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Feasibility studies of post‑processing techniquesto strengthen 3D printed UAV wing structures

Govdeli, Yunus; Ravindrababu, Suraj; Erdal, Kayacan; Ong, Gerard Zi Quan

2018

Ong, G. Z. Q., Govdeli, Y., Ravindrababu, S., & Kayacan, E. (2018). Feasibility studies ofpost‑processing techniques to strengthen 3D printed UAV wing structures. Proceedings ofthe 3rd International Conference on Progress in Additive Manufacturing (Pro‑AM 2018),244‑249. doi:10.25341/D4WW2K

https://hdl.handle.net/10356/88551

https://doi.org/10.25341/D4WW2K

© 2018 Nanyang Technological University. Published by Nanyang Technological University,Singapore.

Downloaded on 03 Aug 2021 17:00:17 SGT

ABSTRACT: To strengthen 3D printed unmanned aerial vehicle parts, this paper aims to investigate the feasibility of two post-processing techniques, namely heat treatment and protective coating, with polylactic acid being the base material of comparison. For a higher degree of crystallinity, the heat treatment is done at various temperature and duration above the glass-transition point. Simultaneously, the protective coating strength of finishing and bonding type epoxies are compared. The results indicate that heat treatment increases the elastic modulus but also decreases the flexural strength. The scanning electron microscope images observed at the filament level show the presence of cavities between and within layers after the heat treatment. Protective coating increases the flexural strength and elastic modulus of the printed samples where the bonding type epoxy performed better than the finishing type.

KEYWORDS: 3D printing, post-processing techniques, heat treatment, protective coating

INTRODUCTION

Unmanned aerial vehicle (UAV) manufacturing has been on the rise in the last decade due to their wide range of military applications in surveillance (Mehndiratta et. al), imagery and remote sensing (Cummings, McKee, Kulkarni, & Markandey, 2017) as well as civilian applications such as first-aid (Nedjati, Vizvari, & Izbirak, 2016) and agricultural monitoring (Xiang, & Tian, 2016).Due to the different nature of each mission, UAVs are often designed and customized in a mission-specific manner (Govdeli, Wong, & Kayacan, 2016). As a freeform manufacturing method, 3D printing provides the flexibility to modify working prototypes and to validate new and intricate designs. Among various 3D printing techniques, fused deposition modelling (FDM) is the most preferred method as it can process a wide range of thermoplastics and it is widely available in commercial market (Goh et al., 2017). On the other hand, FDM involves heating of athermoplastic filament until it flows in a continuous form while extruding it out via a nozzle. It creates a cross-sectional profile within each layer on the horizontal (XY) plane while building the model upwards in the vertical Z axis by stacking the layers of the extruded filament. Thus, the mechanical properties of a 3D printed model are different along the XY-plane and Z-axis.

UAV parts are usually printed spanwise along the Z-axis of the printer. This is due to the size constraints of desktop 3D printers and the elimination of supporting structures while printing the airfoils. The layer adhesion and the strength along the Z-axis are therefore important in ensuring that the UAV is able to withstand the bending stresses generated on the wing during a flight mission. However, Z-axis is the weakest printing direction among all three possible directions as the layers are not continuous and are stacked on top of each other (Ahn et. al., 2002).Consequently, post-processing methods are involved in bettering the structural performance of the 3D printed components.

FEASIBILITY STUDIES OF POST-PROCESSING TECHNIQUES TO STRENGTHEN 3D PRINTED UAV WING STRUCTURES

ZI QUAN ONG1, GOVDELI 1;2, RAVINDRABABU 2, ERDAL KAYACAN1;2

1School of Mechanical and Aerospace Engineering, 2Singapore Center for 3D printing (SC3DP),

Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

ZI QUAN GERARD ONG1, YUNUS GOVDELI1;2, SURAJ RAVINDRABABU2, ERDAL KAYACAN1;2

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Proc. Of the 3rd Intl. Conf. on Progress in Additive Manufacturing (Pro-AM 2018) Edited by Chee Kai Chua, Wai Yee Yeong, Ming Jen Tan, Erjia Liu and Shu Beng TorCopyright © 2018 by Nanyang Technological UniversityPublished by Nanyang Technological University ISSN: 2424-8967 :: https://doi.org/10.25341/D4WW2K

In this paper, two post-processing techniques are evaluated for their feasibility in strengthening 3D printed UAV wing structures: heat treatment and protective coating. Heat treatment involves annealing of the printed models to a temperature above glass transition and below the melting point, with the intention of promoting the formation of crystalline structures within the polymer (Balani, Verma, Agarwal, & Narayan, 2015). Compared to the amorphous state, crystalline structures improve mechanical properties such as strength and stiffness (Yu, Liu, Xie, Chen, & Li, 2008). On the other hand, interlamellar and interspherulitic connections could be weakened and severe cracks may be formed under loading (Han, Han, & Dong, 2013). Besides, the heating temperature and the duration also have varying effects on the strength (Srithep, Nealey, & Turng, 2013). Therefore, in this paper, an investigation is done to understand heating effects on the 3D printed PLA samples. In addition, at the protective coating stage, a layer of an epoxy matrix is applied onto the printed model so that the maximum bearable loading before failure is increased. Epoxy is a common medium used as a finishing agent to smoothen out the rough surfaces of 3D printed models as well as to facilitate the bonding between the contact surfaces. This paper seeks to find out the effectiveness of various types of epoxy in absorbing additional loading force. METHODOLOGY Preparation of the test specimens 3D printing: In order to test the effect of the above-mentioned post-processing techniques on strength, 3-point bending test is selected to determine the maximum flexural stress as well as the modulus of elasticity. ASTM D790 test specimens are 3D printer and the material selected is 3mm diameter PLA filament manufactured by eSUN. The slicing is done using the commercial software CURA using the following print settings: 0.2mm layer height, 0.4mm wall thickness, 100% infill density, 45° infill angle, 205°C printing temperature, 60°C build plate temperature, 40mm/s print speed and 100m/s travel speed. Other settings of the printer are left as standard values in the software. Heat treatment: For the heat treatment experiments, the test specimens are printed in two orientations: lengthwise along the Z-axis and flat along the XY-plane as shown in Fig. 1, so that the effects on the layer adhesion and within each layer are monitored, respectively. For protective coating experiments, the specimens are only printed lengthwise in the Z-axis. The amount of heat treatment is quantified using two variables: temperature and duration. Since PLA has a glass transition temperature of 60-65°C and melting temperature of 180°C, heating is done at selected temperatures of 70, 110 and 140°C. The duration of the heating is selected to be 30 minutes, 60 minutes, 2 hours, 4 hours and 8 hours. Coating: Coating of specimens is done using epoxy from 4 different manufacturers: Smooth-on (XTC-3D), Zap (Zpoxy), Elmers and Environtex Lite. An additional set of specimens are also coated using spray on plastic primer, as a comparison to the strength of epoxy. The specimens are also weighed to find out the amount of epoxy used, for calculation of the specific strength of the epoxy.

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composition which leads to a stronger bond so that the samples are able to withstand some higher loading force. External application of coating can also be used to reduce the amount of infills within the wing structure, thus providing extra space on the UAV for additional mechanisms or electronics. Future work could be done to investigate how the enlargement of cavities during heating is prevented or reduced, possibly via some initial filament bonding mechanisms during printing to reduce free volume between layers or via a highly controlled heat treatment procedure. Research on protective coating can also be done to optimize the location to apply such coating so as to further reduce weight while strengthening critical areas of the structure. ACKNOWLEDGEMENT

Singapore under its Medium-Sized Centre funding scheme. We also wish to acknowledge the funding support for this project from Nanyang Technological University under the Undergraduate Research Experience on Campus (URECA) programme. REFERENCES

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