Decreasing the shear stress-induced in-plane molecular alignment by unprecedented stereolithographic delay in three-dimensional printing

Abstract

The shear stress-induced in-plane molecular alignment in 3D printed polymer is decreased with an increased stereolithographic delay. The molecular alignment represented by the through-thickness (perpendicular to the layers) permittivity (100 Hz) is reduced by 39% to approach that of the bulk polymer. Consequently, the through-thickness piezoelectric coupling coefficient is decreased by 27%. Two types of resins (Resin 1, acrylate ester resin, viscosity 95 cP; Resin 2, methacrylate resin, viscosity 2950 cP) that differ in the viscosity are used for the synthesis of 3D printed components. Resin 2 experiences higher values of the shear stress, resulting in more molecular alignment. During the period of the delay, molecular orientation randomization occurs and is more sluggish for Resin 2. The degree of molecular alignment is more pronounced for 3D printed components with smaller layer thickness (25–50 μm), as the shear stress experienced is higher. The proposed analytical model predicted that the shear stress responsible for the molecular alignment for the component with a layer thickness of 25 μm and a platform speed of 3 mm/s is 38 and 1200 kPa for Resins 1 and 2, respectively. The corresponding peak normal compressive stress (unprecedentedly measured) is 5.6 and 22 kPa, respectively. Moreover, the stress increases abruptly as the piston approaches its lowest position and decreases after reaching the peak stress. The rate of stress decrease is more pronounced for Resin 1.