Common Additive Manufacturing Research Articles

Photocurable poly(ethylene glycol) as a bioink for the inkjet 3D pharming of hydrophobic drugs

Binder jetting and material extrusion are the two most common additive manufacturing techniques used to create pharmaceutical tablets. However, their versatility is limited since the powder component is present throughout the dosage forms fabricated by binder jet 3D printing and material extrusion 3D printing requires high operating temperatures. Conversely, material jetting allows for compositional control at a voxel level and can dispense material at room temperature. Unfortunately, there are a limited number of materials that are both printable and biocompatible. Therefore, the aim of this study was to engineer photocurable bioinks that are suitable for hydrophobic active pharmaceutical ingredients and have rapid gelation times upon visible light exposure. The resulting bioinks were comprised of poly(ethylene glycol) diacrylate (250 Da) as the crosslinkable monomer, Eosin Y as the photoinitiator, and methoxide-poly(ethylene glycol)-amine as the coinitiator. Additionally, poly(ethylene glycol) (200 Da) was added as a plasticizer to modulate the drug release profiles, and Naproxen was used as the model drug due to its high hydrophobicity. Various bioink formulations were dispensed into the bottom half of blank preform tablets – made via direct compression – using a piezoelectric nozzle, photopolymerized, and capped with the top half of the preform tablet to complete the pharmaceutical dosage form. Results from the release studies showed that drug release can be modulated by both the percent of poly(ethylene glycol) diacrylate in the formulation and the light exposure time used to cure the bioinks. These bioinks have the potential to expand the library of materials available for creating pharmaceutical tablets via inkjet printing with personalized drug dosages.

Surface quality and forming characteristics of thin-wall aluminium alloy parts manufactured by laser assisted MIG arc additive manufacturing

An innovative additive manufacturing (AM) system using low power pulsed laser assisted MIG arc welding (L-M) was proposed to manufacture metal products. With the purpose of revealing how width and height dimension of the manufactured thin-wall component are affected by the laser power, the present study has been carried out. The width decreased with the increasing of the laser power within a certain range of laser power, and the height increased proportionally under the equal deposition rate. The width and height fluctuation reduced while adding the low power laser, both the standard deviation decreased by more than 50％ when the laser power was 400 W. The coefficient of materials utilization was up to 91.12%, and increased by more than 15% while using L-M based AM to fabricate thin-wall parts with a proper laser power. In comparison with the common GMAW-based AM method, the L-M based AM method shows feasibility to manufacture a narrower thin-wall component with better surface quality and higher stability, and also with higher deposition efficiency.

Cold spray deposition for additive manufacturing of freeform structural components compared to selective laser melting

Recent advances in the field of cold spray have put forward the potential of this deposition technique to be used as a non-thermal additive manufacturing process with significantly high deposition rates. In this study, we use the additive manufacturing potential of cold spray for fabrication of freestanding three-dimensional Inconel 718 samples, which is a challenging material for cold spray due to its high hardness and limited deformability. Additionally, we fabricated samples with similar geometry using one of the most common additive manufacturing methods, i.e. selective laser melting. Microstructural characteristics, distribution of residual stresses, porosity and structural integrity of the cold spray deposited samples were compared with those obtained by selective laser melting before and after different heat treatments. The results of the first time study of axial fatigue strength of cold spray deposited freeform samples indicate the notable efficiency of cold spray for fabrication of freestanding objects for structural components, with similar characteristics to those obtained from laser based additive manufacturing technique and even comparable to bulk material properties. The low working temperature of the cold spray method, suggests it as a promising additive manufacturing technique with a high potential to address many challenges regarding laser based approaches.

