Polymer Additive Manufacturing Research Articles

Foam 3D Printing of Thermoplastics: A Symbiosis of Additive Manufacturing and Foaming Technology.

Due to their light‐weight and cost‐effectiveness, cellular thermoplastic foams are considered as important engineering materials. On the other hand, additive manufacturing or 3D printing is one of the emerging and fastest growing manufacturing technologies due to its advantages such as design freedom and tool‐less production. Nowadays, 3D printing of polymer compounds is mostly limited to manufacturing of solid parts. In this context, a merged foaming and printing technology can introduce a great alternative for the currently used foam manufacturing technologies such as foam injection molding. This perspective review article tackles the attempts taken toward initiating this novel technology to simultaneously foam and print thermoplastics. After explaining the basics of polymer foaming and additive manufacturing, this article classifies different attempts that have been made toward generating foamed printed structures while highlighting their challenges. These attempts are clustered into 1) architected porous structures, 2) syntactic foaming, 3) post‐foaming of printed parts, and eventually 4) printing of blowing agents saturated filaments. Among these, the latest approach is the most practical route although it has not been thoroughly studied yet. A filament free approach that can be introduced as a potential strategy to unlock the difficulties to produce printed foam structures is also proposed.

An integrated prediction model for processing related yield strength of extrusion-based additive manufactured polymers

As is well acknowledged that mechanical properties of material extrusion (ME) additive manufacturing polymers are highly related to processing parameters. Meanwhile, the yield strength is one of the key properties of a material, which critically affects a component’s manufacturing and further application. To reveal the effect of printing parameters such as layer thickness, printing speed and deposition orientation on the yield strength of ME polymers without extensive experimental researches, a novel, integrated prediction model accounting for processing-structure-property relationship is put forward. In this model, processing related representative volume element (RVE) of unidirectional ME polymers is firstly estimated by combining the volume conservation method and thermal-sintering model. Next, assuming raw polymers to be homogenous, isotropic, and ideal elastic-plastic, a multiscale approach integrating Hill model and homogenization theory is presented to acquire the stress-strain curves of anisotropic ME polymers under externally applied loadings. In this procedure, the dependence of RVE’s estimated anisotropic parameters in Hill model on their external loading states is paid attention. By this means, the three-dimensional yield behavior of ME polymers, including the off-axis yield strength, can be obtained. The validity of the proposed method is examined by comparing with existing test data in literature or our measured ones. Finally, explicit discussions are made. Many valuable conclusions are drawn, which may be of unique significance for the future processing optimization and quality improvement of final products.

Photopolymer formulation towards large scale additive manufacturing of autoclave capable tooling

Large-scale polymeric additive manufacturing (AM) is emerging as a method for producing prototype tooling in the aerospace industry. However, material options for oven/autoclave-capable tooling are currently limited to costly thermoplastics such as polyetherimide (PEI) and polyphenylene sulfide (PPS). Photopolymers can be formulated for oven/autoclave use and have advantages over thermoplastics such as ambient processing and rapid curing under low-energy ultraviolet light emitting diode (UV LED) lamps. This study presents initial photopolymer formulation work towards large-scale AM with bisphenol A ethoxylate dimethacrylate (Bis-EMA), a vinyl ester suitable for moderate autoclave/oven-curing temperatures (100–135 °C, 212–275°F). Bis-EMA photopolymers were prepared and cured in 3 mm thick specimens by UV irradiation to simulate large-scale AM conditions. Bis-EMA was also blended with pentaerythritol triacrylate (PETIA) to determine the influence of a high-functionality acrylate on cure response and material properties. For the same UV dose, an increase in PETIA content up to 50 wt% increased the Vickers Hardness Number (VHN) on the irradiated surface from 22.6 to 25.0. It also improved the ratio of top to bottom surface hardness from 65% to 91%, indicating possible improvement in through-thickness cure response. The addition of PETIA decreased the tensile modulus from 2.8 GPa to 2.1–2.3 GPa, and PETIA content above 20 wt% reduced the glass transition temperature (T g ) below the Bis-EMA control. A blend of 80 wt% Bis-EMA with 20 wt% PETIA yielded an optimal balance of improved through-thickness hardness over the Bis-EMA control while retaining thermal stability. To investigate suitability for oven/autoclave-capable tooling, this 80:20 photopolymer blend was reinforced with fiberglass and used to produce UV cured demonstration tooling via a pseudo-additive process. The tools were subjected to five oven cure molding cycles, and laser scanning showed an average deviation < 0.05 mm, indicating negligible degradation in the tool geometry. The hardness values of the tool stock material did not change significantly with thermal cycling, and the T g increased from 154 °C to 172 °C. This study is the first report of Bis-EMA/PETIA blends towards use in photopolymers for large-scale AM. The data confirm that Bis-EMA photopolymer formulations can achieve adequate through-thickness cure at the layer thicknesses typical of large-scale AM. An 80:20 Bis-EMA/PETIA blend was thermally stable within the low volume thermal cycles expected of a 3D printed prototype tool.

