SPE Library

The SPE Library contains thousands of papers, presentations, journal briefs and recorded webinars from the best minds in the Plastics Industry. Spanning almost two decades, this collection of published research and development work in polymer science and plastics technology is a wealth of knowledge and information for anyone involved in plastics.

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Conference Proceedings

Novel Approach to Improve the Performance of TPV via Self-Nano Reinforced Network Structure
Guralp Ozkoc, Ph.D., March 2023

Thermoplastic vulcanizates (TPVs) are highperformance polymeric materials classified as thermoplastic elastomers, and contain a continuous thermoplastic matrix with crosslinked elastomers as a dispersed phase. TPVs combine the high elasticity of crosslinked elastomers and the easy processability and recyclability of thermoplastics. The most widely produced TPV type is polypropylene (PP)/ethylene propylene diene monomer (EPDM), which is the focus of this study. In this study, polyhedral oligomeric silsesquioxane nanoparticles containing reactive side groups were used as coagents for PP/EPDM TPV system for the first time in the literature. The peroxide crosslinked PP/EPDM/POSS system was dynamically vulcanized in a lab-scale micro-compounder. The mechanical properties of samples were determined by tensile, hardness, and compression set analyses. Scanning electron microscope (SEM) and atomic force microscope (AFM) were used to evaluate the phase morphology. The results showed that nano-reinforced network structure improved the performance of the TPV materials.

In-Situ Rubber Nanofibrillation of Compatibilized High-Density Polyethylene/Thermoplastic Polyester Ether Elastomer With Superior Melt Strength and Foamability
Chul Park, Ph.D., March 2023

High-density polyethylene (HDPE) exhibits poor melt strength which limits its widespread application especially where it is exposed to an elongational deformation flow in processes such as film blowing, melt spinning, and foaming. In this study, by taking advantage of in-situ nanofibrillation of thermoplastic polyester ether elastomer (TPEE) in HDPE matrix, we improved the rheological properties as well as the foamability of HDPE. TPEE consists of a hard crystalline segment of polybutylene terephthalate (PBT) and a soft amorphous segment of polyether. The polarity of these two groups causes TPEE to be thermodynamically incompatible with non-polar HDPE. Therefore, styrene/ethylene-butylene/styrene copolymer grafted maleic anhydride (SEBS-g-MA) as a compatibilizer was used for reducing the interfacial tension between two blend components. In the first step, a 10% masterbatch of HDPE/TPEE with and without compatibilizer was prepared employing a twin screw extruder. Next, to fabricate fiber-in-fiber composites, the 10% masterbatch was diluted and processed by spunbonding. Scanning electron microscopy (SEM) revealed that not only the spherical size of HDPE/TPEE decreased significantly after SEBS-g-Ma inclusion, but also a much smaller TPEE nanofiber size (60-70nm for 5%TPEE) was achieved. Moreover, the extensional rheological results showed strain-hardening behavior for both compatibilized and non-compatibilized stretched samples at earlier times, at a given extensional rate, compared to the unstretched counterparts. It is worth mentioning that the improvement of extensional rheological properties was more pronounced for compatibilized samples compared to the non-compatibilized ones. This can be attributed to smaller nanofiber size and consequently higher aspect ratio as well as a more robust 3D fibrillated network. Finally, batch foaming was conducted to investigate the foamability of fibrillated nanocomposites.

