Extrusion Additive Manufacturing Research Articles

3D bioartificial stretchable scaffolds mimicking the mechanical hallmarks of human cardiac fibrotic tissue

Human cardiac fibrotic tissues are characterized by a higher stiffness relative to healthy cardiac tissues. These tissues are unable to spontaneously contract and are subjected to passive mechanical stimulation during heart functionality. Moreover, scaffolds that can recapitulate the in vivo mechanical properties of the cardiac fibrotic tissues are lacking. Herein, this study aimed to design and fabricate mechanically stretchable bioartificial scaffolds with biomimetic composition and stiffness comparable to human cardiac fibrotic tissues. Poly(ε-caprolactone) (PCL) scaffolds with a stretchable mesh architecture were initially designed through structural and finite element method (FEM) analyses and subsequently fabricated by melt extrusion additive manufacturing (MEX). Scaffolds were surface functionalized by 3,4-dihydroxy-DL-phenylalanine (DOPA) polymerization (polyDOPA) to improve their interaction with natural polymers. Scaffold pores were then filled with different concentrations (5%, 7%, and 10% w/v) of gelatin methacryloyl (GelMA) hydrogels to support in vitro human cardiac fibroblasts (HCFs) 3D culture, thereby producing bioartificial PCL/GelMA scaffolds. Uniaxial tensile mechanical tests in static and dynamic conditions (1 Hz; 22% maximum strain) demonstrated that the bioartificial scaffolds had in vivo-like stretchability and similar stiffness to that of pathological cardiac tissue (tailored as a function of the number of PCL scaffold layers and GelMA hydrogel concentration). In vitro cell tests on bioartificial scaffolds using HCF-embedded GelMA hydrogels under static conditions displayed increasing cell viability, spreading, and cytoskeleton organization with decreasing GelMA hydrogel concentration. Moreover, α-smooth muscle actin (α-SMA)-positive cells were detected after 7 days of culture in static conditions followed by another 7 days of culture in dynamic conditions and not in HCF-loaded scaffolds cultured in static conditions for 14 days. These results suggested that in vitro culture under cyclic mechanical stimulations could induce an HCF phenotypic switch into myofibroblasts.

Mechanical evaluation of elastomeric thermoplastic polyurethane additively manufactured triply periodic minimal surface area lattice structures for adjustable cushioning properties

Purpose Cushioning is a useful material property applicable for a range of applications from medical devices to personal protective equipment. The current ability to apply cushioning in a product context is limited by the appropriateness of available materials, with polyurethane foams being the current gold standard material. The purpose of this study is to investigate additively manufactured flexible printing of scaffold structures as an alternative. Design/methodology/approach In this study, this study investigates triply periodic minimal surface (TPMS) structures, including Gyroid, Diamond and Schwarz P formed in thermoplastic polyurethane (TPU), as a possible alternative. Each TPMS structure was fabricated using material extrusion additive manufacturing and evaluated to ASTM mechanical testing standard for polymers. This study focuses attention to TPMS structures fabricated for a fixed unit cell size of 10 mm and examine the compressive properties for changes in the scaffold porosity for samples fabricated in TPU with a shore hardness of 63A and 90A. Findings It was discovered that for increased porosity there was a measured reduction in the load required to deform the scaffold. Additionally, a complex relationship between the shore hardness and the stiffness of a structure. It was highlighted that through the adjustment of porosity, the compressive strength required to deform the scaffolds to a point of densification could be controlled and predicted with high repeatability. Originality/value The results indicate the ability to tailor the scaffold design parameters using both 63A and 90A TPU material, to mimic the loading properties of common polyurethane foams. The use of these structures indicates a next generation of tailored cushioning using additive manufacturing techniques by tailoring both geometry and porosity to loading and compressive strengths.

Crystallisation Dynamics in Large-Scale Extrusion Additive Manufacturing: An Analysis with and without Temperature Modification.

