of a conventional ink-jet printer and locally cured by a UV source. The interested reader of the variety of technologies is encouraged to review ASTM F2792-12a1 as well as Wong and Hernandez5.
AM provides many key advantages over conventional manufacturing technologies. These include an ability to create unique geometries and to print objects with moving parts without part-specific tooling or in many cases, the assembly of the final part. In order to enable an increased level of multi-functionality of parts fabricated using AM, advancements in two critical areas must occur: 1) the increase of printer-compatible materials with a diverse range of physical properties; and 2) integration of processing steps within AM systems beyond material deposition or fusion. The work presented in this paper describes two strategies employed in the use of two polymeric based AM technologies: 1) FDM; and 2) SL, to achieve a higher level of functionality, usefulness, and benefit to society of additive manufacturing. The work presented here is not an exhaustive review of the area of polymeric AM, but rather an overview of the material and process development work performed at the W.M. Keck Center for 3D Innovation (keck.utep.edu).
Multi-material Additive Manufacturing
The combination of multiple material types in a single printed component is a key aspect to achieving multifunctionality. As demonstrated by Choi, et al.6 as well as Wicker and MacDonald7 multi-material stereolithography was achieved by the development of an SL system with four individual vats on a rotary system which facilitated printing of components of up to four individual resin types. When combined with other manufacturing methods, namely direct write (DW); where conductive and insulating inks and pastes are deposited via printing methodologies, polymeric AM technologies have been used to create the novel genre of 3D structural electronics8-13. Here, the stop-start ability of the AM process allows for the deposition of conductive or passive media along with the insertion of commercial-off-the-shelf electronic components such as microprocessors, accelerometers, magnetometers, etc. Figure 1 shows examples of structural electronic components.
Figure 1. Examples of structural electronic components
The evolution of the structural electronics manufacturing process involving thermoplastic materials has led to the development of wire embedding processes where wire is directly inserted into 3D printed thermoplastic substrates by way of an ultrasonic or ohmic heating method. An example of 3D structural electronics created through the integration of this wire embedding technique with FDM is shown in Figure 2; further details can be found our group’s other publications14-17.
Figure 2. Example of a 3D structural electronic device manufactured by integrating a wire embedding process with thermoplastic 3D printing
In addition to allowing inserting conductive paths into the thermoplastic, there are several other benefits, namely the ability to locally strengthen the material, which arise from this process. As a reinforcement, wire can be embedded into 3D printed parts either during an interruption of the printing process or after the printing. The degree of reinforcement is dependent on wire materials, orientation, gage, and degree of embedding. With the use of composite theory, the expected performance of the composite (polymer matrix and wire reinforcement) can be calculated so that performance metrics are satisfied. The example below demonstrates a wire embedding process for reinforcing 3D printed plastic parts along the layer stacking direction by using a nickel chromium alloy (Nichrome or NiCr).
Here, a Fortus 400mc (Stratasys, Eden Prairie, MN) equipped with T16 tips and polycarbonate (PC) was used to build ASTM D638 Type I specimens in the ZXY orientation. For more information pertaining to coordinate-terminology related to AM processes, we suggest the reader refer to the ASTM standard 52921-1318. To prevent specimens from falling over during the building process, surround support was used. All other processing parameters were not changed from their default settings. After fabrication, a sample set of five specimens were conditioned at 25 ± 2°C and 50 ± 10% relative humidity for at least 40 hours. The specimens were then tested under tensile loading conditions in accordance to ASTM D638 using an Instron 5866 material testing machine using a 10kN load cell and a ramp rate of 5mm/min.
Another sample of five specimens was subjected to the wire embedding process in which five Nichrome wires (28 AWG) were embedded axially on two, opposite sides of the specimen. The wire embedding tool uses thermal energy to simultaneously heat the plastic surface and copper wire. The head is mounted on an automation motion system to facilitate the embedding process directly from a CAD file. In the same fashion, the specimens were conditioned and tensile tested.
The results of these experiments are shown in Figure 3 where it can be seen that the main effect of the reinforcement was noted in the ultimate tensile stress (UTS). In Figure 3, each bar represents the average of five specimens and the error bars represent plus or minus one standard deviation. In this case, the average UTS of the wire-reinforced specimens was 41% greater than the non-reinforced specimens. Likewise, there was an increase in ductility and elasticity by the inclusion of wires.
Figure 3. Mechanical properties of polycarbonate (PC) and NiCr-wire-reinforced PC specimens: a) ultimate tensile strength, b) % elongation at maximum load, and c) modulus of elasticity
From this example it can be concluded that the inclusion of NiCr wires in PC specimens provided a 41% increase in UTS. This is of particular interest since the interlayer bonding is the weakest location within the fabricated part19,20. With the wire embedding process demonstrated here, one can imagine a structure such as an Unmanned Aerial Vehicle (UAV) where the locations susceptible to failure, like where a wing joins to the fuselage, can be reinforced with embedded wires to extend the life of a structure.
