Experimental and Numerical Investigations on the Influence of the Inter-layer Time in Powder Bed Fusion of PA12

In the field of plastics, powder bed fusion using laser-beam (PBF-LB) represents the most industrially relevant additive manufacturing technique. To increase the productivity of the PBF process, reduced build times are of great interest. Shorter build times are economically desirable, but their influence on e.g., mechanical properties is not fully understood. Densification in PBF-LB of polymers is slow in comparison to metals due to the low ratio of surface tension to viscosity. The application of a subsequent powder layer prior to complete densification of the underlying layer can result in significant residual porosity in the part. On the other hand, faster repeated laser irradiation can lead to higher melt temperatures and thus faster densification. In this study, the influence of the inter-layer time (ILT) on the resulting part properties of poly-amide 12 (PA12) was investigated. For this purpose, experiments were carried out and numerical models were developed and compared. An experimental setup was developed to study the stepwise thickness increase of the first ten layers as well as the mechanical and density properties of multi-layer samples. Numerical simulations were performed complementary to the experimental investigations. Therefore, a simulation tool was developed and implemented for direct comparison with experimental observations. The one-dimensional finite difference solver for this purpose considers laser irradiation and melting of the powder bed as well as heat conduction, densification and crystallization of the polymer melt. Multiple layers can be simulated under realistic process conditions. We found that the competition between the time scales of densification and crystallization on one hand and the ILT on the other hand results in a wide range of porosity profiles.