Ventilated surface-based lattice structures designed for polymer powder bed fusion process

Verma S., Kumar A., Lin S., Jeng J.

• Design of novel lattice structure to address powder removal challenges in polymer powder-based AM processes. • Ventilated 3D lattice structure designed to enable parallel and cross-flow of powder. • CFD simulation can show internal flow channels and overall flow resistance due to the design of various lattice structures. • The powder removal experiment can evaluate the powder flowability in various lattices. • Novel lattice structure's anisotropic behavior proven by FEA and experimental compression tests. The laborious work of post-processing powder removal from lattice structures made by polymer powder-based additive manufacturing (AM) process is still a major challenge and requires in-depth study. Here, a novel 3D honeycomb shaped lattice structure with ventilated holes bio-inspired from polyhedral plant cells has been designed to eliminate powder entrapment in constricted inter-cellular regions of surface-based lattice structure. The ventilation enables cross flow of powder within the lattice structure, resulting in easy and complete removal of entrapped powder. The computational fluid dynamics (CFD) method was used to understand the flow paths in various surface based lattice structures. An experimental setup was also designed to calculate the powder flowability from these lattices. The Finite Element Analysis (FEA) method and compression tests of various lattice structures were done to benchmark the strength of newly designed lattice structure. This innovative lattice structure can be used in biomedical, heat-exchanger, food and drug delivery systems and light-weighting applications.

Kumar A., Collini L., Ursini C., Jeng J.

This study analyses the energy absorption and stiffness behaviour of 3D-printed supportless, closed-cell lattice structures. The unit cell design is bioinspired by the sea urchin morphology having organism-level biomimicry. This gives rise to an open-cell lattice structure that can be used to produce two different closed-cell structures by closing the openings with thin or thick walls, respectively. In the design phase, the focus is placed on obtaining the same relative density with all structures. The present study demonstrates that closure of the open-cell lattice structure enhances the mechanical properties without affecting the functional requirements. Thermoplastic polyurethane (TPU) is used to produce the structures via additive manufacturing (AM) using fused filament fabrication (FFF). Uniaxial compression tests are performed to understand the mechanical and functional properties of the structures. Numerical models are developed adopting an advanced material model aimed at studying the hysteretic behaviour of the hyperelastic polymer. The study strengthens design principles for closed-cell lattice structures, highlighting the fact that a thin membrane is the best morphology to enhance structural properties. The results of this study can be generalised and easily applied to applications where functional requirements are of key importance, such as in the production of lightweight midsole shoes.

Verma S., Kumar A., Lin S., Jeng J.

Bhat C., Kumar A., Jeng J.

The properties of lattice structure are influenced by cell morphology, cell size, relative density and tessellations. In this study, the concept of tessellation has been evaluated for structural and functional properties of additively manufactured lattice structures using PA-12 material. The tessellation design of unit lattice cell is inspired by arrangement of atoms in crystal structures. The strategy of mimicking these different arrangements at atomic level to generate mesoscale cellular structures is termed as ‘Atomic Tessellation’. In the current study, arrangement of metallic crystal structures: BCC, FCC and HCP were compared with conventional periodically tessellated SC structure using sea-urchin unit cell for mechanical, energy absorption and structural behaviour properties. The tessellated lattice structures were printed with hybrid AM technology using HP-MJF 4200. The significant effect of tessellations was observed during compression testing of printed samples in terms of their stress-strain behaviour. Lattice structures such as SC, BCC, FCC and HCP_90 shows ‘degradation-prone deformation behaviour’ which is similar to stretch dominated behaviour in structural elements (thin struts and thin walls). On the other hand, HCP tessellation (HCP_0) converts ‘degradation-prone deformation behaviour’ into ‘progressive deformation behaviour’ that is similar to bending dominated behaviour in structural elements. Moreover, significant effects of tessellations were also observed in load bearing and energy absorption properties. Although the study has attempted to introduce the design concept of atomic tessellations from structural point of view, further studies are required to strengthen tessellation based design principles for obtaining different structural and functional properties. • The novel concept of ‘Atomic Tessellation’ have been introduced in design of cellular lattice structures. • Metallic crystal structures: SC, BCC, FCC and HCP were designed in mesoscale using Sea Urchin (SU) inspired unit cell. • Mechanical properties and energy absorption capacity was affected by tessellation strategy. • Tessellation strategy converts stretch-dominated behavior of lattice structure to bending dominated behavior and vice versa. • HCP tessellated structure behaves as bending dominated in 0˚orientation whereas as stretch dominated behavior was observed in 90˚orientation.

