Material aging of Polyamide 12 (Laurinlactam) is a very common problem in laser sintering (LS). For stable process conditions, recycled material used in previous processes should be refreshed with 30–50% virgin powder material. However, even by following these refreshing strategies, material quality drops to an insufficient level after several process cycles which leads to poor part quality showing orange peel or poor mechanical properties when processed. In order to avoid this, a quality assurance system has been established to provide recommendations for robust process conditions and material qualities. A detailed study on aging processes in LS comparing two different machines was performed in order to analyze correlations between material quality, process parameters and part properties. Energy input allowing for robust processing conditions should be in a range between 0.325 and 0.42 J/mm3 showing almost identical values for both machines. Optimal material quality ranges was found to be machine specific, while the lower limit lies between 20 and 25 cm3/ 10 min for both machines used. Additionally, material aging characteristics in an oven and a LS machine were compared, in order to simulate material aging in the LS process by simple experiments in an oven. POLYM. ENG. SCI., 54:1540–1554, 2014. © 2013 Society of Plastics Engineers

In laser sintering (LS) un-molten Polyamide 12 (PA12) powder is usually re-used (recycled) in further processes. However, LS processing time at powder bed temperature leads to material property changes. As a consequence, un-molten PA12 powder that is re-used or recycled in further processes leads to process and part properties deviations. In this context, powder particle size, shape and distribution is assumed to affect surface roughness and porosity of LS parts. In order to investigate this process effect on changes in powder size, shape and distribution, PA12 powder was systematically aged in a vacuum oven at conditions close to the LS process. According to this procedure, polymeric powder was obtained with aging times up to 120 hours and analyzed by dynamic image analysis. At first, fresh powder was investigated as a reference. The effect of LS processing time and temperature, i.e. powder bed temperature of approx. 174°C was measured with respect to changes in size distribution and shape whereas particles were considered of size up to 500μm. The influence of LS processing time at powder bed temperature was found to be neither significant on changes in particle size nor distribution. With respect to particle shape, a higher deviation to the reference was observed for particle size bigger than 100 μm and longer aging times. Consequently, influences on particle shape changes on surface roughness are assumed to be more likely than influences on part porosity due to LS processing conditions.

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