Multi-Objective Optimization of Additive Manufacturing Process

This paper presents an approach to analyze the critical drawbacks and attributes of Additive Manufacturing (AM) simultaneously to find the best manufacturing parameters to fabricate the AM products. In this study, Fused Deposition Modeling (FDM) is investigated as a common AM technology. For this purpose, a multi-optimization problem is formulated according to the analysis of FDM technology. In this problem, layer thickness and part orientation are determined as the decision variables which are the important parameters of manufacturing. As objective functions, production time and material mass are considered and the surface roughness of FDM products and mechanical behavior of material are defined as the constraint functions. Different methodologies are developed to model the AM criteria according to these decision variables. To find the optimal solutions for manufacturing, Non-Dominated Sorting Genetic Algorithm-II (NSGA-II) is used. Finally, a case study highlighted the reliability of the proposed approach.

Discrete element simulations of powder-bed sintering-based additive manufacturing

Additive manufacturing (AM) is the processing of jointing raw materials to make objects, usually in a layer-by-layer fashion, from 3D data. It has become a promising tool for a wide range of applications in recent decades. Nevertheless, qualification and certification remains the major challenge for the AM. One possible approach to address this challenge is using computer simulations as an examination and prediction tools. In this work, we present a prototype discrete element model aimed to replicate one of the most common AM process, the powder-bed sintering. This DEM model is designed to incorporate heat conduction, phase transformation and inter-particle sintering that mimics the powder-bed sintering-based AM process. The DEM-based thermal conductance and bond neck growth models were verified against a closed form analytical solution and the Coblenz model, respectively. Numerical simulations are provided to examine the mechanical properties of the printed objects.

Review of Heat Exchangers Enabled by Polymer and Polymer Composite Additive Manufacturing

ABSTRACTThis paper presents an overview of the most common polymer additive manufacturing processes, including vat photopolymerization, material jetting, sheet lamination, powder bed fusion, and fused filament fabrication. Next, the general strengths and challenges of the common methods are discussed and are followed by discussions on efforts to increase thermal performance of polymers used with the various manufacturing methods. Finally, heat exchangers enabled by polymer additive manufacturing are reviewed to show that novel designs in metal, ceramic, and polymer heat exchangers can be made possible by the unique properties of polymers and the advantages offered by additive manufacturing. By reporting what has been proven possible—and the associated challenges—we hope to stimulate the community and further development in a burgeoning field enabled by polymer additive manufacturing.

A metal additive manufacturing method: semi-solid metal extrusion and deposition

Different existing metal additive manufacturing (AM) processes have many shortcomings and a lot of activities are centered on improving the final printed metal product. A new method of metal additive manufacturing has been proposed in this paper. Unlike most of the metal additive manufacturing processes, it does not use metal powder. The proposed method utilizes semi-solid metal (SSM) forming combined with deposition process as used in the most common additive manufacturing process for polymers. SSM forming is a promising near net shape technology with several advantages, such as being a porosity-free product and having reduced shrinkage, controlled microstructure, and excellent mechanical performance. Implementation of this technology in metallic additive manufacturing improves the mechanical properties and cost savings. Complicated time-dependent behavior of SSM makes it a challenging issue to utilize in additive manufacturing. A wire feedstock as a metallic filament was employed for the proposed process. However, some material preparations were necessary to get the desired rheological properties at the deposition head. In this study, the strain-induced melt-activated (SIMA) process was applied on a low-melting-temperature Sn-Pb alloy to obtain the desired globular feedstock microstructure. Then, preconditioned wire was fed in a thixo-extruder, which was designed and built for the proposed method in this research. A semi-melted alloy was deposited on the moving substrate to build a metallic part layer by layer. Various parameters of SIMA and thixo-extrusion processes, including wire thermomechanical cycle, feed rate, solid fraction, nozzle and chamber geometry, and others, were examined experimentally, and a sustainable semi-solid metal extrusion and deposition (SSMED) process was achieved. Finally, an acceptable metallurgical layer bonding was obtained at the interface of the deposited layers, with good mechanical properties of the fabricated parts. The new proposed method seems to have great potentials for metal additive manufacturing parts.