Experimental Characterization and Analysis of the In-Plane Elastic Properties and Interlaminar Fracture Toughness of a 3D-Printed Continuous Carbon Fiber-Reinforced Composite.

The use of continuous fiber as reinforcement in polymer additive manufacturing technologies enhances the mechanical performance of the manufactured parts. This is the case of the Carbon-Fiber reinforced PolyAmide (CF/PA) used by the MarkForged MarkTwo® 3D printer. However, the information available on the mechanical properties of this material is limited and with large variability. In this work, the in-plane mechanical properties and the interlaminar fracture toughness in modes I and II of Markforged’s CF/PA are experimentally investigated. Two different standard specimens and end-tabs are considered for the in-plane properties. Monolithic CF/PA specimens without any additional reinforcement are used for the interlaminar fracture toughness characterization. Two different mode I specimen configurations are compared, and two different test types are considered for mode II. The results show that prismatic specimens with paper end-tabs are more appropriate for the characterization of the in-plane material properties. The use of thick specimens for mode I fracture toughness tests complicates the characterization and can lead to erroneous results. Contrary to what has been reported in the literature for the same material, fracture toughness in mode I is lower than for mode II, which agrees with the normal tendency of traditional composite materials.

High strain-rate compression behavior of polymeric rod and plate Kelvin lattice structures

The compressive high strain-rate behavior of polymeric Kelvin lattice structures with rod-based or plate-based unit cells was investigated through experimental techniques and finite element simulations. Polymeric lattice structures with 5x5x5 unit cell geometries were manufactured on the millimeter scale using vat polymerization additive manufacturing and tested at low (0.001/s) and high (1000/s) strain-rates. High strain-rate experiments were performed and validated for a viscoelastic split-Hopkinson (Kolsky) pressure bar system (SHPB) coupled with high-speed imaging and digital image correlation (DIC). Experimental results at both low and high strain-rates show the formation of a localized deformation band which was more prevalent in low relative density specimens and low strain-rate experiments. Strain-rate effects of lattice specimens strongly correlate with effects of the base polymer material; both bulk polymer and lattice specimen demonstrated strain-rate hardening, strain-rate stiffening, and decreased fracture strain under dynamic loading. Results show mechanical failure properties and energy absorption depended strongly on the relative density of the lattice specimen and exhibited distinct scaling between relative density and geometry type (rod, plate) and loading rate. High relative density plate-lattices demonstrated inferior mechanical properties to rod-lattices; however, there exists a critical relative density for a given mechanical property (17%–28%) below which plate-lattices outperform rod-lattices of similar mass. High strain-rate explicit finite element simulations were performed and showed good agreement with the mechanical failure trends and deformation modes observed in the experiments.

Assessing Leachable Cytotoxicity of 3D-Printed Polymers and Facile Detoxification Methods.

