Thermoplastic 3D Printing Research Articles

Digital Light 3D Printing of Double Thermoplastics with Customizable Mechanical Properties and Versatile Reprocessability.

Digital light processing (DLP) is a 3D printing technology offering high resolution and speed. Printable materials are commonly based on multifunctional monomers, resulting in the formation of thermosets that usually cannot be reprocessed or recycled. Some efforts are made in DLP 3D printing of thermoplastic materials. However, these materials exhibit limited and poor mechanical properties. Here, a new strategy is presented for DLP 3D printing of thermoplastics based on a sequential construction of two linear polymers with contrasting (stiff and flexible) mechanical properties. The inks consist of two vinyl monomers, which lead to the stiff linear polymer, and α-lipoic acid, which forms the flexible linear polymer via thermal ring-opening polymerization in a second step. By varying the ratio of stiff and flexible linear polymers, the mechanical properties can be tuned with Young's modulus ranging from 1.1GPa to 0.7MPa, while the strain at break increased from 4% to 574%. Furthermore, these printed thermoplastics allow for a variety of reprocessability pathways including self-healing, solvent casting, reprinting, and closed-loop recycling of the flexible polymer, contributing to the development of a sustainable materials economy. Last, the potential of the new material in applications ranging from soft robotics to electronics is demonstrated.

Tuning the rheological properties of paraffin-wax ceramic feedstocks for deposition with thermoplastic 3D printing

Droplet deposition with material-jetting methods such as thermoplastic 3D printing (T3DP) depends greatly on the rheological properties of the feedstocks. This study investigated the effect of particle interactions and the degree of weak flocculation on the shear thinning behaviour, the yield stress and the storage/loss moduli of paraffin-wax-based feedstocks containing 40 vol% of zirconia (3Y-TZP) micron-sized powder. Steric stabilization of the feedstocks was provided by varying the ratios of the surfactants with different chain lengths, i.e., stearic acid (2.4 nm) and Solsperse 3000® (10 nm), which in turn affected the dynamics of the droplet formation and the quality of the layers when jetting non-Newtonian, thermoplastic ceramic feedstocks. The results of the study extend the guidelines for the processing of printable feedstocks used in T3DP additive manufacturing.

Physics-constrained multi-objective bayesian optimization to accelerate 3d printing of thermoplastics

The printing outcome of vat photopolymerization (VPP) of thermoplastics largely depends on physicochemical properties of monomers and their compositions in resins, which also greatly determine the material properties, e.g., tensile strength (σT) and toughness (UT)and phase transition temperature (Tg). A methodology for optimizing the resin formulation is of paramount importance in realizing highly printable thermoplastics with balanced σT/UT and target Tg while remaining largely underexplored. Herein, we introduce a multi-objective Bayesian optimization (MOBO) algorithm under two physics informed constraints (printability and Tg) to optimize two conflicting properties: σT and UT. The two constraints are formulated as two machine learning (ML) models, which are trained with weight ratios of the six monomers and physics informed (PI) descriptors derived from their physiochemical parameters. Dimensional reduction analysis reveals that the algorithm avoids recommendation of the monomer ratios that do not pass the two constraints. The printing failure rate is reduced from 16% in the background experiments to 3% in the recommended experiments. Within only 36 iterations (72 samples), the MOBO algorithm successfully identifies five sets of ratios leading to Pareto optimal of σT and UT. Due to the constraint in Tg they show appropriate Tg for shape memory application. The partial dependence analysis indicates that σT and UT depend on both the ratios and physiochemical features of the monomers. These results underscore capability of such a smart decision-making algorithm—with constraints that are not fully understood but can be informed by prior knowledge—in planning the experiments from the vast design space, thus holding a great promise for broader applications in materials design and manufacturing.

Molecular mass engineering for filaments in material extrusion additive manufacture

Abstract3D printing of thermoplastics through local melting and deposition via material extrusion additive manufacturing provides a simple route to the near net‐shape manufacture of complex objects. However, the mechanical properties resulting from these 3D printed structures tend to be inferior when compared to traditionally manufactured thermoplastics. These unfavorable characteristics are generally attributed to the structure of the interface between printed roads. Here, we illustrate how the molecular mass distribution for a model thermoplastic, poly(methyl methacrylate) (PMMA), can be tuned to enhance the Young's modulus of 3D printed plastics. Engineering the molecular mass distribution alters the entanglement density, which controls the strength of the PMMA in the solid state and the chain diffusion in the melt. Increasing the low molecular mass tail increases Young's modulus and ultimate tensile strength of the printed parts. These changes in mechanical properties are comparable to more complex routes previously reported involving new chemistry or nanoparticles to enhance the mechanical performance of 3D printed thermoplastics. Controlling the molecular mass distribution provides a simple route to improve the performance in 3D printing of thermoplastics that can be as effective as more complex approaches.

