The Effect of Physical Aging and Degradation on the Re-Use of Polyamide 12 in Powder Bed Fusion

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Abstract
Powder bed fusion (PBF) is an additive manufacturing (AM) technique which offers efficient part-production, light-weighting, and the ability to create complex geometries. However, during a build cycle, multiple aging and degradation processes occur which may affect the reusability of the Polyamide 12 (PA-12) powder. Limited understanding of these phenomena can result in discarding re-usable powder unnecessarily, or the production of parts with insufficient properties, both of which lead to significant amounts of waste. This paper examines the thermal, chemical, and mechanical characteristics of PA-12 via an oven storage experiment that simulates multi jet fusion (MJF) conditions. Changes in the properties of PA-12 powder during oven storage showed two separate, time-dependent trends. Initially, differential scanning calorimetry showed a 4.2 °C increase in melting temperature (Tm) and a rise in crystallinity (Xc). This suggests that secondary crystallisation is occurring instead of, or in addition to, the more commonly reported further polycondensation process. However, with extended storage time, there were substantial reductions in Tm and Xc, whilst an 11.6 °C decrease in crystallisation temperature was observed. Fourier transform infrared spectroscopy, a technique rarely used in PBF literature, shows an increased presence of imide bonds—a key marker of thermo-oxidative degradation. Discolouration of samples, an 81% reduction in strength and severe material embrittlement provided further evidence that thermo-oxidative degradation becomes the dominant process following extended storage times beyond 100 h. An additional pre-drying experiment showed how moisture present within PA-12 can also accelerate degradation via hydrolysis.

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