Dizon J.R., Gache C.C., Cascolan H.M., Cancino L.T., Advincula R.C.

Additive manufacturing, commonly known as 3D printing, is an advancement over traditional formative manufacturing methods. It can increase efficiency in manufacturing operations highlighting advantages such as rapid prototyping, reduction of waste, reduction of manufacturing time and cost, and increased flexibility in a production setting. The additive manufacturing (AM) process consists of five steps: (1) preparation of 3D models for printing (designing the part/object), (2) conversion to STL file, (3) slicing and setting of 3D printing parameters, (4) actual printing, and (5) finishing/post-processing methods. Very often, the 3D printed part is sufficient by itself without further post-printing processing. However, many applications still require some forms of post-processing, especially those for industrial applications. This review focuses on the importance of different finishing/post-processing methods for 3D-printed polymers. Different 3D printing technologies and materials are considered in presenting the authors’ perspective. The advantages and disadvantages of using these methods are also discussed together with the cost and time in doing the post-processing activities. Lastly, this review also includes discussions on the enhancement of properties such as electrical, mechanical, and chemical, and other characteristics such as geometrical precision, durability, surface properties, and aesthetic value with post-printing processing. Future perspectives is also provided towards the end of this review.

Piedra-Cascón W., Krishnamurthy V.R., Att W., Revilla-León M.

Objective To review the elements of the vat-polymerization workflow, including the 3D printing parameters, support structures, slicing, and post-processing procedures, as well as how these elements affect the characteristics of the manufactured dental devices. Data Collection of published articles related to vat-polymerization technologies including manufacturing workflow description, and printing parameters definition and evaluation of its influence on the mechanical properties of vat-polymerized dental devices was performed. Sources Three search engines were selected namely Medline/PubMed, EBSCO, and Cochrane. A manual search was also conducted. Study selection The selection of the optimal printing and supporting parameters, slicing, and post-processing procedures based on dental application is in continuous improvement. As well as their influence on the characteristics of the additively manufactured (AM) devices such as surface roughness, printing accuracy, and mechanical properties of the dental device. Results The accuracy and properties of the AM dental devices are influenced by the technology, printer, and material selected. The printing parameters, printing structures, slicing methods, and the post-processing techniques significantly influence on the surface roughness, printing accuracy, and mechanical properties of the manufactured dental device; yet, the optimization of each one may vary depending on the clinical application of the additively manufactured device. Conclusions The printing parameters, supporting structures, slicing, and post-processing procedures have been identified, but additional studies are needed to establish the optimal manufacturing protocol and enhance the properties of the AM polymer dental devices. CLINICAL SIGNIFICANCE The understanding of the factors involved in the additive manufacturing workflow leads to a printing success and better outcome of the additively manufactured dental device.

Khan H.M., Karabulut Y., Kitay O., Kaynak Y., Jawahir I.S.

As industries are increasingly adopting the laser powder bed fusion (LPBF) additive manufacturing (AM) process, it is crucial to understand the process closely and to find ways to improve the final...

Abou-Ali A.M., Al-Ketan O., Lee D., Rowshan R., Abu Al-Rub R.K.

Advances in additive manufacturing triggered a paradigm shift in the design of functional components allowing for complex topology-driven cellular lattices to be incorporated for the aim of reducing weight, enhancing multi-functionality, and facilitating manufacturability. In this paper, the compressive mechanical behavior of different polymeric lattices based on triply periodic minimal surfaces (TPMS) are investigated both experimentally and computationally. The behavior of two classes of TPMS lattices are investigated; sheet- and ligament-based lattices. Samples are fabricated using the laser powder bed fusion technique, selective laser sintering, and characterized using micro-Computed Tomography (micro-CT) and Scanning Electron Microscopy (SEM). A finite-deformation hyperelastic-viscoplastic-damage constitutive model is calibrated and employed to capture the full compressive behavior of lattices. The computational results are compared to and validated against corresponding experimental results. Results show that sheet-based polymeric TPMS lattices exhibit a stretching-dominated mode of deformation and prove to have superior stiffness and strength as compared to TPMS ligament-based lattices. The numerical simulations are in good agreement with experimental results for ligament-based lattices while significant deviation from experimental results is observed for the sheet-based lattices which is attributed to uncertainty in measuring the actual relative density and relatively higher manufacturing defects.