Large-Scale Material Extrusion (LS-MEX) is increasingly being used in small-scale production and prototyping due to its ability to create components in new temporal and spatial dimensions. However, the use of this manufacturing process poses microscopic and macroscopic challenges not encountered in previous small-scale production systems. These challenges arise primarily from the prolonged retention of heat in the material, which leads to insufficient strength in the extruded strands at the macrostructural level. As a result, the component can collapse, a phenomenon known as 'slumping'. Thermal energy also influences microstructural changes, such as crystallisation kinetics, which affect properties such as the strength and stiffness of the final product. The duration and dynamics of thermal energy are influenced by manufacturing parameters and the possible use of additional peripheral equipment, which affects component quality. In this study, the influence of thermal energy on structural processes through simulations of polyamide 6 with 40% carbon fibres (PA6 wt.%40 CF) is investigated. The results show that by adjusting the process parameters and using modification units, the thermal profile of the material can be accurately controlled, which allows the microstructural processes to be precisely controlled. This leads to the targeted modification of the macroscopic material properties. The focus of this work is on the combination of numerical simulations of the LS-MEX process with semi-empirical methods for the analysis of crystallisation processes. The application of the Nakamura model, which is used throughout similar investigations, allows a detailed description and prediction of the crystallisation kinetics during the manufacturing process. The study shows that the absolute degree of crystallisation can be determined with simplified assumptions using a combination of thermal simulations and semi-empirical approaches. It was found that the absolute degree of crystallisation increases from the outer interface of the strand to the print bed across the cross-section. This can be attributed to the specific thermal boundary conditions and the resulting temperature profiles at different points.

Optimization of the extrusion rate for different layer thicknesses to achieve controlled mechanical properties in MEX 3D printing of low-alloy steel

Composite extrusion modeling is an advanced material extrusion additive manufacturing process that requires fine-tuning of printing parameters for printing injection molding feedstocks. In order to achieve comparable high-quality parts at different layer thicknesses, the parameters must be optimized for the respective layer thicknesses. In this work, the printing parameters for AISI 8740 low-alloy steel injection molding material are optimized for four different layer thicknesses. The extrusion multiplier values for each layer thickness are tuned, allowing fine adjustment of the printer's screw rotation speed at every layer thickness, resulting in all final parts' maximum densities (≥ 98 %) with high accuracies. Scanning electron microscope images, surface roughness measurements, and printing precision tests proved the good and comparable high-quality of all optimized printed parts. Young's modulus, tensile and yield strengths also raised by ∼24 %, ∼16 % and ∼15 %, respectively, compared to the non-optimized printed sintered part. The achieved tensile strength range of (∼888–947) MPa and yield strength range of (∼574–586) MPa of all optimized printed sintered parts were also higher than those provided by the feedstock manufacturer. This promising approach significantly reduces printing time and manufacturing costs without compromising the printed final parts' quality and mechanical properties. The adjusting screw rotation speed methodology allows broader applicability across diverse material extrusion processes.

Improving a tool-path optimisation method in material extrusion additive manufacturing by data clustering and collapsing

Improving a tool-path optimisation method in material extrusion additive manufacturing by data clustering and collapsing

From Formulation to Application: Effects of Plasticizer on the Printability of Fluoro Elastomer Compounds and Additive Manufacturing of Specialized Seals.

This work investigated material extrusion additive manufacturing (MatEx AM) of specialized fluoroelastomer (FKM) compounds for applications in rubber seals and gaskets. The influence of a commercially available perfluoropolyether (PFPE) plasticizer on the printability of a control FKM rubber compound was studied using a custom-designed ram material extruder, Additive Ram Material Extruder (ARME), for printing fully compounded thermoset elastomers. The plasticizer's effectiveness was assessed based on its ability to address challenges such as high compound viscosity and post-print shrinkage, as well as its impact on interlayer adhesion. The addition of the PFPE plasticizer significantly reduced the FKM compound's viscosity (by 70%) and post-print shrinkage (by 65%). While the addition of the plasticizer decreased the tensile strength of the control compound, specimens printed with the plasticized FKM retained 34% of the tensile strength of compression-molded samples, compared to only 23% for the unplasticized compound. Finally, the feasibility of seals and gaskets manufacturing using both conventional and unconventional additive manufacturing (AM) approaches was explored. A hybrid method combining AM and soft tooling for compression molding emerged as the optimal method for seal and gasket fabrication.