Dual Crosslinked Antioxidant Mixture of Poly(vinyl alcohol) and Cerium Oxide Nanoparticles as a Bioink for 3D Bioprinting.

Three-dimensional (3D) bioprinting has made it possible to fabricate structures with intricate morphologies and architectures, which is considered difficult to do when using other conventional techniques like electrospinning. Although the 3D printing of thermoplastics has seen a huge boom in the past few years, it has been challenging to translate this technology to cell-based printing. A major limitation in bioprinting is the lack of inks that allow for the printing of 3D structures that meet the biological requirements of a specific organ or tissue. A bioink is a viscous polymer solution that cells are incorporated into before printing. Therefore, a bioink must have specific characteristics to ensure both good printability and biocompatibility. Despite the progress that has been made in bioprinting, achieving a balance between these two properties has been difficult. In this work, we developed a multimodal bioink that serves as both a cell carrier and a free radical scavenger for treating peripheral nerve injury. This bioink comprises poly(vinyl alcohol) (PVA) and cerium oxide nanoparticles (also called nanoceria (NC)) and was developed with a dual crosslinking method that utilizes citric acid and sodium hydroxide. By employing this dual crosslinking method, good printability of the bioink and shape fidelity of the bioprinted structure were achieved. Additionally, a cell viability study demonstrated that the cells remained compatible and viable even after they underwent the printing process. The combination of this PVA/NC bioink and the dual crosslinking method proved to be effective in enhancing printability and cell biocompatibility for extrusion-based bioprinting applications.

Large-scale hollow-core 3D printing (HC3DP): A polymer 3D printing technology for large-scale ultralightweight components

Material extrusion (ME) has become a popular 3D printing method for large-scale applications such as in the automotive, naval, aerospace, and architecture sectors. It allows the production of complex, customized parts without the need of labour-intensive molds. The increasing need to produce larger and larger parts has been addressed with the introduction of large pellet extrusion systems to increase the extrusion rate. With these extruders the production time is not limited by the extrusion rate of the extruder (kg/h) but by the time the material needs to cool down before the next layer can be printed consecutively. Furthermore, larger bead dimensions result in increased material usage and, therefore, might affect the environmental impact of the printed element. In this study, a method for large extrusion 3D printing based on hollow beads is proposed, to address the limitations of speed, cooldown and maximize the extrusion rate of thermoplastic 3D printing. This paper introduces Hollow-Core 3D printing (HC3DP) and compares it to off-the shelf large-scale pellet extruders and their maximum achievable extrusion rate for a given geometry. A comparative analysis of cooldown characteristics between conventional thermoplastic extrusion 3D printing and HC3DP validates that the latter increases the maximum possible extrusion rate. Additionally, the successful 3D printing of large-scale beads with multiple polymer feedstocks is demonstrated. Then the data associated with printing speed and material usage is presented and discussed in comparison with thermoplastic and other large-scale 3D printing methods. The fabrication of large-scale hollow-core beads at a dimension of 24×20mm is showcased with an extrusion rate of 7250 mm3/s (1308 material, 5941 air). HC3DP for large-scale applications is validated through the production of 2-meter tall cylinders with a diameter of 400 mm in less than two hours. Furthermore, bespoke die geometries that subdivide the inside of hollow 3D-printed beads are presented. To successfully print those, the concept of differentiated internal air pressure per print point is introduced. Finally, 3PT bending tests are conducted to compare different die geometries. In conclusion, this study presents a printing method that highlights the benefits of hollow core 3D printing for large-scale applications, showcasing its positive impact on both the printing process and resulting part properties. By addressing the limitations of traditional approaches, HC3DP pushes polymer 3D printing into a scale relevant for architecture and construction, offering similar extrusion rates as concrete 3DP.

PneuPrint: 3D printing on inflatables

This research investigated the feasibility of thermoplastic 3D printing on inflatable membranes. Five experiments were performed in an iterative process through design and manufacture (1), computational simulation and 3D scanning (2), and robotic fabrication on the pneumatic formwork (3). These experiments ranged from small to large-scale 3D printing. Experiment 1 demonstrated the small-scale feasibility of the process and the need to integrate an air-pressure control loop. Experiment 2 investigated the technique transfer from small to large-scale. Experiment 3 analyzed the deviation and shape accuracy of the inflatable membrane. Experiment 4 identified the required fabrication settings and compatibility between the membrane and the 3D printing material. Finally, Experiment 5 demonstrated the design and fabrication potential of large-scale 3D printed elements on pneumatic formworks. The results proved high potential for building freeform design elements for architectural applications on pneumatic formworks.

