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polymers Article Analysis of Tribo-Charging during Powder Spreading in Selective Laser Sintering: Assessment of Polyamide 12 Powder Ageing Effects on Charging Behavior Nicolas Hesse 1,2,3, Maximilian A. Dechet 1,2,3 , Juan S. Gómez Bonilla 1,2,3, Christian Lübbert 1,2, Stephan Roth 3,4, Andreas Bück 1,2,3, Jochen Schmidt 1,2,3 and Wolfgang Peukert 1,2,3,* 1 2 3 4 * Correspondence: wolfgang.peukert@fau.de; Tel.: +49-9131-8529400 Received: 19 February 2019; Accepted: 29 March 2019; Published: 3 April 2019 Institute of Particle Technology (LFG), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Cauerstraße 4, D-91058 Erlangen, Germany; nicolas.hesse@fau.de (N.H.); maximilian.dechet@fau.de (M.A.D.); juan.s.gomez@fau.de (J.S.G.B.); christian.luebbert@fau.de (C.L.); andreas.bueck@fau.de (A.B.); jochen.schmidt@fau.de (J.S.) Interdisciplinary Center for Functional Particle Systems, Friedrich-Alexander-Universität Erlangen-Nürnberg, Haberstraße 9a, D-91058 Erlangen, Germany Collaborative Research Center 814—Additive Manufacturing, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Am Weichselgarten 9, D-91058 Erlangen, Germany; s.roth@blz.org Bayerisches Laserzentrum GmbH (blz), Konrad-Zuse-Straße 2-6, D-91052 Erlangen, Germany Abstract: Powder flowability is key to achieving high process stability and part quality by application of smooth and dense layers in selective laser sintering (SLS). This study sheds light on the rarely investigated effect of tribo-electric charge build-up during powder delivery in the SLS process. This is achieved by a novel approach to quantify electrostatic potentials during doctor blading. The presented model setup is used in combination with charge spectrometry and impedance spectroscopy to investigate the alterations in tribo-electric charging behavior for the most commonly used laser sintering material polyamide 12 in its virgin and aged, c.f. reused, states. We show that the electrostatic charge build-up is significantly enhanced for aged polymer powder material, likely contributing to altered performance in SLS processing. Keywords: additive manufacturing; selective laser sintering; electrostatics; polymer ageing; polyamide 12; powder layer formation; surface potential; charge distribution; tribo-charging 1. Introduction The term Additive Manufacturing (AM) summarizes a variety of techniques that allow the manufacturing of parts from viscous, liquid, powdered or layered materials without the use of any tools or molds. AM enables easy individualization of parts and offers high freedom of design unmatched by any conventional manufacturing method, thus making it ideal and superior to subtractive methods for the manufacture of complex parts at low piece numbers [1]. AM technologies incorporate various polymer processing techniques. Among them, fused filament fabrication (FFF), stereolithography (SLA) and binder jetting have become feasible in recent years and allow the production of parts with a wide variety of properties. However, the mechanical part properties still have to be improved for high loads [2]. One AM technique capable of producing dense parts with superior mechanical performance, thus favored in industrial environment is the powder-based selective laser sintering Polymers 2019, 11, 609; doi:10.3390/polym11040609 www.mdpi.com/journal/polymersPDF Image | Tribo-Charging during Powder in Selective Laser Sintering
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