Defect Evolution and Mitigation in Metal Extrusion Additive Manufacturing: from Deposition to Sintering

Material extrusion additive manufacturing of metals has found widespread application and is typically thought of as a facile technique to create metal components. This can be undertaken without the need for energy sources typically used in direct energy deposition and powder bed fusion processes. Despite recent advances in achieving net shape and improvements to material/component integrity, these processes are characterized by performance limiting defects which often emerge as a result of deposition strategy and are retained post sintering. Although the debinding and sintering steps have been investigated for metal injection molding technology, quantitative studies of defect evolution are lacking. This work proposes an approach to study the pore evolution behavior from deposition to sintering through X-ray Computed Tomography (XCT) image processing. Quantitative results of pore shrinking, partial and full closure through backtracking of individual pores from the sintered to the green state are presented. It is shown that where pores are larger than 4 × 10-6 mm3, equals to the D90 of the feedstock metal particles, these cannot be fully closed upon sintering and will survive to limit the performance of the final part. The average pore shrinkage is by a factor of 5.6 and partial closure happens in pores with minimum Feret diameter of 33 μm, equals to 160% of the D90 powder particle, or less. The utilization of overfill as a proposed technique in the literature for the elimination of macro pores is investigated, yielding disparate and unsatisfactory results. This discrepancy can be attributed to the insufficient formation of micro-channels within the pore structure during the deposition process of the aqueous feedstock, leading to regions of weak grain bonding. To evaluate the contribution of these to undermining material integrity, tensile tests of linear and circular deposition strategies are undertaken. Experiments show that track interface length plays a major role in determining elongation to failure and tensile strength. It is concluded that the limitation to the mechanical properties of metal paste extrusion components originates from the emergence of defects and their characteristics after the completion of the processing, as opposed to being influenced by the metallurgy prior or post sintering.

Material extrusion additive manufacturing of WC-9Co cemented carbide

Additive manufacturing (AM) cemented carbide is primarily challenged by critical issues of difficult-to-eliminate pores, cracks, deleterious phases, and non-uniform microstructure, which result in poor mechanical properties. This study utilizes material extrusion (MEX) AM technology to prepare a WC-9Co cemented carbide. The results show that green body printing and solvent debinding process significantly affect its stacked voids, interlayer bonding defects, and debound cracks. These green body defects are eliminated through liquid phase flow and WC particles flow-rearrangement during sintering, resulting in Co-rich regions or Co pools, abnormal growth of WC grains, and residual pores. A green body free of visible defects is prepared using a feedstock with a powder loading of 54 vol.%, a printing temperature of 150 ℃, a microfilament overlap ratio of 30%, and a printing-layer thickness of 0.1mm. Followed by a two-step solvent debinding and continuous thermal debinding-vacuum-pressure sintering, a nearly fully dense WC-9Co cemented carbide is fabricated, with a uniform microstructure of two phases and free of pores, cracks, and Co pools. Its Vickers hardness, transverse rupture strength, and fracture toughness reach 1525±3 HV30, 3492±45MPa, and 20.4±0.5MPa·m1/2, respectively, which are significantly better than those of similar reports and could be comparable to similar cemented carbides prepared by powder metallurgy. This study could provide a new technological path for developing high-performance cemented carbide complex-structured components.

Fused granulate fabrication of injection molding inserts from high-performance ULTEM 9085™ thermoplastic for cosmetic packaging industry

AbstractRTIM refers to the integration of rapid tooling (RT) using additive manufacturing (AM) with injection molding (IM). Due to the use of raw material (pellet), screw extrusion, fused granulate fabrication (FGF) allows for a cost-effective and versatile production of RTIM inserts especially compared to other material extrusion (MEX) AM processes, such as fused filament fabrication (FFF). This study 3D printed RTIM inserts out of high-performance thermoplastic polyetherimide (PEI) (ULTEM 9085) in granular form, using an in-house developed FGF system. A cosmetic compact was used as a case study part with a simplified design. A dynamic mechanical analysis (DMA) conducted on FGF ULTEM 9085 demonstrated that the strength of the inserts is sustained up to 140 °C, which suits injection molding using polypropylene (PP). Optical profilometry of the FGF 3D printed RTIM inserts demonstrated that when using a 0.4 mm nozzle, the flat surfaces produced had microscopic gaps larger than 10–1 mm. These gaps are sufficiently large so that melt of low viscosity polymer is able to flow through, which leads to undesired part flash. The IM experiments confirmed that the inserts were capable of producing PP parts but with the predicted flash. Simulated part deflection differed both on geometry and magnitude from the actual deflection measured by optical profilometry. A total of 36 prototype parts were produced before the inserts failed due to poor inter-layer bond strength. FGF RTIM using ULTEM 9085 is deemed suitable for prototype part production. Improving the inter-layer bond strength and decreasing part complexity could increase the number of parts produced.