Improving surface finish and wear resistance of additive manufactured nickel-titanium by ultrasonic nano-crystal surface modification

Nickel-titanium (NiTi) alloys have great potential to be used as biomedical implants or devices due to their unique functional properties (i.e., shape memory properties and superelastic behavior). The machining difficulty associated with NiTi alloys is impeding their wide application. Additive manufacturing (AM), however, provides an alternative method to manufacture NiTi structures. One major concern associated with NiTi devices fabricated in this route is the potential for the release of toxic Ni ions due to the poor surface finish as well as high surface porosity. In this study, NiTi samples were produced using selective laser melting, the most common AM techniques. Then, an innovative surface processing technique, ultrasonic nano-crystal surface modification (UNSM), was used to mitigate the potential for the Ni ions release. By simultaneous ultrasonic striking and burnishing, UNSM can significantly improve surface finish and decrease surface porosity. In addition, UNSM induces plastic strain which in turn hardens the surface layer. The synergistic effect of better surface finish, lower subsurface porosity, and a hardened surface layer resulted in higher wear and corrosion resistance. It is therefore expected that UNSM can be potentially used to treat biomedical devices.

Additive Manufacturing of Metallic Materials: A Review

In this review article, the latest developments of the four most common additive manufacturing methods for metallic materials are reviewed, including powder bed fusion, direct energy deposition, binder jetting, and sheet lamination. In addition to the process principles, the microstructures and mechanical properties of AM-fabricated parts are comprehensively compared and evaluated. Finally, several future research directions are suggested.

Overhanging Features and the SLM/DMLS Residual Stresses Problem: Review and Future Research Need

A useful and increasingly common additive manufacturing (AM) process is the selective laser melting (SLM) or direct metal laser sintering (DMLS) process. SLM/DMLS can produce full-density metal parts from difficult materials, but it tends to suffer from severe residual stresses introduced during processing. This limits the usefulness and applicability of the process, particularly in the fabrication of parts with delicate overhanging and protruding features. The purpose of this study was to examine the current insight and progress made toward understanding and eliminating the problem in overhanging and protruding structures. To accomplish this, a survey of the literature was undertaken, focusing on process modeling (general, heat transfer, stress and distortion and material models), direct process control (input and environmental control, hardware-in-the-loop monitoring, parameter optimization and post-processing), experiment development (methods for evaluation, optical and mechanical process monitoring, imaging and design-of-experiments), support structure optimization and overhang feature design; approximately 143 published works were examined. The major findings of this study were that a small minority of the literature on SLM/DMLS deals explicitly with the overhanging stress problem, but some fundamental work has been done on the problem. Implications, needs and potential future research directions are discussed in-depth in light of the present review.

Temperature Mapping of 3D Printed Polymer Plates: Experimental and Numerical Study

In Fused Deposition Modeling (FDM), which is a common thermoplastic Additive Manufacturing (AM) method, the polymer model material that is in the form of a flexible filament is heated above its glass transition temperature (Tg) to a semi-molten state in the head’s liquefier. The heated material is extruded in a rastering configuration onto the building platform where it rapidly cools and solidifies with the adjoining material. The heating and rapid cooling cycles of the work materials exhibited during the FDM process provoke non-uniform thermal gradients and cause stress build-up that consequently result in part distortions, dimensional inaccuracy and even possible part fabrication failure. Within the purpose of optimizing the FDM technique by eliminating the presence of such undesirable effects, real-time monitoring is essential for the evaluation and control of the final parts’ quality. The present work investigates the temperature distributions developed during the FDM building process of multilayered thin plates and on this basis a numerical study is also presented. The recordings of temperature changes were achieved by embedding temperature measuring sensors at various locations into the middle-plane of the printed structures. The experimental results, mapping the temperature variations within the samples, were compared to the corresponding ones obtained by finite element modeling, exhibiting good correlation.