Chemical Formulary, Dual UV & Heat Curability, and Thermo Property & Behavior of a Medical-Grade, UV-Curable Epoxy Adhesive
Xiaoping Guo, Ph.D., March 2023

In the present paper, we have studied thermal properties and thermo-chemical stability of a medical-grade adhesive comprised of a cationic, cycloaliphatic epoxy resin system by using differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) techniques. Then, we have explored UV curability of the adhesive by performing a series of the UV-cure experiments using special photo- DSC (or p-DSC) technique and investigated relevant relationship of resultant thermal properties and thermochemical stability of such UV-cured adhesive materials with the underlying UV irradiances during UV curing. Thereafter, we have further examined thermal curability for various post-UV cured adhesive materials by conducting a series of the thermal-cure experiments and measured the ultimate glass transition temperatures of resultant adhesive materials at various “fully-cured” states with using a conventional DSC technique. According to these thermal analysis tests, p-DSC UV-cure experiments, and DSC thermal-cure experiments, we are able to thoroughly understand effects of UV irradiances applied during UV curing on dual UV-thermal curability and resultant thermal properties of various resultant adhesive materials at the “fully-cured” solid states to provide pertinent scientific insights on relevant adhesive handling and processing operations in making medical devices.

Justification of a Molten Polymer Process Change to a Legally Marketed Medical Device Via Comparative Statistical Analysis of Thermal Stabilities of Material
Xiaoping Guo, Ph.D., March 2023

In an attempt to attest and justify that a polymeric medical device as possibly affected by some polymer process change(s) of device manufacturing would be substantially equivalent to relevant legally-marketed counterpart in view of its biological safety and functional effectiveness, a practical approach for statistically evaluating inherent thermo-chemical stability of polymer, namely activation energy of thermal degradation that is closely dependent of the underlying thermal history of device manufacturing, is proposed in compliance to relevant regulations and industrial guidelines. Accordingly, a series of thermogravimetric analysis (TGA) experiments can be comparatively conducted per ASTM E1641 standard practice and then kinetically studied to determine the measured activation-energy means for some “test” polymer samples taken from affected device, compared to some “control” polymer samples taken from relevant legallymarketed device, using the so-called Ozawa-Flynn-Wall analytical method. The statistical equivalence or superiority of affected device, compared to legally-marketed device, can be then technically assessed by performing the pertinent two-sample (Welch) t-test on any statistical differences in the so-measured activation-energy means between the affected and “control” polymer samples.

Multicomponent Injection Molding With Liquid Silicone Rubber (LSR) and Acrylonitrile-Butadiene-Styrene (ABS) for Medical Device Applications
Mohammad Ali Nikousaleh, Ralf-Urs Giesen, Ph.D., March 2023

Multi-component injection molding of liquid silicone rubber (LSR) with thermoplastics, such as PBT or polyamide, is used in the manufacturing process for many components in the automotive industry and in the field of sanitary technology. Due to its hypoallergenic properties, biocompatibility, and resistance to the majority of liquid medications, liquid silicone rubbers are a promising alternative material for use in medical applications. They can be used over a wide temperature range and they are physiologically well tolerated and can be sterilized in various ways. Standard thermoplastics, such as acrylonitrile butadiene styrene (ABS), cannot be overmolded with silicone rubbers in injection molding because of their low heat deflection temperature. With the right production method that combines the processing of silicone rubber and thermoplastics, it would be possible to replace the formerly expensive production and assembly of individual components. Such an integrated production technology makes it possible to realize high-performance new products economically and at the same time, to improve product safety for the patient through simplified, more highly automated and higher-quality production. In this investigation, we applied ABS grades, approved for medical applications, to show how ABS-LSR test specimens regarding the VDI guideline 2019 could be produced using variothermal mold heating and special surface treatment of ABS. Here we will show the development and challenges of a new 2C-molding technology for LSR – thermoplastic parts. For the quality of the later product, the adhesion between thermoplastic and LSR is the decisive feature and depends not only on the injection molding process, but also on the material pairing and the treatment process. Here, we succeeded to manufacture multifunctional products for medical devices through various partial pretratment methods of the thermoplastic surface. In addition, the effect of sterilization (gamma and eto) and artificial aging (humidity and temperature) and of such components on the adhesive bond is indicated.

