Characterization and analysis of homogeneous and sandwich PLA/ABS specimens produced via the FDM printing process for UAV structural elements

The present new work focuses on the use of the Fused Deposition Modelling (FDM) technique and the desktop 3D printers for the production of structural elements of small Unmanned Aerial Vehicles (UAVs). The considered UAV is a multirotor and multipurpose modular drone which has been patented by the first author in collaboration with the Politecnico di Torino. The proposed drone, called PoliDrone, has only 8 basic elements which can be combined in different ways and numbers in order to obtain 12 different configurations. Each basic element can be produced by means of desktop 3D printers based on the FDM technology. The possible choices can be 3, 4, 6 and 8 arm configurations with the possibility of a single rotor per arm, double rotors per arm and a combination rotor/inflatable element per arm. The inflatable elements can be positioned at the bottom of each arm to allow the protection in the case of landings in the water or forced landings. The payload is always protected and it can be changed in accordance with the chosen mission. A large number of arms and rotors allows a great stability connected with a reduced endurance. On the contrary, a reduced number of arms and rotors allows a long endurance connected with a reduced stability. Desktop 3D FDM printers are usually employed for the production of non structural objects. The innovative idea of the present work is the use of this technology to produce structural elements employed in the construction of small UAVs. Mechanical stresses are not excessive for small multirotor UAVs. Therefore, the FDM technique combined with polymers, such as the ABS (Acrylonitrile Butadiene Styrene) and the PLA (PolyLactic Acid), can be successfully employed to produce structural components. In order to achieve this target, this work is devoted to the statistical study of the performance of a desktop 3D printer to understand the process development and its boundary limits of acceptance. Mechanical and geometrical properties of ABS and PLA specimens are evaluated by means of a capability analysis which allows both mechanical and dimensional performance identifications. Experimental collected data are used to determine statistically stable limits. The ABS and PLA specimens are produced using appropriate geometries for tensile and compression experimental tests. Moreover, such tests are conducted for several specimens produced using different directions for the deposition of the material via the FDM technology. In the preliminary project of PoliDrone, ABS has been chosen as the structural material because of its high mechanical properties combined with a reduced weight. However, a first drone prototype has been produced using the PLA because this material is easier to be printed even if its mechanical properties are lower than those of the ABS. The first prototype of PoliDrone produced in PLA has made its first flight on 4th of July 2015. In order to use the FDM technology and the ABS and PLA materials, it is necessary to know the mechanical properties and the dimensional accuracy of specimens obtained via FDM. The mechanical properties are fundamental for a correct structural analysis and optimization of the drone for the actual loads and employed materials. This study is necessary because the filling percentage of ABS or PLA and the manufacturing process influence the mechanical properties of the finished pieces. The dimensional accuracy is necessary to provide essential information on the tolerances to use in the project. The dimensional behaviour is strictly dependent on the specific used 3D printer. Furthermore, a capability study is proposed to understand the statistical behaviour of 3D printers. Therefore, this work is focused on both the mechanical and dimensional characterization and on the capability analysis based on the Six Sigma process. For both tensile and compressive tests, Young Modulus, maximum stress at rupture and stress at proportional limit are determined. These values can be used with confidence as inputs in the UAV project. Future studies will also consider bending tests combined with different directions of deposition for the construction of ABS and PLA specimens via the FDM printing. A novelty of the proposed work is the realization of 3D printed sandwich specimens with external skins in PLA and an internal core in ABS or external skins in ABS and an internal core in PLA. Sandwich configurations could give an important weight reduction without significant decreases of mechanical properties.