Infrared thermography of welding zones produced by polymer extrusion additive manufacturing

In common thermoplastic additive manufacturing (AM) processes, a solid polymer filament is melted, extruded though a rastering nozzle, welded onto neighboring layers and solidified. The temperature of the polymer at each of these stages is the key parameter governing these non-equilibrium processes, but due to its strong spatial and temporal variations, it is difficult to measure accurately. Here we utilize infrared (IR) imaging – in conjunction with necessary reflection corrections and calibration procedures – to measure these temperature profiles of a model polymer during 3D printing. From the temperature profiles of the printed layer (road) and sublayers, the temporal profile of the crucially important weld temperatures can be obtained. Under typical printing conditions, the weld temperature decreases at a rate of approximately 100°C/s and remains above the glass transition temperature for approximately 1s. These measurement methods are a first step in the development of strategies to control and model the printing processes and in the ability to develop models that correlate critical part strength with material and processing parameters.

Additive manufacturing of fibrous sub-micron poly(e-caprolactone) scaffolds for tissue engineering

Event Abstract Back to Event Additive manufacturing of fibrous sub-micron poly(ε-caprolactone) scaffolds for tissue engineering Gernot Hochleitner1, Tomasz Jüngst1, Toby D. Brown2, Kathrin Hahn1, Claus Moseke1, Franz Jakob3, Paul D. Dalton1 and Jürgen Groll1 1 University of Würzburg, Department for Functional Materials in Medicine and Dentistry, Germany 2 Queensland University of Technology, Institute of Health and Biomedical Innovation, Australia 3 University of Würzburg, Orthopedic Center for Musculoskeletal Research, Germany Introduction: The field of additive manufacturing (AM) has attracted increasing interest over the last decade, in part for tissue engineering (TE) approaches. In our study we use an emerging type of AM technology, termed Melt Electrospinning Writing (MEW), a process that combines the characteristic benefits of 3D printing and electrospinning. When using an electrohydrodynamic drawing process, thermoplastic melts can be deposited to well defined scaffold structures, even with sub-micron sized fiber diameters (817 ± 165 nm) as we recently observed[1],[2]. Materials and Methods: A custom-built device was used to manufacture scaffolds made of poly(ε-caprolactone) (PCL), a biodegradable polymer with a history of clinical use. In order to investigate the influence of the instrumental parameters, the feeding pressure (0.5–4.0 bar), heating temperature (80–120°C), acceleration voltage (2.0–10.0 kV), spinneret diameter (21 G; 23 G; 25 G; 27 G; 30 G; 33 G), collector distance (1–10 mm) and collector speed (1000–9000 mmmin-1) was systematically varied and optimized. To confirm cell adhesion to the scaffolds, we printed scaffolds directly onto hydrophilized (NCO-sP(EO-stat-PO)coated) glass slides and seeded primary human mesenchymal stromal cells (hMSC) to them afterwards[1]. Results and Discussion: Through the adjustment of instrumental parameters, PCL fibers with diameters ranging from 50 µm to the sub-micron level (817 ± 165 nm) can be deposited to highly uniform scaffolds (figure 1 A, B, C). Thus, MEW can be used to tailor scaffold architecture and to define its specific surface, which is of high interest for degradation or cell-interaction processes. Figure 1: A, B & C: MEW PCL scaffolds with a box structure consisting of 2x50 layers of sub-micron fliaments. D: HMSCs adhered to a scaffold after 4 days in vitro[1]. We observed excellent scaffold adhesion to the microscope slide that could withstand media changes and up to 2 weeks in vitro. The MSCs adhered well on the scaffolds, forming circular structures, even when the box spacing is only 90µm (figure 1 D). Further, the hydrophilic coatings of the glass slides not only prevented cell adhesion to the underlying surface, so that specific scaffold cell interactions could be observed[1]. Conclusion: While common AM methods, such as fused deposition modeling, allow a fabrication of fibers in a range of 100 µm microns and more, MEW can print sub-micron fibers with medical grade polymers. In contrast to solution electrospinning, drawing fibers from melts enables manufacturing without the use of often toxic solvents highly interesting for TE or medical approaches[1]. German Academic Exchange Service (DAAD, project No. 54417792); European research council (ERC consolidator grant Design2Heal, contract No. 617989); Professor Dietmar W Hutmacher from Institute of Health and Biomedical Innovation (Queensland University of Technology) for his support of TDB for MEW device construction; Björn Schulte from the Dept of Macromolecular Chemistry (RWTH Aachen) for SEC analysis

