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Polymers 2019, 11, 609 5 of 12 for a circular projection area on the powder bed of varying size. Measurements were performed with a constant probe to surface spacing corresponding to a spot size at the powder bed surface of approximately six millimeters in diameter. To identify the beginning of the applied powder layer precisely, a polyethylene terephthalate (PET) foil was positioned in front of the grounded metal plate. The foil was triboelectrically charged manually to exhibit a strong negative charge. This allows an easy detection of the beginning of the positively charged polyamide powder layer by observing the zero crossing in the measurement files. Measurements over extended periods at zero potential have shown, that the accuracy of the instrumental setup is limited by the recording of the voltmeters output signal. The standard deviation of the detected signal noise is 0.34 V with a maximum deviation of ±0.97 V. Typically, the relative measuring error of voltmeter devices increases at low input signals, therefore the given values are considered as worst-case scenario. 2.5. Charge Spectrometry Charge (q) to particle size (d) ratios are investigated by means of charge spectrometry (q/d-meter, Epping GmbH, Neufahrn bei Freising, Germany), schematically visualized in Figure 1b. The device uses low pressure to accelerate a pulsed particulate stream into an electric field with static field strength. According to Equation (2): →→ Fel = q·E (2) where a charged particle’s trajectory is diverted by Fel, the force the electric field exerts on it. Here, q represents the charge of the particle and E the strength of the electric field. The force resisting this diversion is the drag force F = ρF|→v |→v πd2c (3) d 2 rel rel4 d where vrel is the particle’s velocity relative to the surrounding fluid of density ρF, cd is the Reynolds number dependent drag coefficient and d is the particle’s diameter. By balancing Equations (2) and (3) and gravity, particle trajectories can be calculated by integrating the balance of forces for their non-stationary movement. According to their movement pattern and polarity, particles are deposited on specimen slides that are mounted in front of the field-generating capacitor plates. By optically analyzing the slides with a scanning camera system, the projection area of the particles can be determined. Therefore, it is possible to calculate their charge-to-diameter ratio (q/d) from the position on the slides when flow conditions and electric field strength are known. Before any measurement, samples are mixed with magnetic iron carrier particles and activated by vortex mixing for 10 s at 2500 rpm (Digital Vortex Mixer model 945312, VWR international BVBA, Leuven, Belgium), resulting in more defined q/d-distributions than omitting powder preparation. The iron particles are held back in the sample container by a magnetic field and therefore do not get deposited on the specimen slides. Each measurement has been performed three times with several thousand particles detected in each measurement to guarantee statistical relevant data. A more exhaustive description of the concept of the measurement device and its unique selling point, the used free air beam technique, can be found in Epping et al. [29]. Particle size distributions are inherently measured by determining the q/d ratio of deposited polymer particles via charge spectrometry. Sizes are obtained by counting the number of pixels obscured by a charged particle on a specimen slide and their diameter is calculated by assuming round spheres, resulting in the same simplification of the particles morphology as the laser diffraction measurements. 3. Results and Discussion 3.1. Powder Spreader Integrated Voltmeter Device Figure 2 illustrates the progression of the electrostatic surface potential along the direction of layer deposition. Both virgin Figure 2a and used Figure 2b powders have been investigated. The samePDF Image | Tribo-Charging during Powder in Selective Laser Sintering
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