Molecular Dynamics Simulation and Multiscale Micropolar Modelling for 3D Printed Biodegradable Polymers

Fused Deposition Modelling (FDM) is an additive manufacturing technique used for rapid prototyping to produce complex geometric parts in advanced applications such as tissue engineering, aerospace, and electronics. FDM printers work by heating filament and extruding it through a nozzle layer by layer onto a build plate to create the desired object. This printing process causes voids between the filaments, resulting in inferior mechanical properties. However, higher porosities can be beneficial in applications such as designing scaffolds for tissue engineering. At the micro level, the structure of the FDM components consists of polymer filaments that are partially interconnected and voids. Therefore, the mechanical properties are governed by the material of the filaments and the shape and density of the voids, which in turn are influenced by the printing process parameters. This work presents a multiscale modelling method for predicting the mechanical properties of components manufactured with FDM. At the finer scale (micro), the mechanical properties of individual filaments are extracted using molecular dynamics simulations (MD). Polylactic acid (PLA), as one of the most commonly used filaments in FDM 3D printing, is considered the base polymer material. Its ease of use, wide availability, and biodegradability are some of the attractive properties of PLA. At the coarse scale (macro), the obtained properties of the PLA filaments are considered together with the voids for determining the overall mechanical properties of the component. However, direct modelling and discretisation of such media at the microlevel, with microstructural voids, can lead to cumbersome calculations. Therefore, equivalent constitutive laws at the macrolevel are needed that can globally account for microscopic features while ensuring efficient computation. Since classical continuum theory cannot account for the internal length scale of materials, non-classical continuum field theories that are able to preserve memory of the internal structure at the fine level can circumvent this limitation. In the present work, in particular, the micropolar continuum is used at the second scale to homogenise the heterogeneous structure. The mechanical parameters of the continuum are derived based on the strain energy equivalence of a porous 2D geometry representing the cross-section of the final part with the corresponding micropolar model under prescribed loadings. The effects of various features, such as void patterns and sizes, which originate from printing parameters, on the material properties are investigated by numerical simulations. The developed mechanical model provides a framework for the design of 3D-printed PLA components with desired mechanical properties as a function of printing parameters.