Leveraging the Chemistry and Properties of Polycarbonate to Achieve Maximum Productivity, Lower Energy Consumption, Reduce Waste and Lower Carbon Footprint
Joshua Wagner, March 2023

In addition to polymers based on non-fossil feedstocks that help reduce carbon footprint or blends that incorporate recycled content to reduce waste, there additional strategies a manufacturer can pursue to further lower energy consumption and material usage. In this presentation, we delve into polycarbonate materials chemistry and property profile to point out cases where the material and process innovations come together to maximize productivity and lower energy consumption or even lower material consumption to reduce waste. How these fit together in a greater context of plastics manufacturers looking to be part of an emerging circular economy will also be discussed.

Investigation Of The Induced Electric Field Molecular Alignment Of Sliver-Based Solvate Ionic Liquid (SIL) And Silver Nanoparticles (AgNPs) In A Toughened Epoxy Resin Composite
Ahmed Al-Qatatsheh, Anna Sokolova, Nishar Hameed, March 2023

We report using the coordinated silver (I) complex based on SIL in a toughened epoxy resin composite to enable electrical and thermomechanical properties. The toughened epoxy resin was aligned at the molecular level utilizing an electric field, demonstrating a relatively high electric conductivity, energy storage, and rapid curing behaviour that can save energy, reduce unnecessary heat, and optimize capital and operating costs. Applying Small Angle Neutron Scattering (SANS), our work thoroughly studied the effect of alignment changes on the silver (I) complex and AgNPs under an applied electrical field and assessed the stability of the alignment after the electric field was constantly removed. Furthermore, the in-situ SANS investigation of the kinetic effects under external impulse influence helped identify the clusters of silver under an external electric field in various composite matrices. This technology can be used for accurate noninvasive blood circulation; increasing material electrical conductivity by applying induced electric field molecular alignment can tremendously increase sensor sensitivity. This approach opens the door to the next generation of thermoset polymers with multifunctional properties.

Direct Measurement of the Thermal Conductivity Depending on the Thermal Crystallisation Conditions of Injection Moulding processes Using Flash-DSC
Jonathan Alms, Hakan Çelik, Christian Hopmann, March 2023

During the production of injection moulded components made of semi-crystalline thermoplastics, the material is locally exposed to different thermal conditions and thermal histories. While in the injection phase the surface layer material that gets in direct contact with the cold mould wall solidifies at cooling rates of up to 700 K/s, the core layer material solidifies at cooling rates of ~1 K/s, especially for thick-walled components. This results in variation of crystallisation degree throughout the thickness of the component. Typically, an increasing crystallisation degree is related to an increase in thermal conductivity of a polymer. Since the heat of the whole component is transferred through the surface layers, where a low crystallisation degree is expected, the prevailing reduced thermal conductivity effects the injection moulding process significantly. To measure the crystallisation degree dependent thermal conductivity, a method using a Flash- DSC is presented and tested with isotactic polypropylene. To reduce effects of the Flash-DSC measurement itself a large parameter sweep is used to calibrate the measurement instrument. Using the Flash-DSC, however, delivered an inverse relation of thermal coefficient and crystallisation degree which contrasts with literature, which expects a direct proportional relation.

