Abstract
In this study, the development of a polyoxymethylene (POM) feedstock material for the powder bed fusion (PBF) of polymers is outlined. POM particles are obtained via liquid-liquid phase separation (LLPS) and precipitation, also known as the solution-dissolution process. In order to identify suitable POM solvent systems for LLPS and precipitation, in the first step, a solvent screening based on solubility parameters was performed, and acetophenone and triacetin were identified as the most promising suitable moderate solvents for POM. Cloud point curves were measured for both solvents to derive suitable temperature profiles and polymer concentrations for the solution-dissolution process. In the next step, important process parameters, namely POM concentration and stirring conditions, were studied to elucidate their effect on the product’s properties. The product particles obtained from both aforementioned solvents were characterized with regard to their morphology and size distribution, as well as their thermal properties (cf. the PBF processing window) and compared to a cryo-milled POM PBF feedstock. Both solvents allowed for precipitation of POM particles of an appropriate size distribution for PBF for polymer concentrations of at least up to 20 wt.%. Finally, a larger powder batch for application in the PBF process was produced by precipitation from the preferred solvent acetophenone. This POM powder was further analyzed concerning its flowability, Hausner ratio, and mass-specific surface area. Finally, test specimens, namely a complex gyroid body and a detailed ornament, were successfully manufactured from this feedstock powder showing appropriate bulk solid and thermal properties to demonstrate PBF processability. In summary, a processable and suitable POM PBF feedstock could be developed based on the non-mechanical solution dissolution process, which, to the authors’ best knowledge, has not been reported in previous studies.
Highlights
The term additive manufacturing (AM) is comprised of different technologies, which all have in common that the parts are built layer-by-layer, in contrast to, e.g., “traditional” subtractive manufacturing methods [1], the technologies themselves vary greatly with respect to the underlying processes and materials used
These advantages come with some drawbacks, as the requirements on the powder feedstock material used for powder bed fusion (PBF) are very demanding [6]
Very few types of PBF feedstock powders are commercially available so far, and the market is still dominated by polyamides, especially polyamide 12 (PA12) [13], making up at least 90 % of the total market share, of which the largest fraction is produced via a solution-dissolution process [14,15]
Summary
The term additive manufacturing (AM) is comprised of different technologies, which all have in common that the parts are built layer-by-layer, in contrast to, e.g., “traditional” subtractive manufacturing methods [1], the technologies themselves vary greatly with respect to the underlying processes and materials used. The most widely applied technology in polymer AM, when functional parts of excellent mechanical properties are desired, is PBF, known as selective laser sintering (SLS) [5]. In PBF, first, a homogeneous layer of feedstock powder is spread onto a building platform. A laser selectively fuses the polymer material according to the cross-section of the part to be printed in this layer. The surrounding non-fused powder acts as a support structure, making complex geometries and undercuts accessible by means of PBF. These advantages come with some drawbacks, as the requirements on the powder feedstock material used for PBF are very demanding [6]. Very few types of PBF feedstock powders are commercially available so far, and the market is still dominated by polyamides, especially polyamide 12
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