Inspired by Fused Filament Fabrication (FFF) Additive Manufacturing (AM), we use Molecular Dynamics (MD) simulations to investigate the early stages of the formation of the weld between two polymer films prepared at different temperatures - one above and one below the dilatometric glass transition temperature. We identify three stages of welding: (i) surface approach and formation of the initial contact, (ii) surface adjustment, and (iii) interdiffusion. Surface interactions affect film roughness, polymer conformation, and interfacial temperature during the initial stage. As the two layers come into contact, heat transfer equilibrates the system in an asymmetric way: the hot film cools down more slowly than the cold film heats up. When the films are allowed to exchange heat with the environment, most of the effects of the temperature difference at the interface terminate during the initial surface adjustment, before polymer interdiffusion begins at around the bulk Rouse time. However, if the films are isolated, the onset of interdiffusion occurs earlier for films prepared at different temperatures compared to films prepared at the same temperature. This indicates the importance of thermal relaxation across the interface between welding films, and suggests mechanisms to improve the weld strength.

Material extrusion is a common additive manufacturing process that subjects polymers to non-steady deformation and thermal processing to build a customized part. The mechanical properties of these parts are often worse than those of injection-molded specimens due to failures at or near the weld zone between extrudate layers. Chain orientation is often cited as a contribution to mechanical weakness at the weld, and it is therefore of critical importance to develop strategies to quantify the magnitude and location of residual chain orientation as a function of printing conditions. Here we use birefringence imaging to characterize the spatial variation in residual stress and residual chain orientation in a glassy polylactide. A combination of retardance measurements and sample thickness measurements provide a measure of birefringence as a function of position. As-printed samples show a nearly uniform birefringence background of approximately 7 x 10 -5 and higher birefringence near the weld region at lower nozzle temperatures and faster printing speeds. We propose two origins to the birefringence: one due to residual chain orientation, and the other due to residual stresses that occur when the sample cools non-uniformly on the build plate. Annealing the sample at 65 degrees C (slightly above the glass transition temperature) allows us to relax the residual stress without removing the orientation-based birefringence or crystallizing the sample. The residual orientation shows a strong power-law dependence on the Weissenberg number based on the characteristic timescales for flow in the nozzle and polymer chain reptation.

Over 50% of plastic waste comes from a single class of polymers called polyolefins. Most recycling strategies fail to preserve the broad molecular mass distribution of these polyolefins, from which they derive their unique combination of processability and mechanical strength. Here, we show that incorporation of urethane-based dynamic bonds into high-density polyethylene (HDPE) can circumvent this issue, by strengthening the amorphous phase through interchain supramolecular interactions, as opposed to traditional entanglements from high molecular mass chains. We show that many key properties of these dynamic HDPE polymers, including percent crystallinity, melting temperature, lamellae and amorphous thicknesses, and bond association, are determined by the distribution of bond-to-bond spacings along the chain backbone and are predicted using polymer physics theories. Moreover, we find that a mixed backbone HDPE dynamic polymer exhibits mechanical properties that exceed HDPE and approach the strain-hardening behavior seen in ultrahigh molecular weight polyethylene along with a unique display of long-range supramolecular order that persists in the melt state. This work illuminates key principles governing how the placement of dynamic bonds influences bulk material properties and provides a framework for toughening semicrystalline polymers and designing chemical recycling processes based on controlling bond-spacing distributions.

The chemical gel point that occurs in the resin matrix of thermoset composites is crucial to the design of manufacturing process parameters. However, the formation of a physical network of filler material can affect the viscoelastic response of the composite so strongly that conventional rheological indicators of the chemical gel point (like power-law stress relaxation) are no longer observed. Additionally, many industrially relevant thermoset composites have a small linear viscoelastic region, limiting the utility of the high-strain multiwave measurement approach that was developed to monitor the frequency-dependent behavior of rapidly evolving materials. Here, we pair frequency-dependent properties obtained by low-strain Optimally Windowed Chirp (OWCh) measurements with existing rheology-conversion relationships to apply time-cure superposition to the loss tangent of epoxy-amine resins filled with unreactive particles. We show that this advanced rheological approach allows us to track the relative change in the relaxation time, providing another way to identify the elusive chemical gel point of these thermoset composites. The results allow us to assess the applicability of several gel point criteria to crosslinking composites, including the G'-G” crossover, frequency-independent tan δ, peak in tan δ, and divergence of the relaxation time. Investigation of a resin with a weak filler network reveals all of these gel point criteria, with the frequency-independent tan δ providing the best agreement with the easily identifiable gel point of the neat resin. For resins with a strong physical filler network, frequency independence of tan δ does not occur, but the divergence of the relaxation time matches the gel point of the neat resin and the peak in tan δ.