Mass Spec Solutions for Polymer Analysis
Mark Arnould, March 2023

Mass Spectrometry (MS) has become an indispensable tool for polymer analysis and has been widely used to study polymer structure and composition, end-groups and additives, molecular weight distribution, degree of polymerization, and so on. MS analysis is extremely sensitive, allowing the detection and identification of minor polymer components and synthesis by-products, as well as low-level impurities and products of decomposition. Matrix Assisted Laser Desorption Ionization (MALDI) MS is a well-established method of polymer characterization that continues to be developed and improved with new generations of MS instruments, bringing new analytical capabilities and enhanced performance. Modern MALDI-MS instruments generate rich chemical information highly specific for polymer structural analysis, copolymer composition and complex polymer mixtures characterization, and can even be used for imaging of synthetic polymer surfaces. Because of its unique capabilities, this technology has been widely used in a great variety of polymer analysis applications in both academic and industrial settings. In some cases, MALDI-MS is the only technique that can provide the information required to solve a practical problem. It allows for rapid MS analysis where no prior sample treatment or extensive separation is needed, including characterization of challenging insoluble polymers. TIMS technology has redefined the capabilities of Ion Mobility separation by providing an unmatched combination of resolution, speed, robustness and sensitivity. In polymer analysis applications, the timsTOF instruments expand the analytical boundaries by combining the TIMS technology with ultra-high-performance MS and providing an additional dimension for separation of complex polymer mixtures and structural analysis of challenging polymer compositions. Compatible with HPLS-ESI, GC-APCI and MALDI workflows, Bruker timsTOF fleX is a go-to multitool for a modern polymer lab.

Analytical Characterization to Evaluate Coating Efficiency of Fertilizers
Praveen Boopalachandran, March 2023

There are unique opportunities to develop coatings for non-urea fertilizers and provide desired performance such as enhanced controlled nutrient release and dust resistance. The key contributions from this work; provided advanced analytical solutions to evaluate fertilizer coating quality and developed quantitative QC tools for nutrient release and dust resistance. In conclusion, developed hydrophobic polyurethane fertilizer coating solutions that provides significant shelf stability and controlled nutrient release.

Failure Analysis of Glass Fiber Reinforced Plastics
Jeffrey Jansen, March 2023

The strength that glass reinforcement can impart to plastic materials is phenomenal. Glass fiber reinforced plastics offer enhanced mechanical properties, particularly strength and stiffness over unfilled materials. Their use is widespread in a wide variety of applications where mechanical integrity is essential. However, this benefit is not without its challenges. This presentation will focus on the investigation of failures of components manufactured from glass fiber reinforced plastics. The goal of a failure analysis is to identify the mechanism and cause of the component failure - to distinguish how and why the part broke. This presentation will explore the challenges unique to glass fiber reinforced materials and techniques that can be used to gain the maximum information from these failures.

Rheo-Raman Spectroscopic Study of Flow-Induced Crystallization of Polyethylene
Takumitsu Kida, March 2023

We developed a rheo-Raman spectroscopic system by combining a Raman spectroscope and rheometer to investigate the flow-induced crystallization behavior of polyethylene. Conformational changes that occurred during the flow-induced crystallization such as the formation of consecutive trans sequences or crystalline structure can be detected using Raman spectroscopy. We confirmed that no crystallization takes place at 130 ºC without shear flow because the fraction of the consecutive trans sequences and the crystalline structure was almost zero for 60 min. In the case of the flow-induced crystallization at 130 ºC with a shear flow of 100 s-1 for 30 s, the fraction of the long-consecutive trans sequences composed of more than 10 trans conformers increased with increasing time while the crystallinity was almost zero after applying the shear flow to the sample. Moreover, the long-consecutive trans sequences were formed as the precursor of the crystalline structure only at the shear rate with the Weisenberg number, which is the product of the shear rate and the Rouse relaxation time, greater than unity. These results suggest that the long-consecutive trans sequences are formed as precursors of the crystalline structure due to the stretching of the molecular chains under shear flow.

Investigating the Rapid Solidification Involved in Thermoplastic Processing Using Fast Scanning Calorimetry and Beyond
Xiaoshi Zhang, Ph.D., March 2023

Processing of thermoplastics during injection molding and blow molding usually includes rapid cooling with rates up to 103 K/s and solidification at high supercooling. Fast scanning calorimetry (FSC), an advanced calorimetry, is able to cover high processing rates and wide temperature windows by just using a few nanograms of the sample. With the advent of FSC, the crystallization fingerprint of many thermoplastics has been revealed. In this work, we expand the existing capability of FSC by coupling it with other techniques, including micro-IR spectroscopy (Micro-IR), atomic force microscopy (AFM), polarized optical microscopy (POM), and X-ray computed tomography (XCT). Polymorphism and morphology transition associated with processing conditions will be discussed in polyamide 66, polyamide 6, poly (ether ether) ketone and its composites. A more accurate simulation of plastic solidification can be achieved using fast scanning calorimetry and related technology.