Polymeric Materials Development
Combining technologies such as SL and FDM with other manufacturing methods is one way to expand the applicability of polymeric AM systems. Advancements can be made through bulk material augmentation. Efforts to manipulate the physical properties of SL materials were demonstrated by Sandoval et al.21, 22 where multi-walled carbon nanotubes were combined with photo-curable resins. Though the weight percent of nanotubes added to the resin was small (0.05% and 0.5%), a substantial increase of storage modulus of the material was observed at temperatures above 200°C.
The creation of polymer matrix composites (PMC)s and the creation of novel polymeric blends are two approaches that have been used to advance material extrusion 3D printing based on FDM technology. The goal of both of these approaches has been to create material systems with a broad range of physical properties while retaining compatibility with current FDM-type 3D printing systems. Both approaches have relied heavily on thermoplastic extrusion via a twin screw extruder/compounder system (Collin ZK 25T). We have successfully demonstrated a capability to create 3D printable polymer matrix composites from acrylonitrile butadiene styrene (ABS) and PC with a broad range of filler materials such as metals, metal oxides, and plant fibers 19, 20, 23 and also demonstrated an ability to manipulate the elasticity of ABS through blending with the thermoplastic rubber, styrene ethylene butylene styrene (SEBS) 24.
Part of this research included work to perform failure analysis in order to understand the effect of additives on the fracture behavior of a given novel polymeric material system. The main method for achieving this has been the performing of fractography on the fracture surfaces of 3D printed test coupons with the aid of a scanning electron microscope (SEM). Examples include the documentation of a brittle fracture mode caused by loading ABS with TiO219,20 though, on the other hand, loading PC with tungsten did not significantly influence the fracture surface morphpology23. Key to our work has been the use of variants of SEBS where the effect of adding this material to ABS has been the emergence of a fracture surface indicative of a large amount of plastic deformation and dominated by the presence of fibrils19,20,24.
Of pertinence to 3D printing has been the understanding of the mechanical property anisotropy of these novel thermoplastic material systems. Inherent to 3D printing is a difference in mechanical properties depending on build orientation19,20. Our work has found that a ternary polymeric blend composed of ABS, SEBS, and ultra high molecular weight polyethylene (UHMWPE) yields a material which exhibits similar mechanical properties for samples printed in the XYZ and ZXY build direction;19,20 specifically for a blend created from a mixture of (by weight ratio) 75:25:10 ABS:UHMWPE:SEBS. The compounding of these three polymeric systems led to a polymer/polymer composite where UHMWPE particles were distributed within a matrix composed of blended ABS and SEBS due to the lack of miscibility of UHMWPE within this system20,24. As seen in Figure 4, the presence of UHMWPE particles and the rheological characteristics of this ternary blend leads to random failure rather than failure within the inter-print layer zone which is the main mode of failure for samples printed in the ZXY direction. While the ternary blend yields tensile test data that are lower than samples printed from the ABS base resin alone, the strategy may lead to the development of materials with isotropic qualities without a loss of overall mechanical strength. Another advantage of the ternary blend over ABS was found to be the capability to print smoother inclined planes as verified by surface roughness measurements24,25.
Figure 4. a) SEM micrograph of a 3D printable ternary polymeric blend composed of ABS, SEBS, and UHMWPE particles of this blend as compared to ABS
Other notable achievements have been the development of 3D-printable radiation shielding material based on PC where the attenuation of the material can be tuned by the loading (by weight percent) of tungsten powder23 and the development of printable ferrimagnetic material through the addition of the iron oxide, magnetite (Fe3O4) to ABS25. In both instances bonding of the filler material was improved with the aid of a silane coupling agent; a process we borrowed from the manufacturing process of glass reinforced polymers. The key point of augenting the physical properties of 3D printable materials is that when they are used in conjunction with one another, a pathway to multifunctionality can be realized as seen in Figure 5 where an ABS/SEBS blend is used in conjunction with the magnetite loaded ABS to make a simple magnet driven actuator.