Khrapov D., Koptyug A., Manabaev K., Léonard F., Mishurova T., Bruno G., Cheneler D., Loza K., Epple M., Surmenev R., Surmeneva M.

An ultrasonic vibration post-treatment procedure was suggested for additively manufactured lattices. The aim of the present research was to investigate mechanical properties and the differences in mechanical behavior and fracture modes of Ti6Al4V scaffolds treated with traditional powder recovery system (PRS) and ultrasound vibration (USV). Scanning electron microscopy (SEM) was used to investigate the strut surface and the fracture surface morphology. X-ray computed tomography (CT) was employed to evaluate the inner structure, strut dimensions, pore size, as well as the surface morphology of additively manufactured porous scaffolds. Uniaxial compression tests were conducted to obtain elastic modulus, compressive ultimate strength and yield stress. Finite element analysis was performed for a body-centered cubic (BCC) element-based model and for CT-based reconstruction data, as well as for a two-zone scaffold model to evaluate stress distribution during elastic deformation. The scaffold with PRS post treatment displayed ductile behavior, while USV treated scaffold displayed fragile behavior. Double barrel formation of PRS treated scaffold was observed during deformation. Finite element analysis for the CT-based reconstruction revealed the strong impact of surface morphology on the stress distribution in comparison with BCC cell model because of partially molten metal particles on the surface of struts, which usually remain unstressed.

Hunter L.W., Brackett D., Brierley N., Yang J., Attallah M.M.

The issue of trapped powder within a part made using powder bed fusion additive manufacturing (AM) is one of the ‘dirty secrets’ of AM, yet it has not received significant attention by the research community. Trapped powders limit the application of AM for complex geometries, including heat exchangers and dies with conformal cooling channels. Being able to detect and remove trapped powder from the build is a necessary step to avoid downstream processing and performance challenges. In this work, ‘powder challenge geometries’ with complex internal features were fabricated via laser powder bed fusion (L-PBF) and electron beam selective melting (EBSM) and were used to assess the effectiveness of several powder removal and inspection methods. Hand-held ultrasonic polishing was explored as a powder removal technique and was shown to effectively clear extremely elongated channels that grit-blasting (the current industry standard) cannot clear. X-ray computed tomography (XCT) and weighing were used to inspect and quantitatively assess the effectiveness of powder removal techniques on the challenge geometries. Using the lesser known ‘vacuum boiling’ powder removal process and the more common ultrasonic bathing process, trapped L-PBF powder was easily removed from the deep channels. Conversely, trapped EBSM powder was difficult to remove using ultrasonic polishing as the powder was sintered inside the channels. It was shown that the powder recovered by the ultrasonic polishing process had size distributions, surface chemistry, morphology and porosity similar to the virgin powder. It is suggested, on these bases, that the recovered powder could likely be recycled without detrimental effects on the process operation.

Kang D., Park S., Son Y., Yeon S., Kim S.H., Kim I.

Due to the development of additive manufacturing (AM), lattice structure which cannot be fabricated by the conventional manufacturing process or have shape restriction has attracted much attention. We propose a new lightweight design method using two types of lattice structures considering the manufacturability in the metal selective laser melting (SLM) and structural characteristics. Firstly, the specific procedure for the proposed design method is presented. In order to apply the two lattice structures, relative density criterion is derived by fabricating experiments using metal SLM process and analyzing geometry according to relative density. The optimal relative density distribution is calculated by performing the topology optimization with minimum relative density using a commercial software package. This proposed method is computationally and experimentally validated by a three-point bending test. Simultaneously, the same procedure is applied to uniform lattice for comparison with the proposed method. This proposed design has a 46% increase in stiffness, a relative flexural rigidity of 35% compared to the solid material, and has a deformation mode different from the uniform lattice. This design sets the standard for using two lattice structures and gives a new perspective on lightweight design with lattice structures.

Diegel O., Nordin A., Motte D.