Additive manufacturing of polymers is gaining momentum in health care industries by providing rapid 3D printing of customizable designs. Yet, little is explored about the cytotoxicity of leachable toxins that the 3D printing process introduced into the final product. We studied three printable materials, which have various mechanical properties and are widely used in stereolithography 3D printing. We evaluated the cytotoxicity of these materials through exposing two fibroblast cell lines (human and mouse derived) to the 3D-printed parts, using overlay indirect contact assays. All the 3D-printed parts were measured toxic to the cells in a leachable manner, with flexible materials more toxic than rigid materials. Furthermore, we attempted to reduce the toxicity of the 3D-printed material by employing three treatment methods (further curing, passivation coating, and Soxhlet solvent extraction). The Soxhlet solvent extraction method was the most effective in removing the leachable toxins, resulting in the eradication of the material's toxicity. Passivation coating and further curing showed moderate and little detoxification, respectively. Additionally, mechanical testing of the materials treated with extraction methods revealed no significant impacts on its mechanical performances. As leachable toxins are broadly present in 3D-printed polymers, our cytotoxicity evaluation and reduction methods could aid in extending the selections of biocompatible materials and pave the way for the translational use of 3D printing.

Adaptive Conformal Cooling of Injection Molds Using Additively Manufactured TPMS Structures.

In injection molding, cooling channels are usually manufactured with a straight shape, and thus have low cooling efficiency for a curved mold. Recently, additive manufacturing (AM) was used to fabricate conformal cooling channels that could maintain a consistent distance from the curved surface of the mold. Because this conformal cooling channel was designed to obtain a uniform temperature on the mold surface, it could not efficiently cool locally heated regions (hot spots). This study developed an adaptive conformal cooling method that supports localized-yet-uniform cooling for the heated region by employing micro-cellular cooling structures instead of the typical cooling channels. An injection molding simulation was conducted to predict the locally heated region, and a mold core was designed to include a triply periodic minimal surface (TPMS) structure near the heated region. Two biomimetic TPMS structures, Schwarz-diamond and gyroid structures, were designed and fabricated using a digital light processing (DLP)-type polymer AM process. Various design parameters of the TPMS structures, the TPMS shapes and base coordinates, were investigated in terms of the conformal cooling performance. The mold core with the best TPMS design was fabricated using a powder-bed fusion (PBF)-type metal AM process, and injection molding experiments were conducted using the additively manufactured mold core. The developed mold with TPMS cooling achieved a 15 s cooling time to satisfy the dimensional tolerance, which corresponds to a 40% reduction in comparison with that of the conventional cooling (25 s).

Dynamic fracture behaviour of additively manufactured polymers and composites under ballistic impact

Recent technological advancements led to active research in the field of additive manufacturing (AM) thanks to its ability to fabricate complex structures with reduced material waste. While many studies focused on quasi-static performance of AM materials and structures, investigations on the dynamic performance of these materials are still limited, especially for polymers and composites. The aim of this research is to investigate the dynamic fracture behaviour of AM polymer composites under ballistic impact. Nylon reinforced with short carbon fibres was selected, and fused deposition modelling was used to fabricate solid quasi-isotropic structures. Quasi-static tensile and compression tests were carried out based on ASTM standards to characterise the elastic-plastic properties of the manufactured material. Based on these tests, the material’s deformation and fracture behaviour can be considered elastic-brittle. Further, dynamic mechanical analysis was conducted under a frequency sweep at room temperature to examine the viscoelastic properties; the material exhibited high elastic behaviour. Then, ballistic impact tests were performed on solid plates using a gas gun with 9 mm spherical steel projectiles at velocities ranging from 95 m s-1 to 120 m s-1. Post-experimental observation revealed the complete target perforation, with a circular hole at the front of the plates and delamination at the back. The dynamic experimental conditions were reproduced within the finite-element simulations, employing Hashin damage criteria via a user subroutine. A similar perforation was achieved at the front of the plate.