High-crystal-quality nanoferrites with short surface ligands (oleic acid) were recently shown to exhibit enhanced sensitivity and spatial resolution, likely due to chain formation (uniaxial assemblies of particles) for magnetic particle imaging (MPI). Here, we develop a simple one-pot thermal decomposition approach to produce ferrite (iron oxide) magnetic nano-objects (MNOs) that strongly interact magnetically and have good synthetic reproducibility. The ferrite MNOs were physically characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and dynamic light scattering. The MNOs were magnetically characterized by magnetometry and magnetic particle spectroscopy (MPS) to study their interactions, dynamics, and suitability for spatially resolved magnetic thermometry. The MNOs were synthesized in a range of sizes between 12 nm and 27 nm, showing that MNOs below a minimum size do not exhibit dynamic interactions/significant increased response and that a larger field is required for chain formation as size increases. In addition to size effects, we explore the role of ligand length, environment (liquid vs solid), and concentration on the proposed chain formation. The experimental results were then correlated to micromagnetic simulations to gain further insight into the formation of chains. Compared to some existing MPI tracers, our ferrite MNOs exhibit enhanced signal (up to about 37×) and spatial resolution (up to about 9×) under certain limited (ferrite-MNO optimal) field and frequency conditions used. MPS as a function of temperature and drive field amplitude was performed, showing promise for spatially resolved thermometry. These results confirm the importance of tuning the frequency and amplitude of the drive field for optimal imaging/thermal performance.

The Additive Manufacturing Benchmark Test Series (AM Bench) provides rigorous measurement data for validating additive manufacturing (AM) simulations for a broad range of AM technologies and material systems. AM Bench includes extensive in situ and ex situ measurements, simulation challenges for the AM modeling community, and a corresponding conference series. In 2022, the second round of AM Bench measurements, challenge problems, and conference were completed, focusing primarily upon laser powder bed fusion (LPBF) processing of metals, and both material extrusion processing and vat photopolymerization of polymers. In all, more than 100 people from 10 National Institute of Standards and Technology (NIST) divisions and 21 additional organizations were directly involved in the AM Bench 2022 measurements, data management, and conference organization. The international AM community submitted 138 sets of blind modeling simulations for comparison with the in situ and ex situ measurements, up from 46 submissions for the first round of AM Bench in 2018. Analysis of these submissions provides valuable insight into current AM modeling capabilities. The AM Bench data are permanently archived and freely accessible online. The AM Bench conference also hosted an embedded workshop on qualification and certification of AM materials and components.

Purpose A main cause of defects within material extrusion (MatEx) additive manufacturing is the nonisothermal condition in the hot end, which causes inconsistent extrusion and polymer welding. This paper aims to validate a custom hot end design intended to heat the thermoplastic to form a melt prior to the nozzle and to reduce variability in melt temperature. A full 3D temperature verification methodology for hot ends is also presented. Design/methodology/approach Infrared (IR) thermography of steady-state extrusion for varying volumetric flow rates, hot end temperature setpoints and nozzle orifice diameters provides data for model validation. A finite-element model is used to predict the temperature of the extrudate. Model tuning demonstrates the effects of different model assumptions on the simulated melt temperature. Findings The experimental results show that the measured temperature and variance are functions of volumetric flow rate, temperature setpoint and the nozzle orifice diameter. Convection to the surrounding air is a primary heat transfer mechanism. The custom hot end brings the melt to its setpoint temperature prior to entering the nozzle. Originality/value This work provides a full set of steady-state IR thermography data for various parameter settings. It also provides insight into the performance of a custom hot end designed to improve the robustness of melting in MatEx. Finally, it proposes a strategy for modeling such systems that incorporates the metal components and the air around the system.