Material extrusion additive manufacturing of zirconia: from filament characterisation to Weibull statistics

PurposeThis study aims to investigate the performance of filament-based material extrusion additive manufacturing (MEX), combined with debinding and sintering, as a novel approach to manufacturing ceramic components.Design/methodology/approachA commercial ZrO2 filament was selected and analysed by infra-red (IR) spectroscopy, rheology and thermo-gravimetry. The influence of the print parameters (layer thickness, flow rate multiplier, printing speed) and sintering cycle were investigated to define a suitable printing and sintering strategy. Biaxial flexure tests were applied on sintered discs realised with optimised printing strategies, and the results were analysed via Weibull statistics to evaluate the mechanical properties of printed components. The hardness and thermal conductivity of sintered components were also tested.FindingsLayer thickness and flow rate multiplier of the printing process were proved to have significant effect on the density of as-printed parts. Optimised samples display a sintered density >99% of the theoretical density, 20% linear sintering shrinkage, a characteristic flexural strength of 871 MPa with a Weibull modulus of 4.9, a Vickers hardness of 12.90 ± 0.3 GPa and a thermal conductivity of 3.62 W/mK. Gyroids were printed for demonstration purposes.Originality/valueTo the best of the authors’ knowledge, this work is the first to apply biaxial flexure tests and Weibull statistics to additively manufactured MEX zirconia components, hence providing comparable results to other additive technologies. Moreover, fractography analysis builds the connection between printing defects and the fracture mechanism of bending. This study also provides guidelines for fabricating high-density zirconia components with MEX.

Water Diffusion in Additively Manufactured Polymers: Analysis of the Capillary Effect

Additive manufacturing (AM) is an advanced manufacturing method that produces objects by sequential layering. Material extrusion AM (MEAM) with continuous-fibre reinforcement is becoming more widely used in naval structures, which are exposed to the marine environment. However, the water diffusion process and the effect of water ageing on the mechanical performance of AM materials are not yet well understood because of their complex internal structure, caused by defects generated during manufacturing. Current research on diffusion is mostly based on experimental methods for conventionally manufactured materials without considering AM-induced defects. The objective of this study is to explore how the defects inherent to MEAM affect water diffusion in a composite material by the capillary effect. Results from a numerical study of capillary flow in MEAM polymer are applied as a boundary condition in the subsequent finite-element analysis. The study illustrates that flow in the capillary reaches the steady state quicker compared to the saturation time in the matrix, predicted by Fick’s diffusion equation. It is demonstrated that the capillary effect can significantly affect the water diffusion in MEAM parts and reduce the saturation time to one-third compared to the case without accounting for this effect.

A new design of star auxetic metastructure with enhanced energy-absorption under various loading rates: Experimental and numerical study

Cellular structures are widely used in various industrial applications due to their lightweight and high-strength properties. However, conventional cellular structures have some limitations in terms of energy absorption and load-bearing capacity. Therefore, novel cellular structures with enhanced performance are needed. This study introduces and evaluates two novel auxetic metastructures: Star Triangular Auxetic (STA) and Star Triangular Auxetic - Double Arrow (STA-DA). The metastructures are fabricated by material extrusion additive manufacturing and subjected to quasi-static axial crushing test at three loading rates. The tests are analyzed and validated by using digital image correlation and finite element modeling. The results show that the energy absorption of the STA structure is 19% higher than the STA-DA structure due to progressive damage and negative Poisson’s ratio. However, the STA-DA structure has higher load-bearing capacity than STA due to its stiffness which is 170% higher than the average stiffness of STA. The proposed auxetic metastructures show promising potential for industrial applications that require lightweight and high-strength structures with enhanced energy absorption.