Crack path direction in plane-strain fracture toughness assessment tests of quasi-brittle PLA polymer and ductile PLA-X composite

Plane-strain fracture toughness is one of the main parameters in linear elastic fracture mechanics and its purpose is to show the material capability to withstand load while having a defect. Main validity aspect for such an assessment is to provide a wide enough crack front to enable plane-strain condition. Nonetheless, in FDM (Fused Deposition Modeling), due to structural anisotropy caused by polymer material properties and Additive Manufacturing (AM) process parameters, more validity aspects must be met. During the plane-strain fracture toughness test a crack must follow a straight line from initiation up to the point of structural failure. Plane-strain fracture toughness assessment is conducted according to the ASTM D5045-14 standard for testing of polymer materials. Tests are performed on SENB (Single Edge Notched Bend) specimens, made from two similar polymer materials: quasi-brittle PLA and ductile PLA-X composite. Specimens are manufactured with four different AM process parameters, i.e., layer height, infill density, printing orientation and one specimen batch was dried before testing.

Printing Ready Topology Optimization for Material Extrusion Polymer Additive Manufacturing.

Additive manufacturing-oriented topology optimization features in the extreme geometric complexity that magnifies the product functional performance. However, the increased geometric complexity makes postprocessing of the designs technically nontrivial and sometimes inefficient because of too many structural details. To address this issue, this article presents a novel printing-ready topology optimization method whereby the topological designs can be directly exported in the format of a printing-ready G-code, which saves the postprocessing efforts of stereo lithograph (STL) model generation, model slicing, and tool path planning. More importantly, the slicing and tool path information can be tracked all the time during optimization to facilitate the evaluation of the tool path-related material constitutive model, for example, the fiber-reinforced composites, so as to improve the numerical analysis accuracy and the design result optimality. Finally, three case studies are performed to test the postprocessing efficiency of the printing-ready approach and the multi-scale design case, which demonstrates the outstanding high efficiency characteristic of the proposed approach.

Shape memory polymers for additive manufacturing: An overview

Additive manufacturing (AM), or 3D printing, has arisen as a booming technology for the development of more creative devices and potential functional applications in biomedical, electronics, sensors, and tissue engineering through the pioneering combination of shape memory polymers (SMPs). However, restricted printable materials and poor qualities of existing 3D printing materials in terms of mechanical strength, curing speed, and thermomechanical shape memory have impeded the adoption of SMPs in 3D printing. Several SMPs are available in the market in response to these drawbacks. This paper is contributing a review in advancement of polyurethanes as SMP polymers in 3D printing.

Impacts of ingested MWCNT-Embedded nanocomposites in Japanese medaka (Oryzias latipes)

Polymer nanocomposites combine the versatile, lightweight characteristics of polymers with the properties of nanomaterials. Polyethylene terephthalate glycol (PETG) is commonly used in polymer additive manufacturing due to its controllable transparency, high modulus, and mechanical properties. Multi-walled carbon nanotubes (MWCNTs) add tensile strength, electrical conductivity, and thermal stability. The increased use of nanocomposites has led to concern over potential human health risks. We assessed morphologic alterations to determine impacts of ingested abraded nanocomposites compared to its component materials, pristine MWCNTs (1000 mg/L) and PETG. Adult transparent Japanese medaka (Oryzias latipes) were administered materials via oral gavage in 7 doses over 16 days. In vivo observations revealed altered livers and gallbladders following exposure to pristine MWCNTs and nanocomposites. Subsequent histologic sections showed fish exposed to pristine MWCNTs had highly altered biliary structures, and exposure to nanocomposites resulted in hepatocellular alteration. Thyroid follicle proliferation was also observed in fish exposed to materials containing MWCNTs. Transmission electron microscopy of livers showed that hepatocytes of fish exposed to MWCNTs had widespread swelling of rough endoplasmic reticulum, pronounced lysosomal activity, and swelling of intrahepatic biliary passageways. Fish exposed to nanocomposites had areas of degenerated hepatocytes with interspersed cellular debris. Each analysis showed that fish exposed to pristine PETG were most similar to controls. These results suggest that MWCNTs are the source of toxicity in abraded nanocomposite materials but that nanocomposites may also have some unique effects. The similarities of many teleost and mammalian tissues are such that these findings may indicate human health risks.