Figure 5. A simple 3D-printed magnet-driven actuator
Aspects of Equipment
While our group has access to state-of-the-art material extrusion AM equipment manufactured by Stratasys, the rheological differences of our novel material systems as compared to stock 3D printer filament20,24,25 oftentimes necessitates the use of open source material extrusion 3D printers. In most instances, we have relied upon desktop grade material extrusion 3D printers, namely a MakerBot Replicator 2X (MakerBot Industries, Brooklyn, NY, USA), a LulzBot Taz 4 (Aleph Objects, Inc., Loveland, CO, USA) and a Rostock Max (SeeMe CNC, Goshen, IN, USA). In general, there are typically two types of deposition systems found on desktop-grade material extrusion 3D printers: 1) a Bowden extrusion system where the drive mechanism is far away from the liquifier; and 2) a direct drive extrusion system where the filament drive mechanism is directly above the liquefier (Figure 6a and 6b respectively). Figure 6a) Material extrusion 3D printer with a Bowden-type filament drive system located away from the liquiefierassembly and b) a direct drive 3D printer where the drive system for the filament is integrated with the liquiefier in a single print head assembly
The latter extrusion system is similar to that found on industrial grade FDM systems. We have had more success utilizing direct drive-based systems when printing elastomeric material systems developed by our group due to the lack of rigidity of the material which leads to incompatibility with Bowden-type systems. We have borrowed heavily from the learnings of DIY users of 3D printing technology as the current state is an extremely large community of practice composed of hobbyists and entrepreneurs pushing the development of new technologies. Key examples have been the modification of direct drive systems (Fig. 7a), the integration of custom drive gear systems and the print head modifications of with a high temperature liquefier (E3D V6, E3D-Online Limited, Chalgrove Oxfordshire, UK) to enable print temperatures on the order of 400?C (Fig. 7b) where most stock desktop printers are only capable of print temperatures on the order of 260°C to 280°C due to the fact that ABS and PLA are currently the materials most widely utilized by desktop grade systems.
Figure 7. DIY community inspired modifications: a) cantilever system added to a direct drive-type material extrusion 3D to increase the pressure on the filament by the drive gear and b) addition of a high-temperature liquefier and cooling system to a Bowden-type material extrusion 3D printer
The development of new manufacturing approaches and 3D printable polymer systems with a wide range of physical properties is resulting in a constant flow of new applications for the production of polymer products with 3D printing.
The development of integrated fabrication systems where advanced wire embedding techniques, micromachining, and component insertion are combined with polymer material extrusion 3D printing in a single manufacturing platform is anticipated.
3D printable polymeric materials development is focusing on developing system with tunable electromagnetic, thermal, mechanical, and rheological properties. This is being achieved by compounding new polymer blends containing filler materials with the given desired physical properties. Additionally, the development of polymeric systems in which the mechanical properties are less sensitive to build orientation are providing materials with greater tensile strength.
Our future research efforts are designed to facilitate the rapid advancement of required technology that demands an integration of polymer science, novel manufacturing techniques, and advanced engineering. The community of hobbyists and the DIY community cannot be ignored as their contributions in this area can be easily integrated into research efforts.
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David A. Roberson, Ph.D.
Dr.Roberson is an Assistant Professor in the Department of Metallurgical and Materials Engineering at The University of Texas at El Paso. He currently directs the Polymer Extrusion Lab in the W.M. Keck Center for 3D Innovation where he performs research related to the development of novel polymer matrix composites and polymer blends for additive manufacturing applications.
Prior to his academic career, Dr. Roberson spent eight years working as an engineer in the semiconductor industry for Intel Corporation (2001-2006) and Qimonda NA (2006-2009). Dr. Roberson earned his B.S. in Metallurgical and Materials Engineering (1999), his M.S. in Metallurgical and Materials Engineering (2001), and his Ph.D. in Materials Science and Engineering (2012) from The University of Texas at El Paso, USA.
Mr. Espalin is the Center Manager for the W.M. Keck Center for 3D Innovation. He received the B.S. and M.S. degrees in mechanical engineering from the University of Texas at El Paso (UTEP), in 2010 and 2012, respectively, and is currently pursuing a Ph.D. in Materials Science and Engineering.
Mr. Espalin's research within additive manufacturing has focus on fused deposition modeling, multi-material fabrication, embedded electronics, and hybrid manufacturing.
Ryan Wicker, Ph.D., P.E.
Dr. Wicker is the endowed Mr. and Mrs. MacIntosh Murchison Professor of Mechanical Engineering at the University of Texas at El Paso (UTEP), and Director and Founder of the UTEP W.M. Keck Center for 3D Innovation (Keck Center). He is also Editor-in-Chief and Founding editor of Additive Manufacturing, an Elsevier journal. Ryan received degrees in mechanical engineering from The University of Texas at Austin (B.S., 1987) and Stanford University (M.S., 1991, and Ph.D., 1995), worked at General Dynamics Fort Worth Division (1987-1989), and has spent his entire academic career at UTEP.
The Keck Center (www.keck.utep.edu) represents a world-class research facility that focuses on the use and development of additive manufacturing technologies for fabricating 3D objects that are plastic, metal, ceramic, of bio-compatible materials, composite materials, or that contain electronics. Major research efforts are underway at the Keck Center in the areas of additive manufacturing technology development; closed-loop process control strategies for additive manufacturing; additive manufacturing of various powder metal alloy systems; development of new polymers for use in additive manufacturing; and 3D structural electronics in which electronics, and thus intelligence, are fabricated within additive manufacturing-fabricated structures.