Grain size prediction for stainless steel fabricated by material extrusion additive manufacturing

Most traditional powder melting-based additive manufacturing (AM) technologies generally yield high-performance parts at the expense of additional cost and energy consumption. As an economical and efficient alternative method, material extrusion (ME) that deploys polymer-based filaments with highly filled metal particles offers the possibility of scalable, low-cost fabrication of metal components. As a critical step, sintering governs the mechanical strength and geometry of the final parts. The grain growth behavior induced during sintering should be well controlled to achieve the parts with the desired mechanical performance. Due to the nature of AM, grain growth kinetics could also involve extremely complex grain boundary migration and atomic diffusion mechanisms caused by the heterogeneous pore distribution. In this work, an analytical model was developed to predict and understand the grain growth behavior of stainless steel (SS) 316L built by ME-based sintering-assisted AM. Such a model accounts for anisotropic viscosity parameters calibrated through a three-dimensional dilatometry test, enabling the prediction of grain size evolution during sintering. Grain growth kinetic parameters were further identified by the grain size data at the heating and holding stages. To validate and generalize the model, we also conducted the grain size evolution prediction under different sintering temperature profiles for as-built SS 316L specimens. This work will provide scientific insights into the grain growth behavior of metal parts built by this AM technique and its understanding can be readily transferred to other sintering-assisted AM processes like binder jetting and ink writing for metal structure creation.

Box-Behnken modeling to optimize the engineering response and the energy expenditure in material extrusion additive manufacturing of short carbon fiber reinforced polyamide 6

The field of production engineering is constantly attempting to be distinguished for promoting sustainability, energy efficiency, cost-effectiveness, and prudent material consumption. In this study, three control parameters (3D printing settings), namely nozzle temperature, travel speed, and layer height (LH) are being investigated on polyamide 6/carbon fiber (15 wt%) tensile specimens. The aim is the optimum combination of energy efficiency and mechanical performance of the specimens. For the analysis of the results, the Box-Behnken design-of-experiment was applied along with the analysis of variance. The statistical analysis conducted based on the experimental results, indicated the importance of the LH control setting, as to affecting the mechanical strength. In particular, the best tensile strength value (σB = 83.52 MPa) came from the 0.1 mm LH. The same LH, whereas caused the highest energy consumption in 3D printing (EPC = 0.252 MJ) and printing time (PT = 2272 s). The lowest energy consumption (EPC = 0.036 MJ) and printing time (PT = 330 s) were found at 0.3 mm LH. Scanning electron microscopy was employed as a part of the manufactured specimens’ 3D printing quality evaluation, while Thermogravimetric analysis was also conducted. The modeling approach led to the formation of equations for the prediction of critical metrics related to energy consumption and the mechanical performance of composite parts built with the MEX 3D printing method. These equations proved their reliability through a confirmation run, which showed that they can safely be applied, within specific boundaries, in real-life applications.Graphical abstract

Investigation on the forming process of polylactic acid in material extrusion additive manufacturing technique

Investigation on the forming process of polylactic acid in material extrusion additive manufacturing technique

Effects of aging and shot peening on surface quality and fatigue properties of material extrusion additive manufactured 17-4PH stainless steel

Material extrusion additive manufacturing (MEX) gains increasing interest due to its cost-effectiveness. However, concerns arise regarding inherent MEX characteristics potentially affecting mechanical properties. This study investigates the effects of shot peening and heat treatment on MEX 17-4PH stainless steel. After shot peening, surface nanocrystalline and compressive residual stress are induced. Aging results in higher hardness and tensile strength with lower elongation, while shot peening results in higher surface hardness with minimal change in elongation. For fatigue properties of as-sintered specimens, Al2O3 particle contamination dominates at high cycle regimes, while other MEX surface inhomogeneities dominate at low cycle regimes. Furthermore, as-aged properties are dominated by all surface inhomogeneities, resulting in the non-improvement of fatigue performance compared to the as-sintered specimens. However, shot peening on the as-sintered specimens significantly enhances fatigue properties through the formation of surface nanocrystalline structures and the induction of compressive residual stress, albeit with the occurrence of wrapped corners acting as fatigue crack initiation sites. Remarkably, the combination of aging and shot peening yields the most improvement in fatigue performance, exhibiting behaviours akin to as-sintered specimens with shot peening but without the generation of wrapped corners. Instead, fatigue crack initiation sites shift to subsurface region’s inhomogeneities.

