Relationship between programming stress and residual strain in FDM 4D printing

Abstract

4D printing with fused deposition modeling (FDM) enables the production of smart structures using smart materials that can change their shape over time. During the printing process, stresses are introduced into the structure that are relieved when exposed to an external stimulus, in this case, when raising the temperature above the glass-transition temperature TTrans . This article investigates the relationship between stress and strain during 4D printing. We mounted the print platform on load cells to measure the forces in x-direction during printing. Flat hinges made of PLA are printed and are activated by immersion in hot water, which causes bending of the hinge areas. We varied nozzle temperature, print speed, and melt zone length to investigate their influence on programming force and post-activation curvature. Programming force and curvature translate into stress and strain, when specimen geometry is taken into account. The results are approximated by a linear relationship between programming stress and recovery strain. However, these gradients are different for each printing parameter. Lower nozzle temperature, shorter melt zone and higher print speed all result in higher forces and higher curvatures. However, increasing the forces by raising print speed results in a smaller increase in curvature than when the same increase in force is applied by lowering nozzle temperature. This is partly due to the heating and cooling process of the structure, which in turn depends on the printing parameters.