Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A49: Additive Manufacturing of Soft Materials: Novel Characterization and Processing StrategiesFocus
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Sponsoring Units: DPOLY GSOFT DFD GSNP Chair: Jon Seppala, National Institute of Standards and Technology Room: BCEC 252A |
Monday, March 4, 2019 8:00AM - 8:12AM |
A49.00001: In-situ X-ray and thermal imaging of 3D printed PLA Miriam Rafailovich, Yuval Shmueli, Jiaolong Jiang, Yuchen Zhou, Guangcui Yuan, Sushil K Satija, Sungsik Lee, Taejin Kim, Gad Marom, Dilip Gersappe In-situ WAXS together with FLIR imaging was performed during 3D FDM printing of PLA filaments. The results highlighted the importance of the temperature profiles during printing on the structure/property relationships of the samples. Printing along short axis resulted in increased thermal retention, higher degrees of crystallinity and mechanical strength relative to samples printed along the long axis. Neutron reflectivity used to construct a model of the interdiffusion between filaments as a function of time and temperature. Lattice Boltzmann calculations were used determine the temperature of the filament as a function of nozzle temperature and extrusion speed. The thermal conduction between filaments in the vertical and horizontal direction was measured at four different nozzle temperatures and the interdiffusion was determined by scanning electron microscopy. The data showed large difference during printing between adjacent filaments. Fusion, was shown to occur when the diffusion length exceeded Rg of PLA, occurred first in horizontal direction when the nozzle temperature exceeded 215°C and in the vertical direction when it exceeded 245°C. |
Monday, March 4, 2019 8:12AM - 8:24AM |
A49.00002: XPCS in operando Monitoring of Dynamic Recovery in 3D Printed Thermoset Nanocomposites Kyle Johnson, Lutz Wiegart, Andrew Abbott, Hilmar Koerner Additive manufacturing has made big strides in metals and polymer-based material processing applications; however, the processing of structural composites is still a challenge that requires a number of technical engineering and material improvements. State of the art, off the shelf, feedstock materials do not offer sufficient thermal and mechanical properties to compete with resin systems that are currently used in conventional polymer matrix composites. The goal of this work is to capture multiscale and temporal morphology and dynamics within thermosetting composite inks to get a better understanding of parameters that govern the printing process and establish a processing-structure-performance relationship that enables scientists and engineers to design optimal materials and processes to obtain additively manufactured parts with properties that, at minimum, come close to those known for conventional polymer matrix composites. Herein we use X-ray Photon Correlation Spectroscopy to reveal both morphology and dynamics of a thermoset composite ink composed of a rheology modifier (layered-silicate Cloisite 30B) and epoxy resin (EPON 826) during the printing process in real time and at critical locations along the print processing path. |
Monday, March 4, 2019 8:24AM - 8:36AM |
A49.00003: Processing, Morphology, and Crosslink Network in Model Liquid Crystalline Thermosets for Additive Manufacturing Edward Trigg, Hilmar Koerner The additive manufacturing (AM) of high-temperature thermosetting resins would enable rapid functional prototyping and on-demand production of high-performance aircraft parts; however, limitations of current feedstock materials have prevented AM of such parts to date. The use of liquid crystalline thermosetting polymers could overcome several critical challenges in fused deposition modeling, such as tuning the rheology during print and cure and improving final mechanical properties. The shear-alignment of liquid crystal domains by the nozzle could also allow for the controlled design of anisotropic properties. Here, a model liquid crystal thermoset system based on epoxide chemistry is studied with respect to AM processing. We examine the effect of shear on morphology before, during, and after crosslinking, both in a controlled, uniform shear environment and at the printing nozzle. We explore how viscosity and final morphology (and therefore final properties) can be controlled by exploiting liquid crystallinity. |
Monday, March 4, 2019 8:36AM - 9:12AM |
A49.00004: Feedstock development and in-operando experiments for 3D printing of polymer matrix composites for demanding defense applications Invited Speaker: Hilmar Koerner Additive manufacturing, especially extrusion-based 3D printing, plays a critical role in areas where traditional composite manufacturing is too labor intensive, costly or even impossible. The ability to produce previously un-manufacturable designs combined with attractive economic and lead time benefits has led to significant interest in industry and government to invest in polymer-based 3D printing. Complexity enabled capabilities, part reduction and rapid prototyping are key drivers for DoD applications. While the current state-of-the-art in engineering solutions is progressing at a fast pace, the physics-based understanding of the process is lacking and at best poorly implemented in commercial machines. This inevitably leads to the issue of poor reproducibility with vastly different results from the same equipment conducted in different laboratories, between two pieces of the same equipment within a single laboratory and even between builds in different build plate locations of the same equipment. The fast, non-equilibrium processing space necessitates implementation of novel in-situ metrology controls. In addition, demanding applications that require materials to survive higher temperatures and survive extreme conditions with optimal thermo-oxidative stability are not available in commodity polymer feedstock. This presentation summarizes our efforts in designing and creating new feedstock materials for polymer matrix composite manufacturing via fused deposition modeling (high temperature thermosets) or direct write processes (carbon fiber reinforced epoxy thermosets), including advanced concepts for real-time, in-operando characterization of materials during the additive manufacturing process. The ultimate goal is to provide real-time data for closed-loop feedback control that leads to a sufficiently robust process. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A49.00005: In-situ X-ray and thermal characterization of nanocomposites in FDM 3D printing Yuval Shmueli, Sungsik Lee, Taejin Kim, Gad Marom, Dilip Gersappe, Miriam Rafailovich In-situ synchrotron WAXS simultaneously with high resolution infra-red imaging were used to study the correlation between the extrusion parameters, the filaments deposition directionality and the internal structure of the nanocomposite in 3D printing by placing an “open-walled” FDM printer in the beamline. We used microbeam synchrotron SAXS to study the variance in the crystalline macrostructure formed as function of radial position in the filaments (from core to adjacent interfaces). We used PLA and PP as the polymeric matrix and graphene nano-platelets and hexagonal boron nitride as the fillers due to their excellent mechanical properties and the potential in thermal management applications. We observed the effect of extrusion shear forces on the orientation of the nanoparticles and the influence of the particle/polymer interactions on the polymer crystallization. We show how thermal properties improved by directionality and transcrystallization. We used Raman, electron microscopy and rheological techniques to study the interactions between the polymer matrix and the nanoparticles. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A49.00006: Molecular weight dependence of weld formation in material extrusion additive manufacturing Jon Seppala Material extrusion (MatEx) additive manufacturing (AM), after several decades of development, is now an established production method for small volume or highly complicated parts. While MatEx has transitioned from prototyping to end use production, little is known about the mechanisms that dominate strength development between layers. Previously we reported on a framework for determining weld time and weld strength of MatEx processed welds, comparing those results to traditional polymer-polymer weld formation. Here we extend that work by systematically varying the weight average molecular weight (Mw) of entangled bisphenol-A-polycarbonate (PC) MatEx filaments and measuring weld time and weld strength. The resulting weld formation will be discussed in the context of traditional polymer-polymer welding and weld thickness during the unique shear and thermal history produced by the MatEx process. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A49.00007: Geometrical and Mechanical Characterization of Interlayer Bonding Quality in Fused Filament Fabrication Lichen Fang, Yishu Yan, Ojaswi Agarwal, Kevin Hemker, Sung Kang Fused filament fabrication (FFF) is one of the most popular additive manufacturing processes. However, advanced applications of FFF are still limited by the large variation of mechanical property and mesoscale structural geometry of printed parts. To develop a fundamental understanding of those issues, we focus on the interlayer bonding region of Polycarbonate FFF samples, and perform full 3D geometrical characterizations using X-ray micro computed tomography (Micro-CT). The result reveals large geometry variance brought by different printing conditions, including parameters like nozzle movement speed and layer thickness, and environmental factors like chamber temperature and humidity. The findings are further validated by mechanical peel/tear tests, showing the direct relationship between bonding zone geometry and bonding strength. The outcomes could guide printing parameter selections, as well as provide validations for future theoretical and simulation models. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A49.00008: Effect of processing on semicrystalline morphology in the additive manufacturing of poly(lactic acid) Anthony Kotula, Jonathan Seppala, Claire McIlroy Material extrusion additive manufacturing processes force molten polymer through a printer nozzle at high (>100 s−1) wall shear rates prior to cooling and crystallization. These high shear rates can lead to flow-induced crystallization in common polymer processing techniques, but the magnitude and importance of this effect is unknown for additive manufacturing. Of critical importance in semicrystalline polymer additive manufacturing is the semicrystalline morphology near the weld between extruded layers, which can affect mechanical properties. Here, we present a systematic study of printing conditions on the semicrystalline morphology of parts printed from poly(lactic acid) (PLA). The slow crystallization kinetics of PLA generate parts with low crystallinity, however a secondary annealing process at high temperatures (140 °C) generate a space-filled spherulitic texture, with smaller spherulites near the weld zone between the extruded layers. This spherulite size distribution is attributed to a higher nucleation density templated into the part by the temperature and deformation history of the printing process. We show that theoretical modeling of flow-induced crystallization processes can predict the spatial distribution of spherulites observed in experiment. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A49.00009: Flow-Enhanced Crystallisation in Fused Filament Fabrication Claire McIlroy, Anthony Kotula, Jonathan Seppala, Richard Stephen Graham The most common 3D printing method is known as fused filament fabrication (FFF). This process involves melting a thermoplastic, followed by layer-by-layer extrusion, cooling and re-solidification. The main concern with FFF is the strength at the welds between printed filaments; bulk strength is never achieved in these regions and the reason is currently unclear. Advancing FFF relies on a molecular understanding of how thermoplastics behave during the printing process. We employ a non-isothermal molecularly-aware model for FFF processing of a semi-crystalline polymer melt to show how typical FFF conditions can stretch the polymer molecules prior to cooling. Enhanced nucleation due to residual polymer stretch leads to accelerated crystallisation times at the surface of a deposited filament, whilst the bulk of the filament is governed by slower quiescent kinetics. Consequently we find a cross-sectional variation in the crystal morphology of single filament, with smaller spherulites forming in an outer skin layer. Furthermore, our model of a multi-filament walls reveals significant variations in crystal morphology from filament to filament. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A49.00010: Implications of Crystallization on the Performance of Polyphenylene Sulfide with Material Extrusion Additive Manufacturing Meisha Shofner, Emily Fitzharris, David Rosen Among the additive manufacturing (AM) techniques available, material extrusion additive manufacturing (MEAM) has been widely researched due to its increased availability and relatively simple manufacturing method. This AM method has generally been limited to amorphous polymeric materials as feedstocks, but more recent research has investigated high performance semicrystalline polymers as potential feedstocks with varying levels of success. In this work, we have used two techniques to evaluate how the crystallization of a promising feedstock, polyphenylene sulfide (PPS), affects its use with MEAM. We have used fast scanning calorimetry (FSC) to more fully understand how the crystallization of PPS is affected by cooling rates relevant to MEAM. Additionally, we have used the Taguchi method to analyze the mechanical properties of printed parts when exposed to post-processing heat treatment steps. Overall, the results showed that PPS can be used effectively with MEAM and that its crystallization kinetics provide unique benefits to its processing with MEAM. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A49.00011: Facilitating Improved Isotropy in Fused Deposition Modeling Utilizing UV Initiated Reactive Processing Neiko Levenhagen, Mark Dadmun Minimizing anisotropy in parts prepared by fused deposition modeling (FDM) remains a key area of research in the development of robust and mechanically useful 3D printed objects. Due to the bulky nature of polymer chains and the complex thermal environment experienced by adjacent filaments, interaction of polymer chains in between layers is minimized. Weak interfaces and poor layer adhesion results. In recent years, our group has addressed these issues through the introduction of low molecular weight surface segregating additives (LMW-SuSAs). LMW-SuSAs are smaller than the polymer chains of the neat material and can more readily diffuse and entangle in adjacent layers. In the current research, we report, bimodal blends containing linear and 3-arm PLA LMW-SuSAs terminated with methacrylate groups and crosslinked by UV irradiation. In situ irradiation of the printed layers results in drastic increases in the transverse tensile stresses of the printed layers up to ~140% and ~200% for the linear and 3-arm LMW-SuSAs respectively. Additional experiments examine the effect of UV power on the process. By controlling the UV power, printed parts can be prepared with minimal interfilamentous voids and substantially robust interfaces. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A49.00012: Axisymmetric Simulation of Viscoelastic Filament Thinning and Laser-Induced Forward Transfer with the Oldroyd-B Model Emre Turkoz, Luc Deike, Craig Arnold A fundamental understanding of the filament thinning of viscoelastic fluids is important in practical applications such as spraying and printing of complex materials. Here, we present direct numerical simulations of the two-phase axisymmetric momentum equations using the volume-of-fluid technique for interface tracking and the log-conformation transformation to solve the viscoelastic constitutive equation. The numerical results for the filament thinning are in excellent agreement with the theoretical description developed with a slender body approximation. We show that the off-diagonal stress component of the polymeric stress tensor is important and should not be neglected when investigating the later stages of filament thinning. In addition, we use this numerical model to simulate the blister-actuated laser-induced forward transfer process, which is a nozzle-less laser-based printing technique. We reveal the effect of viscoelasticity on the ejected droplet size, derive criteria for optimum printing conditions, and compare the numerical results with experiments. |
Monday, March 4, 2019 10:48AM - 11:00AM |
A49.00013: Laser sintering of polymer particle pairs studied by in-situ visualization Prakhyat Hejmady, Ruth Cardinaels, Lambèrt van Breemen, Patrick D Anderson A novel in-house developed experimental setup is used to perform laser sintering experiments on polystyrene (PS) particle doublets while performing in-situ visualization of the sintering dynamics. From the recorded images, the evolution of the growth of the neck radius formed between both particles is analyzed as a function of time. Sintering conditions such as heating chamber temperature, laser pulse energy and duration, laser spot size and particle size are precisely controlled and systematically varied. A non-isothermal viscous sintering model is developed that allows to qualitatively predict the observed effects of the various parameters. The sintering kinetics is determined by a complex interplay between the transient rheology caused by the finite relaxation times of the polymer and the time-dependent temperature profile which also affects the polymer viscosity. The combination of a full material characterization with sintering experiments under well-defined conditions has resulted in a general understanding of the effects of material and process parameters on laser sintering. |
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