A Review on Process Parameter Optimization in Material Extrusion Additive Manufacturing using Thermoplastic

Material extrusion is one of the most commonly used additive manufacturing techniques which utilizes thermoplastics as the building material. The quality and performance of parts produced via material extrusion depends highly on the process parameters selected. This paper reviews the work done by various researchers on optimizing key process parameters like extrusion temperature, layer thickness, infill percentage, infill pattern etc. in material extrusion 3D printing using thermoplastics like PLA, ABS and nylon. The effects of these parameters on properties like surface roughness, dimensional accuracy, mechanical strength are discussed. Statistical tools like Taguchi method and response surface methodology that have been applied for parameter optimization are also reviewed. The review highlights the need for further process optimization studies considering part design, build orientation and post processing to unleash the full potential of material extrusion 3D printing..

Quantitative Insight into the Compressive Strain Rate Sensitivity of Polylactic Acid, Acrylonitrile Butadiene Styrene, Polyamide 12, and Polypropylene in Material Extrusion Additive Manufacturing

AbstractHerein, a research and engineering gap, i.e., the quantitative determination of the effects of the compressive loading rate on the engineering response of the most popular polymers in Material Extrusion (MEX) Additive Manufacturing (AM) is successfully filled out. PLA (Polylactic Acid), ABS (Acrylonitrile Butadiene Styrene), PP (Polypropylene), and PA12 (Polyamide 12) raw powders were evaluated and melt-extruded to produce fully documented filaments for 3D printing. Compressive specimens after the ASTM-D695 standard were then fabricated with MEX AM. The compressive tests were carried out in pure quasi-static conditions of the test standard (1.3 mm/min) and in accelerated loading rates of 50, 100, 150, and 200 mm/min respectively per polymer. The experimental and evaluation course proved differences in engineering responses among different polymers, in terms of compressive strength, elasticity modulus, toughness, and strain rate sensitivity index. A common finding was that the increase in the strain rate increased the mechanical response of the polymeric parts. The increase in the compressive strength reached 25% between the lowest and the highest strain rates the parts were tested for most polymers. Remarkable variations of deformation and fracture modes were also observed and documented. The current research yielded results with valuable predictive capacity for modeling and engineering modeling, which hold engineering and industrial merit.

Mechanical properties of lightweight 3D-printed structures made with carbon-filled nylon

AbstractMaterial extrusion additive manufacturing is a widely used 3D-printing process involving depositing molten thermoplastic materials layer by layer to create a 3D object. Combining material extrusion with composites creates strong, lightweight, and functional parts with unique properties. This study uses chopped carbon fiber reinforcement to investigate polyamide's thermal, rheological, and mechanical properties. The study includes an analysis of the material's thermal properties via differential scanning calorimetry and its flow behavior via rotational rheometry. This study provides a comprehensive understanding of carbon-filled nylon PA material's economic and mechanical properties, which will help optimize its performance for various applications. Tensile and flexural tests were used to appraise the material's strength and stiffness under different loading conditions. A cost analysis was performed to compute the specimen cost as a function of orientation and infill density. The aim was to understand how the type and strategy of infill design impact the material's mechanical properties, helping optimize the performance of components and evaluating its cost.

Investigation of the Effectiveness of Silicon Nitride as a Reinforcement Agent for Polyethylene Terephthalate Glycol in Material Extrusion 3D Printing.

Polyethylene terephthalate glycol (PETG) and silicon nitride (Si3N4) were combined to create five composite materials with Si3N4 loadings ranging from 2.0 wt.% to 10.0 wt.%. The goal was to improve the mechanical properties of PETG in material extrusion (MEX) additive manufacturing (AM) and assess the effectiveness of Si3N4 as a reinforcing agent for this particular polymer. The process began with the production of filaments, which were subsequently fed into a 3D printer to create various specimens. The specimens were manufactured according to international standards to ensure their suitability for various tests. The thermal, rheological, mechanical, electrical, and morphological properties of the prepared samples were evaluated. The mechanical performance investigations performed included tensile, flexural, Charpy impact, and microhardness tests. Scanning electron microscopy and energy-dispersive X-ray spectroscopy mapping were performed to investigate the structures and morphologies of the samples, respectively. Among all the composites tested, the PETG/6.0 wt.% Si3N4 showed the greatest improvement in mechanical properties (with a 24.5% increase in tensile strength compared to unfilled PETG polymer), indicating its potential for use in MEX 3D printing when enhanced mechanical performance is required from the PETG polymer.