All Additive manufacturing (AM) technologies require post-processing to produce parts that are ready for use. This post-processing can range from support material removal, to surface quality improvement, to colouring and painting, and to aging for polymer parts and heat-treatment for metal parts. Throughout the AM industry there is a vast amount of tacit knowledge in the area of post-processing but there, currently, exists very little documentation on the various post-processing methods for different AM technologies and materials. This leads to time being wasted by companies having to individually learn and develop post-processing methods. This chapter aims to correct this.

Šoškić Z., Monti G.L., Montanari S., Monti M., Cardu M.

The paper presents a model of the production costs of the multi-jet-fusion technology that is based on a model of production costs of the selective laser sintering technology. The model is developed using the methodology of analysis of the event-driven process chain, which consists of modeling, batch assembly, setup, building, removal, and blasting activities. Production costs of each of the activities are separated to direct (labor, material, and energy) costs and indirect (equipment, overheads, and other indirect) costs. The developed model represents a basis for the development of algorithms and software tools for the calculation of the production costs of the multi-jet-fusion technology, since it defines all the necessary inputs and calculation procedures that enable the calculation of the total costs of a batch of products. Besides, the paper presents a procedure for the estimation of production costs that are attributed to a single product or product type.

Zhang L., Feih S., Daynes S., Chang S., Wang M.Y., Wei J., Lu W.F.

Designing metallic cellular structures with triply periodic minimal surface (TPMS) sheet cores is a novel approach for lightweight and multi-functional structural applications. Different from current honeycombs and lattices, TPMS sheet structures are composed of continuous and smooth shells, allowing for large surface areas and continuous internal channels. In this paper, we investigate the mechanical properties and energy absorption abilities of three types of TPMS sheet structures (Primitive, Diamond, and Gyroid) fabricated by selective laser melting (SLM) with 316 L stainless steel under compression loading and classify their failure mechanisms and printing accuracy with the help of numerical analysis. Experimental results reveal the superior stiffness, plateau stress and energy absorption ability of TPMS sheet structures compared to body-centred cubic lattices, with Diamond-type sheet structures performing best. Nonlinear finite element simulation results also show that Diamond and Gyroid sheet structures display relatively uniform stress distributions across all lattice cells under compression, leading to stable collapse mechanisms and desired energy absorption performance. In contrast, Primitive-type structures display rapid diagonal shear band development followed by localized wall buckling. Lastly, an energy absorption diagram is developed to facilitate a systematic way to select optimal densities of TPMS structures for energy absorbing applications.

Tancogne-Dejean T., Diamantopoulou M., Gorji M.B., Bonatti C., Mohr D.

In lightweight engineering, there is a constant quest for low-density materials featuring high mass-specific stiffness and strength. Additively-manufactured metamaterials are particularly promising candidates as the controlled introduction of porosity allows for tailoring their density while activating strengthening size-effects at the nano- and microstructural level. Here, plate-lattices are conceived by placing plates along the closest-packed planes of crystal structures. Based on theoretical analysis, a general design map is developed for elastically isotropic plate-lattices of cubic symmetry. In addition to validating the design map, detailed computational analysis reveals that there even exist plate-lattice compositions that provide nearly isotropic yield strength together with elastic isotropy. The most striking feature of plate-lattices is that their stiffness and yield strength are within a few percent of the theoretical limits for isotropic porous solids. This implies that the stiffness of isotropic plate-lattices is up to three times higher than that of the stiffest truss-lattices of equal mass. This stiffness advantage is also confirmed by experiments on truss- and plate-lattice specimens fabricated through direct laser writing. Due to their porous internal structure, the potential impact of the new metamaterials reported here goes beyond lightweight engineering, including applications for heat-exchange, thermal insulation, acoustics, and biomedical engineering.

Orak A., Kalyon A.

This study investigates the impact of six distinct lattice structures (Gyroid, Diamond, Schwarz, SplitP, Lidinoid, and Kelvin) on the mechanical performance and biocompatibility of CoCrMoW-based tibial implants. Finite element analysis was conducted using nTopology to simulate compression tests under a 3100 N load, evaluating important parameters such as stress, strain, displacement, reaction forces, mass, volume, and porosity. Global stiffness values were calculated based on the test results. The effect of lattice structure designs on global stiffness properties was determined. The results show that lattice structures improve the mechanical strength and biocompatibility of tibial implants and reduce the mass and stiffness value, consequently improving osseointegration and long-term stability.