Effect of PA12 powder reuse on coalescence behaviour and microstructure of SLS parts

This work examines one polyamide 12 (PA12) powder grade but in different re-using states (i.e., virgin, aged-virgin mixed, and in-process aged powders) for selective laser sintering (SLS). This is to clarify the effect and the corresponding mechanisms of in-process ageing on thermal and coalescence behaviour (during SLS) as well as on the resulting crystallinity, microstructure and mechanical properties of single layer specimens. It is shown that powder ageing increases the viscosity and the molecular weight. After melting and similarly SLS, the crystallinity of powders drastically decreases and also the crystal structure changes from alpha- to gamma-form. Thereafter, the SLS parts from the aged powder show a decreased crystallinity. Further to crystallinity, formation of aggregated pieces after ageing expands the melting intervals. This is also observed under hot stage microscopy where the aged aggregates remain unmolten before heating to higher temperatures (whilst the virgin powder totally melts/coalesces at lower temperatures). Due to the nucleation effects of these aged aggregates during crystallisation, the microstructure is altered from fine fibrillar spherulites within large amorphous regions to coarse spherulite crystals spread throughout the matrix. The above observations and their effect on tensile and shear-punch properties are well discussed and postulated.