To describe a new method for the automatic generation of process parameters for fused filament fabrication (FFF) across varying machines and materials. We use an instrumented extruder to fit a function that maps nozzle pressures across varying flow rates and temperatures for a given machine and material configuration. We then develop a method to extract real parameters for flow rate and temperature using relative pressures and temperature offsets. Our method allows us to successfully find process parameters, using one set of input parameters, across all of the machine and material configurations that we tested, even in materials that we had never printed before. Rather than using direct parameters in FFF printing, which is time-consuming to tune and modify, it is possible to deploy machine-generated data that captures the fundamental phenomenology of FFF to automatically select parameters.

The tear test is widely used to measure the fracture toughness of thin rubber sheets and polymer films. More recently, the tear test has been applied to polymer materials produced by melt extrusion additive manufacturing to measure the fracture toughness of a single weld between two printed (extruded) filaments. This paper presents a finite element modeling study of the tearing of a weld between two printed filaments to investigate the mechanics of the tear test and the effects of geometry and material properties on the measured tear energy. The mechanical behavior of the printed filaments was described by a viscoplastic model for glassy polymers and the weld was represented using cohesive surface elements and the Xu-Needleman traction-separation relationship. The geometric model and the material parameters were chosen based on experimental measurements. The tear energy varied with the specimen dimensions, the curvature of the printed filaments, the yield stress relative to the cohesive strength of the weld, and the post-yield stress drop. The effects of the hardening modulus were small. These factors altered the viscoplastic dissipation in the material ahead of the propagating crack tip. The results showed that viscoplastic dissipation could constitute a large fraction of the tear energy and is strongly affected by the specimen dimensions and the geometry and material properties of the printed filament. There was also considerable mode mixty in the tear energy. The findings can be used to design tear tests to measure the intrinsic fracture toughness of the weld.

Die-swell is a flow effect that occurs in polymer extrusion whereby the material experiences rapid stress and dimensional changes upon exiting the nozzle orifice. Material extrusion additive manufacturing is no exception, and this effect influences the final dimensions of the printed road and imparts residual stresses. Die-swell is measured via a custom test cell that uses optical and infrared cameras and an instrumented hot end with an infeed pressure load cell. The instrumented hot end is mounted onto a stationary extruder above a conveyor to simulate printhead translation at steady state conditions for a wide range of volumetric flow rates. Investigated factors for an acrylonitrile butadiene styrene (ABS) filament include volumetric flow rate (0.9 mm3/s to 10.0 mm3/s), hot end temperature setpoint (200-250 degrees C), and nozzle orifice diameter (0.25-0.60 mm). The die-swell increases as a function of the volumetric flow rate and shear stress but decreases as a function of the hot end temperature setpoint and nozzle orifice diameter. For modelling, an implementation of the Tanner model for dieswell displays good agreement with experimental results. The model also demonstrates that the same proportionality constant, kN1, which relates first normal stress difference to shear stress, can be used for different nozzle orifice diameters with the same length to diameter ratios, and that kN1 increases as a function of hot end temperature setpoint as expected with the rheological concept of time temperature superposition.

The explosive growth of additive manufacturing (AM) is matched by an equally strong push in research and development to support those applications. The relative complexity of these manufacturing processes and the materials they generate still elicit a great need for better understanding of the interactions between the fabrication parameters (e.g., material deposition rate, applied thermal energy, etc.) and the material development and evolution. Complex problems require complex tools, and with the boom in AM, the development of computational models and simulations to predict all aspects of these processes has grown in parallel. Incredible diversity exists in the types of models, the level of physical or computational complexity, the range of materials, and applications. But all require something similar: input of or reference to relevant physical values achieved through accurate measurements. Measurements are essential, not just in the development of AM models, but in the testing and validation of their predictions.