Additive Manufacturing of Micro‐sized Parts

AbstractAdditive Manufacturing (AM) or 3D printing defines a family of technologies for making objects of almost any shape from virtual 3D model data through additive processes in which successive layers of material are laid down. Laser‐based AM processes like laser‐sintering, selective laser‐melting or stereolithography, differ in the materials that can be used as well as the smallest features that can be achieved in the final parts. Driven by the trend of miniaturization, common high‐resolution AM technologies like stereolithography reach their limitations, when the typical layer thickness in the range of 50 to 150 μm does not suffice to meet application demands. This is where the innovative technique of 3D laser lithography provides a significant advantage. The process of two‐photon polymerization enables the fabrication of high‐resolution 3D micro‐objects with heights from a few hundred nanometers up to several millimeters and layer thicknesses well below 1 μm.

Standardization in additive manufacturing: activities carried out by international organizations and projects

Standards have to satisfy the needs of the different groups represented, such as industrial, trade, and consumer groups of all of the countries involved. Most experts agree that the lack of additive manufacturing (AM) standards is a key point to take into account in the barriers to broad adoption of AM. Although over the past two decades several entities and groups of experts have demanded the development of specific standards for additive manufacturing, the most important steps forward have been taken in the last few years, mainly through the actions of international organizations such as International Organization for Standardization (ISO) and American Society for Testing and Materials (ASTM), with the support of technical groups and projects focused on the standardization of AM. This work, which is successfully providing new standards for AM, is expected to be reinforced by a global agreement between ASTM and ISO with the aim of collaboration on common AM standards. This paper presents a summary and review of actions carried out so far by different organizations and projects, based on the work of several road maps and workshops, with the aim of developing new standards in this particular field.

Accuracy prediction in fused deposition modeling

Fused deposition modeling (FDM) is a common additive manufacturing (AM) technology able to fabricate physical prototypes directly from virtual model without geometrical complexity limitations. Initially used to create concept models to help product design stage, FDM developed as regard materials, accuracy, and the overall quality of the output improved, becoming suitable for end use. At present, it is employed in process chains to significantly shorten product development times and costs and to produce parts in small and medium batch. A critical drawback which inhibits its diffusion is the obtainable accuracy. Only few indications, relating the dimensional deviations, exist, and they are conflicting each other, not allowing a reliable prediction. In this paper, a geometrical model of the filament, dependent upon the deposition angle and layer thickness, has been developed in order to predict the obtainable part dimensions. The model has been validated by an experimental campaign. The specimens have been investigated by means of profilometer analysis in order to study macrogeometrical and microgeometrical aspects. Finally, a case study highlighted the reliability of the model. The direct implication of this work is the capability, in process planning, to know in advance if the FDM part dimensions will satisfy the specification and the component will fit with others. Moreover, this model can be employed to choose the suitable manufacturing strategy in order to comply with industrial constrains and scopes.

Selective Laser Melting

AbstractManufacturing enterprises in highwage countries such as Germany are increasingly being exposed to global competition. These enterprises are confronted by an increasing demand for individualized, high quality as well as low‐cost products [1]. Additive manufacturing methods have accelerated their technological capability in the last few years and provide great potential to fulfill these challenges. The most common additive manufacturing method is called Selective Laser Melting (SLM), developed at the Fraunhofer Institute for Laser Technology ILT [2].