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Polymers 2019, 11, x FOR PEER REVIEW 7 of 13 of the spreading process. Since electrostatic-related defects in powder layer are presumed to be most likely found at highly charged positions in the powder bed, the most critical value in this process mimicking measurement is the highest occurring surface potential. While there seems to be a Polymers 2019, 11, 609 7 of 12 converging value for virgin powder, the used powder with altered properties does not reach a limiting value of maximum surface potential within a reasonable amount of repeated powder spreTaadbilneg1.opMeerantiocnosn.duRcetpiveiatitesdobatpapinliecdatbioynismpareedaonfceimspeocrtrtoasnccoepyfomremasaunreymseinmtsploefrvliargseinrasnindtering macahgiendesPlAik1e2thfoerovnaeryuisnegdffroeqrupeonwcidese.r aVgainlugesexinpebrrimaceknets irneptrheisewntotrhke. Isttaunsdesaradbdlaedveia-tbioanseodvpeor wder sprefiavdeemrethasautreamppenlites.sacertainamountofpowder,whichisnotexactlyequaltotheamountof powder needed for a single layer. Therefore, the surplus powder is spread multiple times while Conductivity at 10−2 Hz Conductivity at ~102 Hz Conductivity at 107 Hz beingMexaeterrtieadl to triboelectric charging conditions in each spreading process. Eventually, new [×10−14 S/cm] [×10−10 S/cm] [×10−5 S/cm] powder will be applied to the remainder of the powder that has already been spread several times. Virgin powder 2.22 (±0.13) 1.47 (±0.06) 2.65 (±0.08) However, machines that are more sophisticated use an overflow mechanism that removes surplus Used powder 1.55 (±0.06) 1.31 (±0.05) 2.43 (±0.04) powder from the building chamber after being applied once. In this case, the relevant measurement is the first application of powder which can be considered less critical in comparison to repeated Moreover, the data displayed in Figure 2 shows that multiple applications of the same powder in applications as the provided data shows. close temporal proximity should be avoided when aiming for a lower influence of electrostatics during powder layer formation. This result is apparent for both, virgin and used powder and is clarified Table 1. Mean conductivities obtained by impedance spectroscopy measurements of virgin and aged in Figure 3a by means of the maximum surface potential measured for up to 20 repetitions of the PA12 for varying frequencies. Values in brackets represent the standard deviation over five spreading process. Since electrostatic-related defects in powder layer are presumed to be most likely measurements. found at highly charged positions in the powder bed, the most critical value in this process mimicking Conductivity at 10−2 Hz Conductivity at ~102 Hz Conductivity at 107 Hz measuMreamterniatlis the highest occurring surface potential. While there seems to be a converging value for [×10−14 S/cm] [×10−10 S/cm] [×10−5 S/cm] virgin powder, the used powder with altered properties does not reach a limiting value of maximum Virgin powder 2.22 (±0.13) 1.47 (±0.06) 2.65 (±0.08) surface potential within a reasonable amount of repeated powder spreading operations. Repeated Used powder 1.55 (±0.06) 1.31 (±0.05) 2.43 (±0.04) applications are of importance for many simpler laser sintering machines like the one used for powder aging experiments in this work. It uses a blade-based powder spreader that applies a certain amount In Figure 3b the charge relaxation behavior is depicted for a spatially static measurement over of powder, which is not exactly equal to the amount of powder needed for a single layer. Therefore, several hours. Analogue to earlier experiments the layer formation was done at a constant powder the surplus powder is spread multiple times while being exerted to triboelectric charging conditions deposition speed of 10 mm/s for homogeneous layer formation. The observed surface potential in each spreading process. Eventually, new powder will be applied to the remainder of the powder decays exponentially and reaches an uncharged state after approximately 12 h. Therefore, the that has already been spread several times. However, machines that are more sophisticated use an relaxation times of triboelectric charges are well beyond the times between consecutive layer overflow mechanism that removes surplus powder from the building chamber after being applied applications in the laser sintering process, where a new layer is applied every few minutes. once. In this case, the relevant measurement is the first application of powder which can be considered less critical in comparison to repeated applications as the provided data shows. (a) (b) FFigiguurere3.3.(a(a))Maaxximimuumssuurrfafacceeppootetenntitaiallmeeaassuurreeddfoforraahhigighhnnuumbbeerroof frerpepeeaateteddaappplilcicaatitoionnssoof f vvirigrgininanadnudseudsePdA1P2Ap1o2wpdoerws.d(ebr)s.Re(bla)xaRteiolanxoaftitohne eolfecthroestealteictsruorsftactiecpsouternfatciael opvoetrentitmiael. oVvoeltrmteimtere. pVrobltemheatsernpotrobbeenhamsonvoetdberenlamtivoevlyedtoretlhaetipvoelwydtoerthlaeypeorwindtehrilsamyeraisnutrhemis emnet.asEuxrpeemriemnte.nEtxhpaesribmeent chonasdubecteendcaotn3d0u%ctreedlaattiv3e0%hurmeliadtitvye. humidity. In Figure 3b the charge relaxation behavior is depicted for a spatially static measurement over several hours. Analogue to earlier experiments the layer formation was done at a constant powder deposition speed of 10 mm/s for homogeneous layer formation. The observed surface potential decays exponentially and reaches an uncharged state after approximately 12 h. Therefore, the relaxation timesPDF Image | Tribo-Charging during Powder in Selective Laser Sintering
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