Growth of marine biofilms and macrofouling organisms on biocide-infused, 3D-printed thermoplastics

3D printing has become widely used to rapidly prototype and manufacture novel or bespoke objects or replacement components in a wide range of marine industries, engineering, and research. 3D-printed objects are subject to marine biofouling, impacting their operation and longevity. Application of antifouling paints or coatings adds costly and time-consuming steps and may interfere with the function of fine surface features, counteracting some of the benefits of 3D-printing technology. We measured the antifouling performance of two 3D-printing thermoplastics embedded with antifouling biocides to create 3D-printed materials with inherent antifouling properties: 1) polycaprolactone (PCL) mixed with the organic biocide dichlorooctylisothiazolinone (DCOIT) and extruded as 3D-printing filament, and 2) a commercial polylactic acid (PLA) 3D-printing filament with embedded copper powder. Settlement plates printed from these thermoplastics (“PCL-DCOIT” and “PLA-Cu”, respectively) and deployed in temperate, coastal marine water for 17 weeks during summer remained free of macrofouling. A biofilm developed, and 16S and 18S rRNA metabarcoding analyses revealed that early stage biofilms (at 5 and 12 weeks) had dramatically altered assemblage structures of both prokaryotes and eukaryotes compared to natural biofilms. The assemblage on PCL-DCOIT had reduced microbial diversity, strong dominance of Proteobacteria and chlorophytes, and almost complete absence of Flavobacteriia, Cyanobacteria, and diatoms. In contrast, the biofilm on PLA-Cu had a dominance of Flavobacteriia over Proteobacteria, and resistance to chlorophytes, yet similar to PCL-DCOIT it resisted Cyanobacteria and diatoms. Such alterations to biofilm microbial assemblages could influence microbial dynamics, biofilm growth, and settlement cues to which biofouler propagules respond. At 17 weeks, the two biocide-embedded thermoplastics completely resisted macrofouling, equally well as three commercial antifouling coatings (Intercept 8500, Hempaguard X7, Hempasil X3); however, PCL-DCOIT was more extensively covered by a microalgal film (79%, evidently chlorophytes) than were the commercial coatings, and PLA-Cu had the most settled detritus (100% cover). Biofilm assemblages on the commercial coatings were investigated for comparison, with PCL-DCOIT standing out due to its almost complete resistance to Flavobacteriia. Thermoplastic 3D-printing filaments with embedded biocides show promise for producing 3D-printed objects with inherent antifouling properties, avoiding or lessening the need to apply antifouling coatings, and possibly extending their service lifetime.

Effect of infill strategy on the flexural strength of 3Y-TZP bars fabricated by thermoplastic 3D printing (T3DP)

Thermoplastic 3D printing (T3DP) is a ceramic additive-manufacturing (AM) process that uses a thermoplastic feedstock. During T3DP, parts are manufactured by rapid deposition of micron-sized droplets, which fuse and solidify to form lines that fuse further to form layers. This building logic allows modifying the direction of droplet (and line) deposition, and can affect the mechanical properties. Here, we have studied, for the first time, the effect of 0° and 90° infill strategies as well as their combination (0° + 90°) on the strength of fabricated and sintered 3Y-TZP bending bars. The 0° infill strategy had the highest characteristic strength and Weibull modulus. Unlike fused filament fabrication, the strength and Weibull modulus for the 90° infill strategy were not significantly inferior to 0° strategy, due to the better fusion of subsequent lines. However, the 0° + 90° infill strategy had the least strength due to alternating infill strategies each layer.

Natural and Synthetic Polymer Fillers for Applications in 3D Printing—FDM Technology Area

This publication summarises the current state of knowledge and technology on the possibilities and limitations of using mineral and synthetic fillers in the field of 3D printing of thermoplastics. FDM technology can be perceived as a miniaturised variation of conventional extrusion processing (a microextrusion process). However, scaling the process down has an undoubtful drawback of significantly reducing the extrudate diameter (often by a factor of ≈20–30). Therefore, the results produced under conventional extrusion processing cannot be simply translated to processes run with the application of FDM technology. With that in mind, discussing the latest findings in composite materials preparation and application in FDM 3D printing was necessary.

