Expanding the capabilities of laser-based powder bed fusion of polymers through the use of electrophotographic powder application

Abstract

Generating multimaterial parts, reaching higher efficiency in powder consumption, and decoupling of powder application behavior from powder properties such as powder flowability are key aspects for using electrophotographic powder application (EPA) in laser-based powder bed fusion of polymers (PBF-LB/P). Moreover, EPA allows the layer thickness to be reduced from around 100–150 μm, depending on respective particle size distribution, in the case of conventional doctor blade or roller-based powder application methods to the diameter of the applied polymer particles (typically between 50 and 130 μm). This can have positive effects on the interlayer connection and, therefore, the mechanical properties of the additively manufactured part because less powder volume has to be fused with the already generated underlying part. Moreover, due to the above-mentioned independence of EPA from powder flowability, the addition of flow aids, such as nano silica, can be reduced to a minimum or even avoided completely. This is the first comprehensive study on resulting properties of parts generated by PBF-LB/P using EPA taking into account both the reduction in layer thickness and reduced addition of flow aids. In addition to improving mechanical properties of generated parts, the independence of powder flowability, in particular, offers the possibility of qualifying currently unsuitable materials for PBF-LB/P. For this purpose, besides widely employed polyamide 12 (PA12), a polypropylene (PP) powder is used that is very difficult to process in conventional PBF-LB/P and can only be applied there with the help of flow aids.

Innovative Process Strategies in Powder-Based Multi-Material Additive Manufacturing

“…First, the powder and a photoconductive plate (PCP, aluminum plate coated with photoconductive material) are homogeneously charged [28][29][30]. In EPA, powder particles can be charged via gas discharge or triboelectrically [31][32][33]. While gas discharge-based powder charging does not require any further functionalization of the particles in terms of their charging behavior, for triboelectric charging, the particles need to be functionalized with charge-control agents (CCAs) [33].…”

“…While gas discharge-based powder charging does not require any further functionalization of the particles in terms of their charging behavior, for triboelectric charging, the particles need to be functionalized with charge-control agents (CCAs) [33]. However, as described in [31][32][33], triboelectric charging offers some considerable advantages compared with gas discharge-based powder charging. In the case of triboelectric powder charging, the powder flowability, which, in many cases, renders the usage of certain powders impossible for PBF-LB/P, does not play any role in powder application.…”

“…As demonstrated in [27], controlling the electric field applied in terms of field strength and especially shape is crucial for powder deposition to achieve high dimensional accuracy and coverage of the deposited powder pattern. Moreover, the accumulation of charges accompanied by a progressive decrease in the electric field strength of the powder deposition field needs to be compensated for [32]. This necessitates a strategy for compensating for charge accumulation independently of the already generated part's thickness.…”

“…Although changing the polarity after each powder deposition would be even more efficient regarding the compensation for charge accumulation, a certain duration is needed for the reversion of polarity by the high-voltage power supply. In [32], changing the operation mode from CAD to DAD after every second powder deposition was demonstrated to have a good trade-off between duration for the reversion of polarity and still maintaining significant part height growth. A further aspect of increasing the powder deposition efficiency is the development and implementation of a piezoelectric excitation device in [35].…”

“…Beyond that, the number of powder materials applicable for PBF-LB/P can be increased due to the independency of the powder flowability-especially in the case of triboelectric charging. In [32], the parts generated by EPA-PBF-LB/P could be validated to show at least the same or better mechanical properties compared with parts generated by conventional PBF-LB/P (cf. Figure 2A,B).…”