Folding behavior of thermoplastic hinges fabricated with polymer extrusion additive manufacturing

Abstract

Due to the layer-by-layer nature of additive manufacturing, fabricated parts suffer from an anisotropic behavior with reduced mechanical performance when compared to traditional manufacturing. One specific mechanical property, folding endurance, requires both low flexural strength and simultaneously high elongation to achieve the flexibility needed to sustain repetitive bending. The present work provides an analysis of selected thermoplastics’ flexural capacity, including nylon (PA), polyethylene terephthalate (PETG), polylactide (PLA), thermoplastic polyurethane (TPU), polypropylene (PP), polyethylene (PE), and a TPR blend (ABSMG94: SEBS-g-MA 25:75), in order to evaluate the maximum number of folding cycles and load capacity sustained by a living hinge. A fractographic analysis was performed using scanning electron microscopy and computed tomography. Similar to the performance of injected molded products, the experimental results demonstrated that three of the tested materials behaved well in the context of a large number of folding cycles prior to an eventual detachment into two pieces; TPR blend, 244,424 cycles; PP endured one million cycles; and TPU, more than two million cycles, while the remaining materials failed to survive more than 1000 cycles. The hinges failure analysis revealed a wide variety of fracture morphologies and failure modes. In regard to the load capacity, PLA, PETG, and nylon provided the highest results in the ultimate strength of an axial static force applied (790.61 N, 656.06 N, and 652.75 N respectively), while the TPR blend was the highest (398.44 N) of the elastomeric materials (PP, TPU, and TPR blend). The evaluated materials demonstrated enough flexibility for use in specific applications such as stretchable electronics and wearable applications.