In selective laser sintering (SLS) predominantly polyamide 12 powder (PA12) is used. Often, after the sintering process approximately 70 % of powder, which has not been sintered, remains. The flow properties of this used powder have been affected because of the thermal stress during the sintering process. Due to the high price of new PA12 powder, the used and aged powder must be recycled by refreshing with new powder. This powder mixture can be reused in a next sintering process. However, due to the repeating refreshing process of the used powder, mixtures are produced whose quality is difficult to define. They differ greatly in their flow properties, which in turn has a negative effect on the component quality. In order to generate a good and reproducible powder quality, the thermal aging behavior of PA12 powder has been examined in detail by the authors. Based on these results, a recycling process could be developed step by step, by which the thermal aging can be reversed. A chemical agent is added to the used powder which is able to cleave the long polymer chains of the used powder at its amide bonds. Depending on the kind of chemical agent and its concentration, the flow properties of the used powder can be adjusted selectively, which also makes the quality of these mixtures reproducible. By means of this recycling process, the used powder has better flow and sintering properties and thus, can be reused several times. Thereby, it contributes to material and cost efficiency and thus, leads to an increased cost effectiveness of selective laser sintering.
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