Manufacturing process-driven structured materials (MPDSMs): design and fabrication for extrusion-based additive manufacturing

PurposeThis paper aims to explore the design and fabrication of meso-scale Manufacturing Process-Driven Structured Materials (MPDSMs). These are designed, architected materials where the prime design requirement is manufacturability. The concepts are applied to those fabricated using fused deposition modeling or fused filament fabrication (FDM/FFF), a thermoplastic polymer additive manufacturing (AM) process. Three case studies were presented to demonstrate the approach.Design/methodology/approachThe paper consists of four main sections; the first developed the MPDSMs concept, the second explored manufacturability requirements for FDM/FFF in terms of MPDSMs, the third presented a practical application framework and the final sections provided some case studies and closing remarks.FindingsThe main contributions of this study were the definition and development of the MDPSMs concept, the application framework and the original case studies. While it is most practical to use a well-defined AM process to first explore the concepts, the MPDSMs approach is neither limited to AM nor thermoplastic polymer materials nor meso-scale material structures. Future research should focus on applications in other areas.Originality/valueThe MPDSMs approach as presented in this concept paper is a novel method for the design of structured materials where manufacturability is the prime requirement. It is distinct from classic design-for-manufacturability concepts in that the design space is limited to manufacturable design candidates before the other requirements are satisfied. This removes a significant amount of schedule and costs risk from the design process, as all the designs produced are manufacturable within the problem tolerance.

Effects of wood flour and MA-EPDM on the properties of fused deposition modeling 3D-printed poly lactic acid composites

Fused deposition modeling (FDM) 3D printing technology is the most common system for polymer additive manufacturing (AM). Recent studies have been conducted to expand both the range of materials that can be used for FDM and their applications. As a filler, wood flour was incorporated into poly lactic acid (PLA) polymer to develop a biocomposite material. Composite filaments were manufactured with various wood flour contents and then successfully used for 3D printing. Morphological, mechanical, and biodegradation properties of FDM 3D-printed PLA composites were investigated. To mitigate brittleness, 5 phr of maleic anhydride grafted ethylene propylene diene monomer (MA-EPDM) was added to the composite blends, and microstructural properties of the composites were examined by scanning electron microscopy (SEM). Mechanical strength tests demonstrated that elasticity was imparted to the composites. Additionally, test results showed that the addition of wood flour to the PLA matrix promoted pore generation and further influenced the mechanical and biodegradation properties of the 3D-printed composites. An excellent effect of wood flour on the biodegradation properties of FDM 3D-printed PLA composites was observed.

Design and characterization of AM sensorized capsules for drug delivery devices

Long-acting drug delivery systems able to sustain the release of therapeutics in a controllable manner can provide several advantages in the treatment of chronic diseases. Given the high drug concentration in the reservoir, an inward osmotic fluid transport occurs across the membrane, which counters the outward diffusion of drugs. The resulting osmotic pressure buildup may be sufficient to cause the failure of implants with associated risks to patients. Confidently assessing the osmotic pressure buildup requires testing in vivo. Here, using metal and polymer additive manufacturing processes, we designed and developed implantable drug reservoirs with embedded strain sensors to directly measure the osmotic pressure in drug delivery implants in vitro and in vivo.

Review of Powder Bed Fusion Additive Manufacturing for Metals

Additive manufacturing (AM) as a disruptive technology has received much attention in recent years. In practice, however, much effort is focused on the AM of polymers. It is comparatively more expensive and more challenging to additively manufacture metallic parts due to their high temperature, the cost of producing powders, and capital outlays for metal additive manufacturing equipment. The main technology currently used by numerous companies in the aerospace and biomedical sectors to fabricate metallic parts is powder bed technology, in which either electron or laser beams are used to melt and fuse the powder particles line by line to make a three-dimensional part. Since this technology is new and also sought by manufacturers, many scientific questions have arisen that need to be answered. This manuscript gives an introduction to the technology and common materials and applications. Furthermore, the microstructure and quality of parts made using powder bed technology for several materials that are commonly fabricated using this technology are reviewed and the effects of several process parameters investigated in the literature are examined. New advances in fabricating highly conductive metals such as copper and aluminum are discussed and potential for future improvements is explored.