Embedded 3D printing enables the manufacture of soft, intricate structures. In the technique, a nozzle is embedded into a viscoelastic support bath and extrudes filaments or droplets. While embedded 3D printing expands the printable materials space to low-viscosity fluids, it also presents new challenges. Filament cross-sections can be tall and narrow, have sharp edges, and have rough surfaces. Filaments can also rupture or contract due to capillarity, harming print fidelity. Through digital image analysis of in situ videos of the printing process and images of filaments just after printing, we probe the effects of ink and support rheology, print speeds, and interfacial tension on defects in individual filaments. Using model materials, we determine that if both the ink and support are water-based, the local viscosity ratio near the nozzle controls the filament shape. If the ink is slightly more viscous than the support, a round, smooth filament is produced. If the ink is oil-based and the support is water-based, the capillary number, or the product of the ink speed and support viscosity divided by the interfacial tension, controls the filament shape. To suppress contraction and rupture, the capillary number should be high, even though this leads to trade-offs in roughness and roundness. Still, inks at nonzero interfacial tension can be advantageous, since they lead to much rounder and smoother filaments than inks at zero interfacial tension with equivalent viscosity ratios.

Extrusion-based bioprinting is a powerful tool for fabricating complex cell-laden constructs. Embedded ink writing (EIW) is an extrusion-based printing technique wherein a nozzle embedded into a support bath writes continuous filaments. Because it allows for low-viscosity inks, EIW is particularly useful for bioprinting. One of the largest challenges in extrusion-based bioprinting is limiting the damage that cells experience inside the nozzle. Longer shear stress durations and higher shear stress magnitudes lead to more damage. Shape fidelity is also critical for bioprinting. Filaments in EIW can exhibit defects such as sharp edges and large aspect ratios, which can lead to porosity, surface roughness, and poor mechanical properties in the final part. We use numerical computational fluid dynamics simulations in OpenFOAM to evaluate whether common shear stress mitigation techniques improve cell viability without causing shape defects. Critically, we find that using a conical nozzle, increasing the nozzle diameter, decreasing the print speed, and decreasing the ink viscosity can improve the viability of stress magnitude-sensitive cells, but using a conical nozzle, increasing the nozzle length, and decreasing the print speed can increase damage in stress duration-sensitive cells. Additionally, using a conical nozzle or a larger nozzle can lead to larger shape defects in printed filaments. Material selection and printing parameter selection in embedded bioprinting should take into account allowable shape defects, allowable cell damage, and cell type.

Embedded 3D printing, wherein fluid inks are extruded into support baths, has enabled the manufacture of complex, custom structures ranging from cell-laden tissue analogues to soft robots. This method encompasses two techniques: embedded ink writing (EIW), where filaments are extruded, and embedded droplet printing (EDP), where droplets are suspended. Materials for embedded 3D printing can be Newtonian, but often both the ink and the support bath are yield stress fluids, following elastic behavior below the yield stress and shear-thinning, viscous behavior above the yield stress. The effect of surface tension on print quality has been debated, as inks have been printed into supports at high and low surface tensions. In order to guide material selection for embedded 3D printing and identify key scaling relationships that influence print quality, this study investigates the role of ink rheology, support rheology, and surface tension on the morphology of single filaments. Numerical simulations in OpenFOAM demonstrate that at low viscosities, surface tension controls the filament morphology. Where capillarity is suppressed, the ratio of the local ink and support viscosities and the shape of the yield surface in the support control the filament shape. Herschel-Bulkley support fluids (yield stress fluids) produce more stable, accurately positioned filaments than Newtonian supports. In the short term, non-zero surface tensions can suppress filament shape defects in EIW and are essential for producing droplets in EDP.

The elastic response of homogeneous isotropic materials is most commonly represented by their Young's modulus (E), but geometric variability associated with additive manufacturing results in materials that are neither homogeneous nor isotropic. Here we investigated methods to estimate the effective elastic modulus (Eeff) of samples fabricated by fused filament fabrication. We conducted finite element analysis (FEA) on printed samples based on material properties and CT-scanned geometries. The analysis revealed how the layer structure of a specimen altered the internal stress distribution and the resulting Eeff. We also investigated different empirical methods to estimate Eeff as guides. We envision the findings from our study can provide guidelines for modulus estimation of as-printed specimens, with the potential of applying to other extrusion-based additive manufacturing technologies.

