PSI - Issue 42

Manuel Sardinha et al. / Procedia Structural Integrity 42 (2022) 1098–1105 Manuel Sardinha / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Fused Filament Fabrication Additive Manufacturing (AM) enables the production of lightweight structures with enhanced mechanical performance, while simultaneously eliminating requirements of manufacturing stages such as assemblies and the production of molds, which often enables savings in costs and process time [15]. Design for Additive Manufacturing (DfAM)-related approaches are among the essential tools needed to fully exploit the increased design freedom provided by AM capabilities [16], [17]. Design for Manufacturing (DfM) can be better described as a set of rules to adapt product designs to manufacturing capabilities, by introducing previously unforeseen requirements that constrain the available workspace in the early stages of product development. Among AM procedures, Fused Filament Fabrication (FFF) is the most employed for polymeric materials. Even if the development and characterization of materials for FFF has seen impressive research and commercial attention in the last decade, designing for FFF implies the consideration of stages that go beyond material processing and should have equally present constraints regarding tessellation of models; part build orientation; need of support structures; slicing parameters; toolpath generation; and requirements of post-processing. Notably, FFF intrinsic characteristics mean its functioning is based on thermoplastic materials which are easy to process and easily recycled. Among materials that have been employed in the manufacturing of NPTs, polyurethane (PU) based filaments are, by far, the most common [18]. The work by Sharma et al. [19] analyses the wear behaviour of FFF TPU parts, useful to estimate the life expectancy of these types of tires. Regarding material usage, by geometrically manipulation of the cellular structures that compose a typical NPT, the macro-properties of tires and its functionality goal can be address without the need of multiple materials, which, again, increases the recyclability of the manufactured parts, and promotes circularity economical aspects of the process. When it comes to fused filament fabricated NPTs, the first DfAM consideration to address is build orientation. Orientation has an influence on total part cost, build time, the need for supports, the part’s surface quality , and strength. Generally, to promote radial stiffness, the radial spokes should be aligned with the plane of each built layer, which will imply that inter-layer weaknesses will be mostly orthogonal to the load along the radial direction. Considering this, and even if not always the most adequate or functionally efficient, bi-dimensional (2D) or quasi 2D designs are the perfect fit for reliable and accurate production. Moreover, curvatures along the height of the build process will inevitably lead to a degree of a staircase effect that will diminish the correctness of the intended geometry and could be the source of part fragility [16]. Tread geometry might be considered an exception since even if the accuracy of a surface is compromised during the extrusion, the natural wear of the tire will lessen this effect. Considering the overwhelming amount of proposed fused filament fabricated PU tires, it is inevitably to address some of its processing requirements. Namely, due to the flexible nature of this filament, it often implies the use of very slow and constant material deposition speeds (when compared with other commonly used FFF materials). Reliability can benefit from a reduction of filament retraction stages and maximized cooling. In-build filament retraction can usually avoid stringing effects from material slipping out of the nozzle during purely travel movements. To avoid this when using PU, it is adequate to force the travel movements to be within the geometry of the part. Considering both speed and material stringing, direct extrusion is preferred in opposition to Bowden-type configurations. Conceptually, this workflow is very much in line with the principles of DfAM considering the slicing and toolpath generation procedures and is an additional advantage example of 2D-based designs. These assumptions and other commonly accepted process requirements are meant to be an application example and demonstrate the methodology behind the evaluation criteria that was followed to perform the qualitative analysis present in Table 1. Furthermore, such considerations do not advocate, as an example, that 3D-based designs should not be produced with FFF, since this is a versatile and easily adaptable process, but rather anticipate production difficulties and highlight the importance to adapt product designs to manufacturing capabilities. Other design consideration such as minimum part thickness or multiplicity between nozzle diameter and part features dimensions are naturally advised, they will not be considered as criteria for this summary, since they usually are application agnostic. 3.1. Design for Fused Filament Fabrication of NPTs considerations