Design and performance evaluation of multifunctional midsole using functionally gradient wave springs produced using multijet fusion additive manufacturing process

Additive manufacturing (AM) is getting more attention and is considered a better choice of fabrication in almost every industry due to its unlimited design freedom, optimization, light-weight and customization. Currently, manufacturing constraints, limited design freedom and customization issues restrict manufacturers from designing specific shoes for each particular application e.g., walking, running, hiking, sports, etc. However, AM can design and fabricate a multifunctional, usable single shoe-midsole for all aforementioned applications. This study aims to design a multifunctional shoe midsole incorporated with functionally gradient wave springs (FGWS) at the critical areas of foot pressure (heel, forefoot and toe) measured using the F-scan system (insole plantar pressure measurement system). The non-critical areas were designed by a gradient cellular structure with graded unit cells as per load requirement. The designed midsole with FGWS was printed by an HP MultiJet Fusion printer using PA 12 (polyamide). A LISA printer (selective laser sintering) using TPU (thermoplastic polyurethane) material was also used to check its manufacturability. Compression testing, each spring up to 80% of its compressible distance studied the load-bearing capacity, energy absorption, stiffness and cushioning properties. The load-bearing capacity of FGWS was decreased and energy absorption was enhanced by compressing each spring with EVA (Ethylene-vinyl acetate) foam while traditionally manufactured non-contact metal wave springs showed a lower load-bearing capacity than AM (contact) wave springs. Moreover, the fatigue properties of both were compared after 100 cycles of compression, which found a good response of AM wave springs. Experimental compression results revealed a gradual smooth response of load against compression, i.e., cushioning and more energy absorption, validated by finite element analysis (FEA) with very little deviation.