Pressure Orientation-Dependent Recovery of 3D-Printed PLA Objects with Varying Infill Degree

Abstract

Poly(lactic acid) is not only one of the most often used materials for 3D printing via fused deposition modeling (FDM), but also a shape-memory polymer. This means that objects printed from PLA can, to a certain extent, be deformed and regenerate their original shape automatically when they are heated to a moderate temperature of about 60–100 °C. It is important to note that pure PLA cannot restore broken bonds, so that it is necessary to find structures which can take up large forces by deformation without full breaks. Here we report on the continuation of previous tests on 3D-printed cubes with different infill patterns and degrees, now investigating the influence of the orientation of the applied pressure on the recovery properties. We find that for the applied gyroid pattern, indentation on the front parallel to the layers gives the worst recovery due to nearly full layer separation, while indentation on the front perpendicular to the layers or diagonal gives significantly better results. Pressing from the top, either diagonal or parallel to an edge, interestingly leads to a different residual strain than pressing from front, with indentation on top always firstly leading to an expansion towards the indenter after the first few quasi-static load tests. To quantitatively evaluate these results, new measures are suggested which could be adopted by other groups working on shape-memory polymers.

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.…”