Powder ageing of Polyamide 6 in laser sintering and its effects on powder and component characteristics

The increasing importance of Laser Sintering (LS) for the industry brings a rising number of available materials for this process. One example is the enhanced use of alternative polyamide types like polyamide 6 (PA 6). For polyamide 12 (PA 12), the powder ageing effect is well-known and of great importance from an economical point of view. However, relatively little is known about the powder ageing behavior of newer LS materials. Hence this paper focuses on the experimental powder ageing study of PA 6 in the LS process, both on powder and part level. The powder was reused in total seven times without refreshing and constant sinter parameters were used to produce the samples. Morphological, thermal and rheological measurements of the powder showed a rising number of agglomerates, an apparent increase in the powder crystallinity, and an increase in the viscosity by a factor of 7. These increases consequently affected the density and the mechanical properties of the parts, latter were determined by tensile tests. Between the first and the last iteration, a decrease of the Young's Modulus of 60 % and the tensile strength of 70 % was observed. Besides the decline of mechanical properties, the geometrical dimensions of the last iteration were only 60 % of the first iteration, the mass was halved and the surface quality suffered from the orange peel effect. The results prove that similar powder ageing effects known for PA 12, and here especially the polyamide post condensation, also take place in LS printing of PA 6.