PSI - Issue 49

Theodoros Marinopoulos et al. / Procedia Structural Integrity 49 (2023) 81–87 T. Marinopoulos et al./ Structural Integrity Procedia 00 (2023) 000 – 000

85 5

4. Discussion Generally, the mechanical properties of AM parts produced with material extrusion are determined by several factors. The traditional method of toolpath generation using slicing software prints a shell following the inner and outer surfaces of the geometry, then fills the middle part between these walls with an infill structure [16]. Toolpaths produced with slicing software are erratic and introduce stress raisers in the printed part such as transitions between the infill and the shells, pores when the printer nozzle accelerates, stringing across surfaces due to travel moves, and under-extrusion from changes in nozzle pressure [18,19,22,24]. Such stress raisers result in lower and less predictable failure loads that otherwise would not be expected from the material. Additionally, the material-extrusion process results in an inherent anisotropy due to the presence of filament-scale geometrical features such as grooves between extruded filaments [25], leading to parts often failing due to delamination between layers [26]. Also, slicing-software toolpaths typically have large grooves and small contact areas between filaments. The results of this study can find a very important application in the 3D printing of variable-thickness structures. As topology optimization was explored in the past for the design of lighter and structurally compliant sockets [27], the suggested new designs include variable wall thickness with a reduced amount of material in non-load-bearing regions (Fig. 4). In such cases, manufacturing challenges can arise due to limitations of the traditional slicing algorithms. As illustrated in Fig. 5, the change in the width of the printed area can result in abrupt ends of the printing paths or induce additional porosity due to geometrically constrained infill paths. The advantage of the use of the customized printing path is illustrated in Fig. 5, where a single pass of the printer nozzle can deposit material for the total thickness. Coupled with the ability of FullControl software to control the extrusion rate and, thus, the area covered by the extruded material at any given time, the variable extrusion width can be constantly achieved, resulting in a single uninterrupted contact area between layers. In such a way the internal porosity due to the infill pattern can be eliminated and a wider angle introduced between the layers that showed to contribute to the improved mechanical behavior of the final product.

Fig. 4. Novel socket design produced by material removal based on FEA and topology optimization. Variable thickness is achieved ranging from 0.5 mm to 4.5 mm.

5. Conclusions In this study, the effect of alternative printing paths on the mechanical performance of prosthetic sockets was investigated using ISO 10328 guidelines for static ultimate-strength test. Confirming the initial hypothesis, the use of the developed slicer bypass in order to create custom printing paths resulted in the increased load-bearing performance of the produced sockets. It was observed that eliminating the effect of structural defects such as porosity and interlayer surface notches can improve the mechanical behavior of the sockets. Additional benefits of this method include a considerably reduced manufacturing time as well as increased consistency and reproducible socket fit thanks to the digitalization of the socket development process.