Optimizing and Monitoring UV-Cure Process by DSC, DEA and DMA
Yanxi Zhang, Ph.D., March 2023

A variety of questions may arise in the UV-curing process of polymeric materials. For example, when does UV-curing start? When is UV-curing complete? What is the reactivity of the resin? What is the glass transition temperature after curing? Which photo initiator does show best performance? How does mechanical property of the cured material change in UV-curing process? Differential Scanning Calorimetry (DSC), Dielectric Analysis (DEA) and Dynamic Mechanical Analysis (DMA) offer effective means to help to answer these questions. DSC measures reaction enthalpy and degree of cure initiated by radiation. DEA allows for the measurement of changes in the dielectric properties related with ion mobility and dipole alignment during cure. Compared with DSC, DEA is good for fast cure system because data acquisition rate is less than 5ms and more sensitive to small change in cure process when close to the end of cure. DMA measures modulus changes during UV-curing process. These thermal analysis methods are indispensable in both R&D and quality control in the area of UV cure.

Compressive Behavior of Additively Manufactured Elastomeric Thermoplastic Polyurethane Honeycomb Structures With 2D Density Gradients
Mohammad Ahmed, March 2023

The advent of additive manufacturing (AM) brought in new dimensions to the research and development efforts of cellular polymeric structures by offering design freedom, resulting in tailorable architected structures optimized for specific applications. This work proposes a two-dimensional (2D) density gradient approach to design graded honeycomb structures for energy absorption applications. Graded honeycomb structures having three levels of density gradients (low, medium, and high) and their uniform density honeycomb equivalents were manufactured using material extrusion (MatEx) based fused filament fabrication (FFF) AM process. The material used for the FFF process was thermoplastic polyurethane (TPU) elastomer (Polyflex). The relative density of the structures was in the range of 0.259 – 0.346. A comparative study of the compressive behavior of the graded and regular honeycomb structures was carried out using in-plane quasi-static compression tests. Unlike regular honeycomb structures, all the graded honeycombs showed gradual stepwise deformation. Compared to their honeycomb equivalent counterparts, the high gradient honeycomb showed significantly different force-displacement profile compared to medium and low gradient honeycombs. While high gradient honeycomb showed higher maximum crushing force compared to the honeycomb equivalent, medium and low gradient honeycombs showed higher crush force efficiency. The experimental results were evaluated and compared with non-linear finite element analysis (FEA) simulation results. The hyperelastic properties of the TPU material were defined using Mooney-Rivlin constitutive model. The simulation results agreed well with the experimental results. The proposed 2D gradient parametric design methodology, coupled with the experimental and simulation results, can broaden the knowledgebase of graded honeycomb design principles, thus providing unique opportunities to develop and improve additively manufactured light-weight structures for commercial applications, ranging from automotive and transportation to healthcare and consumer products.

Structure Development of Semi-Crystalline Polymers in Laser Based Powder Bed Fusion
Simon Cholewa,,reas Jaksch, Dietmar Drummer, March 2023

The impact of melt hardening at low melt undercooling and under atmospheric pressure creates boundary conditions that have yet to be extensively studied since traditional techniques do not require such information. However, for powder bed fusion of polymers, the transition from the melt after exposure to an elastically dominant melt is critical as the crystallization in the building phase occurs under these conditions yielding stresses due to crystallization volume shrinkage. As a result, a process-adapted evaluation is required to determine how long the molten polymer remains viscously dominant, and the point where the stresses are stored in the melt. Therefore, the crystallization of semi-crystalline melt is investigated in this work using rheological data in conjunction with FTIR microscopy. A modified measurement setup of the rheometer with an ATR crystal allows a simultaneous description of crystallization by FTIR spectroscopy and measurement of the rheological behavior of the material. A comparison between the different techniques indicates that the increase in viscoelastic properties during crystallization begins at low degrees of crystallinity. It is determined that the solidification of the melt is detectable at relatively low degrees of crystallization conversion and that no stresses are accumulated in the material until this point.