Development and Characterization of Field Structured Magnetic Composites

Polymer composites containing ferromagnetic fillers are promising for applications relating to electrical and electronic devices. In this research, the authors modified an ultraviolet light (UV) curable prepolymer to additionally cure upon heating and validated a permanent magnet-based particle alignment system toward fabricating anisotropic magnetic composites. The developed dual-cure acrylate-based resin, reinforced with ferromagnetic fillers, was first tested for its ability to polymerize through UV and heat. Then, the magnetic alignment setup was used to orient magnetic particles in the dual-cure acrylate-based resin and a heat curable epoxy resin system in a polymer casting approach. The alignment setup was subsequently integrated with a material jetting 3D printer, and the dual-cure resin was dispensed and cured in-situ using UV, followed by thermal post-curing. The resulting magnetic composites were tested for their filler loading, microstructural morphology, alignment of the easy axis of magnetization, and degree of monomer conversion. Magnetic characterization was conducted using a vibrating sample magnetometer along the in-plane and out-of-plane directions to study anisotropic properties. This research establishes a methodology to combine magnetic field induced particle alignment along with a dual-cure resin to create anisotropic magnetic composites through polymer casting and additive manufacturing.

Hybrid additive and subtractive manufacturing of multi-material objects

Purpose This study presents a method for fabricating multi-material objects using a hybrid additive and subtractive approach. By hybridizing the material composition in addition to the fabrication process, functional requirements can be met more effectively than through homogenous material parts produced using a single manufacturing process. Development of multi-material objects consisting of dissimilar materials that have been hampered by a lack of a structural interface compatible with in-envelope hybrid additive and subtractive manufacturing. Design/methodology/approach This research presents a novel method for producing multi-material components through in-envelope hybrid additive and subtractive manufacturing. This study attempts to address the absence of a metal-polymer interface by integrating polymer additive manufacturing into a five-axis mill. The ability of the polymer additive system to reproduce overhang geometries is assessed with different levels of cooling. The relationship between structural performance, cooling and material flow rate is evaluated for the deposited carbon fiber reinforced acrylonitrile butadiene styrene. Findings A mechanically interlocking root structure is developed to form an interface between a machined aluminum region and a polymer region of an object. The tensile strength of the metal-polymer object is measured and found to be on the same order of magnitude as the bulk three-dimensional printed polymer. Originality/value By targeting the material properties to the local functional requirements within a part and taking advantage of both additive and subtractive manufacturing processes, this study will enable broader design options and optimization of performance metrics.

Multimaterial Vat Polymerization Additive Manufacturing

Multimaterial Vat Polymerization Additive Manufacturing

Compaction behavior of powder bed fusion feedstock for metal and polymer additive manufacturing

Purpose Powder bed-based additive manufacturing (AM) is a promising family of technologies for industrial applications. The purpose of this study is to provide a new metrics based on the analysis of the compaction behavior for the evaluation of flowability of AM powders. Design/methodology/approach In this work, a novel qualification methodology based on a camera mounted onto a commercially available tap density meter allowed to assess the compaction behavior of a selection of AM materials, both polymers and metals. This methodology automatizes the reading of the powder height and obtains more information compared to ASTM B527. A novel property is introduced, the “tapping modulus,” which describes the packing speed of a powdered material and is related to a compression/vibration powder flow. Findings The compaction behavior was successfully correlated with the dynamic angle of repose for polymers, but interestingly not for metals, shedding more light to the different flow behavior of these materials. Research limitations/implications Because of the chosen materials, the results may lack generalizability. For example, the application of this methodology outside of AM would be interesting. Originality/value This paper suggests a new methodology for assessing the flowing behavior of AM materials when subjected to compression. The device is inexpensive and easy to implement in a quality assurance environment, being thus interesting for industrial applications.