Investigation of the Shape-Memory Properties of 3D Printed PLA Structures with Different Infills

Abstract

Polylactic acid (PLA) belongs to the few thermoplastic polymers that are derived from renewable resources such as corn starch or sugar cane. PLA is often used in 3D printing by fused deposition modeling (FDM) as it is relatively easy to print, does not show warping and can be printed without a closed building chamber. On the other hand, PLA has interesting mechanical properties which are influenced by the printing parameters and geometries. Here we present shape-memory properties of PLA cubes with different infill patterns and percentages, extending the research reported before in a conference paper. We investigate the material response under defined quasi-static load as well as the possibility to restore the original 3D printed shape. The quasi-static flexural properties are linked to the porosity and the infill structure of the samples under investigation as well as to the numbers of closed top layers, examined optically and by simulations. Our results underline the importance of designing the infill patterns carefully to develop samples with desired mechanical properties.

Infill Designs for 3D-Printed Shape-Memory Objects

“…Apparently, these infill patterns are better suited for bumpers, etc., which are normally damaged once or only a few times, as compared to the other samples showing a strong deviation between the first and the following cycles. The gyroid pattern shows a wave-like shape of the first force-deflection curve, similar to previous experiments with this pattern [15][16][17], and a relatively strong deviation between the first and second cycles, while the following cycles show relatively similar forces at identical deflections. However, none of the curves are smooth, which can be attributed to breaks of small connections occurring again and again.…”

“…For the infill patterns ZHR100 and OC15, the residual strain is firstly negative, meaning that the recovery process slightly over-compensates the deformation in the first three-point bending tests. This behavior was also found in previous tests, depending on the orientation of the impact [15][16][17].…”

“…The gyroid infill was found advantageous in another study working with cubes in which a linear object was pressed till half of the original height [15][16][17]. Here, it shows, by far, the lowest maximum force.…”

“…In previous experiments of our group, cubes printed with different infill patterns and infill ratios were cold deformed along a linear impact area [15][16][17]. Here, the strong impact (with a depth of half the cube height) resulted generally in broken bonds which weakened the structure in subsequent test cycles.…”

“…More common structures were investigated by different groups, e.g., honeycomb, 3D honeycomb or gyroid structures, which are typical infill patterns, offered by many slicer programs [14][15][16][17]. Here, however, the general problem occurred that recovery after cold deformation was never perfect, with recovery ratios clearly below 100% due to undesired broken bonds.…”