Comparison of the Anisotropy of the Mechanical Properties of Injection Moulded and Additively Manufactured Parts
Johannes Austermann, Rainer Dahlmann, March 2023

Based on its mouldless, layer-wise manufacturing principle, screw-extrusion-based Additive Manufacturing (AM) allows for the efficient and economical production of thermoplastic prototype parts. During manufacturing, thermoplastic pellets are molten in a single-screw extruder and discharged through a nozzle. As the extruder is moved by a kinematic, the melt is subsequently locally discharged in a strand- and layer wise fashion to successively build up a part, similar to established AM processes such as the Fused Filament Fabrication (FFF). In contrast to FFF, standard thermoplastic pellets can be processed, as a single-screw extruder instead of a heated nozzle is used for plasticising the material. Thus, enabling injection moulding (IM) prototypes to be manufactured from series IM grade materials, including filled materials such as talc-filled polypropylene. However, the layer-wise additive manufacturing leads to anisotropic mechanical part properties in terms of strength and stiffness, which differ from the properties of the final IM-part, currently limiting the use of AM-parts to concept- and geometric-prototypes. These properties not only result from lower part strength orthogonal to the direction of deposition due to incomplete healing between adjacent strands, but also from a difference in filler-orientations, based on the process specific flow behaviour of the melt during processing. To extend the use of parts manufactured in screw extrusion AM to functional- or even technical prototypes, for which the mechanical properties are crucial, an understanding of these differences in the anisotropic mechanical behaviour of AM- and IM-parts is necessary. In the scope of this work, the quasi-static tensile and flexural properties as well as the high-speed tensile properties of additively, screw-extrusion-based manufactured and injection moulded parts are investigated, taking into consideration differences in the filler orientation between the manufacturing processes. To account for the anisotropy, testing is performed in several directions relative to the direction of deposition in AM or the direction of flow in IM. Furthermore, optical investigations are performed to assess the impact of filler orientations. The investigations are performed by manufacturing 1BA tensile test specimens from a 20 wt.% talc filled IM grade polypropylene material in screw-based AM and IM, which are subsequently used to perform quasi-static tensile and high-speed tensile testing. In addition, test specimens in accordance with DIN EN ISO 178 are manufactured for flexural testing. To allow for comparability, the test specimens are indirectly manufactured, i.e. both in AM as well as IM plate geometries are produced, from which the test specimens are milled. The AM parts are tested parallel and orthogonal to the strand-direction as well as at an angle of 30° and 60° relative to the strands. For IM, testing is carried out parallel and orthogonal to the direction of flow. In addition, µCT and microscopic investigations are conducted to analyse the orientation of the filler. While the results show an anisotropy in strength and stiffness for both IM and AM specimens, the anisotropy of these properties is significantly more pronounced in case of AM. This is based on the higher degree of filler orientation in the strands of the AM-parts. At the same time, only a partial orientation of the fillers in flow direction can be determined for IM-parts, showing that the fillers used can impact the comparability of AM and IM-prototypes. Additionally, it is shown that a higher comparability of the part properties is possible in the case of a quasi static load, compared to high-speeds of load application, limiting the use of AM-prototypes to such load cases.

