Effect of printing speed and part geometry on the self-deformation behaviors of 4D printed shape memory PLA using FDM

Programmable shape memory polylactic acid (SMPLA) fabricated using fused deposition modeling (FDM) technique has attracted broad interest in recent years. However, the effect of printing speed and part aspect ratio on the self-deformation behaviors of parts have not been well studied. In this work, the effect of printing speed and part geometry on the shape recovery behavior was studied. The thermal history was obtained by implementing finite element analysis (FEA) models using different printing speeds. The corresponding residual stress gradient between top layers and bottom layers were analyzed and found critical to the heating-bending behavior. Self-deformation occurs when re-heating the parts, showing interesting shape memory effect that can be tuned using different printing speeds and part aspect ratio. With a smaller part thickness or a higher printing speed, more obvious self-deformation/bending behavior can be observed. Stress gradient Δσ_y and temperature distribution obtained from simulation suggest a strong correlation between Δσ_y and bending angle β, validating the experimental observations. Up to 50 cycles of heating/bending- cooling- reheating cyclic tests were performed to study the repeatability and reversibility of the part self-deformation behavior. Digital image correlation analysis was used to analyze the self-deformation behavior during the first heating cycle. Based on the analysis of the printing speed on the self-deformation behavior, a bio-inspired stem tendril-like gripper was fabricated using a printing speed of 90 mm/s that presents the largest deformation. The bionic gripper possesses a fast response which is <2.5 s to grab pine needles with a diameter of ~0.5 mm, showing good potentials used as actuators/sensors.