3D printing is a popular fabrication technique because of its ability to produce complex architectures. Melt-based 3D printing is widely used for thermoplastic polymers like poly(caprolactone) (PCL), poly(lactic acid) (PLA), and poly(lactic-co-glycolic acid) (PLGA) because of their low processing temperatures. However, traditional melt-based techniques require processing temperatures and pressures high enough to achieve continuous flow, limiting the type of polymer that can be printed. Solvent-cast printing (SCP) offers an alternative approach to print a wider range of polymers. Polymers are dissolved in a volatile solvent that evaporates during deposition to produce a solid polymer filament. SCP, therefore, requires optimizing polymer concentration in the ink, print pressure, and print speed to achieve desired print fidelity. Here, capillary flow analysis shows how print pressure affects the process-apparent viscosity of PCL, PLA, and PLGA inks. Ink viscosity is also measured using rheology, which is used to link a specific ink viscosity to a predicted set of print pressure and print speed for all three polymers. These results demonstrate how this approach can be used to accelerate optimization by significantly reducing the number of parameter combinations. This strategy can be applied to other polymers to expand the library of polymers printable with SCP.

Amorphous polymers exhibit a viscoplastic strain hardening behavior at large strains. To describe this hardening behavior, we have developed an effective temperature model for the nonequilibrium behavior of amorphous polymers that incorporate the effects of network orientation and relaxation at large plastic deformation. The development of network orientation is introduced as a backstress that produces kinematic hardening in the stress response, while network relaxation describes the effects of temperature and strain rate on the hardening response. The model was applied to simulate the thermomechanical behavior of polycarbonate (PC) to determine the model parameters from standard dynamic frequency sweep and differential scanning calorimetry tests. The simulation results showed that the model can quantitatively capture the dependence of the hardening modulus on strain, strain rate, and temperature, as well as the unloading and reloading behavior measured in uniaxial compression tests. We applied the model to investigate the effect of plastic dissipation on the peeling of a polymer filament from a rigid substrate to guide the design of peel tests to measure the intrinsic fracture toughness of polymers fabricated by melt extrusion additive manufacturing processes.

Biofabrication has been adapted in engineering patient-specific biosynthetic grafts for bone regeneration. Herein, we developed a three-dimensional (3D) high-resolution, room-temperature printing approach to fabricate osteoconductive scaffolds using calcium phosphate cement (CPC). The non-aqueous CPC bioinks were composed of tetracalcium phosphate, dicalcium phosphate anhydrous, and Polyvinyl butyral (PVB) dissolved in either ethanol (EtOH) or tetrahydrofuran (THF). They were printed in an aqueous sodium phosphate bath, which performs as a hardening accelerator for hydroxyapatite formation and as a retainer for 3D microstructure. The PVB solvents, EtOH or THF, affected differently the slurry rheological properties, scaffold microstructure, mechanical properties, and osteoconductivity. Our proposed approach overcomes limitations of conventional fabrication methods, which require high-temperature (>50 °C), low-resolution (>400 μm) printing with an inadequate amount of large ceramic particles (>35 μm). This proof-of-concept study opens venues in engineering high-resolution, implantable, and osteoconductive scaffolds with predetermined properties for bone regeneration.

Polymer chain orientation is crucial to understanding the polymer dynamics at interfaces formed during thermoplastic material extrusion additive manufacturing. The flow field and rapid cooling produced during material extrusion can result in chains which are oriented and stretched, which has implications for interdiffusion and crystallization. Polarized Raman spectroscopy offers a non-destructive and surface sensitive method to quantify chain orientation. To study orientation and alignment of chains in 3D printed polycarbonate filaments, we used a combination of polarized Raman spectroscopy and birefringence ( $$\Delta n$$ ) measurements. By changing the orientation of the sample with respect to polarization of incident radiation, we probe changes in the ratio between orientation-dependent vibration modes and orientation-independent modes. We used principal component analysis (PCA) and partial least squares (PLS) regression to develop correlations for birefringence and Raman measurements in samples that were pulled at different draw ratios (DRs). PCA was used to differentiate between orientation-dependent and orientation-independent modes, while PLS regression was used to calculate birefringence from Raman measurements of 3D printed samples. Birefringence measurements were compared to the polycarbonate intrinsic birefringence of 0.2, to estimate the degree of orientation. We find that measured values of birefringence underestimate orientation compared to Raman measurements.