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Polymers 2019, 11, 609 2 of 12 process (SLS) [3], in which components are built in a layer-by-layer fashion. Each consecutive layer is heated to a temperature closely below the melting point of the used polymer and then melted by a laser in accordance to the desired structural design [4,5]. Typically, semi-crystalline thermoplastic powders with a narrow size distribution and a mean diameter of about 50 μm are deposited by means of blade or roller coating to form a layer with a thickness of roughly 100 μm [5,6]. It is pivotal that the powder layers exhibit a smooth surface and a high packing density to ensure low part porosity. Hence, good powder flowability is key to achieve high and homogeneous production quality. Powder flowability and packing density can be tailored by classification of the powders (c.f. removal of fines), shape control (thermal and mechanical rounding) [7,8] and application of flowing aids. Additionally, the diversity of applicable polymers is limited by the stringent requirements on the material’s thermal, chemical and rheological properties like the sintering window [9], melt viscosity [10], isothermal crystallization [11] or ageing behavior [12,13]. While there is a variety of available materials, such as polystyrene (PS), poly (methyl methacrylate) (PMMA), polypropylene (PP), high-density polyethylene (HDPE), polyaryletherketones (PEAK), polyamide 11 (PA11) [14] and polyamide 6 (PA6), the overwhelming share in the market is occupied by polyamide 12 (PA12), which does not only exhibit good powder but also excellent thermal properties. The factors influencing the powder-rheological behavior are manifold, ranging from morphological conditions like particle size distribution, shape or roughness to the physicochemical material properties determining the interparticulate interactions. Therefore, knowledge of the powder conditions during layer deposition is crucial to gain a better understanding of the underlying principles. One aspect often neglected and not yet fully understood is the role of electrostatic charge build-up during polymer powder handling operations resulting from prolonged contact, collision or friction. Both homogeneous and inhomogeneous material interactions may result in charge separation on the involved species. For the explanation of these effects, three complementary concepts are commonly used, i.e., the electron, ion and material transfer model. A comprehensive overview and deeper insights into the governing mechanisms resulting in triboelectric charging is given e.g., by Matsusaka et al. [15] and Lacks et al. [16]. The concept of electron transfer is well-established and extensively reviewed for the interaction of metals with non-oxidized surfaces. The triboelectric charging of metals is usually not critical in technical applications due to the high conductivity of involved species and, therefore, the fast transport of charges away from the contacting area. This behavior allows for an easy measurement of the transferred charges, which are considered to be a result from electron transfer processes. The amount of charge transferred is dependent on the contact potential difference arising from different work functions of the contacting metals [15,17]. Modifications to this model have been made to incorporate insulator contacts by assigning an effective work function to them or utilizing the concept of higher energy level surface electrons, so-called surface states. While these theories yield good agreement for some experimental observations, there are cases where they do not hold (see [15] and references therein). Ion transfer as a reason for triboelectric charging is considered when weakly bound or mobile ions are involved in material contact. One prevalent example is the thin water layer adsorbed on many surfaces that can be responsible for exchanging ionic species during contact events [17]. Material transfer from one body to another can arise from impact, adhesion or friction between them and it can include charge transfer if the transferred material carries any charge. It has been shown that polymer particles impacting a metal wall generate a current by leaving behind material on the wall [18]. Charge transfer between adhering and separated polymer particles can be assessed by AFM measurements whereby the single particle charge can be determined from the force-separation curve [19]. In the case of highly functionalized particles, like those used in SLS a material transfer of flow aids or other additives is conceivable. While there is significant research on the generalized principles of contact electrification of powders for changing basic conditions like humidity [20], the influence of triboelectric charging for polymers in varying reactor designs [21,22] and experimental set-ups to replicate reactor or fluidizedPDF Image | Tribo-Charging during Powder in Selective Laser Sintering
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