Voids and tensile properties in extrusion-based additive manufacturing of moisture-cured silicone elastomer

The tensile strength and strain properties as well as failure modes in silicone dumbbell specimens fabricated by extrusion-based additive manufacturing are investigated. Effects of process parameters, specifically the infill direction (0°, ±45°, and 90° relative to the tensile direction) and adjacent line spacing on the void formation and ultimate tensile strength are studied and compared to the baseline of stamped silicone specimens. The additive manufactured specimens with ±45° and 90° infill direction and either the minimal or small void extrusion configuration had the strongest ultimate tensile strength (average ranged from 1.44 to 1.51 MPa). This strength is close to that of the sheet stamped specimens which have an average ultimate tensile strength of 1.63 MPa. As the void size became larger and more elongated in shape, the average ultimate tensile strength significantly reduced to 1.15 and 0.90 MPa for specimens with ±45° and 90° infill direction, respectively. Counterintuitively, specimens with 0° infill direction were consistently the worst performing due to the tangency voids and poor edge surface finish resulting from the toolpath. We show that, to maximize ultimate tensile strength of silicone parts made by extrusion-based additive manufacturing, it is important to select process parameters which minimize the elongated voids, infill tangency voids, and surface edges. If these conditions can be achieved, the infill direction does not play a significant role in tensile strength of the tensile specimen.