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Polymers 2019, 11, 609 3 of 12 bed conditions [23], the investigation of triboelectric phenomena during deposition processes used in additive manufacturing remains elusive. This study’s aim is introducing a method and a model setup to quantify the electrostatic potentials during powder spreading by means of blade coating. Moreover, the technique represents a novel way for an in-line measurement and controlling system for powder application strategies that rely on electrostatic interactions, such as an electrophotographic multi-material powder deposition [24]. The applicability of the model setup is demonstrated for polyamide laser sintering powders. It is utilized to examine the triboelectric behavior of virgin and aged laser sintering powder. Due to the elevated temperatures during processing, polymer powders can exhibit altered properties even though they are not used for part generation [25,26]. Especially polyamides suffer from high temperatures because of solid-state post-condensation, resulting in a prolonged molecular chain length [12,13,27]. For economic reasons these aged powders are reused by mixing with virgin material. The effects leading to changes in powder flowability for these powders are not yet sufficiently understood and will be discussed on the basis of measurements with the aforementioned set-up and by means of charge to particle size ratios obtained by charge spectrometry for both, virgin and used PA12 powders. 2. Materials and Methods 2.1. Materials The polymer material used in this study is a commercially available polyamide 12 (PA12) laser sintering powder (PA2200, EOS GmbH, Krailling, Germany). Two kinds of conditioning were investigated, the virgin and an aged powder state, which has gone through three consecutive building processes in a FORMIGA P110 system (EOS GmbH, Krailling, Germany), each having a building time of 15.5 h, resulting in a cumulated building time of 46.5 h. This does not consider the heating-up and unregulated cooling-down phases. Powder has not been refreshed between processes. The building chamber temperature applied was 168 ◦C and the withdrawal chamber temperature was set to 150 ◦C. In each process, tensile strength bars and charpy test specimen have been built with a volumetric filling degree of 7.4% of the building chamber. 2.2. Laser Diffraction Particle size distributions have been determined by laser diffraction (Mastersizer 2000, Malvern Panalytical Ltd., Kassel, Germany). For aerosol generation a dry dispersion unit (Scirocco 2000, Malvern Panalytical Ltd., Kassel, Germany) was operated at a pressure of two bar. The values given in this paper are calculated by averaging over five consecutive measurements. For evaluation, a Rayleigh-Mie model assuming spherical particles is used. 2.3. Impedance Spectroscopy Impedance analyses have been carried out using the dielectric spectrometer Alpha-Analyzer (Novocontrol, Montabaur, Germany) equipped with a ZGS head. Prior to measurements, samples have been prepared by pelletising at 300 MPa pressure (MP150, Maassen GmbH, Reutlingen, Germany) with a 25-millimetre diameter press die set. For each pellet 600 mg of polymer powder were used, resulting in a thickness of 1.4 mm. The pellet samples have been mounted between two disc-shaped electrodes connected to the impedance analyser. Measurements have been performed at ambient conditions (19 ◦C and 30% relative humidity) in the frequency range 10−2 Hz to 20 MHz at 1 Volt root mean square (Vrms). 2.4. Powder Spreader Integrated Voltmeter Device A novel lab-scale device for the investigation of electrostatic charge build-up on laser sintering powders during powder deposition is depicted in Figure 1a. The set-up consists of an automated film applicator (Coatmaster 510, Erichsen GmbH & Co. KG, Hemer, Germany) positioned inside a glove boxPDF Image | Tribo-Charging during Powder in Selective Laser Sintering
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