Synergistic material extrusion 3D-printing using core–shell filaments containing polycarbonate-based material with different glass transition temperatures and viscosities

Abstract The application of 3D printing of thermoplastics by Material Extrusion (MatEx) has commonly been limited by their poor mechanical strength that results from voids and weak interfaces between printed layers. Here, we demonstrate that core–shell structured filaments made of polycarbonate-based thermoplastics can achieve synergistic improvement in their interfacial bonding from the combination of high-glass transition temperature (T g)/high-viscosity core and low-T g/low-viscosity shell. Tensile strength along the printing direction was enhanced with the core–shell filaments. Layer-interfacial bonding strength as determined by Izod impact tests of the 3D printed parts is significantly improved by using filaments either with only a core–shell T g mismatch or both T g/viscosity core–shell mismatch. The mechanical behavior can be rationalized in terms of improved inter-layer molecule diffusion by a low T g/viscosity shell, better printability at higher temperature due to the core with higher melt strength, and better bulk mechanical strength of high-viscosity/T g core.

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.

Thermoplastic 3D printing technology using a single filament for producing realistic patient-derived breast models

Objective. This work describes an approach for producing physical anthropomorphic breast phantoms from clinical patient data using three-dimensional (3D) fused-deposition modelling (FDM) printing. Approach. The source of the anthropomorphic model was a clinical Magnetic Resonance Imaging (MRI) patient image set, which was segmented slice by slice into adipose and glandular tissues, skin and tumour formations; thus obtaining a four component computational breast model. The segmented tissues were mapped to specific Hounsfield Units (HU) values, which were derived from clinical breast Computed Tomography (CT) data. The obtained computational model was used as a template for producing a physical anthropomorphic breast phantom using 3D printing. FDM technology with only one polylactic acid filament was used. The physical breast phantom was scanned at Siemens SOMATOM Definition CT. Quantitative and qualitative evaluation were carried out to assess the clinical realism of CT slices of the physical breast phantom. Main results. The comparison between selected slices from the computational breast phantom and CT slices of the physical breast phantom shows similar visual x-ray appearance of the four breast tissue structures: adipose, glandular, tumour and skin. The results from the task-based evaluation, which involved three radiologists, showed a high degree of realistic clinical radiological appearance of the modelled breast components. Measured HU values of the printed structures are within the range of HU values used in the computational phantom. Moreover, measured physical parameters of the breast phantom, such as weight and linear dimensions, agreed very well with the corresponding ones of the computational breast model. Significance. The presented approach, based on a single FDM material, was found suitable for manufacturing of a physical breast phantom, which mimics well the 3D spatial distribution of the different breast tissues and their x-ray absorption properties. As such, it could be successfully exploited in advanced x-ray breast imaging research applications.

Environmentally Benign Flame Retardant Polyamide‐6 Filament for Additive Manufacturing

AbstractThe engineering applications of thermoplastic 3D printing filaments are currently limited by their inherent flammability, especially in fields such as automotive and aerospace, which require high standards for material safety. Polyamide‐6 (PA6) is of particular interest for additive manufacturing but is a very flammable thermoplastic known to spread fires due to its aggressive melt‐dripping. A flame retardant (FR) composite filament composed of PA6, ammonium polyphosphate, and aluminum phosphate is shown to be printable under identical conditions to commercially available PA6. Calorimetry measurements reveal that the composite filament generates a 7000% increase in char yield, along with 47% and 31% decreases in peak heat release rate and total heat release, respectively. Additionally, open flame testing demonstrates a significantly reduced capacity for this filament to spread fire to other nearby combustible materials. This unique FR additive system represents an important step toward improving the safety and utility of 3D printed parts.

Additive manufacturing of ceramics from thermoplastic feedstocks

Ceramics are a class of materials delicate to shape due to their inherent inability to plastically deform. Conventional shaping processes utilizing thermoplastic feedstock that mimic plastic deformation were successful in providing ceramic parts with high quality, but with limited shape complexity. Additive manufacturing (AM) has revolutionized the shaping of ceramics, allowing for realization of components with increased complexity and precision. This paper reviews the current state of AM technologies with emphasis on the techniques based on modelling of thermoplastic feedstocks. Namely, fused filament fabrication (FFF) is widespread and cost-effective AM technology, however, limited by relatively low resolution and poor surface finish. On the other hand, thermoplastic 3D printing (T3DP), emerged recently as a high-precision, high surface finish AM technology. With further development in these processes as well as materials that can be shaped, it is expected that thermoplastic feedstock based processes will be more prominent in AM of ceramics.

Additive-Free and Support-Free 3D Printing of Thermosetting Polymers with Isotropic Mechanical Properties.

