Deformation Behavior of 3D Printed Auxetic Structures of Thermoplastic Polymers: PLA, PBAT, and Blends

Abstract

Auxetic structures have a negative Poisson’s ratio and therefore expand transversely to the direction of loading instead of tapering. This unique behavior is not caused by the materials used, but by the structure, and thus offers completely new functionalities and design possibilities. As a rule, auxetic structures have a very complex geometry, which makes cost-effective production possible only by means of additive manufacturing processes. Due to the high design freedom of the strand deposition method, it makes sense to manufacture auxetic structures using this process. Therefore, in this project, polylactide acid (PLA), polybutylene adipate terephthalate (PBAT), and blends of the two polymers were produced and characterized. Filaments of the two polymers and a blend were extruded, processed into auxetic structures by strand deposition process (SDP), and investigated for their properties, primarily their Poisson’s ratio. The Poisson’s ratio was determined and the influence of the material on it was identified. A specific number of 5 × 5 unit cells has been found to be ideal for investigation. Dual printed specimens showed a similar auxetic behavior as the specimens made of pure PBAT. Likewise, multiple loading and unloading of the structure is possible. Furthermore, in-situ computed tomography revealed the detailed characterization of the initial state, including the warpage of the structures, damage, and traced auxetic behavior in detail.

Deformation Behavior Investigation of Auxetic Structure Made of Poly(butylene adipate-co-terephthalate) Biopolymers Using Finite Element Method

“…PLA and PBAT belong to thermoplastics among elastomer, duroplastic, and thermoplastic. The current work focuses on the PBAT deformation properties/behaviors achieved from experiments [ 40 ] and FE simulations. Here, only a short introduction is given.…”

“…The compounding was carried out on an EBVP25 twin screw extruder (O.M.C., Saronno, Italy). The screw design of the compounding process included different kneading and mixing elements [ 40 ]. A 30 × 25D single-screw extruder (COLLIN Lab and Pilot Solutions GmbH, Maitenbeth, Germany) was used for filament extrusion.…”

“…The filaments were extruded with a diameter of 1.75 mm at a speed of 25 1/min. The filament diameter was continuously determined and checked with an ODAC 15XY laser measuring device (Zumbach Electronic AG, Orpund, Switzerland) [ 40 ].…”

“…For structures with more than 49 (7 × 7) cells, the required material and printing time would be large, i.e., neither economical nor time efficient. The experimental results showed that a structure with 5 × 5 cells is preferable [ 40 ], since its deformation behavior is similar to the one from 7 × 7 cells, while 3 × 3 could not accurately catch the deformation behavior. On the other hand, it needs only about one-half of the materials, compared to 7 × 7 cells, without considering the clamping jaws.…”

“…The currently used metamaterial is commercially available. For a detailed description of the experiments, refer to Hufert et al [ 40 ]. The finite element (FE) calculations are performed on tensile and auxetic specimens.…”