Numerical Simulation and Experimental Investigation of the Flow Behavior in Material Extrusion Additive Manufacturing
Julian Kattinger, March 2023

The melting of a plastic filament in an FFF extruder is characterized by the fact that there is hardly any frictional heating, and instead heat conduction and radiation between the nozzle wall and the filament plays the major role. Experiments have shown that these heat transfer mechanisms limit material heating and thus the overall production rate. For this reason, many efforts have been made to capture the melting behavior of the filament through analytical models, numerical simulations or experiments. This presentation focuses on a CFD simulation of non-Newtonian and non-isothermal polymer flow through the nozzle of a fused filament fabrication printer. The simulations were performed for a wide range of filament velocities at different nozzle temperatures and then compared with two different types of experiments. A comparison with experimentally measurements of the force required to push the filament through the nozzle showed that the assumptions used for the simulations are suitable to predict the melting and flow behavior in the relevant processing range. In addition, an experimental method was used to allow in-situ observation of melt flow in a printing nozzle using X-ray micro-computed tomography. In this way, it was possible to study the velocity distribution in the nozzle and to gain insights into the melting mechanism that can be used for future modeling approaches.

Strategic Cost and Sustainability Analyses of Injection Molding and Material Extrusion Additive Manufacturing
David Kazmer, Ph.D., March 2023

Economic and environmental costs are assessed for four different plastics manufacturing processes, including stock and upgraded material extrusion 3D printers, as well as cold and hot runner molding. Characterization indicated the larger stock 3D printer had a melting capacity of 14.4 ml/h while the smaller but upgraded printer had a melting capacity of 36 ml/h. 3D printing at these maximum melting capacities resulted in specific energy consumption (SEC) of 16.5 and 5.28 kWh/kg, respectively, with the latter value being less than 50% of the lowest values reported in the literature. Even so, analysis of these processes found them to be only 2.8 and 3.5% efficient, respectively, relative to theoretical minimum energy requirements. By comparison, all-electric injection molding with a cold runner mold had a specific energy consumption of 0.205 kWh/kg and was 54% efficient relative to the theoretical minima. Breakeven analyses considering the cost and carbon footprint of mold tooling found injection molding provided lower costs at a production quantity around 70,000 units and a lower carbon footprint at a production quantity around 10,000 units. Parametric analysis of model inputs indicates that the breakeven quantities are robust with respect to carbon tax incentives but highly dependent on mold costs, labor costs, and part size.

Effective Wall Thickness for Computational Modeling of Polymer Extrusion Additive Manufacturing
Saratchandra Kundurthi, Mahmoodul Haq, Abdifitah Adan, March 2023

Polymer material extrusion additive manufacturing processes like fused filament fabrication (FFF) are increasingly being used for structural applications. Accordingly, there is a growing need for computational modeling to characterize and predict the process output and printed part performance under load. Prior studies have shown that the modulus and strength in the build direction (Z-direction) are sensitive to the surface bead shapes and can vary extensively depending on the print settings used. This presents a challenge for part-level (macro-scale) finite element analysis (FEA) because the material properties required for such models can vary from part to part or even different locations within the same part. The use of stress concentration factors is a critical step in computing effective material properties to be used in macro-scale numerical models. However, theoretical stress concentration factors (kt) published in literature for material extrusion AM are limited to tensile loading only. In this work, we demonstrate how the kt from tensile loading can be extended to other load cases. Meso-scale FEA was used to perform parametric studies with varying bead shapes. The models were subjected to pure bending loads as well as bending loads combined with shear loads. The stress concentrations were then evaluated, but with multiple iterations of the wall thickness used for nominal stress calculations. The results were compared to the results from pure tensile loading, and it was observed that the choice of wall thickness is trivial for tensile loads but is critical for bending loads. An equation for effective wall thickness was derived that yields consistent stress concentration factors for any bead shape, irrespective of the applied load. The results were also compared with the effective wall thickness for calculating the Z-direction modulus as published in literature. Ultimately, separate recommendations for effective wall thickness are presented for calculating modulus, strength, and the actual geometry used in macro-scale FEA models.

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