The democratization of thermoplastic 3D printing is rooted in the ease of processing enabled by economical melting and shaping. Thermosetting polymers, on the other hand, have not enjoyed this advantage given that thermosetting resins cannot hold their shape without cross-linking or excessive fillers, and once cross-linked, they cannot be extruded for printing. Due to this formidable challenge, thus far, 3D printing of thermosetting polymers has been limited to the photopolymerization of specialized photosensitive resins or extrusion of resins loaded with large fractions (as high as 20 wt %) of rheology modifiers. Here, we report a rheology-modifier- and photoinitiator-free process for the 3D printing of a pure commercial epoxy polymer, without any resin modification and using a conventional 3D printer. A low-cost non-Newtonian support material that switches between solid-fluid states under a nozzle shear stress enables the printing of complex 3D structures and the subsequent and ″one-step″ curing. Our results show that the one-step curing eliminates the often-compromised interlayer adhesion common in layer-by-layer 3D printing processes and results in unprecedented isotropic mechanical properties (strength, elastic modulus, tensile toughness, and strain to failure). This in-bath print and cure (IBPC) 3D printing process for thermosetting polymers is low-cost, scalable, high-speed (nozzle speeds exceeding 720 cm/min), and high-resolution (down to 220 μm filament size). We demonstrate potential applications for hobbyists, structural and aerospace components, and fiber-reinforced composites, among others.

3D Printing of Metal/Metal Oxide Incorporated Thermoplastic Nanocomposites With Antimicrobial Properties.

Three-dimensional (3D) printing has experienced a steady increase in popularity for direct manufacturing, where complex geometric items can be produced without the aid of templating tools, and manufacturing waste can be remarkably reduced. While customized medical devices and daily life items can be made by 3D printing of thermoplastics, microbial contamination has been a serious obstacle during their usage. A very clever approaches to overcome this challenge is to incorporate antimicrobial metal or metal oxide (M/MO) nanoparticles within the thermoplastics during or prior to 3D printing. Many M/MO nanoparticles can prevent contamination from a wide range of microorganism, including antibiotic-resistant bacteria via various antimicrobial mechanisms. Additionally, they can be easily printed with thermoplastic without losing their integrity and functionality. In this mini review, we summarize recent advancements and discuss future trends related to the development of 3D printed antimicrobial thermoplastic nanocomposites by addition of M/MO nanoparticles.

Multi-material Ceramic-Based Components - Additive Manufacturing of Black-and-white Zirconia Components by Thermoplastic 3D-Printing (CerAM - T3DP).

To combine the benefits of Additive Manufacturing (AM) with the benefits of Functionally Graded Materials (FGM) to ceramic-based 4D components (three dimensions for the geometry and one degree of freedom concerning the material properties at each position) the Thermoplastic 3D-Printing (CerAM -T3DP) was developed. It is a direct AM technology which allows the AM of multi-material components. To demonstrate the advantages of this technology black-and-white zirconia components were additively manufactured and co-sintered defect-free. Two different pairs of black and white zirconia powders were used to prepare different thermoplastic suspensions. Appropriate dispensing parameters were investigated to manufacture single-material test components and adjusted for the additive manufacturing of multi-color zirconia components.

Multi-material Ceramic-Based Components – Additive Manufacturing of Black-and-white Zirconia Components by Thermoplastic 3D-Printing (CerAM - T3DP)

Multi-material Ceramic-Based Components – Additive Manufacturing of Black-and-white Zirconia Components by Thermoplastic 3D-Printing (CerAM - T3DP)

3D printing metals like thermoplastics: Fused filament fabrication of metallic glasses

Abstract Whereas 3D printing of thermoplastics is highly advanced and can readily create complex geometries, 3D printing of metals is still challenging and limited. The origin of this asymmetry in technological maturity is the continuous softening of thermoplastics with temperature into a readily formable state, which is absent in conventional metals. Unlike conventional metals, bulk metallic glasses (BMGs) demonstrate a supercooled liquid region and continuous softening upon heating, analogous to thermoplastics. Here we demonstrate that, in extension of this analogy, BMGs are also amenable to extrusion-based 3D printing through fused filament fabrication (FFF). When utilizing the BMGs’ supercooled liquid behavior, 3D printing can be realized under similar conditions to those in thermoplastics. Fully dense and amorphous BMG parts are 3D printed in ambient environmental conditions resulting in high-strength metal parts. Due to the similarity between FFF of thermoplastics and BMGs, this method may leverage the technology infrastructure built by the thermoplastic FFF community to rapidly realize and proliferate accessible and practical printing of metals.