A Round Robin study for Selective Laser Sintering of polyamide 12: Microstructural origin of the mechanical properties

The mechanical and microstructural investigation of polymer parts (polyamide 12) fabricated by Selective Laser Sintering as part of a Round Robin initiative is presented. The paper focuses on the microstructural analysis of the Round Robin samples and their evaluation regarding their effect on mechanical properties with respect to each other. Therefore optical microscopy on microtomed samples, X-ray computed tomography and Differential Scanning Calorimetry is used to determine the morphology of residual particle cores and of internal pores. The mechanical tensile testing revealed a high variability of the ductility of the samples among the used machines and a distinctive anisotropic mechanical response. Especially the quite brittle characteristic along the building direction has shown to be still a crucial challenge for the process. However, one machine delivered samples with outstanding ductility with total elongation values of about 21% along the building direction and of about 32% planar to the layer. This result was back traced to a distinctive pore and residual particle morphology which is characterized by low pore concentration, the absence of coplanar pore or residual particle arrangements and the highest degree of particle melting measured. Furthermore, the analysis depicts that both features, pores and residual particles, contribute to the mechanical properties significantly and that they are not necessarily linked since they can vary independently in a certain range depending on the machine configuration.