materials Article FDM Layering Deposition Effects on Mechanical Response of TPU Lattice Structures Chiara Ursini * and Luca Collini   Citation: Ursini, C.; Collini, L. FDM Layering Deposition Effects on Mechanical Response of TPU Lattice Structures. Materials 2021, 14, 5645. https://doi.org/10.3390/ma14195645 Academic Editors: Michele Bacciocchi and Abbas S. Milani Received: 7 September 2021 Accepted: 23 September 2021 Published: 28 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Engineering and Architecture, University of Parma, Viale delle Scienze 181/A, 43124 Parma, Italy; luca.collini@unipr.it * Correspondence: chiara.ursini@unipr.it Abstract: Nowadays, fused deposition modeling additive technology is becoming more and more popular in parts manufacturing due to its ability to reproduce complex geometries with many different thermoplastic materials, such as the TPU. On the other hand, objects obtained through this technology are mainly used for prototyping activities. For this reason, analyzing the functional behavior of FDM parts is still a topic of great interest. Many studies are conducted to broaden the spectrum of materials used to ensure an ever-increasing use of FDM in various production scenarios. In this study, the effects of several phenomena that influence the mechanical properties of printed lattice structures additively obtained by FDM are evaluated. Three different configurations of lattice structures with designs developed from unit cells were analyzed both experimentally and numerically. As the main result of the study, several parameters of the FDM process and their correlation were identified as possible detrimental factors of the mechanical properties by about 50% of the same parts used as isotropic cell solids. The best parameter configurations in terms of mechanical response were then highlighted by numerical analysis. Keywords: additive manufacturing; fused deposition modeling; lattice structures; TPU; layering 1. Introduction As opposed to traditional production, which is mainly subtractive in nature, additive manufacturing represents the idea of future production, thanks to its considerable savings in materials and the extended possibilities in producing complex geometries. The major advantage of the fused deposition modeling (FDM) additive process lies in the customized production of printed objects. At the same time, various limitations prevent their domi- nance in the production of fully functional mechanical components, for example, the limited size of produced parts, but the cost should not be underestimated as well [1]. Furthermore, many printing process parameters have an influence on the resulting microstructure of printed objects, as observed by Ziemian et al. and Durgun et al. and discussed below [2,3]. FDM additive process is based on the extrusion of material and on the principle of stacking layer by layer, in order to create parts that can have sophisticated 3D geometries. Due to this principle of deposition of the material, this technology presents some pitfalls. In fact, by building FDM parts from bottom to top, the material in the current layer solidifies before the next one is placed on it [4], causing incomplete interlayer adhesion and often leaving voids in solidified structure, resulting in a decrease in mechanical performance [5]. In addition to this, a combination of several parameters such as raster orientation, air gap, bead width, color, model temperature, infill, etc. causes a decrease in the compressive strength of FDM products [6,7]. The layering effect represents the main challenge in additive manufacturing with fused deposition technology and will be analyzed in depth in the discussion below. In this study, the printed parts under examination were lattice structures made of the repetition of three different unit cells with the same relative density—namely, open cell, closed thin-walled cell, and closed thick-walled cell. The unit cells repeated with the Materials 2021, 14, 5645. https://doi.org/10.3390/ma14195645 https://www.mdpi.com/journal/materials