Polymer Additive Manufacturing Research Articles

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.

Rethinking production of machine tool bases: Polymer additive manufacturing and concrete

Cast iron and steel weldments are the most common machine tool base elements. However, both construction methods have associated disadvantages for domestic machine tool manufacturers. This paper documents the investigation of an alternative method for machine tool base production using concrete to fill an additively manufactured polymer mold, where the motion components are attached to the concrete base after the initial concrete curing. Modal testing results for a three-axis, vertical spindle prototype indicate high damping and stiffness can be achieved using the concrete base construction. Advantages are reduced cost and lead time compared to traditional methods.

Influence of Build Orientation, Geometry and Artificial Saliva Aging on the Mechanical Properties of 3D Printed Poly(ε-caprolactone)

Additive manufacturing of polymers has evolved from rapid prototyping to the production of functional components/parts with applications in distinct areas, ranging from health to aeronautics. The possibility of producing complex customized geometries with less environmental impact is one of the critical factors that leveraged the exponential growth of this processing technology. Among the several processing parameters that influence the properties of the parts, the geometry (shape factor) is amid less reported. Considering the geometric complexity of the mouth, including the uniqueness of each teething, this study can contribute to a better understanding of the performance of polymeric devices used in the oral environment for preventive, restorative, and regenerative therapies. Thus, this work aims to evaluate 3D printed poly(ε-caprolactone) mechanical properties with different build orientations and geometries. Longitudinal and transversal toolpaths produced specimens with parallelepiped and tubular geometry. Moreover, as it is intended to develop devices for dentistry, the influence of artificial saliva on mechanical properties was determined. The research concluded that the best mechanical properties are obtained for parallelepiped geometry with a longitudinal impression and that aging in artificial saliva negatively influences all the mechanical properties evaluated in this study.

Stress-constrained topology optimization for material extrusion polymer additive manufacturing

Abstract This paper presents a comprehensive numerical and experimental study on stress-constrained topology optimization for Fused Deposition Modeling (FDM) additive manufacturing. The qp method is employed to avoid the singularity issue of stress-constrained problems. The P-norm function with stability transformation is adopted to build the global stress constraint with iterative corrections to eliminate the gap between the maximum local stress and the P-norm stress. The Heaviside projection is employed to generate clear-cut 0–1 designs. Two benchmark examples have been studied with the numerical algorithm. Experiments are performed on the topologically optimized MBB beam to investigate the impact of the FDM process parameters, including deposition path direction, building direction, and slicing layer height, on the resulted structural strength. The stress-constrained designs without and with Heaviside projection are comparatively tested with experiments. The stress-minimization designs subject to different P-norm parameters are compared both numerically and experimentally. Experiments show that the deposition path direction and the building direction evidently affect the derived structural strength. Moreover, overthin structural members may severely degrade the structural strength due to manufacturing and loading uncertainties.

Interlayer Bonding Capability of Additively Manufactured Polymer Structures under High Strain Rate Tensile and Shear Loading.

Additive manufacturing of polymers via material extrusion and its future applications are gaining interest. Supporting the evolution from prototype to serial applications, additional testing conditions are needed. The additively manufactured and anisotropic polymers often show a weak point in the interlayer contact area in the manufacturing direction. Different process parameters, such as layer height, play a key role for generating the interlayer contact area. Since the manufacturing productivity depends on the layer height as well, a special focus is placed on this process parameter. A small layer height has the objective of achieving better material performance, whereas a larger layer height is characterized by better economy. Therefore, the capability- and economy-oriented variation was investigated for strain rates between 2.5 and 250 s−1 under tensile and shear load conditions. The test series with dynamic loadings were designed monitoring future applications. The interlayer tensile tests were performed with a special specimen geometry, which enables a correction of the force measurement. By using a small specimen geometry with a force measurement directly on the specimen, the influence of travelling stress waves, which occur due to the impact at high strain rates, is reduced. The interlayer tensile tests indicate a strain rate dependency of additively manufactured polymers. The capability-oriented variation achieves a higher ultimate tensile and shear strength compared to the economy-oriented variation. The external and internal quality assessment indicates an increasing primary surface profile and void volume content for increasing the layer height.

Polymeric drug delivery systems by additive manufacturing.

Additive manufacturing (AM) is gaining interests in drug delivery applications, offering innovative opportunities for the design and development of systems with complex geometry and programmed controlled release profile. In addition, polymer-based drug delivery systems can improve drug safety, efficacy, patient compliance, and are the key materials in AM. Therefore, combining AM and polymers can be beneficial to overcome the existing limitations in the development of controlled release drug delivery systems. Considering these advantages, here we are focusing on the recent developments in the field of polymeric drug delivery systems prepared by AM. This review provides a comprehensive overview on a holistic polymer-AM perspective for drug delivery systems with discussion on the materials, properties, design and fabrication techniques and the mechanisms used to achieve a controlled release system. The current challenges and future perspectives for personalized medicine and clinical use of these systems are also briefly discussed.