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Polymers 2019, 11, 609 10 of 12 Table 3. Characteristic particle sizes of PA12 laser sintering powders for different ageing states and measurement methods. Method laser diffraction laser diffraction q/d-meter q/d-meter Material Virgin powder Used powder Virgin powder Used powder x10,0 30 μm 30 μm 28 μm 26 μm x50,0 42 μm 42 μm 51 μm 48 μm x90,0 62 μm 63 μm 71 μm 67 μm 4. Conclusions In this work, an innovative setup for the analysis of electrostatic surface potential build-up, during a powder spreading process was presented. The combination of the powder delivery process during SLS and with an electrostatic surface potential measurement leads to novel insights into the in-situ triboelectric charge build-up during powder application. The presented measurement principle can easily be transferred and integrated into an SLS machine to produce real time data at building chamber temperature for monitoring of critical layer formation processes during development of new powder systems or to monitor the spatial distribution of surface potential and thus, surface charge in the building area. Moreover, by applying our set-up for the analysis of virgin and aged PA12 powder, we could show for the first time that ageing also affects the charging behavior of the powder material. The differences between virgin and aged powder material in impedance (c.f. smaller conductivity of aged powder) and charge spectroscopy measurements (c.f. larger mean q/d ratio for aged material) are enhanced when looking at the integral behavior of the bulk material under processing conditions (c.f. higher surface potentials for tribo-charging of aged powder as compared to virgin powder). Effects for less optimized materials in earlier development stages are expected to be even more incisive. Whereas mostly exclusively flowability and viscosity, respectively molecular weight distribution, were assessed in terms of ageing so far, we could show that also significant changes in triboelectric properties during powder ageing occur. These have to be taken into consideration for the development of powder recycling strategies and the assessment of powder reusability. Author Contributions: N.H. planned the experiments, conducted the experiments, analyzed the data and wrote the paper draft. All authors discussed the data and the paper draft. M.A.D., J.S.G.B., C.L., J.S. and A.B. reviewed and edited the manuscript. W.P. supervised the research, supported and reviewed the manuscript. Funding: The authors thank the German Research Foundation (DFG) for funding Collaborative Research Center (CRC) 814 Additive Manufacturing, sub-projects A01, A02, A03, B06 and T01. The APC was funded by DFG and Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) in the funding program Open Access Publishing. Acknowledgments: The authors want to thank Andreas Jaksch (Institute of Polymer Technology (LKT), Friedrich- Alexander-Universität Erlangen-Nürnberg (FAU)) for conducting powder ageing experiments and therefore suppling the used powders. The authors also want to thank the German Research Foundation for funding the Collaborative Research Center 814 (CRC 814), sub-projects A01, A02, A03, B06 and T01. We acknowledge support by Deutsche Forschungsgemeinschaft and Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) within the funding program Open Access Publishing. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript and in the decision to publish the results. References 1. Gibson, I.; Rosen, D.; Stucker, B. Additive Manufacturing Technologies. 3D Printing, Rapid Prototyping and Direct Digital Manufacturing, 2nd ed.; Springer: New York, NY, USA; Heidelberg, Germany; Dodrecht, The Netherlands; London, UK, 2015. 2. Ligon, S.C.; Liska, R.; Stampfl, J.; Gurr, M.; Mülhaupt, R. Polymers for 3D Printing and Customized Additive Manufacturing. Chem. Rev. 2017, 117, 10212–10290. [CrossRef] [PubMed] 3. Kruth, J.; Mercelis, P.; van Vaerenbergh, J.; Froyen, L.; Rombouts, M. Binding mechanisms in selective laser sintering and selective laser melting. Rapid Prototyp. J. 2005, 11, 26–36. [CrossRef]PDF Image | Tribo-Charging during Powder in Selective Laser Sintering
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